Hydrophobic hydration and the anomalous partial molar volumes in ethanol-water mixtures

Abstract

The anomalous behavior in the partial molar volumes of ethanol-water mixtures at low concentrations of ethanol is studied using molecular dynamics simulations. Previous work indicates that the striking minimum in the partial molar volume of ethanol VE as a function of ethanol mole fraction XE is determined mainly by water-water interactions. These results were based on simulations that used one water model for the solute-water interactions but two different water models for the water-water interactions. This is confirmed here by using two more water models for the water-water interactions. Furthermore, the previous work indicates that the initial decrease is caused by association of the hydration shells of the hydrocarbon tails, and the minimum occurs at the concentration where all of the hydration shells are touching each other. Thus, the characteristics of the hydration of the tail that cause the decrease and the features of the water models that reproduce this type of hydration are also examined here. The results show that a single-site multipole water model with a charge distribution that mimics the large quadrupole and the p-orbital type electron density out of the molecular plane has "brittle" hydration with hydrogen bonds that break as the tails touch, which reproduces the deep minimum. However, water models with more typical site representations with partial charges lead to flexible hydration that tends to stay intact, which produces a shallow minimum. Thus, brittle hydration may play an essential role in hydrophobic association in water.

Temperature dependent dynamics in water-ethanol liquid mixtures

“…Mixtures of water and alcohols are one of the simplest systems in which there is a competition between the hydrophobic and hydrophilic (hydrogen-bonding) interaction in defining the properties of the system. It is also well-known that the thermodynamic and transport properties (diffusion coefficient, reorientation correlation time) of their mixtures show an anomalous behavior [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The anomalies of liquid water are more pronounced in the low temperature regime [16][17][18][19].…”

“…In this shell the strength of H-bond would be significantly stronger than in bulk water. There are a quite a lot of theoretical and experimental evidence for [1,5,7,8,14] and against [12,15] this model in the literature Furthermore, it is known that near the hydrophobic surface the translational and orientational motions of water molecules are retarded. Some of the authors connected the activation energy of the reorientational motion to the energy of H-bond breaking [35,43,50].…”

Clustering mechanism of ethanol-water mixtures investigated with photothermal microfluidic cantilever deflection spectroscopy

“…At very low concentrations, water molecules are reported to be densely packed around the hydrophobic tail of alcohol resulting in a compressive effect from water which causes the minimum in partial molar volume at X EtOH = 0.07 28 . At this concentration all water are in the hydration shell of EtOH 28 .…”

“…At very low concentrations, water molecules are reported to be densely packed around the hydrophobic tail of alcohol resulting in a compressive effect from water which causes the minimum in partial molar volume at X EtOH = 0.07 28 . At this concentration all water are in the hydration shell of EtOH 28 . Perceptible higher amplitude of deflection at 20 wt% for gauche conformer could be as a result of such existence of waters in the hydration shell of EtOH which possibly make the presence of gauche conformer more favourable than anti in the mixture.…”

Hydrogen Bond Networks in Binary Mixtures of Water and Organic Solvents

“…a stronger network between the water molecules surrounding a hydrophobic/amphiphile molecule, 4,9,11,14,23−29…”