Kinetics of Triton-X100 Transfer Across the Water/Dodecane Interface: Analysis of the Interfacial Tension Variation

The interfacial tension of water/dodecane interface is investigated by the pendant drop technique during the transfer of a surfactant molecule, the Triton X-100. This molecule is initially present in one of the liquid phases, and the measurement of the interfacial tension as a function of time permits us to probe the dynamic incoming and outgoing surfactant at the interface and to deduce a rate of transfer. As a rule, after the contact of both solutions, the interfacial tension exhibits a steep initial decrease, passes through a minimum, and then levels off at a value that depends on the initial surfactant concentration in the drop. The time delay before reaching the equilibrium state is due to the limitant step of the surfactant crossing over the liquid/liquid interface. Moreover, the interfacial tension evolution depends strongly on the phase where the Triton-X100 is initially dispersed. Indeed, the global kinetics is found to be faster when Triton X-100 transfers from dodecane toward water as compared to the reverse pathway. To quantify the limiting factors of the interfacial transfer mechanism, two theoretical models describing the time evolution of the system are presented and discussed. The first one is based on a diffusion-controlled adsorption/desorption at the liquid/liquid interface, whereas the second one also takes into account an interfacial chemical adsorption/desorption reaction. This latter permits us to give an estimation of importance of the chemical processes, which occurs at the interface in the overall kinetics.