In the Laboratory
Conductivity, a Simple Method to Probe Micellar Solutions Steven J. Bachofer Saint Mary’s College, Moraga, CA 94575 Typical undergraduate physical chemistry courses appropriately spend considerable amounts of time on thermodynamics, chemical kinetics, and quantum and statistical mechanics. Surface chemistry and interfacial phenomena are discussed briefly, if at all. This is unfortunate because current biomedical research indicates that molecular recognition at the membrane interface and transport across the membrane are complex interfacial phenomena that will be critical research areas in the future. To more adequately prepare students, exposure to surface chemistry and interfacial phenomena should be increased. Considering that developments in computational physical chemistry are also vying for more prominence, expansion of the curriculum to include more surface chemistry and interfacial studies is not too likely. A possible solution to this problem is to present surface chemistry and interfacial phenomena in the physical chemistry lab. Surface and interfacial chemistry experiments can be developed that illustrate the concepts studied in traditional physical chemistry experiments and also highlight important specific features of the surface and interfacial chemistry. Specifically, the study of the chemistry of water-soluble surfactants provides a large number of physical properties that show significant changes with the formation of micellar aggregates. A few experiments focusing on the determination of the critical micelle concentration (CMC) have been reported in this Journal using various methods (1–3). Beyond determining the CMC, students could be introduced to changes in molecular fluorescence caused by changes in solvation in micellar systems. Students can also be introduced to micellar catalysis as an extension of traditional chemical kinetics experiments. A few experiments focusing on these two topics have also been presented in this Journal (4–6). Our conductivity experiment allows a student class to readily obtain a set of CMC values and an estimate of the fractional ionization constant for the micelle using inexpensive instrumentation and materials. Furthermore, the student class could use the set of CMC values as an introduction to Quantitative Structure–Activity Relationships (QSAR) methods. Our conductivity experiment involves studying a cationic surfactant, tetradecyltrimethylammonium, with an organic benzoate counterion that will be referred to as TTA+/X-benzoate. We suggest that the instructor prepare the isolated surfactant with the organic counterion.1 If this preparation appears too time-consuming, then an instructor could select typical anionic surfactants with either inorganic or organic counterions [e.g. sodium dodecylsulfate (SDS), 1-dodecanesulfonic acid (7), etc.]. The designated concentration range for the surfactant study would need to be modified, but the CMC values of common surfactants are listed in reference resources (8). The students can obtain data of sufficient precision to be confident of their CMC values and to estimate the fractional ionization constant for the micellar aggregate regardless Presented at the 13th Biennial Conference on Chemical Education, Bucknell University, Lewisburg, PA, August 1994.
of which surfactant is selected (9). However, the introduction to QSAR methodology must be excluded if the TTA +/X-benzoate surfactants with various substituted benzoate anions are not selected. Our experiment has the students, first of all, study the conductivity properties of sodium benzoates and then, secondly, investigate those of the surfactant system. The class can study a number of different counterions with the same tetradecyltrimethylammonium (TTA+ ) cationic surfactant and comment on the relative ability of different benzoate anions to stabilize micelle formation. Experimental Procedure
Physical Chemistry Conductivity Lab This conductivity experiment will involve experimental measurements on two related chemical systems, the sodium salt of a substituted benzoate and the TTA+/Xbenzoate surfactant with the same substituted benzoate as the sodium salt. The conductivities of the sodium substituted benzoate salt samples will be used to determine the molar conductance of the salt at infinite dilution. The conductivity measurements of the TTA+/X-benzoate surfactant system will focus on determining the CMC and the fractional ionization constant (α) for the micelle from Evan’s equation (9, 10). The specific conductance measurements will be obtained utilizing a water bath to maintain the temperature at 25 °C, since conductivity is temperature-dependent. Students should equilibrate their samples in the water bath for 10 min before obtaining the measurements. The samples can be obtained by preparing a stock solution with the selected sodium substituted benzoate salt and from the provided TTA+ /X-benzoate surfactant stock solution. Students should prepare all of their diluted samples for one substituted benzoate system on one day before measuring the conductivity on another day to save time on both portions of the laboratory. Students should measure the conductivity on each sample at least twice.
