Coadsorption of sodium dodecyl sulfate and medium-chain alcohols at the air-water interface

The adsorption of sodium dodecyl sulfate (SDS) and n-dodecanol from aqueous solutions of the pure and mixed surfactants at the air-water surface is studied by equilibrium surface pressure measurements, surface pressure transients and Brewster angle microscopy. The adsorption layers of SDS and n-dodecanol show fundamental differences in the phase behaviour. The adsorption parameters of both components are determined. Under appropriate conditions, a phase transition at which condensed phase textures are fanned, occurs in the adsorption layers of n-dodecanol. The adsorption layers of surface-chemically purified SDS exist only in a fluid-like state without a phase transition under formation of condensed phase domains. Coadsorption of both surfactants is only investigated in the range of trace amounts of n-dodecanol. Depending on the mixing ratio and the system conditions (bulk concentration, temperature), a phase transition can or cannot occur. At absence of a phase transition, comparable surface concentrations of both components are calculated based on a orthogonal collocation solution for a two-component system. The adsorption properties are completely different when a phase transition occurs. Condensed phase domains of n-dodecanol formed, after the phase transition point, grow finally to a homogeneous condensed phase which replaces completely SDS. (C) 2000 Elsevier Science B.V. All rights reserved.