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Nanoparticles or drug carriers which can selectively bind to cells expressing receptors above a certain threshold surface density are very promising for targeting cells overexpressing specific receptors under pathological conditions. Simulations and theoretical studies have suggested that such selectivity can be enhanced by functionalizing nanoparticles with a bimodal polymer monolayer (BM) containing shorter ligated chains and longer inert protective chains. However, a systematic study of the effect of these parameters under tightly controlled conditions is still missing. Here, we develop well-defined and highly specific platforms mimicking particle–cell interface using surface chemistry to provide a experimental proof of such selectivity. Using surface plasmon resonance and atomic force microscopy, we report the selective adsorption of BM-functionalized nanoparticles, and especially, a significant enhanced selective behavior by using a BM with longer protective chains. Furthermore, a model is also developed to describe the repulsive contribution of the protective brush to nanoparticle adsorption. This model is combined with super-selectivity theory to support experimental findings and shows that the observed selectivity is due to the steric energy barrier which requires a high number of ligand–receptor bonds to allow nanoparticle adsorption. Finally, the results show how the relative length and molar ratio of two chains can be tuned to target a threshold surface density of receptors and thus lay the foundation for the rational design of BM-functionalized nanoparticles for selective targeting.

Dual-responsive poly-(N-isopropylacrylamide) (PNIPAM) microgels surface-functionalized with polyethylene glycol (PEG) or poly-2-dimethylaminoethyl methacrylate (PDMAEMA) were developed to enable the swelling behavior and surface properties of the microgels to be tuned independently. The thermo-triggered swelling and pH-triggered surface properties of the microgels were investigated in aqueous suspensions using dynamic light scattering and on substrates using the surface forces apparatus. Grafting polymer chains on the microgel surface did not impede the thermo-triggered swelling behavior of the microgels in suspensions and immobilized on substrates. An unprecedented decoupling of the swelling behavior and surface properties could be obtained. More particularly, the thermo-triggered swelling behavior of the PNIPAM underlying microstructure could be tuned below and above the phase transition temperature with no change in the surface potential and adhesion provided by the surface non-responsive PEG.

A series of acrylamide-based water-soluble (co)polymers was synthesized and they were investigated as flocculants of model kaolin suspensions and mature fine tailings of oil sands. The effects of molar mass, charge density, and polymer concentration on flocculation efficiency were studied by monitoring the initial settling rate during sedimentation. Hydrolyzed polyacrylamide (HPAM) with high molar mass and intermediate acrylic acid contents (0.14–0.41 mol/mol (14–41 mol%)) performed better in flocculation tests on kaolin suspensions requiring lower dose for maximum initial settling rate than native polyacrylamide (PAM). Surface force measurements showed that at low polymer concentrations (1 ppm), the partially-adsorbed polymer induced a bridging attraction between the mica surfaces. Increasing the polymer concentration to 10 and 50 ppm caused purely repulsive forces. The presence of anionic groups in HPAM led to stronger repulsion, which was also demonstrated by the higher viscosity and larger hydrodynamic radius of the charged polymer. The charge-induced increase in the viscosity of polymer solutions was suppressed by the screening effect of salts in a buffer solution and reducing the viscosity is desirable in the injection of flocculants in the industrial process. This article is protected by copyright. All rights reserved

Normal and friction forces between immobilized two-dimensional (2D) homogeneous non-close-packed colloidal arrays made of different particles are compared in aqueous media. Soft pH-responsive (microgels) and nonresponsive hard (silica) particles of different sizes were used to create the 2D arrays. The results show that the friction of soft responsive structured layers can be successfully modulated by varying the pH, with a friction coefficient varying by nearly 3 orders of magnitude (10–2 to 1). This important change in lubricating properties is mainly correlated with the particle swelling behavior, i.e., the friction coefficient decreasing exponentially with an increase in the swelling ratio regardless of the size, surface coverage, and degree of ionization of the particles. In addition, the robustly attached microgel particles were able to sustain high pressure (up to 200 atm) without significant surface damage. The 2D arrays of nonresponsive hard particles also gave rise to a very low friction coeffic...

The tribological properties of two novel biomimetic multihierarchical polymers, synthesized by covalently linking single bottlebrush polymers onto a hyaluronic acid (HA) backbone, were investigated in the boundary lubrication regime using the surface forces apparatus. The polymers were immobilized on flat substrates, and their lubrication properties and wear resistance were investigated in aqueous media in the absence of a polymer reservoir (i.e., no free polymer chains in the surrounding medium) in order to better reveal the underlying mechanism of surface-attached biomimetic polymers. The effects of composition, structure, and, more particularly, surface attachment (physisorbed vs chemisorbed) on the tribological properties were investigated and compared with other biomimicking systems reported in the literature. The covalently surface attached bottlebrushes allowed wear resistance between sliding surfaces to be significantly improved, compared to physisorbed bottlebrushes, with a constant coefficient of friction (10

International audience; In this work, we have investigated the behavior under shear and compression of mica surfaces coated with poly(N-isopropylacrylamide) cationic microgels. We have observed the emergence of velocity dependent, shear-induced normal forces, which can be large enough to entrain a fluid film that separates the surfaces out of contact, driving the dynamic system from conditions of boundary to hydrodynamic lubrication. By implementing a feedback-loop control on the surface separation, we were able to quantify the magnitude of the lift force as a function of the surface separation and driving speed. Our results illustrate how elastohydrodynamic effects can play an important role in the lubrication of compliant surfaces, providing pathways for control of friction and wear.

