Stability of Sodium Dodecyl Sulfate Micelles in the Presence of a Range of Water-Soluble Polymers: A Pressure-Jump Study Dibakar Dhara and Dinesh O. Shah* Center for Surface Science and Engineering, Departments of Chemical Engineering and Anesthesiology, UniVersity of Florida, GainesVille, Florida 32611 ReceiVed: August 24, 2000; In Final Form: NoVember 17, 2000 The effects of a range of nonionic and ionic water-soluble polymers on the micellar stability of sodium dodecyl sulfate were investigated by the pressure-jump method. The presence of polymer that interacts with SDS was found to decrease the micellar stability drastically. Surface activity of the polymers was correlated with their ability to reduce the micellar stability. The decrease in micellar stability was explained by the formation of premicellar aggregates (or submicelles) of surfactants in the presence of polymer, which facilitated the micelle formation-disintegration process. Introduction In any situation where surfactant must adsorb at a newly created interface, whether it is at air/liquid or solid/liquid interface, it is supplied to the new interface by the diffusion of monomer molecules. When a surface is suddenly expanded, the stability of the micelles affect the ability of the solution to supply the monomers to the new surface. The less stable the micelles, the greater the monomer flux to the new surface/interface will be. Hence, the stability of micelles should play a significant role in processes where new interfaces are constantly generated. It has been experimentally verified that the micellar stability indeed plays an important role in various technological processes such as foaming, 1 bubble dynamics, 2 wetting time of cotton, 3 solubilization and detergency, 4 emulsion droplet size, 5 and thin film stability. 6 Therefore, understanding the factors affecting micellar stability is key to influencing the dynamic surface or interfacial tension and also the technological processes controlled by them. Shah and co-workers have showed that the stability of sodium dodecyl sulfate micelles can be affected by additives such as short and long chain alcohols, 7,8 oppositely charged surfactants, 9 tetraalkylammonium chlorides, 10 and antifoams. 11 However, the effect of polymers on the micellar stability has not been explored well. Since the polymers are added in various surfactant formulations, it will be very interesting to investigate the dynamic behavior of surfactant micelles in the presence of polymers. Besides, the surfactant-polymer systems in aqueous solution are also intriguing from both fundamental as well as practical points of view. These complex mixtures find extensive industrial applications in areas related to mineral processing, foaming control, medicine, food, detergency, enhanced oil recovery, etc. They are also of interest in formulation and conditioning of cosmetics, biological, pharmaceutical, and fine chemistry ap- plications. From the fundamental point of view, understanding the nature of the surfactant-polymer interactions that lead to the formation of a complex and the physical structure and stability of this complex is fascinating and not clearly established yet. Thus interaction between surfactants and polymers has been the subject of active research for the last three decades and it has also been focused on in some of the recent reviews. 12,13 Most studies on polymer-surfactant interaction are based upon equilibrium data, i.e., the effect of polymers on the critical micellar concentration of the surfactant and the aggregation number of the micelles. 14-17 The effect of polymers on the dynamic behavior of micellar solutions has not been studied in depth. 18-21 Such a study is important in understanding the dynamic behavior of surfactant micelles in the presence of polymers and also in understanding the fundamentals of polymer-surfactant interaction. In a recent paper we have reported the effect of poly(ethylene glycols) on sodium dodecyl sulfate micelles in the concentration range of 50-600 mM. 22 The present paper is an extension of that work, intended as a detailed study of the effect of a range of water-soluble polymers on the micellar stability over a wide range of surfactant concentrations. Experimental Section Materials. SDS (99% purity), from Sigma Chemical Co., was used as received. The source, average molecular weights (when available), and abbreviations of the polymers used in this study are listed in Table 1. The average molecular weights are as given by the suppliers. The polydispersity data are not available. Deionized water was used in all the experiments. Pressure-Jump Experiments. The slow relaxation time (τ 2 ) of SDS micelles was measured using a pressure-jump apparatus with conductivity detection from Dia-Log GmbH (Duesseldorf, Germany) by recording the change in conductivity that results from micelle formation or disintegration. 24,25 The surfactant solution was pressurized up to 100-130 bar and the solution was allowed to reach its new equilibrium state (at high cmc). Subsequently, the pressure was suddenly released to ambient pressure (initial cmc). The slow relaxation time τ 2 was then calculated from the exponential decay in electrical conductivity. The conductivity of the surfactant sample solution held in the conductivity cell attached to the pressure chamber was compared to a reference cell containing KCl solution of the same conductivity. In the pressure-jump instrument used here, the pressure falls following the rupture of a thin metal diaphram, * Corresponding author. Phone: (352) 392-0877.Fax: (352) 392-0127. E-mail: shah@che.ufl.edu. 7133 J. Phys. Chem. B 2001, 105, 7133-7138 10.1021/jp003072l CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001