Structural insights into pyridinium-based dicationic surfactants at harsh conditions: Influence of spacer groups on thermal stability and surface properties

The structural properties of surfactants are essential for understanding their behavior and sustainability under extreme conditions, such as high temperatures and varying salinities. In this study, four pyridinium-based dicationic surfactants with different hydrophilic and hydrophobic spacer groups were synthesized and structurally characterized. Thermogravimetric analysis (TGA) and surface tension measurements were conducted to investigate the influence of spacer groups on thermal stability and surface properties. The results demonstrated that all four surfactants demonstrated remarkable solubility in various aqueous environments-seawater (SW), deionized water (DW), and formation water (FW)-at both ambient and elevated temperatures (90 °C), with no phase separation or cloudiness observed after three weeks. TGA results revealed high thermal stability, with decomposition temperatures close to 300 °C, highlighting their robustness for applications requiring sustainable materials with extended lifespans. Surface tension and critical micelle concentration (CMC) measurements showed that surfactants with hydrophilic spacers achieved lower surface tension and more efficient micelle formation than those with hydrophobic spacers. Notably, the dicationic surfactant with a hydroxy spacer exhibited the lowest CMC, indicating a tightly packed micelle structure, advantageous for efficient resource utilization. The lowest surface tension value of 31.790 mNm⁻¹ was obtained for GS-NH. These findings provide valuable insights into the structural design of dicationic surfactants for sustainable engineering, particularly for applications like CO₂ storage and enhanced oil recovery, where thermal resilience and salt tolerance are critical.