Synthesis, Characterization, and Physicochemical Performance of Nonionic Surfactants via PEG Modification of Epoxides of Alkyl Oleate Esters

The synthesis of surfactants from fatty acid esters via epoxide chemistry has been known for its accompanying challenges, which usually involve the use of toxic homogeneous catalysts in the ring-opening reaction step and generation of many side reaction products. This paper presents environmentally benign routes to a library of nonionic surfactants via a three-step synthesis involving transesterification of methyl oleate to alkyl oleates, epoxidation of the oleate alkene, and solventless heterogeneously catalyzed ring opening of the epoxides with poly(ethylene glycols) of varying chain length under a short reaction time (60 min). The processes were highly atom efficient and afforded a minimum surfactant yield of 80% with limited or negligible side reaction products. The intermediate molecules and synthesized surfactants were purified and comprehensively characterized, including physicochemical measurements: dynamic surface tension and equilibrium surface tension. Additionally, the hydrophilic-lipophilic balance (HLB) concept was used to comprehensively scan through the polarity behaviors of the surfactants' head and tail in solution as a prediction of their end use. The results showed that surfactants have a critical micelle concentration (CMC) lower than 0.1 mg/ml as the alkyl oleate increases in length from ethyl to decyl and that the lower-molecular-weight surfactants reached equilibrium faster than the higher-molecular-weight surfactants. HLB results showed that the surfactants can be applied as oil-in-water emulsifiers, detergents, solubilizers, and wetting agents. In general, the synthesized surfactants potentially possess switchable properties for use in industrial formulations, as the alkyl chain length and the ethylene oxide number in the surfactant's structure are varied.