Anal. Chem. 1999, 71, 1270-1277
Characterization of Chemical Selectivity in Micellar Electrokinetic Chromatography. 4. Effect of Surfactant Headgroup Mark D. Trone and Morteza G. Khaledi*
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
The influence of surfactant headgroups on migration behavior in micellar electrokinetic chromatography is examined. Using linear solvation energy relationships (LSER) and functional group selectivity studies, the effect of six anionic headgroups on chemical selectivity is characterized. The sodium dodecyl surfactants of the sulfate [SO4-], sulfonate [SO3-], carboxylate [CO2-], carbonyl valine [OC(O)NHCH(CH(CH3)2)CO2-], and sulfoacetate [OC(O)CH2SO3-] anions are investigated. Solute size and the hydrogen-bond-donating ability of the micellar phase play the most significant roles in solute retention in all of the surfactants studied. While solute-micelle hydrogen bonding plays a dominant role in the observed selectivity, the dipolarity and polarizability of the micellar phase also have a small influence. The results also suggest that the hydrogen-bond-accepting ability for surfactants is inversely proportional to the proton acidity (pKa) of its headgroup. The observed hydrogen-bond-donating ability and dipolarity of surfactant systems are believed to be a result of the water that resides near the micelle surface. Since its introduction by Terabe et al. in 1985,1 micellar electrokinetic chromatography (MEKC) has gained much attention. MEKC expands the usefulness of capillary electrophoresis by separating uncharged solutes on the basis of their differential partitioning into a micellar pseudostationary phase.2 The primary driving force behind solute retention in MEKC is the hydrophobic interaction between solutes and the micellar phase. However, other, more specific types of interactions between solutes and micelles also influence solute retention and selectivity. In fact, this is the primary reason for the observed variations in selectivity between different pseudophases in MEKC. Complex mixtures can be separated using various single and mixed surfactant systems in MEKC. However, the standard practice for choosing the composition of the pseudostationary phase solution has been trial and error. To achieve a better understanding of the factors that control selectivity, one should first understand the nature of the interactions. Previously, we reported modeling MEKC retention using linear solvation energy relationships (LSER).3,4 Kamlet and Taft (1) Terabe, S.; Otsuka, K.; Ando, T. Anal. Chem. 1985, 57, 834. (2) Khaledi, M. G. In High Performance Capillary Electrophoresis; Theory, Techniques, and Applications; Khaledi, M. G., Ed.; Wiley-Interscience: New York, 1998; p 77.
1270 Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
and co-workers initially developed LSER to describe solvation effects on physicochemical processes.5-7 There are several reports that have used variations of this model to characterize retention in gas and liquid chromatography, as outlined in a review by Carr.8 Abraham9,10 has since used gas chromatography to improve the accuracy of some of the solute descriptor values and modified the LSER model reported by Kamlet and Taft and co-workers. According to Abraham’s modified version of the LSER equation, retention in MEKC, log k ′, can be described as
log k′ ) c + mVx + rR2 + sπ2* + aΣR2 + bΣβ2
(1)
The Vx, R2, π2*, Σβ2, and ΣR2 terms are the solute descriptors, where Vx represents the solute characteristic volume in (cm3‚ mol-1/100),11 R2 is the solute’s excess molar refraction divided by 10,12 and π2* represents the solute dipolarity. The Vx and R2 terms have been reduced by factors of 100 and 10, respectively, to bring them to scale with the other parameters. The solute hydrogen-bond-accepting and solute hydrogen-bond-donating abilities are represented by Σβ2 and ΣR2, respectively. The subscript 2 simply signifies that these parameters are solute descriptors. The coefficients of these descriptors (m, r, s, b, and a) are related to the contribution of the micellar phase toward each type of interaction. The cohesiveness and the dispersive interactions of the micellar phase are related to m.13 The ability of the micelle to interact with the n- or π-electrons of the solute is described by r. This term is essentially related to the polarizability of the pseudostationary phase. The dipolarity of the micellar phase is represented by s. The a and b terms describe the ability of the micelle to form hydrogen bond interactions with solute molecules, (3) Yang, S.; Khaledi, M. G. Anal. Chem. 1995, 67, 499. (4) Yang, S.; Khaledi, M. G. J. Chromatogr. A 1995, 692, 301. (5) Kamlet, M. J.; Abraham, M. H.; Doherty, R. M.; Taft, R. W. J. Am. Chem. Soc. 1984, 106, 464. (6) Kamlet, M. J.; Doherty, R. M.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. CHEMTECH 1986, 566. (7) Abraham, M. J.; Doherty, R. M.; Kamlet, M. J.; Taft, R. W. Chem. Br. 1986, 22, 551. (8) Carr, P. W. Microchem. J. 1993, 48, 4. (9) Abraham, M. H. Chem. Soc. Rev. 1993, 22, 73. (10) Abraham, M. H. Pure Appl. Chem. 1993, 65, 2503. (11) McGowan, J. C.; Abraham, M. H. Chromatographia 1987, 23, 243. (12) Abraham, M. H.; Whiting, G. S.; Doherty, R. M., Shuely, W. J. J. Chem. Soc., Perkin Trans. 2 1990, 1451. (13) (a) Tan, L. C.; Carr, P. W.; Abraham, M. H. J. Chromatogr. A 1996, 752, 1. (b) Abraham, M. H.; Whiting, G. S.; Fuchs, R.; Chambers, E. J. J. Chem. Soc., Perkin Trans. 2 1990, 291. 10.1021/ac9809736 CCC: $18.00
© 1999 American Chemical Society Published on Web 02/25/1999
