Binding of Cationic Surfactants by Carrageenans. A Study of the Influence of Polyelectrolyte Charge Density†
Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology,
University of Ljubljana, Aškerčeva 5, P. O. Box 537, SI-1000 Ljubljana, Slovenia
Binding isotherms for the binding of dodecyl- (DPC) and cetylpyridinium chloride (CPC) to κ-, ι-, and λ-carrageenan were determined in aqueous solution without added salt by using a potentiometric technique based on surfactant-selective membrane electrodes. The potentiometric curves in the case of CPC were used in the first place to determine the number of charges on the monomer unit of polysaccharides. This procedure showed that κ- and ι- carrageenan samples have structures close to the ideal ones, whereas λ-carrageenan sample bears around 2.8 (instead of ideally 3) charges per disaccharide repeating unit. The CAC values, determined from the binding isotherms, are around two orders of magnitude lower in the CPC case than in the DPC one. At the same time, CPC is practically completely associated with all polyions, whereas the amount of bound DPC is lower and depends on the carrageenan charge density. However, the degree of binding in the plateau region of binding is very similar for both surfactants. This finding can be explained by proposing lamellar type of ordering between rigid carrageenan chains and surfactants. The experimental CAC values were compared with those predicted by a theoretical approach based on the cell model of a polyelectrolyte solution with two monovalent counterions of different size. Asatisfactory agreement between experimental and calculated values was obtained.
Keywords: Surfactant-polyelectrolyte interactions, binding isotherms, carrageenans, charge density, critical association concentration