As-synthesized oleic amido propyl betaine surfactant mixture, that was produced through a “direct formulation through synthesis” process, exhibited ultra-low oil/water interfacial tension (IFT) values as low as 3.5 × 10−4 mN/m when dissolved in seawater at a reservoir temperature of 96 °C. The as-synthesized surfactant, which was left untreated, had a slightly cloudy appearance when mixed with seawater. Polar solvents were introduced to this surfactant to improve its aqueous solubility by changing its overall hydrophilicity, particularly on the oil/water interface. In this study, two types of glycol ether co-solvent, i.e., ethylene glycol monobutyl ether and diethylene glycol monobutyl ether, were used at different concentrations in a single application and as a mixture of co-solvents at a certain ratio. The behavior of the as-synthesized surfactant with the presence of these co-solvents was investigated. As a result, it showed that the co-solvent helps in solubility improvement and alters the interfacial tension behavior of the surfactant. Ethylene glycol monobutyl ether was found to be efficient in maintaining an ultra-low IFT value of the surfactant. However, the aqueous solubility of the surfactant was not significantly improved. In contrast, the addition of diethylene glycol monobutyl ether showed improvement of the aqueous solubility of the surfactant, but it tends to increase the IFT above ultra-low value. Based on this understanding, a set of co-solvent mixing ratios were tested, and the results showed further improvement in both the oil/water interfacial tension behavior and surfactant aqueous solubility. The most stable oil/water IFT of 3.36 × 10−3 mN/m and clearer surfactant solutions were obtained for ratio C at 35 wt.% presence of co-solvent.
Improved As-Synthesized Oleic Amido Propyl Betaine Surfactant Mixture for Stable Ultra-Low Interfacial Tension: Effect of Mixed Co-Solvents
