DIFFUSION-LIMITED INTERPRETATION OF THE INDUCTION PERIOD IN THE RELAXATION IN SURFACE-TENSION DUE TO THE ADSORPTION OF STRAIGHT CHAIN, SMALL POLAR GROUP SURFACTANTS - THEORY AND EXPERIMENT

The relaxation in surface tension due to the adsorption of bulk-soluble, unbranched, long chain surfactants with small polar groups at the air-water interface is often characterized by an initial induction period in which the surface tension relaxes very slowly. In this study, the origin of this induction in the surface tension relaxation is attributed to intermolecular cohesive forces among the adsorbed surfactant molecules which develop as the surface coverage increases. Surfactant molecules with long, slender hydrocarbon chains and small polar groups are subject to strong, attractive van der Waals forces when surface crowding causes interchain contact. Two models are constructed to account for this cohesion. In the first, intermolecular attraction leads to the formation of a liquid phase from a gaseous state. The induction period arises as the liquid state is forming, and addition of further molecules by diffusion is not accompanied by a change in the surface pressure. In the second model, the intermolecular attraction causes a cooperative adsorption as the activation energy for desorption increases faster with surface coverage than for adsorption. The induction period arises as the presence of cohesion lowers the surface pressure, offsetting the effect of the large increase in surface concentration due to the cooperative adsorption. Equations of state and adsorption isotherms necessary to describe this cooperative adsorption/phase transition behavior are developed, and theoretical solutions of the diffusion limited mass transfer to a fresh surface coupled with these isotherms are presented. Experimental verification of these ideas is obtained by studying the adsorption of aqueous solutions of 1-decanol at the air-water interface. Surface tension relaxation profiles for 1-decanol are obtained by using pendant bubble tensiometry enhanced by video digitization, and these profiles compare favorably with the numerical solutions obtained by using the developed models.