A New Look at the Isomeric Purity and Its Impact on the Thermotropic Properties of Alkylbenzene Sulfonate-Derived Catanionic Surfactants

Thermotropic polymorphism of catanionic surfactants and its dependence on molecular structure was studied in seven catanionic surfactants with varying cationic and anionic chain lengths that were synthesized using isomerically pure para-alkylbenzene sulfonates as the anionic moiety and quaternary ammonium as its cationic counterpart. Thermal and structural properties of these materials were studied using TGA, DSC, POM, and VTXRD. A comparison of the thermotropic properties of these compounds with those of ortho/para mixture compounds derived from commercial alkylbenzene sulfonates is presented. Catanionic surfactants synthesized from corresponding alkylbenzene sulfonates exhibited similar thermal stabilities that did not show a dependence on the chain length or the structural isomerism of the anionic moiety. All the isomerically pure compounds, except one, formed hexagonal crystalline phases at high temperatures. Their polymorphism depended on the number of alkyl chains, while the crystal formation was controlled by headgroup interactions. This behavior is different than that of surfactants synthesized from ortho/para mixtures. The temperature range over which polymorphs were formed in these materials showed a complex dependence on the isomeric purity of the para-alkylbenzene sulfonates, chain length, and number of alkyl chains.