Thermotropic phase behaviour of long-chain alkylmaltosides

Abstract

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-β-D-maltoside (C₁₄G₂) and n-hexadecyl-β-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 °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 °C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.