A study of micellization and the thermodynamic properties of a series of aqueous sodium cyclohexyl alkanoate surfactants Judith A. MacInnis, D. Gerrard Marangoni, and R. Palepu Abstract: The micellization and the thermodynamic properties of a series of sodium cyclohexyl alkanoates of the gen- eral formula C 6 H 11 (CH 2 ) n COONa (where n = 1, 2, 3, 4) were investigated employing conductivity, density, surface ten- sion, viscosity, speed of sound, luminescence probing experiments, and Fourier transform infrared (FT-IR) spectroscopy. The critical micelle concentrations (cmcs) and the aggregation numbers indicate that these surfactants have high cmc values and low aggregation numbers when compared to other single-headed surfactants (most notably the sodium alkanoates). Thermodynamic properties, obtained from the mass action model, indicate that micellization is spontaneous and entropically driven. The saturation area per molecule, the free energies of adsorption, and the efficiency and effec- tiveness of adsorption were determined through surface tension measurements. The presence of the cylcohexyl ring ap- pears to influence the surface properties of micellization. Both the effectiveness and the efficiency of these surfactants, in lowering the surface tension of water, are lower than that of the straight chain alkanoates. Key words: thermodynamics, micellization, aggregation numbers, speed of sound, and spectroscopy. Résumé : Faisant appel à la conductivité, la densité, la tension superficielle, la viscosité, la vitesse du son, des expériences de luminescence ainsi que la spectroscopie infrarouge par transformation de Fourier (« FT-IR »), on a étudié la micellisation et les propriétés thermodynamiques d’une série de cyclohexylalcanoates de sodium de formule générale C 6 H 11 (CH 2 ) n COONa (où n = 1, 2, 3, 4). Par comparaison avec les valeurs correspondantes pour d’autres agents de surface à une seule tête (plus particulièrement les alcanoates de sodium), les concentrations micellaires cri- tiques (cmc) sont élevées alors que les nombres d’agrégation sont faibles. Les propriétés thermodynamiques, obtenues à partir du modèle d’action de masse, indiquent que la micellisation est spontanée et sous l’influence de l’entropie. On a déterminé la superficie de saturation, les énergies libres d’adsorption et l’efficacité de l’adsorption à l’aide de mesures de la tension superficielle. La présence de noyaux cyclohexyles semble influencer les propriétés de surface de micellisation. L’efficacité de ces agents de surface à diminuer la tension superficielle dans l’eau est inférieure à celle des alcanoates en chaîne droite. Mots clés : thermodynamique, micellisation, nombres d’agrégation, vitesse du son, spectroscopie. MacInnis et al. 1232 1. Introduction The study of micellization of aqueous solutions of surfac- tants in the literature is well established (1–18). The depend- ence of interfacial activity and micellization on the structure of the hydrophobic part of the surfactant is an area of re- search that has been receiving increased attention in recent years. The bulk properties of n-alkanoate surfactants with linear alkyl chains have been extensively reported in the lit- erature (19–24), however, very little attention has been given to the alkanoate surfactants that contain a cyclohexyl ring. The sodium cyclohexyl alkanoate surfactants have structures similar to those of the petroleum acids. The latter contain mainly short alkyl chains that are attached to various saturated, partially saturated, and aromatic structures. These com- pounds play an important role in the reduction of interfacial tension during alkaline water flooding of oil reservoirs (10). Employing techniques such as density, viscosity, conduc- tivity, surface tension, speed of sound, FT-IR spectroscopy, and fluorescence spectroscopy, this paper investigates the bulk thermodynamic and micellar properties of a family of alkanoate surfactants, which contain a cyclohexyl ring in place of a long hydrocarbon chain to address the effects of the cyclohexyl ring and of increasing alkyl chain length on these properties. 2. Experimental The cyclohexyl acids were purchased from Aldrich (99% pure). They were dissolved in deionized water, equivalent to triply glass-distilled, and were then converted to the cor- responding sodium salts. All of the dried salts were soxhlet extracted with diethyl ether for 2–5 days, dried and stored Can. J. Chem. 78: 1222–1232 (2000) © 2000 NRC Canada 1222 Received April 4, 2000. Published on the NRC Research Press website on September 1, 2000. J.A. MacInnis, D.G. Marangoni, and R. Palepu. 1 Department of Chemistry, St. Francis Xavier University, P.O. Box 5000, Antigonish, NS B2G 2W5, Canada. 1 Author to whom correspondence may be addressed. Telephone: (902) 867-3886. Fax: (902) 867-2414. e-mail: rpalepu@stfx.ca