Impact of Surfactant Structure on NAPL Mobilization and Solubilization in Porous Media

Additives such as surfactants are commonly used to enhance the remediation of oil-contaminated rocks. Although the majority of crude oils are light nonaqueous phase liquids (LNAPLs), they often contain heavy molecules such as asphaltenes that are classified as dense nonaqueous phase liquids (DNAPLs). Surfactants are able to reduce the interfacial tension between water and LNAPLs and enhance their mobilization. Furthermore, the formation of microemulsions by surfactants can promote the solubilization of DNAPLs and restore the wettability of contaminated surfaces. Numerous studies have indicated that surfactants can promote the cleanup of oil-contaminated rocks; however, the impact of surfactant structure on solubilization and mobilization is still unclear. In this study, we investigated the remediation of heterogeneous aquifer rocks using four different nonionic surfactants: n-dodecyl β-d-maltoside, Triton X-100, Biosoft N1-7, and Saponin. The goal was to develop an improved understanding of the role of the surfactant molecular structure on nonaqueous phase liquids (NAPLs) removal through solubilization and mobilization. Each of these surfactants has a unique structural characteristic in its hydrophilic or hydrophobic segment. Dodecyl β-d-maltoside contains hydroxyl groups in its hydrophilic segment, which tend to form strong hydrogen bonds, while Triton X-100 has branched-chain alkyl groups in its hydrophobic segment that are more soluble in NAPL. Alkyl ethoxylated surfactants such as N1-7 display the simplest structure, while saponin with a heavy and complex structure cannot solubilize NAPLs as fast as other surfactants. Through measurements of phase behavior, dynamic interfacial properties, adsorption, spontaneous imbibition, thin sections analysis, and high resolution transmission electron microscopy, we showed that microemulsions formed by these surfactants are able to mobilize LNAPLs, especially in the presence of branched-chain alkyl groups in the hydrophobic segments (such as those in Triton X-100), which promoted higher reduction in the interfacial tension between NAPL and brine. Micellar solubilization, on the other hand, was favored by the hydroxyl groups in hydrophilic segments (such as those in n-dodecyl β-d-maltoside), which were able to form strong hydrogen bonds at interfaces and favor the desorption of DNAPL from mineral surfaces. Following a different trend from the other surfactants, saponin with a higher solubility in brine showed a tendency to self-aggregate and form micron-size clusters of microemulsions, which slowed down the NAPL remediation.