A Microscopic Hydrophobicity Parameter

Article abstract of DOI:10.1021/ja00271a029

p-Nitrophenyl laurate at 1x1E-5 M in water forms aggregates within which the ester groups hydrolyze slowly (about 1E3 less than a short-chain monomer).Salts of the general structure RNMe3⁺X^- disrupt or destroy the aggregates; the ester groups are thereby "deshielded", and the observed hydrolysis rate increases.The magnitude of the rate increase at a given salt concentration depends on R: the more hydrophobic the R group, the greater the rate enhancement.This observation provided the basis of a "microscopic" hydrophobicity parameter MH which was evaluated for 25 different Rs (e.g., MH=0.73, 0.97, and 1.33 for R=ethyl, n-butyl, and n-hexyl).MH values were used to assess the role of branching, unsaturation, cyclization, aromaticity, halogenation, etc., in hydrophobic association.The parameters correlate well with Hansch ? values for aliphatic substituents but not for aromatic groups.Since the MH scale is based on the specific binding of one molecule to another, it may be well suited for modeling association among bioactive species.