Molecular design and thermodynamic insights into TnSm-type gemini surfactants for advanced interfacial applications

In this study, nine novel cationic gemini surfactants were designed, synthesized, and comprehensively characterized for potential application in chemical enhanced oil recovery (EOR). The molecular architecture was systematically varied by combining three alkyl chain lengths (C12, C14, C16) with three polymethylene spacers (2, 3, and 4 carbon atoms), enabling a detailed investigation of structure-property relationships. Successful synthesis was confirmed via Fourier Transform Infrared Spectroscopy (FTIR), Proton Nuclear Magnetic Resonance (¹H-NMR), Thermogravimetric Analysis (TGA), and Dynamic Light Scattering (DLS), which provided insights into morphological features, thermal stability, and aggregation behavior. Surface tension and conductivity measurements revealed that the critical micelle concentration (CMC) decreases with increasing alkyl chain length, but increases with spacer length, highlighting the interplay between hydrophobic and hydrophilic interactions. Thermodynamic analysis indicated that both micellization and adsorption are spontaneous processes, with micellization predominantly entropy-driven. Notably, adsorption was significantly more favorable, contributing to substantial interfacial tension reduction, a key requirement for efficient EOR. Furthermore, DLS confirmed the tunability of micelle and aggregate formation as a function of surfactant architecture and concentration, offering a rational pathway for designing tailored surfactant systems under demanding oil recovery conditions.