In the Laboratory
Structure and Solvent Properties of Microemulsions Civia A. Katz, Zachary J. Calzola, and Jeremiah K. N. Mbindyo* Department of Chemistry, Millersville University, Millersville, PA 17551-0302; *
[email protected] Microemulsions are technologically important fluids that can reduce the use of toxic organic solvents (1, 2). They are prepared by mixing water, oil, and a surfactant. Sometimes, a fourth component termed a co-surfactant may also be added. The resulting mixture is a clear, optically transparent, and thermodynamically stable liquid. Despite the growing importance of microemulsions as environmentally friendly solvents, there are few published experiments that can be used in the undergraduate laboratory class room to investigate their properties. The experiment described here is aimed at bridging this gap. Theoretical Background To understand the structure of microemulsions, it is helpful to first consider the formation of micelles because both have similar structure at the nanometer scale but microemulsions are more complex. Surfactants, also known as “surface-active agents” are key components of micelles and microemulsions. They are compounds that have distinct hydrophobic and hydrophilic regions in their structure. An example is sodium dodecyl sulfate (SDS) shown in Figure 1. It is this category of compounds that gives detergents their cleaning power. If a drop of a long-chain surfactant dissolved in a non-polar solvent is added to water, it will spread. If the solvent evaporates, a thin layer of the surfactant known as a Langmuir film is formed on top of the water. This film can be compressed into a compact layer in which the tail groups are in the air and the head groups are in solution. Langmuir films can be transferred onto a glass slide or other solids and the process repeated to form thin films with precisely controlled thickness and orientation. Such ordered films assembled on solid substrates are known as Langmuir–Blodgett films. At low concentrations in aqueous solutions, surfactants are randomly distributed in water (Figure 2A). However, at higher concentrations, the molecules assemble into spherical aggregates called micelles. The hydrophobic tails are found in the core of the micelles and the hydrophilic head groups on the outside (Figure 2B). The concentration at which micelles form is called the critical micelle concentration (CMC). The diameter of SDS micelles is about 2 nm. If a non-polar liquid is added to a solution containing micelles, it will dissolve in the cores of the micelles where the surrounding environment is hydrophobic. As more oil is added, the micelles swell. Eventually, a pool of oil is formed at the center
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of each micelle, transforming the micelle solution into a microemulsion. The droplet size in microemulsions is generally in the range of 20–100 nm. Microemulsions are classified according to their structure. The three common types are oil-in-water, water-in oil, and bicontinuous microemulsions (Figure 2C, D, and E, respectively). Microemulsions are excellent solvents. They can dissolve both polar and non-polar compounds. Because of this property, they have found applications in areas such as drug delivery (3–5), synthesis of nanoparticles (6, 7), oil recovery (8), and as a medium for organic synthesis (9, 10), destruction of chemical warfare agents (11), toxic heavy metals (12), and persistent organic pollutants (13, 14). Pollutant remediation is the clean up of pollutants from the environment especially soils and water. Remediation of sediments is particularly important because some adsorbed pollutants bioconcentrate in aquatic organisms (15, 16). Their accumulation in the human body through consumption of seafood may cause some types of cancer, immune system dysfunction, and reproductive health and growth disorders (17, 18). In this experiment, students prepare a microemulsion and compare the solubility of sudan III dye in the microemulsion and in dodecane. They examine the effectiveness of the two solvents in extracting the adsorbed dye from soil and investigate the structure of micelles and microemulsions using pyrene as a probe molecule. The structure of micelles and microemulsions can be studied using fluorescence probes (19–21). These are compounds such as pyrene that have a fluorescence spectrum that is sensitive to the solvent environment (22, 23). Fluorescence occurs when electrons in the first excited electronic state (S1) of a molecule lose some energy through collision processes then relax to the ground state, releasing photons with lower energy than the exciting radiation (24). Because pyrene will dissolve preferentially in hydrophobic regions, changes in its fluorescence spectrum can signal the formation of micelles or microemulsions.
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Figure 1. Structure of sodium dodecyl sulfate showing the hydrophobic dodecyl chain and the hydrophilic sulfate head group.
Figure 2. Schematic illustration of the structure of micelles and microemulsions: (A) solution of a surfactant below CMC, (B) solution of a surfactant above the CMC, (C) oil-in-water microemulsion, (D) water-in-oil microemulsion, and (E) bicontinuous microemulsion. Note that the figure is not drawn to scale.
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Chemicals Sodium dodecyl sulfonate (SDS), 1-pentanol, dodecane, sudan III, and pyrene are available from Fisher Scientific. CAS numbers and links to MSDS are provided in the online supplement.
