Cationic gemini surfactant with cleavable spacer: Emulsion stability

In this study, a cationic gemini surfactant with cleavable spacer and two monomeric single chain counterparts were synthesized and used. The critical micelle concentration (CMC) was measured by tensiometry for these surfactants. The emulsion stability of (Toluene 30:70 Water) emulsions in the presence of these surfactants and at various concentrations was determined by two dispersion testers (i.e. Turbiscan^® under gravity force and Lumisizer^® analytical centrifuge). The surfactant critical concentration for the emulsion stability was found to be at 2 × CMC. The emulsion stability in the presence of these surfactants followed the same trend and remained almost constant at surfactant concentration (2–6 × CMC). The gemini surfactant with lower CMC value is far better an emulsion stabilizer on concentration basis than the monomeric surfactants. The phase separation velocities of the emulsions as a function of the relative centrifugal force were fitted with linear and power correlations. The results showed that the estimated separation velocities at earth's gravity (1 × g) are larger than the measured values by Turbiscan for all of the surfactants. While the accelerated phase separation by Lumisizer is a fast and easy method for testing the emulsion stability and comparing the predicted shelf lives, the extrapolation and estimation of the real emulsion stability should be considered cautiously.