Dynamic wetting and dewetting by aqueous solutions containing amphiphilic compounds

Abstract

The kinetics of wetting and dewetting of a glass surface by an aqueous solution containing an amphiphilic compound, C₁₈OE₈₄, has been investigated by following the capillary rise of the liquid. Typically, an overshoot of the liquid entering the capillary was observed, before relaxation towards the equilibrium height occurred. At concentrations much below and above the critical micellar concentration (cmc) the magnitude of the overshoot is small, while at intermediate concentrations (close to the cmc) the overshoot is more pronounced. The kinetics of the relaxation towards the equilibrium height depends on the concentration, with an increase in the relaxation rate at concentrations above the cmc. The observations are explained by a reduced (nonequilibrium) surface excess of the surfactant at the liquid/vapor interface. The transport of surfactant molecules to this surface depends on parameters such as concentration and diffusion constants (molecular weights). Since the concentration of the surfactant used is low, owing to the low cmc, the actual concentration in the capillary can be significantly lower as the surfactant is depleted from solution owing to adsorption on the walls of the capillary. To account for this effect, the adsorption isotherm for the solid/liquid interfaces was determined by ellipsometry. At high concentrations the change in the height of the capillary rise with time has a smooth profile, while for low and intermediate concentrations (below and close to the cmc) the behavior is more complex. An abrupt stop at a height corresponding to a certain value of the surface tension is observed. This behavior is discussed in terms of dynamic contact angle and surface tension effects due to the kinetics of surfactant adsorption that occurs close to the three-phase line at the solid/vapor and solid/liquid interfaces. The wetting kinetics varies significantly, depending on the preparation and treatment of the capillaries before the experiment.