where b represents the hydrogen-bond-donating ability and a represents the hydrogen-bond-accepting ability of the micellar phase. The major contributor to the regression constant, c, is the phase ratio of the separation system. Increasing the phase ratio reduces the magnitude of c but does not affect the other LSER coefficients.3 It is also important to note that the hydrogen-bonddonating ability of a micellar system describes the sharing of a proton and should not be confused with proton dissociation acidity. The original Kamlet and Taft LSER model used for those investigations did not use the R2 solute descriptor in eq 1. That model also used the intrinsic volume of the solute rather than the McGowan characteristic volume, which was developed later.11 Yang and Khaledi first reported the usefulness of LSER modeling of solute retention in MEKC for characterization of chemical selectivity of micellar pseudophases.3 Applying the original Kamlet and Taft LSER model, the interaction behavior of sodium dodecyl sulfate (SDS), lithium perfluorooctane sulfonate (LiPFOS), and sodium cholate (SC) was examined. The results from this initial investigation suggested that the most important surfactant interactions were based on the solute size (m coefficient) and hydrogen bonding (b coefficient). This observation is similar to the previously reported results for GC and HPLC.8 The m coefficient was large for all of the surfactants, but the magnitude of m was quite similar for all of the surfactant systems except for LiPFOS. The hydrogen-bond-donating capability of the surfactant (b coefficient), however, had the most influence on selectivity differences. Other reports from this laboratory extended the LSER characterization of selectivity in MEKC to other monosurfactant systems, polymeric pseudophases, and liposomes as well as mixed systems of hydrocarbon/fluorocarbon surfactants, hydrocarbon/bile salt surfactants, SDS/polymer, and SDS/organic modifiers.14,15,47-49 Muijselaar et al. have used the solute retention index and Kamlet and Taft’s model to describe selectivity in various single and anionic/nonionic mixed surfactant systems.16 These papers also concluded that the ability of the surfactant system to donate hydrogen bonds (b coefficient) played the most critical role in determining selectivity in MEKC separations. In addition, Poole and Poole have recently applied Abraham’s revised LSER model (eq 1) and solute descriptor values to our data as well as their own to characterize various surfactant systems.17,18 They found that using Abraham’s model improved the fit of the data but came to the same conclusion about the factors that have the largest influence on selectivity in MEKC. Near the time of Yang and Khaledi’s paper, other groups also published reports using LSER to analyze micellar systems. Quina et al.19 and Abraham et al.20 used literature values for the waterSDS micelle partition coefficients and Abraham’s revised LSER model to describe solubilization in SDS solutions. More recently, Carr et al. measured the SDS-water partition coefficient by (14) Yang, S.; Bumgarner, J. G.; Khaledi, M. G. J. Chromatogr. A 1996, 738, 265. (15) Khaledi, M. G.; Bumgarder, J. G.; Hadjmohammadi, M. J. Chromatogr. A 1998, 802, 35. (16) Muijselaar, P. G.; Claessens, H. A.; Cramers, C. A. Anal. Chem. 1997, 69, 1184. (17) Poole, S. K.; Poole, C. F. Anal. Commun. 1997, 34, 57. (18) Poole, S. K.; Poole, C. F. Analyst 1997, 122, 267. (19) Quina, F. H.; Alonso, E. O.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708. (20) Abraham, M. H.; Chadha, H. S.; Dixon, J. P.; Rafols, C.; Treiner, C. J. Chem. Soc., Perkin Trans. 2 1995, 887.
headspace GC for a set of test solutes.21 After applying Abraham’s model to calculate the LSER coefficients for their SDS system, they compared their results to the results of Khaledi, Quina, and Abraham. Although three different methods and two different versions of the LSER model were compared, all four studies came to the same conclusions showing the robustness of the LSER model. Another important conclusion can be drawn from this comparison. Although Carr, Quina, and Abraham used the SDSwater partition coefficient and our group used the MEKC retention factor, the LSER coefficients were similar. The only significant difference is in the LSER constant. This observation is not surprising, since the retention factor is directly proportional to the partition coefficient, and it is further evidence that, although the phase ratio is dependent on the surfactant concentration, selectivity is not. One conclusion from all of the previous reports should be addressed further. The micelle cohesive and dispersive properties as well as its hydrogen-bond-donating ability are important in solute-micelle interactions. However, as has been discussed previously, the cohesive and dispersive properties have little influence on selectivity in MEKC. Therefore, to understand selectivity in MEKC, it is important to understand how the surfactant structure influences hydrogen-bonding properties. This group has initiated a systematic investigation on the effects of surfactant structure on chemical selectivity. As an initial step, this paper examines the effect of the headgroup for six anionic sodium dodecyl surfactants: sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDSu), sodium dodecyl carboxylate (SDC), sodium dodecyl phosphate (SDP), sodium (S)-N-dodecoxycarbonyl valine (SDCV), and sodium lauryl sulfoacetate (SLSA). Although both LSER models gave similar results, the revised model (eq 1) and descriptors were used. EXPERIMENTAL SECTION All of the test solutes were purchased from Aldrich (Milwaukee, WI). The test solutes and their solvation parameters are listed in Table 1. Sodium dodecyl sulfate (Sigma, St. Louis, MO), sodium dodecanesulfonate (Lancaster, Windham, NH), mono-n-dodecyl phosphate (Lancaster), n-tridecanoic acid (Lancaster), and (S)N-dodecoxycarbonyl valine (Waters Corp., Milford, MA) were all used as received from the manufacturer without further purification. Sodium lauryl sulfoacetate was graciously donated by Dr. Sahori Takeda at the Osaka National Research Institute and was used as received. The headgroup structures for these surfactants are shown in Figure 1. All of the solutions contained a 10 mM phosphate buffer (pH ) 7). SDS, SDSu, and SLSA were available as their sodium salts. The solutions of these surfactants were made by first dissolving the solid in about 4 mL of Milli-Q water. Exactly 2 mL of a 50.0 mM phosphate stock buffer (pH ) 7) was then pipetted into this solution, followed by dilution to 10.00 mL with Milli-Q water. Because of its higher Krafft point, SDSu had to be warmed to 35 °C before the solid would dissolve. SDC, SDP, and SDCV were received as acids and were titrated with a 1:1 mole ratio of NaOH. This ensured that the surfactant would be charged and that all solutions would have the same ionic strength and counterion (21) Vitha, M. F.; Dallas, A. J.; Carr, P. W. J. Colloid Interface Sci. 1997, 187, 179.
Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
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Table 1. Test Solutes and Their Solvation Descriptorsa
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 a
solutes
Vx
π2*
Σβ2
ΣR2
R2
benzene toluene ethylbenzene propylbenzene p-xylene chlorobenzene bromobenzene iodobenzene 4-chlorotoluene biphenyl naphthalene 1-methylnaphthalene acetophenone benzonitrile nitrobenzene methyl benzoate ethyl benzoate 4-chloroanisole 4-nitrotoluene 4-chloroacetophenone methyl 2-methylbenzoate phenyl acetate 3-methylbenzyl alcohol phenethyl alcohol benzyl alcohol phenol 4-methylphenol 4-ethylphenol 4-fluorophenol 4-chlorophenol 4-bromophenol 4-chloroaniline 3-chlorophenol 3-methylphenol 3-bromophenol 3,5-dimethylphenol
0.716 0.857 0.998 1.139 0.998 0.839 0.891 0.975 0.980 1.324 1.085 1.226 1.014 0.871 0.891 1.073 1.214 1.038 1.032 1.136 1.214 1.073 1.057 1.057 0.916 0.775 0.916 1.057 0.793 0.898 0.950 0.939 0.898 0.916 0.950 1.057
0.52 0.52 0.51 0.50 0.52 0.65 0.73 0.82 0.67 0.99 0.92 0.90 1.01 1.11 1.11 0.85 0.85 0.86 1.11 1.09 0.87 1.13 0.90 0.83 0.87 0.89 0.87 0.90 0.97 1.08 1.17 1.13 1.06 0.88 1.15 0.84
0.14 0.14 0.15 0.15 0.16 0.07 0.09 0.12 0.07 0.22 0.20 0.20 0.48 0.33 0.28 0.46 0.46 0.24 0.28 0.44 0.43 0.54 0.59 0.66 0.56 0.30 0.31 0.36 0.23 0.20 0.20 0.31 0.15 0.34 0.16 0.36
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.33 0.30 0.33 0.60 0.57 0.55 0.63 0.67 0.67 0.30 0.69 0.57 0.70 0.57
0.610 0.601 0.613 0.604 0.613 0.718 0.882 1.188 0.705 1.360 1.360 1.344 0.818 0.742 0.871 0.733 0.689 0.838 0.870 0.955 0.772 0.661 0.815 0.784 0.803 0.805 0.820 0.800 0.670 0.915 1.080 1.060 0.909 0.822 1.060 0.820
Solute descriptors from ref 46.