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Preparation of an SDS Microemulsion To prepare the microemulsion, 17.10 mL of water is added to 10.05 g of SDS and mixed by stirring with a magnetic stirring bar. Dodecane, 3.95 mL, is then added followed by 24.70 mL of pentanol, as a co-surfactant. The mixture is stirred continuously until it becomes clear and transparent. Solubility of Sudan III Dye in SDS Microemulsion and in Dodecane A stock standard solution of sudan III dye in SDS microemulsion is prepared in two steps: (i) by dissolving 5.0 mg of the dye in 10.0 mL of SDS microemulsion and (ii) diluting 500 μL of the solution to 5.0 mL with SDS microemulsion. Working standards are prepared by diluting 100, 200, 400, 600, and 800 μL of stock standard solution to 2.0 mL using the microemulsion. The absorbance of each solution is then measured using a spectrophotometer and calibration graphs plotted. A saturated solution of sudan III dye is prepared by adding 5.0 mL of SDS microemulsion to 60 mg of the dye in a vial. The solution is centrifuged, a portion of the supernatant diluted 10,000 fold with SDS microemulsion, and the absorbance measured. A saturated solution of the dye in dodecane is prepared and diluted with the microemulsion in the same way and the absorbance measured. The concentration of the saturated solutions of the dye in each solvent is then calculated from the measured absorbance, the linear regression equation of the calibration curve, and the dilution factor. Removal of Contaminants Adsorbed on Soils Contaminated soil, containing 20 mg∙g sudan III dye, is prepared by the instructor (details in the online supplement). The soil, 0.100 g, is mixed with 1.00 mL of SDS microemulsion in a microfuge vial and centrifuged. A portion of supernatant, 100 μL, is diluted to 1.00 mL with SDS microemulsion and the absorbance measured. The same procedure is repeated using dodecane as the extracting solvent and the SDS microemulsion as the diluent. Fluorescence of Pyrene in Microemulsion and Micelles A working solution of pyrene is prepared by dissolving 5.0 mg of pyrene in 10.00 mL of methanol then diluting 100 μL to 2.00 mL with methanol. A solution of SDS above the CMC is prepared by diluting 650 μL of a 100 mg∙mL SDS stock solution with 2300 μL of water in a cuvette. Pyrene working solution, 50 μL, is then added and the fluorescence spectrum obtained using the Fluoromax 2 spectrometer. The conditions used are excitation wavelength of 334 nm, slit width 1.0 nm, integration time 1.0 s in increments of 0.5 nm, and a scan range of 360–410 nm. The fluorescence spectrum of pyrene in a solution of SDS below the CMC is also measured. The solution is prepared by mixing 250 μL of SDS stock solution with 2700 μL of water and adding 50 μL of the pyrene working solution. 264
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Materials and Methods
y = 0.0159 x R 2 = 0.9895
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Concentration / (Nmol/L) Figure 3. Beer–Lambert law plot of sudan III dye in SDS microemulsion. The equation of the regression line is used to calculate the concentration of saturated solutions from measured absorbance taking into account dilution factors.
Finally, 50 μL of the 25 μg∙mL pyrene working solution is added to 2950 μL of SDS microemulsion and the fluorescence spectrum obtained. Hazards Methanol, pentanol, and dodecane are flammable and should not be handled near open flames. Sodium dodecyl sulfate, pentanol, sudan III, and dodecane are irritants. Pentanol has a strong unpleasant smell. Students should wear eye protection and gloves at all times and avoid skin contact with all the chemicals. The solids should be transferred into weighing vials in a functional hood to avoid generating dust. The microemulsion and pentanol should be handled in the hood. Pyrene is a possible carcinogen. The solid compound should only be handled by the instructor. Use gloves and eye protection and transfer into weighing vials in a functional hood. All the waste generated in the experiment should be consolidated as aqueous waste and labeled properly for appropriate disposal with other lab waste. Results and Discussion A typical calibration curve for sudan III in microemulsion is shown in Figure 3. Based on the calibration plot, the measured absorbance, and dilution factors, the solubility of sudan III is determined to be 6.1 g∙L in SDS microemulsion and 8.2 g∙L in dodecane. Thus, although the microemulsion contains 20% water by weight and only 5% dodecane, the solubility of sudan III in both solvents is comparable. The effectiveness of the microemulsion and dodecane in extracting sudan III dye adsorbed on soil are compared in Figure 4. The results show that the microemulsion is better than dodecane in extracting the dye. The fluorescence emission spectra of pyrene in SDS microemulsion, SDS micellar solution, and SDS aqueous solution below the CMC are shown in Figure 5. There are five strong peaks that are numbered I–V in order of decreasing energy. The ratio between the intensities of peak I and peak III is low for pyrene dissolved in non-polar solvents compared to pyrene in polar solvents of similar carbon chain length (23). From Figure 5, the observed ratios of peak I and peak III are 1.9 for pyrene
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Figure 5. Fluorescence spectra of pyrene in (A) aqueous solution of SDS below the critical micelle concentration, (B) SDS micelle solution, and (C) SDS microemulsion.
