Solvation processes in steam: Ab initio calculations of ion–solvent structures and clustering equilibria

Abstract

Reports of the high ion content of steam and low-density supercritical fluids date back to the work of Carlon [Carlon H. R. (1980) Ion content of air humidified by boiling water. J. Appl.Phys.51, 171–173], who invoked ion and neutral-water clustering as mechanism to explain why ions partition into the low-density aqueous phase. Mass spectrometric, vibrational spectroscopic measurements and quantum chemical calculations have refined this concept by proposing strongly bound ion–solvent aggregates and water clusters such as Eigen- and Zundel-type proton clusters H 3O +·(H 2O) m and the more weakly bound water oligomers (H 2O) m . The extent to which these clusters affect fluid chemistry is determined by their abundance, however, little is known regarding the stability of such moieties in natural low-density high-temperature fluids. Here we report results from quantum chemical calculations using chemical-accuracy multi-level G3 (Curtiss–Pople) and CBS-Q theory (Peterson) to address this question. In particular, we have investigated the cluster structures and clustering equilibria for the ions H 3 O + · ( H 2 O ) m ( H 2 S ) n , NH 4 + · ( H 2 O ) m ( H 2 S ) n and H 3S +·(H 2O) m (H 2S) n , where m ⩽ 6 and n ⩽ 4, at 300–1000 K and 1 bar as well as under vapor–liquid equilibrium conditions between 300 and 646 K. We find that incremental hydration enthalpies and entropies derived from van’t Hoff analyses for the attachment of H 2O and H 2S onto H 3O +, NH 4 + and H 3S + are in excellent agreement with experimental values and that the addition of water to all three ions is energetically more favorable than solvation by H 2S. As clusters grow in size, the energetic trends of cluster hydration begin to reflect those for bulk H 2O liquids, i.e. calculated hydration enthalpies and entropies approach values characteristic of the condensation of bulk water (Δ H o = −44.0 kJ mol −1, Δ S o = −118.8 J K mol −1). Water and hydrogen sulfide cluster calculations at higher temperatures indicate that a significant fraction of H 3O +, NH 4 + and H 3S + ions exists as solvated moieties.