concentrations. They were then fully dissolved in water before addition of the necessary amount of phosphate stock solution. Again, because of the higher Krafft points of SDP and SDC, these two surfactants had to be dissolved at 35 °C before addition of the phosphate stock solution. All of the surfactant solutions were made fresh every day, filtered through a 0.45-µm polypropylene filter, and sonicated for at least 2 min before being used. All MEKC experiments were performed on a laboratory-built CE system equipped with a 0-30-kV power supply (Series EH, Glassman High Voltage, Inc., Whitehouse Station, NJ), an Acutect 500 variable-wavelength UV-visible detector, and a HewlettPackard HP3394A integrator. A 50-µm-i.d., 375-µm-o.d. fused silica capillary (Polymicro Technologies, Phoenix, AZ) was used. The total length of the capillary was 63 cm, with an effective length of 51 cm. A positive voltage of 25 kV was applied throughout the experiments. A circulating oil bath and two 250-mL jacketed beakers were used to keep the buffer reservoirs and the separation zone of the capillary at the same temperature. Since the Krafft points for SDC, SDSu, and SDP are all above room temperature, the capillary and buffer reservoirs were maintained at 36 °C for all of the surfactant systems. The solutes were introduced to the capillary by a 3-s hydrodynamic injection at the anodic end of the capillary. The retention factor for the 36 test solutes was measured three times for each surfactant system using eq 2.1 In this equation, teo is the migration 1272 Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
k′ )
(tr - teo) teo(1 - tr/tmc)
(2)
time of an unretained solute, tr is the solute retention time, and tmc represents the migration time of the micelle. Solutes were detected at 214 nm. Methanol was used as the electroosmotic flow (teo) marker and was measured from the time of injection to the first deviation from the baseline. The migration time of the micelle (tmc) was determined using n-dodecanophenone as a marker. RESULTS AND DISCUSSION Linear Solvation Energy Relationships. The influence of the surfactant headgroup on solute-micelle interactions was investigated by comparing six anionic surfactants, all of which are comprised of a 12-member hydrocarbon tail and a sodium counterion. The headgroups include sulfate [SO4-], sulfonate [SO3-], carboxylate [CO2-], phosphate [P(OH)O3-], carbonyl valine [OC(O)NHCH(CH(CH3)2)CO2-], and sulfoacetate [OC(O)CH2SO3-] (Figure 1). A LSER comparison between SDS and SDSu has already been examined, but the older descriptor values were used.17 Therefore, for the sake of comparison, the SDSu system was reexamined using the newer descriptors and the same conditions as the other surfactants. The retention behavior of the 36 test solutes in each surfactant system was examined. The solutes and their descriptors are listed in Table 1. Although the solutes exhibit a wide range of sizes and polarities, they are roughly classified into three groups on the basis of their hydrogen-bonding characteristics. Because of the detection limitations, all the solutes were benzene derivatives, which inherently are weak hydrogen bond acceptors (β e 0.20) due to their aromatic rings. However, these solutes can generally be considered to be non-hydrogen-bond-donating (NHB) solutes. Therefore, the solutes were categorized as non-hydrogen bond donors (1-12), hydrogen bond acceptors (13-24), and hydrogen bond donors (25-36). The LSER results for the different headgroups are listed in Table 2. The separation conditions were identical for each surfactant: 40 mM surfactant, 10 mM phosphate buffer (pH ) 7), 36 °C. All of the surfactant systems yielded high LSER correlation coefficients (R2 g 0.98). The coefficients represent differences in interactive properties of the micellar pseudostationary phase and the bulk aqueous phase. Using this model, as the coefficient becomes more positive, the micellar phase participates in that type of interaction more readily. For example, the more positive (or less negative) the b coefficient, the better that micellar phase acts as a hydrogen bond donor. It is important to note that the coefficients are used most reliably as qualitative values for the comparison of different MEKC systems. The regression constant, c, is large and negative for all of the surfactant systems studied. The most significant contributor to the constant is the phase ratio of the system. As mentioned previously, increasing the phase ratio decreases the magnitude of the system constant (makes it less negative) but does not affect the other coefficients in the LSER model.3 Therefore, it should influence the retention of all the analytes in roughly the same manner and has little effect on selectivity. Cohesiveness/Dispersion. The m coefficient is large and positive for all of the surfactants studied, indicating that the
Figure 1. Headgroup structures of the surfactants used in this investigation. Table 2. Headgroup Effect on Migration Behavior in MEKCa surfactant
c
m
s
SDS
-1.85
SDSu
-1.92
SDC
-1.95
SDP
-1.92
SDCV
-1.65
SLSA
-1.82
2.86 (0.09) 2.84 (0.10) 2.96 (0.12) 3.01 (0.11) 2.94 (0.12) 2.96 (0.12)
-0.31 (0.06) -0.42 (0.07) -0.39 (0.08) -0.55 (0.08) -0.61 (0.08) -0.37 (0.08)
b
a
-1.70 -0.15 (0.07) (0.04) -1.78 -0.02 (0.09) (0.04) -1.77 0.23 (0.10) (0.05) -2.00 0.15 (0.09) (0.05) -2.38 0.11 (0.10) (0.05) -2.39 0.10 (0.10) (0.05)
r
R2
0.25 (0.06) 0.33 (0.07) 0.15 (0.08) 0.24 (0.08) 0.42 (0.08) 0.41 (0.08)
0.989 0.984 0.978 0.984 0.985 0.984
a Numbers in parentheses indicate the standard deviation for each coefficient.
