Adsorption of organic molecules on silica surface.

A broad range of manufactured products and biological fluids are colloids. The ability to understand and control the processes (of scientific, technological and industrial interest) in which such colloids are involved relies upon a precise knowledge of the electrical double layer. The traditional approach to describing this ion cloud around colloidal particles has been the Gouy-Chapman model, developed on the basis of the Poisson-Boltzmann equation. Since the early 1980s, however, more sophisticated theoretical treatment have revealed both quantitative and qualitative deficiencies in the Poisson-Boltzmann theory, particularly at high ionic strengths and/or high surface charge densities. This review deals with these novel approaches, which are mostly computer simulations and approximate integral equation theories based on the so-called primitive model. Special attention is paid to phenomena that cannot, be accounted for by the classic theory as a result of neglecting ion size correlations, such as overcharging, namely, the counterion concentration in the immediate neighborhood of the surface is so large that the particle surface is overcompensated. Other illustrative examples are the nonmonotonic behavior of the electrostatic potential and attractive interactions between equally charged surfaces. These predictions are certainly remarkable and, on paper, they can have an effect on experimentally measurable quantities (for instance, electrophoretic mobility). Even so, these new approaches have scarcely been applied in practice. Thus a critical survey on the relevance of ion size correlation in real systems is also included. Overcharging of macroions can also be brought about by adsorption of oppositely charged polyelectrolytes. Noteworthy examples and theoretical approaches for their are also briefly reviewed.

Overcharging in colloids: beyond the Poisson-Boltzmann approach.

Formation of polyelectrolyte–surfactant complexes on surfaces

The adsorption of poly(vinylamine) (PVA) on poly(styrene sulfate) latex particles is studied, and its consequences on the charging behavior and suspension stability are investigated. The adsorption process is assessed by batch depletion experiments and time-resolved electrophoretic mobility measurements. The adsorption of PVA appears to be basically irreversible. The rate of adsorption decreases with decreasing polymer dose. At low polymer dose, the polymer coverage corresponds to the amount of the polyelectrolyte added, while at high polymer dose, the polymer coverage saturates the surface. Stability ratios are determined by dynamic light scattering, and strongly depend on the polymer dose and salt level. The aggregation is rapid near the isoelectric point (IEP), and it slows down when moving away from it. The charge neutralization is highly nonstoichiometric with charging ratios (CR) larger than unity, meaning that several charges on an adsorbed polyelectrolyte chain are necessary to neutralize a single charge on the particle surface. By comparing the IEP for particles and polyelectrolytes of different charge densities, we find a strong dependence of the CR on the mismatch between the average distances between individual charges on the surface and on the polyelectrolyte. A simple model is proposed to explain this trend.

Super-stoichiometric charge neutralization in particle-polyelectrolyte systems.

Streaming potential studies of colloid, polyelectrolyte and protein deposition.

https://www.pse.umass.edu/sites/default/files/santore/documents/jcis247.pdf

The relationship between polymer/substrate charge density and charge overcompensation by adsorbed polyelectrolyte layers.

This work addressed the bound train fraction of an adsorbing model polyelectrolyte ((dimethylamino)ethyl methacrylate, DMAEMA), for variations in pH, which altered the relative charge densities of the polymer and substrate (silica). In addition to the weakly basic and acidic natures of the polyelectrolyte and substrate, respectively, this model system had the characteristic that adsorption was driven exclusively by electrostatic attractions. Therefore, at pH's where the polymer or the substrate was not charged, adsorption did not occur. At pH 6, which represented the extreme of a sparsely charged substrate and a densely charged polyelectrolyte with condensed counterions, the train fraction approached unity and was coverage-independent. At these conditions, the maximum bound train mass of 0.7 mg/m2 exceeded previous reports for the maximum train mass of adsorbed nonionic polymers. At higher pH's, the substrate charge density increased while the polymer became sparsely charged. In this limit, the train frac...

Yongwoo Shin

Papers

The relationship between polymer/substrate charge density and charge overcompensation by adsorbed polyelectrolyte layers.

Influence of Charge Density and Coverage on Bound Fraction for a Weakly Cationic Polyelectrolyte Adsorbing onto Silica