Anal. Chem. 1997, 69, 1852-1860
Micelle Polymers as Pseudostationary Phases in MEKC: Chromatographic Performance and Chemical Selectivity Christopher P. Palmer*,† and Shigeru Terabe
Faculty of Science, Himeji Institute of Technology, Kamigori, Hyogo, 678-12 Japan
Two micelle polymers, sodium polyundecenyl sulfate and sodium polyundecylenate, have been synthesized, purified, and characterized as pseudostationary phases for micellar electrokinetic chromatography. The polymers have been characterized by size exclusion chromatography, laser light scattering, and electrokinetic chromatography. When employed as pseudostationary phases, the micelle polymers were found to provide selective and efficient separations. In aqueous buffers, the chromatographic properties of the polymers were similar to those of sodium dodecyl sulfate (SDS) micelles. The efficiency was found to be similar to that of SDS micelles. The electrophoretic mobility of the polymers was greater than SDS micelles, providing an extended migration time range. The chemical selectivity of the two polymers was found to be very similar. However, both of the polymers are more polar than SDS micelles. In buffers modified with high concentrations of organic solvents, the chromatographic properties of sodium polyundecenyl sulfate were found to be superior to those of SDS micelles. Separations of polynuclear aromatic hydrocarbons in acetonitrile- and methanol-modified buffers are much improved using the sulfate polymer relative to SDS micelles. The structural selectivity of the undecenyl sulfate polymer pseudostationary phase for the separation of polynuclear aromatic hydrocarbons in organic-modified buffer media was found to be affected by the organic modifier used. Introduced in 1984 by Terabe et al.,1 micellar electrokinetic chromatography (MEKC) is a modification of capillary electrophoresis that separates charged or neutral compounds based on their relative affinity for the lipophilic interior and/or the ionic exterior of a micellar pseudostationary phase. Due to electrophoretic effects, negatively charged micelles formed from anionic surfactants such as sodium dodecyl sulfate (SDS) migrate at a rate slower than that of the electroosmotic flow. The rate of migration of an analyte therefore depends on its partition coefficient between the micelles and the electroosmotically pumped aqueous phase. This has proven to be a powerful tool for the separation and analysis of a variety of analytes (e.g., refs 2-5). †
Current address: Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, NM 87801 (1) Terabe, S.; Otsuka, K.; Ichikawa, K.; Tsuchiya, A.; Ando, T. Anal. Chem. 1984, 56, 111-3. (2) Vindevogel, J.; Sandra, P. Introduction to Micellar Electrokinetic Chromatography; Hu ¨ thig: Heidelberg, Germany, 1992.
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The migration of an analyte can be described mathematically in a manner very similar to that used in other forms of chromatography. The number of theoretical plates, N, or the efficiency, is calculated in the same manner. The equation for the migration factor, k′, must be modified, however, because of the mobility of the pseudostationary phase. The equation becomes6
k′ )
tmig - t0 t0(1 - (tmig/tmc))
where tmig is the migration time of the analyte, t0 is the migration time of a neutral solute which does not interact with the micelles, and tmc is the time required for a micelle to travel the length of the capillary. tmc is usually measured using an analyte that spends all of its time associated with the micellar pseudostationary phase but can be estimated using a homologous series and the reiterative approach of Bushey and Jorgenson.7 There are several significant limitations associated with MEKC. MEKC is characterized by a limited migration time range: all analytes must have migration times between t0 and tmc. Additionally, hydrophobic analytes present problems because, due to high partition coefficients, they tend to have migration times close to tmc with very high k′ values. Finally, micellar pseudostationary phases have limited stability, being in a state of equilibrium with the free surfactant in the surrounding buffer medium. The micellar equilibrium is characterized by the critical micelle concentration of free surfactant (cmc) and the aggregation number, or the number of surfactant monomers assembled in a single micelle. Since the micelle is the pseudostationary phase in MEKC, the migration factor is directly related to the volume of the micelle, Vmc, through
