Solid-state phase behaviour of dodecylglycosides

Abstract

The solid-state phase behaviour of lyophilised n-dodecyl-β- d-glucoside (β-C 12G 1), n-dodecyl-β- d-maltoside (β-C 12G 2) and n-dodecyl-β- d-maltotrioside (β-C 12G 3) has been investigated by differential scanning calorimetry (DSC) and X-ray techniques. For β-C 12G 1, lyophilisation results in a formation of a crystalline anhydrate. The lamellar spacing (37 Å) is consistent with an alkyl chain packing in which the chains are not interdigitated. At 80 °C, the material melts into a lamellar liquid crystal with a lamellar spacing of 32 Å, which suggests that the non-interdigitated chain packing of the crystalline state is retained in the liquid crystal. In contrast, lyophilisation of β-C 12G 2 and β-C 12G 3 results in the formation of a glassy state, best described as a frozen version of the lamellar liquid crystal. For β-C 12G 2, the lamellar spacing in the glass and liquid crystal suggests interdigitation of the alkyl chains. The glass transition temperature was found to be 65 °C for β-C 12G 2 and 100 °C for β-C 12G 3, which compares favourably with the glass transition of the parent carbohydrates. A second crystalline modification of β-C 12G 1 was prepared by precipitation from an aqueous solution at temperatures below the Krafft point (38 °C). For this modification, the lamellar distance (24 Å) is consistent with interdigitated alkyl chains. At 50 °C, the crystalline material melts into a liquid crystalline phase. The material also readily loses water and rapidly re-crystallises to the anhydrate. The amount of water lost upon drying is consistent with the idea that the material is a monohydrate of β-C 12G 1. The drying and re-crystallisation processes give rise to ‘pre-transitions’ in the DSC thermograms and illustrate the importance of careful control of water in any analysis of the phase behaviour of alkylglycosides.