Applying SAFT-type models for the anomalous properties of water: Successes and challenges

Water is the most important, but at the same time the most anomalous substance on earth. Due to its anomalies it is a challenge even for advanced equations of state like PC-SAFT, CPA and SAFT-VR Mie to predict water’s anomalous properties. It is believed by some researchers that water’s anomalous properties are the result of structural fluctuations. The recently developed DAPT model is consistent with this idea and it is actually able to predict a considerable amount of water’s anomalies, like the density maximum and speed of sound maximum, which is promising, but we show in this work that it produces also some unrealistic results like minima and maxima for isochoric heat capacity. Also DAPT’s accuracy falls at high temperatures/high pressure regions where water behaves like a simple fluid. This reveals further the complexity of the problem since a successful model should treat water as an anomalous and as a simple fluid when necessary. Moreover, all of the association models fail to reproduce some of water’s structural properties and especially the tetrahedral fraction. It may be that a more accurate structural representation of water and the inclusion of steric hindrance and cooperativity effects could provide a more realistic model for water.