Conventional surfactants encounter limitations for application in oil reservoirs; however, combining surface-active ionic liquids (SAILs) with conventional surfactants presents an opportunity to enhance the interfacial properties of crude oil–water systems, giving also economic benefits. Accordingly, blends of a long-chain cationic imidazolium-based SAIL, namely, 1-dodecyl-3-methylimidazolium chloride, [C12mim][Cl], and the anionic conventional surfactant, sodium dodecyl sulfate were investigated here. Initial experiments with individual surfactants revealed efficient adsorption and consistent adsorption parameters. Subsequently, the use of mixtures showed synergistic effects for interfacial tension reduction of up to 86.0%, and critical micelle concentration reduction of 72.1% compared to the linear contribution of individual components. These improvements were observed at the optimal SAIL mole fraction of 0.3 and the mixture concentration of 0.003 mol dm−3, resulting in interfacial tension reduction from 29.1 to 1.6 mN m−1 as well as achieving a low critical micelle concentration of 2.7 × 10−3 mol dm−3 coinciding with 83.6% synergy. These findings underscore the favorable interactions between oppositely charged components in the mixtures, amplifying their activity beyond the linear contributions of the individual surfactants. Additionally, theoretical assessments using the Gibbs adsorption equation and the Rosen model provided insight into the adsorption behavior of both the individual surfactants and their mixtures, together with reasonable variations in the corresponding parameters.
