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

We have determined the complex dielectric spectra of 2-propanol/water mixtures 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 concentrations, 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, ΔGiE, ΔHiE, and ΔSiE [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 positive values, which suggests an increase of the “time averaged” number 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 isopropyl unit than that for the linear n-propyl moiety in preventing “disturbing fifth neighbor water molecules” from acting as new H-bond partners for a hydroxyl group of alcohols. The two pertinent maxima in ΔH2PAE and ΔS2PAE at X(molar fraction of 2-propanol)∼0.03 and ∼0.07 support the view that around X∼0.05, water molecules are pushed out of the first coordination shell of the nonpolar group into the bulk due to solute–solute association, commonly called hydrophobic interaction. In X⩾0.15, ΔH2PAE and ΔS2PAE become nearly zero, suggesting the generation of zigzag H-bonded alcohol chains similar to the pure alcohol. The analysis of the effective correlation factor, geff, revealed a strong structural perturbation effect of water in 0.5⩽X⩽1.0.