Search

Complex coacervate core micelles (C3Ms) are formed by mixing aqueous solutions of a charged (bio)macromolecule with an oppositely charged-neutral hydrophilic diblock copolymer. The stability of these structures is dependent on the ionic strength of the solution; above a critical ionic strength, the micelles will completely disintegrate. This instability at high ionic strengths is the main drawback for their application in, e.g. , drug delivery systems or protein protection. In addition, the stability of C3Ms composed of weak polyelectrolytes is pH-dependent as well. The aim of this study is to assess the effectiveness of covalent crosslinking of the complex coacervate core to improve the stability of C3Ms. We studied the formation of C3Ms using a quaternized and amine-functionalized cationic-neutral diblock copolymer, poly(2-vinylpyridine)- block -poly(ethylene oxide) (QP2VP- b -PEO), and an anionic homopolymer, poly(acrylic acid) (PAA). Two different core-crosslinking strategies were employed that resulted in crosslinks between both types of polyelectrolyte chains in the core ( i.e. , between QP2VP and PAA) or in crosslinks between polyelectrolyte chains of the same type only ( i.e. , QP2VP). For these two strategies we used the crosslinkers 1-ethyl-3-(3'-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and dimethyl-3,3'-dithiopropionimidate dihydrochloride (DTBP), respectively. EDC provides permanent crosslinks, while DTBP crosslinks can be broken by a reducing agent. Dynamic light scattering showed that both approaches significantly improved the stability of C3Ms against salt and pH changes. Furthermore, reduction of the disulphide bridges in the DTBP core-crosslinked micelles largely restored the original salt-stability profile. Therefore, this feature provides an excellent starting point for the application of C3Ms in controlled release formulations.

Encapsulation of proteins can have advantages for their protection, stability, and delivery purposes. One of the options to encapsulate proteins is to incorporate them in complex coacervate core micelles (C3Ms). This can easily be achieved by mixing aqueous solutions of the protein and an oppositely charged neutral-hydrophilic diblock copolymer. However, protein-containing C3Ms often suffer from salt-inducible disintegration due to the low charge density of proteins. The aim of this study is to improve the salt stability of protein-containing C3Ms by increasing the net charge of the protein by tagging it with a charged polypeptide. As a model protein, we used CotA laccase and generated variants with 10, 20, 30, and 40 glutamic acids attached at the C-terminus of CotA using genetic engineering. Micelles were obtained by mixing the five CotA variants with poly( N -methyl-2-vinyl-pyridinium)- block -poly(ethylene oxide) (PM2VP 128 - b -PEO 477 ) at pH 10.8. Hydrodynamic radii of the micelles of approximately 31, 27, and 23 nm for native CotA, CotA-E20, and CotA-E40, respectively, were determined using dynamic light scattering (DLS) and fluorescence correlation spectroscopy (FCS). The encapsulation efficiency was not affected using enzymes with a polyglutamic acid tail but resulted in more micelles with a smaller number of enzyme molecules per micelle. Furthermore, it was shown that the addition of a polyglutamic acid tail to CotA indeed resulted in improved salt stability of enzyme-containing C3Ms. Interestingly, the polyglutamic acid CotA variants showed an enhanced enzyme activity. This study demonstrates that increasing the net charge of enzymes through genetic engineering is a promising strategy to improve the practical applicability of C3Ms as enzyme delivery systems.

Bacterial alginate-like exopolymers (ALE) gels have been used in this work as a model for the extracellular polymeric matrix of biofilms. Aim was to relate the mechanical properties and strength of this matrix that make biofilms as persistent to cleaning as they are, to the complex cohesive molecular interactions involved. Mechanical properties of the gels as a function of CaCO 3 concentration were investigated using dynamic and static rheology. Gels with relatively low CaCO 3 concentrations, between 100 μmol and 300 μmol per g ALE, were found to exhibit similar viscoelastic behaviour as real biofilms, with elastic moduli between 50 Pa and 100 Pa and dissipation factors between 0.2 and 0.3. Increasing CaCO 3 concentrations resulted in an increase of the elastic modulus up to 250 Pa, accompanied by an increase in brittleness. At a CaCO 3 concentration of 1250 μmol per g ALE this trend stopped, probably due to disturbance of the continuous ALE network by precipitation of salts. Therefore, overdosing of Ca salts can be an adequate approach for the removal of biofouling. All gels exhibited permanent strain hardening under medium strain, and their mechanical properties showed dependency on their strain history. Even after application of an oscillatory strain with 200% amplitude that caused the gel structure to collapse, the gels recovered 65 to 90% of their original shear modulus, for the major part within the first 20 s. Recovery was slightly less for gels with high CaCO 3 concentration. In creep tests fitted with a Burgers model with multiple Kelvin elements at least three different interactions in the ALE gels could be distinguished with characteristic retardation times in the range of 10, 100 and 1000 s. Further identification of the mechanisms underlying the gel mechanics will allow the development of targeted strategies to undermine the mechanical strength of biofouling and aid the cleaning process., (Copyright © 2021 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The microbial transglutaminase (mTG) was used to improve the stability of the naringenin-loaded β-casein micelles (CNMs). The formation of cross-linked CNMs was confirmed by SDS-PAGE electrophoresis, showing a decrease in monomeric β-CN levels with increasing crosslinking time. Dynamic light scattering (DLS) showed that after crosslinking the particle size distribution did not change upon dilution, suggesting occurrence of intra-crosslinking. Fluorescence spectroscopy and circular dichroism (CD) showed that crosslinking induced only minor changes in the structure. Finally, release of naringenin in buffer at pH 7.4 demonstrated a slower release from the cross-linked micelles compared to the untreated micelles. In addition, the cross-linked micelles exhibited a partial resistance to pepsin enzyme. We conclude that crosslinking with mTG is a suitable method to modulate naringenin release kinetics from β-CN micelles and improves the potential of these micelles as delivery systems targeted to the small intestine., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Mapping the topological phase behaviour of lipids in aqueous solution is time consuming and finding the ideal lipid system for a desired application is often a matter of trial and error. Modelling techniques that can accurately predict the mesomorphic phase behaviour of lipid systems are therefore of paramount importance. Here, the self-consistent field theory of Scheutjens and Fleer (SF-SCF) in which a lattice refinement has been implemented, is used to scrutinize how various additives modify the self-assembled phase behaviour of monoolein (MO) and 1,2-dioleoyl-phosphatidylcholine (DOPC) lipids in water. The mesomorphic behaviour is inferred from trends in the mechanical properties of equilibrium lipid bilayers with increasing additive content. More specifically, we focus on the Helfrich parameters, that is, the mean and Gaussian bending rigidities (κ and [small kappa, Greek, macron], respectively) supplemented with the spontaneous curvature of the monolayer (Jm0). We use previously established interaction parameters that position the unperturbed DOPC system in the lamellar Lα phase ([small kappa, Greek, macron] < 0, κ > 0 and Jm0 ≈ 0). Similar interaction parameters position the MO system firmly in a bicontinuous cubic phase ([small kappa, Greek, macron] > 0). In line with experimental data, a mixture of MO and DOPC tends to be in one of these two phases, depending on the mixing ratio. Moreover we find good correlations between predicted trends and experimental data concerning the phase changes of MO in response to a wide range of additives. These correlations give credibility to the use of SF-SCF modelling as a valuable tool to quickly explore the mesomorphic phase space of (phospho)lipid bilayer systems including additives.

