Atomistic simulations of the wetting behavior of nanodroplets of water on homogeneous and phase separated self-assembled monolayers

The wetting behavior of water nanodroplets on homogeneous and phase separated self-assembled monolayers (SAMs) composed of CH3(CH2)10SH and HOCH2(CH2)10SH on Au(111) was studied using molecular dynamics simulation. A simple model is introduced for the SAM where only the top eight or ten atomic layers are considered. For the homogeneous monolayers the CH3– and HOCH2–terminated chains are uniformly mixed. With χp denoting the mole fraction of HOCH2-terminated chains, we report the equilibrium contact angles of water droplets composed of 4000 molecules on homogeneous monolayers for χp = 0, 0.25, 0.5, 0.75, and 1. Good agreement is seen between the simulation results and experimental data for millimetre-size drops. For χp = 0.5, the contact angle and base diameter of the droplet are found to be 51.9 ± 1.3° and 10.2 ± 0.2 nm, respectively. Large deviations from the Cassie model are observed while the Israelachvili-Gee equation gives a better prediction. For the monolayers with χp = 0 and 0.25, we estimate the line tension through the drop-size dependence of the contact angle and find good agreement with theoretical predictions. For the phase separated monolayers, the HOCH2-terminated chains are organized into randomly-located circular domains which are embedded in a background matrix of CH3-terminated chains. With χp = 0.5, we consider two domain sizes with the diameters of the small and large domains being 1 and 2 nm. The water contact angle is found to be 63.1° for the monolayer with small domains and 68.1° for the monolayer with large domains. The results for the homogeneous and phase separated monolayers with χp = 0.5 indicate that the nanoscale wetting behavior of the SAM is sensitive to its nanostructure.