Despite the simplicity of a single water molecule, water is highly complex, possessing a range of properties untypical for a liquid. Until now, even the most advanced thermodynamic models have failed to describe these anomalous phenomena. However, resent progress in applied thermodynamics at DTU Chemical and Biochemical Engineering is about to change the picture. By enabling state-of-the-art molecular theory for associations to accommodate multiple hydrogen-bonding schemes, a model can satisfactorily represent the anomalous properties of water. Water is the most essential fluid on our planet, and it plays important roles in a wide variety of fields. An example of an anomalous property is the fact that water becomes less dense when it enters its solid phase, ice. Water has a density maximum at 4 °C, which makes ice float in water, and this behavior plays an important role in controlling temperature distribution and vertical circulation and increasing the chances of survival of organisms in water. Further, water has a very high heat capacity, which prevents large temperature fluctuations and maintains uniform body temperatures, important for biochemical processes. Water’s heat capacity and compressibility present minima as temperature changes.
Successful introduction of parameter set
Research on water at the Applied Thermodynamics Center for Energy Resources Engineering Center (AT-CERE) at DTU Chemical and Biochemical Engineering can be traced back to before 2000. At that time, a robust parameter set for modeling the phase behavior of water with the Cubic Plus Association equation of state was developed. Unlike many other models which have many different sets of water parameters from different groups, the parameter set of this model has been kept and successfully used for all important applications over the past 25 years. However, this model – just like other advanced thermodynamic models – failed to describe the anomalous properties of water. With the award of the ERC (European Research Council) Advanced Grant “New Paradigm in Electrolyte Thermodynamics” to Professor Georgios M. Kontogeorgis, water research at AT-CERE was boosted. Since water is the most important and commonly used solvent in electrolyte solutions, water research was identified as a key element in the research program on electrolyte thermodynamics. Especially two questions were addressed. Firstly, to which extent can the state-of-the-art molecular dynamics represent the anomalous properties of water? And secondly, will a phenomenological two-state theory model suffice? In the meantime, fundamental research using molecular dynamics simulation, supported by PetroChina, was conducted to investigate the relationship between the structure and properties of water. Here, new methods on how to identify and quantify locally favored tetrahedral structures in different water models were proposed.
Water as a two-state liquid
These research activities led to the conclusion, that the dynamics of different structures need to be considered in modern molecular thermodynamics models to correctly capture the anomalous properties of water as well as to accurately describe the phase equilibria of water-containing mixtures. With the award of the Villum Experiment project “Is Water a two-state liquid?” to Professor Georgios M. Kontogeorgis significant further progress is made. According to initial results, with two different hydrogen-bonding possibilities in dynamic equilibrium, the modern molecular thermodynamics models can satisfactorily represent the anomalous properties of water such as density maxima and compressibility minima. The water research at AT-CERE will continue with the aim of developing a general model which can accurately describe water's anomalous thermodynamic properties and predict the phase behavior of water-containing multicomponent and multiphase systems.
