Bulk and interfacial thermodynamics of ammonia, water and their mixtures

Ammonia is a promising energy carrier for the green transition, but its hygroscopicity and toxicity necessitate in-depth understanding of its interaction with water. This work examines the bulk and interfacial thermodynamics of the ammonia–water system. Parameters for three equations of state are fitted to experimental data and compared to parameters from literature: PC-SAFT, Cubic Plus Association and Peng–Robinson. Peng–Robinson stands out as most accurate for bulk thermodynamics. Introducing a temperature-dependent volume shift for water with Peng–Robinson yields a highly accurate model without introducing problematic inconsistencies, with errors of 0.05% for saturation pressures, and 0.5% for liquid densities. For the mixture, Peng–Robinson with a two-parameter Huron–Vidal mixing rule reproduces measurements mostly within their uncertainties, whereas the standard mixing rules for PC-SAFT and CPA are less accurate. A literature review of surface tension measurements of ammonia–water mixtures reveals that accurate measurements exist only at ambient temperature. We apply density gradient theory and density functional theory based on PC-SAFT, finding that both models fail at reproducing qualitative features of the surface tensions and adsorptions of dilute solutions of aqueous ammonia. Whereas bulk properties are well characterized, understanding and describing the interfacial thermodynamics of the ammonia–water system demands more work both on the experimental and modeling side.