Water on hydroxylated silica surfaces: Work of adhesion, interfacial entropy, and droplet wetting

Abstract

In the last few years, much attention has been devoted to the control of the wettability properties of surfaces modified with functional groups. Molecular dynamics (MD) simulation is one of the powerful tools for microscopic analysis providing visual images and mean geometrical shapes of the contact line, e.g., of nanoscale droplets on solid surfaces, while profound understanding of wetting demands quantitative evaluation of the solid-liquid (SL) interfacial tension. In the present work, we examined the wetting of water on neutral and regular hydroxylated silica surfaces with five different area densities of OH groups ρ OH A , ranging from a nonhydroxylated surface to a fully hydroxylated one through two theoretical methods: thermodynamic integration (TI) and MD simulations of quasi-two-dimensional equilibrium droplets. For the former, the work of adhesion needed to quasi-statically strip the water film off the solid surface was computed by the phantom wall TI scheme to evaluate the SL interfacial free energy, whereas for the latter, the apparent contact angle θapp was calculated from the droplet density distribution. The theoretical contact angle θ YD and the apparent one θapp, both indicating the enhancement of wettability by an increase in ρ OH A , presented good quantitative agreement, especially for non-hydroxylated and highly hydroxylated surfaces. On partially hydroxylated surfaces, in which θ YD and θapp slightly deviated, the Brownian motion of the droplet was suppressed, possibly due to the pinning of the contact line around the hydroxyl groups. Relations between work of adhesion, interfacial energy, and entropy loss were also analyzed, and their influence on the wettability was discussed.

Quantifying interfacial tensions of surface nanobubbles: How far can Young's equation explain?

“…Furthermore, we evaluated the apparent contact angle of the nanobubbles θ app and that of the nanodroplet θ droplet by least-squares circular fitting to a contour of the gas–liquid interface away from the wall. 30,33–35 Masuduzzaman and Kim reported the importance of the definition of the solid–liquid boundary position for the evaluation of the apparent contact angle. 41 In this study, the solid–liquid interface was defined as the position where the fluid density becomes zero which gives a consistent mechanical balance near the three-phase contact lines.…”

“…2. Specifically, the liquid–gas and solid–gas interfacial tensions were calculated based on Bakker's equation, 30–37 which describes the relationships between the fluid stress anisotropy and interfacial tensions. The solid–liquid interfacial tension was quantified by the thermodynamic integration (TI) method that calculates the minimum work needed to separate the liquid from the solid surface per area, which corresponds to the interfacial free energy.…”

“…The solid–liquid interfacial tension was quantified by the thermodynamic integration (TI) method that calculates the minimum work needed to separate the liquid from the solid surface per area, which corresponds to the interfacial free energy. 30,34,35,43 These types of calculation methods of interfacial tensions are called the “mechanical route” 32 and “thermodynamic route”, 30,34,35,43 respectively. The details of the methods used for the calculation of interfacial tensions are described in the Methods section.…”

“…This is an implementation of the TI method for the calculation of the solid–liquid interfacial free energy. 34,43…”

“…In this study, by integrating well-proven mechanical and thermodynamic insights 30–37 into MD simulations, we quantified the liquid–gas, solid–gas, and solid–liquid interfacial tensions of nitrogen surface nanobubbles at graphite/water interfaces. The resulting contact angles evaluated by substituting these three interfacial tensions into Young's equation agreed well with those measured from the apparent shapes of the surface nanobubbles.…”