Ultrasonic Absorption Studies of Aqueous Solutions of Cetyltrimethylammonium Bromide and 2,6-O-Dimethyl-β-cyclodextrin

The results of ultrasonic absorption and conductance studies of 1:1 complexes of 2,6-O-dimethyl-β-cyclodextrin (β-DMCD) and cetyltrimetylammonium bromide (CTAB) at 25°C are reported. CTAB was chosen as a guest species to eliminate any contribution to the experimental sound absorption from the monomer surfactant–micelle exchange process. The resulting sound absorption is considered to be primarily due to the 1:1 complex of β-DMCD:surfactant, and its amplitude is observed to be smaller than that of β-DMCD in aqueous solution. This suggests that the intermolecular interaction between the host and the guest species makes the structure of the 1:1 complex more rigid than that of β-DMCD and, consequently, the degrees of freedom contributing to sound absorption are reduced. This is in contrast to the results observed for β-CD:surfactant 1:1 complexes, for which it appears that the intermolecular guest/host interactions in the complex loosen the more rigid structure of β-CD, which exists because of the presence of more intramolecular hydrogen bonding sites in β-CD than in modified β-CD. This increases the sound absorption of the complex relative to β-CD.