The effect of temperature on the adsorption of non-ionic surfactants and non-ionic surfactant mixtures at the air-water interface

Neutron reflectivity has been used to study the effect of temperature on the adsorption of non-ionic surfactants and non-ionic surfactant mixtures at the air-water interface. For a range of polyoxyethylene non-ionic surfactants the adsorption is invariant with temperature in the range 25 to 50 c; however some subtle changes in the structure of the adsorbed layer are observed. For the non-ionic mixture of monododecyltriethylene glycol, C(12)EO(3), and monododecyloctaethylene glycol, C(12)EO(8), measurements over a wide concentration range, from below to above the cmc (10(-5) to 10(-2) M), provide a direct qualitative confirmation of the abrupt change in surface composition at the cmc due to the onset on mixed micelle formation predicted recently by Nikas, Puvvada, and Blankschtein, Langmuir, 8 (1992) 2680. For the same non-ionic mixtures and at concentrations below and above the cmc (at concentrations of 1.25 x 10(-5) and 5 x 10(-4) M) a change in surface composition with increasing temperature is observed. The surface becomes more rich in the least surface active component, the C(12)EO(8). In the same temperature range the total adsorption from the mixture at the interface decreases slightly.