Molecular dynamics simulations of the wetting behavior of nanodroplets of water and aqueous surfactant solutions on solid substrates of varying surface energy

Trisiloxane surfactants have the unique ability to promote the rapid and complete wetting of aqueous drops on hydrophobic, hydrocarbon substrates. Molecular dynamics simulations have been conducted to elucidate the mechansim by which these surfactants operate.;The substrates used in this work are graphite and self-assembled monolayers (SAMs) of varying surface energy. Before investigating aqueous surfactant droplets, pure droplets of water on SAMs composed of 1-undecanethiol (CH 3(CH2)10SH) and 11-mercapto-1-undecanol (HOCH 2(CH2)10SH) on Au(111) are considered. With chi p denoting the mole fraction of HOCH2-terminated chains, the equilibrium contact angles of water droplets consisting of 4000 molecules on homogeneous monolayers are reported for chip = 0, 0.25, 0.5, 0.75, and 1. Good agreement is seen between the simulation results and experimental data. Phase separated monolayers are also considered. The nanoscale wetting behavior of the SAM is found to be sensitive to its nanostructure.;Wetting simulations of droplets of aqueous trisiloxane (M(D'E4OH)M) and alkyl polyethoyxlate (C12E4) solutions were conducted. Spherical and cylindrical droplet shapes were considered. While the C 12E4 systems showed a behavior that is consistent with a simple wetting theory based on the Young equation, the trisiloxane systems showed a different behavior. For initial surface concentrations above the maximum packing concentration the M(D'E4OH)M/H2O droplet did not spread appreciably. When the initial surface concentration was below the maximum packing concentration the trisiloxane droplet showed increased spreading but the final contact angle was still large.;The interfacial and bulk properties of aqueous trisiloxane and alkyl polyethoxylate surfactant solutions were investigated. The surfactant molecules of a 35.2 wt% M(D'E4OH)M solution, which were randomly distributed throughout the solution at initialization, were found to self-assemble into a bilayer in less than 50 ns at 375 K. The alkyl polyethyoxylate surfactant was also found to form a bilayer on the same time scale. The adsorption isotherms at 298.15 K for each surfactant system were determined. In the case of the M(D'E4OH)M monolayer, the simulation results were found to overestimate the experimental values while for C12E4 the computed tensions are found to be within one standard deviation of the experimental data for each surface concentration considered.