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
* Corresponding author: E-mail:
ksenija.kogej@fkkt.uni-lj.si
Abstract
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