Ion association in concentrated NaCl brines from ambient to supercritical conditions: results from classical molecular dynamics simulations

Abstract

Highly concentrated NaCl brines are important geothermal fluids; chloride complexation of metals in such brines increases the solubility of minerals and plays a fundamental role in the genesis of hydrothermal ore deposits. There is experimental evidence that the molecular nature of the NaCl–water system changes over the pressure–temperature range of the Earth's crust. A transition of concentrated NaCl–H₂O brines to a ″hydrous molten salt″ at high P and T has been argued to stabilize an aqueous fluid phase in the deep crust.

In this work, we have done molecular dynamic simulations using classical potentials to determine the nature of concentrated (0.5–16 m) NaCl–water mixtures under ambient (25 °C, 1 bar), hydrothermal (325 °C, 1 kbar) and deep crustal (625 °C, 15 kbar) conditions. We used the well-established SPCE model for water together with the Smith and Dang Lennard-Jones potentials for the ions (J. Chem. Phys., 1994, 100, 3757). With increasing temperature at 1 kbar, the dielectric constant of water decreases to give extensive ion-association and the formation of polyatomic (Na_nCl_m)^n−m clusters in addition to simple NaCl ion pairs. Large polyatomic (Na_nCl_m)^n−m clusters resemble what would be expected in a hydrous NaCl melt in which water and NaCl were completely miscible. Although ion association decreases with pressure, temperatures of 625 °C are not enough to overcome pressures of 15 kbar; consequently, there is still enhanced Na–Cl association in brines under deep crustal conditions.