Structure and extent of adsorbed gelatin on acrylic latex and polystyrene colloidal particles

The extent and structure of the adsorbed layer of gelatin onto colloidal particles is measured using dynamic light scattering, small-angle neutron scattering, and dilution viscometry. Adsorption isotherms are measured using solution depletion of fluorescein-5-isothiocyanate tagged gelatin. The results show that the adsorbed gelatin layer at saturation on acrylic-based latex (pH 6.5, 0.01 M sodium acetate) has approximately twice the average density of the free gelatin in solution, while for the adsorption onto polystyrene latex (pH 8.7, deionized water), it has nearly the same average density. The energetics of gelatin affinity is calculated from the temperature dependence of the adsorption equilibrium constant. It was found that the enthalpy of adsorption of a net negatively charged gelatin onto a negatively charged surface is favorable. An additional, favorable entropy of adsorption is measured for the polystyrene latex, for which hydrophobic interactions are important. Selective, competitive adsorption of nonionic and anionic surfactants confirms the specific source of the adsorption interactions. (C) 1998 Academic Press.