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Solutions of semiflexible polymers confined in cylindrical pores with repulsive walls are studied by Molecular Dynamics simulations for a wide range of polymer concentrations. Both the case where both lengths are of the same order and the case when the persistence length by far exceeds the contour length are considered, and the enhancement of nematic order along the cylinder axis is characterized. With increasing density the character of the surface effect changes from depletion to the formation of a layered structure. For binary 50 : 50 mixtures of the two types of polymers an interplay between surface enrichment of the stiffer component and the isotropic-nematic transition is found, and a phase separated structure with cylindrical symmetry occurs, with the isotropic phase located around the cylinder axis. For melt densities the mixed nematic phase forms at the wall a layer with a screw-like structure of a tilted smectic phase. The observed behavior is tentatively interpreted in terms of the competition of the chain orientational entropy with entropy of mixing and excluded volume due to the wall.

Magnetic composite materials i.e. elastomers, polymer gels, or polymer solutions with embedded magnetic nanoparticles are useful for many technical and bio-medical applications. However, the microscopic details of the coupling mechanisms between the magnetic properties of the particles and the mechanical properties of the (visco)elastic polymer matrix remain unresolved. Here we study the response of a single-domain spherical magnetic nanoparticle that is suspended in a polymer solution to alternating magnetic fields. As interactions we consider only excluded volume interactions with the polymers and hydrodynamic interactions mediated through the solvent. The AC susceptibility spectra are calculated using a linear response Green-Kubo approach, and the influences of changing polymer concentration and polymer length are investigated. Our data is compared to recent measurements of the AC susceptibility for a typical magnetic composite system [Roeben et al., Colloid and Polymer Science, 2014, 2013--2023], and demonstrates the importance of hydrodynamic coupling in such systems., 10 pages, 7 figures

The impact of brush molecular weight distribution on the conformation and response of weak polyacid brushes was investigated. We show that weight-average degree of polymerization (Nw) and dispersity (Đ) alter the pH-responsive conformation of poly(acrylic acid) (PAA) brushes grafted to silica nanoparticles. We quantified the average brush length (lb) at various pH using dynamic light scattering. The lb of low-Nw PAA brushes (Nw = 45) increased as Đ was increased from 1.09 to 1.69, but lb for the high-Nw PAA brushes (Nw ≈ 813) did not vary substantially when Đ was increased from 1.23 to 1.76. This result indicates that the presence of a small fraction of long chains in a broad dispersity brush has a greater impact on lb when Nw is low. Additionally, the extent of pH-response in lb/lb,max increased with Nw or Đ (when Nw is low), where the maximum brush length (lb,max) was lb measured at pH 10. The scaling of lb/lb,max with degree of dissociation (α), however, was influenced by Nw but not Đ, indicating the low- and high-Nw PAA brushes were in the quasi-neutral brush (q-NB) and salted brush (SB) regimes, respectively. Differing behaviors in the pH-response and α-response of lb at low Nw arose from subtle differences in dissociation behaviors among brushes of varying Đ. At low Đ, the low-Nw brush adopted a pH-independent extended conformation due to strong excluded volume interactions, whereas the conformation of the high-Nw brush varied from collapsed to stretched with increasing pH arising from electrostatic interactions. At high-Đ, we suggest the brush conformation also varied from collapsed to stretched with increasing pH regardless of Nw.

In this paper, a novel approach is proposed based on the accurate computation of the inaccessible volume and the corresponding surface area which is defined by the locus of points where a ligand molecule can be placed so that it “touches” a protein molecule at a preset minimum interatomic distance without resulting in overlaps between the atoms of the protein and the atoms of the ligand. The proposed approach can be considered an extension of the widely used concept of the solvent accessible surface area (SASA). The SASA is defined as the surface where a solvent molecule can be in contact with the initial one without any overlaps. This excluded volume interaction is evaluated by treating atoms as hard core spheres, with the limitation of the solvent molecule being represented as a single sphere. In the proposed concepts of the molecular accessible surface (MASA) and the molecular inaccessible volume (MIV) we have practically removed this limitation and all atoms, both in the initial and the “inserted” molecules, are represented as hard spheres. In this paper we focus our examples on biological systems, especially on studying protein–ligand systems, since we expect that this will be one of the most promising fields of applications where the MASA and MIV extensions of the SASA will be of practical and immediate use. Therefore, the MASA and MIV are evaluated based on the surface generated by the ligand while it is being rolled over on all the atoms of the protein without penetrating them. Identification of the inaccessible volume of each candidate protein–ligand pair is also provided in the context of this study, along with the boundary surface where the ligand can be placed so as to be in “contact” with the protein. The proposed concepts of the MASA and MIV are expected to significantly enhance the ability to investigate specific protein–drug interactions by explicitly taking into account the polyatomic nature of a ligand. Several trials have been conducted using the analytical method of Dodd and Theodorou leading to accurate volume and surface area measurements of an arbitrary set of fused spheres in systems of various scales.

Biomolecules evolve and function in the intracellular crowded environment that is densely packed with macromolecules. Yet, a microscopic understanding of the effects of such an environment on the conformational preferences of biomolecules remains elusive. While prior investigations have attributed crowding effects mainly to the excluded volume (size) effects of the crowders, very little is known about the effects exerted due to their chemical interactions. In this study, crowding effects of tri-alanine peptides on the collapse equilibria of generic hydrophobic polymer are investigated using molecular dynamics simulations. The role of weak, non-specific, attractive polymer-crowder interactions in modulating the polymer collapse equilibria is examined. The results highlight that crowding effects can lead to polymer compaction as well as unfolding depending on the strength of polymer-crowder interaction energy. Strongly interacting crowders weaken hydrophobic collapse (or unfold the polymer) at high volume fractions and induce polymer collapse only under dilute conditions. Weakly interacting crowders induce polymer collapse at all crowder concentrations. Interestingly, the thermodynamic driving forces for polymer collapse are remarkably different in the two cases. Strongly and weakly interacting crowders induce collapse by preferential adsorption and preferential depletion respectively. The findings provide new insights into the possible effects of interplay of intermolecular interactions in a crowded environment. The results have implications in understanding the impact of crowding in altering free energy landscapes of proteins.

