DYNAMIC SURFACE-TENSION OF AQUEOUS SURFACTANT SOLUTIONS .7. PHYSICAL SIGNIFICANCE OF DYNAMIC PARAMETERS AND THE INDUCTION PERIOD

The equation describing dynamic surface tension in aqueous media at constant surfactant concentration, (gamma(0) - gamma(t))/(gamma(t) - gamma(m)) = (t/t*)(n), where gamma(0) is the surface tension of the solvent, gamma(t) at time t, and gamma(m) at mesoequilibrium, and t* and n are constants, is transformed mathematically to a Frumkin-type equation, pi(t) = pi(m)/[1 + 3.73t(i)(n)exp(-n 1n t)], where pi(t) and pi(m) are the surface pressure at time t and mesoequilibrium, respectively, and t(i) is the time at the end of the induction (initial) period of the surface tension reduction. It is suggested that n is related to the difference between the energies of adsorption and desorption and that t(i) is related to the surface coverage during the induction period. More than 20 systems were studied, including anionic, nonionic, and zwitterionic surfactants. The value of n increases with increases in the hydrophobic character of the surfactant while t(i) increases with increases in the tightness of packing of the adsorbed molecules at the interface. There is a linear relationship between In t(i) and 1n (Gamma(i)/C), where Gamma(i) is the surface (excess) concentration at t(i), with a slope of approximately 2, suggesting diffusion-controlled adsorption in all systems studied. Surface coverage at t(i) in all the systems studied was 0.62 +/- 0.05 of the maximum possible, which suggests an explanation for the observation that surfactants with larger surface areas/molecule decrease surface tension more rapidly than similar molecules with smaller areas/molecule. (C) 1995 Academic Press, Inc.