We describe how a long-range repulsive interaction can surreptitiously modify the effective geometry of approaching compliant surfaces, with significant consequences on friction. We investigated the behavior under shear and compression of mica surfaces coated with poly(N-isopropylacrylamide) pNIPAM-based cationic microgels. We show that local surface deformations as small as a few nanometers must be considered to understand the response of such surfaces under compression and shear, in particular when the range of action of normal and friction forces are significantly different, as is often the case for macromolecular lubrication. Under these conditions, a subtle interplay between normal forces and surface compliance may significantly reduce friction increment by limiting the minimum approach of the surfaces under pressure. We found that stiffening of compressed microgels confined in the region of closest approach make it increasingly difficult to reduce the gap between the mica surfaces, limiting the defo...

Controlling surface coverage and stability of supported aminoalkylsilane monolayers on silica-based substrates still remains a challenge for the development of biosensors and nanomaterials. We have developed protocols using simple surface chemistry and self-assembly from solution without stringent deposition conditions to covalently attach monolayers of 11-aminoundecyltriethoxysilane (AUTES) onto mica and silica substrates. The resulting self-assembled monolayers (SAMs) exhibited excellent hydrolytic stability. The long alkyl chain together with the large grafting density and homogeneity enhanced the monolayer stability by preventing the Sisilane O Sisurface bonds from hydrolysis over a wide range of pH values (2–10) for long time periods (up to 8 days). The control over the surface density of amino groups was achieved and the reactivity of the amino SAMs was confirmed by covalently attaching carboxyl-functionalized nanoparticles on the SAMs. The immobilized nanoparticles exhibited the same hydrolytic stability as that of the SAMs. The AUTES SAMs prepared in this study exhibited the best hydrolytic stability of similar systems reported so far.

Hybrid nanoparticles (NPs) integrated with dye molecules have attracted interest for biomolecular detection, due to their effortless fabrication, timely operation, controllable specific recognition and low-cost. In this study, hybrid core–shell NPs made of a metal–dye complex (AgNPs@[Ru(bpy)3]2+) core and a chitosan (CS) shell exhibiting a selective fluorescence quenching effect were successfully prepared using a cost-effective wet-chemical approach. The physico-chemical properties of NPs were determined by spectroscopy and light scattering measurements. The bio-affinity of the AgNPs@[Ru(bpy)3]2+/CS was evaluated in aqueous media using sialic acid (SA) as the target molecule in the presence of different monosaccharides and anionic biomolecules as interferents. A significant fluorescence quenching of hybrid NPs was observed in aqueous solutions of SA with interferents, while no significant quenching effect was detected in SA-free interferent solutions. The selective binding of SA to the particles resulted from favorable electrostatic interactions and inter-molecular hydrogen bonding with the functional groups of CS. The hybrid NP system displayed a good sensitivity for SA with a detection limit of 5.1 nM and a concentration dependent fluorescence quenching for SA concentrations ranging from 25 nM to 3.2 μM. This hybrid NP system represents a promising alternative probe for detecting sialic acid in complex samples.

We report the first successful preparation of polyelectrolyte brushes using an ATRP initiator that was covalently grafted to silica and mica substrates via an organophosphonic acid. Covalent attachment of the initiator to silica and mica and the subsequent synthesis of polyacrylic acid (PAA) and poly(sulfopropyl methacrylic acid) brushes by water mediated-ATRP were confirmed by ATR-FTIR, ellipsometry, AFM, and contact angle measurements. The initiator–substrate bond was robust and could resist a large range of pH in the absence of salt. Interactions between PAA brushes anchored to mica via the organophosphonic acid initiator were investigated using a surface forces apparatus. The results confirmed the robustness of the initiator–mica bond as the brushes could resist shearing and compression under relatively high applied loads.