cohesiveness and the dispersive interactions of micellar microenvironments play significant roles in solute retention.13 However, all of the surfactants possess similar m coefficients. This is evidence that, although the molecular size of the solute plays a significant role in its retention, it does not explain the observed differences in selectivity. Dipolarity and Polarizability. Three of the solute-micelle interactions that have the smallest influence on solute retention are the dipole-dipole, dipole-induced dipole (s coefficient), and induced dipole-induced dipole interactions (r coefficient). Both of these terms are small relative to the m and b coefficients, but they do indicate some rather significant differences between micellar phases. A less negative s coefficient shows that the pseudostationary phase is more dipolar. Therefore, LSER suggests that the dipolarity of SDS is only marginally higher than those of SDSu, SDC, and SLSA but significantly greater than those of SDP and SDCV. According to Table 2, the ability to participate in dipole-dipole interactions is the only significant difference between the larger headgroups of SLSA and SDCV. This is not surprising, as the presence of the large isopropyl moiety of SDCV causes it to be noticeably less dipolar than the other five surfactants. The ability of the micelle to interact with or become polarized by solute n- and π-electrons is related to the r coefficient.21,40 As r becomes more positive, the more easily the micellar phase can adapt to (or become polarized by) neighboring solute n- and π-electrons. Typically, a molecule whose electrons are less tightly
bound (e.g., the electrons are at some distance from the nuclei) is considered to have a larger polarizability.45 According to Table 2, SDC shows the weakest interaction with solute electron pairs. This is probably a consequence of the higher charge density of the compact CO2- headgroup. At the other extreme, the carbamate moiety of SDCV and the ester group of SLSA are both easily polarized, improving their interaction with the n- and π-bonding electrons of solutes. This type of interaction can be especially important for solutes with bulky halogenated or nitro substituents. Hydrogen Bonding. As discussed previously, hydrogen bond interactions between solutes and micelles have been shown to play a significant role in selectivity.3,4,14-21 The large b coefficients indicate that the hydrogen-bond-donating ability of the micellar phase has a large influence on solute-micelle interaction. Although the small a coefficients suggest that the hydrogen bond acceptor strength of the micelles is one of the least important contributors to retention, the differences in a values between some of the surfactants (e.g., SDS and SDC) are quite large. Not surprisingly, there is a direct relationship between the dissociation constant (pKa) and the hydrogen-bond-accepting ability (a coefficient) of the surfactant headgroup. As the pKa of the headgroup moiety increases, the a coefficient of the surfactant becomes more positive. However, a similar correlation does not hold for the hydrogen-bond-donating acidity (b coefficient) of the micelles. It is important to remember that LSER shows that multiple interactions are responsible for the solute retention, but the observed selectivity behavior can be almost entirely rationalized through hydrogen bond interactions. To investigate the hydrogen bond influences more closely, a plot of the logarithm of the solute capacity factors in different surfactants can be made (Figures 2-6). The NHB solutes should form a straight line with a slope of unity. HBD solutes should deviate from this line toward the surfactant that is the stronger hydrogen bond acceptor, and HBA solutes should deviate from this line toward the micellar phase that is the stronger hydrogen bond donor. Figures 2-6 compare each of the surfactants against the more commonly used SDS. SDSu and SDS differ structurally only by a sulfate ester oxygen. Looking at Table 2, one can see that LSER does not predict a large difference as a result of removing the bridging oxygen. The more negative b and s coefficients indicate that SDSu is a slightly weaker hydrogen bond donor and a less dipolar pseudostationary phase than SDS. Similarly, the less negative a term suggests that SDSu is a better hydrogen-bond-accepting base than SDS. A plot Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
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Figure 2. Solute retention comparison between SDSu and SDS for NHB (solid square), HBA (solid triangle), and HBD (box with ×) solutes. The solid line represents the linear regression data for NHB solutes: log k ′(SDSu) ) 0.998 log k ′(SDS) - 0.08 (R2 ) 0.997).
Figure 5. Solute retention comparison between SDCV and SDS for NHB (solid square), HBA (solid triangle), and HBD (box with ×) solutes. The solid line represents the linear regression data for NHB solutes: log k ′(SDCV) ) 1.000 log k ′(SDS) + 0.05 (R2 ) 0.997).
Figure 3. Solute retention comparison between SDC and SDS for NHB (solid square), HBA (solid triangle), HBD (box with ×) solutes. The solid line represents the linear regression data for NHB solutes: log k ′(SDC) ) 1.010 log k ′(SDS) - 0.15 (R2 ) 0.990).
Figure 6. Solute retention comparison between SDC and SDCV for NHB (solid square), HBA (solid triangle), and HBD (box with ×) solutes. The solid line represents the linear regression data for NHB solutes: log k ′(SDC) ) 0.982 log k ′(SDCV) - 0.27 (R2 ) 0.989).
Figure 4. Solute retention comparison between SDP and SDS for NHB (solid square), HBA (solid triangle), and HBD (box with ×) solutes. The solid line represents the linear regression data for NHB solutes: log k ′(SDP) ) 0.986 log k ′(SDS) - 0.11 (R2 ) 0.977).
a slight difference between NHB and HBA solutes. The HBD solutes, however, show a clear deviation above the line, indicating a stronger interaction with SDC micelles. A similar plot (not shown) of solute retention of SDC versus SDSu reveals a single straight line (R2 ) 0.989) for both NHB and HBA solutes, with a slope of unity. There is still a large enough difference between the LSER a coefficients of SDC and SDSu to observe a significant deviation from the regression line for HBD solutes, indicating more favorable interaction with the SDC micelles. The phosphate headgroup of SDP shows some rather significant differences when compared to that of SDS. Not only is there a large difference between the micelle polarities, as mentioned previously, but the hydrogen-bonding interactions are also unique. SDP forms a poor hydrogen-bond-donating and moderate hydrogenbond-accepting pseudostationary phase. A plot of solute retention in SDP versus SDS is shown in Figure 4. It shows that HBA solutes are retained poorly in SDP solutions relative to the retention in SDS solutions, confirming the differences observed in the LSER results. However, the differences in retention for HBD solutes is not as large as one might expect when looking at the a coefficients from the LSER analysis. This is a good illustration that multiple types of solute-micelle interactions determine solute retention. Although SDP is a better hydrogen-bond-accepting phase than SDS, it forms a much less polar microenvironment. Therefore, the polar HBD solutes (relative to NHB solutes) will have a slight tendency to prefer the SDS phase. This will partially
of solute retention in SDSu versus solute retention in SDS (Figure 2) indicates that there is not a significant difference in selectivity between the two surfactants. There is, however, a modest tendency for the HBA solutes to interact more strongly with SDS. The carboxylate headgroup of SDC is a weaker Bro¨nstedLowry acid relative to the sulfate headgroup of SDS. Therefore, as the LSER a coefficients suggest, SDC is a stronger hydrogen bond acceptor. When compared to SDC, SDS is only a marginally better hydrogen-bond-donating phase but a much weaker hydrogen bond acceptor. Figure 3 confirms these results, showing only 1274 Analytical Chemistry, Vol. 71, No. 7, April 1, 1999
Figure 7. Headgroup effect on the elution order and selectiviy for SDS (A), SDSu (B), SDC (C), SDP (D), SDCV (E), and SLSA (F). The test mixture was comprised of 4-bromophenol (1), 4-chloroacetophenone (2), bromobenzene (3), and ethyl benzene (4). The elution window (tmc/teo) is similar for all electropherograms.