k′ ) K(Vmc/Vaq) where K is the distribution coefficient and Vaq is the volume of the aqueous phase excluding the volume of the micelle. The volume of the micelle is given as
Vmc ) νj(Csrf - cmc) where νj is the partial specific volume of the surfactant and Csrf is (3) Terabe, S.; Chen, N; Otsuka, K. In Advances in Electrophoresis; Chrambach, A., Dunn, M. J., Radola, B. J., Eds.; VCH: Weinheim, Germany 1994; Vol. 7, pp 87-153. (4) Monnig, C. A.; Kennedy, R. T. Anal. Chem. 1994, 66, 280R-314R. (5) St. Claire, R. L., III Anal. Chem. 1996, 68, 569R-86R. (6) Terabe, S.; Otsuka, K.; Ando, T. Anal. Chem. 1985, 57, 834-41. (7) Bushey, M. M.; Jorgenson, J. W. Anal. Chem. 1989, 61, 491-3. S0003-2700(96)00801-3 CCC: $14.00
© 1997 American Chemical Society
the total concentration of the surfactant. The cmc varies with the analytical conditions: it is affected by changes in the temperature, salt concentration, pH, and the concentration and nature of buffer additives. The dependence of the cmc on the temperature is particularly problematic since the application of the electric field across the capillary causes Joule heating and a rise of the temperature inside the capillary, even when it is thermostated.8-11 A change in the temperature will cause a change in the cmc, the distribution coefficient, and the viscosity of the buffer. Because of the dependence of the migration factor on the cmc and the distribution coefficient, temperature effects can be expected to be more serious in MEKC than in capillary electrophoresis. A second serious impact of the equilibrium status of the micellar phase is that it limits the flexibility of the technique in terms of the choice of analytical conditions. The surfactants must have a relatively low cmc, limiting the choice of surfactants considerably. Additionally, the effect of organic additives on the cmc and structure of micelles adds complications for the analysis of hydrophobic compounds. Micelle formation may be eliminated altogether by the addition of organic modifiers at concentrations above 20-30%.12,13 Reports of separations using SDS micelles in buffers modified with high concentrations of organic modifiers may be due to premicellar aggregation.14,26 Recently, several authors have reported the use of covalently stabilized high molecular surfactants, or micelle polymers, as pseudostationary phases in MEKC.15-29 These compounds provide very stable pseudostationary phases with zero cmc. The structure and concentration of the phase does not change with changes in the analytical conditions. The structures can be used in the presence of relatively high amounts of organic modifier,15,16,20,21,24-27 and often afford unique selectivity relative to micelles of SDS.15,16,25 In similar work, resorcarenes have been (8) Wa¨tzig, H. Chromatographia 1992, 33, 445-8. (9) Bello, M. S.; Chiari, M.; Nesi, N.; Righetti, P. G.; Saracchi, M. J. Chromatogr., A 1992, 625, 323-30. (10) Terabe, S.; Katsura, T.; Akada,Y.; Ishihama, Y.; Otsuka, K. J. Microcolumn. Sep. 1993, 5, 23. (11) Knox, J. H.; McCormack, K. A. Chromatographia 1994, 38, 207-13. (12) Magid, L., Solvent Effects on Amphiphilic Aggregation. In Solution Chemistry of Surfactants; Mittal, K., Ed.; Plenum Press: 1979; Vol. 1, 427-453. (13) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 3320-30. (14) Vindevogel, J.; Sandra, P. Anal. Chem. 1991, 63, 1530-6. (15) Palmer, C. P.; McNair, H. M. J. Microcolumn Sep. 1992, 4, 509-14. (16) Palmer, C. P.; Khaled, M. Y.; McNair, H. M. J. High Resolut. Chromatogr. 1992, 15, 756-62. (17) Ozaki, H.; Terabe, S.; Ichihara, A. J. Chromatogr., A 1994, 680, 117-23. (18) Terabe,S.; Ozaki, H.; Tanaka, Y. J. Chin. Chem. Soc. 1994, 41, 251-7. (19) Wang, J.; Warner, I. M. Anal. Chem. 1994, 66, 3773-6. (20) Palmer, C. P.; Terabe, S. Kuromatogurafi 1995, 16, 98-9. (21) Ozaki, H.; Ichihara, A.; Terabe, S. J. Chromatogr., A 1995, 709, 3-10. (22) Ozaki, H.; Itou N.; Terabe, S.; Takada, Y.; Sakairi, M.; Koizumi, H. J. Chromatogr., A 1995, 716, 69-79. (23) Dobashi, A.; Hamada, M.; Dobashi, Y. Anal. Chem. 1995, 67, 3011-7. (24) Yang, S. Y.; Bumgarner J. G.; Khaledi, M. G. J. High Resolut. Chromatogr. 1995, 18, 443-5. (25) Palmer, C. P.; Terabe, S. J. Microcolumn Sep. 1996, 8, 115-21. (26) Tanaka, N.; Fukutome,T.; Hosoya, K; Kimata, K.; Araki, T. J. Chromatogr., A 1995, 716, 57-67. (27) Tanaka, N.; Fukutome,T.; Tanigawa, T.; Hosoya, K; Kimata, K.; Araki, T.; Unger, K. K. J. Chromatogr., A 1995, 699, 331-41. (28) Palmer, C. P.; Terabe, S.; Ozaki, H. Development of Molecular Micelles as Pseudo-Stationary Phases in MEKC. Presented at the 17th International Symposium on Capillary Chromatography and Electrophoresis, Wintergreen, VA, May 7-11, 1995. (29) Shamsi, S. A.; Mathison, S. M.; Dewees, S.; Wang, J.; Warner, I. M. The Utility of a Non-Chiral Polymeric Surfactant as a Co-Modifier in Cyclodextrin Modified Micellar Electrokinetic Capillary Chromatography. Pittcon’96, Chicago, IL, March 3-8, 1996; Poster 84P.
