Determination of Critical Micelle Concentration of Aerosol-OT Using Time-of-Flight Secondary Ion Mass Spectrometry Fragmentation Ion Patterns

Abstract

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.

Do electrospray mass spectra of surfactants mirror their aggregation state in solution?

“…[1][2][3][4] Following this idea, a method for the determination of the critical micelle concentration (CMC) of sodium bis(2-ethylhexyl) sulfosuccinate (AOTNa) in carbon tetrachloride by collecting TOF-SIMS spectra has been described. [5] However, in contrast with this latter report and consistently with the intense solvent evaporation involved in electrospray ionization, we have observed the occurrence of surfactant aggregates using starting solutions below the CMC of AOTNa and independently either on the ionization technique (ESI-MS and MALDI-MS) or the polarity of the solvent used (water/methanol mixture or n-hexane). [6] Concerning the surfactant structural organization in gas phase, it has been claimed that micellar structures in the starting solution can be preserved in the high vacuum conditions of the mass spectrometer and can survive the phase transition, without changing the orientation of the surfactant molecules.…”

“…observed the occurrence of species with an average weight formally corresponding to that of the aggregates in solution . Following this idea, a method for the determination of the critical micelle concentration (CMC) of sodium bis(2‐ethylhexyl) sulfosuccinate (AOTNa) in carbon tetrachloride by collecting TOF‐SIMS spectra has been described . However, in contrast with this latter report and consistently with the intense solvent evaporation involved in electrospray ionization, we have observed the occurrence of surfactant aggregates using starting solutions below the CMC of AOTNa and independently either on the ionization technique (ESI‐MS and MALDI‐MS) or the polarity of the solvent used (water/methanol mixture or n ‐hexane) …”

Degrees of freedom effect on fragmentation in tandem mass spectrometry of singly charged supramolecular aggregates of sodium sulfonates

“…These species have been selected as typical representatives of noncovalent aggregate forming substances, more simple but structurally similar to the surfactant sodium bis(ethylhexyl)sulfosuccinate (AOT). AOT aggregates have been observed under ESI, fast atom bombardment, matrix assisted laser desorption ionization, and ToF‐SIMS conditions both as positive and negative ions. Self‐assembling sodiated AOT aggregates have also been investigated in vacuo by molecular dynamics simulation .…”

Mass Spectrometry Study of Multiply Negatively Charged, Gas-Phase NaAOT Micelles: How Does Charge State Affect Micellar Structure and Encapsulation?

“…In contrast with enormous work devoted to NaAOT aggregates in solution, only a handful of investigations have been reported concerning NaAOT aggregates as charged species in the gas phase [8][9][10][11][12][13][14][15][16], including electrospray ionization (ESI) and matrix-assisted laser desorption/ionization mass spectra [10,14], time-offlight secondary ion mass spectra [16] and infrared multiple photon dissociation spectra [13] of singly and doubly positively charged NaAOT aggregates. These measurements revealed reverse micelle-like structures for these aggregates.…”