Structuring of Polyelectrolyte (NaPSS) Solutions in Bulk and under Confinement as a Function of Concentration and Molecular Weight

Parametric Study of Colloidal Particle Confinement near a Surface in the Presence of DLVO and Structural Interactions Using Brownian Dynamic Simulations

Total internal reflection microscopy (TIRM) has become a crucial technique for understanding the surface interactions and dynamics of Brownian colloidal particles near a surface. However, for select colloidal systems, experimental limitations associated with TIRM can occlude exploration of nano- and submicrometer colloids dispersed in complex or structured fluids. It should be possible to use Brownian dynamic simulations to quantify, explore, or circumvent these limitations to extend the TIRM technique further. A Brownian dynamics algorithm based on the Langevin equation was utilized to identify favorable colloidal systems for conducting TIRM experiments in electrolyte and nonadsorbing polyelectrolyte solutions. In electrolyte solution, the motion of polystyrene and silica particles of nanometer- and micrometer-sized radii was simulated near a glass slide in the presence of retarded van der Waals and electric double-layer forces to develop potential energy profiles. In the case of nonadsorbing polyelectrolyte solutions, a structural force was also implemented into the simulation, and the influence of structural interactions on particle confinement was explored as a function of particle size, particle density, and polyelectrolyte concentration. In electrolyte solutions, our results were able to identify the minimum particle size required for TIRM experiments as well as insight into particle selection based on material density. For structural or oscillatory forces, our results show that prior to conducting TIRM experiments, Brownian dynamics simulation can be used to select the appropriate particle size, material, and polyelectrolyte concentration range where the colloidal particle can sample multiple structural energy wells without confinement. These results provide insight into the colloidal system suitable to experimentally study near-surface particle diffusion dynamics for a range of separations in the presence of structural interactions.

Overlap Concentration of Sodium Polystyrene Sulfonate in Solution

The overlap concentration c* of sodium polystyrene sulfonate in water is calculated using multichain atomistic and coarse grained (CG) simulations for a range of chain lengths. Fully atomistic molecular dynamics simulations are carried out for N = 32-192 monomers. The CG model was parameterized to match the end-to-end distance from the atomistic simulations at small N and allows us to simulate a much larger N. Treating the hydrophobic backbone by inclusion of attraction between monomers is an essential addition to the CG model. The simulation c* are in agreement with experimental data, yet at c*, the chains are not fully stretched, even for N as large as 1200. This implies that none of the experimental systems are in the scaling regime and to reach the scaling regime for NaPSS chains much longer than N = 1200 are required.

Shape and Structure Formation of Mixed Nonionic-Anionic Surfactant Micelles

Aqueous solutions of a nonionic surfactant (either Tween20 or BrijL23) and an anionic surfactant (sodium dodecyl sulfate, SDS) are investigated, using small-angle neutron scattering (SANS). SANS spectra are analysed by using a core-shell model to describe the form factor of self-assembled surfactant micelles; the intermicellar interactions are modelled by using a hard-sphere Percus–Yevick (HS-PY) or a rescaled mean spherical approximation (RMSA) structure factor. Choosing these specific nonionic surfactants allows for comparison of the effect of branched (Tween20) and linear (BrijL23) surfactant headgroups, both constituted of poly-ethylene oxide (PEO) groups. The nonionic–anionic surfactant mixtures are studied at various concentrations up to highly concentrated samples (ϕ ≲ 0.45) and various mixing ratios, from pure nonionic to pure anionic surfactant solutions. The scattering data reveal the formation of mixed micelles already at concentrations below the critical micelle concentration of SDS. At higher volume fractions, excluded volume effects dominate the intermicellar structuring, even for charged micelles. In consequence, at high volume fractions, the intermicellar structuring is the same for charged and uncharged micelles. At all mixing ratios, almost spherical mixed micelles form. This offers the opportunity to create a system of colloidal particles with a variable surface charge. This excludes only roughly equimolar mixing ratios (X≈ 0.4–0.6) at which the micelles significantly increase in size and ellipticity due to specific sulfate–EO interactions.