When proteins in aqueous solutions are exposed to solid substrates, they adsorb due to the dynamic interplay of electrostatic, van der Waals, and hydration interactions and do so in a rather irreversible fashion, which makes protein recovery troublesome. Here, we use a gold electrode as the solid substrate and modulate the surface potential to systematically induce protein adsorption as well as partial desorption. We use different methods such as surface plasmon resonance, atomic force microscopy, and electrowetting and show that biasing the electrode to more negative potentials (by -0.4 V compared to the open-circuit potential at pH 6) results in an increased adsorption barrier of 6 kJ mol -1 for the negatively charged protein β-lactoglobulin. Further, we clearly demonstrate that this is due to an increased double layer potential of -0.06 V and an increase in hydration repulsion. This indicates that an electric potential can directly influence surface interactions and thus induce partial β-lactoglobulin desorption. These observations can be the basis for biosensors as well as separation technologies that use only one trigger to steer protein ad- and desorption, which is low in energy requirement and does not generate large waste streams, as is the case for standard protein separation technologies.

The properties of biofilm EPS are determined by the multiple interactions between its constituents and the surrounding environment. Because of the high complexity of biofilm EPS, its constituents' characterisation is still far from thorough, and identification of these interactions cannot be done yet. Therefore, we use gels of bacterial alginate-like exopolysaccharides (ALEs) as a model component for biofilm EPS in this work. These gels have been examined for their cohesive properties as a function of CaCl 2 and KCl concentration. Hereto, ALE gel layers were formed on membranes by dead-end filtration of ALE solutions. Accumulation of the cations Ca 2+ and K + in the gels could be well predicted from a Donnan equilibrium model based on the fixed negative charges in the ALE. This suggests that there is no specific binding of Ca 2+ to the ALE and that on the time scale of the experiments, the Ca 2+ ions can distribute freely over the gel and the surrounding solution. The concentration of fixed negative charges in the ALE was estimated around 1 mmol/g VSS (volatile suspended solids, organic mass) from the Donnan equilibrium. Moreover, an accumulation of H + was predicted. Gels with more CaCl 2 in the supernatant were more compact and bore a higher osmotic pressure than those with less CaCl 2 , revealing the role of Ca 2+ ions in the network crosslinking. It is hypothesised that this mechanism later transitions into a rearrangement of the ALE molecules, which eventually leads to a fibrous network structure with large voids., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The self-consistent field theory of Scheutjens and Fleer is implemented on a grid with (lattice) sites that are smaller than the segment size. In this quasi lattice-free implementation we consider united atom-like molecular models and study bilayer self-assembly of phospholipids in a selective solvent (water). We find structural as well as mechanical parameters for these bilayers. The mean (κ) and Gaussian ([small kappa, Greek, macron]) bending moduli, as well as the spontaneous curvature of the monolayer (Jm0), are computed for the first time following a grand canonical ensemble route. Results are in line with previous estimates for mechanical parameters that at the time could not be made following this correct route. This proves that the mean bending modulus is only a very weak function of the membrane tension. We performed a systematic study on the effects of model parameter variations. The mean bending modulus generally grows with increasing bilayer thickness. As expected Jm0 and [small kappa, Greek, macron] behave oppositely with respect to each other and for classical phospholipids assumes values near zero. As an example, an increase in the lipophilic to hydrophilic ratio in the lipids, may cause the Gaussian bending rigidity to switch sign from negative to positive, while - not necessarily at the same point - the spontaneous curvature of the monolayer may switch sign from positive to negative. Together with other investigated trends, these results point to mechanisms of how topological phase transitions of the lipid bilayer membranes may be regulated in the biological context, which correlates with known lipid phase behaviour.

Small vesicle aggregates as a model for primitive cellular assemblies or for application as multi-compartment drug delivery systems recently received a lot of interest, yet controlling the aggregation of vesicles to predetermined aggregate sizes remains quite a challenge. We show that this type of control is possible by using a combination of two different linker systems: streptavidin-biotin and C18-pNIPAm. The latter linker is a thermoresponsive surfactant, which below its lower critical solution temperature (LCST) of 32 °C acts as barrier on the outside of the vesicles preventing aggregation, even in the presence of other linkers. Above the LCST however, C18-pNIPAm collapses, becomes sticky and thus acts as a linker inducing aggregation. By working at low vesicle concentrations and tuning the C18-pNIPAm/lipid ratio, the aggregation is by design limited. When the temperature drops below the LCST again, the aggregation is reversed. However, this is not the case if other linkers are present. The collapse of C18-pNIPAm above the LCST provides close contact between vesicles, allowing other linker molecules to connect them. By combining the reversible 'switch-like' aggregation properties of C18-pNIPAm, with the irreversible linkage between biotinylated lipids and streptavidin, it is possible to control the size of the aggregates step by step using a simple temperature program.

Encapsulation of charged proteins into complex coacervate core micelles (C3Ms) can be accomplished by mixing them with oppositely charged diblock copolymers. However, these micelles tend to disintegrate at high ionic strength. Previous research showed that the addition of a homopolymer with the same charge sign as the protein improved the stability of protein-containing C3Ms. In this research, we used fluorescence correlation spectroscopy (FCS) and dynamic light scattering (DLS) to study how the addition of the homopolymer affects the encapsulation efficiency and salt stability of the micelles. We studied the encapsulation of laccase spore coat protein A (CotA), a multicopper oxidase, using a strong cationic-neutral diblock copolymer, poly( N -methyl-2-vinyl-pyridinium iodide)- block -poly(ethylene oxide) (PM2VP 128 - b -PEO 477 ), and a negatively charged homopolymer, poly(4-styrenesulfonate) (PSS 215 ). DLS indeed showed an improved stability of this three-component C3M system against the addition of salt compared to a two-component system. Remarkably, FCS showed that the release of CotA from a three-component C3M system occurred at a lower salt concentration and over a narrower concentration range than the dissociation of C3Ms. In conclusion, although the addition of the homopolymer to the system leads to micelles with a higher salt stability, CotA is excluded from the C3Ms already at lower ionic strengths because the homopolymer acts as a competitor of the enzyme for encapsulation.

Lipid vesicles are widely used as model systems to study biological membranes. The self-assembly of such vesicles into vesicle pairs provides further opportunity to study interactions between membranes. However, formation of vesicle pairs, while subsequently keeping their colloidal stability intact, is challenging. Here, we report on three strategies that lead to stable finite-sized aggregates of phospholipid vesicles: (i) vesicles containing biotinylated lipids are coupled together with streptavidin, (ii) bridging attraction is exploited by adding cationic polymers (polylysine) to negatively charged vesicles, and (iii) temperature as a control parameter is used for the aggregation of vesicles mixed with a thermo-sensitive surfactant. While each strategy has its own advantages and disadvantages for vesicle pair formation, the latter strategy additionally shows reversible limited aggregation: above the LCST of pNIPAm, vesicle pairs are formed, while below the LCST, single vesicles prevail. Mixing protocols were assessed by dynamic and static light scattering as well as fluorescence correlation spectroscopy to determine under which conditions vesicle pairs dominate the aggregate size distribution. We have strong indications that without subsequent perturbation, the individual vesicles remain intact and no fusion or leakage between vesicles occurs after vesicle pairs have formed.

The use of microbial fuel cells (MFCs) for wastewater treatment fits in a circular economy context, as they can produce electricity by the removal of organic matter in the wastewater. Activated carbon (AC) granules are an attractive electrode material for bioanodes in MFCs, as they are cheap and provide electroactive bacteria with a large surface area for attachment. The characterization of biofilm growth on AC granules, however, is challenging due to their high roughness and three-dimensional structure. In this research, we show that 3D magnetic resonance imaging (MRI) can be used to visualize biofilm distribution and determine its volume on irregular-shaped single AC granules in a non-destructive way, while being combined with electrochemical and biomass analyses. Ten AC granules with electroactive biofilm (i.e. granular bioanodes) were collected at different growth stages (3 to 21 days after microbial inoculation) from a multi-anode MFC and T 1 -weighted 3D-MRI experiments were performed for three-dimensional biofilm visualization. With time, a more homogeneous biofilm distribution and an increased biofilm thickness could be observed in the 3D-MRI images. Biofilm volumes varied from 0.4 μL (day 4) to 2 μL (day 21) and were linearly correlated (R 2  = 0.9) to the total produced electric charge and total nitrogen content of the granular bioanodes, with values of 66.4 C μL -1 and 17 μg N μL -1 , respectively. In future, in situ MRI measurements could be used to monitor biofilm growth and distribution on AC granules., (Copyright © 2019 The Authors. Published by Elsevier Ltd.. All rights reserved.)