We study the temporal and structural development of colloid films during vertical drying using the lattice Boltzmann (LB) simulation. The dispersed particles moving in Brownian motion have excluded volume and hydrodynamic interactions in the film. The concentrated colloidal film formed by solvent evaporation is modeled as an uniaxial compression of colloids with a planar moving interface. The simulation studies are carried out over a wide range of Péclet number (Pe), the relative ratio between the evaporation rate and the diffusion rate of colloids. The results clearly demonstrate a temporal variation of colloid concentration as the evaporation rate increases. In the case of high Pe, the increase of colloid concentration in the top layer creates structural features that can be distinguished along the height of the film, and eventually can induce a large tensile stress in the layer. However, surprisingly, the colloids are maximally crystallized in the case of moderate Pe. The LB simulation results are further compared with those from previous studies of the Brownian Dynamics (BD) simulation and the continuum model for the evaporation film. The LB and BD results match well both at low and high Pe limits. The qualitatively significant differences between LB and BD simulations at a moderate Pe indicate that hydrodynamic interactions (HIs) play an important role in this Pe. The presence of HIs induces a greater reduction of diffusion than under geometrical restriction alone, and the effect is conspicuous when particles are driven both by diffusion and by advection.

Determinations of solute-cosolute interactions from chromatography have often resulted in problems, such as the "antibinding" (or a negative binding constant) between the solute and micelle in micellar liquid chromatography (MLC) or indeterminacy of salt-ligand binding strength in high-performance affinity chromatography (HPAC). This shows that the stoichiometric binding models adopted in many chromatographic analyses cannot capture the non-specific nature of solvation interactions. In contrast, an approach using statistical thermodynamics handles these complexities without such problems and directly links chromatographic data to, for example, solubility data via a universal framework based on Kirkwood-Buff integrals (KBI) of the radial distribution functions. The chromatographic measurements can now be interpreted within this universal theoretical framework that has been used to rationalize small solute solubility, biomolecular stability, binding, aggregation and gelation. In particular, KBI analysis identifies key solute-cosolute interactions, including excluded volume effects. We present (i) how KBI can be obtained directly from the cosolute concentration dependence of the distribution coefficient, (ii) how the classical binding model, when used solely as a fitting model, can yield the KBIs directly from the literature data, and (iii) how chromatography and solubility measurements can be compared in the unified theoretical framework provided via KBIs without any arbitrary assumptions about the stationary phase. To perform our own analyses on multiple datasets we have used an "app". To aid readers' understanding and to allow analyses of their own datasets, the app is provided with many datasets and is freely available on-line as an open-source resource.

There is growing support for the idea that the in vivo folding process of knotted proteins is assisted by chaperonins, but the mechanism of chaperonin assisted folding remains elusive. Here, we conduct extensive Monte Carlo simulations of lattice and off-lattice models to explore the effects of confinement and hydrophobic intermolecular interactions with the chaperonin cage in the folding and knotting processes. We find that moderate to high protein-cavity interactions (which are likely to be established in the beginning of the chaperonin working cycle) cause an energetic destabilization of the protein that overcomes the entropic stabilization driven by excluded volume, and leads to a decrease of the melting temperature relative to bulk conditions. Moreover, mild-to-moderate hydrophobic interactions with the cavity (which would be established later in the cycle) lead to a significant enhancement of knotting probability in relation to bulk conditions while simultaneously moderating the effect of steric confinement in the enhancement of thermal stability. Our results thus indicate that the chaperonin may be able to assist knotting without simultaneously thermally stabilizing potential misfolded states to a point that would hamper productive folding thus compromising its functional role.

Recently there has been a revival of interest in the basic structure of the aqueous or “hydrated” electron, e−(aq). According to the conventional picture, this species occupies a cavity or excluded volume in the structure of liquid water, with a characteristic absorption spectrum ascribable to s → p excitations of a particle in a quasi-spherical box. This traditional picture has been questioned over the past few years, however, on the basis of a one-electron pseudopotential model that predicts a more delocalized spin density and no distinct cavity. This Perspective reviews the known experimental properties of e−(aq) along with attempts to reproduce and understand them using both one-electron models and many-electron quantum chemistry calculations. The overwhelming weight of the evidence continues to support the conventional excluded-volume picture of the aqueous electron.

In this paper, we develop a theory to account for the effect of excluded volume (EV) interactions in the strong stretching theory (SST) based description of pH-responsive polyelectrolyte (PE) brushes. The existing studies have considered the PE brushes to be present in a θ-solvent and hence have neglected the EV interactions; however, such a consideration cannot describe the situations where the pH-responsive brushes are in a "good" solvent. Secondly, we consider a more expanded form of the mass action law, governing the pH-dependent ionization of the PE molecules, in the SST description of the PE brushes. This expanded form of the mass action law considers different values of γa3 (γ is the density of chargeable sites on the PE molecule and a is the PE Kuhn length) and therefore is an improvement over the existing SST models of PE brushes as well as other theories involving pH-responsive PE molecules that always consider γa3 = 1. Our results demonstrate that the EV effects enhance the brush height by inducing additional PE inter-segmental repulsion. Similarly, the consideration of the expanded form of the mass action law would lead to a reduced (enhanced) brush height for γa3 1). We also quantify variables such as the monomer density distribution, the distribution of the ends of the PE brush, and the EDL electrostatic potential and explain their differences with respect to those obtained with no EV interactions or γa3 = 1.

We report the effect of neutral macromolecular crowders poly(ethylene glycol) (PEG) (8 kDa) and Ficoll (70 kDa) on liquid-liquid phase separation in a polyuridylic acid (polyU)/spermine complex coacervate system. The addition of PEG decreased both the amount of spermine required for phase separation and the coacervation temperature (TC). We interpret these effects on phase behavior as arising due to excluded volume and preferential interactions on both the secondary structure/condensation of spermine-associated polyU molecules and on the association of soluble polyU/spermine polyelectrolyte complexes to form coacervate droplets. Examination of coacervates formed in the presence of fluorescently-labeled PEG or Ficoll crowders indicated that Ficoll is accumulated while PEG is excluded from the coacervate phase, which provides further insight into the differences in phase behavior. Crowding agents impact distribution of a biomolecular solute: partitioning of a fluorescently-labeled U15 RNA oligomer into the polyU/spermine coacervates was increased approximately two-fold by 20 wt% Ficoll 70 kDa and by more than two orders of magnitude by 20 wt% PEG 8 kDa. The volume of the coacervate phase decreased in the presence of crowder relative to a dilute buffer solution. These findings indicate that potential impacts of macromolecular crowding on phase behavior and solute partitioning should be considered in model systems for intracellular membraneless organelles.

