Influence of Hydrated Silica Surfaces on Interfacial Water in the Presence of Clathrate Hydrate Forming Gases

Abstract

We study the hydrated silica−water interface in the presence of methane or carbon dioxide gas with molecular dynamics simulations. The simulations are performed with a limited amount of water, which forms a meniscus between two hydroxylated silica surfaces separated by 40 to 60 Å. Simulations were performed with the remaining space of the simulation cell left empty or filled with different numbers of methane or carbon dioxide gas molecules. The meniscus is used to determine the contact angle between water and silica in the absence and presence of the gases. The distribution profiles of the water and gas phases are determined over the duration of the simulation. The water number density in the layers adjacent to the silica is higher, and these layers are more structured and less mobile compared with water layers far from the surface. Additionally, the concentrations of the gases are significantly higher at the liquid and silica interfaces than in other locations in the gas phase. We speculate that the enhanced concentration of gases at the interface along with the extended contact area (curved meniscus compared with flat interface in the absence of silica surfaces) between water and guest molecules at the meniscus as well as lesser mobility of water molecules near the silica surface may provide a mechanism for the heterogeneous nucleation of the clathrate hydrate in water-wetting porous medium.

Molecular Simulation of CO2- and CO3-Brine-Mineral Systems

“…In the presence of CO 2 , to our knowledge, only two atomistic simulations have probed adsorbed water films, on forsterite (Kerisit et al 2012) and quartz (Bagherzadeh et al 2012). In atomistic simulations, film thickness can be controlled by imposing the vapor pressure of water in the gas phase (Stöckelmann and Hentschke 1999;Rahaman et al 2008;Malani and Ayappa 2009), the total number of water molecules in the simulation cell (Romero-Vargas Castrillón et al 2011;Kerisit et al 2012), or the radius of curvature r m of a bubble of water vapor or CO 2 in the simulated system (Wensink et al 2000;Bagherzadeh et al 2012). This last property is related to the relative vapor pressure P/P sat through the Kelvin equation (Wensink et al 2000): where γ wg is the interfacial tension between liquid water and the non-wetting phase (CO 2 or water vapor) and V m is the molar volume of water.…”

“…The uncharged silica surface (with fully protonated silanol groups) simulated by Leroch and Wendland (2012) is representative of acidic conditions, the point of zero net charge of silica being located near pH 3 (Wang et al 2012b). Figure 11b includes data obtained in systems where P/P sat < 1 was imposed by placing a bubble of water vapor (Wensink et al 2000;Bagherzadeh et al 2012) or CO 2 (Bagherzadeh et al 2012) in contact with the adsorbed water film. Experimental and simulation results show that at P/P sat ~ 0.7, negatively charged)silica surfaces carry two water monolayers (Asay et al 2009), uncharged silica surfaces (representative of pH ~ 3) carry one Figure 11.…”

“…The figure shows that adsorbed water molecules are attracted to the forsterite surface because of their affinity for structural Mg 2+ ions (light gray spheres). (b) Compilation of MD and GCMC simulation predictions of the thickness of adsorbed water films on flat mineral surfaces at T = 298 to 300 K on silica (Bagherzadeh et al 2012;Leroch and Wendland 2012;Wensink et al 2000), calcite (Rahaman et al 2008), and mica (Malani and Ayappa 2009). Values of film thickness and P/P sat in the simulations of Wensink et al (2000) and Bagherzadeh et al (2012) were estimated by visual inspection of MD simulation snapshots and by applying Equation (1) with γ gw = 54.7 mN m −1 and r m = 1.1 ± 0.2 nm (Wensink et al 2000) or with γ gw = 80.1 or 70.1 mN m −1 (in the absence or presence of CO 2 , respectively) and r m = 1.93 ± 0.4 nm (Bagherzadeh et al 2012) [γ gw values were roughly estimated from the results of Vega and deMiguel (2007) water monolayer (Leroch and Wendland 2012), and uncharged silica surfaces exposed to a bulk CO 2 fluid carry zero water monolayers (Bagherzadeh et al 2012).…”

“…These interfacial tensions determine the wetting angle through the well-known Young equation: In the absence of a well-tested set of force fields for CO 2 -water-mineral wettability studies, Iglauer et al (2012b) used a combination of CO 2 and water models (EPM2, TIP4P/2005) that predicts γ wg within ~10 mN m −1 (Nielsen et al 2012), but they selected a silica-water interaction model (van Beest et al 1990) that overestimates the strength of the silica-water interaction (in the absence of CO 2 , they predicted θ = 0° for water on a silica surface that carries only siloxane functional groups, whereas the experimental value for a fully dehydroxylated silica surface is about 42°; Lamb and Furlong 1982). Bagherzadeh et al (2012) used a water model (TIP4P/Ice) that significantly overestimates the surface tension of liquid water (Vega and de Miguel 2007) and a silica-water interaction model (Lopes et al 2006) that was not designed for use with the TIP4P/Ice water model and that performs less than optimally when used with at least one other water model (Skelton et al 2011). In recent years, the CLAYFF model (Cygan et al 2004b) has emerged as a remarkably accurate model of the interaction between silicate minerals and liquid water (Bourg and Sposito 2010;Ferrage et al 2011;Marry et al 2011;Skelton et al 2011;Bourg and Steefel 2012;Leroch and Wendland 2012), but this model has been exclusively used with two water models (the SPC and SPC/E water models; Berendsen et al 1981Berendsen et al , 1987) that underestimate the surface tension of liquid water (Vega and de Miguel 2007;Nielsen et al 2012).…”

Dynamic measurements of hydrate based gas separation in cooled silica gel

“…Linga et al also found that the gas uptake was found to be significantly higher for the CO 2 /H 2 /C 3 H 8 systems in a fixed bed column compared to a stirred tank reactor [32]. Recently, molecular dynamics (MD) simulations are carried out for the investigation of hydrate formation in silica [33].…”