Study of Salts The goal for the first system studied will involve determining the molar conductance of the X-benzoate anion at infinite dilution. A 0.0100 M sodium X-benzoate stock solution should be prepared from the substituted benzoic acid and standardized NaOH solution. The water used for all reagents and dilutions should be of 2 MΩ resistance or better. Students are cautioned to add only a stoichiometric amount of NaOH because one wants to measure the conductivity of the sodium benzoate salt alone. Approximately 250 mL of this stock should be sufficient for the study of salts depending on the necessary volume of conductivity sample cell. The samples of the sodium X-benzoate should be prepared by volumetric dilution to give concentrations of 0.0100, 0.0075, 0.0050, 0.0025, 0.0020, 0.0010, and 0.0005 M. The molar conductance values will be computed from the specific conductance values and plotted versus the [sodium X-benzoate]1/2. The
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molar conductance at infinite dilution is obtained by extrapolation to [sodium X-benzoate] equals zero. The student should use a linear regression from a spreadsheet program to determine the molar conductance of the salt at infinite dilution. The molar conductance of the X-benzoate anion at infinite dilution (Λ anion,∞) is computed by subtracting the tabulated molar conductance of the sodium cation at infinite dilution from the extrapolated value of the sodium benzoate salt. The resulting value for the X-benzoate anion will be utilized in the second portion of the experiment to calculate the fractional ionization constant (α) for the micellized surfactant system.
Study of Micellar System A cationic micellar system is studied in the second portion of the experiment. Students will determine the critical micellization constant (CMC) value and the fractional ionization constant (α) for the TTA+/X-benzoate. The CMC is the concentration at which monomeric surfactant molecules can thermodynamically self-aggregate, forming the molecular aggregates called micelles. The specific conductivity increases incrementally with increasing surfactant concentration before the CMC, and upon micellization, the incremental increase will change to some value smaller than before micellization. Rosen notes that numerous physical properties show drastic changes with the onset of micelle formation (11). The tetradecyl-trimethylammonium bromide (TTAB), which is commercially available, forms micelles at a concentration of 3.50 mM. The TTA +/X-benzoate surfactants would be predicted to have CMC values lower than TTAB’s CMC value. Students are provided with a 30-mM TTA+/X-benzoate stock solution and they do a serial dilution to generate their conductivity samples (below and above the CMC). The surfactant concentration range of the conductivity samples should be from 0.20 to 5.0 mM. The concentration should be increased by 0.20-mM increments from 0.20 to 2.0 mM and by 0.4-mM increments from 2.0 to 5.0 mM for most TTA+/X-benzoate surfactants unless the parent acid of the selected benzoate counterion is strongly hydrophobic. Instrumentation We have used a Markson 1062 digital conductivity meter for conductivity measurements. The digital conductivity meter was calibrated with potassium chloride standards (0.0100 and 0.0050 M KCl) to determine the cell constant. Students are directed to read the appropriate portions of the conductivity meter manual to learn how to calibrate the meter. Calculations The experimental salt data can be tabulated in a spreadsheet and the molar conductivity versus [sodium Xbenzoate]1/2 plotted. The micellar data can also be tabulated in a spreadsheet and used to yield a plot of specific conductivity versus concentration which has two linear regions. By linear regression analysis, students can obtain the best 1 The cationic surfactant with benzoate counterions (TTA+/X-benzoate) are prepared from commercial tetradecyltrimethylammonium bromide (TTAB) using a hydroxide charged ion exchange column to produce TTA+OH–, which is then reacted with a stoichiometric amount of benzoic acid. To determine the concentration of TTA+OH–, the Epton two-phase surfactant titration was employed using sodium dodecylsulfate (SDS) as the titrant. (References: Reid, V. W.; Longman, G. F., Heinerth, E. Tenside 1967, 4, 292; Smith, W. B. J. Soc. Cosmet. Chem. 1963, 14, 513.)
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slopes and intercepts for the two linear portions. The intersection of the two lines is the CMC for that surfactant at 25 °C. The slopes of the lines before and after micelle formation are needed to determine the fractional ionization constant, α, which is the fraction of surfactant molecules in the micellar aggregate that do not have bound counterions. The α values can be calculated from the specific conductance data using the Evans equation (9, 10): 1000 × S2 = [(n–m)2/n(4/3)] × (1000×S1 – Λanion,∞) + [(n–m)/n] × Λanion,∞
where S1 and S2 = slopes of the specific conductivity versus surfactant concentration (M) plot before (S1) and after (S2) the CMC; n = aggregation number =70; m = micellar-bound counterions; α = [(n–m)/n] = fraction ionization constant; and Λanion,∞ = molar conductivity at infinite dilution of the anion. Students can rewrite this equation in terms of α and can utilize the data to solve the resulting quadratic equation: 0 = [n(2/3) × (1000 × S1 – Λanion,∞)] × (α2) + Λanion,∞ × α – 1000 × S2
Students should report their CMC and a values. The micellar systems studied by this group of students are TTA+/benzoate, TTA+ /p-toluate, TTA+ /o-toluate, TTA+/mchlorobenzoate, and TTA +/o-chlorobenzoate. Individual students should make their results available to other students to facilitate answering the questions below.
