Molecular Simulation of water confined in nanoporous Ca-silica
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1227-JJ08-05
Molecular Simulation of water confined in nanoporous Ca-silica Patrick A. Bonnaud1, Benoît Coasne2 and Roland J.-M. Pellenq1, 3 1 Centre Interdisciplinaire des Nanosciences de Marseille, Aix-Marseille Université, UPR 3118 CNRS, Campus de Luminy – case 913, 13288 Marseille cedex 9 France. 2 Institut Charles Gerhardt Montpellier, Université Montpellier 2, place E. Bataillon cc 003, 34095 Montpellier cedex 5 France. 3 Massachusetts Institute of Technology, Department of Civil & Environmental Engineering, 77 Massachusetts Ave., Cambridge, MA 02139, US.
ABSTRACT This paper reports on a molecular simulation study of the thermodynamics, structure and dynamics of water confined at ambient temperature in charged silica nanopores of a width H = 10 and 20 Å. The adsorption isotherms for water resemble those observed for experimental samples; the adsorbed amount increases continuously in the multilayer adsorption regime until a jump occurs due to capillary condensation of the fluid within the pore. Strong layering of water in the vicinity of the silica surfaces is observed as marked density oscillations are seen up to 8 Å from the surface in the density profiles for confined water. Our results also indicate that the Ca2+ counterions remain in a space close to the silica surface whatever the pore width and the adsorbed amount of water. For all pore sizes and adsorbed amounts, the self-diffusivity of confined water is lower than the bulk due to the strong hydrophilic nature of the pore surface. Our results also suggest that the self-diffusivity of confined water is sensitive to the adsorbed amount of water molecules. INTRODUCTION The behavior of confined and vicinal water significantly differs from the bulk [1,2,3]. As a result, important research efforts have been devoted to understand the effect of confinement and surface forces on the thermodynamics and dynamics of water [for a brief review, see ref. 4]. Among the important literature published on this topic, several authors have reported studies of water confined in nanoporous silicas with pores of a simple geometry such as slit pores [5,6,7,8,9] or cylindrical pores [10,11,12,13,14,15,16,17] or disordered porous networks such as Vycor [18,19,20,21,22,23,24,25]. Other authors have investigated the thermodynamics and dynamics of water and ions (electrolytes) confined in nanoporous materials [26,27,28,29,30]. In this paper, we investigate by means of molecular simulation the thermodynamics, dynamics, and structure of water confined in charged silica nanopores of a simple slit geometry. The negatively charged surface (with no OH surface groups) is compensated by Ca2+ counterions. Two pore sizes, H = 10 and 20 Å, are considered in order to address the effect of confinement. The thermodynamics of confined water is first studied using Grand Canonical Monte Carlo (GCMC) simulations, which allow us calculating the adsorption isotherm (adsorbed amount versus relative pressure or humidity) and isosteric heat of adsorption curves (differential enthalpy as a function of lo
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