Effect of hydrophobic and hydrophilic groups on physicochemical properties of nonionic gemini surfactants for oilfield application

In recent years, the continuous surge in developing stable surfactants capable of withstanding harsh reservoir conditions has attracted considerable attention for enhanced oil recovery (EOR) applications. In this context, four new nonionic gemini surfactants ( NGS-1 , NGS-2 , NGS-3 , and NGS-4 ) with different tail groups and different ethoxy units were synthesized. The surfactants were prepared using various alkyl ethoxylated alcohols as tails and α,α′-dibromo-p-xylene as the spacer, and their structures were confirmed by NMR and FTIR. The thermal stability was evaluated using a thermal gravimetric analyzer (TGA), while solubility tests were performed in deionized water and saline water. Furthermore, surface properties, such as critical micelle concentration (CMC), surface tension at CMC (γ CMC ), maximum surface excess (Г max ), and minimum area per molecule (A min ), Gibbs free energy of micellization ( \(\Delta {G}_{mic}^{^\circ })\) and Gibbs free energy of adsorptions ( \(\Delta {G}_{adv}^{^\circ })\) were determined. The results showed that most synthesized surfactants displayed excellent solubility, except for NGS-3 , which contained fewer number of ethoxy units. However, the incorporation of some additional ethoxy units in the surfactant structure ( NGS-4 ) improved solubility under saline conditions. TGA thermograms demonstrated that the thermal decomposition temperatures of all surfactants were above the reservoir temperature (90 °C). The CMC results indicated that surfactants with long linear alkyl chains exhibited lower CMC values compared to those with branched alkyl chains. These results revealed the impact of the chemical structures of these surfactants in achieving the desired characteristics for the oilfield application in harsh conditions.