Phase Behavior of the Mixed Systems of α-, β-Cyclodextrin and Cationic-Anionic Surfactants

The phase behavior of mixtures of alpha-, beta-cyclodextrin (alpha-, beta-CD) and equimolarly mixed CnH2n+1N(C2H5)(3)Br-CnH2n+1SO3,4Na (CnNE-CnSO3,4Na, n = 8, 10, 12) was studied. CDs could not only destroy the precipitates of cationic-anionic surfactants by forming 1:1 CD/surfactant complexes, but also induce precipitation by forming 2:1 CD/surfactant complexes, depending on the concentration, CD types, and chain lengths of surfactants. Precipitation of 2: 1 complexes was ascribed to the synergism of electrostatic attractions and intermolecular hydrogen bonding. For the initially turbid C12NE-C12SO4Na, it turned clear and then formed precipitates upon continuous addition of beta-CD, but the addition of alpha-CD only induced precipitation. It showed that alpha-CD and the chain length of C-12 were more favored than beta-CD and C-10, respectively, to form precipitates of CD/surfactant complexes. XRD showed that the precipitates exhibited channel-type structure. The increase of temperature could induce the dissolution of precipitates, due to the enhancement of solubility instead of decomplexation of CD/surfactant complexes. The addition of salts could dissolve the precipitates owing to the shielding effect of salts on the interactions between opposite-charged headgroups. The sequence of their ability that NaI > KBr > NaBr > NaCl was consistent with Hofmeister series.