Is water a two-state liquid

Water is hypothesized to be a mixture two states. In this project the two-state theory will be implemented into the Statistical Associating Fluid Theory.

Water is of huge relevance in chemistry, biology and engineering. Its molecular structure and geometry have been established while many properties are known with such accuracy that we can now state that over 50 properties are “anomalous”; no other liquids behave like this.


Many of these anomalous properties are of thermodynamic origin (density maximum and minimum heat capacity with respect to temperature).


It has been known as a concept for over a century that water may consist of two “states” in some form of equilibrium with each other. But it has been during the last 20 years with the advanced experimental studies of Lars Pettersson and Anders Nilsson that this theory has regained serious attention.


Published extensively in prestigious journals [1-3], this theory advocates that water is a two-state liquid, having so-called low-density (LDL) and high-density liquid (HDL) regions, with most of water molecules being in the form of rings or chains and only a smaller percentage (15-20% depending on temperature) in tetrahedral form.


This theory is based on a very wide range of data from diverse advanced spectroscopic and diffraction methods, but it is far from accepted by all [4-6].


In this project we will attempt to incorporate the two-state theory of water in the Statistical Associating Fluid Theory (SAFT). The SAFT framework introduced by Chapman et al. [7] has led to the development of a new family of Equations of State in the last 30 years, and is the basis of two of the most successful EoS currently, Cubic plus Association (CPA) and Perturbed-Chain-Statistical Associating Fluid Theory (PC-SAFT).


The implementation of the two-state-theory in SAFT will allow the re-evaluation of the water thermodynamic properties using an engineering model that accounts for the two different states of water. If the anomalous properties of water are successfully captured, this could serve as further proof of the validity of the water-two-state theory.


1. Wernet, Ph. et al., 2004. Science, 304, 995.

2. Perakis, P. et., 2017. PNAS, 114(31): 8193-8198

3. Pettersson, L.G.M. et al., 2016. Chemical Reviews, 116: 7459-7462.

4. Smith, J.D. et al., 2004. Science, 306: 851-853.

5. Teixeira, J., 2019. Substantia 3(2) Suppl. 3, 57-63.

6. Soper, A.K., 2019. J. Chem. Physics, 150, 234503.

7. Chapman, W. G. et al., 1990. Ind. Eng. Chem. Res. 29 (8), 1709–1721.



Georgios M. Kontogeorgis

Xiaodong Liang





Nefeli Effrosyni Novak
DTU Chemical Engineering


Georgios Kontogeorgis
DTU Chemical Engineering
+45 45 25 28 59


Xiaodong Liang
Associate Professor
DTU Chemical Engineering
+45 45 25 28 77