SURFACE LIGHT-SCATTERING-STUDIES OF SOLUBLE BLOCK-COPOLYMER SURFACTANTS AT THE AIR WATER INTERFACE/

Thermally excited capillary waves of the poly(ethylene oxide)-block-po ly(dimethylsiloxane) copolymer surfactant solutions have been studied by surface light scattering (SLS). The surface tension measured by the Wilhelmy plate method agrees well with that calculated from the surfa ce light scattering data using the Kelvin equation at low and high con centrations, while the deviation is observed in the concentration rang e between 4 x 10(-7) to 2 x 10(-6) g/mL. This deviation is considered as due to surface adsorption and relaxation in the monolayer. The surf ace elastic modulus (epsilon') deduced from the SLS spectrum exhibits a maximum at a concentration around 7.8 x 10(-7) g/mL. The epsilon' pr edicted by the diffusion exchange model of Lucassen shows a serious di screpancy with our data. In addition, the epsilon '' value is found to be negative in some regions and it displays a minimum for concentrati ons below 3 x 10(-6) g/mL. The negative values of epsilon '' have also been previously reported, and it may be due to the inadequacy of the dispersion equation currently used to interpret the SLS result.