used as pseudostationary phases for electrokinetic chromatography in the presence of high concentrations of organic modifiers.30 These studies have demonstrated the utility of micelle polymers and their potential advantages over conventional micelles as pseudostationary phases for MEKC, but limited studies have been performed to determine the effects of polymerization or polymer chemistry on the chromatographic performance and chemical selectivity of pseudostationary phases. A greater understanding of the effects of polymer structure on performance will facilitate the development and introduction of new polymeric pseudostationary phases. Many micelle polymers have been reported in the literature for a variety of commercial applications,31,32 and these structures might be adapted to MEKC to provide a wide range of chemical selectivities. We have investigated the effects of polymerization and polymer chemistry on the selectivity and performance of two micelle polymers in an effort to gain a greater understanding of these factors. Larrabee and Sprague reported in 1979 that sodium 10undecylenate can be polymerized by γ irradiation, but only above the cmc.33 The surfactant evidently undergoes reaction at the tail end of the lipid chain in the interior of the micelle to form an oligomer of 10 monomer units. Solutions of the resulting oligomer behave similarly to micellar solutions of the free surfactant. Others have confirmed these results.34-37 Spectroscopic studies using spin-labeled substrates35 and pyrene37 have shown that the polymer does solubilize hydrophobic compounds. The microenvironment of the solubilized substrates is more polar35,37 and the substrates have less mobility35 than when solubilized by micelles of sodium 10-undecylenate. This has been interpreted to mean that hydrophobic compounds are less able to penetrate the more structured core of the micelle polymer.35,37 Oligomerized sodium 10-undecylenate has been employed previously as a pseudostationary phase in MEKC,15,16 as has the sulfate analog.20,25,28,29,30 These phases provide high-efficiency separations, unique selectivity relative to SDS micelles, and high stability in the presence of both methanol and acetonitrile. The undecylenate analog is limited, however, by the carboxylate head groups, whose pH chemistry limits the electrophoretic mobility and solubility of the phase at pH’s below 8. Additionally, it remains unclear whether the differences in chemical selectivity between this oligomer and SDS micelles are related to the chemistry of the head group or are the result of the polymerization procedure or the size of the polymers. In this report, sodium undecylenate was polymerized and sodium 10-undecenyl sulfate was synthesized and polymerized. The structures of the monomers are presented in Figure 1. The resulting polymers have been purified of low molecular weight impurities and characterized by size exclusion chromatography. The chemical selectivity of the two polymers for the separation of substituted benzene and naphthalene compounds and polynuclear aromatic hydrocarbons has been compared to one another (30) Ba¨chmann, K.; Bazzanella, A.; Haag, I.; Han, K.-Y.; Arnecke, R.; Bo¨hmer V.; Vogt, W. Anal. Chem. 1995, 67, 1722-6. (31) Anton, P.; Ko¨berle P.; Laschewsky, A. Makromol. Chem. 1993, 194, 1-27. (32) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 3-85. (33) Larrabee, C. E.; Sprague, E. D. J. Polym. Sci. Polym. Lett. 1979, 17, 74951. (34) Durairaj, B.; Blum, F. D. Langmuir 1989, 5, 370-2. (35) Sprague, E. D.; Duecker, D. C.; Larrabee, C. E., Jr. J. Am. Chem. Soc. 1981, 103, 6797-800. (36) Arai, K.; Sugita, J.; Ogiwara, Y. Makromol. Chem. 1987, 188, 2511-6. (37) Paleos, C. M.; Stassinopoulou, C. I.; Malliaris,A. J. Phys. Chem. 1983, 87, 251-4.
