ADSORPTION-KINETICS OF DECANOL AT THE AIR-WATER-INTERFACE

The adsorption kinetics is studied for 1-decanol by using pendant bubble tensiometry enhanced by video digitization. Cohesive forces between decanol molecules adsorbed onto an air-water interface play an important role in the sorption kinetics. Intermolecular cohesive forces among the adsorbed molecules raise the energy barrier for desorption of the adsorbed molecules into the bulk sublayer and correspondingly lower the desorption rate. For adsorption onto an initially clean interface, a reduction in the desorption rate affects the tail end of the process and diffusion is the rate-controlled step. After the establishment of equilibrium, adsorbed monolayers at the air-water interface are compressed by shrinking the air bubble slightly. The subsequent re-equilibration, affected by the lower desorption rate during the entire period of the process, is controlled by both diffusion and sorption kinetics. A diffusion coefficient is computed by comparing the dynamic surface tension profiles of clean adsorption with numerical solutions of bulk surfactant diffusion equation and a generalized Frumkin adsorption isotherm. The adsorption and desorption rate constants are determined from the experimental relaxation of re-equilibration.