Anal. Chem. 1997, 69, 4271-4274
Determination of Critical Micelle Concentration Values Using Capillary Electrophoresis Instrumentation Alejandro Cifuentes,† Jose L. Bernal,† and Jose C. Diez-Masa*,‡
Laboratory of Analytical Chemistry, University of Valladolid, Paseo de la Magdalena s/n, 47011 Valladolid, Spain, and Institute of Organic Chemistry (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain
The purpose of this work is to demonstrate the usefulness of capillary electrophoresis (CE) instrumentation for determining values of critical micelle concentration (cmc) of surfactants. The approach essentially consists of a CE version of the traditional method of measuring values of cmc by conductivity. Namely, the different conductivities of ionic surfactants in solution depending on their aggregation state, i.e., as monomers or micelles, and the effect on the electrical current as usually measured in a CE apparatus are employed to determine the cmc values. The cmc of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) is obtained in several media such as water, aqueous solutions containing salts, organosaline solutions, and aqueous solutions containing β-cyclodextrin. The cmc values for SDS and CTAB under these conditions are in good agreement with those reported in the literature. Advantages and drawbacks of this procedure as well as its implications in micellar electrokinetic chromatography are discussed. From our results, it is deduced that the present method can be used with high confidence to determine values of cmc in a fast and easy way. In the early 1930s, Bury and co-workers1,2 established the term “critical micelle concentration” (cmc), defining it as a concentration range below which surfactant is in solution as a monomer and above which practically all additional surfactant added to the solution forms micelles. As mentioned by Mukerjee and Mysels,3 cmc is probably the simplest way of describing the colloid and surface behavior of a surfactant solute. Moreover, this value determines the industrial usefulness and biological activity of detergents as well as some other interesting surfactant features like solute-solvent and solute-solute interactions. Directly related to the solute-solvent interactions in micellar media is the application of surfactants in separation science.4 That is, the different associations or solubilizations of analytes by aqueous micelles are employed for their separation in, e.g., liquid chromatography, liquid-liquid extractions, cloud point extractions, * Corresponding author. Fax: 34-1-5644853. E-mail:
[email protected]. † University of Valladolid. ‡ Institute of Organic Chemistry (CSIC). (1) Davies, D. G.; Bury, C. R. J. J. Chem. Soc. 1930, 2263-2270. (2) Grindley, J.; Bury, C. R. J. J. Chem. Soc. 1929, 679-684. (3) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentration of Aqueous Surfactan Systems; National Bureau of Standards: Washington, DC, 1970. (4) Armstrong, D. W. Sep. Purif. Methods 1985, 14, 213-304. S0003-2700(97)00696-3 CCC: $14.00
© 1997 American Chemical Society
membrane techniques, etc. In 1984, Terabe and co-workers5 introduced a new separation mode of capillary electrophoresis (CE) using surfactants. This new CE mode, called micellar electrokinetic chromatography (MEKC), allows the separation of analytes without charge under the influence of an electric field, while improving the solubility of highly hydrophobic substances in the separation buffer. Both effects are the result of the use of surfactants, mainly sodium dodecyl sulfate (SDS), which are added to the separation buffer at concentrations higher than their cmc. As described in the literature,6-10 MEKC has found numerous applications in the separation of drugs, metabolites, and other small molecules and also in the field of biopolymers. In MEKC, as well as in any other separation technique using amphiphilic molecules, a good knowledge of the cmc value of the surfactant employed in the actual separation media is mandatory to optimize and even to carry out the analysis. Moreover, it has to be stressed that cmc values depend on the separation conditions, e.g., ionic strength, temperature, additives, etc. In order to determine such values, a large variety of methods have been traditionally employed,3 e.g., conductivity, light scattering, surface tension, spectrophotometry, etc. More recently, other new methods, such as speed of sound11 or NMR,12 have been shown to be useful for this type of determination. MEKC has been also applied to the determination of cmc values.13-15 In some works, plots of capacity factor, k′, versus concentration of surfactant have been employed,13,14 obtaining the cmc value by extrapolating to k′ ) 0. However, this method does not seem to be reliable enough to obtain cmc values, as mentioned