We examine the phase behaviour of colloidal suspensions of hard board-like particles (HBPs) as a function of their shape anisotropy, and observe a fascinating spectrum of nematic, smectic, and columnar liquid-crystalline phases, whose formation is entirely driven by excluded volume effects. We map out the phase diagram of short and long HBPs by gradually modifying their shape from prolate to oblate and investigate the long-range order of the resulting morphologies along the phase directors and perpendicularly to them. The intrinsic biaxial nature of these particles promotes the formation of translationally ordered biaxial phases, but does not show solid evidencethat it would, per se, promote the formation of the biaxial nematic phase. Our simulations shed light on the controversial existence of the discotic smectic phase, whose layers are as thick as the minor particle dimension, which is stable in a relatively large portion of our phase diagrams. Additionally, we modify the Onsager theory to describe the isotropic-nematic phase transition offreely rotating biaxial particles as a function of the particle width, and find a relatively strong firstorder signature, in excellent agreement with our simulations. In an attempt to shed light on the elusive formation of the biaxial nematic phase, we apply this theory to predict the uniaxial-biaxialnematic phase transition and confirm, again in agreement with simulations, the prevailing stability of the translationally ordered smectic phase over the orientationally ordered biaxial nematic phase.

Worm-like micelles (WLMs) are nano-scale self-assemblies widely used for viscosity enhancement, for drug delivery, and in personal care products. The stability of WLMs under variable ionic and surfactant concentrations is important to applications of these fascinating materials. In this work it is demonstrated that a virial approach can be used to understand and predict WLM stability. A mixed surfactant system consisting of sodium laureth sulfate and cocoamidopropyl betaine was examined using small-angle neutron scattering. A linear relationship between the second virial coefficient, A2, and the salt to surfactant ratio, Θs-s, is derived and demonstrated. The Θs-s-dependent term is described via association/dissociation kinetics of salt ions between the bulk and an ion cloud surrounding the WLMs yielding, , where Keq is the association/dissociation constant, nsurf is the molar surfactant concentration, b is the molar excluded volume, M the WLM molar mass, Θs-s is the salt-surfactant ratio and Θ is the salt-surfactant ratio where the interactions are solely determined by the excluded volume. The ratio b/M2 is independent of WLM contour length. The exponent ϑ is found to be approximately 5/4 in agreement with polymer scaling laws for the semi-dilute regime in good solvents.

The effect of excluded volume (EV) interactions on the manifestation of the local dynamics in the spin-lattice NMR relaxation in dendrimers has been studied by using Brownian dynamics simulations. The study was motivated by the theory developed by Markelov et al., [J. Chem. Phys., 2014, 140, 244904] for a Gaussian dendrimer model without EV interactions. The theory connects the experimentally observed dependence of the spin-lattice relaxation rate 1/T(1)H on the location of NMR active groups with the restricted flexibility (semiflexibility) of dendrimers. Semiflexibility was introduced through the correlations between the orientations of different segments. However, these correlations exist even in flexible dendrimer models with EV interactions. We have simulated coarse-grained flexible and semiflexible dendrimer models with and without EV interactions. Every dendrimer segment consisted of two rigid bonds. Semiflexibility was introduced through a potential which restricts the fluctuations of angles between neighboring bonds but does not change orientational correlations in the EV model as compared to the flexible case. The frequency dependence of the reduced 1/T(1)H(ωH) for segments and bonds belonging to different dendrimer shells was calculated. It was shown that the main effect of EV interactions consists of a much stronger contribution of the overall dendrimer rotation to the dynamics of dendrimer segments as compared to phantom models. After the exclusion of this contribution the manifestation of internal dynamics in spin-lattice NMR relaxation appears to be practically insensitive to EV interactions. For the flexible models, the position ωmax of the peak of the modified 1/T(1)H(ωH) does not depend on the shell number. For semiflexible models, the maximum of 1/T(1)H(ωH) for internal segments or bonds shifts to lower frequencies as compared to outer ones. The dependence of ωmax on the number of dendrimer shells appears to be universal for segments and bonds in dendrimer models with and without EV interactions.

In this work a new density functional theory framework is developed to predict the salt-concentration dependent swelling state of charged microgels and the local concentration of monovalent ions inside and outside the microgel. For this purpose, elastic, solvent-induced and electrostatic contributions to the microgel free energy are considered together with the free energy of the ions. In addition to the electrostatic interaction, the model explicitly considers both the microgel-ion excluded-volume (steric) repulsion and the ionic correlations, in such a way that the formalism is consistent with the Hypernetted-Chain Closure approximation (HNC). We explore the role that the solvent quality, chain elasticity, salt concentration and microgel bare charge play on the swelling state, the effective charge and on the ionic density profiles. Our results show that the microgel-ion steric exclusion foments the increase of the particle size up to 10 %. The role that the steric effect plays on the counterion distribution becomes more important when the microgel approaches the shrunken configuration, developing an accumulation peak at the microgel interface and a reduction in the inner core of the microgel that induce a significant increase of the microgel effective charge. We further find that deep inside the particle charge electroneutrality is achieved and a Donnan potential corrected by the steric exclusion is established., 15 pages, 8 figures

We report on the synthesis of [123I]-iodometomidate from a soluble polymer-supported precursor. The precursor was linked through a soluble polymer via a silyl linker polymer-supported precursor and released [123I]-iodometomidate. The radiotracers were purified based on a combination of size-exclusive and normal-phase chromatography to provide [123I]-iodometomidate in good quality.

Ionic liquids exhibit fascinating nanoscale morphological phases and are promising materials for energy storage applications. Liquid crystalline order emerges in ionic liquids with specific chemical structures. Here, we investigate the phase behaviour and related ionic conductivities of dry ionic liquids, using extensive molecular dynamics simulations. Temperature dependence, properties of polymeric tail and excluded volume symmetry of the amphiphilic ionic liquid molecules are investigated in large scale systems with both short and long-range Coulomb interactions. Our results suggest that by adjusting stiffness and steric interactions of the amphiphilic molecules, lamellar or 3D continuous phases result in these molecular salts. The resulting phases are composed of ion rich and ion pure domains. In 3D phases, ion rich clusters form ionic channels and have significant effects on the conductive properties of the observed nano-phases. If there is no excluded-volume asymmetry along the molecules, mostly lamellar phases with anisotropic conductivities emerge. If the steric interactions become asymmetric, lamellar phases are replaced by complex 3D continuous phases. Within the temperature ranges for which morphological phases are observed, conductivities exhibit low-temperature maxima in accord with experiments on ionic liquid crystals. Stiffer molecules increase the high-conductivity interval and strengthen temperature-resistance of morphological phases. Increasing the steric interactions of cation leads to higher conductivities. Moreover, at low monomeric volume fractions and at low temperatures, cavities are observed in the nano-phases of flexible ionic liquids. We also demonstrate that, in the absence of electrostatic interactions, the morphology is distorted. Our findings inspire new design principles for room temperature ionic liquids and help explain previously-reported experimental data.

