A far infrared study of water diluted in hydrophobic solvents

The far infrared (FIR) absorption spectra of water diluted in various hydrophobic solvents (cyclohexane, n-heptane, carbon tetrachloride, benzene and its fluorinated derivatives) are reported. It is shown that the spectra are due essentially to the rotational motion of monomeric water in a solvent cage, the molecule behaving like a quasi free rotor when dissolved in cyclohexane and n-heptane, a slightly hindered rotor in carbon tetrachloride and a highly perturbed rotor in benzene. In the latter case our findings corroborate recent experimental and theoretical analyses concerning the specificity shown by the water-benzene ‘complex’. We also show that the FIR absorption spectrum is particularly sensitive to the presence of associated water in these solutions. Because of the very low solubility of water in the solvents investigated it is found that the presence of associated water is in fact the signature of incomplete equilibration of the mixture. Well equilibrated solutions show no evidence of association.