Langmuir 2005, 21, 79-86
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Influence of pH, Ionic Strength, and Temperature on Self-Association and Interactions of Sodium Dodecyl Sulfate in the Absence and Presence of Chitosan Masubon Thongngam and D. Julian McClements* Biopolymers and Colloids Research Laboratory, Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003 Received May 25, 2004. In Final Form: September 30, 2004 Chitosan is a cationic biopolymer that has many potential applications in the food industry because of its unique nutritional and physicochemical properties. Many of these properties depend on its ability to interact with anionic surface-active molecules, such as surfactants, phospholipids, and bile acids. The purpose of this study was to examine the influence of pH (3 and 7), ionic strength (0-200 mM NaCl), and temperature (10-50 °C) on the interactions between a model anionic surfactant (sodium dodecyl sulfate, SDS) and chitosan using isothermal titration calorimetry, selective surfactant electrode, and turbidity measurements. At pH 3 and 30 °C, SDS bound strongly to chitosan to form an insoluble complex that contained about 4-5 mmol of SDS/1 g of chitosan at saturation. When SDS and chitosan were mixed at pH 7 they did not interact strongly, presumably because the biopolymer had lost most of its positive charge at this pH. However, when SDS and chitosan were mixed at pH 3 and then the solution was adjusted to pH 7, the SDS remained bound to the chitosan. The presence of NaCl (0-200 mM) in the solutions decreased the critical micelle concentration (cmc) of SDS (in both the absence and the presence of chitosan) but had little influence on the amount of SDS bound to chitosan at saturation. The cmc of SDS and the amount of SDS bound to the chitosan at saturation were largely independent of the holding temperature (10-40 °C). Nevertheless, the enthalpy changes associated with micelle dissociation were highly temperaturedependent, indicating the importance of hydrophobic interactions, whereas the enthalpy changes associated with SDS-chitosan binding were almost temperature-independent, indicating the dominant contribution of electrostatic interactions. This study provides information that may lead to the rational design of chitosanbased ingredients or products with specific nutritional and functional characteristics, for example, cholesterol lowering.
Introduction Chitosan is a cationic biopolymer that has many potential biological and industrial applications, including cholesterol lowering, heavy metal chelation, wastewater treatment, texture modification, encapsulation, and emulsion stabilization.1-8 Many of these applications depend on the interactions between chitosan and anionic surfaceactive substances, for example, small molecule surfactants, phospholipids, or bile acids. The rational application of chitosan for these applications, therefore, depends on a better understanding of the origin and nature of chitosananionic surfactant interactions. The purpose of this study was to examine the influence of solution and environmental conditions (pH, ionic strength, and temperature) on chitosan-sodium dodecyl sulfate (SDS) interactions to provide a better understanding of the characteristics of chitosan-anionic surfactant interactions. * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Shahidi, F.; Arachchi, J. K. V.; Jeon, Y. J. Trends Food Sci. Technol. 1999, 10, 37-51. (2) Rinaudo, M.; Domard, A. In Chitin and chitosan; Skjak-Braek, G., Anthonsen, T., Sandford, P., Eds.; Elsevier Applied Science: London, 1989. (3) Claesson, P. M.; Ninham, B. W. Langmuir 1992, 8, 1406-1412. (4) Kubota, N.; Kikuchi, Y. Macromolecular complexes of chitosan. In Polysaccharides: structural, diversity and functional versatility; Dumitriu, S., Ed.; Marcel Dekker: New York, 1998. (5) Ravi Kumar, M. N. V. Bull. Mater. Sci. 1999, 22, 905-915. (6) Jeuniaux, C.; Voss-Foucart, M. F.; Poalicek, M.; Bussers, J. C. In Chitin and Chitosan; Skjak-Braek, G., Anthonsen, T., Sandford, P., Eds.; Elsevier Applied Science: London, 1989. (7) Herrera, F. P.; Mata-Segreda, J. F. Rev. Biol. Trop. 1996-1997, 44/45 (3/1), 613-614. (8) Schulz, P. C.; Rodriguez, M. S.; Del Blanco, L. F.; Pistonesi, M.; Agullo, E. Colloid Polym. Sci. 1998, 276, 1159-1165.
Chitosan is a (1f4)-linked 2-amino-2-deoxy-β-D-glucan derived from fully or partially deacetylated chitin.1 It has three types of reactive functional groups: an amino group at the C-2 position (pKa ∼ 6.3-7) and primary and secondary hydroxyl groups at the C-3 and C-6 positions, respectively.1,3,9 At relatively low pH (
