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Determination of Critical Micelle Concentration of Aerosol-OT Using Timeof-Flight Secondary Ion Mass Spectrometry Fragmentation Ion Patterns Sarah A. Burns, Paul L. Valint, Jr., and Joseph A. Gardella, Jr.* Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14260 Received June 30, 2009. Revised Manuscript Received August 31, 2009 Aggregation patterns and fragmentation ion data from thin film preparations of the anionic surfactant sodium bis(2ethylhexyl) 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 [AOTþNa]þ 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 [AOTnþNa]þ. 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 [AOT3þNa]þ 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 2.0 10-5 and 3.0 10-5 molar.
Introduction The aggregation and formation of reverse micelles by the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in apolar solution is a phenomenon that has been widely investigated.1-8 and has given rise to applications for reverse micelles ranging from hydrophilic drug delivery to the synthesis of nanostructures.9-17 The variety of applications is based on spontaneous formation of micelles above the critical micelle concentration (CMC), the concentration of AOT in an apolar *Corresponding auther. E-mail:
[email protected]. Tel.: þ1 716 645 1499. (1) Maitra, A. N.; Eicke, H. F. J. Phys. Chem. 1981, 85, 2687–91. (2) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans.2 1981, 77, 601– 29. (3) Jain, T. K.; Maitra, A. Colloids Surf. 1989, 36, 87–95. (4) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409–16. (5) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95–193. (6) Moran, P. D.; Bowmaker, G. A.; Cooney, R. P.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1995, 11, 738–43. (7) Yoshino, A.; Okabayashi, H.; Yoshida, T.; Kushida, K. J. Phys. Chem. 1996, 100, 9592–9597. (8) Boissiere, C.; Brubach, J. B.; Mermet, A.; de Marzi, G.; Bourgaux, C.; Prouzet, E.; Roy, P. J. Phys. Chem. B 2002, 106, 1032–1035. (9) Ono, T.; Goto, M. Curr. Opin. Colloid Interface Sci. 1997, 2, 397–401. (10) Goto, M.; Ono, T.; Horiuchi, A.; Furusaki, S. J. Chem. Eng. Jpn. 1999, 32, 123–125. (11) Mitra, S.; Maitra, A. N. Int. J. Surf. Sci. Technol. 2000, 16, 316–330. (12) Melo, E. P.; Aires-Barros, M. R.; Cabral, J. M. S. Biotechnol. Annu. Rev. 2001, 7, 87–129. (13) Ono, T.; Kawakami, K.; Goto, M.; Furusaki, S. J. Mol. Cat. B: Enzym. 2001, 11, 955–959. (14) Melo, E. P.; Baptista, R. P.; Cabral, J. M. S. J. Mol. Cat. B: Enzym. 2003, 22, 299–306. (15) Gupta Reeta, R.; Jain Swantrant, K.; Varshney, M. Colloids Surf., B. 2005, 41, 25–32. (16) Chavanpatil, M. D.; Handa, H.; Mao, G.; Panyam, J. J. Biomed. Nano. 2007, 3, 291–296. (17) Biasutti Maria, A.; Abuin Elsa, B.; Silber Juana, J.; Correa, N. M.; Lissi Eduardo, A. Adv. Colloid Interface Sci. 2008, 136, 1–24.
11244 DOI: 10.1021/la902343r
solution in the presence of water.4,18-21 One application involves the reverse micelle encapsulating and protecting a molecule that is dissolved into the water from the surrounding apolar solvent. Studies of encapsulated molecules have focused on characterization of the encapsulation and release kinetics of a molecule inside the reverse micelle.22-25 The physical and chemical properties of AOT and molecules encapsulated within AOT have been investigated with spectroscopic methods such as infrared (IR) spectroscopy, nuclear magnetic resonance, and fluorescence spectroscopy. These methods have been used to probe the changes in the physical nature of water or an encapsulated molecule within the core of the micelle when the concentration of AOT is raised above the CMC. IR spectroscopic analysis is the most widely used instrumental method for AOT encapsulation and CMC analysis. An easily distinguishable IR absorbance for the SO3 headgroup and water interaction during the encapsulation process is used for determination of CMC and related formulation questions.4,6,8,20 These studies have found that, when AOT is dissolved in apolar solvents with no added water above the CMC, there is an increase in the interaction between the SO3- and sodium counterion. When additional water is added to the AOT solution, there is an increased separation between the sodium counterions and the SO3- groups. This indicates that the reverse micelle is formed with (18) Muto, S.; Meguro, K. Bull. Chem. Soc. Jpn. 1973, 46, 1316–1320. (19) Maitra, A. J. Phys. Chem. 1984, 88, 5122–5. (20) D’Angelo, M.; Onori, G.; Santucci, A. J. Phys. Chem. 1994, 98, 3189–93. (21) Hossain, M. J.; Hayashi, Y.; Shimizu, N.; Kawanishi, T. J. Chem. Eng. Soc. Jpn. 1996, 29, 381. (22) Changez, M.; Varshney, M. Drug Dev. Ind. Pharm. 2000, 26, 507. (23) Degim, I. T.; Celebi, N. Curr. Pharm. Des. 2007, 13, 99–117. (24) Ono, T.; Goto, M. Curr. Opin. Colloid Interface Sci. 1997, 2, 397–401. (25) Melo, E. P.; Aires-Barros, M. R.; Cabral, J. M. Biotechnol. Annu. Rev. 2001, 87–129.
