Multiple insights call for revision of modern thermodynamic models to account for structural fluctuations in water

Abstract

Modern thermodynamic models incorporate the concept of association (hydrogen bonding) and they can describe very satisfactorily many properties of water containing mixtures. They have not been successful in representing water's anomalous properties and this work provides a possible explanation. We have analyzed and interpreted recent experimental data, molecular simulation results, and two‐state theory approaches and compared against the predictions from thermodynamic models. We show that the dominance of the tetrahedral structure implemented in modern thermodynamic models may be the reason for their failure for describing water systems. While this study does not prove the two‐state theories for water, it indicates that a high level of tetrahedral structure of water is not in agreement with water's anomalous properties when used in thermodynamic models.

Investigation of the Alcohols and Water Hydrogen Bonding Structure via Monomer Fraction Studies

“…The disagreements are not limited to spectroscopic measurements but also to molecular simulation studies. The simulations by Dixit et al and others present hydrogen bond numbers in the high end while the recent simulations by Muthachikavil et al , for strong hydrogen bonds are in close agreement with the data of Nilsson . Muthachikavil et al , used the iAMOEBA force field.…”

“…The equivalence of sites can be checked if we have simulation data for all the k-times bonded fractions, as then we can back-calculate X A from all equations in eqs or and conclude whether these X A fractions are identical. We have provided this discussion in refs , , and it can be concluded that, for water, there is rather good agreement; thus, we have the approximate equivalence of sites in almost all cases (except for the X 3 fraction). The conclusion was the same for the various force fields considered.…”

“…For theory estimates, the density values from the NIST Chemistry Webbook (IAPWS-95 formulation) , are used. iAMOEBA estimates refer to 1 bar and were published by Tsochantaris et al Symbols are data from Luck (infrared spectroscopy) and Fouad et al (molecular simulations using the iAMOEBA and TIP4 P /2005 water models). Luck published results for the free-site fraction X A and monomer fraction X 0 at saturation.…”

“…22,63 As already discussed, the origin of water's thermodynamic behavior is believed to be connected to structural fluctuations in water. 22 In fact, molecular simulation models can predict results that are much more consistent with the published data of Nilsson and Mallamace as shown by Tsochantaris et al 22 The fact that the RSP theory does not incorporate this concept might be the reason why this model fails in predicting these properties. This indicates indirectly that a two-state assumption is needed in order to represent the low tetrahedral fraction data of Figure 8 and such data cannot be represented with a homogeneous Three adjustment attempts are displayed.…”

“…Association equations of state (EoS), belonging to SAFT (statistical associating fluid theory), quasi-chemical, and other concepts, can predict monomer fractions and other k -times bonded fractions assuming some characteristics for the hydrogen bonding interactions, such as number of associating sites and structural orientation. − There are several literature studies on the topic, and numerous articles have presented comparisons of such monomer fraction data against model predictions, especially for alcohols and water for which such data are available. − Alcohols are typically modeled using two or three sites, while water and glycols are often modeled assuming four sites. Recently, molecular simulation studies have generated, in addition to monomer fractions, k -times bonded fractions − for alcohols and water.…”