Bicanonical Monte Carlo simulation of the structural properties of Cl −(H 2O) N clusters using entropy data based model

Abstract

The entropy and the structural properties of chloride anion-water clusters changes upon varying their size have been investigated from Monte Carlo simulation results on the chloride ion hydration shell formation at room temperature and at the temperature typical of polar stratosphere. The interaction model includes water-water ST2 [F.H. Stillinger, A. Rahman, J. Chem. Phys., 60 (1974) 1545. [1]] potential and our ion-water potential, which takes SPC [H.J.C. Berendsen, J.P. M Postma, W.F. van Gunsteren, J. Hermans, in B. Pullman (Eds.), Intermolecular Forces, Reidel, 1981, p. 331. [2]] three-site geometry. Also the interaction model takes into account the polarizations of an ion and molecules in the field of the ion. The parameters of ion–water interaction potential are fitted to the experimental data on free energies and entropy of the first attachment reactions of water molecules to a cluster in vapour. The effect of ousting the ion to the surface of the cluster is observed even after switching off many-body polarization interaction. The non-monotonous behaviour of the entropy of small clusters at stratospheric temperatures as a function of cluster size is a manifestation of ‘magic numbers’ effect. At low pressures the hydration shell consists of a single layer and only in the vapour close to saturation the formation of different hydration spheres begins. The overall anion hydration shell has pronounced radial and orientational flaky structures. Establishing orientational order in the cluster has cooperative character and is influenced to an essential degree by hydrogen bonds between the molecules. The presence of the ion reduces the length of most probable water molecules rings.