Thermotropic Liquid Crystals of Double-Chain Zwit-terionic Surfactants (C16)2NCnS

A series of double-chain sulfobetain zwitterionic surfactants with varying carbon number of the intercharge group (spacer), (C16H33)2NCH3(CH2)nSO3, where n=2, 3, 4, and 6, referred to as (C16)2NCnS, were synthesized. The influence of the spacer length on the thermal behaviors of (C16)2NCnS in their pure state was studied using thermogravimetry (TG), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). Thermogravimetric analysis (TGA) showed that the thermal stability of (C16)2NCnS lowered with increasing the spacer length. All compounds showed a complex polymorphism. Only for (C16)2NC4S and (C16)2NC6S, thermotropic liquid crystals were observed using POM. The former exhibited a smectic A (SmA) phase, whereas the latter formed a hexagonal columnar phase. These liquid crystals obtained both in the cooling and the second heating scans provided compelling evidence for the thermal stability of these compounds.