The cooperative dynamics of the H-bond system in 2-propanol/water mixtures: Steric hindrance effects of nonpolar head group

Abstract

We have detd. the complex dielec. spectra of 2-propanol/water mixts. employing time domain reflectometry for the entire mixing range in 0.1≤ν/GHz≤25 at 20 °C, 25 °C, and 30 °C. At the specific concns., a frequency range was expanded up to 89 GHz with the help of waveguide interferometry. The excess activation free energy, ΔGE, enthalpy, ΔHE, and entropy, ΔSE, and their partial molar quantities, ΔGEi, ΔHEi, and ΔSEi [i=2-propanol (2PA) and water (W)] were derived from the relaxation time of the cooperative process, τ1. In the water-rich region, ΔHE and ΔSE exhibit conspicuous behavior with large pos. values, which suggests an increase of the "time averaged" no. and the strength of H-bonds. τ1 Becomes considerably larger for 2-propanol/water than that for 1-propanol/water, resulting in ∼10% larger maxima of ΔH and ΔS. This indicates that the steric hindrance caused by the nonpolar group is more efficient for the branched iso-Pr unit than that for the linear Pr moiety in preventing "disturbing fifth neighbor water mols." from acting as new H-bond partners for a hydroxyl group of alcs. The two pertinent maxima in ΔHE2PA and ΔSE2PA at X(molar fraction of 2-propanol)∼0.03 and ∼0.07 support the view that around X∼0.05, water mols. are pushed out of the first coordination shell of the nonpolar group into the bulk due to solute-solute assocn., commonly called hydrophobic interaction. In X≥0.15, ΔHE2PA and ΔSE2PA become nearly zero, suggesting the generation of zigzag H-bonded alc. chains similar to the pure alc. The anal. of the effective correlation factor, geff, revealed a strong structural perturbation effect of water in 0.5≤X≤1.0.