The frictional properties of a thin hexadecane film confined between two atomically smooth surfaces of mica were studied using the surface forces apparatus equipped with a 3D actuator-sensor attachment specially designed to investigate static and dynamic forces in three orthogonal directions simultaneously. The use of this attachment allows the relative alignment of the reciprocal sliding motion to be changed by an angle of 90 while maintaining the film under the same confinement conditions. The effects of the commensurability of the confining mica surfaces as well as the relative sliding direction on the frictional behavior of the hexadecane film were determined for different temperatures (18-29 C) and sliding velocities (4 nm/s to 4 μm/s). The confined hexadecane film exhibited smooth sliding friction whose amplitude increased with the commensuration of the surfaces. A progressive evolution in the kinetic friction force toward a steady-state value was observed over reciprocal sliding motion for given experimental conditions of applied load, sliding velocity and environmental temperature. This friction evolution shows to be dependent on the sliding history of the film and could result from a partial molecular ordering, occurring during shear. Refereed/Peer-reviewed

International audience; As double stranded, single stranded siRNA (ss-siRNA) has demonstrated gene silencing activity but still requires efficient carriers to reach its cytoplasmic target. To better understand the fundamental aspect driving the complexation of ss-siRNA with nanocarriers, the interactions between surfaces of various compositions across a ss-siRNA solution were investigated using the Surface Forces Apparatus. The results show that ss-siRNA can adsorb onto hydrophilic (positively and negatively charged) as well as on hydrophobic substrates suggesting that the complexation can occur through hydrophobic interactions and hydrogen bonding in addition to electrostatic interactions. Moreover, the binding strength and the conformation of ss-siRNA depend on the nature of the interactions between the ss-siRNA and the surfaces. The binding of ss-siRNA with nanocarriers, such as micelles or liposomes through non-electrostatic interactions was also evidenced by a SYBR ® Gold cyanine dye. We evidenced the presence of interactions between the dye and oligonucleotides already complexed to non-cationic nanovectors biasing the quantification of the encapsulation. These results suggest that non-electrostatic interactions could be exploited to complement electrostatic interactions in the design of nanocarriers. In particular, the different highlighted interactions can be used to complex ss-siRNA with uncharged or anionic carriers which are related to lower toxicity compared to cationic carriers.

Poly(acrylic acid) (PAA) covalently immobilized on glass substrates was made fluorescent by grafting a BODIPY derivative (PMOH) via an ester linkage. Although only nanograms/square centimeter of polymer are understood to be immobilized onto the SiO2 substrate, the fluorophore-tagged polymer was readily visible to the naked eye and its fluorescence was easily detected. The characteristic BODIPY emission, centered at 550 nm, was used to follow the degrafting of PAA from the glass substrates in aqueous solution in real-time using total internal reflection fluorescence (TIRF) microscopy. The substrate−initiator bond hydrolysis and the conditions at which the PAA degrafting occurred were unequivocally confirmed in real-time by TIRF microscopy. No cleavage of the polymer occurred between pH 6.5 and 10.5 in the absence of NaCl. In contrast, polymer degrafting from the substrate occurred at pH ≥ 9.5 when 10 mM NaCl was added to the buffer solution.

Summary: Unprecedented direct polymerization of sodium acrylate (NaA) on mica and silica substrates was undertaken using standard ATRP polymerization conditions at room temperature. The resulting thickness of the poly(sodium acrylate) (PNaA) grafted layer was determined using ellipsometry and AFM.

Poly(acrylic acid) (PAA) and poly(tert-butyl acrylate) (PBA) brushes of various grafting densities were prepared via surface-initiated polymerization of tert-butyl acrylate on mica. PAA was prepared by hydrolyzing the PBA brushes. The swelling behavior of PBA and PAA brushes was studied as a function of grafting density by AFM. The swelling of the polymer layer was found to be higher for PAA in water than for the PBA sample swollen with DFM.

Poly(acrylic acid) (PAA) brushes covalently linked to mica were prepared using the graft from approach in a two-step process: (i) poly(tert-butyl acrylate) (PtBA) brushes were first synthesized by atom transfer radical polymerization directly from an activated mica substrate (ii) followed by hydrolysis to generate PAA brushes. The hydrolysis reaction was confirmed by water contact angle measurements, polymer thickness measurements, and FTIR. The swelling behavior of the brushes in aqueous solutions was measured by examining the change in brush thickness (L), using atomic force microscopy (AFM), as a function of polymer grafting density (σ), pH, and salt (NaCl) concentration (Cs). A sharp transition from collapsed to stretched conformation was found at pH 7.5. For pH ≤ 7, the acrylic acid groups are not dissociated, and no swelling of the polymer layer was observed relative to the dry state, regardless of grafting density and salt concentration. For pH ≥ 7.5, the brushes behaved as charged polymer brushes ...

The behavior of adsorbed surface active molecules at mica surfaces from crude oil and from their extracted asphaltenes dissolved in different solvents was investigated. The interactions between mica surfaces across a crude oil and across asphaltene solutions using different solvents and humidity conditions were measured using the surface forces apparatus. The nature of the interactions between the adsorbed layers strongly depends on their composition and on the presence of dissolved water. The results clearly indicate that the adsorbed layer/oil interface for a crude oil is significantly different from that for an asphaltene/toluene interface.