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Figure 1. Structures of (A) sodium undecylenate and (B) sodium undecenyl sulfate.
and to micelles of SDS. The selectivity of sodium polyundecenyl sulfate for the separation of polynuclear aromatic hydrocarbons in methanol- and acetonitrile-modified buffers has also been compared to that of SDS micelles. EXPERIMENTAL SECTION Synthesis and Characterization. Sodium 10-undecenyl sulfate (SUS) was synthesized from undecen-10-ol (Tokyo Kasei Kogyo, Tokyo, Japan) as reported earlier.25 Sodium undecylenate (Sigma Chemical, St. Louis, Mo) and SUS were polymerized following the procedure of Durairaj and Blum,34 as reported earlier.15 The polymers were collected by precipitation from cold ethanol (Tokyo Kasei), as reported earlier.15,25 Elemental analysis was performed using a Perkin Elmer elemental analyzer and was repeated by an independent laboratory to confirm the results. Elemental analysis of the polymers prepared as described above indicated the presence of inorganic impurities. The polymers were thus further purified by dialysis for 24 h using dialysis tubing (Spectrum, Houston, TX) with a 1000 MW cutoff. The solution remaining within the tubing was freeze-dried. Elemental analysis results for the dialyzed products were consistent with the structures of the polymers, assuming an order of polymerization of 10-12. Proton nuclear magnetic resonance (NMR) characterization of the product was run in deuterated water (Aldrich Chemical Co., Milwaukee, WI) at 400 MHZ on a JEOL (Tokyo, Japan) JNMGX400 spectrometer. Resonances were referenced to water (4.7 ppm). Size exclusion chromatographic analysis was run by an independent laboratory (Kaneka Techno Research, Osaka, Japan) using a Waters (Milford, MA) Ultrahydrogel linear column (7.8 mm × 300 mm; particle size, 6-13 µm; mixed pore size, 1202000 Å) at 40 °C using 80% aqueous 0.4 M sodium nitrate and 20% acetonitrile. Molecular weights for the polymers were calculated using poly(ethylene glycol) molecular weight standards [860 000 and 145 000 MW from Tosoh Chemical (Tokyo) and 6000 and 1000 MW from Kanto Chemical (Tokyo)]. The numberaverage molecular weight (Mn) and weight-average molecular weight (Mw) were calculated using the size exclusion algorithm on the Waters Maxima 8205 chromatography workstation. Dynamic laser light scattering (DLS) was performed to determine the radius of the undecylenate polymer. This was 1854
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performed using a DLS-700 (Photal, Otsuka Electronics, Hirakata, Osaka, Japan) instrument at 25 °C at a concentration of 1.2% (w/ v) in 0.1 M phosphate buffer at pH 7. The solution was filtered through a 0.2 µm membrane filter before analysis. The detection angle of the scattered light was varied from 30 to 120° in 15° intervals. Chromatography. All of the micellar electrokinetic chromatographic experiments were performed using a Beckman (Fullerton, CA) P/ACE 2000 capillary electrophoresis instrument and Beckman Gold software. The detector was operated at 214 nm. The temperature was thermostated at 25 °C. Fused-silica capillaries (PolyMicro Technologies, Phoenix, AZ) of 50 µm i.d. were used for all studies. Buffers were prepared at the reported pH by mixing 25 mM sodium dihydrogen phosphate (Nacalai Tesque) solution with 12.5 mM sodium tetraborate (Nacalai Tesque) buffer. The solutions were prepared in reverse osmosis purified water. All of the organic modified buffers were prepared by volume from the borate buffer and the organic modifier. Acetonitrile and methanol were obtained in special reagent grade from Nacalai Tesque. All buffers were filtered through 0.45 µm filters (Nacalai Tesque) prior to use. Analyte solutions were prepared in the separation buffers at concentrations of ∼25 ppm. All analytes were purchased in the highest grade available. Dimethyl sulfoxide (0.01%) and Sudan IV were used to measure t0 and tmc respectively. tmc values for buffers containing organic modifiers were determined by measuring the migration times of a homologous series of alkyl phenones (C1-C6, C8) and alkyl benzoates (C1-C4) and estimating the tmc using the reiterative calculation procedure of Bushey and Jorgenson.7 A BASIC program was used for the reiterative calculation, and the calculation was repeated until the change in the tmc from the previous iteration was