In the Laboratory
Determination of the Critical Micelle Concentration of Cationic Surfactants An Undergraduate Experiment Xirong Huang,* Jinghe Yang, Wenjuan Zhang, Zhenyu Zhang, and Zesheng An Department of Chemistry, Shandong University, Jinan 250100, P. R. China
Surfactants are amphiphilic compounds consisting of a long-chain hydrocarbon “tail” and a polar (often ionic) “head”. In aqueous solutions, surfactant molecules can arrange themselves into organized molecular assemblies known as micelles if the concentration of the surfactant exceeds a certain value. The concentration at which micelles form is called the critical micelle concentration (cmc). The formation of micelles greatly changes the properties of their medium, thereby affecting the chemical reactions taking place in the medium. In recent years, there has been considerable interest in studies of the effect of micelles on chemical reactions, or micellar catalysis (1–3). It is therefore useful to introduce undergraduate students to some basic concepts and physicochemical properties of surfactants, with a special emphasis on their aggregates. Knowing about these helps students to answer everyday-life questions (e.g., explain the washing power of a detergent) and arouses their interest in colloidal chemistry. Therefore, the determination of cmc’s of ionic surfactants is of pedagogical interest. In the past, methods for the determination of the cmc of ionic surfactants were based on the abrupt change of some physicochemical properties such as conductivity or surface tension in proximity to the cmc of surfactants (4). As an undergraduate physicochemical laboratory experiment, these methods (5, 6 ) have disadvantages in terms of relevancy of experimental content and technique to companion physicochemical laboratory experiments (e.g., experiments on kinetics), vitality in the program (introduction of new developments in colloidal chemistry), information contained, complexity and availability of instruments, and toxicity of reagents. In this paper, a novel method based on micellar catalysis is developed for undergraduates. Cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) catalyze the redox reaction between H2O2 and bromopyrogallol red (BPR), a triphenylmethane dye, and this allows for the measurement of cmc’s for CTAB and CPB. The indicator reaction is monitored by a fixed-time kinetic spectrophotometric technique. As an undergraduate physicochemical laboratory experiment, the present method has some attractive features. Experimental Procedure The absorption spectra are recorded on a Shimadzu UV240 spectrophotometer using a matched pair of 10-mm quartz cells. The cell compartment of the spectrophotometer is thermostated by circulating water (25 °C) from its accessories. Unless otherwise stated, all solutions are prepared by weighing. Triply distilled water is used as the solvent. CPB and CTAB are purified by recrystallization. BPR (E. Merck) *Corresponding author.
is used as received. The concentrations of BPR, CPB, and CTAB solutions are 1.0 × 10᎑4 mol/L, 3.0 × 10᎑3 mol/L, and 3.0 × 10᎑3 mol/L, respectively. A solution of approximately 0.8 mol/L of H2O2 is prepared by diluting the commercially available H2O2 (ca. 30% w/v) (A. R.) and is then standardized with KMnO4. The following procedure is used. To a 10-mL graduated tube with stopper, add an aliquot of an aqueous surfactant solution (3.0 × 10 ᎑3mol/L) (warm the solution whenever needed to avoid precipitation), 2.0 mL of 1.0 × 10᎑4 mol/L BPR, 0.5 mL of 0.01 mol/L H2SO 4, and an appropriate volume of distilled water to give a total volume of 7.0 mL. After mixing, add 3.0 mL of ca. 0.8 mol/L H2O2 and, at the same time, start the stopwatch. Quickly mix well and immediately transfer the solution to a cell. Then record the absorbance value at a time of 5 min and a wavelength of 574 nm for CTAB or 580 nm for CPB against distilled water. Results and Discussion
Selection of Experimental Conditions Acidity The rates of the indicator reaction differ greatly at different pH values (adjusted with H 2SO 4). As the acidity decreases, the reaction rate decreases. One-half milliliter of 0.01 mol/L H2SO4 has proven optimal. At such acidity (pH = 3.6), H2O2 can barely oxidize BPR within 10 min. However, in the presence of micelles of a cationic surfactant (CSF), the indicator reaction has a moderate reaction rate. Fortunately, at this acidity the effect of electrolyte (H2SO4) on the cmc is negligible. Dye Bromopyrogallol red (BPR), a triphenylmethane dye, is suitable for our purpose because the association complex of BPR with CSF is stable at the optimal pH. Moreover, the molar absorptivity of the complex is great, assuring enough accuracy of the absorbance data (the error of a spectrophotometer is a minimum when absorbance values fall between 0.2 and 0.8) even when a relatively low dye concentration is used to minimize the influence of the dye on the cmc. Oxidant The reasons why we choose H2O2 as the oxidizing agent are that (i) H2O2 is uncharged and its reaction product is water; therefore, the effect of H2O2 on the cmc is negligible; and (ii) the change in absorbance after the formation of micelles is significant (a plot of absorbance versus time shows that the indicator reaction is a pseudo-first-order reaction). In the presence of micelles of a CSF, the reaction rate increases with the increase of H2O2. To prevent any negative effect of H2O2 on the determination of cmc, it is advisable to add
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H2O2 in small amounts on the premise that the change of the reaction rate in the proximity of the cmc is large enough to indicate the onset of micelle formation. The use of dilute H2O2 solution minimizes the sampling error caused by the poor stability of the concentrated H2O2 solution.
