Interfacial Behavior and Synergistic Effects in Mixed Systems of Fatty Alcohol Polyoxyethylene Polyoxypropylene Ether Sodium Sulfate and DTAB

Mixed C 1214 E 9 P 3 S/DTAB systems show pronounced synergistic surface activity. • The lowest CMC and highest activity occur at a molar ratio of 3:7. • Attractive interactions dominate micellization in the mixed system. Surfactants are widely used in detergency, emulsification, and a variety of interfacial processes, and mixed ionic surfactant systems have recently attracted considerable attention due to their tunable self-assembly and interfacial properties that are not accessible in single-component systems. However, a detailed understanding of synergistic interactions and structure–property relationships in anionic–cationic mixed systems remains incomplete. Here, we investigate aqueous mixtures of sodium alcohol polyoxyethylene polyoxypropylene ether sulfate (C 1214 E 9 P 3 S) and dodecyl trimethyl ammonium bromide (DTAB) at constant total concentration and varying molar ratios. Static and dynamic surface tension, zeta potential, and foaming properties were systematically measured, and the interfacial behavior was analyzed using Rubingh’s regular solution theory and thermodynamic analysis. The mixed systems exhibit substantially lower critical micelle concentration (CMC) and surface tension at CMC than the corresponding single surfactants, indicating pronounced synergistic effects in adsorption and self-assembly. The strongest interfacial activity is observed at a C 1214 E 9 P 3 S:DTAB molar ratio of 3:7, where the CMC reaches a minimum and surface activity is maximized. Negative interaction parameters derived from the Rubingh model confirm strong attractive interactions between the two surfactants, with stronger association in the micellar phase than at the air–water interface. In addition, systematic variations in surface tension, zeta potential, and foam properties demonstrate that charge neutralization and interfacial packing jointly govern macroscopic behavior. These results provide a coherent picture of the synergistic mechanism in mixed ionic surfactant systems and suggest their potential utility in interfacial formulation processes such as detergency and emulsification. Download: Download high-res image (164KB) Download: Download full-size image