Anion association and micelle formation in solutions of hydrotropic and short-chain carboxylates

Abstract

Abstract A survey is given of studies on the association processes, the changes in structure and the formation of micelles in solutions of alkali metal salts of various carboxylic acids. The substances investigated are normal and branched alkanoates with 1–8 carbon atoms, and typical hydrotropes such as hydroxybenzoate. The experimental methods have been measurement of activities of carboxylate ions, counterions, and solvent by potentiometry, vapor pressure osmometry and gas chromatography; calorimetric determination of enthalpies of mixing; NMR studies of relaxation times for protons and line widths of signals from counterions; dilatometric measurements of changes in density on mixing and potentiometric investigations of hydrolytic equilibria. In order to simplify the evaluation of the results, the solutions were given a high content of electrolyte, mostly by adding sodium chloride to an ionic strength of 3 c. It has been established that association occurs in solutions of all alkanoates except acetate and propionate; the results are less conclusive for the benzoates. The initially formed aggregates contain from 2 to 4 anions. Larger aggregates (micelles) are formed by alkanoates with more than four carbon atoms. The results support the view that these association processes are mainly results of structural changes in the solvent, i.e., the aggregates are held together by hydrophobic bonding.