We developed a hybrid Monte Carlo self-consistent field technique to model physical gels composed of ABA triblock copolymers and gain insight into the structure and interactions in such gels. The associative A blocks of the polymers are confined to small volumes called nodes, while the B block can move freely as long as it is connected to the A blocks. A Monte Carlo algorithm is used to sample the node configurations on a lattice, and Scheutjens-Fleer self-consistent field (SF-SCF) equations are used to determine the change in free energy. The advantage of this approach over more coarse grained methods is that we do not need to predefine an interaction potential between the nodes. Using this MC-SCF hybrid simulation, we determined the radial distribution functions of the nodes and structure factors and osmotic compressibilities of the gels. For a high number of polymers per node and a solvent-B Flory-Huggins interaction parameter of 0.5, phase separation is predicted. Because of limitations in the simulation volume, we did however not establish the full phase diagram. For comparison, we performed some coarse-grained MC simulations in which the nodes are modeled as single particles with pair potentials extracted from SF-SCF calculations. At intermediate concentrations, these simulations gave qualitatively similar results as the MC-SCF hybrid. However, at relatively low and high polymer volume fractions, the structure of the coarse-grained gels is significantly different because higher-order interactions between the nodes are not accounted for. Finally, we compare the predictions of the MC-SCF simulations with experimental and modeling data on telechelic polymer networks from literature.

The encapsulation of proteins into complex coacervate core micelles (C3Ms) is of potential interest for a wide range of applications. To address the stability and dynamic properties of these polyelectrolyte complexes, combinations of cyan, yellow, and blue fluorescent proteins were encapsulated with cationic-neutral diblock copolymer poly(2-methyl-vinyl-pyridinium) 128 - b-poly(ethylene-oxide) 477 . Förster resonance energy transfer (FRET) allowed us to determine the kinetics of C3M formation and of protein exchange between C3Ms. Both processes follow first-order kinetics with relaxation times of ±100 s at low ionic strength ( I = 2.5 mM). Stability studies revealed that 50% of FRET was lost at I = 20 mM, pointing to the disintegration of the C3Ms. On the basis of experimental and theoretical considerations, we propose that C3Ms relax to their final state by association and dissociation of near-neutral soluble protein-polymer complexes. To obtain protein-containing C3Ms suitable for applications, it is necessary to improve the rigidity and salt stability of these complexes.

In order to study how acidic pro-peptides inhibit the antimicrobial activity of antimicrobial peptides, we introduce a simple model system, consisting of a 19 amino-acid long antimicrobial peptide, and an N-terminally attached, 10 amino-acid long acidic model pro-peptide. The antimicrobial peptide is a fragment of the crotalicidin peptide, a member of the cathelidin family, from rattlesnake venom. The model pro-peptide is a deca (glutamic acid). Attachment of the model pro-peptide only leads to a moderately large reduction in the binding to- and induced leakage of model liposomes, while the antimicrobial activity of the crotalicidin fragment is completely inhibited by attaching the model pro-peptide. Attaching the pro-peptide induces a conformational change to a more helical conformation, while there are no signs of intra- or intermolecular peptide complexation. We conclude that inhibition of antimicrobial activity by the model pro-peptide might be related to a conformational change induced by the pro-peptide domain, and that additional effects beyond induced changes in membrane activity must also be involved.

Adhesion of emulsified oil droplets to a surface plays an important role in processes such as crossflow membrane filtration, where the oil causes fouling. We present a novel technique, in which we study oil droplets on a model surface in a flow cell under shear force to determine the adhesive force between droplets and surface. We prepared an emulsion of hexadecane and used hydrophilic and hydrophobic glass slides as model surfaces. Different surfactants were used as emulsifiers: negatively charged sodium dodecyl sulphate (SDS), positively charged hexadecyltrimethylammonium bromide (CTAB) and nonionic Triton X-100. We evaluate the role of the surfactant, the glass surface properties and the ionic strength of the emulsion. We found a minimum in the adhesion force between droplets and surface as a function of surfactant concentration. The charged surfactants cause a lower droplet adhesion compared to the nonionic surfactant. The flow cell technique presented here proved to be very useful in understanding the interaction between oil droplets and a surface.

Encapsulation of proteins can be beneficial for food and biomedical applications. To study their biophysical properties in complex coacervate core micelles (C3Ms), we previously encapsulated enhanced green fluorescent protein (EGFP) and its monomeric variant, mEGFP, with the cationic-neutral diblock copolymer poly(2-methyl-vinyl-pyridinium) n - b -poly(ethylene-oxide) m (P2MVP n - b -PEO m ) as enveloping material. C3Ms with high packaging densities of fluorescent proteins (FPs) were obtained, resulting in a restricted orientational freedom of the protein molecules, influencing their structural and spectral properties. To address the generality of this behavior, we encapsulated seven FPs with P2MVP 41 - b -PEO 205 and P2MVP 128 - b -PEO 477 . Dynamic light scattering and fluorescence correlation spectroscopy showed lower encapsulation efficiencies for members of the Anthozoa class ( an FPs) than for Hydrozoa FPs derived from Aequorea victoria ( av FPs). Far-UV CD spectra of the free FPs showed remarkable differences between av FPs and an FPs, caused by rounder barrel structures for av FPs and more elliptic ones for an FPs. These structural differences, along with the differences in charge distribution, might explain the variations in encapsulation efficiency between av FPs and an FPs. Furthermore, the av FPs remain monomeric in C3Ms with minor spectral and structural changes. In contrast, the encapsulation of an FPs gives rise to decreased quantum yields (monomeric Kusabira Orange 2 (mKO2) and Tag red fluorescent protein (TagRFP)) or to a p K a shift of the chromophore (FP variant mCherry)., Competing Interests: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Complex coacervate core micelles (C3Ms) are colloidal structures useful for encapsulation of biomacromolecules. We previously demonstrated that enhanced green fluorescent protein (EGFP) can be encapsulated into C3Ms using the diblock copolymer poly(2-methyl-vinyl-pyridinium) 41 -b-poly(ethylene-oxide) 205 . This packaging resulted in deviating spectroscopic features of the encapsulated EGFP molecules. Here we show that for monomeric EGFP variant (mEGFP) micellar encapsulation affects the absorption and fluorescence properties to a much lesser extent, and that changes in circular dichroism characteristics are specific for encapsulated EGFP. Time-resolved fluorescence anisotropy of encapsulated (m)EGFP established the occurrence of homo-FRET (Förster resonance energy transfer) with larger transfer correlation times in the case of EGFP. Together, these findings support that EGFP dimerizes whereas the mEGFP mainly remains as a monomer in the densely packed C3Ms. We propose that dimerization of encapsulated EGFP causes a reorientation of Glu222, resulting in a pK a shift of the chromophore, which is fully reversible after release of EGFP from the C3Ms at a high ionic strength.

