Impact and structure of water in aqueous octanol mixtures: Hz-GHz dielectric relaxation measurements and computer simulations

Abstract

Temperature (283 ≤ T/K ≤ 323) dependent dielectric relaxation (DR) measurements of aqueous binary mixtures of octanol at three different water mole fractions (XH2O = 0.05, 0.10, 0.20) were performed employing two different frequency windows: 0.00002 ≤ν/MHz≤ 10 and 0.2 ≤ν/GHz≤ 50. In addition, DR measurements were carried out for pure octanol and water saturated octanol at 298 K. All these measurements showed, irrespective of solution temperature, a small decrease of the static dielectric constant (εs) upon addition of water in octanol. Two-step relaxation process with time constants ∼ 3–0.3 ns and ∼ 120–40 ps characterized the measured DR spectra with no signature of bulk water-like DR time constant (∼10 ps). Differential scanning calorimetric (DSC) measurements of octanol + water mixtures reflect only water concentration dependent depression of octanol freezing temperature. Our computer simulations of aqueous octanol mixtures with SPC/E water suggests a dramatic decrease of the average number of water-water H-bonds 〈NHBw-w〉 per water molecule and a spectacular increase of water populations possessing OOO angles distributed over 0°≤θ≤ 60°. Consequently, a negative value for the average tetrahedral order parameter (〈Q〉) and an εs as low as ∼ 2 for water have been predicted. A severe damage to the global tetrahedrality of water in these mixtures has been predicted and the angle distributions corresponding to negative and positive Q values explicitly shown. These simulation results therefore explain the microscopic origin for small effects of water at low mole fractions on εs of octanol + water binary mixtures.