We propose an interpolation formula (the EV-WLC relation) for the force-extension behavior of wormlike chains in the presence of hard-core excluded volume interactions, analogous to the classic interpolation formula from Marko and Siggia for ideal wormlike chains. Using pruned-enriched Rosenbluth method (PERM) simulations of asymptotically long, discrete wormlike chains in an external force, we show that the error in the EV-WLC interpolation formula to describe discrete wormlike chains is systematically smaller than the error in the Marko-Siggia interpolation formula, except for the saturation region in which both formulas have the same limiting behavior. We anticipate that the EV-WLC interpolation formula will prove useful in the coarse-graining of wormlike chain models for dynamic simulations. Related results for the excess free energy due to excluded volume provide strong support for the physical basis of the Pincus regime.

The microphase separation of amphiphilic dendrimer copolymers in a selective solvent with different excluded volume effects (αS) is investigated using three-dimensional real space self-consistent field theory. The morphological transition of disorder-to-order and order-to-order is observed by systematically regulating the excluded volume effect parameter, interaction parameter of block species, and the spacer length of the second generation of the dendrimer. The ordered segregates of the dendrimer solution are observed with a stronger excluded volume effect due to the strong depletion effect of solvent on the dendrimer. The relative magnitude between hydrophobic block B and hydrophilic block C is very important for microphase separation: when they are equal (NB = NC), a structural shift from vesicles to micelles has been found upon increasing the interaction parameter, and the region of disordered morphology is controlled by the interfacial free energy (Uint); when NB > NC, the vesicular morphologies overwhelmingly appear in the ordered region and then NC increases to close to NB, and the ordered aggregates take a shift from vesicles to micelles. Furthermore, the amphiphilic block C of the dendrimer is intended to enlarge to NC > NB, the micellar morphology is dominant in the ordered regime with a stronger excluded volume effect, which contributes to the decrease in the hydrophobic block repulsion that is affected by the decrease in the entropic free energy (−TS). The knowledge obtained from the microphase separation of dendrimer solution induced by the excluded volume effect of selective solvent is full of referential significance in understanding the morphological transition from vesicles to micelles for the amphiphile in the field of soft matter.

We study structural properties of a ring polymeric melt confined in a film in comparison to a linear counterpart using molecular dynamics simulations. Local structure orderings of ring and linear polymers in the vicinity of the surface are similar to each other because the length scale of surface-monomer excluded volume interactions is smaller than the size of an ideal blob of the ring. In a long length scale, while the Silberberg hypothesis can be used to provide the physical origin of the confined linear polymer results, it no longer holds for the ring polymer case. We also present different structural properties of ring and linear polymers in a melt, including the size of polymers, the adsorbed amount, and the coordination number of a polymer. Our observation reveals that a confined ring in a melt adopts a highly segregated conformation due to a topological excluded volume repulsion, which may provide a new perspective to understand the nature of biological processes, such as territorial segregation of chromosomes in eukaryotic nuclei.

Domain movements play a fundamental and critical role in the specific biological function that multidomain proteins have evolved to perform. A significant amount of research has been carried out to investigate the effects of macromolecular crowding agents, mostly on single domain proteins, thereby furthering our appreciation for the crowding phenomenon. However similar studies on proteins having multiple domains are relatively scarce. Using the plasma protein human serum albumin (HSA) as the protein of interest, we have probed the influence of dextran based crowding agents (Dextran 6, Dextran 40, and Dextran 70) on the relative movements of domains I and II using FRET, with Trp-214 in domain II acting as the donor and acrylodan (Ac) covalently attached to Cys-34 of domain I as the acceptor. Amongst the higher molecular weight crowders, while both Dextran 70 and Dextran 40 induced a significant decrease in the distance between the aforesaid domains, however for the latter macromolecular crowder (Dextran 40), beyond 50 g L(-1), no change in domain separation was observed even up to concentrations of 175 g L(-1). On the other hand, contrary to our expectations, Dextran 6, having the highest packing density by virtue of it being the smallest crowding agent used, provided an asymmetric excluded volume which resulted in forced elongation of HSA along the Trp-Ac FRET axis. Additionally both chemical and thermal studies performed at varying concentrations of the chemical denaturant, urea, reveal unusual movements of the two domains, an aspect that can have important implications with regard to HSA being an avid transporter of fatty acids, with the binding of latter being known to invoke appreciable domain displacements. We hypothesise that we see a distinct crossover from entropy dominated depletion effects in the case of Dextran 6 to significant enthalpic contribution for Dextran 70 with Dextran 40 lying midway between these two crowders, having characteristics of both.

In this paper we present a computational approach, based on NVT molecular dynamics trajectories, that allows the direct evaluation of the protein partial molecular volume. The results obtained for five different globular proteins demonstrate the accuracy of this computational procedure in reproducing protein partial molecular volumes, providing quantitative characterization of the hydration shell in terms of the protein excluded volume, hydration shell ellipsoidal volume and related solvent density. Remarkably, our data indicate for the hydration shell a ≈10% solvent density increase with respect to the liquid water bulk density, in excellent agreement with the available experimental data.

We report the phase behaviour of polymer/fullerene/solvent ternary mixtures and its consequence for the morphology of the resulting composite thin films. We focus particularly on solutions of polystyrene (PS), C60 fullerene and toluene, which are examined by static and dynamic light scattering, and films obtained from various solution ages and thermal annealing conditions, using atomic force and light microscopy. Unexpectedly, the solution phase behaviour below the polymer overlap concentration, c*, is found to be described by a simple excluded volume argument (occupied by the polymer chains) and the neat C60/solvent miscibility. Scaling consistent with full exclusion is found when the miscibility of the fullerene in the solvent is much lower than that of the polymer, giving way to partial exclusion with more soluble fullerenes (phenyl-C61-butyric acid methyl ester, PCBM) and a less asymmetric solvent (chlorobenzene), employed in photovoltaic devices. Spun cast and drop cast films were prepared from PS/C60/toluene solutions across the phase diagram to yield an identical PS/C60 composition and film thickness, resulting in qualitatively different morphologies in agreement with our measured solution phase boundaries. Our findings are relevant to the solution processing of polymer/fullerene composites (including organic photovoltaic devices), which generally require effective solubilisation of fullerene derivatives and polymer pairs in this concentration range, and the design of well-defined thin film morphologies.

A ring polymer is a classical model to explore the behaviors of biomacromolecules. Compared with its linear counterpart in shear flow, the ring polymer should be more sensitive to excluded volume and hydrodynamic interaction attributed to the absence of chain ends. We carried out multiparticle collision dynamics combined with molecular dynamics simulation to study the effects of excluded volume and hydrodynamic interaction on the behaviors of ring polymers in shear flow. The results show that in the absence of the strong excluded volume interaction, the ring polymer prefers a two-strand linear conformation with high deformation and orientation in the flow-gradient plane, and the tank-treading motion is nearly negligible. Ring polymers without excluded volume show no significant difference from linear polymers in the scaling exponents for the deformation, orientation and tumbling motion. We also observed that the hydrodynamic interaction could efficiently slow down the relaxation of ring polymers while the scaling exponents against the Weissenberg number have rarely been affected.