To investigate processes possibly underlying accumulation and ecological effects of plastic nano-particles we have characterized their interaction with the cell wall of green algae. More specifically, we have investigated the influence of particle surface functionality and water hardness (Ca 2+ concentration) on particle adsorption to algae cell walls. Polystyrene nanoparticles with different functional groups (non-functionalized, -COOH and -NH 2 ) as well as coated (starch and PEG) gold nanoparticles were applied in these studies. Depletion measurements and atomic force microscopy (AFM) showed that adsorption of neutral and positively charged plastic nanoparticles onto the cell wall of P. subcapitata was stronger than that of negatively charged plastic particles. Results indicated that binding affinity is a function of both inter-particle and particle-cell wall interactions which are in turn influenced by the medium hardness and particle concentration. Physicochemical modelling using DLVO theory was used to interpret the experimental data, using also values for interfacial surface free energies. Our study shows that material properties and medium conditions play a crucial role in the rate and state of nanoparticle bio-adsorption for green algae. The results show that the toxicity of nanoparticles can be better described and assessed by using appropriate dose metrics including material properties, complexation/agglomeration behavior and cellular attachment and adsorption. The applied methodology provides an efficient and feasible approach for evaluating potential accumulation and hazardous effects of nanoparticles to algae caused by particle interactions with the algae cell walls., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The Scheutjens-Fleer self-consistent field (SF-SCF) theory is used to study complexation between two oppositely charged polyelectrolytes across an interface formed by two solvents, here called oil and water. The focus is on the composition and the lateral stability of such interfacial coacervate. One polyelectrolyte is chosen to be oil soluble and the other one prefers water, whereas the counter and salt ions are taken to distribute ideally over all phases. There exists an electrostatic associative driving force for the formation of the coacervate phase which increases with decreasing ionic strength and may be assisted by some specific affinity between the associating units and an effective poor solvency for the coacervate. As with respect to the lateral stability an unusual wetting scenario, called pseudo-partial wetting, presents itself, which results from interactions on two different length scales. On the segmental length the screening of oil-water contacts promotes the wetting by the coacervate: a pre-wetting jump-like transition takes place off-coexistence from a microscopically thin to a mesoscopically thin film. Usually this implies complete wetting. However, the mesoscopically thin film is exposed to long-ranged attractive electrostatic interactions and therefore cannot grow to macroscopic dimensions upon approach towards coexistence. Hence the system remains partial wet. The bulk correlation length controls the thickness of the mesoscopically thin film and as a result the wetting transition occurs extremely close to the bulk critical point. We therefore expect that a thick coacervate film typically is laterally inhomogeneous: there are drops on top of a mesoscopically thin coacervate film. This conclusion qualitatively explains the experimental observation that such a coacervate film scatters visible light., (Copyright © 2016 Elsevier B.V. All rights reserved.)

We investigated the growth of biosynthetic protein polymers with templated curvature on pluronic nanospheres. The protein has a central silk-like block containing glutamic residues (S(E)) and collagen-like end-blocks (C). The S(E) blocks stack into filaments when their charge is removed (pH <5). Indeed, at low pH curved and circular fibers are formed at the surface of the nanospheres, which keep their shape after removal of the pluronics. The data reveal the mechanism of the templated fibril-growth: The growth of protein assemblies is nucleated in solution; small protein fibrils adsorb on the nanospheres, presumably due to hydrogen bond formation between the silk-like blocks and the pluronic PEO blocks. The surface of the pluronic particles templates further growth. At relatively low protein/pluronic weight ratios, only a fraction of the nanospheres bears protein fibers, pointing to a limiting amount of nuclei in solution. Because the nanospheres capture fibrils at an early stage of growth, they can be used to separate growth and nucleation rates in protein fibril formation. Moreover, the nanoparticle-templated growth of stable curved fibers opens ways to build proteinaceous nanocapsules from designed protein polymers.

We present the design, preparation, and characterization of two types of complex coacervate core micelles (C3Ms) with cross-linked cores and spectroscopic labels and demonstrate their use as diffusional probes to investigate the microstructure of percolating biopolymer networks. The first type consists of poly(allylamine hydrochloride) (PAH) and poly(ethylene oxide)-poly(methacrylic acid) (PEO-b-PMAA), labeled with ATTO 488 fluorescent dyes. We show that the size of these probes can be tuned by choosing the length of the PEO-PMAA chains. ATTO 488-labeled PEO113-PMAA15 micelles are very bright with 18 dye molecules incorporated into their cores. The second type is a (19)F-labeled micelle, for which we used PAH and a (19)F-labeled diblock copolymer tailor-made from poly(ethylene oxide)-poly(acrylic acid) (mPEO79-b-PAA14). These micelles contain approximately 4 wt % of (19)F and can be detected by (19)F NMR. The (19)F labels are placed at the end of a small spacer to allow for the necessary rotational mobility. We used these ATTO- and (19)F-labeled micelles to probe the microstructures of a transient gel (xanthan gum) and a cross-linked, heterogeneous gel (κ-carrageenan). For the transient gel, sensitive optical diffusometry methods, including fluorescence correlation spectroscopy, fluorescence recovery after photobleaching, and super-resolution single nanoparticle tracking, allowed us to measure the diffusion coefficient in networks with increasing density. From these measurements, we determined the diameters of the constituent xanthan fibers. In the heterogeneous κ-carrageenan gels, bimodal nanoparticle diffusion was observed, which is a signpost of microstructural heterogeneity of the network.

Protein encapsulation with polymers has a high potential for drug delivery, enzyme protection and stabilization. Formation of such structures can be achieved by the use of polyelectrolytes to generate so-called complex coacervate core micelles (C3Ms). Here, encapsulation of enhanced green fluorescent protein (EGFP) was investigated using a cationic-neutral diblock copolymer of two different sizes: poly(2-methyl-vinyl-pyridinium)41-b-poly(ethylene-oxide)205 and poly(2-methyl-vinyl-pyridinium)128-b-poly(ethylene-oxide)477. Dynamic light scattering and fluorescence correlation spectroscopy (FCS) revealed a preferred micellar composition (PMC) with a positive charge composition of 0.65 for both diblock copolymers and micellar hydrodynamic radii of approximately 34 nm. FCS data show that at the PMC, C3Ms are formed above 100 nM EGFP, independent of polymer length. Mixtures of EGFP and nonfluorescent GFP were used to quantify the amount of GFP molecules per C3M, resulting in approximately 450 GFPs encapsulated per micelle. This study shows that FCS can be successfully applied for the characterization of protein-containing C3Ms.

Algal blooms can seriously affect the operation of water treatment processes including low pressure (micro- and ultra-filtration) and high pressure (nanofiltration and reverse osmosis) membranes mainly due to accumulation of algal-derived organic matter (AOM). In this study, the different components of AOM extracted from three common species of bloom-forming algae (Alexandrium tamarense, Chaetoceros affinis and Microcystis sp.) were characterised employing various analytical techniques, such as liquid chromatography - organic carbon detection, fluorescence spectroscopy, fourier transform infrared spectroscopy, alcian blue staining and lectin staining coupled with laser scanning microscopy to indentify its composition and force measurement using atomic force microscopy to measure its stickiness. Batch culture monitoring of the three algal species illustrated varying characteristics in terms of growth pattern, cell concentration and AOM release. The AOM produced by the three algal species comprised mainly biopolymers (e.g., polysaccharides and proteins) but some refractory compounds (e.g., humic-like substances) and other low molecular weight acid and neutral compounds were also found. Biopolymers containing fucose and sulphated functional groups were found in all AOM samples while the presence of other functional groups varied between different species. A large majority (>80%) of the acidic polysaccharide components (in terms of transparent exopolymer particles) were found in the colloidal size range (<0.4 μm). The relative stickiness of AOM substantially varied between algal species and that the cohesion between AOM-coated surfaces was much stronger than the adhesion of AOM on AOM-free surfaces. Overall, the composition as well as the physico-chemical characteristics (e.g., stickiness) of AOM will likely dictate the severity of fouling in membrane systems during algal blooms., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Self-assembly of polypeptides into fibrillar structures can be initiated by planar surfaces that interact favorably with certain residues. Using a coarse-grained model, we systematically studied the folding and adsorption behavior of a β-roll forming polypeptide. We find that there are two different folding pathways depending on the temperature: (i) at low temperature, the polypeptide folds in solution into a β-roll before adsorbing onto the attractive surface; (ii) at higher temperature, the polypeptide first adsorbs in a disordered state and folds while on the surface. The folding temperature increases with increasing attraction as the folded β-roll is stabilized by the surface. Surprisingly, further increasing the attraction lowers the folding temperature again, as strong attraction also stabilizes the adsorbed disordered state, which competes with folding of the polypeptide. Our results suggest that to enhance the folding, one should use a weakly attractive surface. They also explain the recent experimental observation of the nonmonotonic effect of charge on the fibril formation on an oppositely charged surface [C. Charbonneau et al., ACS Nano 8, 2328 (2014)].

