PC-SAFT-TS-CAF: a revised two state model for pure water and binary aqueous mixtures

Water is one of the most abundant substances on earth, but it is still not entirely understood. >50 of water’s properties show unusual behavior with characteristic extrema, which indicate two distinct regions, where water either behaves like a simple fluid or has anomalous behavior. A possible explanation is the two-state theory of water, that proposes that water forms different clusters, one with high density and one with low density which may even form two distinct phases at low temperatures.

In a previous work we have derived the framework to incorporate the two-state theory of water in the Statistical-Associating-Fluid-Theory (SAFT), by assuming water is an ideal solution of high-density water molecules (HDW) and low-density water molecules (LDW) which are in chemical equilibrium, thus re-formulating the association term. We have incorporated the new association term in the context of the Perturbed Chain – SAFT (PC-SAFT), arriving to a two-state SAFT Equation of State, PC-SAFT-Two-State (PC-SAFT-TS).

In this work, we are building upon our previous work and revising the PC-SAFT-TS, by assuming the equilibrium constant between HDW and LDW is only temperature dependent and by fitting specific cross-association parameters between HDW and LDW. The new model, PC-SAFT-TS-cross-association-fitted (CAF) can predict the characteristic extrema of water where water shows anomalous behavior and is superior to the original PC-SAFT. Binary aqueous mixtures are studied with this new framework (water-hydrocarbons and water-alcohols) and PC-SAFT-TS-CAF manages to a) capture the solubility minimum of hydrocarbons in water in a straightforward manner and b) improve water-alcohol vapor-liquid and liquid-liquid equilibria at the temperature range where water behaves as an anomalous liquid.