There has been much interest in the dimensional properties of double-stranded DNA (dsDNA) confined to nanoscale environments as a problem of fundamental importance in both biological and technological fields. This has led to a series of measurements by fluorescence microscopy of single dsDNA molecules under confinement to nanofluidic slits. Despite the efforts expended on such experiments and the corresponding theory and simulations of confined polymers, a consistent description of changes of the radius of gyration of dsDNA under strong confinement has not yet emerged. Here, we perform molecular dynamics (MD) simulations to identify relevant factors that might account for this inconsistency. Our simulations indicate a significant amplification of excluded volume interactions under confinement at the nanoscale due to the reduction of the effective dimensionality of the system. Thus, any factor influencing the excluded volume interaction of dsDNA, such as ionic strength, solution chemistry, and even fluorescent labels, can greatly influence the dsDNA size under strong confinement. These factors, which are normally less important in bulk solutions of dsDNA at moderate ionic strengths because of the relative weakness of the excluded volume interaction, must therefore be under tight control to achieve reproducible measurements of dsDNA under conditions of dimensional reduction. By simulating semi-flexible polymers over a range of parameter values relevant to the experimental systems and exploiting past theoretical treatments of the dimensional variation of swelling exponents and prefactors, we have developed a novel predictive relationship for the in-plane radius of gyration of long semi-flexible polymers under slit-like confinement. Importantly, these analytic expressions allow us to estimate the properties of dsDNA for the experimentally and biologically relevant range of contour lengths that is not currently accessible by state-of-the-art MD simulations.

Polyelectrolyte (PE) brushes are a special class of polymer brushes (PBs) containing charges. Polymer chains attain "brush"-like configuration when they are grafted or get localized at an interface (solid-fluid or liquid-fluid) with sufficiently close proximity between two-adjacent grafted polymer chains - such a proximity triggers a particular nature of interaction between the adjacent polymer molecules forcing them to stretch orthogonally to the grafting interface, instead of random-coil arrangement. In this review, we discuss the theory, synthesis, and applications of PE brushes. The theoretical discussion starts with the standard scaling concepts for polymer and PE brushes; following that, we shed light on the state of the art in continuum modelling approaches for polymer and PE brushes directed towards analysis beyond the scaling calculations. A special emphasis is laid in pinpointing the cases for which the PE electrostatic effects can be de-coupled from the PE entropic and excluded volume effects; such de-coupling is necessary to appropriately probe the complicated electrostatic effects arising from pH-dependent charging of the PE brushes and the use of these effects for driving liquid and ion transport at the interfaces covered with PE brushes. We also discuss the atomistic simulation approaches for polymer and PE brushes. Next we provide a detailed review of the existing approaches for the synthesis of polymer and PE brushes on interfaces, nanoparticles, and nanochannels, including mixed brushes and patterned brushes. Finally, we discuss some of the possible applications and future developments of polymer and PE brushes grafted on a variety of interfaces.

The dynamics of an elastic network model for the enzyme 4-oxalocrotonate tautomerase is studied in a system crowded by mobile macromolecules, also modeled by elastic networks. The system includes a large number of solvent molecules, as well as substrate and product molecules which undergo catalytic reactions with this hexameric protein. The time evolution of the entire system takes place through a hybrid dynamics that combines molecular dynamics for solute species and multiparticle collision dynamics for the solvent. It is shown that crowding leads to subdiffusive dynamics for the protein, in accord with many studies of diffusion in crowded environments, and increases orientational relaxation times. The enzyme reaction kinetics is also modified by crowding. The effective Michaelis constant decreases with crowding volume fraction, and this decrease is attributed to excluded volume effects, which dominate over effects due to reduced substrate diffusion that would cause the Michaelis constant to increase.

Macroscopic properties of suspensions, such as those composed of globular particles (e.g., colloidal or macromolecular), can be tuned by controlling the equilibrium aggregation of the particles. We examine how aggregation -- and, hence, macroscopic properties -- can be controlled in a system composed of both globular particles and long, flexible polymer chains that reversibly bind to one another. We base this on a minimal statistical mechanical model of a single aggregate in which the polymer chain is treated either as ideal or self-avoiding, and, in addition, the globular particles are taken to interact with one another via excluded volume repulsion. Furthermore, each of the globular particles is taken to have one single site to which at most one polymer segment may bind. Within the context of this model, we examine the statistics of the equilibrium size of an aggregate and, thence, the structure of dilute and semidilute suspensions of these aggregates. We apply the model to biologically relevant aggregates, specifically those composed of macromolecular proteoglycan globules and long hyaluronan polymer chains. These aggregates are especially relevant to the materials properties of cartilage and the structure-function properties of perineuronal nets in brain tissue, as well as the pericellular coats of mammalian cells., 8 pages, 3 figures, Soft Matter (2015)

We study the diffusion of a tracer particle, which moves in continuum space between a lattice of excluded volume, immobile non-inert obstacles. In particular, we analyse how the strength of the tracer-obstacle interactions and the volume occupancy of the crowders alter the diffusive motion of the tracer. From the details of the partitioning of the tracer diffusion modes between trapping states when bound to obstacles and bulk diffusion, we examine the degree of localisation of the tracer in the lattice of crowders. We study the properties of the tracer diffusion in terms of the ensemble and time averaged mean squared displacements, the trapping time distributions, the amplitude variation of the time averaged mean squared displacements, and the non-Gaussianity parameter of the diffusing tracer. We conclude that tracer-obstacle adsorption and binding triggers a transient anomalous diffusion. From a very narrow spread of recorded individual time averaged trajectories we exclude continuous type random walk processes as the underlying physical model of the tracer diffusion in our system. For moderate tracer-crowder attraction the motion is found to be fully ergodic, while at stronger attraction strength a transient disparity between ensemble and time averaged mean squared displacements occurs. We also put our results into perspective with findings from experimental single-particle tracking and simulations of the diffusion of tagged tracers in dense crowded suspensions. Our results have implications for the diffusion, transport, and spreading of chemical components in highly crowded environments inside living cells and other structured liquids., Comment: 12 pages, 9 figues, RevTeX4

Recent experimental and computational results have suggested that attractive interactions between a chaperonin and an enclosed substrate can have an important effect on the protein folding rate: it appears that folding may even be slower inside the cavity than under unconfined conditions, in contrast to what we would expect from excluded volume effects on the unfolded state. Here we examine systematically the dependence of the protein stability and folding rate on the strength of such attractive interactions between the chaperonin and substrate, by using molecular simulations of model protein systems in an idealised attractive cavity. Interestingly, we find a maximum in stability, and a rate which indeed slows down at high attraction strengths. We have developed a simple phenomenological model which can explain the variations in folding rate and stability due to differing effects on the free energies of the unfolded state, folded state, and transition state; changes in the diffusion coefficient along the folding coordinate are relatively small, at least for our simplified model. In order to investigate a possible role for these attractive interactions in folding, we have studied a recently developed model for misfolding in multidomain proteins. We find that, while encapsulation in repulsive cavities greatly increases the fraction of misfolded protein, sufficiently strong attractive protein-cavity interactions can strongly reduce the fraction of proteins reaching misfolded traps.