To perform its barrier function, the lipid bilayer membrane requires a robust resistance against pore formation. Using a self-consistent field (SCF) theory and a molecularly detailed model for membranes composed of charged or zwitterionic lipids, it is possible to predict structural, mechanical, and thermodynamical parameters for relevant lipid bilayer membranes. We argue that the edge energy in membranes is a function of the spontaneous lipid monolayer curvature, the mean bending modulus, and the membrane thickness. An analytical Helfrich-like model suggests that most bilayers should have a positive edge energy. This means that there is a natural resistance against pore formation. Edge energies evaluated explicitly in a two-gradient SCF model are consistent with this. Remarkably, the edge energy can become negative for phosphatidylglycerol (e.g., dioleoylphosphoglycerol) bilayers at a sufficiently low ionic strength. Such bilayers become unstable against the formation of pores or the formation of lipid disks. In the weakly curved limit, we study the curvature dependence of the edge energy and evaluate the preferred edge curvature and the edge bending modulus. The latter is always positive, and the former increases with increasing ionic strength. These results point to a small window of ionic strengths for which stable pores can form as too low ionic strengths give rise to lipid disks. Higher order curvature terms are necessary to accurately predict relevant pore sizes in bilayers. The electric double layer overlap across a small pore widens the window of ionic strengths for which pores are stable.

We studied the interactions of silica and titanium dioxide nanoparticles with phospholipid membranes and show how electrostatics plays an important role. For this, we systematically varied the charge density of both the membranes by changing their lipid composition and the oxide particles by changing the pH. For the silica nanoparticles, results from our recently presented fluorescence vesicle leakage assay are combined with data on particle adsorption onto supported lipid bilayers obtained by optical reflectometry. Because of the strong tendency of the TiO2 nanoparticles to aggregate, the interaction of these particles with the bilayer was studied only in the leakage assay. Self-consistent field (SCF) modeling was applied to interpret the results on a molecular level. At low charge densities of either the silica nanoparticles or the lipid bilayers, no electrostatic barrier to adsorption exists. However, the adsorption rate and adsorbed amounts drop with increasing (negative) charge densities on particles and membranes because of electric double-layer repulsion, which is confirmed by the effect of the ionic strength. SCF calculations show that charged particles change the structure of lipid bilayers by a reorientation of a fraction of the zwitterionic phosphatidylcholine (PC) headgroups. This explains the affinity of the silica particles for pure PC lipid layers, even at relatively high particle charge densities. Particle adsorption does not always lead to the disruption of the membrane integrity, as is clear from a comparison of the leakage and adsorption data for the silica particles. The attraction should be strong enough, and in line with this, we found that for positively charged TiO2 particles vesicle disruption increases with increasing negative charge density on the membranes. Our results may be extrapolated to a broader range of oxide nanoparticles and ultimately may be used for establishing more accurate nanoparticle toxicity assessments and drug-delivery systems.

We present a complete toolbox to use responsive ionic liquid (IL) emulsions for extraction purposes. IL emulsions stabilized by responsive microgels are shown to allow rapid extraction and reversible breaking and re-emulsification. Moreover, by using a paramagnetic ionic liquid, droplets can be easily collected in low magnetic fields.

We report the surface-nucleated self-assembly into fibrils of a biosynthetic amino acid polymer synthesized by the yeast Pichia pastoris. This polymer has a block-like architecture, with a central silk-like block labeled SH, responsible for the self-assembly into fibrils, and two collagen-like random coil end blocks (C) that colloidally stabilize the fibers in aqueous solution. The silk-like block contains histidine residues (pKa≈6) that are positively charged in the low pH region, which hinders self-assembly. In aqueous solution, CSHC self-assembles into fibers above a pH-dependent critical nucleation concentration Ccb. Below Ccb, where no self-assembly occurs in solution, fibril formation can be induced by a negatively charged surface (silica) in the pH range of 3.5-7. The density of the fibers at the surface and their length are controlled by a subtle balance in charge between the protein polymer and the silica surface, which is evidenced from the dependence on pH. With increasing number density of the fibers at the surface, their average length decreases. The results can be explained on the basis of a nucleation-and-growth mechanism: the surface density of fibers depends on the rate of nucleation, while their growth rate is limited by transport of proteins from solution. Screening of the charges on the surface and histidine units by adding NaCl influences the nucleation-and-growth process in a complicated fashion: at low pH, the growth is improved, while at high pH, the nucleation is limited. Under conditions where nucleation in the bulk solution is not possible, growth of the surface-nucleated fibers into the solution--away from the surface--can still occur.

To understand how lipid architecture determines the lipid bilayer structure and its mechanics, we implement a molecularly detailed model that uses the self-consistent field theory. This numerical model accurately predicts parameters such as Helfrichs mean and Gaussian bending modulus kc and k̄ and the preferred monolayer curvature J(0)(m), and also delivers structural membrane properties like the core thickness, and head group position and orientation. We studied how these mechanical parameters vary with system variations, such as lipid tail length, membrane composition, and those parameters that control the lipid tail and head group solvent quality. For the membrane composition, negatively charged phosphatidylglycerol (PG) or zwitterionic, phosphatidylcholine (PC), and -ethanolamine (PE) lipids were used. In line with experimental findings, we find that the values of kc and the area compression modulus kA are always positive. They respond similarly to parameters that affect the core thickness, but differently to parameters that affect the head group properties. We found that the trends for k̄ and J(0)(m) can be rationalised by the concept of Israelachivili's surfactant packing parameter, and that both k̄ and J(0)(m) change sign with relevant parameter changes. Although typically k̄ < 0, membranes can form stable cubic phases when the Gaussian bending modulus becomes positive, which occurs with membranes composed of PC lipids with long tails. Similarly, negative monolayer curvatures appear when a small head group such as PE is combined with long lipid tails, which hints towards the stability of inverse hexagonal phases at the cost of the bilayer topology. To prevent the destabilisation of bilayers, PG lipids can be mixed into these PC or PE lipid membranes. Progressive loading of bilayers with PG lipids lead to highly charged membranes, resulting in J(0)(m) >> 0, especially at low ionic strengths. We anticipate that these changes lead to unstable membranes as these become vulnerable to pore formation or disintegration into lipid disks.

Because of their ease of preparation and versatile modification opportunities, complex coacervate core micelles (C3Ms) may be a good alternative for expensive diffusional probes, such as dendrimers. However, C3Ms are unstable at high salt concentrations and may fall apart in contact with other polymers or (solid) materials. Therefore, we designed and characterized small (15 nm radius), stable fluorescent C3Ms. These were formed by electrostatic interactions between poly(ethylene oxide-methacrylic acid) (PEO-PMAA) and fluorescently labelled poly(allylamine hydrochloride) (PAH) and irreversible cross-linking of the core through amide bonds. We compared the properties of the cross-linked and non-cross-linked micelles. The radii of the two types of micelles were quite similar and independent of the ionic strength. Surprisingly, both were found to be stable at salt concentrations as high as 1.5 M. However, unlike the non-cross-linked C3Ms, the stability of the cross-linked C3Ms is independent of the pH. As a first example of their application as diffusional nanoprobes, we present results on the diffusion of the fluorescent micelles measured in xanthan solutions using fluorescence recovery after photobleaching (FRAP).