shadow the hydrogen-bonding differences. It must also be noted that, although all of the HBD solutes listed in Table 1 have large R values, they also all have rather significant acceptor properties (high β values). This is inherent in all HBD solutes and is difficult to avoid. Thus, there is also competition between the hydrogenbond-donating and -accepting properties of these solutes. Of the six surfactants studied, SDCV has the largest headgroup. Although it has the same charge-carrying carboxylate anion as SDC, the additional carbamate moiety proves to have a substantial effect on solute retention. As discussed previously, it causes the micellar environment to be much less polar and better at accommodating solute electron pairs relative to the other surfactants. The carbamate group also has significant effects on the hydrogen-bonding nature of the micelles. The LSER coefficients show SDCV to be the weakest hydrogen-bond-donating phase in this study and a moderate hydrogen bond acceptor. The retention of hydrogen-bonding analytes in SDCV and SDS is compared in Figure 5. The HBA solutes tend to be retained significantly longer in the SDS micellar phases. Again, the HBD solutes do not deviate from the NHB solutes as much as might be expected. The same circumstances that were discussed for SDP are still applicable in this situation. A more dramatic result is shown in Figure 6. A plot of solute retention in SDC versus SDCV yields some of the most striking deviations from NHB solute linearity. Both HBD and HBA solutes show a large preference for SDC micelles. Although the two surfactants have the same charge center on their headgroups (e.g., carboxylate group), they possess quite different selectivity characteristics. Sodium lauryl sulfoacetate possesses the sulfonate charge center on its headgroup similar to the case with SDSu but shows significant differences because of its ester functionality. The ester group reduces the hydrogen-bond-donating ability of SLSA. It also improves the hydrogen bond acceptor strength of the surfactant. This is probably because the ester functional group increases the pKa of the sulfonic acid headgroup through inductive effects. Although they are structurally different from the other surfactants in this study, SLSA and SDCV form micelles that have very similar interactive properties (Table 2).
The elution pattern of a test mixture in the different surfactant solutions is presented in Figure 7. Note the selectivity differences between the surfactant systems. As the hydrogen-bond-donating strength of the surfactant decreases, the retention of 4-chloroacetophenone (peak 2) also decreases. Comparing the electropherograms from SDSu (Figure 7B) and SDC (Figure 7C) shows the importance of the relatively small a coefficient. Although both surfactants have similar polarity and hydrogen-bond-donating properties, an elution order reversal is observed for 4-bromophenol (peak 1) and 4-chloroacetophenone because of the hydrogen-bondaccepting strength of SDC micelles. It should also be noted that, although SDC, SDP, SDCV, and SLSA all have the same elution order, differences in selectivity are still observed. Functional Group Selectivity. Examining the functional group selectivity for solutes can also be a useful method to investigate solute-micelle interactions in MEKC. Since the retention factor is directly proportional to the partition coefficient, the functional group selectivity, τ, can be defined as the ratio of retention factors between a substituted benzene and benzene.3 The difference in free energy of transfer of a functional group from the aqueous phase to the micellar phase, ∆∆G, can then be determined using eq 3.