Thickness of the particle-free layer near charged interfaces in suspensions of like-charged nanoparticles

When a suspension of charged nanoparticles is in contact with a like-charged water-solid interface, next to this interface a particle-free layer is formed. The present study provides reliable measurements of the thickness of this particle-free layer with three different techniques, namely optical reflectivity, quartz crystal microbalance (QCM), and direct force measurements with atomic force microscopy (AFM). Suspensions of negatively charged nanoparticles of different size and type are investigated. When the measured layer thickness is normalized to the particle size, one finds that this normalized thickness shows universal inverse square root dependence on the particle volume fraction. This universal dependence can be also derived from Poisson-Boltzmann theory for highly asymmetric electrolytes, whereby one has to assume that the nanoparticles represent the multivalent coions.

Dynamic stabilisation during the drainage of thin film polymer solutions

The drainage and rupture of polymer solutions was investigated using a dynamic thin film balance. The polymeric nature of the dissolved molecules leads to significant resistance to the deformation of the thin liquid films. The influence of concentration, molecular weight, and molecular weight distribution of the dissolved polymer on the lifetime of the films was systematically examined for varying hydrodynamic conditions. Depending on the value of the capillary number and the degree of confinement, different stabilisation mechanisms were observed. For low capillary numbers, the lifetime of the films was the highest for the highly concentrated, narrowly-distributed, low molecular weight polymers. In contrast, at high capillary numbers, the flow-induced concentration differences in the film resulted in lateral osmotic stresses, which caused a dynamic stabilisation of the films and the dependency on molecular weight distribution in particular becomes important. Phenomena such as cyclic dimple formation, vortices, and dimple recoil were observed, the occurrence of which depended on the relative magnitude of the lateral osmotic and the hydrodynamic stresses. The factors which lead to enhanced lifetime of the films as a consequence of these flow instabilities can be used to either stabilise foams or, conversely, prevent foam formation.

Nonelectrostatic Adsorption of Polyelectrolytes and Mediated Interactions between Solid Surfaces

Polymer-mediated interaction between two solid surfaces is directly connected to the properties of the adsorbed polymer layers. Nonelectrostatic interactions with a surface can significantly impact the adsorption of polyelectrolytes to charged surfaces. We use a classical density functional theory to study the effect of various polyelectrolyte solution properties on the adsorption and interaction between two like-charged surfaces. Our results show that nonelectrostatic interactions not only enhance polyelectrolyte adsorption but can also result in qualitatively different salt effects with respect to the adsorbed amount. In particular, we observe decreasing, increasing, and a previously unreported nonmonotonic behavior in the adsorbed amount of polymer with added salt under the conditions studied, although the nonmonotonic regime only occurs for a narrow range in the parameter space. With sufficient nonelectrostatic adsorption, the adsorbed polymer layers produce a long-range repulsive barrier that is strong enough to overcome dispersive interactions that cause surfaces to attract. Concurrently, a short-range bridging attraction is observed when the two polyelectrolyte layers span both the surfaces. Both the repulsive barrier and bridging attraction depend on the charge density of the polymer backbone and the bulk salt concentration but not on the chain length in the semidilute regime studied.

Untangling superposed double layer and structural forces across confined nanoparticle suspensions

The description of forces across confined complex fluids still holds many challenges due to the possible overlap of different contributions. Here, an attempt is made to untangle the interaction between charged surfaces across nanoparticle suspensions. Interaction forces are measured using colloidal-probe atomic force microscopy. The experimental force profiles are considered as a superposition of double layer and structural forces. In order to independently describe the decay of the double layer force, the ionic strength of the suspension is determined by electrolytic conductivity measurements. Jellium approximation is used to define the impact of the fluid on screening the surface potential. There, the nanoparticles are considered homogeneously distributed across the fluid and screening is only carried out via the particles counterions and added salt. The structural force follows a damped oscillatory profile due to the layer-wise expulsion of the nanoparticles upon approach of both surfaces. The description of the oscillatory structural force is extended by a depletion layer next to the confining surfaces, with no nanoparticles present. The thickness of the depletion layer is related to the electrostatic repulsion of the charged nanoparticles from the like-charged surfaces. The results show that the total force profile is a superposition of independent force contributions without any mutual effects. Using this rather simple model describes the complete experimentally determined interaction force profiles very well from surface separations of a few hundred nanometres down to the surfaces being almost in contact.

