Thermotropic phase behaviour of long-chain alkylmaltosides.

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C14G2) and n-hexadecyl-beta-D-maltoside (C16G2) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C14G2 has the same melting point as that of C16G2, namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C14G2 crystallises from water at room temperature, whereas the hemihydrate of C14G2 crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.

Solid-state phase behaviour of dodecylglycosides.

The solid-state phase behaviour of lyophilised n-dodecyl-beta-D-glucoside (beta-C(12)G(1)), n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) and n-dodecyl-beta-D-maltotrioside (beta-C(12)G(3)) has been investigated by differential scanning calorimetry (DSC) and X-ray techniques. For beta-C(12)G(1), lyophilisation results in a formation of a crystalline anhydrate. The lamellar spacing (37 Angstroms) is consistent with an alkyl chain packing in which the chains are not interdigitated. At 80 degrees C, the material melts into a lamellar liquid crystal with a lamellar spacing of 32 Angstroms, which suggests that the non-interdigitated chain packing of the crystalline state is retained in the liquid crystal. In contrast, lyophilisation of beta-C(12)G(2) and beta-C(12)G(3) results in the formation of a glassy state, best described as a frozen version of the lamellar liquid crystal. For beta-C(12)G(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 degrees C for beta-C(12)G(2) and 100 degrees C for beta-C(12)G(3), which compares favourably with the glass transition of the parent carbohydrates. A second crystalline modification of beta-C(12)G(1) was prepared by precipitation from an aqueous solution at temperatures below the Krafft point (38 degrees C). For this modification, the lamellar distance (24 Angstroms) is consistent with interdigitated alkyl chains. At 50 degrees 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 beta-C(12)G(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.