Dynamics of hydration of alcohols and diols in aqueous solutions

The O-17 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 degrees C. For the alcohol solutions, the concentration dependence of T-1 has been determined in the temperature range from 1 to 50 degrees C. The value of n(h)[(tau(c)(h)/tau(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, tau(c)(h), of water molecules around the solute molecules have been estimated and compared with that of pure water tau(c)(0). The value of tau(c)(h)/tau(c)(0) = 2.22 for (BuOH)-O-t at 25 degrees C is the largest among the solutes investigated. The tau(c)(h)/tau(c)(0) values of the n-alcohols and diols increase with increasing ASA and become almost constant. The tau(c)(h)/tau(c)(0) values decrease with increasing temperature and approach constant value above 40 degrees C. The temperature dependence of tau(c)(h)/tau(c)(0) for (BuOH)-O-t is the largest. The activation energy of tau(c)(h) for n-alcohols has the maximum value at ASA approximate to 240 Angstrom(2). These results are discussed bg considering the molecular sizes. The tau(c)(h)/tau(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).