Structural and Double Layer Forces between Silica Surfaces in Suspensions of Negatively Charged Nanoparticles

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with atomic force microscopy (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the nonexponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits extraction of the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.

Oscillatory structural forces between charged interfaces in solutions of oppositely charged polyelectrolytes

Forces between negatively charged micron-sized silica particles were measured in aqueous solutions of cationic polyelectrolytes with an atomic force microscope (AFM). In these oppositely charged systems, damped oscillatory force profiles were systematically observed in systems at higher polyelectrolyte concentrations, typically around few g L-1. The wavelength of these oscillations is decreasing with increasing concentration. When the wavelength and concentration are normalized with the cross-over concentration, universal power-law dependence is found. Thereby, the corresponding scaling exponent changes from 1/3 in the dilute regime to 1/2 in the semi-dilute regime. This dependence is the same as in the like-charged systems, which were described in the literature earlier. This common behavior suggests that these oscillatory forces are related to the structuring of the polyelectrolyte solutions. The reason that the oppositely charged systems behave similarly to like-charged ones is that the former systems undergo a charge reversal due to the adsorption of the polyelectrolytes to the oppositely charged surface, whereby sufficiently homogeneous adsorbed layers are being formed. The main finding of the present study is that at higher polyelectrolyte concentrations such oscillatory forces are the rule, including the oppositely charged ones.

Surface forces and interaction mechanisms of soft thin films under confinement: a short review

Surface forces of soft thin films under confinement in fluids play an important role in diverse biological and technological applications, such as bio-adhesion, lubrication and micro- and nano-electromechanical systems. Understanding the involved interaction mechanisms underlying the adhesion behaviors and tribological performances (i.e., friction and lubrication) of various confined soft thin films is significant in the development of both fundamental science and practical technologies. In this review, the fundamentals of surface forces are briefly presented. The widely utilized force measurement techniques including surface forces apparatus (SFA), atomic force microscopy (AFM) and spacer layer interferometry tribometer techniques are introduced. The advances in the fundamental understanding of a wide range of adhesion and tribological phenomena have been reviewed, in terms of the intermolecular and surface interaction mechanisms involved. The influences of various factors such as confined film properties, experimental conditions (e.g., normal load, and sliding velocity) and environmental variables (e.g., salts, salinity, additives and pH) on the adhesion, friction or lubrication forces of confined soft thin films are presented. The correlation between adhesion hysteresis and friction/lubrication behaviors has been discussed. Some of the challenging issues remaining and future perspectives are also provided.

Interaction of Different Charged Polymers with Potassium Ions and Their Effect on the Yield Stress of Highly Concentrated Glass Bead Suspensions

The interaction of different charged polymers, namely anionic polycarboxylate superplasticizer (PCE) and neutral polyethylene glycol (PEG) with potassium ions, and their effect on the yield stress of highly concentrated glass bead suspension (GBS), were studied under different concentrations of potassium ions ([K⁺]). It was found that, compared to the neutral PEG, the negatively charged PCE can be adsorbed on glass beads (GB), and then decreases the yield stress of GBS. The increasing concentration of free polymer in the interstitial liquid phase with the increased polymer dosage leads to the higher yield stress of GBS, which may be caused by the higher depletion force. In addition, this effect is also related to the charge density of the polymer and the [K⁺] in the solution. Along with the increase in [K⁺], the yield stress of GBS increases significantly with the addition of PCE, but this cannot be observed with PEG, which indicates that potassium ions can interact with negatively charged PCE instead of the neutral PEG. At last, the interparticle forces between two single GB with adsorbed PCE in solutions containing [K⁺] and PCE were measured by colloidal probe atomic force microscopy to better understand the interaction of the charged polymer with counterions.