Homogeneous polystyrene monolayers covalently end-attached on mica and silica surfaces were obtained using a "graft to" methodology. The grafting was achieved via nucleophilic substitution between silanol groups (Si-OH) containing surface and monochlorosilyl terminated polystyrene (PS). Different parameters, such as surface activation, grafting reaction time, polymer concentration, nature of solvent, and presence of catalyst, were investigated to determine the optimal conditions for creating very homogeneous and stable polymer monolayers. Ellipsometry, atomic force microscopy (AFM), surface forces apparatus (SFA), and contact angle measurements were used to characterize the polymer-grafted layers. An efficient plasma activation procedure was established to create a maximum number of silanol groups on mica surfaces without increasing the surface roughness. Surface reactivity was investigated by grafting trimethylchlorosilane (TMS) on OH-activated mica and silica. The maximum TMS surface coverage on activated mica is similar to that observed for silica. The stability of covalently attached TMS and PS layers in toluene and water were investigated. Both grafted layers (TMS and PS) partially detached from the mica and silica surfaces when immersed in water. Hydrolysis of the siloxane bond between the monochlorosilyl groups and the surface is the most probable cause of layer degrafting. The degrafting was much slower with the long PS polymer chains, compared to the small TMS molecules, which may act as a protective layer against hydrolysis.

Thermosensitive polymer nanospheres based on N,N-diethylacrylamide and 2-hydroxyethyl methacrylate (HEMA) have been prepared, characterized, and conjugated with biotin. The thermosensitivity of poly(N,N-diethylacrylamide) was enhanced by the incorporation of HEMA up to about 40 mol %. Atomic force microscopic images show that these particles can be closely packed even without the surface charges as in the latex particles. Biotinylation reduces the thermosensitivity of the copolymer nanospheres. The biotinylated hydrogel nanospheres showed a reduction in size upon binding with streptavidin, indicating the formation of a less hydrophilic conjugate. No aggregation of the biotinylated particles due to the cross-linking effect of streptavidin was observed. This size change could be reversed by the addition of free biotin to the system. The interaction is specific, and no such changes were observed when streptavidin was replaced by bovine serum albumin.

A biofunctionalized graphene oxide (GO) nanosheet with improved physicochemical properties is useful for electrocatalysis and sensor development. Herein, a new class of functionalized GO with a chemically anchored biomolecule glucosamine is developed. Structural and chemical analyses confirm the glucosamine anchoring. Ultraviolet irradiation transforms the surface chemistry of GO. Glucosamine-anchored GO nanosheets exhibit improved cyclic voltammetric and amperometric sensing activity toward the model redox probe, ruthenium(II) and N-acetylneuraminic acid, respectively. The biomolecular anchoring and ultraviolet irradiation helped to tune and enhance the properties of GO, which may find multiple applications in optimizing sensor platforms.

The behavior of three copolymers of N-isopropylacrylamide (NIPAM), methacrylic acid (MAA), and hydrophobic moiety was studied at phospholipid monolayer/subphase interfaces. The hydrophobic moieties, N-terminal dioctadecylamine (DODA) and random octadecylacrylate (ODA), were used as anchoring groups. The interactions between a 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC) monolayer and the copolymers were studied using the Langmuir balance technique. The effect of subphase pH, distribution of anchors along the copolymer chain, and copolymer molecular weight on the nature of the interactions between the copolymer chains and the DSPC monolayer were investigated. A first-order kinetics model was used to analyze the copolymers adsorption at the DSPC monolayer/subphase interface and allowed the interaction area between the copolymer chains and the DSPC monolayer, A(x), to be determined. The interaction area appears to depend on the subphase pH and the copolymer molecular weight. On decreasing pH, the interaction area of high molecular weight copolymers increases significantly; this is consistent with the copolymer chain phase transition from an extended coil to a collapsed globule while pH is lowered. In the latter conformation, strong hydrophobic attractive interactions between the copolymer chains and the hydrophobic part of the DSPC monolayer favor the copolymer intercalation, which could eventually provoke the phospholipidic layer destabilization or rupture.

Dynamic shear oscillation measurements at small strains are used to characterize the polymerization process in situ and the viscoelastic properties of cross-linked polyacrylamide hydrogels. Hydrogels are synthesized by free-radical redox polymerization of acrylamide (8 wt %) for different concentrations of cross-linker, N,N‘-methylenebis(acrylamide) (BIS), at different temperatures. Both elastic modulus G‘ and viscous modulus G‘‘ are measured in real time during the gelation which takes place directly between parallel rheometer plates. The elastic modulus G‘ remains constant with frequency, G‘(ω) ≈ cte, and is significantly larger than G‘‘(ω), characteristic of a well-developed cross-linked polymer network. Temperature scanning of the elastic modulus shows that G‘(T) is a linear relationship with a proportionality value that depends on the polymerization temperature Tpol. This observation is in agreement with the classical theory of rubberlike elasticity, i.e., G‘ = neRT where ne is the active network lin...

