Direct Measurement of Normal and Shear Forces between Surface-Grown Polyelectrolyte Layers

Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing

Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.

Underscreening in concentrated electrolytes: re-entrant swelling in polyelectrolyte brushes

Hypersaline environments are ubiquitous in nature and are found in myriad technological processes. Recent empirical studies have revealed a significant discrepancy between predicted and observed screening lengths at high salt concentrations, a phenomenon referred to as underscreening. Herein we investigate underscreening using a cationic polyelectrolyte brush as an exemplar. Poly(2-(methacryloyloxy)ethyl)trimethylammonium (PMETAC) brushes were synthesised and their internal structural changes and swelling response was monitored with neutron reflectometry and spectroscopic ellipsometry. Both techniques revealed a monotonic brush collapse as the concentration of symmetric monovalent electrolyte increased. However, a non-monotonic change in brush thickness was observed in all multivalent electrolytes at higher concentrations, known as re-entrant swelling; indicative of underscreening. For all electrolytes, numerical self-consistent field theory predictions align with experimental studies in the low-to-moderate salt concentration regions. Analysis suggests that the classical theory of electrolytes is insufficient to describe the screening lengths observed at high salt concentrations and that the re-entrant polyelectrolyte brush swelling seen herein is consistent with the so-called regular underscreening phenomenon.

Entropy-Enhanced Mechanochemical Activation for Thermal Degrafting of Surface-Tethered Dry Polystyrene Brushes

Dry polymer brushes have attracted great attention because of their potential utility in regulating interface properties. However, it is still unknown whether dry polymer brushes will exhibit degrafting behavior as a result of thermal annealing. Herein, a study of the conformational entropy effect on thermal degrafting of dry polystyrene (PS) brushes is presented. For PS brushes with an initial grafting density (σ_pⁱⁿⁱ) of 0.61 nm^-2, degrafting behavior was observed at 393 K, and the equilibrium σ_p was approximately 0.14 nm^-2 at 413 K. However, for brushes with σ_pⁱⁿⁱ ≤ 0.14 nm^-2, thermal degrafting was not observed even if the temperature was increased to 453 K. Furthermore, we found that the degrafting rate was faster for PS brushes with higher σ_pⁱⁿⁱ and higher molecular weights when σ_pⁱⁿⁱ > 0.14 nm^-2. Our findings confirmed that degrafting is a mechanochemical activation process driven by tension imposed on bonds that anchor the chains to the surface, and the process is amplified by conformational entropy.

Interaction Forces on Polyampholytic Hydrogel Gradient Surfaces

Rational design and informed development of nontoxic antifouling coatings requires a thorough understanding of the interactions between surfaces and fouling species. With more complex antifouling materials, such as composites or zwitterionic polymers, there follows also a need for better characterization of the materials as such. To further the understanding of the antifouling properties of charge-balanced polymers, we explore the properties of layered polyelectrolytes and their interactions with charged surfaces. These polymers were prepared via self-initiated photografting and photopolymerization (SIPGP); on top of a uniform bottom layer of anionic poly(methacrylic acid) (PMAA), a cationic poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) thickness gradient was formed. Infrared microscopy and imaging spectroscopic ellipsometry were used to characterize chemical composition and swelling of the combined layer. Direct force measurements by colloidal probe atomic force microscopy were performed to investigate the forces between the polymer gradients and charged probes. The swelling of PMAA and PDMAEMA are very different, with steric and electrostatic forces varying in a nontrivial manner along the gradient. The gradients can be tuned to form a protein-resistant charge-neutral region, and we demonstrate that this region, where both electrostatic and steric forces are small, is highly compressed and the origin of the protein resistance of this region is most likely an effect of strong hydration of charged residues at the surface, rather than swelling or bulk hydration of the polymer. In the highly swollen regions far from charge-neutrality, steric forces dominate the interactions between the probe and the polymer. In these regions, the SIPGP polymer has qualitative similarities with brushes, but we were unable to quantitatively describe the polymer as a brush, supporting previous data suggesting that these polymers are cross-linked.