On the origin of oscillatory interactions between surfaces mediated by polyelectrolyte solution

We use a numerical implementation of polymer classical density functional theory with an incompressibility condition to study the system consisting of nonadsorbing polyelectrolytes confined by two planar surfaces and quantify the effective interaction between the two planar surfaces as a function of the polyelectrolyte and salt concentrations. Our results indicate that for the uncharged surfaces (and weakly charged surfaces), the effective interaction primarily consists of a short-range attraction due to the depletion followed by repulsion due to the electric double layer overlapping and electrostatic correlations. For salt-free and low salt concentration systems, the magnitude of the repulsion barrier is determined by the overlap between the electric double layers, while at relatively high salt concentrations, the magnitude of the repulsion barrier is determined by the electrostatic correlations. Due to the competition between the electric double layer and the electrostatic correlations, the magnitude of the repulsion barrier varies nonmonotonically. In contrast, a mean-field Poisson-Boltzmann treatment of the electrostatics predicts a monotonically decreasing repulsion barrier with increasing salt concentration. At moderate salt concentrations, our theory predicts oscillatory interaction profiles. A comparison with the mean-field Poisson-Boltzmann treatment of electrostatics suggests that the oscillations are due primarily to electrostatic correlations.

Viscosity of Semidilute and Concentrated Nonentangled Flexible Polyelectrolytes in Salt-Free Solution

We report viscosity data of nonentangled sodium polystyrene sulfonate (NaPSS) in salt-free aqueous solution as a function of polymer concentration ( c) and degree of polymerization ( N). Different empirical equations are examined and found not to describe the semidilute solution viscosity over a wide concentration range and/or to yield values of [η] that do not match dilute solution measurements. Deviations from the scaling prediction of η_sp ∝ c^1/2 (Fuoss' law) are observed at high concentrations. Specifically, we find η_sp ≈ N^1.26 c^1/2 e^{1.4 c} in the semidilute regime, which agrees with the scaling prediction only for c ≲ 0.02 M. The viscosity data presented in this study and in earlier reports show a high degree of consistency. A comparison with diffusion measurements for NaPSS in salt-free solution by Oostwal and co-workers suggests that the disagreement between the scaling theory and experiments does not arise solely from the concentration dependence of the monomeric friction coefficient.

Bridging the gap between two different scaling laws for structuring of liquids under geometrical confinement

Structural forces are a phenomena obtained in liquids of one-component (e.g. for organic solvents) and two-components (colloidal dispersions), alike. So far, those two systems were discussed separately, using two different scaling laws. In this review article, an attempt is made to bridge the gap between both scaling laws by defining the scaling limit for two-component systems. Colloidal probe atomic force microscopy (CP-AFM) is used to measure structural forces in suspensions of silica nanoparticles (NPs) of three different sizes. In these two-component systems (solid NPs suspended in water), oscillatory behaviour can be obtained in the force vs. separation profiles. The wavelength λ is larger than the actual particle diameter d and rather depends on the particles' volume fraction ϕ following the inverse cubic root law λ∝ϕ^-13. It is shown that the real particle diameter d can be determined by a gedankenexperiment by extrapolating the fitted wavelength λ from the structural force measurements at a specific particle concentration to a particle volume fraction ϕ of 52% - the packing factor for simple cubic packing - using the well-known inverse cubic root scaling law. This extrapolation can be interpreted as a transition from a two-component system towards a one-component-like problem. In this case, particles are in contact and the wavelength λ is equal to the particle diameter d, λ = d as for one-component systems. The determined diameters d of the different silica nanoparticles agree well with independent measurements using transmission electron microscopy (TEM), validating the used approach. The proposed method can be extended to numerous dispersions of spherical nano-sized objects, for which structural forces can be measured.

