Thermodynamic properties of ammonia–water mixtures in the vapor–liquid, critical, and Widom regions

Due to growing fertilizer demand and new energy applications, ammonia-water mixtures are poised to become even more important than they already are. Under milder conditions, high-accuracy equations of state adequately describe mixture properties, but data confirming their accuracy above 500 K and 10 MPa are lacking. Molecular simulations, which have historically focused on near-ambient conditions, are utilized in this effort to address this deficiency. The simulation methods are first validated against literature data, after which the critical properties of the pure fluids and seven binary mixtures are estimated. For the estimates, a simple, theory-based technique applying critical scaling laws is developed, which is particularly suitable for data from molecular simulation or other methods that yield the chemical potential. The ascertained critical parameters are used to study the Widom region, where six thermodynamic response functions are sampled to locate the Widom line. Thermodynamic data are thus captured from the triple point to the critical point and beyond, exceeding 1000 K and 110 MPa. Qualitative structural insight is offered in the form of simulation snapshots, alluding to density fluctuations at the critical point, which gradually vanish after reaching five times the critical pressure.