Scaling and Interactions of Linear and Ring Polymer Brushes via DPD Simulations

Single and double layers of polymer coated surfaces are investigated by means of Dissipative Particle Dynamics (DPD), focusing on the difference between grafted ring and linear chains. Several different surface coverages σ , as well as chain lengths N and brush separations D, are analyzed for athermal, i.e., good solvent, conditions. The size in the form of the radius of gyration R g , the shape as asphericity δ ∗ , and orientation β ∗ , as well as density profiles as functions of distance from grafting plane ρ ( z ) , are studied. The effect of an added bond repulsion potential to suppress bond crossing in DPD is analyzed. Scaling laws of R g and its components R g ⊥ and R g ∥ are investigated. We find R g ∝ N ν , ν = 0.588 for surface coverages below the overlap surface concentration σ ∗ . For σ > σ ∗ we find R g ⊥ ∝ N ν ⊥ , ν ⊥ ≅ 1 and R g ∥ ∝ N ν ∥ , ν ∥ = 1 / 2 of ring brushes with the standard DPD model and ν ∥ ≅ 2 / 5 with added bond repulsion. The σ dependence of the radius of gyration was found to be R g ∝ σ μ with μ = 1 / 3 for surface coverages grater than σ ∗ . The perpendicular component R g ⊥ scales independent of the bond repulsion potential as R g ⊥ ∝ σ μ ⊥ , μ ⊥ = 1 / 3 , whereas the scaling of the parallel component exhibits a topological repulsion dependence R g ∥ ∝ σ μ ∥ , μ ∥ = - 1 / 12 for standard DPD and μ ∥ = - 1 / 6 for bond repulsion.

Tribological Properties of Double-Network Gels Substituted by Ionic Liquids

Since human body joints have a gel-like structure with low friction that persists for several decades, hydrogels have attracted much interest for developing low-friction materials. However, such advantages can hardly be realized in industrial usage because water in the gel evaporates easily and the gel deswells. The substitution of water with an ionic liquid (IL) is one of the effective ways to overcome this problem. In this study, we substituted water in a double network (DN) hydrogel with 3-ethyl-1-methyl-imidazolium ethylsulfate (EMI-EtSulf), a hydrophilic IL, via a simple solvent exchange method to obtain a DN ion gel. A compressive test and thermogravimetric analysis showed that the DN ion gel has a high compression fracture stress and improved thermal properties, with the difference in 10% loss of temperature being ΔT10 = 234 °C. A friction test conducted using a reciprocating tribometer showed that the friction of a glass ball/DN ion gel was relatively higher than that of a glass ball/DN hydrogel. Because the minimum coefficient of friction (COF) value increased after substitution, the increase in polymer adhesion caused by the electrostatic shielding of the surface moieties of glass and poly 2-acrylamidomethylpropanesulfonic acid (PAMPS) was considered the main contributor to the high friction. As the COF value decreased with increasing temperature, the DN ion gel can achieve low friction via the restriction of polymer adhesion at high temperatures, which is difficult in the DN hydrogel owing to drying.

A Versatile Method for the Distance-Dependent Structural Characterization of Interacting Soft Interfaces by Neutron Reflectometry

Interactions between soft interfaces govern the behavior of emulsions and foams and crucially influence the functions of biological entities like membranes. To understand the character of these interactions, detailed insight into the interfaces' structural response in terms of molecular arrangements and conformations is often essential. This requires the realization of controlled interaction conditions and surface-sensitive techniques capable of resolving the structure of buried interfaces. Here, we present a new approach to determine the distance-dependent structure of interacting soft interfaces by neutron reflectometry. A solid/water interface and a water/oil interface are functionalized independently and initially macroscopically separated. They are then brought into contact and structurally characterized under interacting conditions. The nanometric distance between the two interfaces can be varied via the exertion of osmotic pressures. Our first experiments on lipid-anchored polymer brushes interacting across water with solid-grafted polyelectrolyte brushes and with bare silicon surfaces reveal qualitatively different interaction scenarios depending on the chemical composition of the two involved interfaces.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Expanding the Polymer Mechanochemistry Toolbox through Surface-Initiated Polymerization