less than 0.1% or the correlation coefficient (r2) was greater than 0.9999 (typically 3050 iterations). Migration factors were calculated using the tmc from the migration of sudan IV when available (aqueous selectivity studies). Otherwise, capacity factors were calculated using the mobility of the polymer as calculated from the tmc obtained from the homologous series method. Methylene selectivities for the homologous series were calculated from the slope of the plot of the log of the capacity factor vs carbon number (RCH2 ) 10m) and thus represent an average for the entire homologous series. RESULTS AND DISCUSSION Synthesis. As previously reported, the synthesis of SUS yielded 79% recovery of purified product. The cmc of this material, determined by conductimetric methods, was found to be 0.0333 ( 0.0003 mol/kg. The synthesis of the SUS polymer yielded 1.4 g (32% recovery) of white granular product. An NMR spectrum of the product yielded resonances with appropriate area for the sulfate R methylene protons, but no resonances for the vinyl protons. Additionally, a resonance was observed which was assigned to R-methylene protons for an alkanol. Integration of this resonance and comparison with that for the sulfate R protons indicated that ∼5% of the sulfate groups were hydrolyzed during the course of the synthesis. The synthesis of the sodium undecylenate (SUA) polymer yielded 25% product, and the NMR confirmed the absence of resonances for the vinyl protons.
Table 1. Results of Elemental Analysis for Original and Purified Polymersa calculated
before dialysis
after dialysis
% carbon % hydrogen
47.5 7.5
SUS Polymer 27.8 4.9
46.5 8.1
% carbon % hydrogen
60.0 9.0
SUA Polymer 51.5 7.9
59.5 9.1
a Calculated results assume 10 monomer units and sulfate end groups.
Table 2. Results of Size Exclusion Chromatography Performed on the Two Polymersa polymer
Mn (g/mol)
Mw (g/mol)
Mw/Mn
SUA SUS
942 1035
1029 1769
1.09 1.71
a Molecular weights are relative to poly(ethylene oxide) reference standards. Mn is the number average, Mw is the weight average, and Mn/Mw is the dispersity.
Characterization of SUS and SUA Polymers. Elemental analysis of the polymer products is presented in Table 1. Clearly the polymers, especially the SUS polymer, were contaminated by an inorganic impurity before dialysis. (Anion exchange chromatography later confirmed the presence of sulfate anions, presumably from the persulfate initiator and/or the hydrolysis of sulfate head groups). Elemental analysis after dialysis is close to the calculated values assuming 10-12 monomer units, indicating that the inorganic impurities had been removed by dialysis. The molecular weights and dispersity for the two polymers, as determined by size exclusion chromatography, are presented in Table 2. These results are only estimates of the true molecular weights, due to the unavailability of suitable molecular weight reference standards. It should be noted, however, that these standards did provide accurate molecular weights for polyacrylate polymers.17 The values indicate relatively small oligomers of 5 monomer units for SUA and 10 monomer units for SUS. The polymerization order for SUA has been previously reported to be ∼10,33 which is similar to the aggregation number of the micelle. The results reported here may be influenced by the tendency of micelle polymers to have relatively small radii for their molecular weight,32 and thus do not disprove the earlier estimates. More importantly, the size exclusion results indicate that these micelle polymers are small relative to SDS micelles, which have an aggregation number on the order of 60-65. Additionally, the results show that the SUS polymer has a greater molecular weight and a greater dispersity than the SUA polymer. It should also be noted here that preliminary results using dynamic laser light scattering indicate that the diameter of the SUA polymer in solution is 37.9 (at 120°) to 62.5 nm (at 30°) with an extrapolated value at 0°, where the Einstein-Stokes equation is valid, of 70 nm. The diameter of SDS micelles in 0.03-0.5 M sodium chloride at 25 °C has been reported to be 1.91-2.79 nm.38-41 These results indicate that the SUA oligomer varies (38) Chang, N. J.; Kaler, E. W. J. Phys. Chem. 1985, 89, 2996-3000. (39) Stigter, D. J. Phys. Chem. 1979, 83, 1670-5. (40) Corti, M.; Degiorgio,V. J. Phys. Chem. 1981, 85, 711-7.
