Dynamic of solitary water in hydrophobic solvents

Abstract

Structural and dynamical properties of D 2O infinitely dilute in supercritical CO 2 have been studied by molecular dynamics simulations. A new intermolecular potential model taking explicitly into account electron donor acceptor (EDA) interactions between water and CO 2 has been developed from ab initio calculations. The analysis of the local ordering between water and CO 2 from simulated radial distribution functions put in evidence specific short-ranged EDA complexes. Taking xenon as a reference inert solvent in which the re-orientational dynamics of D 2O is quasi-free, we comparatively showed that the rotational dynamics of water is weakly anisotropic due to the EDA interactions, which affect more specifically the re-orientational motions of the main symmetry axis. These results have been used to assess the contribution of the vibrational relaxation in the experimental mid-infrared (MIR) profiles associated with the ν 1 symmetric and ν 3 anti-symmetric stretching modes of D 2O. It is found that only the rotational dynamics contribute to the broadening of the IR profiles. However, vibrational processes contribute to the red-shifts of the band centres and the relative intensity enhancements of the ν 1 and ν 3 modes of D 2O. Comparison with another existing model reveals that EDA interactions have to be taken into account to reproduce MIR and FIR measurements. We argue that vibrational relaxation plays an increasing role in the genesis of the MIR profiles as the hydrophilic character of the solvent increases. From this point of view, CO 2 can be classified as an intermediate solvent between xenon and carbon tetrachloride on a hydrophilic solvent scale based upon the solubility criterion.