Molecular interactions between a terminally alkylated pH-sensitive N-isopropylacrylamide copolymer DODA-poly(NIPAM-co-MAA) and a monolayer of distearoylphosphatidylcholine (DSPC) at the air/water interface are investigated using the Langmuir balance technique. The compression isotherms ofthe copolymer monolayer at the air-water interface confirm that the copolymer undergoes a structural transition with a change in pH ranging from an extended coil state at neutral pH to a collapsed globular state at a pH corresponding to the pH of the polymer phase transition. Adsorption kinetics of DODA-poly(NIPAM-co-MAA) in the DSPC monolayer is analyzed using a first-order kinetics model allowing an effective interaction area Ax between DSPC and DODA-poly(NIPAM-co-MAA) molecules to be evaluated. The results clearly indicate that the interaction area increases with a decrease in pH. The results also suggest that the penetration of the DODA-poly(NIPAM-co-MAA) within the phospholipid monolayer is enhanced by a decrease in pH which causes a change in the copolymer structure and an increase in specific attractive interactions between the copolymer and the phospholipid. Therefore, the copolymer can trigger the destabilization or rupture of the phospholipidic layer through a simple variation in its structure associated with a variation in molecular interactions when coupled or inserted within the membrane. This study greatly supports the prospects of the copolymer-functionalized liposomes as stable and tunable carrier systems for in vivo applications in drug delivery.

A surface force balance has been used to investigate the viscosity of salt-free (conductivity) water confined between hydrophilic and between hydrophobic surfaces. We examine the process of jump-in, across the last few nanometres of thin water films, to adhesive contact between the surfaces. We analyse the flow of water out of the gap under slip and no-slip boundary conditions at the confining surfaces. In both cases we find that the effective viscosity of water remains comparable to its bulk value even when it is confined to sub-nanometre thin films.

We studied the association of hydrophobically anchored polyelectrolyte layers with a free oppositely charged surfactant at a hydrophobized solid–liquid interface under partially screened conditions using the surface forces and ellipsometry techniques. Combining results from both measurements, we conclude that the association of surfactant (from solution below the cmc) with partially screened adsorbed polyelectrolyte layers causes a reduction in the surface density of the polymer chains and a subsequent reduction in the extensibility of the chains.

Dual templating using aqueous cetyltrimethylammonium chloride/hydroxide (CTMA Cl/OH) and latex spheres was performed to prepare powders of bimodal silica molecular sieves with mesopore and macropore systems. The CTMA/Si ratio was chosen to generate a cubic mesopore of MCM-48 structure. According to high Brunauer–Emmett–Teller surface area, narrow pore size distribution, X-ray diffraction powder pattern and scanning electron micrographs, the cubic mesophase is located within the macropore walls, which is the characteristic of a hierarchical porous system.

The adsorption kinetics of a diblock copolymer poly(fer£-butyl methacrylate)-6-poly(glycidyl methacrylate sodium sulfonate) on hydrophobic substrate from aqueous solution under different added monovalent salt (NaCl) concentrations was investigated using an ellipsometric technique. The effect of monovalent counterion size on adsorption kinetics of the same copolymer on hydrophobic surfaces was also part of the investigation. The results, in general, indicate that the adsorption process on solid surfaces occurs through the anchoring of hydrophobic chains due to the short-ranged hydrophobic interactions. The kinetic data reveal three distinct stages in the adsorption process: an incubation period, a subsequent fast growth process of the polymer layer, and a plateau (equilibrium) region. These three stages are found to be influenced by salt concentration as well as counterion size. The equilibrium adsorption density increases as a function of salt concentration, . and the dependence is found to be different from the theoretical predictions. The incubation time increases with salt concentration according to a power law dependence, and a simple bound ionic layer formation on the substrate is proposed as a possible explanation for this observation. An attempt has been made to explain the growth process in terms of an Avrami type ordering process. The Avrami analysis indicates that the buildup of polyelectrolyte layer structure depends on added salt conditions. Our kinetic data suggest that the diffusion of the chains to the surface is not the rate-controlling process for adsorption. A slow birth (nucleation) and fast growth of the layer seem to be the determining adsorption process. © 2000 American Chemical Society.

New surfactants derived from crown ethers were synthesized and used as templates in the preparation of inorganic mesostructures. These surfactants having a crown ether group as hydrophilic head yielded silica mesoporous molecular sieves of the M41S family which were transformed with subsequent thermal treatment into a material of particular unprecedented morphology.The crown ether derived surfactants were characterized by H-1 NMR and mass spectroscopy and the mesoporous materials by X-ray diffraction (XRD), nitrogen physisorption, transmission electron microscopy (TEM) and solid state Si-29 NMR spectroscopy. The TEM technique was used to assess the different phases present in the calcined samples. The morphology of these materials is quite different from that of MCM-41 and shows lamellar stackings of silica sheets pillared by minor amounts of hexagonal phase.Using molecular simulations of the surfactants and their cation complexed forms, some detailed conclusions could be drawn regarding the effect of various cations on pore size and shape. (C) 2000 Elsevier Science B.V. All rights reserved.