significantly in size and shape and is larger than SDS micelles. Further work is in progress to determine whether these results are an experimental artifact, are caused by intermolecular aggregation between the SUA polymers,37,42 or are due to different shape or conformation for the polymer (spherical, cylindrical, elipsoidal, etc.). These results will be reported in a later publication. Performance as Pseudostationary Phases. Presented in Figure 2 is the separation of several substituted benzene and naphthalene compounds using SDS micelles and SUS polymer dissolved in pH 7.3 phosphate/borate buffer at concentrations of 0.030 M and 0.83% (w/v) (equivalent to 0.030 M SUS monomer), respectively. It can be seen from this figure that the SUS polymer provides similar efficiency and resolution to SDS. Additionally, the polymer has a higher electrophoretic mobility, as reported in Table 3, and thus provides an enhanced migration time range. The electrophoretic mobilities reported in Table 3 are calculated from tmc values obtained from linearization of homologous series. Measurements of electrophoretic mobility using sudan IV as a micelle marker agree with these values within 4% and indicate lower mobility. It should also be noted from Figure 2 that the selectivity is different for the two pseudostationary phases. The migration order for nitrobenzene and nitroaniline is reversed, and the relative migration of acenaphthol, naphthalene, and naphthaleneethanol is altered significantly. A plot of the migration factors for these substituted benzene and naphthalene compounds as a function of the concentration of the SUS polymer at pH 8.0 is linear and the intercepts are not significantly different from zero, which indicates that the effective cmc for the polymer is zero.17,25 All but the earliest eluting compounds can still be separated at a polymer concentration of 0.3% (equivalent to 0.011 M SUS monomer).25 No break in retention is observed from 0.3 to 1.2%, indicating that intermolecular aggregation of SUS is not occurring in this concentration range or does not significantly affect interactions between the polymer and solutes. Figure 3 shows the separation of some selected cold medicine ingredients and some naphthols using the SUS polymer. The polymer can be used to separate a variety of solutes with efficiency similar to that obtained with SDS micelles. In Figure 4, the logarithms of the migration factors for the analytes in Figures 2 and 3 using SUS oligomer and SDS micelles have been plotted against one another. The labels in the plot indicate the functional groups on the test solutes. This plot allows the comparison of the selectivities of the two pseudostationary phases: if the selectivities were the same one would expect a linear plot with all points falling on the line, while if the selectivities were very different one would expect a scatterplot. In general, the selectivity of the SUS is similar to that of SDS (r2 ) 0.969), although there are some significant deviations from the best-fit line. Polar hydroxyl- and amine-containing compounds all fall above the line (indicating increased affinity for the SUS phase), and the nonpolar compounds fall below the line (indicating increased affinity for the SDS micelles). This indicates that the SUS polymer phase is a significantly more polar phase than the (41) Douhe´ret, G.; Viallard, A. J. Chim. Phys. 1981, 78, 85. (42) Arai, K.; Maseki, Y.; Ogiwara, Y. Makromol. Chem. Rapid Commun. 1987, 8, 563-7.