Surfactants soluble in liquid CO2 are rare and knowledge on interfacial and self-assembly behaviour is fragmented. We found that polyoxyethylene (5) isooctylphenyl ether is interfacially active at the water-liquid CO2 interface. Water-liquid CO2 interfacial tension was measured at various surfactant concentrations at 50 bar and 283 K using the pendant drop method, and a CMC like cusp was observed at a surfactant concentration of ~50 mM in the bulk liquid CO2. This system was modelled applying the self-consistent field theory of Scheutjens and Fleer (SF-SCF). We use a free-volume approach, wherein the chemical potential of the vacancies was linked to the pressure and the molecules were described using a freely-jointed chain model on a united atom level. The model indicates that typically the water-vapour interface is wet by CO2. Interestingly, a window of partial wetting was identified at the water-vapour interface as a function of the chemical potential of the surfactant. The second-order nature of both wetting transitions is attributed to the close proximity to the critical point of the CO2-vapour system. Furthermore, the SF-SCF theory was used to study the self-assembly of the surfactant in bulk CO2 or water, focusing on the three-phase coexistence, that is at P/Psat = 1. Above ~40 mM in the CO2-rich phase, the theory indicates stable water swollen reverse micelles with an aggregation number of ~100. The analysis further shows the stability of compressible CO2-swollen surfactant bilayers in the bulk water phase at elevated surfactant concentrations. Finally it was found that the critical reverse micellar concentration (in liquid CO2) increases and the aggregation number decreases with increasing pressure.

This paper presents experimental and modeling results on water-CO2 interfacial tension (IFT) together with wettability studies of water on both hydrophilic and hydrophobic surfaces immersed in CO2. CO2-water interfacial tension (IFT) measurements showed that the IFT decreased with increasing pressure and the negative slopes of IFT-pressure isotherms decreased with increasing temperature. Water contact angle on a cellulose surface (hydrophilic) immersed in CO2 increased with pressure, whereas the water contact angle on a hydrophobic surface such as hexamethyl disilazane (HMDS) coated silicon surface was almost independent of pressure. These experimental findings were augmented by modeling using the self-consistent field theory. The theory applies the lattice discretization scheme of Scheutjens and Fleer, with a discretization length close to the size of the molecules. In line with this we have implemented a primitive molecular model, with just small variations in the molar volume. The theory makes use of the Bragg-Williams approximation and has binary Flory-Huggins interaction parameters (FH) between CO2, water, and free volume. Using this model, we generated the complete IFT-pressure isotherms at various temperatures, which coincided well with the trends reported in literature, that is, the water-CO2 interfacial tension decreased with increasing pressure for pressures ≤100 bar and became independent of pressure >100 bar. The transition point occurred at higher pressures with increasing temperature. At three-phase coexistence (water-CO2-free volume) and at the water-vapor interface (water-free volume), we always found the CO2 phase in between the water-rich and free volume-rich phases. This means that for the conditions studied, the water-vapor interface is always wet by CO2 and there are no signs of a nearby wetting transition. Calculation of the water contact angle on a solid surface was based on the computed adsorption isotherms of water from a vapor or from a pressurized CO2-rich phase and analysis of surface pressures at water-vapor or water-CO2 coexistence. The results matched reasonably well with the experimental contact angle data. Besides, we also computed the volume fraction profiles of the CO2, H2O, and the V phase, from which the preferential adsorption of CO2 near the hydrophilic surface was deduced.

Liquid CO(2) is a viable alternative for the toxic and environmentally harmful solvents traditionally used in dry-cleaning industry. Although liquid CO(2) dry-cleaning is being applied already at a commercial scale, it is still a relatively young technique which poses many challenges. The focus of this review is on the causes of the existing problems and directions to solve them. After presenting an overview of the state-of-the-art, we analyze the detergency challenges from the fundamentals of colloid and interface science. The properties of liquid CO(2) such as dielectric constant, density, Hamaker constant, refractive index, viscosity and surface tension are presented and in the subsequent chapters their effects on CO(2) dry-cleaning operation are delineated. We show, based on theory, that the van der Waals forces between a model soil (silica) and model fabric (cellulose) through liquid CO(2) are much stronger compared to those across water or the traditional dry-cleaning solvent PERC (perchloroethylene). Prevention of soil particle redeposition in liquid CO(2) by electrostatic stabilization is challenging and the possibility of using electrolytes having large anionic parts is discussed. Furthermore, the role of different additives used in dry-cleaning, such as water, alcohol and surfactants, is reviewed. Water is not only used as an aid to remove polar soils, but also enhances adhesion between fabric and soil by forming capillary bridges. Its role as a minor component in liquid CO(2) is complex as it depends on many factors, such as the chemical nature of fabrics and soil, and also on the state of water itself, whether present as molecular solution in liquid CO(2) or phase separated droplets. The phenomena of wicking and wetting in liquid CO(2) systems are predicted from the Washburn-Lucas equation for fabrics of various surface energies and pore sizes. It is shown that nearly complete wetting is desirable for good detergency. The effect of mechanical action and fluid dynamic conditions on dry-cleaning is analyzed theoretically. From this it follows that in liquid CO(2) an order of magnitude higher Reynold's number is required to exceed the binding forces between fabric and soil as opposed to PERC or water, mainly due to the strong van der Waals forces and the low viscosity of CO(2) at dry-cleaning operational conditions., (Copyright © 2012 Elsevier B.V. All rights reserved.)

The roughness and softness of interacting surfaces are both important parameters affecting the capillary condensation of water in apolar media, yet are poorly understood at present. We studied the water capillary adhesion between a cellulose surface and a silica colloidal probe in hexane by AFM force measurements. Nanomechanical measurements show that the Young's modulus of the cellulose layer in water is significantly less (~7 MPa) than in hexane (~7 GPa). In addition, the cellulose surface in both water and hexane is rather rough (6-10 nm) and the silica probe has a comparable roughness. The adhesion force between cellulose and silica in water-saturated hexane shows a time-dependent increase up to a waiting time of 200 s and is much (2 orders of magnitude) lower than that expected for a capillary bridge spanning the whole silica probe surface. This suggests the formation of one or more smaller bridges between asperities on both surfaces, which is confirmed by a theoretical analysis. The overall growth rate of the condensate cannot be explained from diffusion mediated capillary condensation alone; thin film flow due to the presence of a wetting layer of water at both the surfaces seems to be the dominant contribution. The logarithmic time dependence of the force can also be explained from the model of the formation of multiple capillary bridges with a distribution of activation times. Finally, the force-distance curves upon retraction show oscillations. Capillary condensation between an atomically smooth mica surface and the silica particle show less significant oscillations and the adhesion force is independent of waiting time. The oscillations in the force-distance curves between cellulose and silica may stem from multiple bridge formation between the asperities present on both surfaces. The softness of the cellulose surface can bring in additional complexities during retraction of the silica particle, also resulting in oscillations in the force-distance curves.

