Solution and emulsion behavior of highly soluble carboxylate surfactants with α-substituted phenoxy group

The aqueous solubility of surfactants is an important physicochemical property for their applications, and is expected to be improved by altering the molecular structures. Herein, a series of carboxylate surfactants with α-substituted phenoxy group, namely, sodium α-phenoxy alkyl carboxylates (C n -ph-Na, n  = 12, 14, 16, 18, 20), were synthesized. Their solution and emulsion behaviors were investigated by surface tension, fluorescence probe method, confocal laser scanning microscopy (CLSM) and X-ray diffraction (XRD). Compared with conventional sodium carboxylate surfactants (C n Na), C n -ph-Na surfactants show extremely high water solubility of 1250, 1100, 800, 670 and 100 mmol·L − 1 for n  = 12, 14, 16, 18 and 20, respectively, at 25 °C. The reason for this difference is the presence of the phenoxy group, which leads to the weak cohesive forces between the alkyl tails in the solid state. The high solubility of C n -ph-Na makes it possible to investigate Pickering emulsions stabilized with alumina (Al 2 O 3 ) nanoparticles hydrophobized by C n -ph-Na in situ. Both C 12 -ph-Na and C 14 -ph-Na can induce double phase inversion of the emulsions with increasing surfactant concentrations. The present work demonstrates the feasibility of adding hydrophobic groups to promote the water solubility of surfactants, providing a new approach for the molecular design and extending application of surfactants.