Surface-initiated polymerizations represent a versatile toolbox to generate densely grafted assemblies of chain end-tethered polymers. At sufficiently short interchain distances, surface-grafted polymers are forced into an extended chain conformation, which forms the basis of several unique properties, including their ability to withstand efficiently biofouling or to act as low friction coatings. While the effect on materials properties is well-established, only relatively recently first reports have appeared describing that chain stretching in surface-grafted polymer films also impacts chemical stability/reactivity. This Viewpoint presents surface-initiated polymerization as an alternative polymer mechanochemical tool. The absence of an external force field to induce chain elongation and the possibility to modulate chain stretching by varying brush molecular weight and grafting density, in conjunction with electrostatic interactions and nanoinclusions that may be present inside the polymeric grafts, make surface-initiated polymerization an attractive tool to both study and understand the effects of polymer chain conformation on the stability/reactivity of surface-grafted polymers.

Self-organized polyelectrolyte end-grafted layers under nanoconfinement

Layers of end-grafted weak polyelectrolytes in poor solvent self-organize into a rich variety of structures (such as micelles, micelles coexisting with nonaggregated chains, stripes and layers with solvent-filled holes) due to the subtle competition among hydrophobic, electrostatic and steric interactions and the chemical acid-based equilibria of the weak polyelectrolyte. In this work, a molecular theory has been used to systematically study how nanoconfinement modulates the competition among these interactions and, therefore, dictates the morphology of the self-assembled layer. Two different types of confinement were considered and compared: (i) soft lateral confinement due to increasing surface coverage in a planar polyelectrolyte brush and (ii) hard vertical confinement due to the interaction of a planar polyelectrolyte brush with an opposing surface, as typically found in AFM-colloidal-tip and surface-force-apparatus experiments. It is shown that increasing the surface coverage (soft lateral confinement) or compressing the layer with an opposing wall (hard vertical confinement) have a similar qualitative effect on the morphology of the system: both types of nanoconfinement increase the stability of morphologies that extend in one or two dimensions (such as the homogeneous brush, holes and stripes) over nonextended aggregates (such as hemispherical micelles). However, vertical confinement can also lead to pillar-like structures that are not observed in the absence of the opposing wall. Interestingly, the pillar structures, which bridge the grafting and opposing surfaces, may coexist with metastable structures collapsed to the grafting surface only. This coexistence may help to understand the hysteresis commonly observed in surface-force experiments.

Boundary lubricant polymer films: effect of cross-linking

We have studied the adsorption and lubricant properties of a multifunctional triblock copolymer poly(L-lysine)-b-poly(acrylic acid)-b-poly(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 cross-linking 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.

Ultraviolet Light-Induced Surface-Initiated Atom-Transfer Radical Polymerization

UV light-induced surface-initiated atom-transfer radical polymerization (ATRP) was reported. This method uses TiO₂ nanoparticles as photoactive materials to reduce Cu(II)/L to a Cu(I)/L complex under UV irradiation by a one-electron transfer process for ATRP with multiple usage of monomer solutions. The growth of polymer brushes can be manipulated by either varying the content of photoactive materials or regulating the irradiation intensity, thereby yielding polymer brushes with controllable thickness, composition, and architecture.

Effect of grafted oligopeptides on friction

Frictional and normal forces in aqueous solution at 25 °C were measured between a glass particle and oligopeptide films grafted from a glass plate. Homopeptide molecules consisting of 11 monomers of either glutamine, leucine, glutamic acid, lysine, or phenylalanine and one heteropolymer were each "grafted from" an oxidized silicon wafer using microwave-assisted solid-phase peptide synthesis. The peptide films were characterized using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. Frictional force measurements showed that the oligopeptides increased the magnitude of friction compared to that on a bare hydrophilic silicon wafer but that the friction was a strong function of the nature of the monomer unit. Overall we find that the friction is lower for more hydrophilic films. For example, the most hydrophobic monomer, leucine, exhibited the highest friction whereas the hydrophilic monomer, polyglutamic acid, exhibited the lowest friction at zero load. When the two surfaces had opposite charges, there was a strong attraction, adhesion, and high friction between the surfaces. Friction for all polymers was lower in phosphate-buffered saline than in pure water, which was attributed to lubrication via hydrated salt ions.

