Binding of Cationic Surfactants by Kappa, Iota, and Lambda Carrageenans in Aqueous Solution in the Presence of Sodium Chloride

Ksenija Kogej

Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, P. O. Box 537, SI-1000 Ljubljana, Slovenia
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Binding isotherms for the binding of dodecyl- (DPC) and cetylpyridinium chloride (CPC) to κ-, ι-, and λ-carrageenan were determined in aqueous solution without and with added NaCl at concentrations cNaCl = 0.01, 0.05, and 0.1 mol/L by using a potentiometric technique based on surfactant-selective membrane electrodes. The critical aggregation concentration, CAC, values obtained from the binding isotherms are around two orders of magnitude lower in the CPC case than in the DPC one. They are almost independent of NaCl concentration in κ- and ι-carrageenan solutions, but increase with increasing cNaCl in the λ-carrageenan case. Experimental isotherms were treated by the model of cooperative binding of surfactants by polyelectrolytes. Ionic strength dependence of the cooperativity parameters u and Ku was in agreement with the rather insignificant dependence of CAC on salt concentration. The isotherms of CPC binding by ι- and λ-carrageenan display a highly cooperative character with u values up to almost 800, whereas those for both surfactants in the presence of κ-carrageenan have a considerably lower cooperativity parameter (between 3 and 10). In addition, the isotherms in the CPC/ κ-carrageenan case do not indicate any saturation of the polyion chain with surfactant ions, but proceed to values of the degree of binding larger than 1. Our observations were explained by a lamellar type of ordering of surfactant in the presence of rigid polysaccharide chains.

Keywords: Surfactant-polyelectrolyte interactions, binding isotherms, carrageenans, charge density, critical association concentration, ionic strength