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 formed, 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.