Study of host–guest interaction between ß-cyclodextrin and alkyltrimethylammonium bromides in water

Abstract

Cyclodextrins were found to play important roles in self-assembly systems of surfactants. The interactions between host molecule s-cyclodextrin (CD) and model cationic surfactants, alkyltrimethylammonium bromides with different alkyl chain length: dodecyl-(C12TAB), tetradecyl-(C14TAB) and hexadecyl-(C16TAB) are studied by means of conductivity measurements at 313.2 K. The data obtained indicate that inclusion complexes (CD:S+) had formed, and apparent critical micelle concentration (CMC*) is equivalent to the combined concentrations of surfactant monomers complexed with the CD and that of a free dissolved monomer in equilibrium with the micellized surfactant without CD. Inclusion complexes were characterized by an equilibrium binding constant K 11, which value increases as the length of alkyl chains, and consequently the hydrophobicity, increases. From mathematical model the concentrations of the uncomplexed cyclodextrin, uncomplexed surfactant ion, and inclusion complex in the submicellar, as well as in the micellar range were calculated. The competition between the micellization and complexation processes leads to the existence of a significant concentration of free CD in equilibrium with the micellar aggregates. The percentage of uncomplexed cyclodextrin in equilibrium with the micelles is independent on cyclodextrin concentration for a particular ternary system and is 31, 37, and 34 % for C12TAB/water/s-CD, C14TAB/water/s-CD and C16TAB/water/s-CD, respectively. By using standard Gibbs free energy for micellization and surfactant complexation by CD, we can explain the observed behavior.