We produced several pH-responsive silk-collagen-like triblocks, one acidic and two alkaline. At pH values where the silk-like block is uncharged the triblocks self-assemble into filaments. The pH-induced self-assembly was examined by atomic force microscopy, light scattering, and circular dichroism. The populations of filaments were found to be very monodisperse, indicating that the filaments start to grow from already present nuclei in the sample. The growth then follows pseudo-first-order kinetics for all examined triblocks. When normalized to the initial concentration, the growth curves of each type of triblock overlap, showing that the self-assembly is a generic process for silk-collagen-silk triblocks, regardless of the nature of their chargeable groups. The elongation speed of the filaments is slow, due to the presence of repulsive collagen-like blocks and the limited number of possibilities for an approaching triblock to successfully attach to a growing end. The formation of filaments is fully reversible. Already present filaments can start growing again by addition of new triblocks. The structure of all filaments is very rich in β-turns, leading to β-rolls. The triblocks attain this structure only when attaching to a growing filament., (© 2011 American Chemical Society)

The interaction of biocompatible polyelectrolytes (chargeable poly(amino acids)) with oxidized starch microgel particles has been studied. The aim was to form a polyelectrolyte complex layer around the outer shell of microgel particles filled with functional ingredients to slow down the release of the ingredients from the gel and make this process less sensitive to salt. First, the distribution of positively charged poly(l-lysine) (PLL) of two different molecular weights ("small", 15-30 kDa, and "large", 30-70 kDa) in the negatively charged gel particles was measured. The small PLL distributes homogeneously throughout the gel particles, but the large PLL forms a shell; i.e., its concentration at the outer layer of the particles was found to be much higher than in their core. This shell formation does not occur at a relatively high salt concentration (0.07 M). The large PLL was selected for further study. It was found that upon addition of PLL to lysozyme-loaded gel particles the protein is exchanged by PLL. The exchange rate increases with increasing pH, in line with the increasing electrostatic attraction between the gel and the polyelectrolyte. Therefore, it was decided to use also a negatively charged poly(amino acid), poly(L-glutamic acid) (PGA), to form together with PLL a stable polyelectrolyte complex shell around the gel particles. This approach turned out to be successful, and the PLL/PGA complex layer effectively slows down the release of lysozyme from the microgel particles at 0.05 M salt. In addition, it was found that the PLL/PGA layer protects the gel particle from degradation by α-amylase., (© 2011 American Chemical Society)

In this paper we explored the use of an electrochemical quartz crystal microbalance (QCM) to follow the development of electrochemically active biofilms on electrodes. With this technique it should be possible to monitor simultaneously the increase in biomass and the current generated by the electrogenic bacteria in the biofilm. We monitored the adsorption and the subsequent growth of bacteria that are used in microbial electrolysis cells, on a gold electrode (anode). After attachment it took about 3h for the bacteria to start to grow and develop a biofilm. Although the current was still relatively low, there is a clear correlation with the increase in biomass. The method is promising for the further investigation of the development of biofilms on electrodes (bioelectrodes)., (2010 Elsevier B.V. All rights reserved.)

We investigated the insertion of transmembrane structures in a lipid bilayer and their interactions using self-consistent field theory. The lipids are coarse-grained on a united-atom level and consist of a phosphatidylcholinelike headgroup and two hydrophobic tails. The inclusions, acting as simple models for proteins that span biological membranes, are rigid rods (radius R ) with a hydrophobic surface and hydrophilic end caps. The insertion free energy Omega of an individual rod is strongly regulated by the affinity between its hydrophobic surface and the lipid tails. This affinity also controls the best match of the hydrophobic length of the rod with that of the bilayer. The line tension tau(=Omega/2piR) is practically independent of R . The perturbations in the bilayer as a function of distance from the inclusion, have the shape of a damped oscillation. The wavelength and decay length are related to the elastic properties of the bilayer and do not depend on R . These results are used to analyze how the lipid matrix affects the interaction between transmembrane objects, for computational reasons considering the limit of R-->infinity . Contributions on different length scales can be distinguished: (i) a long-range elastic interaction, which is an exponentially decaying oscillation; (ii) an exponentially decaying repulsion on an intermediate length scale, resulting from the loss of conformational entropy of the lipid tails; and (iii) a short-range interaction due to the finite compressibility of the lipid tails, which manifests either as a depletion attraction if there is no affinity between the tails and the inclusions' surface or, otherwise, as an oscillatory structural force.

In plant cells, Golgi vesicles are transported to the division plane to fuse with each other, forming the cell plate, the initial membrane-bordered cell wall separating daughter cells. Vesicles, but not organelles, move through the phragmoplast, which consists of two opposing cylinders of microtubules and actin filaments, interlaced with endoplasmic reticulum membrane. To study physical aspects of this transport/inhibition process, we microinjected fluorescent synthetic 1,2-dioleoyl-sn-glycero-3-phospho-rac-1-glycerol (DOPG) vesicles and polystyrene beads into Tradescantia virginiana stamen hair cells. The phragmoplast was nonselective for DOPG vesicles of a size up to 150 nm in diameter but was a physical barrier for polystyrene beads having a diameter of 20 and 40 nm and also when beads were coated with the same DOPG membrane. We conclude that stiffness is a parameter for vesicle transit through the phragmoplast and discuss that cytoskeleton configurations can physically block such transit.

At the moment two competing explanations exist for the experimental finding that net negatively charged proteins adsorb on or absorb in negatively charged polyelectrolyte brushes. One explanation is based on the possibility of charge regulation. The idea is that a protein can reverse its charge when it is in the presence of the high electrostatic potential of the brush and then can be inserted. The other explanation relies on the charge anisotropy of proteins, that is, that it carries positively charged and negatively charged patches. The positively charged region gains more energy from interacting with the negative brush than the negative charged patch loses, especially when the charge densities and electrostatic potentials are high, thus providing a net attraction. We present a model in which both mechanisms are combined. We confirm that both charge anisotropy and charge regulation effects on their own can be responsible for protein uptake at the "wrong" side of the isoelectric point (IEP). In addition, we find that the respective effects are additive. Indeed, taking both effects into account results in a stronger attraction between a PE brush and protein at the IEP, and the attraction is found further above the IEP than the individual effects would have made possible. Still, for patchiness to have a strong contribution, the patches need very high charge densities. Therefore, we argue that for most types of protein charge reversal will be the main driving force for adsorption on the wrong side of the IEP, while patchiness will contribute less.

The hydrogen evolution reaction (HER) at Pt-cathodes of microbial electrolysis cells (MEC) has been associated with overpotentials of several hundred millivolts. The high overpotentials challenge the sustainability of an MEC. This paper shows that the HER overpotential at MEC relevant pH values is reduced if buffer is present At 15 A/m2 and 50 mM buffer, the lowest overpotential for phosphate was -0.05 V at pH 6.2, for ammonia was -0.05 V at pH 9.0, for carbonate was -0.09 V at pH 9.3, for Tris(hydroxymethyl)aminomethane was -0.07 V at pH 7.8, and for N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid was -0.08 V at pH 7.2. It was shown that the effect of buffer on the overpotentialis strongly pH dependent Furthermore, experimental data and a mass transport equation showed that by increasing the buffer concentration or linear flow speed (i.e., pump speed), or decreasing the current density (i) the overpotential reduces and (ii) the minimum overpotential is reached at a pH that approaches the buffer dissociation constant (pKa). Thus, to reduce the HER overpotential of an MEC, buffer (i.e., pKa), buffer concentration, linear flow speed, and current density must be well balanced with the expected operational pH.

