KINETIC MEDIUM EFFECTS OF AMPHIPHILIC COSOLUTES BELOW THEIR CRITICAL MICELLE CONCENTRATION - THE EFFECT OF SODIUM N-ALKYL SULFATES ON THE NEUTRAL HYDROLYSIS OF 1-BENZOYL-1,2,4-TRIAZOLE

Kinetic medium effects are presented for sodium n-alkyl sulfates (R = methyl to n-octyl) on the neutral hydrolysis of 1-benzoyl-1,2,4-triazole at 298.15 K in highly aqueous solutions below the critical micellar concentration. The observed rate-decreasing effects are quantitatively analyzed in terms of pairwise solute-solute interactions. It is shown that the effect of the alkyl chains cannot be accounted for on the basis of additivity of pairwise group interactions involving individual methylene moieties. Apparently, the sulfate group shields methylene moieties in close proximity for intermolecular interactions. Methylene groups, further away from the ionic functionality, are shielded less effectively. At the critical micellar concentration of the longer-chain sodium n-alkyl sulfates, the observed medium effects change dramatically as expected on the basis of the onset of micellization. No clear indication for premicellar aggregation was found. The medium effect of the sulfate moiety is estimated by extrapolation of kinetic medium effects to zero methylene groups and appears to be rate-retarding as well.