Association of Hydrotropes in Aqueous Solution Studied by Reaction Kinetics

Abstract

The effect of added hydrotropes on the rates of neutral hydrolysis for 1-benzoyl-3-phenyl-1,2,4-triazole 1 has been studied, together with the concentration dependence of the 1H NMR spectra of the hydrotropes in aqueous solution. Hydrotropes include sodium 4-alkylbenzenesulfonates 2a - e, sodium 4-methoxybenzenesulfonate 2f, sodium 4-hydroxybenzenesulfonate 2g, cesium benzenesulfonate 3, benzamidinium chloride 4, phenyltrimethylammonium bromide 5a, and benzyltrimethylammonium bromide 5b. All hydrotropes, except 2g, induce strong rate-retarding effects, indicative of strong interactions with 1 and of remarkably strong hydrophobic interactions between aromatic moieties. Most hydrotropes show neither spectroscopic nor kinetic evidence for cooperative aggregation in the concentration range studied, i.e., from 0 to 1.4 mol kg-1. Cooperative aggregation is absent because the hydrophobic moieties are too small for hydrophobic interactions to overcome electrostatic repulsion. Lack of aggregation results in high availability of hydrophobic binding sites, thereby accounting for the high solubilizing power characteristic for hydrotropes. However, sodium 4-n-propylbenzenesulfonate 2d and sodium 4-nbutylbenzenesulfonate 2e show cooperative self-association forming highly dynamic loose micellar-type structures.