Wetting of hydrophobic substrates by nanodroplets of aqueous trisiloxane and alkyl polyethoxylate surfactant solutions

Trisiloxane surfactants at low concentrations promote the complete and rapid wetting of aqueous droplets on very hydrophobic (hydrocarbon) substrates. This behavior has not been demonstrated by any other surfactant which explains why the trisiloxanes are referred to as superspreaders. Despite many experimental and theoretical investigations the mechanism of superspreading is not fully understood. Molecular dynamics simulations using all-atom force fields have been conducted to attempt to elucidate the mechanism of superspreading. Spherical nanodroplets containing approximately 10,000 water molecules in the bulk and 475 surfactant molecules at the liquid-vapor interface were placed in the vicinity of a graphite substrate and allowed to spread freely at room temperature. In the trisiloxane case the droplet was found to spread very little, although randomly removing 175 surfactant molecules lowered the final contact angle from 110^° to 80^°. In contrast, an alkyl polyethoxylate surfactant-laden droplet was found to spread significantly further, with the equilibrium contact angle reaching 55^°. Similar results for the two surfactant systems were found for cylindrical nanodroplets spreading on a self-assembled monolayer (SAM). The reasons for the lack of spreading in the trisiloxane case and the simulation challenges associated with these systems are discussed. In support of our arguments we demonstrate that the surfactant molecules of an initially uniform aqueous trisiloxane solution self-assemble into a bilayer in tens of nanoseconds on a graphite substrate. Lastly, in a final set of simulations, neat trisiloxane droplets at 450 K are found to arrange into a layered structure on a methyl-terminated SAM and to form a sand pile-shape on a hydroxyl-terminated SAM.