Dynamics of Water Adsorption onto a Calcite Surface as a Function of Relative Humidity

Abstract

We have developed a molecular dynamics scheme in order to understand the dynamics of water adsorption on calcite surfaces as a function of relative humidity. In contrast to previous studies where either a monolayer or bulk water was assumed to cover the surface, we observe the formation of two to three prominent layers of water depending on the relative humidity. Due to the fact that these simulations are at room temperature, the distribution of water molecules on the surface is inhomogeneous and nonuniform. Our simulation results agree well with recent grazing incidence X-ray diffraction studies. The free energy of adsorption of a single water molecule onto the bare calcite (101̄4) surface is predicted to be −10.6 kcal/mol at room temperature while the enthalpy for the same process is −21.3 kcal/mol. The time scale for the bare calcite surface to become in dynamic equilibrium with water vapor at 100% relative humidity is determined to be close to 6 ns, and the adsorption follows a BET (Brunauer−Emmet−Teller)-like isotherm, in that multilayers form. From our orientational distribution functions, we are able to determine the existence of three binding modes for water. The mobility of water adsorbed on the calcite surface is greater in directions parallel to the surface. Motion perpendicular to the suface is slower. The diffusivity of water significantly increases with increasing relative humidity.

The energetics of nanophase calcite

“…Table 4 gives the summary of the water adsorption calorimetric data. The computed hydration energy value reported from various atomistic simulation techniques range between À79.2 to À138.9 kJ/mol (Liang et al, 1996;de Leeuw and Parker, 1998;Kerisit and Parker, 2004;Rahaman et al, 2008). The enthalpy of water adsorption determined from the water adsorption calorimetric measurements (À96.26 ± 0.96 kJ/mol) falls within the theoretically calculated range.…”

“…6) could possibly be due to distinct modes of water adsorption on different active sites on nanocalcite surface. Several molecular dynamic simulation studies on adsorbed water density distribution on calcite surface (Kerisit and Parker, 2004;Rahaman et al, 2008) have also indicated the existence of three different conformational modes for water adsorption on calcite surface. The two modes within the first layer have two hydrogen atoms of the water molecule pointing either up away from the surface or down toward a carbonate ion.…”

Detection of oxygen isotopic anomaly in terrestrial atmospheric carbonates and its implications to Mars

“…In the case of metal oxides (CaO, MgO, and Fe 2 O 3 ) and carbonates (CaCO 3 and MgCO 3 ), dissociation of water molecules at the interfaces is energetically favorable because hydroxylated surfaces are more stable than metal-terminated surfaces under ambient conditions (15,41,42). Recent experiments and molecular dynamic simulations have shown the free energy of adsorption of water on calcite surfaces (43,44) to be as low as −10.6 kcal mol −1 .…”

Experimental study and isotherm models of water vapor adsorption in shale rocks

“…Some previous studies by Chenevert (1970a,b), Kahr and Madsen (1995) have reported the effect of clay minerals on water adsorption capacity on reservoir rocks. Kerisit and Parker (2004), Rahaman et al (2008) and Schultz et al (2013) proposed that calcite is characterized by high surface area and weak affinity for water. Consequently, shale rocks with calcite will have a relatively low water capacity in the shale samples.…”