Different building modes of α-cyclodextrin/monoalkyl amphiphile complexes

In this study, the impact of the length of the guest molecule alkyl chain and the crystallization conditions on the structural parameters of α-cyclodextrin (α-CD)/monoalkyl complexes was determined. Several procedures to crystallize those complexes were developed for different alkylalcohols as model guest molecules, as a function of temperature. Three different crystalline structures were identified depending on the alkyl chain length, using synchrotron X-ray diffraction (LURE, Orsay, France). In all cases, complexes crystallize in channel-type structures, where α-CD molecules are stacked like coins in a roll and the alkyl chain of the guest compound is embedded in the tubular cavity of the α-CDs. However, depending on the length of the chains and the crystallization conditions, the channels are organized differently. C6-C8 chains give rise to a pseudo-hexagonal lattice, a packing mode already observed for polyiodide complexes [1]. C10-C12 chains crystallize in a triclinic pseudo-monoclinic C2 lattice, while longer chains up to C18 form hexagonal crystals with R3 symmetry. These two novel crystal structures are described. Understanding these structures opens new routes to nanotube formation through amphiphile-driven crystallization of cyclodextrin templates.