Graphene reinforced waterborne polyurethane (PU) composite coatings were fabricated on steel surfaces. When the filler content was 0.4 wt%, self-alignment of graphene was driven by the reduction of the total excluded volume. The superior anticorrosion properties were proven by electrochemical impedance spectroscopy (EIS) analysis for the PU matrix composite coating reinforced by 0.4 wt% of aligned graphene. The interaction mechanism between electrolyte and graphene layers was discussed for the three-dimensional randomly distributed graphene and the in-plane aligned graphene, respectively, to better understand their effects as anticorrosive barriers.

Ab initio and classical simulations were used to study the equilibrium and fluctuating ring diameters for all-silica zeolites SOD, FER, and MFI over the temperature range 300–900 K. Such simulations are important for understanding and predicting zeolite/guest fit, especially for relatively bulky guest species, e.g., those derived from biomass. We simulated equilibrium zeolite structures, IR spectra, thermal expansion coefficients, and ring breathing vibrations to investigate the competition between negative thermal expansion and enhanced vibrational amplitudes with increasing temperature. We find that although negative thermal expansion tends to shrink equilibrium ring sizes with increasing temperature, this trend is nullified by considering ring breathing vibrations, giving effective pore sizes that are roughly constant with temperature, and larger than those extracted from X-ray data. Several force fields were tested and a modified BKS force field was found to give the best agreement with the simulated properties listed above, especially for MFI. Our results are consistent with previous work suggesting that effective zeolite ring sizes are underestimated by using oxygen ionic radii for estimating atomic excluded volume.

The structure of the system consisting of adsorbed cyclic polymer chains and solvent molecules was studied. Our main aim was to check how the percolation threshold in such a system was related to the percolation threshold obtained for a system of “ordinary” flexible linear chains, i.e. how the geometry of objects influenced the percolation threshold of regarded objects and solvent molecules. The macromolecules were represented as sequences of statistical segments restricted to a two-dimensional triangular lattice. The system was athermal and the excluded volume was the only potential introduced. The properties of the system were determined by means of the Monte Carlo method using the cooperative motion algorithm (CMA). The percolation thresholds and critical exponents were determined. It was shown that the percolation threshold of cyclic chains was considerably higher than that of linear chains and depends on the chain length less significantly then for linear polymers.

Active gels are a class of biologically-relevant material containing embedded agents that spontaneously generate forces acting on a sparse filament network. In vitro experiments of protein filaments and molecular motors have revealed a range of non-equilibrium pattern formation resulting from motor motion along filament tracks, and there are a number of hydrodynamic models purporting to describe such systems. Here we present results of extensive simulations designed to elucidate the microscopic basis underpinning macroscopic flow in active gels. Our numerical scheme includes thermal fluctuations in filament positions, excluded volume interactions, and filament elasticity in the form of bending and stretching modes. Motors are represented individually as bipolar springs governed by rate-based rules for attachment, detachment and unidirectional motion of motor heads along the filament contour. We systematically vary motor density and speed, and uncover parameter regions corresponding to unusual statics and dynamics which overlap but do not coincide. The anomalous statics arise at high motor densities and take the form of end-bound localized filament bundles for rapid motors, and extended clusters exhibiting enhanced small-wavenumber density fluctuations and power-law cluster-size distributions for slow, processive motors. Anomalous dynamics arise for slow, processive motors over a range of motor densities, and are most evident as superdiffusive mass transport, which we argue is the consequence of a form of effective self-propulsion resulting from the polar coupling between motors and filaments.

The adsorption of stiff magnetic filaments close to an attractive surface is studied thoroughly via extensive Langevin dynamics simulations (LD). Magnetic filaments are represented by a coarse-grained bead-spring model where each bead bears a point dipole located at its center and the excluded volume interaction is introduced via a soft-core repulsive potential. We find strong evidence for the existence of two transitions as the temperature is lowered. First, the system undergoes a continuum phase transition from the desorbed to the adsorbed state. This transition is followed by a second structural transition that takes place when the filaments are already adsorbed. The adsorption transition is found to be very similar to the one observed for stiff non-magnetic polymer chains [Sintes et al., Macromolecules 2001, 34, 1352-1357] where the chain bending interaction plays a similar role as the magnetic component of the present case. However, the tendency of the magnetic chains to stretch is reversed by a further reduction in temperature and the chains tend to form closed adsorbed loops leading to a second structural transition. A representation of the phase diagram for the adsorption of magnetic filaments is determined here for the first time. We also present a novel way to determine the temperature at which the chain is adsorbed that is based on the analysis of the change in the number of trains, tails and loops developed by the polymer chain during the adsorption process. © 2011 The Royal Society of Chemistry., J. J. Cerd a and C. Holm acknowledge support from the DFG grant HO 1108/12-2 and the Volkswagen foundation. T. Sintes acknowledges Project No. FIS2007-60327 funded by the Spanish MEC.

We examine the issue of whether a low-volume fraction of nano- and microparticles can lead to phase separation in semidilute biopolymer mixtures. To this end, we determine the phase diagrams and ESEM images of aqueous semidilute and weakly structured sodium caseinate-sodium alginate-dextran sulfate (SC-SA-DS) systems in the presence of several charged micro particles with a diameter less, higher, and comparable with the size of the system network holes (2–3 μm).We demonstrate that 3 μm particles (both negatively and positively charged) help to enhance phase separation, and increase the viscoelastic properties of the emulsion, whereas the larger particles affect oppositely, and nanoparticles (210 nm and 910 nm in diameter) do not affect appreciably the phase separation and rheology. Experimental observations suggest that the dominant mechanism responsible for the decreased thermodynamic compatibility in such systems may be the formation of the large charged microparticle (3 μm)–SC/DSS associates in SC enriched phase that leads to decrease their compatibility with alginate due to the effect of the excluded volume interactions between SA and microparticle-DS + SC associates. Decrease in compatibility and increase in viscoelastisity are more pronounced by using highly charged hydrophobic carboxylate modified sulfonate latex (CLM sulfonate) and sulfate latex, and, to a lesser estend by use of aliphatic amino latex, and hydrophilic CLM latex.

