Distinct Behavior of SPC/E, TIP4P/2005, and TIP5P Water Adsorbates in Graphitic Pores

Knowledge of the mechanism of water adsorption in activated carbons is essential for the design of the adsorptive separation process because water molecules form clusters at strong sites within porous structures with subsequent growth to fill the entire pore space, interfering with the removal of undesired components from a gaseous mixture. To this end, computer simulation is a valuable tool to explore the mechanism of water adsorption to delineate the various processes occurring in a given pore. However, there exists many intermolecular potential models for water, each with its own advantages and disadvantages, and an obvious question is raised: do they describe the same macroscopic behavior (isotherm and isosteric heat) and the underlying microscopic mechanism? Three of the most common nonpolarizable, rigid force fields for water, SPC/E, TIP4P/2005, and TIP5P models, are chosen to investigate adsorption of water in graphitic pores. The first two models describe well the thermodynamic properties of the bulk liquid, while TIP5P properly models the tetrahedral structure of water to explicitly account for the two lone pairs of electrons, in contrast to the planar structure of the first two models. With respect to modeling of graphitic pores, we chose two models, one of which has an infinite extent along the directions parallel to the pore walls and the other has a finite extent to explore the effects of the pore opening on the adsorption behavior. It was found that with the appropriate choice of temperatures for each water potential model, only the TIP5P model in finite graphitic pores captures the second-order pore filling mechanism of water in microporous carbons as observed experimentally, suggesting the importance of the molecular shape in the packing of the adsorbate and its role in properly describing the adsorption isotherm and isosteric heat.