by some authors.13 The other MEKC procedure is based on the variation of the effective electrophoretic mobility of a neutral compound with the surfactant concentration,15 which, in turn, is related to the cmc value. However, it is well known that the use of an indicator dye which is included into the micelle can alter the cmc value to some extent.3 In fact, cmc values measured using (5) Terabe, S.; Otsuka, K.; Ichikawa, K.; Tsuchiya, A.; Ando, T. Anal. Chem. 1984, 56, 111-113. (6) Kuhr, W. G. Anal. Chem. 1990, 62, 403R-414R. (7) Kuhr, W. G.; Monnig, C. A. Anal. Chem. 1992, 64, 389R-407R. (8) Monnig, C. A.; Kennedy, R. T. Anal. Chem. 1994, 66, 280R-314R. (9) St. Claire, R. L. Anal. Chem. 1996, 68, 569R-586R. (10) Matsubara, N.; Terabe, S. In Capillary Electrophoresis in Analytical Biotechnology; Righetti, P. G., Ed.; CRC Press: Boca Raton, FL, 1996; pp 155182. (11) Junquera, E.; Tardajos, G.; Aicart, E. Langmuir 1993, 9, 1213-1219. (12) Lee, Y. S.; Woo, K. W. J. Colloid Interface Sci. 1995, 169, 34-38. (13) Terabe, S.; Otsuka, K.; Ando, T. Anal. Chem. 1985, 57, 834-841. (14) Strasters, J. K.; Khaledi, M. G. Anal. Chem. 1991, 63, 2503-2508. (15) Jacquier, J. C.; Desbene, P. L. J. Chromatogr. A 1995, 718, 167-175.
Analytical Chemistry, Vol. 69, No. 20, October 15, 1997 4271
Table 1. Cmc Values Determined by CE and Compared with Those Found in the Literature
CE conditions SDS in water
75 µm i.d., 47 cm; 20 kV
CTAB in water
100 µm i.d., 27 cm; 20 kV
SDS + 20 mM borax
75 µm i.d., 47 cm; 15 kV
SDS + 10 mM β-cyclodextrin
75 µm i.d., 47 cm; 20 kV
SDS + 30 mM NaCl
75 µm i.d., 47 cm; 20 kV
SDS + 5 mM borax + 15% acetonitrile
75 µm i.d., 47 cm; 20 kV
CE results from least-squares fittingb 1170c + 0.90, r2 ) 0.9996 530.5c + 6.21, r2 ) 0.996 6200c + 0.45, r2 ) 0.9997 1500c + 4.80, r2 ) 0.998 577.1c + 43.55, r2 ) 0.975 411c + 44.06, r2 ) 0.995 1159.1c + 0.08, r2 ) 0.9996 505c + 9.73, r2 ) 0.9993 950c + 52.13, r2 ) 0.9991 350c + 54.30, r2 ) 1 1566.1c + 18.26, r2 ) 0.9999 1367.9c + 19.71, r2 ) 0.9992
CE
cmca (mM) literature
refs
8.3
8.1-8.4
3, 32
0.93
0.90-0.98
3, 33
3.1
2.6-3.2
17, 28
14.8
14.0-16.0
25-27
3.6
2.9-3.7
3, 28
7.3
7.7
28
a All determinations were done at 25 °C. b r is the correlation coefficient of the equation i ) Bc + A, where i is the electric current (µA) and c the surfactant concentration (mol/L). The first equation applies for c < cmc and the second one for c > cmc, respectively.
such a procedure are systematically lower than those obtained by other methods.3 Probably due to the aforementioned problems, we have observed through literature that authors using CE apparatus use other instrumentation, e.g., surface tension apparatus,16 conductimetric titration,17,18 etc., to carry out determinations of cmc values when needed. However, according to Tickle et al.,19 it seems possible to determine in an easy way the cmc values of detergents by plotting the electrical current values, as measured by a CE instrument, versus the surfactant concentrations at a given electric field. In this work, we discuss, validate, and extend the usefulness of this CE approach. EXPERIMENTAL SECTION Instrumentation. Measurements were carried out using a P/ACE 5000 HPCE (Beckman Instruments Inc., Fullerton, CA) electrophoresis apparatus controlled by a Pentium 100 MHz personal computer. Two fused silica capillaries (Polymicro Technologies Inc., Phoenix, AZ) were employed. One, with 100 µm i.d., 360 µm o.d., and 27 cm of total length, was used to determine the cmc of cetyltrimethylammonium bromide (CTAB) in water. For all the other experiments, a capillary with 75 µm i.d., 360 µm o.d., and 47 cm of total length was employed. The temperature of the capillary was maintained at 25 °C. The electrical current data were collected and analyzed using System Gold software from Beckman running on the Pentium 100 MHz computer. As stated in the apparatus’ specifications provided by the manufacturer, the minimum detectable variation of the electric current was 0.1 µA. Reagents. Sodium dodecyl sulfate (SDS), sodium chloride, and borax (sodium tetraborate decahydrate) were from E. Merck (Darmstadt, Germany). Acetonitrile was from Scharlau (Barcelona, Spain). Cetyltrimethylammonium bromide (CTAB) was from EGA-Chemie (Steinheim, Germany), and β-cyclodextrin was from Fluka (Madrid, Spain). Procedures. A 50 mM SDS solution was freshly prepared every day by dissolving the required amount of surfactant in (16) Piera, E.; Erra, P.; Infante, M. R. J. Chromatogr. A 1997, 757, 275-280. (17) Saitoh, K.; Kiyohara, C.; Suzuki, N. J. High Resolut. Chromatogr. 1991, 14, 245-248. (18) Terabe, S.; Katsura, T.; Okada, Y.; Ishihama, Y.; Otsuka, K. J. Microcolumn Sep. 1993, 5, 23-33. (19) Tickle, D. C.; Okafo, G. N.; Camilleri, P.; Jones, R. F. D.; Kirby, A. J. Anal. Chem. 1994, 66, 4121-4126.