We investigated the nature and the range of interactions between negatively charged polyelectrolyte-coated surfaces as a function of salt concentration using the surface forces apparatus. The measured force profiles (interaction forces versus separation distance) are purely repulsive and show long-range electrostatic and short-range steric interactions. The measured range of interaction in salt-free as well as in low ionic strengths extends well beyond the contour length of the polyelectrolyte chain. It is shown that the interaction range between the ionic brush layers and the grafting density depend on the ionic strength of the solution. In salt-free solution, the counterions associated with polyelectrolyte cause the chains to stretch and give rise to long-range double-layer electrostatic repulsions between the opposing chains. When salt is added to the system, the electrostatic interactions are partially screened and the polymer chain regains its flexibility and therefore the range of interactions is reduced. The measured total range of interaction exhibits relatively weaker dependence on the salt concentration. We find that our force-distance profiles with added salt in a compressed regime can be very well described by the Pincus scaling model.

The interactions between surfaces across a nonionic surfactant (tetraoxyethylene dodecyl ether, C12E4) solution and its microemulsion in the dilute regime have been directly measured using the surface force apparatus. The results show that the structure of the adsorbed surfactant layer depends dramatically on the size and shape of the self-assembled structures in the bulk solution as well as the chemical nature of the substrate. On hydrophilic surfaces, the adsorption from surfactant solutions is relatively weak and the resulting long-range repulsive force is attributed to entropic confinement and elastic compression of adsorbed micelles. Microemulsion droplets adsorb much more strongly. On hydrophobic surfaces, a compact surfactant monolayer adsorbs leading, to a strong but shorter-ranged force. In contrast to the results with hydrophilic surfaces, the adsorbed surfactant layer is “destabilized” by the presence of oil from the microemulsion solution and is easily expelled upon compression. Quantitative a...

The texture of food is considered related to its rheological behavior when confined within thin films between two shearing or squeezing surfaces. Such thin-film tribological properties may represent foods undergoing massive grinding and deformation during mastication. The thin-film tribological properties rather than the bulk rheological properties correlated with the composition (oil droplets, etc.) and texture of mayonnaise samples. Using the Surface Forces Apparatus (SFA) technique and optical interferometry using fringes of equal chromatic order, we characterized the thin-film morphology and measured tribological and other interfacial properties of mayonnaise emulsions. Results revealed new properties that help differentiate between full fat, light and fat free mayonnaises. This provides an important approach to determine properties of such foods in thin films which cannot be deduced from bulk properties.

14 pages; International audience; We have studied the adsorption and lubricant properties of a multifunctional triblock copolymer poly(L-lysine)-b-poly(acrylic acid)-bpoly(L-lysine). In particular, we investigated the nature of the layer adsorbed under different conditions of polymer and salt concentration and the lubricant properties of the polymer layer before and after its chemical crosslinking by bridging the poly(acrylic acid) blocks. We found that the amount of polymer adsorbed is controlled by the ionic strength and the polymer concentration in the solution. In all cases, the self-assembled polymer layer is a poor lubricant before cross-linking, but the cohesion and load-carrying ability of the layer are substantially improved by this reaction. However, the chemically cross-linked coating has a limited deformation capacity as a consequence of its permanent network nature, and irreversible damage is observed after excessive strain of the film.

A new 3D sensor-actuator attachment for a Surface Forces Apparatus (SFA) allows measurements of friction forces between surfaces in all 3 orthogonal directions. The anisotropic friction forces when shearing two mica surfaces separated by a nanometer-thin film of hexadecane can induce complex transient and steady-state motions involving displacements perpendicular to the applied force and nonzero velocity at all stages of back-and-forth sliding. Refereed/Peer-reviewed

Static and dynamic interaction forces between two thermosensitive polymeric nanoparticle monolayers grafted onto mica surfaces and immersed in water were studied using a surface forces apparatus. The polymeric nanoparticles (NPs) were made of N,N-diethylacrylamide and had a hydrodynamic diameter of ca. 780 nm at 20 °C in aqueous suspension. They were irreversibly grafted onto chemically modified mica surfaces at a constant surface coverage of 2.6 NPs/µm2. The measured normal forces between two opposing NP monolayers were found to be strongly dependent on the temperature. At temperatures lower than the lower critical solution temperature (LCST), the grafted NPs were swollen, and the normal interaction forces between the two NP monolayers were repulsive. Above the LCST, the NPs collapsed, and attractive forces between the NP layers were measured. The swollen NPs were found to exhibit very low friction forces compared to the collapsed ones. The effect of the sliding velocity on the shear stress was investigated, and the results are in agreement with the so-called adhesive friction model developed for rubber friction. Our results suggest that the water content in the contact area and the interdiffusion of polymer chains are important parameters in determining the friction between polymer-bearing surfaces. Refereed/Peer-reviewed

