Molecular Dynamics Study of the Influence of Surfactant Structure on Surfactant-Facilitated Spreading of Droplets on Solid Surfaces

摘要

The spreading of a partially wetting aqueous drop in air on a hydrophobic surface can be facilitated by the adsorption of surfactants from the drop phase onto the air/aqueous and aqueous/hydrophobic solid interfaces of the drop.At the contact line at which these interfaces meet,conventional surfactants with a linear alkyl hydrophobic chain attached to a polar group adsorb onto the surfaces,forming monolayers which remain distinct as they merge at the contact juncture.The adsorption causes a decrease in the interfacial tensions and reduction in the contact angle but the angle remains above zero so the drop is still nonwetting.Trisiloxane surfactants with a T-shaped geometry in which the hydrophobic group is composed of a trisiloxane oligomer with a polar group attached at the center of the chain can give rise to a zero contact angle at the contact line and complete wetting (superspreading).Experimental evidence suggests the adsorption of the T-shaped molecule,in addition to significantly decreasing the tensions of the interfaces (relative to the conventional surfactants),promotes the formation of a precursor film consisting of a surfactant bilayer at the contact line which facilitates the spreading.The aim of this study is to use molecular dynamics to examine if the T-shaped structure can promote spreading by the formation of a bilayer and to contrast this case with that of the linear chain surfactant where complex assembly does not occur.The simulation models the solvent as a monatomic liquid,the substrate as a particle lattice,and the surfactants as united atom structures,with all interactions given by Lennard-Jones potentials.We start with a base case in which the solvent partially wets a substrate comprised of a lattice of particles.We demonstrate that adsorbed T-shaped surfactant monolayers can,when the interaction between the solvent and the hydrophile particles is strong enough,assemble into a bilayer,allowing the drop to extend to a thin planar film.In the case of the flexible linear chain surfactant,there is no interaction between the monolayers on the two interfaces in the case of a strong hydrophile-solvent interaction and less coordination for a weaker interaction.In either case,the monolayers remain distinct,as the surfactant only marginally improves wetting.