The Effect of Molecular Weight of Nonadsorbing Polymer on the Structure of Depletion-Induced Flocs

Repulsive and attractive depletion forces mediated by nonadsorbing polyelectrolytes in the Donnan limit

In mixtures of colloids and nonadsorbing polyelectrolytes, a Donnan potential arises across the region between surfaces that are depleted of the polyelectrolyte and the rest of the system. This Donnan potential tends to shift the polyelectrolyte density profile toward the colloidal surface and leads to the local accumulation of polyelectrolytes. We derive a zero-field theory for the disjoining pressure between two parallel flat plates. The polyelectrolyte is allowed to enter the confined interplate region at the cost of a conformational free energy penalty. The resulting disjoining pressure shows a crossover to a repulsive regime when the interplate separation gets smaller than the size of the polyelectrolyte chain, followed by an attractive part. We find a quantitative match between the model and self-consistent field computations that take into account the full Poisson-Boltzmann electrostatics.

Electroosmosis of Viscoelastic Fluids: Role of Wall Depletion Layer

We investigate electroosmotic flow of two immiscible viscoelastic fluids in a parallel plate microchannel. Contrary to traditional analysis, the effect of the depletion layer is incorporated near the walls, thereby capturing the complex coupling between rheology and electrokinetics. Toward ensuring realistic prediction, we show the dependence of electroosmotic flow rate on the solution pH and polymer concentration of the complex fluid. In order to assess our theoretical predictions, we have further performed experiments on electroosmosis of an aqueous solution of polyacrylamide (PAAm). Our analysis reveals that neglecting the existence of a depletion layer would result in grossly incorrect predictions of the electroosmotic transport of such fluids. These findings are likely to be of importance in understanding electroosmotically driven transport of complex fluids, including biological fluids, in confined microfluidic environments.

Phase behavior of colloid-polymer depletion mixtures with unary or binary depletants

Adding depletants to a colloidal suspension induces an attractive interparticle interaction that can be tuned to obtain desired structures or to probe phase behavior. When the depletant is not uniform in size, however, both the range and strength of the attraction become difficult to predict and hence control. We investigated the effects of depletant bidispersity on the non-equilibrium phase behavior of colloid-polymer mixtures. We added unary or binary mixtures of polystyrene as the depletant to suspensions of charged poly(methyl methacrylate) particles. The structure and dynamics of the particles were compared over three sets of samples with various mixtures of two different polystyrenes whose size varied by an order of magnitude. The structure and dynamics were nearly independent of depletant dispersity if the polymer concentration was represented as a sum of normalized concentrations of each species. Near the transition region between a fluid of clusters and an interconnected gel at intermediate volume fractions, partitioning of polymers in a binary mixture into colloid-rich and polymer-rich phase leads to a slightly different gelation pathway.

Effect of Maillard Conjugates on the Physical Stability of Zein Nanoparticles Prepared by Liquid Antisolvent Coprecipitation

Protein nanoparticles are often not very stable in a complex food matrix because they are primarily stabilized by electrostatic repulsion. In this study, we envisaged the stabilization of zein nanoparticles through Maillard conjugation reactions with polysaccharides of different molecular mass. Zein nanoparticles (0.5% w/v) containing resveratrol (0.025% w/v grape skin extract) were produced by liquid antisolvent precipitation and coated with Maillard conjugates (MC) of sodium caseinate and different molecular mass carbohydrates during particle production. Zein nanoparticles coated with conjugated polysaccharides of 2.8, 37, and 150 kDa had diameters of 198 ± 5, 176 ± 6, and 180 ± 3 nm, respectively. The encapsulation efficiency (∼83%) was not affected by conjugation, but the conjugates significantly improved particle stability against changes in pH (2.0-9.0), CaCl2 addition (up to 100 mM), and heat treatment (30-90 °C, 30 min). Zein nanoparticles coated by MC may therefore be suitable delivery systems for hydrophobic bioactive molecules in a wide range of commercial products.

