Cluster formation and percolation in ethanol-water mixtures

Abstract

Results of systematic molecular dynamics studies of ethanol-water mixtures, over the entire concentration range, were reported previously to agree with experimental X-ray diffraction data. These simulated systems are analyzed in this work to examine cluster formation and percolation, using four different hydrogen bond definitions. Percolation analyses revealed that each mixture (even the one containing 80 mol % ethanol) is above the 3D percolation threshold, with fractal dimensions, d f , between 2.6 to 2.9, depending on concentration.Monotype water cluster formation was also studied in the mixtures: 3D water percolation can be found in systems with less than 40 mol % ethanol, with fractal dimensions between 2.53 and 2.84. These observations can be put in parallel with experimental data on some thermodynamic quantities, such as the excess partial molar enthalpy and entropy.

Ring structure analysis of ethanol–water mixtures

“…The average nullity for Et0S and Et0T hardly differs, while Et60S has substantially larger value than Et60T. This is in agreement with our previous analysis of these mixtures [14,28], where we found that although for this concentration both SWM4-DP and TIP4P-2005 water force field simulations reproduce the experimental total scattering X-ray structure factor equally well, the structure created with the SWM4-DP water model has a higher connectivity.…”

“…This shows that the higher affinity of water for cycle formation due to its ideal connectivity of four increases the chances of ethanol molecules for participating in cycles. The value for Et60S is higher than that for Et60T, indicating higher cyclic connectivity, in agreement with the results of the combined cyclic + acyclic connectivities investigated previously [28]. While the majority of ethanol O atoms have a connectivity of two (see Fig.…”

“…As it was found in another, separate part of this study [28], percolation occurs in all the ethanol-water mixtures and in pure water. The effect of percolation on building the HB network and on the ring perception is discussed in Appendix B.…”

“…Percolation occurs in all the mixtures and in pure water, where each configuration has a percolating main cluster, as well as smaller clusters beside that [28].…”

“…As a continuation of our recent study [28], here detailed analyses of the hydrogen-bonded clusters/network were performed for the most successful MD simulations at each concentration, using four different geometric hydrogen bond definitions. These definitions differ in terms of the presence and/or the cutoff value of the angular constraint.…”