The dynamics of hydrophobic hydration of benzene was examined by NMR for both the solute and the solvent water. The rotational dynamics of water in the benzene hydration shell was observed to be slower than that in the bulk. This idea, in addition to the data on solitary waters in organic solvents, shows that the rotational dynamics of a water correlates positively with the hydrogen-bond strength of the water with the surrounding environment. The temperature dependence of the rotational motion of the solute benzene was examined to probe the cage structure of water around the benzene. It reveals the competition between the exclusion volume effect of the solute and the attractive and directional effects of solvent structure. The similarity and difference between the rotational and translational mobilities of hydrophobic benzene are also discussed.

NMR rotational correlation times t 2 R for heavy-water molecules (D20) were measured by the integrated capillary method in pure water (&) and dilute aqueous solutions of benzene (23 mM) and phenol (55 mM) over a wide range of temperatures including the supercooling range down to -18 "C. The B, coefficient defined by B, = (az2R/ac)t:R (c, concentration) is positive and negative for benzene and phenol, respectively, and its magnitude increases with the decrease in temperature for both solutes. Benzene, an apolar molecule, is classified as a structure maker and phenol, a polar molecule, as a breaker. They are more clearly distinguished in the supercooled region. The correlation time for the shell water molecules, which is shown to be expressed by z ? : " = [I -I-(55/Z)B,]tiR (Z, hydration number), is 4.1 times as large as the tiR value (14.6 ps) in supercooled aqueous solution of benzene (Z = -23) at -18 "C. This means that the hydrophobic hydration of benzene is associated with a remarkable rotational slowdown or increase in the local viscosity.

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