Non-equilibrium exchange kinetics in sequential non-ionic surfactant adsorption: Theory and experiment

Abstract

Vastly different characteristic times of adsorption and desorption of surfactants in multi-component systems can cause non-uniform changes in interfacial concentration and surface tension. The rate constants for these processes can be determined by experiments in pendant drops in which the interface adjacent to a surfactant solution is removed from equilibrium by replacement of the drop subphase. Experiments using two n-alkyl (C 10 and C 14) dimethyl phosphine oxides are presented which measure the desorption coefficients of each in single component experiments and show the impact desorption kinetics have on the dynamic surface tension which occurs during sequential replacement of a surfactant at the air–water interface by exchanging the bulk phase for solution of a second surfactant. Because of finite sorption kinetics, non-equilibrium surface densities prevail during the exchange process. When C 10 DMPO is replaced by C 14 DMPO, the desorption of C 10 DMPO is more rapid than the adsorption of C 14 DMPO and the surface coverage at intermediate times is less than the equilibrium surface coverage, causing the surface tension to exhibit a maximum. When the more slowly desorbing C 14 DMPO is replaced by C 10 DMPO, the surface density displays a maximum at intermediate times. This overcrowding causes the surface tension to exhibit a temporary minimum. The presence of these extrema can be described using theoretical framework which includes finite sorption kinetics. Reasonable agreement between theory and experiment for the surfactants in this study is achieved using only equilibrium adsorption parameters from each of the unary systems and independent measurements of the desorption kinetic coefficients; i.e. no adjustable parameters. These simple paradigms suggest a chemical strategy for temporal control of Marangoni stresses, which could be important in a variety of technologies including foam and emulsion formation and microfluidic applications.