Dynamics of hydration of alcohols and diols in aqueous solutions

Abstract

The ¹⁷ O NMR spin–lattice relaxation times, T ₁ , of the solvent water in aqueous solutions of nine alcohols and five diols have been determined as a function of the concentration at 25°C. For the alcohol solutions, the concentration dependence of T ₁ has been determined in the temperature range from 1 to 50°C. The value of n _h [(τ _c ^h / τ _c ⁰ )-1] which has been obtained from the concentration dependence of T ₁ has been defined as the dynamic hydration number (n _DHN ). The values of the coordination numbers n _h , have been estimated on the basis of the water-accessible surface areas (ASA) of the solute molecules. The rotational correlation times, τ _c ^h , of water molecules around the solute molecules have been estimated and compared with that of pure water τ _c ⁰ . The value of τ _c ^h /τ _c ⁰ =2.22 for Bu ^t OH at 25°C is the largest among the solutes investigated. The τ _c ^h /τ _c ⁰ values of the n-alcohols and diols increase with increasing ASA and become almost constant. The τ _c ^h /τ _c ⁰ values decrease with increasing temperature and approach constant value above 40°C. The temperature dependence of τ _c ^h /τ _c ⁰ for Bu ^t OH is the largest. The activation energy of τ _c ^h for n-alcohols has the maximum value at ASA≈240 Å ² . These results are discussed by considering the molecular sizes. The τ _c ^h /τ _c ⁰ value for a diol was smaller than that of the corresponding n-alcohol. This result supports the concept that hydrophobic hydration is distributed by the OH groups of the solute molecules. The thermodynamic properties of hydration for n-alcohols, butanol isomers and diols are linearly dependent on their n _DHN .