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

Carlos Bistafa,Hiroki Kusudo,Donatas Surblys,Yasutaka Yamaguchi

doi:10.1063/5.0056718

Carlos Bistafa, Hiroki Kusudo + Show 2 more

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https://doi.org/10.1063/5.0056718

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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 ρA OH, ranging from a non-hydroxylated 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 θYD and the apparent one θapp, both indicating the enhancement of wettability by an increase in ρA OH, presented good quantitative agreement, especially for non-hydroxylated and highly hydroxylated surfaces. On partially hydroxylated surfaces, in which θYD 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.

Highlights

Silica (SiO2) is a common substance found in nature, in particular, in the sand, and used since ancient times for the production of glass
As ρOAH increases, the interaction of water molecules with the surface becomes larger, so the phantom wall must exert a larger force to lift the liquid film. This is very clear in Fig. 3: The curve for the non-hydroxylated silica surface (0 nm−2, purple full circles) presents a peak around 30 × 107 N/m2; the peak for the subsequent cases is displaced to larger values of z and presents a monotonic increase, with values of ∼35, 40, and 45 × 107 N/m2 for the intermediate cases of ρOAH = 1.175, 2.350, and 3.525 nm−2, respectively, culminating to the value of about 50 × 107 N/m2 for the case of the fully hydroxylated surface
Different OH area density values were considered, and two theoretical methods were used: thermodynamic integration implemented by the phantom wall and Molecular dynamics (MD) simulations of water droplets on silica surfaces

Summary

Introduction

Silica (SiO2) is a common substance found in nature, in particular, in the sand, and used since ancient times for the production of glass. Besides being the precursor of glass and silicon, more recent applications of silica include fillers, catalysts, and catalyst supports, and one of the most common products is silica gel, which, nowadays, spreads as a desiccant for industrialized food, making use of its efficient interaction with water. A recent approach suggests a possible use of silica nanopores as more efficient desiccant dehumidifiers, in which water is trapped in the nanopore as a liquid.. A recent approach suggests a possible use of silica nanopores as more efficient desiccant dehumidifiers, in which water is trapped in the nanopore as a liquid.1 In such applications, the surface property plays a key role, and thermal and chemical processes can be used to modify the level of hydroxylation on silica surfaces by replacing siloxane bridges (≡Si–O–Si≡). There is experimental evidence that the silanol group distribution on the silica surfaces is not homogeneous, but rather in patches, while the effects of such inhomogeniety on the wetting behavior are not clear

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