4272 Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
Milli-Q water (Millipore Corp., Bedford, MA). Likewise, aqueous solutions of 5 mM CTAB, 20 mM β-cyclodextrin, 40 mM borax, and 60 mM sodium chloride in Milli-Q water were also prepared. Final solutions were obtained by conveniently mixing and diluting these solutions. Cmc values were determined for both surfactants in the following media: CTAB in water, SDS in water, SDS in a 30 mM NaCl aqueous solution, SDS in a 10 mM β-cyclodextrin aqueous solution, SDS in a 20 mM borax (pH 9.2) aqueous solution, and SDS in a 5 mM borax solution containing water-acetonitrile (85: 15 v/v). In Table 1, voltages and capillaries dimensions employed are indicated for each case. These experimental conditions were chosen in order to obtain an adequate value of electric current, as will be discussed below. All determinations were done at 25 °C. At least nine different concentrations of surfactant were monitored to determine the cmc value in each set of given conditions. Prior to measurement of the electric current, the capillary was consecutively rinsed with water and the new surfactant solution for 0.5 min. The high voltage was applied, and after waiting 1 min for equilibration of the CE system, the electric current was measured. THEORY It is well known that micellation of surfactants in aqueous media occurs due to the fact that the reduction of the hydrocarbonwater interface is energetically favored. The critical micelle concentration at which aggregation takes place reflects that the hydrophobic interaction between the hydrocarbonaceous moieties of the surfactant molecules is balanced by the hydration and electrostatic repulsive effects of hydrophilic head groups.20 Thus, hydrophobic forces control the formation of micelles, while electrostatic ones limit the maximum size (aggregation number) that micelles can reach under determined conditions. This equilibrium can be explained through a scheme similar to that used by Evans21 in 1956. Assuming that the micelle is composed of n ionic monomers or amphiphiles (S-) and it carries m co-ions (e.g., Na+) induced within the micelle, at concentration not greatly above the cmc, we can write (20) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1980. (21) Evans, H. C. J. Chem. Soc. 1956, 579-586.