Data manipulation 1. Select a suitable wavelength (574 nm for CTAB or 580 nm for CPB) in the vicinity of the absorption maximum of the dye–CSF complex . 2. Record the absorbance value of the indicator reaction (A2 ) and of its corresponding blank (A1) (without H2O2) at a given time (5 min). 3. Calculate the reaction rates (expressed as |A2 – A1|/5 min) at different concentrations of CSF. 4. Graph the reaction rate versus the final concentration of CSF. 5. Connect the data points near to the cmc by straight lines and determine the cmc graphically. The intercept of the two linear portions of the plot is considered the onset of micelle formation (see Fig. 1 ).
The obtained cmc values are (4.8 ± 0.3) × 10᎑4 mol/L and (4.5 ± 0.3) × 10᎑4 mol/L for CTAB and CPB, respectively.
Applicability Theoretically, the proposed method is applicable to both cationic and anionic surfactants (in the latter case a cationic dye is needed). This method can be used whenever the micelles formed by the surfactant of interest have an obvious positive catalytic effect on a certain indicator reaction. A negative catalytic effect could not be utilized for our purpose because of the inevitable influence of electrolyte on the cmc. Mechanism Micellar catalysis is different from chemical catalysis (1). The former is a physical phenomenon, while the latter is a chemical process. Although micelles themselves are not involved in the indicator reaction, the microenvironment provided by them, especially at the interface, can concentrate reactants involved in the indicator reaction via hydrophobic and electrostatic interactions. The intermediates, products, or both can be stabilized, thereby enhancing the reaction rate. Advantages of the New Method First, it is a novel method based on micellar catalysis rather than micellar solubilization as reported in this Journal (5, 6 ). Micellar catalysis is a relatively new concept and is of interest to students. Because micelles are similar in some respects to cell membranes and micellar catalysis to enzyme catalysis (4), the present method may well be of interest to some instructors who are involved in developing a laboratory for membrane biochemistry or some similar course. Second, its content is informative and related to other companion physicochemical laboratory experiments. The technique used is an already existing one in kinetic experiments; therefore, introduction of the new concept does not result in an increase in the total class hours of experimental
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Reaction Rate / 75 min–1
In the Laboratory
Concentration of CTAB / 10–4 mol/L
Figure 1. Plot of reaction rate at 574 nm vs CTAB concentration.
physical chemistry. The information generated can be used to elucidate the aggregation effect on the physicochemical properties of surfactants and its possible application in science and technology as well. Third, the phenomenon surveyed by the experiment is interesting, intuitive, measurable, and easy to interpret (5). The cmc values can be estimated without the need for data manipulation. Fourth, the method is simple, rapid, and inexpensive. Neither an expensive fluorimeter (5) nor flammable pentane (6 ) is needed. All reagents are commercially available and inexpensive. In addition, inaccurate setting of the wavelength has little influence on the measurement of the reaction rate. Temperature fluctuations under nonthermostated conditions (25 ± 5 °C) gives rise to an uncertainty of only ±7% (RSD). Therefore the method is also applicable for some labs where no thermostating accessories are available. Last but not least, the effect of dye solubility in water on the accuracy of cmc determination is minimized. The cmc values obtained by micellar catalysis are much closer to the literature values (4 ) of 9.2 × 10᎑4 mol/L (CTAB) and 9.0 × 10᎑4 mol/L (CPB) than those (0.9 × 10᎑4 mol/L for CTAB, 0.6 × 10᎑4 mol/L for CPB) determined by us with BPR based on micellar solubilization (7 ). Acknowledgment We gratefully acknowledge financial support from the Key Laboratory for Colloid and Interface Chemistry of the State Education Commission of China at Shandong University. Literature Cited 1. Perez-Bendito, D.; Rubio, S. Trends Anal. Chem. 1993, 12, 9. 2. Corsaro, G. J. Chem. Educ. 1973, 50, 575. 3. Huang, X.; Jie, N.; Han, S.; Zhang, W.; Huang, J. Mikrochim. Acta 1997, 126, 329. 4. Zhao, G. Physical Chemistry of Surfactants; Beijing University Press: Beijing, 1991. 5. Rujimethabhas, M.; Wilairat, P. J. Chem. Educ. 1978, 55, 342. 6. Furton, K. G.; Norelus, A. J. Chem. Educ. 1993, 70, 254. 7. Huang, X.; Han, S.; Zhang, W.; Yin, Y.; Jie, N. Indian J. Chem., in press.
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