Experimental evaluation of additional short ranged repulsion in structural oscillation forces

The paper addresses additional short ranged repulsion in structural oscillation forces between silica surfaces across a suspension of silica nanoparticles. Fit and prediction of the structural oscillation forces usually involve an exponentially decreasing harmonic as introduced by Israelachvili [Israelachvili, Intermolecular & surface forces, Academic Press, San Diego, USA, 1985]. Recently we demonstrated, for aqueous suspensions of silica nanoparticles at various concentrations, that this fit equation is insufficient to describe the structural oscillation forces in its whole range [Schön et al., Beilstein J. Nanotechnol., 2018, 9, 1095-1107]. An additional force acting on short separations leads to the fit parameters scattering widely as well as being dependent on each other and the starting point of the fit. An additional repulsive term was introduced to solve these problems. The additional repulsive force has also been observed by others, in ionic liquids and polyelectrolyte solutions at high ionic strength. It was attributed to the diffusive double layer forces. The rise of the additional repulsion with increasing particle concentration seems to conflict with this interpretation. In this work, colloidal probe atomic force microscopy is used in aqueous suspensions of silica nanoparticles to investigate other contributing factors such as the increasing hydrodynamic drag in the normal direction to the confining surface with increasing particle concentration. A kinetic component to the structural oscillation forces is observed. Furthermore, sodium chloride is used to adjust the ionic strength of two different concentrated silica nanoparticle suspensions. For these systems the additional decay length is compared to the Debye length in the range from low to high ionic strength. A master curve of the additional decay length over Debye length at different ionic strengths, approach speed and particle concentration is produced. It affirms the link between the two and the connection between the additional force and the diffusive double layer forces. The increasing trend for the additional repulsion with increasing particle concentration reveals a synergistic effect of diffusive double layer forces and structural oscillation forces at low to medium ionic strength, which cannot be observed at high ionic strength.

A simple extension of the commonly used fitting equation for oscillatory structural forces in case of silica nanoparticle suspensions

Background: The ordering of molecules or particles in the vicinity of a confining surface leads to the formation of an interfacial region with layers of decreasing order normal to the confining surfaces. The overlap of two interfacial regions gives rise to the well-known phenomenon of oscillatory structural forces. These forces are commonly fitted with an exponentially decaying harmonic oscillation as introduced by Israelachvili (Israelachvili, J. N. Intermolecular & surface forces; Academic Press: San Diego, CA, USA, 1985). From the fit three important parameters are obtained, namely wavelength, amplitude and decay length, which are related to the period, the strength and the correlation length of the oscillatory structural forces, respectively. The paper addresses structural forces between a silica microsphere and a silicon wafer across silica nanoparticle suspensions measured with a colloidal probe AFM. Using the simple fitting procedure with three parameters often leads to underestimation of actually measured forces. The deviation of the fit from the experimental data is especially pronounced at small distances of the confining surfaces and at high concentrations of silica nanoparticles. As a consequence, the parameters of the common fit equation vary with the starting point of the fit. Although the wavelength is least affected and seems to be quite robust against the starting point of the fit, all three parameters show distinct oscillations, with a period similar to the wavelength of the oscillatory structural forces themselves. The oscillations of amplitude and decay length, which are of much higher magnitude, show a phase shift of 180° implying not only a dependence on the starting point of the fit but also on each other. The range affected by this systematic deviation of the fit parameters is much larger than the optically perceived mismatch between fit and experimental data, giving a false impression of robustness of the fit. Results: By introducing an additional term of exponentially decaying nature the data can be fitted accurately down to very small separations and even for high silica nanoparticle concentrations (10 wt %). Furthermore wavelength, amplitude and decay length become independent of the starting point of the fit and in case of the latter two of each other. The larger forces at small separations indicate a more pronounced ordering behavior of the particles in the final two layers before the wall. This behavior is described by the proposed extension of the common fit equation. Conclusion: Thus, the extension increases the accessible data range in terms of separation and concentration and strongly increases the accuracy for all fitting parameters in the system studied here.

