ESTIMATION OF MICELLIZATION PARAMETERS OF AQUEOUS SODIUM DODECYL-SULFATE FROM CONDUCTIVITY DATA

The cmc (critical micelle concentration), counterion binding parameter (beta), and aggregation number (n) of aqueous sodium dodecyl sulfate (SDS) are estimated from conductivity data at various temperatures and salinities. The conductivity models used are based on mass action micellization thermodynamics and the Debye-Huckel-Onsager conductivity theory. With rigorous parameter estimation methods, the models are optimally fit to previous literature data. Moreover, new data were taken with an experimental design which is based on the precise solution of the inverse problem. The average beta and n were determined to be 0.72 and 50, respectively. An increase in temperature from 25 to 60-degrees-C results in a 5% decrease in beta and an increase in the cmc from 8 to 10 mM. The estimated parameters compare well to those based on ion activity, surface tension, and light scattering. The results indicate strongly that SDS micelles contribute substantially to the conductivity but not the effective ionic strength. Literature force-distance measurements for surfaces immersed in aqueous SDS micellar solutions indicated effective ionic strengths which agree with those determined from the conductivity models.