Questions 1. What would you predict the molar conductivity versus the square root of the concentration plot for benzoic acid to look like? Can you support why you predict it to be linear or nonlinear? 2. Speculate on why a micelle forms. Write a sentence or two based on the theory of chemical potentials discussed in your lectures. 3. Do you think that your selected tetradecyltrimethylammonium substituted benzoate surfactant should form a micelle more easily than another student’s system? Why or why not? If you see any trends, then attempt to explain them. Results Students can usually obtain precise data for the studies of the electrolyte salts and the surfactant systems. Typical student conductivity data for TTA+/benzoate and TTA+/ p-toluate are recorded in Table 1 and shown in Figure 1. The resulting CMC and α values are recorded in Table 2. If a number of students pool their data on various TTA+/ para- or meta-substituted benzoates, they can recognize a trend in the CMC values due to the hydrophobicity of the benzoate counterions. However, they are usually confused by CMC values for the TTA+ /ortho-substituted benzoates. The experiment does have an element of discovery at this point! Students have to hypothesize why the ortho-substituted benzoates fail the hydrophobicity model. The propagated most probable error in the CMC values from the linear regression analysis of the conductivity data before and after the CMC is ± 0.1 mM for most CMC values greater than 1 mM. With CMC values limited to two significant figures, the α values were also limited to two significant figures; however, that may be overstating the precision in these values. In our experiment, students were directed only to address the propagated error in the CMC values, since various methods for determining α are known to give somewhat different values (9). Discussion
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gates were done using conductivity measurements exclusively (17). The mixed micellar and shape selective phase transition studies might appear too complex. However, it is a logical extension for student projects after studying a simple ionic surfactant. This conductivity experiment should allow students to recognize that complex equilibrium systems can be studied easily with careful attention to the physical properties!
Table 2. CMC and Fractional Ionization Constant Values for TTA+/X-Benzoatesa Student Values
Instructor's Values
Anion
Conductivity CMC (α) (mM)
Surface Tension Conductivity CMC CMC (α) (mM) (mM)
Benzoate
1.68 (0.14)
1.47 (SM)
1.71 (0.13)
o-Cl,Benzoate
1.80 (0.14)
1.99 (21 °C)
2.14*(0.17)
Specific Conductivity (µΩ-1 cm-1)
m-Cl,Benzoate
0.82 (0.08)
0.76 (25 °C)
—
Anion
p-Cl,Benzoate
—
0.80(22 °C)
0.80 (0.08)
o-Toluate
2.10 (0.17)
1.50 (SM)
—
p-Toluate
1.20 (0.12)
1.05 (24 °C)
1.03*(0.12)
Figure 1. The specific conductivity versus surfactant concentration for TTA+/benzoate and TTA+/ p-toluate surfactants. The sur-
Table 1. Specific Conductivity versus Surfactant Concentration: Typical Student Data at 25 °C Surfactant Concentration (mM)
Benzoate
p-Toluate
0.20
16
14
0.40
27
25
0.60
40
35
o-NO2,Benzoate
—
2.02 (21 °C)
2.16 (0.18)
0.80
—
47
m-NO2,Benzoate
1.40 (0.14)
1.03 (21 °C)
1.04 (0.14)
1.00
60
53
1.20
72
62
p-NO2,Benzoate
—
1.28 (21 °C)
1.26 (0.16)
1.40
85
67
o-Phthalate
1.08 (0.18)
1.04 (25 °C)
—
1.60
98
70
Salicylate
—
0.57 (21 °C)
0.50 (0.12)
2.00
107
74
2.40
114
79
2.80
123
84
3.20
125
89
3.60
134
95
4.00
—
98
factant concentration is in mM and the conductivity is in µΩ-1/cm.
This experiment can serve as a starting point for a physical chemistry student to do additional colloidal chemistry research projects. A direct extension of this experiment would be to study the 1H NMR spectra of these TTA+ /X-benzoate micelles to confirm that the benzoate anions are indeed oriented with respect to the micellar interface, as discussed in the colloidal chemical literature (12, 13). The student conductivity projects could involve studies of changes in the counterion binding when various neutral organic molecules (additives or solubilizates) are present or investigations of shape selective phase transitions for ionic surfactants. Students could also apply conductivity measurements to study mixed micelles. Students could couple conductivity measurements with other methodologies to study micellar systems (12). They could study mixed micelles by light scattering as well as conductivity and compare their data to literature values (14–16). The shape selective phase transition studies by Claude Treiner on mixed micellar aggre-
aCMC values from conductivity measurements are in units of mM and the fractional ionization constants are listed in parentheses for each TTA+/X-benzoate studied. The instructor’s CMC values with the asterisk were the result of only one determination. CMC values from surface tension data were recorded on either isolated TTA+ /X-benzoate surfactant or on stoichiometric mixtures (SM) of TTAB and the sodium X-benzoate at room temperature. Surface tension data recorded at different temperatures were also listed in parentheses.
