Uncovering the physical origin of the difference between aliphatic chain and aromatic ring in the "hydrophobic" effects on partial molar volume

The partial molar volume changes in the transfer of several hydrophobic molecules, which are composed of aromatic rings and an aliphatic chain of different lengths, from carbon tetrachloride to water (DeltaV(hyd)) are calculated using the three-dimensional interaction site model theory of molecular solvation. The theory reproduces recent experimental observations: the addition of a methyl group decreases DeltaV(hyd); in contrast, the addition of an aromatic ring increases DeltaV(hyd). The discrepancy is found to originate from the difference between chain and ring structures rather than that between aliphaticity and aromaticity. Furthermore, a general rule of the variation in DeltaV(hyd) due to the addition of a hydrocarbon is found through the theoretical analysis. An outward addition at the trans position, which is to form chain structure, decreases DeltaV(hyd), while an inward addition at the cis position, which is to form ring structure, increases DeltaV(hyd). This is explained in terms of solvent packing rather than the so-called hydrophobic hydration. The present findings argue against the traditional idea that the hydrophobic hydration can be represented by the observed values of DeltaV(hyd).