Poly(ethylene oxide) star polymer adsorption at the silica/aqueous interface and displacement by linear poly(ethylene oxide)

Multiarm star copolymers with approximately 460 poly(ethylene oxide) (PEO) arms that have a degree of polymerization N = 45 were synthesized via atom transfer radical polymerization (ATRP) of PEO-methacrylate macromonomers in the presence of divinyl benzene cross-linkers. These are an example of molecular or nanoparticulate brushes that are of interest as steric stabilizers or boundary lubrication agents when adsorbed from solution to a solid/aqueous interface. We use ellipsometry to measure adsorption isotherms at the silica/aqueous interface for PEO star polymers and linear PEO chains having molecular weights comparable either to the star polymer or to the individual arms. The compactness of the PEO star polymers (molecular weight 1.2 × 10(6)) yields a saturation surface excess concentration that is approximately 3.5 times greater than that of the high molecular weight (1 × 10(6)) linear PEO. Adsorption of low molecular weight (6000) linear PEO was below the detection limit. Competitive adsorption experiments were conducted with ellipsometry, complemented by independent quartz crystal microbalance with dissipation (QCM-D) measurements. Linear PEO (high molecular weight) displaced preadsorbed PEO star polymers over the course of approximately 1.5 h, to form a mixed adsorbed layer having not only a significantly lower overall polymer surface excess concentration, but also a significantly greater amount of hydrodynamically entrapped water. Challenging a preadsorbed linear PEO (high molecular weight) layer with PEO star polymers produced no measurable change in the overall polymer surface excess concentration, but changes in the QCM-D energy dissipation and resonance frequency suggested that the introduction of PEO star polymers caused a slight swelling of the layer with a correspondingly small increase in entrapped water content.

Strongly Compressed Polyelectrolyte Brushes under Shear

By means of molecular dynamics simulations we provide evidence for pronounced counterion immobilization in strongly compressed polyelectrolyte-brush bilayers, where the counterions represent the vast majority of mobile solvent particles. As a consequence, hydrodynamic effects are strongly suppressed and semidilute bilayers can respond to shear motion like electrically neutral bilayers at melt density. For large, time-independent shear rates, γ̇, the shear force scales as f(γ̇) ∼ γ̇^0.69, in agreement with scaling theory. In this regime, polyelectrolyte-brush bilayers can stabilize highly nonstationary processes, such as the instantaneous inversion of the shear direction. The absence of hydrodynamic flow leads to a suppression of the overshoot for the shear force, which is found for electrically neutral bilayers with the same molecular parameters. We suggest that nature uses this mechanism to optimize biolubrication, for instance in synovial joints.

Tuning the adhesion of silica microparticles to a poly(2-vinyl pyridine) brush: an AFM force measurement study

AFM force measurements have been performed to study the influence of the pH value and salt concentration on the interactions between poly(2-vinyl pyridine) brushes and microsized silica spheres, focusing on attractive and adhesion forces. It was found that the interaction was composed of a repulsive component reflecting the conformation of the brush and an additional attractive force. It can therefore be switched reversibly between purely repulsive at pH 2.5 to strong and medium adhesion by changing the pH value to pH 4 and 6, respectively. Addition of KCl showed different effects: at pH 2.5 high salt concentrations induced an attractive force; at pH 4 the interaction changed from strong attraction in the osmotic brush regime to repulsion and weaker adhesion in the salted brush regime; at pH 6 increase of the KCl concentration weakened the attractive force. These effects could partly be explained by the theory of polyelectrolyte brushes; under some conditions the mechanism of the attractive force is still unclear.

Structure and collapse of a surface-grown strong polyelectrolyte brush on sapphire