nNaS f nNa+ + nS- a (SnNam)(n-m)- + (n - m)Na+
This equilibrium shows that, at concentration of surfactant c > cmc, amphiphiles are mainly in a micellar form, i.e., n molecules of surfactant plus m co-ions will aggregate to form a micelle, while at c < cmc, surfactant will be in a monomeric form, moving freely in solution in a way similar to that of co-ions at these low concentrations. On the other hand, in a CE instrument, applying a voltage V on a capillary of radius r and total length l, filled with a surfactant solution, gives an electric current I according to Ohm’s law:22
V)
l I πr2(σNa+ + σS- + σmic)
where σNa+, σS-, and σmic are the specific conductivities of the coion, amphiphile, and micelle, respectively. It can be easily deduced from the equilibrium shown above that, at c < cmc, the main contribution to the overall conductivity of the solution comes from σNa+ and σS-, while at c > cmc, the main contribution comes from σmic, with a low number of co-ions, e.g., sodium, moving freely in solution at these high concentrations.21 The observed decrease in conductivity of solutions of ionic surfactants above the cmc is explained through both inclusion within the micelle of ions of charge opposite (co-ions) to that of amphiphiles21 and the increase in resistance to migration of the micelle caused by the ionic atmosphere of co-ions surrounding this ordered structure.23 Thus, in CE, by plotting electric current against different surfactant concentrations at a given voltage, experimental points must fit into two lines whose slopes should be different depending on the range of c considered.19 That is, the two slopes correspond to the monomeric and micellar states of the surfactant. By employing simple mathematical procedures as usually applied for the determination of cmc in other methods,3 it will be possible to calculate such a value. In our case, the cmc values were calculated from the intersection point of two straight lines, whose equations were calculated by the linear least-squares method. RESULTS AND DISCUSSION In Figure 1A, the plot of the electric current values obtained for different concentrations of SDS in water at 25 °C is shown. As expected, experimental points fit into two straight lines of different slope whose values are given in Table 1, i.e., 1170 at c < cmc versus 530.5 at c > cmc. As can be seen in Figure 1A, this variation of slope takes place at a SDS concentration of ∼8 mM. Such variation is originated by the conductivity change brought about by the micelle formation, that concentration (or range of concentrations3) being the cmc value. In order to give a more precise value of cmc, the procedure explained under Theory was applied. The experimental data appearing too close to the cmc value, indicated by filled squares in Figure 1, were not included in the cmc calculations, as recommended by Mukerjee and Mysels.3 The intersection point of the two straight lines, i.e., the cmc values of SDS in water at 25 °C, gave a c value equal to 8.3 mM by this procedure. As can be deduced from Table 1, this (22) Foret, F.; Krivankova, L.; Bocek, P. In Capillary Zone Electrophoresis; Radola, B. J., Ed.; VCH: Weinheim, 1993; pp 7-15. (23) Hunter, T. J. Zeta Potential in Colloid Science: Principle and Applications; Academic Press: London, 1981; pp 98-112.
Figure 1. Plots of electric current vs concentration of SDS under different conditions. Fused silica capillary: 75 µm i.d. and 47 cm length. Temperature was kept at 25 °C. Conditions: (A) SDS in water; current measured at 20 kV. (B) SDS + 10 mM β-cyclodextrin; current measured at 20 kV. (C) SDS + 5 mM borax + 15% acetonitrile; current measured at 25 kV.
value is very close to those found in the literature, i.e., from 8.1 to 8.4 mM, depending on the authors, which seems to indicate the usefulness of the present procedure. Similar representation was obtained when the electric current was plotted versus different concentrations of CTAB in water at 25 °C, and these results are also given in Table 1. In this case, the intersection point of the two straight lines gave a cmc value equal to 0.93 mM, which is in good agreement with those values found in the literature under the same conditions, i.e., 0.90-0.98 mM. The versatility of this CE procedure can be easily understood with the CTAB example, as is discussed next. For determining the cmc of CTAB, the capillary of 75 µm i.d. and 47 cm length used in the previous case was substituted by one of 100 µm i.d. and 27 cm of length. This was done because CTAB solutions were of very low concentration, ranging from 0.2 to 1.6 mM. Their low conductivity, together with the high electrical resistance of the 75 µm i.d. capillary employed, resulted in a very low electric current. Namely, under an electric field of 42 kV/m, a variation in the CTAB concentration from 1.2 to 1.6 mM induced a current variation as small as 0.2 µA. Such a value is too close to the minimum current variation detected by the CE instrument used, i.e., 0.1 µA. Moreover, by applying the highest electric field provided by the instrument, 64 kV/m under these conditions, the Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
4273
