Dipole moment, hydrogen bonding and IR spectrum of confined water.

Drastic changes can take place in the structure, dynamics, and thermodynamics of a fluid when it is confined to spaces of molecular dimensions, as compared to its bulk counterpart. Water confinement in the nanometer-scale channels and pores of inorganic open framework materials, such as zeolites, are of great scientific interest. Zeolites are crystalline aluminosilicates, with various controlled pore sizes and connectivities. From a practical point of view, water plays a key role in many applications, including ion-exchange and separation. From a more general point of view the interaction of water with solid surfaces is of key importance for many chemical and physical processes. However, our present understanding of interfacial or confined water at the molecular level is still very limited. Most of the experimental techniques are difficult to carry out in nanometer environments. Theoretical investigation is, thus, a great help for a better understanding of confined water properties. Classical simulations have recently been performed aimed at understanding the structure, dynamics, and thermodynamics of water confined in carbon micropores or nanotubes and zeolites. However, the quantum nature of bonding in water can only be captured by ab initio calculations. Elaborate treatments of the electronic density are usually limited to small clusters (or a few adsorbed molecules) and restricted to equilibrium structures. The Car–Parrinello molecular dynamics method (CPMD) offers an alternative route to capture the electronic properties of molecules, as well as dynamical effects. Such DFT-based ab initio molecular dynamics calculations have been already applied to bulk water, interfacial water on Si surfaces, and zeolitic materials. Most of the CPMD studies of confined water deal with the dynamics of one-dimensional water chains and helices in hydrophilic zeolites with narrow, non-connected, cylinder pores. Herein we use this method for the first time to investigate water confined in a hydrophobic zeolite with large interconnected pores. The LTA zeolite displays supercages of diameter 13 <, connected to one another in a cubic symmetry by 8-ring windows of diameter 7 <, and smaller sodalite cages of diameter 7 <. This zeolite is, thus, a good candidate for studying the effect of confinement in open structure hydrophobic nanoporous materials. We have studied the thermodynamic and electronic properties, and the vibrational infrared spectrum of the confined water. It is worth noticing that while infrared measurements are a powerful tool to explore the structure of confined fluids, IR spectra are often complicated to interpret. Vibrational spectra of confined fluids have been computed some years ago using normal mode analysis on minimal-energy structures or using polarizable force fields and electrooptical models in classical molecular dynamics simulations. Most studies use the velocity auto-correlation function that provides vibrational frequencies but not IR intensities, resulting in a quite different vision of the overall spectrum. This is the first time, to our knowledge, that the dipole moments and IR spectrum of confined water are calculated from ab initio dynamics. This technique was developed recently, based on an estimator of the dipole moment in periodic boundary conditions, and was shown to successfully reproduce infrared spectra of aqueous systems. We have first computed the water adsorption isotherm at 300 K following the Grand Canonical Monte Carlo methodology of Desbiens et al. 22] As it was experimentally observed on other siliceous zeolites, the all-silica LTA is found to be hydrophobic, with liquid intrusion occurring at 70 MPa. The maximum loading is found at 20 water molecules per unit cell. Starting from these classical GCMC configurations we have performed CPMD simulations using plane-wave basis set and norm-conserving pseudopotentials on siliceous LTA with various hydration rates. The effect of confinement on the molecular dipole of water is shown in Figure 1 for two molecular loadings: 15 and 20 H2O molecules per unit cell of zeolite (one supercage plus one sodalite cage). For comparison, the water dipole of the gasphase water molecule and the distribution of dipole moments for 32 bulk water molecules computed using the same DFT and Car–Parrinello parameters are also plotted. The present bulk data are consistent with the original results of Silvestrelli et al. The dipole distribution for 20 confined water molecules has a maximum at 2.9 D, close to the bulk water maximum found at 3.1 D, and displays a smaller band around 2 D, close to the gas-phase value of 1.8 D. This latter peak corresponds to the water molecule located in the small sodalite cage. This seems to show that the polarization induced by the zeolite