Cross-diffusion, the phenomenon in which a gradient in the concentration of one species induces a flux of another chemical species, has generally been neglected in the study of reaction-diffusion systems. We summarize experiments that demonstrate that cross-diffusion coefficients can be quite significant, even exceeding "normal," diagonal diffusion coefficients in magnitude in systems that involve ions, micelles, complex formation, excluded volume effects (e.g., surface or polymer reactions) and other phenomena commonly encountered in situations of interest to chemists. We then demonstrate with a series of model calculations that cross-diffusion can lead to spatial and spatiotemporal pattern formation, even in relatively simple systems. We also show that, in the absence of cross-diffusion among the reacting species, introduction of a nonreactive species that induces appropriate cross-diffusive fluxes with reactive species can lead to pattern formation.

In this work we test the validity of a recent modified Poisson-Boltzmann (MPB) theory that includes ion size effects through a Langmuir-type correction. In particular, we will focus on an analytic charge-potential relationship accounting for such effects. Previous electric double layer (EDL) surveys have demonstrated that the inclusion of ions size in classical EDL theories, based on the Poisson-Boltzmann (PB) equation, can yield considerable improvements. In this sense, the theory we analyze assumes that, as a result of the excluded volume, the ion concentration close to the charged surface cannot exceed a fixed value determined by the close packing fraction. This leads to predictions of counterion concentrations (in this region) smaller than the corresponding PB values. In our opinion, it is worthwhile to test the validity of this novel theory. To this end, computer simulations appear as a useful tool for this kind of task. Our results prove that the above-mentioned analytical expression works fairly well for 1 : 1 electrolytes and large ions, and its predictions can be considerably improved with certain corrections in the estimation of some key parameters. However, it fails for multivalent electrolytes.

Recent work on the thermodynamics of protein denatured states is providing insight into the stability of residual structure and the conformational constraints that affect the disordered states of proteins. Current data from native state hydrogen exchange and the pH dependence of protein stability indicate that residual structure can modulate the stability of the denatured state by up to 4 kcal mol(-1). NMR structural data have emphasized the role of hydrophobic clusters in stabilizing denatured state residual structures, however recent results indicate that electrostatic interactions, both favorable and unfavorable, are also important modulators of the stability of the denatured state. Thermodynamics methods that take advantage of histidine-heme ligation chemistry have also been developed to probe the conformational constraints that act on denatured states. These methods have provided insights into the role of excluded volume, chain stiffness, and loop persistence in modulating the conformational preferences of highly disordered proteins. New insights into protein folding and novel methods to manipulate protein stability are emerging from this work.

The absorption of hard spheres into narrow pores is examined in the framework of Rosenfeld's “fundamental measure” formulation of density functional theory (DFT) for inhomogeneous fluids. The influence of the dimensionality of the confining geometry is assessed by considering the cases of a spherical cavity, an infinite cylindrical channel and an infinite slit. The pores are assumed to be in chemical equilibrium with a reservoir which fixes the chemical potentials of the various species. The hard sphere mixture is considered as a highly simplified model of aqueous solutions, involving a majority component (solvent) and solutes competing for absorption into the pores. It is shown that excluded volume effects alone can lead to very strong selectivities of the pores, for certain ratios of the solute and solvent to pore diameters. The selectivity is strongest for spherical cavities, and is least pronounced in the slit geometry. More complex geometries, including pore edge effects, with dimensions typical of simple ion channels through membranes, are also examined within the same DFT framework. The DFT predictions for the density profiles inside the pores, and the resulting absorbances and selectivities, are tested by grand-canonical Monte Carlo (GCMC) simulations, and good agreement is found.

The diffusion of a model system describing a polydisperse sample has been investigated experimentally. Precise mutual diffusion coefficients were measured, with a Gouy interferometer, for the ternary system hexa(ethylene glycol)(6)–di(ethylene glycol)(2)–water at four values of the concentration ratio β6 = C6/(C6 + C2), and at a constant total concentration C6 + C2 = 0.3000 mol dm−3. The main-term diffusion coefficient, D22, shows a substantial decrease with β6, while D66 varies modestly. Both cross-term diffusion coefficients are positive, the largest, D26, reaches a value that is about 15% of the corresponding main-term. The experimental diffusion coefficients of this system have been interpreted through an extension of the common hard sphere hydrodynamic theory. Good agreement between the experimental diffusion coefficients and those predicted by a set of equations recently proposed by the authors for systems of non-ionic and slightly interacting solutes has been found. These equations, accounting only for the “ excluded volume” effect, succeed in the evaluation of the value of the diffusion coefficients in ternary systems with solute size mismatch. A comparative analysis, with analogous systems investigated previously confirms the efficiency of our predictive procedure.

The region of a lipid membrane, with all hydrocarbon chains equal, embedded in an aqueous solution and interacting with a single DNA molecule, has been modelled. The lattice model used assigned an area characteristic of a lipid molecule in a gel phase to each lattice site. In a fluid phase two lipid molecules occupy three sites. We studied a membrane composed of lipids with phosphatidylcholine (PC) and trimethylammoniumpropane (TAP) headgroups. Lipid headgroup states were enumerated as described elsewhere (Pink et al., Biochim. Biophys. Acta, 1988, 1368, 289). Here charged lipid moieties were represented by point charges inside an excluded volume. The aqueous solution was modelled as a linearized Poisson–Boltzmann system characterized by a Debye screening length, κ−1. We employed standard Monte Carlo computer simulation techniques. We came to the following conclusions. (a) In the absence of DNA, PC and TAP headgroup pairs formed dynamic bound states in a gel phase. These did not occur if the PC was represented as an object carrying no charges. Accordingly, although PC carries zero net charge, it is important to represent the charged moieties explicitly. The gel–fluid phase transition in a 1:1 PC–TAP membrane (with equal hydrocarbon chain lengths) might thus involve not only hydrocarbon chain disordering but also the break-up of the dynamical PC–TAP bound pairs. (b) Increasing TAP concentration resulted in changing the orientation of the PC dipole. (c) DNA binding is a complex process and can involve weak binding even to a pure PC interface (which could, however, be disrupted by membrane undulations not modelled here) and tighter binding when TAP is present. The minimum concentration for the latter depends upon κ. DNA binding at low TAP concentrations would be changed if the PC was represented by a chargeless object. (d) A bound DNA changes the headgroup orientation in its proximity and also results in increased headgroup lateral packing in the immediate neighbourhood of the DNA. The latter could result in denser lateral hydrocarbon chain packing in the neighbourhood of the DNA. Both of these phenomena exhibited a dependence upon the TAP concentration. (e) The average lipid–DNA binding energies per DNA PO2− group can be as large as 2kBT or more depending upon the TAP concentration. (f ) DNA can be unbound by changing the ionic concentration. We compare our results with experimental data and other simulations.

