Determination of critical micelle concentration of aerosol-OT using time-of-flight secondary ion mass spectrometry fragmentation ion patterns.

Aggregation patterns and fragmentation ion data from thin film preparations of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (aka Aerosol-OT (AOT)) near the critical micelle concentration (CMC) in carbon tetrachloride were determined using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Previous work using electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to determine the chemical structure of AOT aggregates was compared to data from ToF-SIMS results from both positive and negative ion spectra. Quasi-molecular ions were detected for AOT in the positive and negative spectra at m/z 467 and 421, respectively, corresponding to [AOT+Na]+ and [AOT-Na]-. Repeating ion patterns assigned to AOT aggregates were detected in the positive spectra from n=3 to n=13, corresponding to the repeating series [AOTn+Na]+. A similar pattern [AOTn-Na]- was observed in the negative ion spectra from n=4 to n=14. ToF-SIMS analysis was also able to detect a previously unreported fragmentation pattern in the mass region below [AOT3+Na]+ when the film was cast from a solution with AOT concentration above the CMC. This pattern is observed starting at m/z 526 and continuing until the n=3 AOT is reached at m/z 1356 in the positive spectra. The pattern of ions is assigned to structures related to the sodium and sulfate ions from the headgroups of an aggregate of AOT molecules. The formation of the low mass pattern is shown to respond only to concentrations above the CMC, and allows for a more precise determination of CMC than previously reported methods. The CMC of AOT in carbon tetrachloride is shown to be between 2.0x10(-5) and 3.0x10(-5) molar.