Surface tension isotherms, adsorption dynamics and dilational visco-elasticity of sodium dodecyl sulphate solutions

Abstract

The adsorption kinetics, adsorption isotherm and the dilational surface rheology at low oscillation frequencies of sodium dodecyl sulphate (SDS) were studied by drop and bubble profile, and bubble pressure tensiometry. The effect of the inevitably present dodecanol in any SDS solution is taken into account by interpreting the experimental data with theoretical models valid for mixed surfactant solutions. In particular the dilational rheology method appears to be very suitable for demonstrating the continuously on-going hydrolysis reaction in a SDS solution, as the resulting dilational elasticities and viscosities change from a frequency dependencies characteristic to SDS to those expected for dodecanol adsorption layers. For surfactants as active as dodecanol, it turns out that all adsorption and rheological quantities depend significantly on the use of drop or bubble profile tensiometry, i.e. the small reservoir of available surface active molecules in a single droplet has to be considered quantitatively to obtain reliable data.