ИСТИНА

We study three amphiphilic fragrances: benzyl acetate, linalool, and citronellol, which behave as surfactants on air/ water interface. All three possess low but appreciable saturated vapor pressures and solubilities in water. We report experimental data for the time relaxation of the surface tension in the regimes of (i) adsorption from vapor toward the surface of a pendant drop of water, and (ii) evaporation from a pre-saturated pendant drop (aqueous solution). The results are analysed theoretically, by combining them with separately obtained equilibrium surface tension isotherms and the equations of state, that relate the surface tension with the adsorption. The theoretical description is based on numerical solution of the diffusion equations in the bulk, coupled with boundary conditions which take into account the existence of kinetic barriers for the transfer to/ from the drop interface by mass exchange with the contiguous sub-surface. Thus, it is shown that the mixed barrier-diffusion control in the vapor and the diffusion control in the drop describe well the experimental data in the first regime, adsorption from gas phase onto an aqueous drop. The respective rate constant kv,ads quantifies the effect of barriers. The regime of evaporation from a pendant drop of aqueous solution follows a convection-enhanced adsorption mechanism from the drop interior, with a desorption rate constant in the gas affected by the simultaneous evaporation of water and the amphiphile. The detailed theory is fully compliant with a simplified „engineering” approach that describes the overall transport kinetics by assigning an effective mass transfer coefficient. The latter is stipulated to proportionate the flux to the gradient in the solute concentration; this model is in accord with the measured data for the dynamic surface tension.