Students can build on knowledge acquired in organic chemistry (Hammett relationships) and begin to understand the concepts of QSAR methodology if enough substituted benzoate anions are studied. Since data on the + TTA /X-benzoate surfactants are not all in the literature, students will record new experimental results, which they usually enjoy. The selection of the benzoate counterions for this experiment requires that an instructor judiciously select anions that will behave differently.2 The instructor can select both hydrophobic and hydrophilic substituted benzoic acids, but should be aware that the CMC and structure of the micellar aggregate are con2
Some hydrophobic benzoate anions readily promote viscoelastic rod shaped micellar aggregates, which in concentrated solutions do not equilibrate rapidly. The salicylate anion has also promoted the appearance of viscoelastic rod-shaped micellar aggregates. Therefore, students are directed to prepare their diluted samples, heat resulting solution to 50 °C, cool to room temperature slowly, and record the conductance after waiting at least overnight to ensure equilibration at the new concentration.
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trolled by the thermodynamic constraints of the packing of the anions into the micelle (18). An instructor can utilize the CMC values reported for a decyltrimethylammonium surfactant with X-benzoates and the CMC values reported for stoichiometric mixtures of cetyltrimethylammonium bromide (CTAB) with sodium X-benzoates to predict effects of TTA+/X-benzoate surfactants (19, 20). The TTA+/salicylate surfactant is known to undergo a sphere-to-rod phase transition near the CMC and yields a viscoelastic phase (21, 22). Although the viscoelastic phase can make preparations of diluted samples difficult, the surfactant’s shear- dependent flow properties make it an interesting system to present to chemical engineering students. In summary, the TTA+/X-benzoate surfactants provide numerous opportunities for students to characterize complex chemical systems with careful attention to the physical properties. Acknowledgment The author expresses his thanks to Research Corporation for funding the research projects involving cationic surfactants. The author also thanks the Saint Mary’s College Faculty Development Fund for providing funding so he could present this laboratory experiment at the 13th Biennial Conference on Chemical Education. I appreciate the time and assistance of my collaborator, Ursula Simonis of San Francisco State University, whose NMR expertise has greatly enhanced my understanding of these surfactant systems. The diligent efforts of the physical chemistry students (Brian Blasquez, Pilar Garcia, Monty Mola, Kim Raymonde, Kristie Schleicher, Sandra Smith, John Spence, and others) who assisted me as the experiment was developed are graciously acknowledged. The general assistance of John Correia of Saint Mary’s College is also graciously acknowledged. Literature Cited 1. Rujimethabhas, M.;Wilairat, P. J. Chem. Educ. 1978, 55, 342. 2. Goodling, K.; Johnson, K.; Lefkowitz, L.; Williams, B. W. J. Chem. Educ. 1994, 71, A8–A12. 3. Furton, K. G.; Norelus, A. J. Chem. Educ. 1993, 70, 254–257. 4. Marzzacco, C. J. Chem. Educ. 1992, 69, 1024–1025. 5. Abuin, E. B.; Lissi, E. A. J. Chem. Educ. 1992, 69, 340–342. 6. Corsaro, G. J.; Smith, J. K. J. Chem. Educ. 1976, 53, 589–590. 7. Matsuoka, K.; Moroi, Y.; Saito, M. J. Phys. Chem. 1993, 97, 13006–13010. 8. Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36, U.S. Dept. of Commerce: Washington, DC, 1971. 9. Sepulveda L.; Cortes, J. J. Phys. Chem. 1985, 89, 5322–5324. 10. Evans, H. C. J. Chem. Soc. 1956, 579–586. 11. Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989; pp 108–110. 12. Mandal, A. B.; Wang, L.; Brown, K.; Verrall, R. E. J. Colloid & Interfac. Sci. 1993, 161, 292–298. 13. Manuscript in progress addressing 1H NMR data for students. 14. Biresaw, G.; McKenzie, D. C.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1985, 89, 5144–5146. 15. Zana, R. J. Colloid & Interfac. Sci. 1980, 78, 330–337. 16. Abuin, E. B.; Lissi, E. A.; Bianchi, N.; Laerte, M.; Quina, F.H. J. Phys. Chem. 1983, 87, 5166–5172. 17. Treiner C.; Makayssi, A. Langmuir 1992, 8, 794–800. 18. Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc. Faraday Trans. 2 1976, 72, 1525. 19. Underwood A. L.; Anacker, E. W. J. Phys. Chem. 1984, 88, 2390–2393. 20. Bachofer, S. J.; Turbitt, R. M. J. Colloid & Interfac. Sci. 1990, 135, 325–334. 21. Anet, F. A. L. J. Am. Chem. Soc. 1986, 108, 7102–7103. 22. Ohlendorf, D.; Interthal, W.; Hoffmann, H. Rheologica Acta 1986, 25, 468–486.
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