Investigation of the disposition of surfactant in a latex by the “rate of dialysis method.” Mechanism of detergent dialysis

The initial rate of dialysis across a cellophane membrane for sodium dodecylsulfate and dodecylbenzenesulfonate proved to be dependent on surfactant concentration without a break at the critical micelle concentration (cmc) and could, therefore, be used to determine free surfactant concentration in latices over a wide range. Equilibrium dialysis is not a suitable tool, because, if the cmc is exceeded in the dialyzate, diffusion comes practically to a dead stop long before equilibration, at least with ordinary detergents. (Dialysis is, however, resumed at a substantial rate if dialyzate and dialyzed solution are replaced by newly prepared solutions of the same concentration.) An explanation for these phenomena is suggested. Free surfactant concentrations measured by rate of dialysis were far below the cmc in a polystyrene latex and far above the cmc in a comparable poly(vinyl acetate) (PVA) latex. When increasing the surfactant‐polymer ratio in a PVA dispersion by post‐addition complete solubilization of the polymer is observed eventually. Concentrations of free surfactant and of solubilized polymer were measured for the intermediate stages. The experimental data indicate a twofold mechanism for the solubilization process: Surfactant penetrates into the interior of PVA particles taking water with it and causing swelling to the point of disintegration, parallel to which single chains from the outer parts of the particles are already disentangled and solubilized. It is concluded that in a PVA latex only a comparatively small portion of the anionic surfactant will be at the particle surface, a relatively large part being in the aqueous phase and in the particle interior, which may account for this surfactant being a poor stabilizer in vinyl acetate emulsion polymerization at high total solids content.