Dynamics of Hydration of Alkylsulfonate Anions in Aqueous Solutions

Abstract

The 17O-NMR spin-lattice relaxation times (T1) of water molecules in aqueous solutions of n-alkylsulfonate (C1 to C6) and arylsulfonic anions were determined as a function of concentration at 298 K. Values of the dynamic hydration number, \((\mathrm{S}^{-}) = n_{\mathrm{h}}^{ -} (\tau_{\mathrm{c}}^{-} /\tau_{\mathrm{c}}^{0} - 1)\), were determined from the concentration dependence of T1. The ratios (\(\tau_{\mathrm{c}}^{ -}/\tau_{\mathrm{c}}^{0}\)) of the rotational correlation times (\(\tau_{\mathrm{c}}^{ -} \)) of the water molecules around each sulfonate anion in the aqueous solutions to the rotational correlation time of pure water (\(\tau_{\mathrm{c}}^{0}\)) were obtained from the nDHN(S−) and the hydration number (\(n_{\mathrm{h}}^{ -} \)) results, which was calculated from the water accessible surface area (ASA) of the solute molecule. The \(\tau_{\mathrm{c}}^{ -}/\tau_{\mathrm{c}}^{0}\) values for alkylsulfonate anions increase with increasing ASA in the homologous-series range of C1 to C4, but then become approximately constant. This result shows that the water structures of hydrophobic hydration near large size alkyl groups are less ordered. The rotational motions of water molecules around an aromatic group are faster than those around an n-alkyl group with the same ASA. That is, the number of water–water hydrogen bonds in the hydration water of aromatic groups is smaller in comparison with the hydration water of an n-alkyl group having the same ASA. Hydrophobic hydration is strongly disturbed by a sulfonate group, which acts as a water structure breaker. The disturbance effect decreases in the following order: \(\mbox{--} \mathrm{SO}_{3}^{-} > \mbox{--} \mathrm{NH}_{3}^{ +} > \mathrm{OH}> \mathrm{NH}_{2}\). The partial molar volumes and viscosity BV coefficients for alkylsulfonate anions are linearly dependent on their nDHN(S−) values.