Dissolution rates of surfactants and granules
Miller, Clarence A.
Doctor of Philosophy
A quantitative penetration scan method was used to study rates of dissolution of pure, noncrystalline anionic surfactants in water. The displacement of phase boundaries from initial surface of contact was found to be proportional to the square root of time, indicating the importance of diffusion. For some surfactants studied such as Aerosol OT (AOT) and 7-phenyl tetradecane sulphonate, myelinic figures grew from the initial surface of contact between surfactant and water toward the aqueous phase. A simple model was developed which included both this swelling and diffusion in the rest of the surfactant-containing region. The usual penetration scan method involving semi-infinite phases was supplemented by a novel modified scan in which only a thin layer of surfactant was used. With the combined results it was possible to obtain effective diffusivities of the liquid crystalline phases of AOT and of the two lamellar phases of 7-phenyl tetradecane sulphonate. Values were of order 10-10 m2/s. Videomicroscopy was used to investigate the mechanisms and rates of dissolution for a system containing the pure nonionic surfactant C12E 4 and the soap sodium oleate. A microinjection technique was used to inject drops of surfactants or surfactant mixtures into water. Although C 12E4 itself does not dissolve in water, dissolution was observed when drops of its mixtures with sufficient oleic acid were injected into alkaline buffer solutions. Formation of sodium oleate during the dissolution process made the surfactant mixture more hydrophilic and hence soluble. A lamellar phase formed upon injection and dissolved by a shrinking core mechanism. A hanging drop slide technique was developed and used to study disintegration of single granules consisting of many zeolite particles bound together with liquid nonionic surfactant. For pure nonionic surfactants and their mixtures, granules disintegrated below the cloud point of the pure surfactant or mixture. Disintegration did not occur when the neat surfactant developed viscous myelinic figures upon contact with water. Nor was it observed when an aqueous phase coexisted with a surfactant-rich L1 phase or L3 (sponge) phase at equilibrium. Similar behavior was observed for commercial nonionic surfactants and their mixtures.