Depletion and double layer forces acting between charged particles in solutions of like-charged polyelectrolytes and monovalent salts

Interaction forces between silica particles were measured in aqueous solutions of the sodium salt of poly(styrene sulphonate) (PSS) and NaCl using the colloidal probe technique based on an atomic force microscope (AFM). The observed forces can be rationalized through a superposition of damped oscillatory forces and double layer forces quantitatively. The double layer forces are modeled using Poisson-Boltzmann (PB) theory for a mixture of a monovalent symmetric electrolyte and a highly asymmetric electrolyte, whereby the multivalent coions represent the polyelectrolyte chains. The effective charge of the polyelectrolyte is found to be smaller than the bare number of charged groups residing on one polyelectrolyte molecule. This effect can be explained by counterion condensation. The interplay between depletion and double layer forces can be further used to predict the phase of the depletion force oscillations. However, this picture holds only at not too elevated concentrations of the polyelectrolyte and salt. At higher salt concentrations, attractive van der Waals forces become important, while at higher polyelectrolyte concentrations, the macromolecules adsorb onto the like-charged silica interface.

Interplay between Depletion and Double-Layer Forces Acting between Charged Particles in Solutions of Like-Charged Polyelectrolytes

Direct force measurements between negatively charged silica particles in the presence of a like-charged strong polyelectrolyte were carried out with an atomic force microscope. The force profiles can be quantitatively interpreted as a superposition of depletion and double-layer forces. The depletion forces are modeled with a damped oscillatory profile, while the double-layer forces with the mean-field Poisson-Boltzmann theory for a strongly asymmetric electrolyte, whereby an effective valence must be assigned to the polyelectrolyte. This effective valence is substantially smaller than the bare valence due to ion condensation effects. The unusual aspect of the electrical double layer in these systems is the exclusion of the like-charged polyelectrolyte from the vicinity of the surface, leading to a strongly nonexponential diffuse ionic layer that is dominated by counterions and has a well-defined thickness. As the oscillatory depletion force sets in right after this layer, this condition can be used to predict the phase of the oscillatory depletion force.

Motion of Molecular Probes and Viscosity Scaling in Polyelectrolyte Solutions at Physiological Ionic Strength

We investigate transport properties of model polyelectrolyte systems at physiological ionic strength (0.154 M). Covering a broad range of flow length scales-from diffusion of molecular probes to macroscopic viscous flow-we establish a single, continuous function describing the scale dependent viscosity of high-salt polyelectrolyte solutions. The data are consistent with the model developed previously for electrically neutral polymers in a good solvent. The presented approach merges the power-law scaling concepts of de Gennes with the idea of exponential length scale dependence of effective viscosity in complex liquids. The result is a simple and applicable description of transport properties of high-salt polyelectrolyte solutions at all length scales, valid for motion of single molecules as well as macroscopic flow of the complex liquid.

Two-Dimensional Aggregation and Semidilute Ordering in Cellulose Nanocrystals

The structural properties and aggregation behavior of carboxymethylated cellulose nanocrystals (CNC-COOH) were analyzed with small angle neutron scattering (SANS), transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS) and compared to sulfuric acid hydrolyzed cellulose nanocrystals (CNC-SO3H). The CNC-COOH system, prepared from single carboxymethylated cellulose nanofibrils, was shown to laterally aggregate into 2D-stacks that were stable both in bulk solution and when adsorbed to surfaces. CNC-SO3H also showed a 2D aggregate structure with similar cross sectional dimensions (a width to height ratio of 8) as CNC-COOH, but a factor of 2 shorter length. SANS and DLS revealed a reversible ordering of the 2D aggregates under semidilute conditions, and a structure peak was observed for both systems. This indicates an early stage of liquid crystalline arrangement of the crystal aggregates, at concentrations below those assessed using birefringence or polarized optical microscopy.

