EVIDENCE OF A HAIRY LAYER AT THE SURFACE OF POLYSTYRENE LATEX-PARTICLES

Polystyrene latexes are treated as model colloids, yet the observed electrokinetic and size behavior do not conform to the predictions of theory. Upon the addition of low concentrations of electrolyte, the electrophoretic mobility and zeta potential of the latex increase, while the hydrodynamic size decreases. One explanation for the anomalous behavior is the presence of a hairy layer at the latex surface. According to the hairy layer model, flexible polymer chains, or hairs, at the surface extend into solution, owing to electrostatic repulsion between the ionic groups which terminate the hairs and the ionic groups which are anchored at the surface. The thickness of the hairy layer is altered by the presence of electrolyte owing to charge shielding. As such, the size and electrokinetic parameters are functions of ionic strength. This model is frequently invoked in the literature; however, to the authors' knowledge, no previous studies have attempted to verify the presence of a hairy layer via simultaneous measurement of both the size and electrokinetic characteristics of a given latex over a range of conditions. Evidence from other investigators indicates that heating a latex to its glass transition temperature yields particles whose electrokinetic behavior is more nearly ideal. Interpreted in terms of the hairy layer model, such results suggest that heat treatment smoothes or removes the hairs. In the current study, the presence of a hairy layer is investigated by comparing size, mobility, critical coagulation concentration, and surface charge density measurements for three different surfactant-free latexes before and after heat treatment. The new results strongly support the existence of a hairy layer. Light scattering measurements indicate that the thickness of the layer may be as great as 7 nm.