Colloid stability of casein-coated polystyrene particles

Abstract

The stability of casein-coated polystyrene latices with respect to aggregation by sodium chloride has been studied at 20 °C and pH 7.3. Initial aggregation rates have been measured by light scattering as a function of ionic strength (100–1000 mol m^–3) for negatively charged particles separately coated with α_s1-casein, β-casein, κ-casein and three commercial caseinate mixtures. Casein adsorption reduces the mean electrophoretic mobility of latex particles. At low surface coverage (ca. 10% saturation), derived critical coagulation concentrations are roughly in accord with predictions from DLVO theory based on changes in zeta potential estimated from experimental mobilities. Near saturation coverage, latices are stabilized against aggregation at all ionic strengths up to 1000 mol m^–3, presumably by a ‘steric’ mechanism. While each pure casein or caseinate mixture shows the same general behaviour, κ-casein appears quantitatively less effective as a steric stabilizer.