Surfactant Self-Assembly ahead of the Contact Line on a Hydrophobic Surface and Its Implications for Wetting

Abstract

The carryover of surfactant ahead of contact lines on hydrophilic surfaces results in unusual wetting behavior of surfactant solutions, such as the autophobic effect. This carryover of surfactant ahead of the contact line is due to the strong interactions between the hydrophilic surface and the headgroup of ionic and nonionic surfactants. However, in the case of a nonionic surfactant and a hydrophobic surface, the possibility of self-assembly of surfactant ahead of the contact line has been neglected because of the absence of any obvious strong interactions and any unusual wetting behavior. In this paper, we provide evidence of carryover of a nonionic surfactant (octaethyleneoxide monododecyl ether, C12E8) ahead (i.e., on the solid−vapor, SV, side) of advancing and receding contact lines on hydrophobic self-assembled monolayers of octadecyltrichlorosilane. This system of a nonionic surfactant and hydrophobic surface was thought to be an unlikely system to show the presence of surfactant ahead of the contact line. However, the presence of a region with higher wettablity ahead of advancing and receding contact lines in condensation figure experiments provides direct evidence of the self-assembly and carryover of nonionic surfactant ahead of the contact line even on a hydrophobic surface. More evidence of this carryover is provided by comparing advancing and receding contact-angle measurements, along with independently determined solid−liquid (SL) and liquid−vapor (LV) surface tensions. These measurements show that the local SV surface tension increases with an increasing bulk surfactant concentration. As a result of this increase in the SV surface tension on a hydrophobic surface, the contact angle is lower than expected from the reduction in the SL and LV surface tensions alone. This phenomenon is “the autophilic effect”, in contrast with the autophobic effect that results in a larger contact angle. Our results also invalidate the assumption of no surfactant ahead of the contact line, a basis commonly used to estimate adsorption on SL interfaces from contact-angle data.