Dynamics of hydration of alcohols and diols in aqueous solutions

Abstract

The ¹⁷O NMR spin–lattice relaxation times, T₁, 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 T₁ has been determined in the temperature range from 1 to 50°C. The value of n_h[(τ_c^h/ τ_c⁰)-1] which has been obtained from the concentration dependence of T₁ 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_c⁰. The value of τ_c^h/τ_c⁰ =2.22 for Bu^tOH at 25°C is the largest among the solutes investigated. The τ_c^h/τ_c⁰ values of the n-alcohols and diols increase with increasing ASA and become almost constant. The τ_c^h/τ_c⁰ values decrease with increasing temperature and approach constant value above 40°C. The temperature dependence of τ_c^h/τ_c⁰ for Bu^tOH is the largest. The activation energy of τ_c^h for n-alcohols has the maximum value at ASA≈240 Ų. These results are discussed by considering the molecular sizes. The τ_c^h/τ_c⁰ 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.

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