Synthesis and Thermotropic Liquid Crystalline Properties of Heterogemini Surfactants Containing a Quaternary Ammonium and a Hydroxyl Group

homologous series of novel heterogemini surfactants, (N, N-Dimethyl-N-[3- (alkyloxy)-2- hydroxypropyl]-alkylammonium bromide (referred to as CmOhpNCn; m, n = 10, 8; 12, 8; 14, 8; 16, 8; 14, 10; 12, 12 and 10, 14) has been synthesized. Their thermotropic phase behavior has been investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), infrared spectroscopy (IR) and X-ray diffraction (XRD). All the compounds showed high thermal stability and formed enantiotropic thermotropic smectic A liquid crystals. The mesophase region was sensitive to the overall length (m + n) of the two hydrophobic tails and was widened with increasing (m + n). When fixing the overall length of the two hydrophobic tails (m + n =24), the liquid crystal region was affected little by the dissymmetry of the two hydrophobic chains. The combination of IR and XRD results suggested the cis configuration of the alkyl chain and the alkoxy chain with respect to the extended N儃CH2儃CH(OH) skeleton in both crystal lattice and liquid crystal mesophase. The folding of the alkoxy chain at the carbon atom connecting the hydroxyl group led to the hydrogen bonding between the hydroxyl and the Br儃 counterion, which is beneficial to stabilizing the liquid crystal phase.