Water adsorption on hydrophilic mesoporous and plane silica substrates: A grand canonical Monte Carlo simulation study

Abstract

The adsorption properties of water on silica surfaces at 300 K are evaluated by way of Grand Canonical Monte Carlo simulations, a well suited technique to investigate thermodynamical properties of interfacial fluids. The PN-TrAZ potential is used to describe atomic interactions between water and substrate, and the SPC model for water-water interactions. To measure confinement effects, we compare adsorption on different plane surfaces to a realistic disordered mesoporous material 1 previously obtained by off lattice reconstructions known to reproduce in a realistic way the geometrical complexity of real high specific surface Vycor, in terms of surface area, chord distribution, and correlation peak in small angle neutron scattering spectra. The adsorption isotherm, and isosteric heat of adsorption are calculated and compared to experimental data found in literature for Vycor. They show good agreement, the PN-TrAZ potential for adsorbate-surface interaction being able to give a satisfactory description of hydrophilic properties of plane and curved silica surfaces. For pressure less than 0.65 times the vapor pressure of SPC water at 300 K, the model shows identical adsorption properties for plane and mesoporous surfaces, validating the t-plot concept for such materials, i.e. the fact that the amount of fluid adsorbed is proportional to the surface area of the substrate irrespective of the pore size, as long as the surface chemistry (the OH density in this case) is kept constant. However, for very low relative pressure (10 -3 ) an analysis of the isosteric heat of adsorption shows the presence of highly energetic sites on the mesoporous Vycor sample probably related to curvature, defects or roughness absent from perfectly plane surfaces. For relative pressure higher than 0.65 a sudden rise in Vycor adsorption is the signature of capillary condensation in the mesoporosity of the material, in agreement with experimental measurements in Vycor. Such a rise is absent on plane surfaces as expected for this confinement-induced capillary condensation.

Molecular simulations of water in hydrophobic microporous solids

“…Molecular simulations is a well suitable to gain a profound understanding of dynamical, structural or thermodynamic properties of polar and non polar fluids confined in nanopores (Auerbach et al 2003;Bussai et al, 2001Bussai et al, , 2002Bussai et al, , 2003Demontis et al 2003;Fleys et al 2004;Desbiens et al 2005;Douguet et al 1996;Fleys and Thompson 2005;Ramachandran et al 2006;Puibasset and Pellenq 2003a).…”

Temperature Effects on Tensile and Compressive Mechanical Behaviors of C-S-H Structure via Atomic Simulation

“…Based on that, the intralayer calcium atoms are allowed to relax by energy minimization using ClayFF potential, followed by all the other atoms and the supercell dimensions. Grand Canonical Monte Carlo method [13] is used to simulate the water molecules adsorption using the same potential at room temperature. The final C-S-H with the chemical composition (CaO) 1.67 (SiO 2 )(H 2 O) 1.69 , the density of 2.45 g/cm 3 , and the reasonable distribution ( 0 = 11.6%, 1 = 65.1%, 2 = 23.3%) is obtained, which is in good agreement with the results from neutral scattering tests [6].…”

Adsorption of Methylene Fluoride and Methylene Chloride at the Surface of Ice under Tropospheric Conditions: A Grand Canonical Monte Carlo Simulation Study

“…Thus, performing a set of GCMC simulations in which the chemical potential of the adsorbate molecules is systematically varied, the number of the adsorbed molecules per surface unit vs. the chemical potential, i.e., the adsorption isotherm can easily be determined from extremely low pressures up to the point of condensation. Indeed, the GCMC method has been successfully applied to calculate the adsorption isotherms of water and other small molecules at various different solid surfaces, such as at carbonaceous materials, 15-23 self-assembled monolayers, 24,25 covalent organic frameworks, [26][27][28] protein crystals, 29 metal oxides, [30][31][32][33] zeolites, [34][35][36][37][38][39][40][41] kaolinite, [42][43][44] and ice. [45][46][47][48][49][50][51][52][53] Further, besides the adsorption isotherms themselves, the structure and energetics of the adsorption layer can also be analyzed in detail in such simulations.…”