Thermotropic liquid crystalline and surface-active properties of n-alkyl α-d-mannopyranosides

Abstract

A series of nonionic surfactants n-alkyl α-d-mannopyranosides (CnMan) with different alkyl chain length (n=6, 7, 8, 9, 10, 12, 14) have been prepared and the influence of the hydrophobic chain length on their thermal stability, thermotropic phase behavior and surface activity was studied, respectively. They showed fine thermal stabilities and smectic mesophase with TGA, DSC and POM methods. Meanwhile, the lamellar phase of CnMan (n=9, 10, 12, 14) was further confirmed by 2D-WAXD, which indicated that the arrangement of liquid crystal units was tilted and interdigitated layer-by-layer away from the layer normal. With increasing alkyl chain length of CnMan (n=9, 10, 12, 14), the layer d-spacing values of smectic phase increased gradually. Moreover, their HLB number, water solubility and surface tension decreased with increasing the alkyl chain length. Furthermore, their foaming properties increased with increasing the alkyl chain length. C9Man had the strongest foaming characteristic and an excellent foaming stability whereas C8Man and C9Man had the lowest surface tension (γCMC) in the same series. In addition, their emulsifying properties depended on the alkyl chain length and the corresponding oil/water system, C8Man and C9Man had stronger emulsifying activity in n-octane/water system, C9Man and C10Man had stronger emulsifying activity in rapeseed oil/water system.