Solid-state inclusion compounds of small amphiphilic molecules (CnEm) in β-cyclodextrin: a study at defined relative humidities

Abstract

Solid-inclusion compounds of ethylene glycol butyl ether (C₄E₁), di(ethylene glycol) butyl ether (C₄E₂) and di(ethylene glycol) hexyl ether (C₆E₂) in β-cyclodextrin (βCD), with general formula represented by the notation {βCD·C_nE_m}, were prepared from aqueous solution and characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), FT-Raman and ¹³C CP MAS NMR spectroscopies, at ambient humidity, as true hydrated microcrystalline inclusion compounds, pointing to a cage structure for {βCD·C₄E₁} and a channel structure for both {βCD·C₄E₂} and {βCD·C₆E₂}. In addition, the inclusion compounds were investigated at several defined relative humidities (RHs). Several relative intensities of βCD Raman bands generally ascribed to C–O stretching and CH₂ bending vibrations are found to be influenced by the presence of guests in the βCD cavity or the increase of ambient humidity, or by both of these factors. PXRD patterns show that crystalline structures are preserved for RHs equal or above 20%. Interestingly, important changes on the multiplicity of resonances and in the dispersions of ¹³C CP MAS NMR chemical shifts values for all the carbon atoms of the βCD macrocycle are observed in passing from RH 15% to 20%, suggesting amorphous structures below RH 20%. Overall, the above findings converge to stress the structural relevance of hydration water in the βCD inclusion compounds, either with a cage packing arrangement or with a channel structure.