Adsorption of DMSO at the free surface of water: Surface excesses and surface potential shifts in the low concentration range

The adsorption of dimethyl sulfoxide (DMSO) at the free surface of aqueous solutions has been studied by means of surface tension and surface potential measurements in the low concentration range up to 3 mol dm(-3). In this range the surface excess (Gamma) cs. concentration plot shows a definite trend to saturation with Gamma(max)=2.2x10(-10) mol cm(-2). This value, well below that for a close-packed monolayer, is explained in terms of adsorption of hydrated DMSO molecules. The shift of the surface potential indicates that adsorbed DMSO molecules point their CH3 groups towards the exterior of the liquid phase. A model is suggested and worked out to describe the adsorption behaviour.