Effect of Hydrocarbon Chain Length on the Performance of Cationic Silicone Surfactants

Three quaternary ammonium cationic silicone surfactants with different carbon-hydrogen chain lengths are synthesized by a two-step process. The molecular structure of the surfactants has been researched by infrared spectroscopy, nuclear magnetic resonance, and mass spectrometry. Subsequently, the effects of carbon-hydrogen chain length on the surface activity, aggregation behavior, and micellization process of surfactants were investigated through methods such as surface tension, electrical conductivity, and dynamic light scattering, providing theoretical guidance for the application of surfactants. It was found that the three silicone surfactants have excellent surface activity, with their critical micelle concentration (CMC) being 9.83, 6.81, and 4.56 mmol·L^-1, respectively, while the γ_CMC (surface tension at CMC) being as low as 20.95, 20.73, and 20.17 mN·m^-1, respectively. As the carbon-hydrogen chain increases, the area occupied by a single silicone surfactant molecule at the gas-liquid interface (A _S) increases, and the CMC values decrease significantly. The size distribution of the three silicone surfactants is unimodal, and they can self-assemble into nonuniform sized spheroidal aggregates. Moreover, the values of ΔG _m ⁰ range from -25.85 to -28.45 kJ·mol^-1, indicating that the micellization process of SiCHs is spontaneous. Based on the values of ΔH _m ⁰ and -TΔS _m ⁰, we can see that the increase in the carbon-hydrogen chain gradually causes the micellization process to shift from entropy-driven to enthalpy-driven.