Electron Spin Resonance Study of Effect of Urea on Microenvironmental Properties of Alkylbenzenesulfonate Micellar Solutions Jingcheng Hao, Taotao Wang, Shuo Shi, Runhua Lu, and Hanqing Wang* Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, People’s Republic of China Received May 6, 1996. In Final Form: October 16, 1996 X The effect of urea on micelle formation and the microenvironmental properties of sodium dodecylben- zenesulfonate micellar solutions has been investigated by using electron spin resonance spectroscopy and surface tension measurement at the air/water interface. Two different nonionic spin probes, 5-doxylstearic acid and the piperidinyl-1-oxy with long hydrocarbon chains (Tempo, C 6-Tempo, C12-Tempo, C16-Tempo), have been used for studying sodium dodecylbenzenesulfonate solutions as a function of surfactant and urea concentrations. The surface tension results show that the addition of urea increases the critical micelle concentration values of the surfactant. The analysis of the nitrogen hyperfine coupling constant (A N) and the correlation time (τ) for the probe motion indicates that urea molecules interact with the polar headgroups of the sodium dodecylbenzenesulfonate micelles and penetrates below the polar headgroups. The addition of urea slightly decreases the micropolarity and strongly increases the microviscosity of the micellar interface. These results are in agreement with the recently reported mechanism where urea molecules replace some water molecules that solvate the hydrophobic chain and the polar headgroups of the surfactant. Introduction Electron spin resonance (ESR) has been used to characterize the micropolarity and microviscosity of the micellar interface. 1-3 Nitroxyl radicals have been widely used in the past for studying the microenvironmental properties and reactivities of assemblies such as micelles 4 and microemulsions. 5 By using suitable nitroxide labeled spin probes that partition into the surfactant aggregates, it is possible to study the micellization and the behavior of micellar aggregates in solutions. The rotational cor- relation times measured from the ESR spectrum reflect the probe mobility and can be used to study the formation and microenvironmental properties of aggregates in solutions. The change of the probe mobility can often yield useful information on the structure of organized molecular assembles. 6 The values of the hyperfine split- ting constants are dependent on the polarity of the probe environment 4 and can be used to extract valuable infor- mation on the probe environment. So, the ESR technique has an obvious advantage over other techniques in studying the microenvironmental properties of micelles as well as other organized molecular assembles. The physicochemical properties of surfactant solutions are extremely interesting. The properties of micellar solutions, such as critical micelle concentration (cmc), aggregation number, micelle size and shape, etc., depend on the balance between “hydrophobic” and “hydrophilic” interactions. For ionic surfactants this balance can be modified in several ways, i.e., salt addition, counterion complexation, addition of alcohols or other substances that can be solubilized into the micelle, change of the solvent, or change of the structure of the solvent itself. Urea has been used as an additive to check on the properties of micellar solutions, and two different mechanisms for urea action have been proposed. 7,8 (i) Urea breaks the “struc- ture” of water to facilitate the solvation of a hydrocarbon chain. (ii) Urea molecules replace some water molecules that solvate the hydrophobic chain and the polar head- group of the amphiphile. Linear alkylbenzenesulfonate (LABS) is one of the commercially important surfactants finding a wide range of industrial applications. 9 It is the largest tonnage anionic after soap. In addition, the presence of a number of isomers and the aromatic ring in the hydrophobic part make the study of this molecule extremely interesting. In recent years, the micellar structure of LABS has been investigated by using a variety of techniques, such as SAXS, 10 fluorescence spectroscopy, 11 and NMR. 12 Some of the important structural features and physicochemical properties of organization of LABS in micelles and comicellization have been well obtained. In a recent study, 13 Wang Jing-He reported the effect of inorganic salts and urea on the viscosity of sodium linear alkyl- benzenesulfonate solution with high concentrations. Urea markedly diminishes the viscosity values of concentrated LABS solutions, and this result is interpreted in terms of forming the adducts between urea and the hydrated individual surfactant and diminishing LABS actual concentration of the equilibria between the hydrated individual and micelles. However, the effect of urea on the microenvironmental physicochemical properties of LABS, the properties of the comicellization of LABS with other anionic surfactants, such as sodium oleate (NaOL) and sodium laurate (NaL), and the effect of the linear * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218. (2) Krishnakumar, S.; Somasundarn, P. J. Colloid Interface Sci. 1994, 162, 425. (3) Ottaviani, F. M.; Baglioni, P.; Martini, G. J. Phys. Chem. 1983, 87, 3146. (4) Berliner, L. J., Ed. Spin labeling. Theory and Applications; Academic Press: New York, 1976. (5) Barelli, A.; Eicke, H. F. Langmuir 1986, 2, 780. (6) Marsh, D. In Membrane spectroscopy; Grell, E., Ed.; Spinger Verlag: Berlin, 1981. (7) Wetlaufer, D. B.; Malik, S. K.; Stoller, L.; Coffin, R. I. J. Am. Chem. Soc. 1977, 99, 2898. (8) Enea, O.; Jolicoeur, C. J. Phys. Chem. 1982, 86, 3370. (9) Berth, P.; Jeschke, P. Tenside 1989, 2, 75. (10) Caponetti, E.; Triolo, R.; Patience, C. H. O.; Johnson, J. S.; Magid, L. J.; Butler, P.; Payne, K. A. J. Colloid Interface Sci. 1987, 116, 200. (11) Binanana, L. W.; Van Os, N. M.; Rumert, L. A. M.; Zana, R. J. Colloid Interface Sci. 1991, 141, 157. (12) Balasubramanian, D.; Shoba, J. J. Phys. Chem. 1986, 90, 2800. (13) Wang, Jing-He Acta Chim. Sin. 1995, 53, 237. 1897 Langmuir 1997, 13, 1897-1900 S0743-7463(96)00444-1 CCC: $14.00 © 1997 American Chemical Society