variation of electric current observed was only ∼0.3 µA. These two low values of electric current reduced substantially the “sensitivity” of the method since concentration steps smaller than 0.3-0.4 mM CTAB were avoided. Therefore, in order to increase the sensitivity of our system, a shorter capillary with 100 µm i.d. was employed. This capillary allowed us to work with smaller concentration steps. Namely, for the same increase in concentration as above (1.2-1.6 mM) and the same electric field of 42 kV/ m, a variation of 0.6 µA was measured. Besides the possibility to manipulate the sensitivity of this method by choosing the adequate capillary size, the high voltage applied can also be selected to improve the sensitivity of the method. However, in this case, systematic errors can arise from the increase in temperature of the solvent due to the Joule effect if heat is not properly dissipated.24 In our case, to overcome this problem, the power generated in the capillary was always kept below 2 W/m, even when, according to the instrument’s specifications, it was able to dissipate up to 5 W/m. Figure 1B shows the plot obtained when the electric current was represented against aqueous solutions containing different concentrations of SDS plus 10 mM β-cyclodextrin. The cmc value of SDS calculated at the intersection of the two straight lines given in Table 1 was 14.8 mM. This value also agrees quite well with those found in the literature, i.e., 14.0-16.0 mM. Further, by comparing the cmc value obtained, 14.8 mM, with that for SDS in water, 8.3 mM, it can be deduced that β-cyclodextrin shifts the cme of SDS to higher values. This effect has already been studied,11,25-27 and it is explained through the inclusion of SDS monomers into the cavity of β-cyclodextrin following an approximately 1:1 stoichiometry. Therefore, this procedure would allow one to determine the cmc of the surfactant in MEKC buffers in some enantiomeric separations where surfactant and cyclodextrin are contained in the buffer. This knowledge can help to optimize the separation conditions, as well as to carry out CE thermodynamic studies18 or any other investigation3,16 for which the cmc value is required. When the electric current was measured for solutions containing different concentrations of SDS plus 5 mM borax and 15% acetonitrile, the plot shown in Figure 1C was obtained. As can (24) (25) (26) (27) (28) (29) (30) (31) (32)
(33)
Cifuentes, A.; Kok, W.; Poppe, H. J. Microcolumn Sep. 1995, 7, 365-372. Aman, E. S.; Serve, D. J. Colloid Interface Sci. 1990, 138, 365-375. Georges, J.; Desmettre, S. J. Colloid Interface Sci. 1987, 118, 192-197. Palepu, R.; Reinsborough, V. C. Can. J. Chem. 1988, 66, 325-332. Jacquier, J. C.; Desbene, P. L. J. Chromatogr. A 1996, 743, 307-314. Lindman, B. In Surfactants; Tadros, Th.F., Ed.; Academic Press: London, 1984; p 83. Kertes, A. S. In Micellation, Solubilization, and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 1, p 445. Osipow, L. I. Surface Chemistry, Theory and Industrial Application; Reinhold Publishing: New York, 1962. Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905912. Fendler, E. J.; Fendler, J. H. In Advances in Physical Organic Chemistry; Gold, V., Ed.; Academic Press: New York, 1970; Vol. 8.
4274
Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
be seen, under these conditions, the change in slope is not as abrupt as that observed in the other two cases, i.e., 1566.1 versus 1367.9, as given in Table 1. This is probably due to the denaturing effect of acetonitrile on micelles, which has been demonstrated to bring about the generation of small surfactant-solvent mixed aggregates.28 This effect is typically explained through the multiple equilibria model,29 also called the stepwise aggregation model.30 Therefore, the change in conductivity or electric current is lower than that observed for the previous cases. Although, under these conditions, to refer to cmc value is not very rigorous, the cmc value was calculated for comparative purposes. From Figure 1C, a value of 7.3 mM was obtained. This value is slightly lower than SDS in water and similar to the value of 7.7 mM found in the literature for the same conditions. We have also included in Table 1 the results obtained for the determination of the cmc of SDS in aqueous solutions containing 20 mM borax (pH 9.2) or 30 mM NaCl. As can be seen, under these conditions, there is also a good agreement between the CE results and those reported in the literature, which seems to corroborate the applicability of this method for cmc determinations. Further, some other possibilities derived from the use of a CE instrument to determine cmc values should be mentioned. For instance, compared to traditional conductimetry measurements, the CE procedure would allow the study in an easy way of the influence of temperature on cmc values based on the good temperature control provided by the CE instruments. Moreover, it can also permit determination of cmc values in an unattended mode, while the volume of surfactant required (and additives if any) for each measurement is much smaller than that normally needed in traditional conductimetry. Besides, unlike other CE procedures,13-15 the present method for determining cmc values does not require dyes nor separations. However, although the method involving electrical conductance is the preferred method for determining the cmc, and far more accurate results have been reported using this method than any other,31 the method is limited to ionic surfactants. It also presents limitations for measurements carried out in highly conductive solutions. ACKNOWLEDGMENT This work was supported by the Commission of the European Communities (Training and Mobility of Researchers, Contract No. ERBFMBICT950003) and by a DGICYT project (PB94-02818-C02C02). The authors thank Dr. F. Ortega, Physical Chemistry Department, Complutense University (Madrid, Spain), for fruitful discussion. Received for review July 1, 1997. 1997.X
Accepted July 10,
AC970696N X
Abstract published in Advance ACS Abstracts, September 1, 1997.