∆∆G ) -RT ln τ
(3)
The differences in free energy of transfer for various solutes are listed in Table 3. A negative value for ∆∆G indicates that the addition of the functional group to the principle solute, benzene, increases its interaction with the micellar phase. As the solutemicelle interaction becomes more favorable with the addition of a substituent, the ∆∆G will become more negative. A more positive ∆∆G value indicates that the addition of the substituent causes the solute-micelle interaction to become less favorable. The first three solutes in Table 3 are NHB alkyl-substituted benzenes. As the alkyl chain length increases, the solute-micelle interaction increases (∆∆G becomes more negative) for all six surfactants, as expected. Homologous series have been used to describe solute interactions in both MEKC and chromatogAnalytical Chemistry, Vol. 71, No. 7, April 1, 1999
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Table 3. Headgroup Effect on Functional Group Selectivitya ∆∆G (kJ/mol) functional group (1) CH3 (2) CH2CH3 (3) CH2CH2CH3 (4) CN (5) NO2 (6) O2CCH3 (7) COCH3 (8) CO2CH3 (9) Cl (10) Br (11) I (12) CH2OH (13) OH
SDS
SDSu
SDC
SDP
SDCV
SLSA
-2.47 (0.03) -4.73 (0.04) -7.30 (0.03) -0.26 (0.03) -0.54 (0.02) -1.20 (0.03) -1.26 (0.04) -2.90 (0.02) -3.10 (0.03) -3.91 (0.02) -5.32 (0.02) 1.23 (0.06) 1.69 (0.05)
-2.43 (0.05) -4.65 (0.02) -7.28 (0.03) -0.01 (0.03) -0.66 (0.10) -0.58 (0.06) -0.62 (0.10) -2.76 (0.02) -3.43 (0.02) -4.31 (0.02) -5.29 (0.02) 1.57 (0.09) 1.35 (0.03)
-2.48 (0.09) -4.18 (0.06) -7.59 (0.06) -0.21 (0.05) -0.36 (0.05) -0.69 (0.08) -0.68 (0.07) -2.99 (0.10) -2.61 (0.07) -3.42 (0.28) -5.11 (0.20) 1.09 (0.23) -0.28 (0.26)
-2.62 (0.04) -5.08 (0.03) -8.10 (0.03) 0.84 (0.10) 0.27 (0.03) 0.06 (0.03) -0.45 (0.05) -2.31 (0.02) -2.71 (0.08) -3.53 (0.03) -5.02 (0.10) 2.00 (0.05) 1.41 (0.05)
-2.51 (0.01) -5.00 (0.01) -7.92 (0.01) 1.28 (0.02) 0.37 (0.01) 1.00 (0.01) 0.76 (0.02) -1.35 (0.01) -3.31 (0.01) -4.19 (0.01) -5.61 (0.02) 2.28 (0.04) 1.56 (0.09)
-2.52 (0.03) -4.95 (0.03) -7.78 (0.03) 0.74 (0.02) -0.36 (0.02) 0.26 (0.02) 0.18 (0.04) -1.87 (0.04) -3.54 (0.03) -4.42 (0.02) -5.84 (0.03) 2.07 (0.04) 1.32 (0.04)
a Numbers in parentheses are the 95% confidence intervals for the ∆∆G values.
raphy.22-28 The free energy difference between homologues (e.g., ethylbenzene/toluene and propylbenzene/ethylbenzene) is also directly related to the hydrophobic selectivity. The transfer of a -CH2- unit from the bulk aqueous phase to the micellar phase is a function of the dipolarity and the cohesive nature of the micelle. A reduction in both will result in a larger free energy difference between homologues. Inspection of the ∆∆G differences between the alkyl benzenes for each surfactant reveals that SDP and SDCV have the largest hydrophobic selectivity. Solutes 4-8 in Table 3 are all strong hydrogen bond acceptors. The overall trend in retention for a given solute is consistent with the LSER results. HBA solutes show stronger interaction with the hydrogen-bond-donating SDS micelles relative to the interactions of the other surfactant systems. The last two solutes in Table 3 represent the hydrogen bond donors. These solutes are a bit more difficult to evaluate because of their hydrogen bond duality. However, the best hydrogen-bond-accepting surfactant, SDC, consistently shows the strongest interaction with these solutes. Table 3 also confirms that SLSA and SDCV micelles differ only in polarity. Since SLSA and SDCV have identical hydrogenbonding properties and the hydrogen-bonding solutes are inher(22) Grushka, E.; Colin, H.; Guichon, G. J. J. Chromatogr. 1982, 248, 325. (23) Colin, H.; Guichon, G.; Yun, Z.; Diez-Masa, J. C.; Jandera, P. J. J. Chromatogr. Sci. 1983, 21, 179. (24) Khaledi, M. G.; Peuler, E.; Ngeh-Ngwainhi, J. Anal. Chem. 1987, 59, 2738. (25) Khaledi, M. G. Anal. Chem. 1988, 60, 876. (26) Yang, S.; Kruk, L.; Khaledi, M. G. J. Chromatogr. 1994, 664, 1. (27) Borgerding, M. F.; Quina, F. H.; Hinze, W. L.; Bowermaster, J.; McNair, H. M. Anal. Chem. 1988, 60, 2520. (28) Borgerding, M. F.; Williams, R. L.; Hinze, W. L.; Quina, F. H. J. Liq. Chromatogr. 1989, 12, 1367.
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Figure 8. Schematic of the surface region of a roughly spherical micelle.
ently polar, both HBA and HBD solutes show more favorable interactions with SLSA micelles. Finally, the halogen-substituted benzenes show the significance of the r coefficient. The poorly polarizable SDC consistently shows the weakest interactions with this set of solutes when compared to the other surfactants. This is especially true when compared to the more easily polarized SDCV and SLSA micelles, which show some of the strongest interactions with the n- and π-electrons of the halogenated benzenes. Source of Interactions. These results and those of previous reports have found clear differences in micelle-solute interaction and therefore selectivity among various types of surfactants.3,4,14-18 A common conclusion is that, although solute size has the most significant effect on its retention in MEKC, the separation selectivity can almost entirely be rationalized by the hydrogen bond interactions. Interestingly, the best hydrogen-bond-donating phases, LiPFOS and SDS, do not contain hydrogen-bond-donating moieties.3 Under the conditions used in MEKC, surfactants with long aliphatic or fluorinated tails form roughly spherical micelles in solution. Figure 8 shows a schematic of a section of this structure in which two distinct regions can be recognized. The first is the hydrophobic core, which consists of the hydrocarbon tails of the surfactant. This region is generally considered to be void of water. The micelle-bulk aqueous phase interface consists of the first two or three carbons of the surfactant tail, the headgroups (the palisade layer), and the condensed counterions (the Stern layer).29,30 A significant amount of water is present in this interfacial region, and this region can be highly structured due to electrostatic effects from the headgroup and counterions as well as inductive effects from the adjacent hydrocarbon moiety.31-36 Spectroscopic studies have shown that solute molecules generally reside in the interfacial region of the micelle and rarely (29) Aamodt, M.; Landgren, M.; Jonsson, B. J. Phys. Chem. 1992, 96, 945. (30) Landgren, M.; Aamodt, M.; Jonsson, B. J Phys. Chem. 1992, 96, 950. (31) Seoud, O. A. J. Mol. Liq. 1997, 72, 85. (32) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J. Phys. Chem. 1982, 86, 3198. (33) Mukerjee, P.; Ko, J.-S. J. Phys. Chem. 1992, 96, 6090. (34) Handa, T.; Matsuzaki, K.; Nakagaki, M. J. Colloid Interface Sci. 1987, 116, 50. (35) Ulmius, J.; Lindman, B. J. Phys. Chem. 1981, 85, 4131. (36) Stigter, D. J. Colloid Interface Sci. 1974, 47, 473.