We have used neutron reflectometry to investigate the behavior of a strong polyelectrolyte brush on a sapphire substrate, grown by atom-transfer radical polymerization (ATRP) from a silane-anchored initiator layer. The initiator layer was deposited from vapor, following treatment of the substrate with an Ar/H(2)O plasma to improve surface reactivity. The deposition process was characterized using X-ray reflectometry, indicating the formation of a complete, cross-linked layer. The brush was grown from the monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC), which carries a strong positive charge. The neutron reflectivity profile of the swollen brush in pure water (D(2)O) showed that it adopted a two-region structure, consisting of a dense surface region ∼100 Å thick, in combination with a diffuse brush region extending to around 1000 Å from the surface. The existence of the diffuse brush region may be attributed to electrostatic repulsion from the positively charged surface region, while the surface region itself most probably forms due to polyelectrolyte adsorption to the hydrophobic initiator layer. The importance of electrostatic interactions in maintaining the brush region is confirmed by measurements at high (1 M) added 1:1 electrolyte, which show a substantial transfer of polymer from the brush to the surface region, together with a strong reduction in brush height. On addition of 10(-4) M oppositely charged surfactant (sodium dodecyl sulfate), the brush undergoes a dramatic collapse, forming a single dense layer about 200 Å in thickness, which may be attributed to the neutralization of the monomers by adsorbed dodecyl sulfate ions in combination with hydrophobic interactions between these dodecyl chains. Subsequent increases in surfactant concentration result in slow increases in brush height, which may be caused by stiffening of the polyelectrolyte chains due to further dodecyl sulfate adsorption.

Effect of salt on the compression of polyelectrolyte brushes in a theta solvent

Classical strong-stretching theory (SST) predicts that, as opposing polyelectrolyte brushes are compressed together in a salt-free theta solvent, they contract so as to maintain a finite polymer-free gap, which offers a potential explanation for the ultra-low frictional forces observed in experiments despite the application of large normal forces. However, the SST ignores chain fluctuations, which would tend to close the gap resulting in physical contact and in turn significant friction. In a preceding study, we examined the effect of fluctuations using self-consistent field theory (SCFT) and illustrated that high normal forces can still be applied before the gap is destroyed. We now look at the effect of adding salt. It is found to reduce the long-range interaction between the brushes but has little effect on the short-range part, provided the concentration does not enter the salted-brush regime. Consequently, the maximum normal force between two planar brushes at the point of contact is remarkably unaffected by salt. For the crossed-cylinder geometry commonly used in experiments, however, there is a gradual reduction because in this case the long-range part of the interaction contributes to the maximum normal force.

Friction between brush layers of charged and neutral bottle-brush macromolecules. molecular dynamics simulations

Using molecular dynamics simulations, we study the lubricating properties of neutral and charged bottle-brush coatings as a function of the compression and shear stresses and brush grafting density. Our simulations have shown that in charged bottle-brush systems under shear there is a layer with excess counterions located in the middle between brush-bearing surfaces. The main deformation mode of the charged bottle-brush layers is associated with the backbone deformation, resulting in the backbone deformation ratio, α, and shear viscosity, η, being universal functions of the Weissenberg number. In the case of neutral bottle-brush systems, in addition to the backbone deformation there is also side chain deformation. The coupling between backbone and side chain deformation violates universality in the deformation ratio, α, dependence on the Weissenberg number and results in scaling exponents varying with the compression stress and brush grafting density. The existence of different length scales controlling deformation of neutral bottle brushes manifests itself in the shear viscosity, η, dependence on the shear rate, ̇γ. Shear viscosity, η, as a function of the shear rate, ̇γ, has two plateaus and two shear thinning regimes. The low shear rate plateau and shear thinning regime correspond to the backbone deformation, while the second plateau and shear thinning regime at moderate shear rates are due to side chain deformation. For both systems the value of the friction coefficient increases with increasing shear rate. The values of the friction coefficient for charged bottle-brush systems are about ten times smaller than corresponding values for neutral systems at the same shear rate.

Interaction between brush layers of bottle-brush polyelectrolytes: molecular dynamics simulations

Interactions between tethered layers composed of aggrecan (charged bottle-brush) macromolecules are responsible for the molecular origin of cartilage biomechanical behavior. To elucidate the role of the electrostatic forces in interaction between bottle-brush layers, we have performed molecular dynamics simulations of charged and neutral bottle-brush macromolecules tethered to substrates. In the case of charged bottle-brush layers, the disjoining pressure P between two brush layers in salt-free solutions increases with decreasing distance D between substrates as P ∝ D(-1.8). A stronger dependence of the disjoining pressure P on the surface separation D was observed for neutral bottle-brushes, P ∝ D(-4.6), in the same interval of disjoining pressures. These scaling laws for dependence of disjoining pressure P on distance D are due to bending energy of the bottle-brush macromolecules within compressed brush layers. The weaker distance dependence observed in polyelectrolyte bottle-brushes is due to interaction between counterion clouds surrounding the bottle-brush macromolecules preventing strong brush overlap.

