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

Abstract

The phase behavior of mixtures of α-, β-cyclodextrin (α-, β-CD) and equimolarly mixed C_nH_2n+1N(C₂H₅)₃Br-C_nH_2n+1SO_3,4Na (C_nNE-C_nSO_3,4Na, 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 β-CD, but the addition of α-CD only induced precipitation. It showed that α-CD and the chain length of C₁₂ were more favored than β-CD and C₁₀, 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.

Keywords:

• Cationic-anionic surfactants
• cyclodextrin
• inclusion complex
• phase behavior

Acknowledgments

This project was financially supported by National Natural Science Foundation of China (No. 20573007).

Notes

^a The molar ratio of cationic to anionic surfactants was 1:1.

^a The amount of precipitates increased with the concentration of α-CD from 11 to 20 mM.

^a The added concentration of salts was 1 M in all systems.

^b It meant that such amount of salts was needed to dissolve the precipitates.

^c KCl was unable to dissolve the precipitates before the formation of its own precipitates because of its relatively low solubility.