Molecular self-assembly in nonionic surfactant-cyclodextrin systems

Complex formation between nonionic poly(etylene oxide)-containing surfactan ts [Tritons X-45 and X-100, poly(ethylene glycol)-1000 monostearate, and Br ij 35] and cyclodextrins (CD) was studied. It was shown that insoluble crys talline substances are formed during the mixing of aqueous saturated soluti ons of alpha-CD and surfactants. The parameters of the crystalline structur e and the composition of complexes are similar to those of molecular neckla ces based on poly(ethylene oxide) (PEO). The influence of CD on surfactant micellization was studied by measuring the surface tension. It was shown th at the formation of complexes between CD and the PEO fragments of surfactan ts results in an increase in the CMC. Two CMCs corresponding to the free an d modified surfactants are registered in the surface tension isotherms of m ixed surfactant-CD solutions. The study of the dependence of the CMC on the CD-to-surfactant molar ratio makes it possible to determine the compositio n of the inclusion complex in the solution. The stoichiometric compositions of complexes in the solution and in the crystalline state are in good agre ement with each other. The localization of the molecules of alpha- and beta -CDs on the Triton X-100 molecule was studied by UV spectroscopy. It was es tablished that alpha-CD molecules occupy the PEO-block, while the molecules of beta-CD occupy the hydrophobic moiety of the surfactant molecule. It wa s demonstrated that a nonstoichiometric complex is formed between alpha-CD and Brij 35 where alpha-CD molecules fill the hydrophobic region of a surfa ctant molecule, which leads to the loss of micelle-forming properties.