Why do aqueous surfactant solutions spread over hydrophobic substrates?

Spreading of aqueous surfactant solution droplets over hydrophobic substrates proceeds in one slow stage at concentration of surfactants below some critical value and in two stages if the surfactant concentration is above the critical value: the fast and relatively short first stage is followed by a slower second stage. It is shown that the kinetics of a slow spreading at concentrations below the critical value and the second stage at concentrations above the critical value are determined by a transfer of surfactant molecules on a bare hydrophobic substrate in front of the moving three-phase contact line (autophilic phenomenon). The latter process results in an increase of the solid-vapour interfacial tension of the hydrophobic solid surface in front of the moving three-phase contact line and spreading as a result. It is proven that the adsorption of surfactant molecules in front of the moving three-phase contact line results in a decrease of the total free energy of the droplet. Hence, the adsorption of surfactants molecules on a bare hydrophobic substrate in front of the moving three-phase contact line is a spontaneous process in spite of an increase of the local solid-vapour interfacial tension. The duration of the first stage of spreading in the case of the surfactant concentration above the critical value correlates well with the duration of adsorption of surfactant molecules onto a liquid-vapour interface. The latter allows assuming that the adsorption on the liquid-vapour interface is the driving mechanism of spreading during the first fast stage of spreading at surfactant concentrations above the critical value. It is discussed why the first stage of spreading does not take place in the case of surfactant concentrations below the critical concentration in spite of the longer duration of adsorption on liquid-vapour interface in this case.