Direct AFM force measurements between air bubbles in aqueous monodisperse sodium poly(styrene sulfonate) solutions

Structural forces play an important role in the rheology, processing and stability of colloidal systems and complex fluids, with polyelectrolytes representing a key class of structuring colloids. Here, we explore the interactions between soft colloids, in the form of air bubbles, in solutions of monodisperse sodium poly(styrene sulfonate) as a model polyelectrolyte. It is found that by self-consistently modelling the force oscillations due to structuring of the polymer chains along with deformation of the bubbles, it is possible to precisely predict the interaction potential between approaching bubbles. In line with polyelectrolyte scaling theory, two distinct regimes of behaviour are seen, corresponding to dilute and semi-dilute polymer solutions. It is also seen that by blending monodisperse systems to give a bidisperse sample, the interaction forces between soft colloids can be controlled with a high degree of precision. At increasing bubble collision velocity, it is revealed that hydrodynamic flow overwhelms oscillatory structural interactions, showing the important disparity between equilibrium behaviour and dynamic interactions.

Effect of polyelectrolytes on (de)stability of liquid foam films

The review addresses the influence of polyelectrolytes on the stabilisation of free-standing liquid foam films, which affects the stability of a whole macroscopic foam. Both the composition of the film surface and the stratification of the film bulk drives the drainage and the interfacial forces within a foam film. Beside synthetic polyelectrolytes also natural polyelectrolytes like cellulose, proteins and DNA are considered.

Interactions between solid surfaces mediated by polyethylene oxide polymers: effect of polymer concentration

Using total internal reflection microscopy (TIRM), we have systematically measured the interactions between a microsphere and a flat hydrophilic surface in the presence of polyethylene oxide (PEO) polymer solution. Our results reveal that PEO significantly mediates the interaction forces between the two surfaces. At low polymer concentration, the interactions between two surfaces in the presence of PEO are mainly dominated by repulsive forces, originating from diffuse layer overlap. At intermediate polymer concentration, a long-range and weak attraction sets in. This force is likely attributed to the depletion attraction due to the presence of free PEO chains in bulk solution; however, a simple hard-sphere AO model fails to precisely describe the attraction. At high polymer concentration where PEO chains overlap, the attraction disappears, and levitation of the microsphere probe is detected. We argue that at this overlapping region, the correlation length of PEO chains is much smaller than the size of single PEO molecule, leading to weakening and disappearing of the depletion attraction. Finally, at very high concentration, oscillatory structural force is obviously found, indicating the significant structural ordering of the PEO chains under confinement.

Oscillatory forces of nanoparticle suspensions confined between rough surfaces modified with polyelectrolytes via the layer-by-layer technique

This paper addresses the systematic study of surface roughness effects on the internal structuring of silica nanoparticle suspensions under confinement. The confining surfaces are modified by physisorption of layers of oppositely charged polyelectrolytes with the so-called layer-by-layer technique. The layer-by-layer technique modifies the surface roughness without changing the surface potential of a multilayer with the same outermost layer, by increasing the number of constituent layers and ionic strength of the polyelectrolyte solutions and by selecting an appropriate pair of polyelectrolytes. The oscillatory forces of nanoparticle suspensions with a particle diameter of 26 nm are measured by a colloidal-probe atomic force microscope (CP-AFM). The characteristic lengths of the oscillatory force, i.e., wavelength, which indicates interparticle distance, and decay length, or particle correlation length, are not affected by the surface roughness. The corresponding reduction in the oscillatory amplitude and the shift in the phase correlate with an increase in surface roughness. Increasing surface roughness further induces a disappearance of the oscillations, and both confining surfaces contribute to the effect of surface roughness on the force reduction. In order to show an oscillatory force, the particles have to show positional correlation over a reasonably long range perpendicular to the surface, and the correlation function should be the same over a larger lateral area. This requires that both the particles and the surfaces have a high degree of order or symmetry; otherwise, the oscillation does not occur. A roughness of a few nanometers on a single surface, which corresponds to about 10% of the nanoparticle diameter, is sufficient to eliminate the oscillatory force.