Composition-dependent dynamical structures of 1-propanol–water mixtures determined by dynamical dielectric properties

Abstract

Using time domain reflectometry, we carried out dielectric relaxation measurements on 1-propanol–water mixtures for the entire concentration range in the frequency range 100 MHz–25 GHz at 20, 25, and 30 °C. We have calculated the excess partial molar activation free energy, enthalpy, and entropy for 1-propanol, ΔG1PAE, ΔH1PAE, and ΔS1PAE, and those for water, ΔGWE, ΔHWE, and ΔSWE from the relaxation times. In the region of X (molar fraction of 1-propanol) ⩾0.14, ΔH1PAE and ΔS1PAE take nearly zero. This means that 1-propanol molecules in the mixtures find themselves in not a very different environment from that in pure liquid. In the water-rich region, ΔH1PAE and ΔS1PAE exhibit two maxima at X∼0.03 and X∼0.06, corresponding roughly to 0.9 and 0.78 g/cm3 of water content, respectively. This fact, together with the results of the molecular dynamics studies of Sciortino et al. suggest the formation of two kinds of saturated hydration structures: the clathrate hydration shells with tetrahedral local arrangements of water molecules around X∼0.03 and nonclathrate shells with large cavities with three-coordinated local arrangements around X∼0.06. Hydrophobic hydration seems to partly share the same mechanism with structural enhancement in pure water by lowering local density.

Communication: Hydrogen bonding interactions in water-alcohol mixtures from X-ray absorption spectroscopy

“…[33][34][35] Dielectric relaxation and nuclear magnetic resonance spectroscopies support the clustering explanation while also revealing reduced liquid mobility. [36][37][38][39][40][41] Using Raman spectroscopy, Ben-Amotz et al have concluded that the observed clustering in alcohol-water mixtures is a result of random, rather than hydrophobic, interactions. 42,43 In contrast, some femtosecond midinfrared spectroscopy experiments have shown that water surrounding the aliphatic groups of the alcohols displays significantly slowed dynamics relative to the bulk, lending support to the model in which the measured negative excess entropy is attributed to water forming clathrate-like structure around the hydrophobic groups of the alcohols.…”

Organic molecules on the surface of water droplets – an energetic perspective

“…Even though molecules like methanol and water can be mixed over the whole concentration range, the methyl groups still tend to cluster to some degree in methanol solutions 26 . Dielectric relaxation measurements of a number of alcohol/water mixtures have revealed mixing to be a complex function of alcohol concentration [27][28][29][30] . Molecular simulation of such mixtures reproduces most features of these mixtures at least qualitatively 31,32 .…”

Hydrophobic and hydrophilic interactions in aqueous mixtures of alcohols at a hydrophobic surface

“…5,6 It is believed that the anomalous properties of water-alcohol mixtures arise from the structuring of water and alcohol molecules around each other in solution. The molecular structure of water and alcohol in solution is not completely understood; both water and alcohol form complex hydrogen bonded networks but the presence of hydrophobic alkyl groups in alcohols complicate the structuring in the solution.…”