The adsorption of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) in poly(ethylene oxide) (PEO) brushes was studied using a fixed-angle optical flow-cell reflectometer. We show that, just as in solution, there is a critical association concentration (CAC) for the surfactants at which adsorption in the PEO brush starts. Above the critical micelle concentration (CMC) the adsorption is found to be completely reversible. At low brush density the adsorption per PEO monomer is equal to the adsorption of these surfactants in bulk solution. However, with increasing brush density, the number of adsorbed surfactant molecules per PEO monomer decreases rapidly. This decrease is explained in terms of excluded volume interactions plus electrostatic repulsion between the negatively charged surfactant micelles. Experimentally, a plateau value in the total adsorption is observed as a function of grafting density. The experimental results were compared to the results of an analytical self-consistent field (aSCF) model, and we found quantitative agreement. Additionally, the model predicts that the plateau value found is in fact a maximum. Both experiments and model calculations show that the adsorption scales directly with the polymerization degree of the polymers in the brush. They also show that an increase in the ionic strength leads to an increase in the adsorbed amount, which is explained as being due to a decrease in the electrostatic penalty for the adsorption of the SDS micelles. The adsorption of SDS micelles changes the interactions of the PEO brush with a silica particle. This is illustrated by atomic force microscopy (AFM) measurements of the pull-off force of a silica particle from a PEO brush: at high enough PEO densities, the addition of SDS leads to a very strong reduction in the force necessary to detach the colloidal silica particle from the PEO brush. We attribute this effect to the large amount of negative charge incorporated in the PEO brush due to SDS adsorption.

Using fixed-angle ellipsometry, we investigate the degree of mass transfer upon vertically dipping a polystyrene surface through a layer of a polystyrene-poly(ethylene oxide) (PS-PEO) block copolymer at the air water interface (Langmuir-Blodgett or LB transfer). The transferred mass is proportional to the PS-PEO grafting density at the air-water interface, but the transferred mass is not equal to the mass at the air-water interface. We find that depending on the chain length of the PEO block only a certain fraction of the polymers at the air-water interface is transferred to the solid surface. For the shortest PEO chain length (PS36-PEO148), the mass transfer amounts to 94%, while for longer chain lengths (PS36-PEO370 and PS38-PEO770), a transfer of, respectively 57% and 19%, is obtained. We attribute this reduced mass transfer to a competition for the PS surface between the PEO block and the PS block. Atomic force microscopy shows that after transfer the material is evenly spread over the surface. However, upon a short heating of these transferred layers (95 degrees C, 5 min) a dewetting of the PS-PEO layer takes place. These results have a significant impact on the interpretation of the results in a number of papers in which the above-described transfer method was used to produce PEO polymer brushes, in a few cases in combination with heating. We briefly review these papers and discuss their main results in light of this new information. Furthermore, we show that, by using Langmuir-Schaeffer (LS, horizontal) dipping, much higher mass transfers can be reached than with the LB method. When the LB or LS methods are carefully applied, it is a very powerful technique to produce PEO brushes, as it gives full control over both the grafting density and the chain length.

Standing room only: Dense polymer brushes can be prepared by adsorbing a diblock copolymer comprising a neutral block and a polyelectrolyte block to an oppositely charged polyelectrolyte brush (see picture). The density of the resulting neutral brush is determined by charge compensation, leading to brush densities well over 1 nm(-2). The diblock copolymer can be desorbed by changing the solution conditions.

The adsorption of bovine serum albumin (BSA) in a planar poly(acrylic acid) (PAA) brush layer has been studied by fixed-angle optical reflectometry. The influence of polymer length, grafting density, and salt concentration is studied as a function of pH. The results are compared with predictions of an analytical polyelectrolyte brush model, which incorporates charge regulation and excluded volume interactions. A maximum in adsorption is found near the point of zero charge (pzc) of the protein. At the maximum, BSA accumulates in a PAA brush to at least 30 vol %. Substantial adsorption continues above the pzc, that is, in the pH range where a net negatively charged protein adsorbs into a negatively charged brush layer, up to a critical pH value. This critical pH value decreases with increasing ionic strength. The adsorbed amount increases strongly with both increasing PAA chain length and increasing grafting density. Experimental data compare well with the analytical model without having to include a nonhomogeneous charge distribution on the protein surface. Instead, charge regulation, which implies that the protein adjusts its charge due to the negative electrostatic potential in the brush, plays an important role in the interpretation of the adsorbed amounts. Together with nonelectrostatic interactions, it explains the significant protein adsorption above the pzc.

We examined the effect of the insertion of a trans-membrane peptide or protein on the thermodynamic, structural and mechanical characteristics of a lipid bilayer using self-consistent field (SCF) theory. The peptide or protein is modeled as a rigid cylindrical body with a hydrophobic surface and two hydrophilic end caps. We first characterise the properties of the undisturbed bilayer consisting of lipids with a hydrophilic phosphatidylcholine-like head group of nine segments and two identical tails ranging from 8 to 19 alkyl segments. The structural properties are given in terms of thickness of the hydrophobic core d(l)0 and the area a0 per lipid. The mechanical or elastic properties are characterised by the mean bending modulus k(c), the saddle splay modulus k and the area compression-expansion modulus of the bilayer k(a) and its monolayers. Furthermore we calculated the spontaneous curvature J0(m) of the individual monolayer, which can be positive or negative depending on the tail length. Subsequently, we focus on the effect of hydrophobic mismatch between the bilayer and inclusion. The free energy of insertion is parabolic in the mismatch. The minimum is at a small negative mismatch, i.e., when the hydrophobic thickness of the rod is smaller than that of the bilayer. This is attributed to conformational restrictions of the lipid tails close to the rod. This results in a positive curvature of the bilayer adjacent to it, even if the hydrophobic thickness of the rod is larger than that of the bilayer. The bilayer deformation has a wave character which decays exponentially. We show that the decay length of this perturbation is the same as the elastic length (kc(d)2/ka)(1/4), provided that for the thickness d of the bilayer d(l)0 is used.

Using an optical reflectometer with impinging-jet system, the adsorption from aqueous solution onto gold of three charged macromolecules has been studied: the strong linear-chain polyelectrolyte polyvinyl pyridine (PVP(+)), the fifth-generation poly(propylene imine) dendrimer DAB-64, which has a pH-dependent charge and a relatively fixed shape, and the protein lysozyme, of which both the charge and the structure-stability are dependent on solution composition. Experimental conditions that have been varied include the adsorbate concentration, electrolyte concentration, pH, and externally applied potential across the gold/solution interface. Making use of the earlier established dependency of the double layer potential of the gold substrate on solution conditions and externally applied potential, the results of measurements as a function of pH and as a function of external potential control are compared. The total set of results enables us to draw conclusions with respect to the relative importance of electrostatic interactions for the adsorption process. PVP(+) adsorption follows the electric potential of the gold/solution interface and is further determined by a rather strong nonelectrostatic affinity between segments and surface. The adsorption behavior of DAB-64 is not quite understood, but electrostatic interactions with the gold surface seem to play a minor role. For lysozyme, surface-induced conformational changes dominate the adsorption process. The extent of spreading of the molecules decreases with increasing polarity of the surface, resulting in a minimum in adsorbed amount around the point of zero potential of the gold.

Colloidal probe atomic force microscopy was used to determine the electric double layer interactions between a gold electrode and a spherical silica probe. The double layer properties of the gold/solution interface were varied through the pH and salt concentration of the electrolyte, as well as by externally applying an electric potential. The double layer potentials ψ(d) of the gold surface were obtained by fitting the force-distance curves according to the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, using earlier obtained values for the double layer potential of the silica probe as input parameter. It was found that the gold electrode combines the features of reversible and polarizable interfaces; i.e., its charge and potential are determined by both the solution pH and the external potential. The pH dependence is attributed to proton adsorption and desorption from oxidic groups on the gold surface. In the potential range studied, ψ(d) varies linearly with the applied potential; the variation in ψ(d) is roughly 10% of that in the applied potential. The potential of zero force (the external potential at which ψ(d) = 0) varies with pH. The various features of the gold/electrolyte interface are described well by an amphifunctional double layer model. The results of this study form the basis of the interpretation of adsorption studies on gold as a function of pH and externally applied potential.