The structure and stability of metal water clusters in high temperature low density fluids

Abstract

Metal-bearing low-density aqueous fluids occur ubiquitously close to and on the Earthís surface over a wide range of temperatures ranging from atmospheric water vapor up to high-pressure supercritical steam. These volatile metal species, however, do not exist as ìanhydrousî moieties but as hydrated metal clusters. Our current understanding of the interaction of metals as well as metal halides with such water clusters in steam and/or low-density supercritical water and in magmatic gases is rudimentary. Quantum chemistry in combination with experiment can provide fundamental insight into the structures, corresponding energies and distribution of these metal water clusters in natural systems and play an important role in our understanding of solute-solvent interactions such as, for instance, in volatile-enriched ore fluids at high temperature. The work here presents new theoretical results for the clustering equilibrium constants K for the reaction M+ (H2O)n + H2O = M+ (H2O)n+1 (M = Co, Ni, Cu, Zn) in the temperature range 298-600K at 1bar and at water vapor saturation pressures for n≤6, using G3 and CBS-x ab initio theory procedures [1, 2]. Our newly calculated cluster geometries and values for K are in good agreement with those obtained from mass spectrometric measurements [3, 4] as well as those obtained from IR experiments [5]. For example, the stepwise binding enthalpies at 298K obtained in this study for n=0 and n=1 for the reaction Cu+ (H2O)n + H2O = Cu+ (H2O)n+1 are -159.1 kJ·mol-1 and -161.9 kJ·mol-1 , respectively, and are in good agreement with results from mass spectrometric measurements (∆H0,1 = -160.6±7.5 kJ·mol-1 and ∆H1,2 = -170.3±6.7 kJ·mol-1) [3]. Calculations for larger metal water clusters, i.e. for Cu+ (H2O)n, Zn+ (H2O)n and Ni+ (H2O)n with n≤6, indicate that stepwise binding enthalpies and entropies asymptotically approach values characteristic of bulk water (i.e., -44.0 kJ·mol-1 for the enthalpy and -118.8 J·K-1·mol-1 for the entropy of H2O condensation). [1] Mayhall et al. (2008) JCP 128, 144122. [2] Montgomery et al., 1999, JCP, 110, 2822. [3] Dalleska et al. (1994) JACS 116, 3519. [4] Magnera et al. (1989) JACS 111, 4100. [5] Iino et al. (2007) JCP 126, 194302.