The controlled grafting density of poly(tert-butyl acrylate) was studied on OH-activated mica substrates via surface-initiated atom-transfer radical polymerization (ATRP). By properly adjusting parameters such as the immobilization reaction time and the concentration of an ATRP initiator, a wide range of initiator surface coverages and hence polymer densities on mica were possible. The covalently immobilized initiator successfully promoted the polymerization of tert-butyl acrylate on mica surfaces. The resulting polymer layer thickness was measured by AFM using a step-height method. Linear relationships of the polymer thickness with respect to the molecular weight of the free polymer and with respect to the monomer conversion were observed, suggesting that ATRP is well controlled and relatively densely end-grafted layers were obtained. The polymer grafting density controlled by adjusting the initiator surface coverage was confirmed by the polymer layer swelling capacity and film thickness measurements.

New active particulate polymeric vectors based on branched polyester copolymers of hydroxy-acid and allyl glycidyl ether were developed to target drugs to the inflammatory endothelial cell surface. The hydroxyl and carboxyl derivatives of these polymers allow grafting of ligand molecules on the polyester backbones at different densities. A known potent nonselective selectin ligand was selected and synthesized using a new scheme. This synthesis allowed the grafting of the ligand to the polyester polymers, preserving its binding activity as assessed by docking simulations. Selectin expression on human umbilical cord vascular endothelial cells (HUVEC) was induced with the pro-inflammatory bacterial lipopolysaccharide (LPS) or with the nonselective inhibitor of nitric oxide synthase L-NAME. Strong adhesion of the ligand decorated nanoparticles was evidenced in vitro on activated HUVEC. Binding of nanoparticles bearing ligand molecules could be efficiently inhibited by prior incubation of cells with free ligand, demonstrating that adhesion of the nanoparticles is mediated by specific interaction between the ligand and the selectin receptors. These nanoparticles could be used for specific drug delivery to the activated vascular endothelium, suggesting their application in the treatment of diseases with an inflammatory component such as rheumatoid arthritis and cancer.

Normal and lateral forces between two opposing monolayers of grafted polymer nanoparticles (NPs) were measured using the Surface Forces Apparatus in a humid atmosphere. The NPs made of N, N-diethylacrylamide and 2-hydroxyethyl methacrylate have a hydrodynamic diameter of ca. 660 nm at 25 degrees C. The effect of surface roughness was studied by creating surface asperities using different NP grafting densities ranging from 0.41 to 2.63 NPs/mum (2). An increase in the NPs grafting density gave rise to an increase in surface roughness and to a deformation of the nanoparticles caused by the lateral pressure between neighboring particles. An elastoplastic behavior of the nanoparticles was observed for large grafting densities, while a purely elastic behavior was observed for small grafting densities. The lateral forces measured between two opposing NP monolayers sliding past each other followed Amontons' law for all grafting densities. The friction coefficient between the surfaces appeared to increase significantly with an increase in surface roughness, which was inherent to an increase in the elastoplastic behavior of the NP monolayers.

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.

We investigated the effect of physical and chemical modifications of mica surfaces induced by water vapor-based plasma treatments on the stability of silanols and grafted alkylsilane monolayers. The plasma-activated substrates were characterized using XPS, TOF-SIMS, and contact angle measurements. They revealed a large surface coverage of silanol groups (Si-OH) and a loss of aluminum atoms compared to freshly cleaved mica surfaces. The stability of plasma-induced silanol groups was investigated by contact angle measurements using ethylene glycol as a probe liquid. The Si-OH surface coverage decreased rapidly under vacuum or thermal treatment to give rise to hydrophobic dehydrated surfaces. The stability of end-grafted monofunctionalized n-alkylsilanes was investigated in different solvents and at different pH using water contact angle measurements. The degrafting of alkylsilanes from the activated mica was promoted in acidic aqueous solutions. This detachment was associated with the hydrolysis of covalent bonds between the alkylsilanes and the mica surface. The monolayer stability was enhanced by increasing the length of the alkyl chains that probably act as a hydrophobic protective layer against hydrolysis reactions. Stable alkylsilane monolayers in water with pH greater than 5.5 were obtained on mica surfaces activated at low plasma pressure. We attributed this stability to the loss of surface Al atoms induced by the plasma treatment.