Published on Web 09/04/2009
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a hydrophilic core and that the size of the micelle can be affected by the volume of water added. These studies have also determined the average number of water molecules that interact with the SO3- and sodium counterion based on the amount of water present in solution. This approach allows the determination of the CMC over a broad concentration range and the physical properties of the reverse micelle, but cannot detail any structural information about individual AOT molecules or head groups. IR spectroscopy and other methods are typically restricted to determining the interaction of encapsulated water within the reverse micelle or characterizing target molecules that have been encapsulated.5 Lyon and Stebbings first reported fast atom bombardment (FAB) mass spectrometry to analyze a variety of anionic surfactants, which included AOT.26 This study reported the detection of aggregates of AOT where [AOTn þ Nanþ1]þ from n = 1 to n = 3. This study was the first to utilize mass spectrometry to determine aggregation numbers and the fragmentation characteristics of anionic surfactants. In a more recent study, electrospray ionization (ESI) mass spectrometry has been used to investigate the aggregation number and conformational changes in micelle structure from micelles, reverse micelles, and protein encapsulating reverse micelles. Sharon et al. reported results from ESI and tandem mass spectrometry documenting structural changes in the gas of cetyltrimethylammonium bromide (CTAB) based normal and reversed micelles.27 CTAB micelles can vary from normal to reversed depending upon the organic solvent-to-water ratio in solution. The differences between the micellelar structures were determined using the fragmentation pattern, which was distinctive of the type of micelle that was formed. In addition, the mass spectra were used to determine the number of CTAB molecules that must be present in order for micellization and subsequently, encapsulation of a protein to occur. Aggregation numbers of 270 CTAB molecules were determined using mass spectrometry which represented instrumental data of the largest surfactant clusters that had been reported. This study also demonstrated the capability of mass spectrometry in surfactant research. ESI along with matrix-assisted laser desorption/ionization (MALDI) mass spectrometry have also been used to determine aggregation numbers of AOT in solution and for the analysis of the chemical structure of AOT in solution. Bongiorno, et al. used ESI coupled with tandem mass spectrometry and MALDI mass spectrometry in order to determine surfactant self-assembly patterns of AOT.28 The AOT and water were dissolved in solvent and then this solution was directly introduced into the ESI system. The aggregation data is representative of what occurred in the droplets of solution which were aerosolized for analysis in the gas phase in the mass spectrometer. This type of analysis can also determine bonding information between AOT aggregates by interpretation of ion structure and fragmentation patterns. This study found that AOT aggregation was observed in the repeating pattern due to the series of [AOTn þ Na]þ ions in the positive ion spectra and [AOTn - Na]- in the negative ion spectra from n = 1 until n = 22. The number of repeating AOT molecules in the aggregate is dependent upon the concentration of AOT in solution where a higher concentration yielded a higher n. The repeating ion detected by both ESI and MALDI methods was [AOTn þ Na]þ; no other ions or series of ions were detected with structures including solvent molecules. The appearance of this (26) Lyon, P. A.; Stebbings, W. L. Anal. Chem. 1984, 56, 8–13. (27) Sharon, M.; Ilag, L. L.; Robinson, C. V. J. Am. Chem. Soc. 2007, 129, 8740– 8746. (28) Bongiorno, D.; Ceraulo, L.; Ruggirello, A.; Liveri, V. T.; Basso, E.; Seraglia, R.; Traldi, P. J. Mass Spectrom. 2005, 40, 1618–1625.
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Letter
one main aggregation pattern indicates that, in the gas phase after desorption using both ESI and MALDI, there is an absence of an AOT-water solvated species. The authors hypothesized that sodium ions acted to bond extended head groups in the aggregates. Analysis by the ESI and MALDI methods can give chemical bonding information from fragmentation data, but that is hindered by the interference of the solvent in ESI and the matrix for MALDI. Time-of-flight secondary ion mass spectrometry (ToFSIMS) does not require the use of a matrix and therefore can be used to analyze the full mass spectrum of AOT aggregates. The analysis is limited to studying the AOT in a thin film solid phase preparation from solution or in high viscosity (e.g., glycerin, as used in FAB SIMS) solution. Thin solution-cast films were prepared from solutions of AOT where the concentrations prior to thin film preparation were above and below the literature value for the solution CMC. ToF-SIMS analysis was used to determine the aggregation pattern and determine, from the fragmentation, proposed ion structures. The results of this analysis show a fragmentation pattern from the thin films of AOT that is dependent upon the solution phase concentration. This fragmentation pattern is only observed when the solution phase concentration is between the proposed range for the CMC of AOT in chloroform (10-3-10-5 M). The solid phase fragmentation pattern provides a new method to determine the CMC of AOT in solution with more specificity than traditional instrumental methods.
Experimental Section AOT Film Preparation. Aerosol-OT (AOT, Docusate Sodium Salt) was purchased from Sigma-Aldrich (St. Louis, MI) and used as received (>99% pure with