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Figure 2. Separation of substituted benzene and naphthalene compounds: (A) 0.83% SUS polymer; (B) 30 mM SDS. Capillary was 50 cm effective length and 57 cm total length, 16.1 kV applied potential. A phosphate/borate buffer at pH 7.3 was employed. Key: (1) nitrobenzene; (2) anisole; (3) p-nitroaniline; (4) o-xylene; (5) m-xylene; (6) naphthylamine; (7) naphthalene methanol; (8) acenaphthenol; (9) naphthalene; (10) naphthaleneethanol; (11) diphenyl ether.
Figure 3. Separation of (A) selected cold medicine ingredients: (1) acetaminophen; (2) caffeine; (3) guaifenesin; (4) phenacetin; (5) ethenzamide; (6) trimetoquinol. (B) Separation of resorcinol and naphthols: (1) resorcinol; (2) 1,6 dinaphthol; (3) 1-naphthol; (4) 2-naphthol. Both separations were conducted in a 57 cm (50 cm effective) capillary at 16.1 kV applied potential in a pH 7.3 phosphate/borate buffer containing 0.83% w/v SUS polymer.
SDS micellar phase. This is consistent with previous observations for the oligomer of sodium 10-undecylenate.15,35,37 Figure 5 presents a similar plot for the comparison of chemical selectivity of SUA oligomer vs SUS oligomer. This plot shows that the SUA oligomer has chemical selectivity very similar to that of the SUS oligomer. This result is somewhat surprising, since the chemical selectivity of micellar phases has been shown to be most dependent on the chemistry of the ionic head groups.43 In this case, two oligomers with different head groups have (43) Nishi, H.; Fukuyama, T.; Matsuo M.; Terabe, S. J. Pharm. Sci. 1990, 79, 519-23.
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virtually the same selectivity while the SDS micelles, with the same head group as the SUS oligomer, provide different selectivity. Presented in Table 3 are several important descriptors of pseudostationary phase performance. The effective electrophoretic mobilities of the two polymers and SDS micelles in aqueous systems show that polymers have somewhat higher electrophoretic mobilities and thus provide a broader migration time range. The mean efficiencies for the separation of the substituted benzene and naphthalene compounds using the three pseudostationary phases show that there is no significant loss in efficiency when the polymers are used relative to SDS micelles.
Table 3. Chromatographic and Electrophoretic Properties of the Three Pseudostationary Phasesa 104 × electrophoretic mobility (cm2/V‚s) SDS micelles 50 mM) SUS polymer (1% (w/v) SUA polymer (1% (w/v)
mean efficiency (theor plates)
methylene selectivityb
-4.147 ( 0.003
200 000 ( 61 000 2.47 ( 0.08
-4.358 ( 0.007
180 000 ( 34 000 2.20 ( 0.06
-4.36 ( 0.02
200 000 ( 13 000 1.99 ( 0.04
a All data were collected with 57cm (50 cm effective length) 50 µm i.d. capillaries with an applied potential of 16.1 kV using a pH 8.4 phosphate/borate buffer. Electrophoretic mobitlity is calculated from the tmc values obtained from linearization of the homologous series.
Figure 5. Logarithm of the capacity factors for the analytes from Figure 2 using 1% w/v SUA vs 1% w/v SUS in pH 8.4 phosphate/ borate buffer. Labels are as in Figure 4.
Figure 4. Logarithm of the capacity factors for the analytes in Figures 2 and 3 using 0.83% SUS polymer vs 30 mM SDS. Analytical conditions are as described in Figures 2 and 3. Labels: et ) ether group, no ) no functional groups; nh ) amine group, oh ) hydroxyl, and ohoh ) two hydroxyl groups. Unlabeled points have multiple functional groups.
There may be a loss in efficiency due to the greater dispersity of the SUS polymer relative to the SUA polymer, although the results are inconclusive. The methylene selectivities (mean selectivity between adjacent compounds in a homologous series) for the three pseudostationary phases are further evidence that the polymers have a more polar character than SDS micelles, as more polar pseudostationary phases would provide less methylene selectivity. The methylene selectivites reported in Table 3 are the average of those obtained for the two homologous series, and the standard deviation reported includes the variation between the two series. The difference in methylene selectivity between the two series was less than (7%. The SUA polymer provides the lowest methylene selectivity, perhaps because of its smaller size or because of the chemistry of the head group. Differences in selectivity between different pairs of homologues was insignificant (