The kinetics of weak gelation is modelled as a system of spherical particles interacting with sticky hard-sphere pair potentials. Gradual cross-linking of primary particles to form disordered non-close-packed aggregates of greater total excluded volume than the separate non-bonded primary particles leads to a steady increase in the effective volume fraction of the sticky hard-sphere system. Characterizing the bonded aggregate structure by a single fractal dimensionality, d f , the model predicts a time-dependent average cluster size which diverges as the system reaches the percolation threshold. The inferred gel time is shown to be rather sensitive to bonded aggregate structure as measured by the value of d f . The model may provide a simple kinetic description of particle gel formation in real systems having a combination of weak and strong particle interactions.

Lattice chains, chain length 200 monomers, generated by the Monte Carlo method, have been studied on a tetrahedral lattice. Real-chain features such as conformational restrictions, variable segment interaction and chain thickness were assumed in the model. The chain vector density distribution function W(R) and the related changes in the Helmholtz energy were calculated as a function of the displacement of the chain ends, R. The force–displacement curves derived by this approach are non-linear with the shape depending on the solvent quality. The energy changes, accompanying the chain deformation, due to (a) conformational isomerization and (b) loss of segment contacts, were computed. In the model used (a) leads to a negative and (b) to a positive energetic contribution, fu, to the total tensile force, f. The mixed energy–entropy character of the single-chain deformation was demonstrated for some representative chains differing in excluded volume and flexibility.

The excluded-volume effect in polymer–mixed-solvent systems has been investigated in the light of the two-parameter theory. These multicomponent polymer systems exhibit some specific features quite different from polymer–single-solvent systems, involving ternary interactions and the synergic action of the mixed solvents at a given composition on polymer dimensions. Both effects are taken into account in order to derive a new expression for the interpenetration ternary function, ψT, and the expansion factor, αs, both being quantitatively evaluated for ternary systems based on polystyrene and poly(methyl methacrylate). In addition, a complementary treatment of the excluded-volume effects, proposed by Wolf, based on the evaluation of the intra- and inter-molecular interactions has been carried out.

A formalism recently proposed to study the excluded volume effect in ternary polymer solutions is used to analyse, in terms of inter- and intra-molecular interactions, mixtures of poly(ethyl methacrylate)(PEMA) in acetonitrile (AcN)–1-chlorobutane (ClBu) and in AcN–BrBu. Viscometric and light-scattering techniques have been used to obtain intrinsic viscosities, second virial coefficients and preferential solvation parameters as a function of the binary solvent composition. Solvent(1)–solvent(2)–polymer(3) contacts have been evaluated using the Flory–Huggins–Pouchly formalism, and the individual contributions of both inter- and intra-molecular segment–segment contacts to the excluded volume are reported and discussed.

Several studies have focused on developing a theory of the excluded volume effect in polymer solutions in good accord with experimental data. There are very few expressions for the interpenetration factor, Ψ, which can be chosen for a given theory of the expansion factor α3s. The combination of Kurata–Yamakawa theory for Ψ with Yamakawa–Tanaka theory for α is in good accord with experimental data on binary polymer systems. However, no self-consistent combination of theories can be chosen to describe the behaviour in ternary polymer systems. A modified expression for the interpenetration factor has been developed for ternary polymer systems. It depends on intrinsic viscosities and on the Kurata–Yamakawa binary interpenetration factor. Good qualitative agreement has been attained by comparing the modified (Ψ, α) plot with experimental data on reported ternary systems. A useful application is the theoretical calculation of second virial coefficients.

A vapour–liquid phase coexistence with vanishing critical volume fraction (empty liquid) is predicted in some model systems of attractive hard ellipsoids. The trends in critical density and temperature do not depend on the ellipsoid shape and the range of the square-well attraction. Our theoretical and simulation studies show transparently that the empty liquid limit is due to the competition between the anisotropic attractive and repulsive interactions. Additionally, the nematic phase is destabilized by the square-well attraction for both prolate and oblate shapes. The demand to maximize the number of bonds while keeping a relatively low excluded volume leads to special structures such as the cubatic and star-shaped cluster phases at low temperatures. The liquid branch of the vapour–liquid binodal is always isotropic for oblates, while it can be isotropic or cluster-like for prolates. Our results predict the occurrence of an empty liquid state in high valence anisotropic colloidal systems.

Direct observation of the Brownian motion of linear and circular duplex DNA in the chain-length range from about 5 to 300 kilo-basepairs (kbp), as reported by Smith, Perkins, and Chu, [Macromolecules, 1996, 29, 1372–1373] and by Robertson, Laib, and Smith, [Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 7310–7314] has indicated an effective mass-scaling exponent for diffusion, νH, of approximately 0.6, and this correlation was interpreted as implying that duplex DNA in this chain-length range can be described as a flexible polymer swollen by excluded volume interactions. However, we present computational evidence, based on our path-integration technique for computing transport properties, that indicates this inference needs to be reconsidered. Because of variable draining effects, the ratio Rh/Rg for random-coil polymers, including DNA, varies gradually throughout the chain-length range of interest, and does not stabilize to an asymptotic value until the chains are extremely long. This slow exponent crossover is particularly extended in stiff chains such as DNA. In particular, we find that if we take duplex DNA to be a worm-like chain comprised of a number of base pairs in the range between 5 and 300 kbp, then the effective exponent νH happens to be about 0.6, while the effective exponent, ν, for the radius of gyration, Rg, is about 0.52 and 0.51, with and without chain excluded volume, respectively. These results were obtained directly for a stiff lattice-chain model that agrees very well with the predictions of the continuum worm-like chain model. Based on these calculations and consistent experimental observations, we suggest that duplex DNA having a length below about 300 kbp is better described as a worm-like chain than a self-avoiding walk.

Three ice structuring proteins (ISPs) with different molecular weights and net charges at pH 7.4 were partitioned in an aqueous anionic/nonionic surfactant two-phase system of SDS and Trition X-114 in the presence of 100 mM sodium chloride. The effects of temperature and SDS concentration on the protein partition coefficients were investigated. It is found that the protein partition coefficients increase with the increase of temperature and SDS concentration. Compared to the reported net dependence of protein partitioning in aqueous mixed surfactant two-phase systems, the observed protein partitioning behavior is driven primarily by excluded volume interactions between the proteins and the surfactant aggregates, rather than electrostatic interactions. Specifically, the ISPs partition preferentially to the micelle-poor phase, where they experience less excluded-volume interactions with the surfactant aggregates. The improvements in protein partitioning to the micelle-poor phase upon the SDS addition appear to result from the growth of the surfactant aggregates by forming mixed micelles, which enhances the excluded-volume interactions between the proteins and the surfactant aggregates.