partition solely into the hydrophobic core of the micelle.33,37-40 Therefore, in addition to interacting with the surfactant molecules in aggregates, the solute is also likely to interact with the water molecules that hydrate the micelles. It has been suggested that the observed hydrogen-bonding and polarity characteristics of surfactants is determined by the water that resides within the micelle palisade layer and/or in the Stern layer at the micellebulk aqueous interface.32-34,39,41 The hydrophobic moiety, headgroup, and counterion can all affect the structure and amount of water in this region. By increasing the length of the surfactant tail or decreasing the electrostatic repulsion of the headgroup (e.g., decreasing the charge density of the headgroup or increasing the counterion coverage), the micelle aggregation number and chain packing increases.42,43 As a result the localized water in the palisade layer will be influenced.33,40,43 This seems to be a reasonable explanation for the observed hydrogen-bond-donating and polarity differences between the surfactants in this study. Varying the headgroup changes the quantity and/or structure of water that the solute comes in contact with while interacting with the micelle. Therefore, changing the surfactant headgroup may influence solute retention both directly through solute-headgroup interaction and indirectly by affecting the amount and structure of the water in the palisade layer. To understand these phenomena more clearly, we have extended the (37) Janzen, E. G.; Coulter, G. A. J. Am. Chem. Soc. 1984, 106, 1962. (38) Wasylichen, R. E.; Kwak, J. C. T.; Gao, Z.; Verpoorte, E.; MacDonald, J. B.; Dickson, R. M. Can. J. Chem. 1991, 69, 822. (39) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 92, 1620. (40) Zacharlasse, K. A.; Nguyen, V. P.; Kozanklewicz, B. J. Phys. Chem. 1981, 85, 2676. (41) Poole, C. F.; Poole, S. K. J. Chromatogr. A 1997, 792, 89. (42) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985. (43) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (44) Grate, J. W.; Abraham, M. H. Sens. Actuators B 1991, 3, 85. (45) Atkins, P. W. Physical Chemistry, 5th ed.; W. H. Freeman & Co.: New York, 1994. (46) Abraham, M. H.; Chadha, H. S.; Whiting, G. S.; Mitchell, R. C. J. Pharm. Sci. 1994, 83, 1085. (47) Bumgardner, J. G. Ph.D. Thesis, North Carolina State University, Raleigh, NC. (48) Leonard, M. S.; Khaledi, M. G. Manuscript in preparation. (49) Agbodjan, A.; Khaledi, M. G. Manuscript in preparation.
investigation to other headgroups for anionic and cationic surfactants as well as initiating studies on how the counterion and hydrophobic tail influence selectivity. In addition, the relationship between the interactive nature and aggregation properties of micellar pseudophases are being examined and will be discussed elsewhere. SUMMARY The results in the study show that significant selectivity differences can be achieved by simply changing the surfactant headgroup (Figure 7). From the free energy transfer studies of non-hydrogen-bonding solutes as well as the LSER model, the most “hydrocarbon-like” pseudostationary phases studied are dodecyl phosphate and dodecoxycarbonyl valine. Hydrophobicity does play an important role in solute-micelle interactions, but hydrogen bonding has the strongest influence on selectivity. The surfactants in decreasing order of HBD strength are SDS g SDSu g SDC > SDP > SDCV ) SLSA. This is particularly true for SDCV and SDP micelles, which have markedly weaker interactions with HBA solutes. Our results also suggest that the observed hydrogenbond-accepting ability (a coefficient) for surfactants is directly proportional to the proton acidity (pKa) of its headgroup. Because of this, SDC micelles are strong hydrogen bond acceptors as well as being moderate hydrogen bond donors. This gives SDC micelles rather unique retention and selectivity characteristics. Finally, our results also indicate that two structurally different surfactants that may be expected to have different selectivity characteristics (e.g., SLSA and SDCV) can have surprisingly similar interaction behavior. ACKNOWLEDGMENT We gratefully thank Dr. Sahori Takeda at the Osaka National Research Institute for donating the sodium lauryl sulfoacetate and Waters Corp. for donating the (S)-dodecoxycarbonyl valine. A research grant from the U.S. National Institutes of Health (GM 38738) is also acknowledged. Received for review August 31, 1998. Accepted January 1, 1999. AC9809736
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