Printing small dots from large drops

Printing of droplets of pure solvents containing suspended solids typically leads to a ring stain due to convective transport of the particles toward the contact line during evaporation of the solvent. In mixtures of volatile solvents, recirculating cells driven by surface tension gradients are established that lead to migration of colloidal particles toward the center of the droplet. In favorable cases, a dense disk of particles forms with a diameter much smaller than that of the droplet. In the latter stages of drying, convective transport of the particles radially toward the contact line still occurs. Two strategies are described to fix the distribution of particles in a compact disk much smaller than the initial diameter of the drying droplet. First, a nanoparticulate clay is added to induce an evaporation-driven sol-gel transition that inhibits convective flow during the latter stages of drying. Second, a nonadsorbing polymer is added to induce depletion flocculation that restricts particle motion after the particles have been concentrated near the center of the droplet. The area of the resulting deposit can be as little as 10% of the footprint of the printed droplet.

Highly enhanced energy conversion from the streaming current by polymer addition

In this contribution, we present for the first time the experimental results of energy conversion from the streaming current when a polymer is added to the working solution. We added polyacrylic acid (PAA) in concentrations of 200 ppm to 4000 ppm to a KCl solution. By introducing PAA, the input power, which is the product of volumetric flow rate and the applied pressure, reduced rapidly as compared to the case of using only a normal viscous electrolyte KCl solution. The output power at the same time remained largely constant, whereby an increase of the streaming current and a decrease of the streaming potential simultaneously occurred. These combined factors led to the massive increase of the energy conversion efficiency. Particularly, the results showed that when PAA was in a 0.01 mM KCl solution, the energy conversion efficiency of the system was enhanced by a factor of 447 (±2%), as compared to the case of the solution containing only 0.01 mM KCl. An enhancement factor of 249 (±4%) was also observed when PAA was added to the higher ionic strength background solution, 1 mM KCl. This finding can have practical use in microchannel-array energy conversion systems. When, instead of the negatively charged PAA, a non-ionic polymer polyethylene oxide (PEO) was added to the solution, no efficiency increase was observed, probably due to polymer wall adsorption.

Latex migration in battery slurries during drying

We used real-time fluorescence microscopy to investigate the migration of latex particles in drying battery slurries. The time evolution of the fluorescence signals revealed that the migration of the latex particles was suppressed above the entanglement concentration of carboxymethyl cellulose (CMC), while it was significantly enhanced when CMC fully covered the surfaces of the graphite particles. In particular, a two-step migration was observed when the graphite particles flocculated by depletion attraction at high CMC/graphite mass ratios. The transient states of the nonadsorbing CMC and graphite particles in a medium were discussed, and the uses of this novel measurement technique to monitor the complex drying processes of films were demonstrated.

Polymer depletion-driven cluster aggregation and initial phase separation in charged nanosized colloids

We study polymer depletion-driven cluster aggregation and initial phase separation in aqueous dispersions of charge-stabilized silica spheres, where the ionic strength and polymer (dextran) concentration are systematically varied, using dynamic light scattering and visual observation. Without polymers and for increasing salt and colloid content, the dispersions become increasingly unstable against irreversible cluster formation. By adding nonadsorbing polymers, a depletion-driven attraction is induced, which lowers the stabilizing Coulomb barrier and enhances the cluster growth rate. The initial growth rate increases with increasing polymer concentration and decreases with increasing polymer molar mass. These observations can be quantitatively understood by an irreversible dimer formation theory based on the classical Derjaguin, Landau, Verwey, and Overbeek pair potential, with the depletion attraction modeled by the Asakura-Oosawa-Vrij potential. At low colloid concentration, we observe an exponential cluster growth rate for all polymer concentrations considered, indicating a reaction-limited aggregation mechanism. At sufficiently high polymer and colloid concentrations, and lower salt content, a gas-liquidlike demixing is observed initially. Later on, the system separates into a gel and fluidlike phase. The experimental time-dependent state diagram is compared to the theoretical equilibrium phase diagram obtained from a generalized free-volume theory and is discussed in terms of an initial reversible phase separation process in combination with irreversible aggregation at later times.

Aggregate morphology and aggregation rate constants for silica dispersions in the presence of salt and polyelectrolyte

It is shown that the addition, over suitable concentration ranges, of mixtures of (nonadsorbing) sodium poly(styrene sulfonate) and potassium chloride, to dispersions of silica particles in water, can lead to very large changes in the sediment height of the resulting aggregates, reflecting similarly large changes in particle packing density within the aggregates. It can also lead to aggregation rates which are considerably faster than the diffusion-controlled rates (by as much as a factor of 2.5), although this enhancement is reduced as the dispersion particle concentration is reduced.

