Thermodynamic properties of water in the water-poor region of binary water plus alcohol mixtures

In our previous thermodynamic studies, we suggested that alcohol molecules in water-poor water + alcohol mixtures exist as alcohol clusters in a form similar to the pure alcohols. Here, we use calorimetry and densitometry to investigate how H2O interacts with alcohol clusters in water-poor binary aqueous mixtures of 12 different alcohols. The composition dependence of the measured excess partial molar enthalpy and volume of water (H-W(E) and V-W(E)), along with entropy data calculated from H-W(E) and literature data for excess chemical potentials, showed that in water-poor solutions of small alcohols such as methanol, ethanol, and 1-propanol, mutual water-water interactions are endothermic, but entropically favorable. Conversely, in long-chain solvents such as 1-octanol and 1-decanol, the interaction is exothermic and entropically unfavorable. We suggest that these observations reflect water-alcohol hydrogen bonding in short-chain solvents and water clustering with more hydrogen bonding than in pure water or "dewetting" in mixtures of the longer alcohols, respectively. The composition dependence of H-W(E) was also used to locate anomalies that specify the boundary between the mixing schemes characterizing the intermediate and the water-poor regions of alcohol + water mixtures.