MIXED ADSORPTION OF ANIONIC AND NONIONIC SURFACTANTS ON LATEX-PARTICLES

A model is presented for the adsorption of a mixture of an anionic and a nonionic surfactant on latex particles. The model is based on the L angmuir equation for adsorption and the regular solution theory for mi xed micelle formation. Good agreement between the experimental data an d the calculated adsorption was obtained with a mixture of sodium dode cyl sulphate (SDS) and nonylphenol ethoxylate (NPE10). The preferentia l adsorption of the more surface active NPE10 is greatest at the onset of micellization and diminishes as the total surfactant concentration increases. The composition of the mixed surfactant at the surface gre atly depends on the total surfactant concentration and on the surface area/solution volume ratio. This is illustrated by the calculated adso rption equilibria at different conditions. Addition of small amounts o f organic solvents (e.g., coalescing aids) increases the critical mice lle concentration (cmc) of NPE10 and decreases the cmc of SDS. The dec rease in the difference between the cmc values of the two surfactants is paralleled by a reduction in the preferential adsorption of the non ionic surfactant in the presence of a coalescing-aid.