Polyelectrolyte brushes were built on mica by anchoring polystyrene-poly(acrylic acid) (PS-b-PAA) diblock copolymers at a controlled surface density in a polystyrene monolayer covalently attached to OH-activated mica surfaces. Compared to physisorbed polymer brushes, these irreversibly attached charged brushes allow the polymer grafting density to remain constant upon changes in environmental conditions (e.g., pH, salt concentration, compression, and shear). The normal interaction and friction forces as a function of surface separation distance and at different concentrations of added salt (NaCl) were investigated using a surface forces apparatus. The interaction force profiles were completely reversible both on loading and receding and were purely repulsive. For a constant polymer grafting density, the influence of the polyelectrolyte charges and the Debye screening effect on the overall interaction forces was investigated. The experimental interaction force profiles agree very well with scaling models developed for neutral and charged polymer brushes. The variation of the friction force between two PAA brushes in motion with respect to each other as a function of surface separation distance appeared to be similar to that observed with neutral brushes. This similarity suggests that the increase in friction is associated with an increase in mutual interpenetration upon compression as observed with neutral polymers. The effect of the PAA charges and added ions was more significant on the repulsive normal forces than on the friction forces. The reversible characteristics of the normal force profiles and friction measurements confirmed the strong attachment of the PAA brushes to the mica substrate. High friction coefficients (ca 0.3) were measured at relatively high pressures (40 atm) with no surface damage or polymer removal.

Mica substrates were activated by a plasma method leading to OH-functionalized surfaces to which an atom transfer radical polymerization (ATRP) radical initiator was covalently bound using standard siloxane protocols. The unprecedented covalently immobilized initiator underwent radical polymerization with tert-butyl acrylate, yielding for the first time end-grafted polymer brushes that are covalently linked to mica. The initiator grafting on the mica substrate was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS), while the change in the water contact angle of the OH-activated mica surface was used to follow the change in surface coverage of the initiator on the surface. The polymer brush and initiator film thicknesses relative to the virgin mica were confirmed by atomic force microscopy (AFM). This was done by comparing the atomic step-height difference between a protected area of freshly cleaved mica and a zone exposed to plasma activation, initiator immobilization, and then ATRP.

Dissipative particle dynamics (DPD) was used to investigate the behavior of two opposing end-grafted charged polymer brushes in aqueous media under normal compression and lateral shear. The effect of polymer molecular weight, degree of ionization, grafting density, ionic strength, and compression on the polymer conformation and the resulting shear force between the opposing polymer layers were investigated. The simulations were carried out for the poly(tert-butyl methacrylate)-block-poly(sodium sulfonate glycidyl methacrylate) copolymer, referred as PtBMA-b-PGMAS, end-attached to a hydrophobic surface for comparison with previous experimental data. Mutual interpenetration of the opposing end-grafted chains upon compression is negligible for highly charged polymer brushes for compression ratios ranging from 2.5 to 0.25. Under electrostatic screening effects or for weakly charged polymer brushes, a significant mutual interpenetration was measured. The variation of interpenetration thickness with separation distance, grafting density, and polymer size follows the same scaling law as the one observed for two opposing grafted neutral brushes in good solvent. However, compression between two opposing charged brushes results in less interpenetration relative to neutral brushes when considering equivalent grafting density and molecular weight. The friction coefficient between two opposing polymer-coated surfaces sliding past each other is shown to be directly correlated with the interpenetration thickness and more specifically to the number of polymer segments within the interpenetration layer.

The purpose of this study was to develop oil-in-water emulsions (100–120 nm in diameter) and to correlate the surface properties of the emulsions with blood residence time and accumulation into neoplastic tissues by passive targeting. We investigated the effect of phospholipid and sphingolipid emulsifiers, hydrogenated soybean phosphatidylcholine (HSPC) and egg sphingomyelin (ESM), in combination with polysorbate 80 (PS-80) and 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine (DSPE)-PEG lipids of various PEG chain lengths and structures in prolonging circulation time and enhancing accumulation into B16 melanoma or C26 colon adenocarcinoma. The relationship between amphiphile molecular packing at the air/water interface on emulsion stability upon dilution in albumin and circulation longevity in vivo was also explored for non-PEGylated emulsions. PEGylation of the droplet surface with 10–15 mol% of DSPE-PEG 2000 or 5000 enhanced the circulation time of the emulsions, however, accumulation was only observed in the C26 tumor model. The tighter molecular packing observed with ESM/PS-80 monolayers at the air/water interface compared to HSPC/PS-80 correlated with improved emulsion stability in vitro, however, enhanced circulation time in vivo was not observed. A better understanding of the relationships between composition and performance will result in improved emulsion-based drug delivery vehicles for cancer therapy.

In contrast to non-associating liquids such as oils or organic solvents, whose viscosity diverges when they are confined by solid surfaces to films thinner than about ten molecular diameters, recent studies reveal that salt-free water remains fluid, with a viscosity close to its bulk value, even when confined to films down to only one or two monolayers thick. For the case of high concentration aqueous salt solutions compressed down to subnanometre films between confining planar surfaces, the hydration sheaths about the ions (trapped between the oppositely charged surfaces) also remain extremely fluid: this behaviour is attributed to the tenacity of water molecules in the hydration layers together with their rapid relaxation/exchange time. Related experiments on highly compressed, polyelectrolyte brushes in aqueous media reveal a remarkable lubricity which is in large measure attributed to similar hydration layers about the charged segments: this water of hydration strongly resists being squeezed out, but at the same time it may rapidly exchange with adjacent water molecules, thereby remaining quite fluid and acting as a molecular lubricant.