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

Abstract

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 χ_p 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 χ_p = 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 χ_p = 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 χ_p = 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 χ_p = 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 χ_p = 0.5 indicate that the nanoscale wetting behavior of the SAM is sensitive to its nanostructure.