Compression of polyelectrolyte brushes in a salt-free theta solvent

This paper examines the normal force between two opposing polyelectrolyte brushes and the interpenetration of their chains that is responsible for sliding friction. It focuses on the special case of semi-dilute brushes in a salt-free theta solvent, for which Zhulina and Borisov (J. Chem. Phys. 107, 5952 (1997)) have derived analytical predictions using the classical strong-stretching theory (SST). Interestingly, SST predicts that the brushes contract as they are compressed together maintaining a polymer-free gap, which provides an explanation for the ultra-low frictional forces observed in experiment. We examine the degree to which the SST predictions are affected by chain fluctuations by employing self-consistent field theory (SCFT). While the normal force is relatively unaffected, fluctuations are found to have a strong impact on brush interpenetration. Even still, the contraction of the brushes does significantly prolong the onset of interpenetration, implying that a sizeable normal force can be achieved before the sliding friction becomes significant.

Stability behavior of anionic spherical polyelectrolyte brushes in the presence of La(III) counterions

In this paper we discuss the stability behavior of spherical polyelectrolyte brushes (SPB) in the presence of trivalent lanthanum counterions. Stability behavior is measured through the rate of coagulation of the SPB as a function of the lanthanum concentration using simultaneous static and dynamic light scattering. As the counterion concentration increases, we observe coagulation of the SPB which in turn leads to a dramatic decrease in the stability of our particles. Since the rate of coagulation is dependent upon the balance between the repulsive interactions and the thermal energy of the diffusing particles (reaction-limited colloidal aggregation; RLCA), we then can relate the measured particle stability to the value of the repulsive potential in the RLCA regime. These "microsurface potential measurements" (MSPM) allow us to measure repulsive energies down to the order of k(B)T. From the repulsive energy of the particles we can then determine precise information about the net surface potential Ψ0 of the SPB as a function of the lanthanum counterion concentration. Moreover, we demonstrate that a simple mean-field model predicts the stability of the SPB in the presence of lanthanum counterions with high accuracy.

Interaction forces between microsized silica particles and weak polyelectrolyte brushes at varying pH and salt concentration

The AFM colloidal probe technique was used to measure the interaction between microsized silica spheres and annealed polyelectrolyte brushes made of poly(acrylic acid) (PAA) and poly(2-vinyl pyridine) (P2VP) in KCl solutions of various pH values and salt concentrations. The interaction energy showed a distance dependence that was affected strongly by the swelling and the electric properties of the brushes. Between PAA brushes and silica particles, a repulsive interaction has been observed for all pH values and salt concentrations reflecting the swelling of the brush with varying pH value and the transition from osmotic to salted brush regime with increasing KCl concentration. Force measurements between P2VP brushes and silica particles revealed a much more complex behavior: a steric repulsion by the swollen brush at low pH values, a complex interplay of attractive and repulsive forces at intermediate pH values and a short-ranged attraction between the collapsed brush and the silica particle at basic pH values and high salt concentrations. The results are interpreted in comparison with the Alexander de Gennes model and zeta potential and ellipsometric measurements.

Surface interactions between two like-charged polyelectrolyte gels

Due to the migration of mobile molecules and ions, a thin diffusive layer of distributed charge--the electric double layer--forms at the interface between a polyelectrolyte gel and a liquid ionic solution. When two polyelectrolyte gels are brought closely together, the electric double layers overlap and interact with each other, resulting in an effective repulsion. The multiphysics-coupling nature of soft gels makes their surface interactions significantly different from the interactions between rigid solids. Using the recently formulated nonlinear theory, this paper develops a continuum model to study the surface interactions between two like-charged polyelectrolyte gels, accounting for the coupled electric, concentration, and deformation fields in both the gels and the liquid. Numerical solutions of the surface interactions are obtained and compared to a qualitative scaling law derived via linearization. The results suggest that the structure of double layers, as well as their interactions, depends not only on the concentration of liquid solutions, but more on the bulk properties of the gels such as stiffness and fixed-charge density. This model also provides insights to the mechanism of the low-friction phenomena on the surface of a polyelectrolyte gel.