Determining the critical micelle concentration in O/W emulsion using the rate constant of hydrolysis for benzyl acetate

An attempt to evaluate the kinetically effective critical micelle concentration CMC of sodium dodecyl sulfate (SDS) in micellar solutions and in O/W emulsions at 40 degrees C and pH 9 utilizing the pseudo first order rate constant of benzyl acetate hydrolysis was implemented. The critical micelle concentration of SDS in micellar solutions was determined by both surface tension measurements utilizing Wilhelmy plate technique and by rate constant of hydrolysis. Hydrolysis reaction of benzyl acetate was monitored in surfactant solutions as well as in o/w emulsions as a function of time. Emulsion droplets were controlled using microfluidizer 110T and oily droplets were separated from the emulsion by ultracentrifugation at (11,500rpm or 9,800g) prior to analysis by high performance liquid chromatography. The value of the critical micelle concentration (CMC) in micellar solutions in the presence of benzyl acetate as determined from the Wilhelmy plate technique was 7.8 x 10(-4) moles/L (CMC in micellar solution was 10 times lower than the value in literature due to use of buffer) while the CMC as determined from the kinetic study was 8.8 x 10(-4) moles/L. In emulsion systems, using 5% mineral oil, the CMC value was 8.6 x 10(-3) moles/L and at 10% oil, the value doubled to 1.73 x 10(-2) moles/L. The above results indicate that kinetics can be used to determine CMC in micellar solutions and in o/w emulsions.