Interaction between cetyltrimethylammonium bromide and β-cyclodextrin:: surface tension and interfacial dilational viscoelasticity studies

The interaction between cetyltrimethylammonium bromide (CTAB) and beta-cyclodextrin (beta-CD) was studied via surface tension and dilational viscoelasticity methods. The effect of sodium bromide and sodium chloride on the interaction between CTAB and beta-CD were studied as well. The surface tension isotherms provided a series of parameters, including apparent critical micelle concentration (cmc*), surface tension at the cmc* (gamma(cmc)), stoichiometry of the complex (R), and the efficiency of adsorption (pC(20) ). The addition of NaBr and NaCl decreases the cmc* of CTAB/beta-CD solution. CTAB molecules enter the cavities of beta-CD molecules thus formed both 1:1 and 1:2 inclusion complexes. From the change of gamma(cmc), it can be seen that the CTAB/beta-CD complexes (1:1) act as short-chain alcohol, which decrease gamma(cmc), but the depression of cmc* is too small to be detected. R first decreases then increases as a function of NaBr and NaCl. Compared to NaCl, NaBr increases R more efficiently. The presence of NaBr and NaCl increases pC(20) of CTAB/beta-CD solution. The results obtained from the dilational viscoelasticity measurements at low dilational frequencies (0.005-0.1 Hz) reveal that the dilational modulus passes through a maxium value with increasing concentration of beta-CD at a given concentration of CTAB. The addition of both NaBr and NaCl decreases the dilational modulus of a given concentration CTAB/beta-CD solution.