BROWNIAN-MOTION AT LIQUID GAS INTERFACES .2. EFFECT OF SURFACTANTS

The effect of surfactants on the Brownian motion of macroparticles flo ating at the liquid-gas interface is discussed. The drag force of a sp here half immersed in a liquid containing soluble surfactants is calcu lated. It is shown that in the cases of small disturbances in the conc entration c is similar to (GAMMA0V/D) < c0 (c0, GAMMA0, initial equili brium distributions; V, velocity of the sphere; D, bulk diffusion coef ficient of the surfactant), the relative decrease of the particle mobi lity with respect to the mobility at a surfactant-free interface is of the order of (alphaE/Dmu) (alpha = (partial derivative GAMMA/partial derivative c)0, coefficient of surfactant distribution; E = (-GAMMA pa rtial derivative gamma/partial derivative GAMMA)0, Gibbs elasticity; m u, dynamic viscosity of the fluid). The correctness of this result is discussed and compared with some typical data from the literature.