Role of nonadsorbing polymers in bacterial aggregation

Bacteria exhibit properties similar to those of nonbiological colloids and can display pairwise attractions when in close proximity. This interaction is governed by the surface chemistry of the cells. We seek to understand bacterial aggregation at the cellular level using Escherichia coli (E. coli) AB1157. Aggregation studies were carried out using 0.5 to 2.5 wt% E. coli AB1157 harvested in different growth phases with varying concentrations of a nonadsorbing polymer, sodium polystyrene sulfonate (SPS). The electrophoretic mobility of E. coli AB1157 in different growth phases was determined using phase-amplitude light scattering. E. coli AB1157 was found to be negatively charged, and the cell surface properties changed in different growth phases. The electrokinetic results correlated well with the different concentrations of nonadsorbing polymer needed to induce depletion aggregation. This shows that a difference in aggregation properties is due to changes in the bacteria electrokinetic properties during their growth.

Depletion-induced phase separation in colloid-polymer mixtures

Phase separation can be induced in a colloidal dispersion by adding non-adsorbing polymers. Depletion of polymer around the colloidal particles induces an effective attraction, leading to demixing at sufficient polymer concentration. This communication reviews theoretical and experimental work carried out on the polymer-mediated attraction between spherical colloids and the resulting phase separation of the polymer-colloid mixture. Theoretical studies have mainly focused on the limits where polymers are small or large as compared to the colloidal size. Recently, however, theories are being developed that cover a wider colloid-polymer size ratio range. In practical systems, size polydispersity and polyelectrolytes (instead of neutral polymers) and/or charges on the colloidal surfaces play a role in polymer-colloid mixtures. The limited amount of theoretical work performed on this is also discussed. Finally, an overview is given on experimental investigations with respect to phase behavior and results obtained with techniques enabling measurement of the depletion-induced interaction potential, the structure factor, the depletion layer thickness and the interfacial tension between the demixed phases of a colloid-polymer mixture.

Viscosity effect on the structural compactness of latex flocs formed under weak depletion attractions

Dilute aqueous dispersions of colloidal polystyrene latex spheres were flocculated by adding a nonadsorbing polymer sample, poly(acrylic acid). The structural compactness of the flocs thus formed was characterized in terms of their mass fractal dimension using the small-angle static light scattering technique. It was found that with low poly(acrylic acid) concentrations and thus weak depletion attraction forces, the dispersion medium viscosity had a marked effect on the floc structure. An increase in the viscosity led to formation of denser flocs. This was revealed in three sets of depletion flocculation experiments: (a) adjusting the background electrolyte concentration at a fixed level of poly(acrylic acid), (b) using water and 30% (w/w) glycerol as the respective solvents, and (c) inducing latex flocculation with two poly(acrylic acids) of different molecular weights at the respective critical polyacid concentrations. Direct force measurements were made with atomic force microscopy to isolate the influence of viscosity on floc structure from that of interparticle interaction energies. We conclude that the formation of denser flocs with increasing medium viscosity can be attributed to the reduced diffusivity of particles in the solution. The latter resulted in an enhanced rate of floc restructuring (through relaxation of attached particles) relative to floc growth.

Direct Measurement of Depletion and Structural Forces in Polydisperse, Charged Systems

The effect of polydispersity in macromolecule size or surface potential on the depletion interaction between a spherical silica particle and a silica flat in solutions containing two different types of nonadsorbing charged spherical macromolecules was studied with an atomic force microscope (AFM). The macromolecules used here were negatively charged nanospheres of either polystyrene or silica. To investigate the effect of size polydispersity, experiments were performed under the condition of either constant macromolecule number density or constant volume fraction as the relative proportions of smaller and larger polystyrene nanospheres in the suspension were varied. Similarly, for the experiments with surface potential polydispersity, the suspensions contained varying fractions of more highly charged (polystyrene) and less highly charged (Ludox silica) nanospheres at constant number density. The experimental results were compared to the predictions of the modified force-balance model of Piech and Walz and semiquantitative agreement was found. In particular, the maximum attraction and repulsion observed in the measured force profiles were found to agree with the predicted trends as the makeup of the macromolecules was varied. The trend in the maximum attraction was also consistent with predictions made using a simple "scaling" analysis derived using the equation for hard-sphere interactions.