(291c) A Molecular Dynamics Study of Superspreading

Trisiloxane surfactants are capable of causing the complete and rapid wetting of aqueous droplets on very hydrophobic substrates. The phenomena is so dramatic that it has been termed superspreading. The focus of this work is to understand the mechanism by which superspreading occurs. All-atom parallel molecular dynamics simulations with full electrostatic interactions have been performed. A spherical nanodroplet consisting of 9997 water molecules and 475 surfactant molecules is placed in the vicinity of a graphite substrate and allowed to spread freely at 298 K. In the case of the superspreading surfactant or TSE4, the surfactant is found to exclude water from the substrate by packing the trisiloxane tail groups tightly. To help illustrate the difference between conventional surfactants and superspreaders, a polyethoxlyate droplet has also been studied. In this case, the C12E4 surfactant molecules are unable to pack as tightly at the solid-liquid interface. This leads to a high-energy interface due in part to the fact that water molecules at this surface have fewer hydrogen bonds than those in the bulk. It is believed that the removal of water from this interface is a key to superspreading. We report the hydrogen bonding structure and density profiles of the solid-liquid interface, the area per molecule, and the spreading rate.