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Solid Crystal Network of Self-Assembled Cyclodextrin and Nonionic Surfactant Pseudorotaxanes

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posted on 09.09.2010, 00:00 by Andrés Guerrero-Martínez, David Ávila, Francisco J. Martínez-Casado, John A. Ripmeester, Gary D. Enright, Luisa De Cola, Gloria Tardajos
The title system allows the straightforward formation of three-dimensional crystals of self-assembled pseudorotaxanes formed by the nonionic surfactant Igepal CO-520 and β-cyclodextrin (β-CD) in aqueous solution. The work involves a combination of X-ray powder diffraction, high resolution electron transmission microscopy, and 13C CP/MAS NMR studies of the solid crystal, supported by single crystal structural analysis. The results indicate a lamellar self-assembly of pseudorotaxanes with preferential orientation and disorder in the structure. For the single crystal, the unit cell was found to be triclinic (P1) and contains a β-CD dimer. The surfactant molecules are located in the channel formed by these dimers along the c axis of the crystal network. The individual pseudorotaxane structure is formed by a dimer of β-CDs threaded by the oxyethylene hydrophilic segment of Igepal CO-520, and a β-CD dimer that binds the hydrophobic region of the surfactant. Thus, as in a CD polyrotaxane structure, this system results in an ordered self-assembly of pseudorotaxanes through the formation of a network of hydrogen bonds between head-to-head β-CD dimers. Moreover, the analysis of the 1H NMR spectra in solutions of pseudorotaxanes formed by β-CD and Igepals with different lengths of the hydrophilic tails indicates equal stoichiometry patterns of both oxyethyelene and hydrophobic regions for the different supramolecules. Whereas the common hydrophobic moiety threads two macrocycles, the ratio between complexed oxyehtlyene segments and β-CD is 2.5 for the hydrophilic tails. All these results show that nonionic surfactants can be used as alternative and effective linear threads to polymers and copolymers in the synthesis of supramolecular polyrotaxane solid crystals with CDs.