Microheterogeneity in binary mixtures of propyl alcohols with water: NIR spectroscopic, two-dimensional correlation and multivariate curve resolution study

Abstract

Separation at a molecular level is responsible for macroscopic structure of liquids and leads to anomalous behavior of the physicochemical properties of mixtures. Here, we present the study of the microheterogeneity in binary mixtures of propyl alcohols with water. Both alcohols are fully miscible with water but due to different structures one may expect some differences in the interaction with water at a molecular level. Thus, a special attention was paid for the changes in the separation of alcohol and water molecules in going from 1-propanol to 2-propanol. Our results indicate that in spite of different molecular structures, the binary mixtures of both alcohols with water reveal similar properties. In the water-poor region the molecules of alcohols are in the same environment as those in the pure liquid alcohols, while the molecules of water do not form separate clusters but are dispersed in the organic phase. At higher water content the molecules of water form clusters interacting with the OH groups of the alcohol's clusters. The transition from the dispersed state to the formation of water clusters is a continuous process. The largest deviation from the ideal mixture appears at an alcohol:water molar ratio of 1:2. The mixtures of both propyl alcohols differ mainly in the degree of hydration of the clusters. The clusters of 1-propanol are less hydrated than those of 2-propanol, hence 1-propanol/water mixture is more heterogeneous. On the basis of the present results and available literature data one can conclude that the extent of the microheterogeneity in the alcohol/water binary mixtures is related to the degree of self-association of the alcohol.