ISOPIESTIC COMPOSITIONS OF AQUEOUS IONIC SURFACTANT SYSTEMS AS A MEASURE OF PREFERENTIAL INTERACTIONS - APPLICATION TO THE DETERMINATION OF MICELLE AGGREGATION NUMBERS BY EQUILIBRIUM ULTRA-CENTRIFUGATION

The determination of reliable micelle aggregation numbers for ionic surfactants by means of equilibrium ultracentrifugation requires a correction for the preferential interactions which occur in multicomponent charged systems. Isopiestic distillation experiments on solutions containing the volatile solvent and the nonvolatile supporting electrolyte and varying amounts of the ionic surfactant lead directly to the desired correction for preferential interactions. Results for the surfactant, sodium dodecyl sulfate (SDDS), in various NaCl(aq) background solutions show that a substantial correction, increasing with increasing NaCl concentration, is required. Results on the surfactant, sodium decylsulfonate (SDS), show a similar correction above the critical micelle concentration (cmc). Data for SDS below the cmc show a much different trend from that above, a condition not evident with SDDS because of the much lower cmc. If the interaction is interpreted as arising strictly from micelle charge, values for the micelle charge can be obtained. Results for SDDS indicate a fractional charge ranging from 0.356 for SDDS in 0.1 m NaCl(aq) to 0.418 in 0.3 m NaCl(aq). Sedimentation equilibrium experiments on SDDS in NaCl(aq) solutions gave aggregation numbers in good agreement with published values though generally higher. These higher results are consistent with the more reliable corrections for charge effects or preferential interactions available from the isopiestic distillation experiments. The aggregation numbers for SDDS increased with increasing NaCl concentration, consistent with published results. Precision density measurements, using a magnetic float densimeter having a precision of 1 ppm, were used to obtain accurate values for the partial specific volumes (v̄) of SDDS in NaCl(aq) and water for use in the ultracentrifuge calculations. The results show a slight increase in 0 with NaCl concentration. Values for the cmc obtained from the density measurements are in excellent agreement with published values. A model relating the increasing fractional charge on the micelle to decreasing micelle surface area to micelle volume ratio as a function of NaCl concentration is presented.