Dissolution rates of surfactants

Abstract

Information on dissolution rates of surfactants is reviewed and discussed. One or more viscous liquid crystalline phases form during dissolution in water of pure liquid nonionic surfactants of the linear alcohol ethoxylate type, as expected from known equilibrium phase diagrams. Dissolution rates are controlled by diffusion at temperatures below a critical solution point T C, where the aqueous micellar solution begins to separate into dilute and concentrated phases. Analysis of linear penetration and drop dissolution experiments yields values of effective diffusion coefficients for each phase. Once the temperature exceeds T C, solubility of surfactant in the aqueous phase falls rapidly to very low values and complete dissolution does not occur. Particularly striking is formation by a swelling process at temperatures somewhat above T C of filaments of the lamellar liquid crystalline phase known as myelinic figures. Phospholipids and other rather lipophilic surfactants also form myelinic figures when contacted with water. A model of their growth is described and applied to the pure anionic surfactant Aerosol OT. Using data from linear penetration experiments, one can calculate effective diffusivities in the liquid crystalline phases (other than the myelins) and the composition of the myelins. Preferential dissolution of more hydrophilic species occurs during dissolution of drops of surfactant mixtures, causing dissolution rates to decrease as the drops become less soluble. Moreover, drop compositions can reach states where new phases form, leading to intriguing behavior such as spontaneous emulsification, anisotropic swelling, and formation of jets. © 2008 Springer-Verlag.