in aqueous SDS solution, 1.2 in SDS micellar solution, and 0.9 in SDS microemulsion (curves A, B, and C respectively). This shows that in the microemulsion, pyrene resides in a mostly hydrophobic environment.
10. Gao, J.; Njue. C. K.; Mbindyo, J. K. N.; Rusling, J. F. J. Electroanal. Chem. 1999, 464, 31–38. 11. Wagner, G. W.; Procell, L. R.; Yang, Y.-C.; Bunton, C. A. Langmuir 2001, 17, 4809–4811. 12. Dantas, C. T. N.; Neto, A. A. D.; Moura, M. C. P.; Neto, E. L. B.; Telemaco, E. P. Langmuir 2001, 17, 4256–4260. 13. Jayanti, S.; Britton, L. N.; Dwarakanath, V.; Pope, G. A. Environ. Sci. Technol. 2002, 36, 5491–5497. 14. Zelina, J. P.; Rusling, J. F. Electrochemical Remediation of Soils. In Encyclopaedia of Electrochemical Analysis and Remediation; Myers, R. A., Ed.; Wiley: New York, 1998; Vol. 3, pp 1567–1563. 15. Jensen, S.; Johnels, A. G.; Olsson, M.; Otterlind, G. Nature 1969, 224, 247–250. 16. Hites, R. A.; Foran, J. A.; Carpenter, D. O.; Hamilton, M. C.; Knuth, B. A.; Schwager S. J. Science 2003, 303, 226–229. 17. Erickson, B. Anal. Chem. 1998, 70, 528A–532A. 18. Maczka, C.; Pang, S.; Policansky, D.; Wedge, R. Environ. Sci. Technol. 2000, 34, 136A–141A. 19. Mays, H. J. Chem. Educ. 2000, 77, 72–76. 20. �������������������������������������������������������������� Dominguez, A.; Fernandez, A.; Gonzalez, N.; Iglesias, E.; Montenegro, L. J. Chem. Educ. 1997, 74, 1227–1231. 21. van Stam, J.; Depaemelaere, S.; De Schryver, F. C. J. Chem. Educ. 1998, 75, 93–98. 22. Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039–2044. 23. Kalyanasundaram, K. Langmuir 1988, 4, 942–945. 24. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry, 8th ed.; Brooks/Cole-Thomson Learning: Belmont, CA, 2004.
Summary Microemulsions are environmentally friendly alternatives to organic solvents with many applications. The experiments described can be used to demonstrate the formation and utility of microemulsions to students. The fluorescence probe experiments are suitable for advanced classes in which students have some background in spectroscopy. Acknowledgments This work was supported by Millersville University start up funds and a Niemeyer–Hodgson research grant to CAK. We are grateful to Thomas C. Greco for helpful discussions. CAK and ZJC are chemistry undergraduate student co-authors. Literature Cited 1. Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Marcel Decker: New York, 1988. 2. Correa, M. A.; Scarpa, M. V.; Franzini, M. C.; Oliveira, A. G. Coll. Surf. B: Biointerfac. 2005, 43, 108–114. 3. Yoshikawa, T.; Moroto, Y.; Kanaoka, E.; Koji, K.; Nishihara, Y.; Masuda, K. J. Controlled Release 2002, 81, 65–74. 4. Nesamony, J.; Kolling, W. M. J. Pharm. Sci. 2005, 94, 1310– 1320. 5. Shiokawa, T.; Hattori, Y.; Kawano, K.; Ohguchi, Y.; Kawakami, H.; Toma, K.; Maitani, Y. Clin. Cancer Res. 2005, 11, 2018– 2025. 6. Ye, X.; Wai, C. M. J. Chem. Educ. 2003, 80, 198–204. 7. Raveendran, P.; Fu, J.; Wallen, S. L. J. Am. Chem. Soc. 2003, 125, 13940–13941. 8. Rosen, M. J.; Wang, H.; Shen, P.; Zhu, Y. Langmuir 2005, 21, 3749–3756. 9. Vaze, A.; Parizo, M.; Rusling, J. F. Langmuir 2004, 20, 10943– 10948.
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