Anal. Chem.
1902, 5 4 , 2233-2238
the particular fragment; the loss of hydrogen chloride has a much greater variation in activation energy than does the loss of the hydrocarbons. This may explain the wide variation in published activation energies for the same processes. The second evolution profile has a much higher activation energy similar to the value found for the thermolysis of isoprene (351, 56-63.05 kcal, which has a similar degree of unsaturation. These results clearly show the usefulness of this technique to study the temperature-resolved pyrolysis of synthetic polymers and currently well-defined biopolymers are being studied.
ACKNOWLEDGMENT We thank Joel Balogh for the construction of the control electronics for the probe and Tatyana Frenkel for helping to obtain the data. The helpful discussions of M. S. B. Munson and R. Lattimer are gratefully acknowledged.
LITERATURE CITED (1) Risby, T. H.; Yorgey, A. L. US. Patent No. 4075475, Feb 21, 1978. (2) Rlsby, T. H.; Yergey, A. L. Anal. Chem. 19781, 50, 326A. (3) Risby, T. H.; Yergey, A. L. J . Phys. Chem. 1876, 8 0 , 2839. (4) . , Yeraev, A. L.:Risbv, T. H.: Golomb. H. M. Blomed. Mass Spectrom. 197&.5, 47. (5) Campana, J. E.; Rlsby, T. H.; Jurs, P. C. Anal. Chim. Acta 1979, 712, 371.. (6) Yergey, J. A. Pt1.D. Thesis, The Pennsylvania State University, Univer-
--
sitv Park. PA. 1981. (7) Wise,€.'M., Ed.-'The Platinum Metals and Thulr Alloys"; The International Nickel Co.. Inc.: New York. 1941. (8) Wegner, J.; Patat, F. J . Po/ym. SC;.1970, 37, 121. (9) Madorsky, S.L.; Strauss, S. J . Res. Natl. Bur. Stand. (U.S.) 1948, 4 0 , 417. (IO) Bradt, P.; Dibeier, V.; Mohler, F. L. J . Res. Natl. Bur. Stand. (U.S.) 1953, 5 0 , 201. (11) Wiiey, R. H.; Smlthson, L. H., Jr. J . Macromol. Sci. Chem. 1968, 2 , 589
(12) %man, A. I n "Thermal Analysis"; Wiedeman, H. G., Ed.; Birkhauser Verlag: Basel, 1972. (13) Stenhagen, E.; Abrahamson, S.; Mclafferty, F. W. "Atlas of Mass Spectral Data"; Wiiey-Interscience: New York, 1969; Voi. 4, p 645.
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(14) Shimizu, Y.; Munson, M. S. B. J . Polym. Sci., Polym. Chem. Ed. 1979, 17, 1991. (15) Cascaval, C. N. Mater. Plast. (Bucharest) 1979, 76,120. (16) Blazso, M.; Varhegyi, G.; Jakab, E. J . Anal. Appl. Pyrolysis 1980, 2 , 177. (17) Oehme, G.;Baudisch, H.; Mix, H. Makromol. Chem. 1976, 177, 2657. (18) Kiran, E.; Glliham, J. K. J . Appl. Polym. Sci. 1977, 2 1 , 1159. (19) Cameron, G. C.; MacCaiium, J. R. J . Macromol. Sci., Rev. Macromol. Chem. 1967, 61. 327. (20) Rudin, A.; Samanta, M. C.; Relliy, P. M. J . Appl. Polym. Sci. 1979, 2 4 , 171. (21) Cameron, G. G.; Meyer, J. M.; McWaiter, I.T. Macromolecules 1978, 7 7 , 696. (22) Cascaval, C. N.;Straus, S.; Brown, D. W.; Florin, R. E. J . Polym. Sci. 1976, 5 7 , 81. (23) Rudin, A.; Samanta, M. C.; Van der Hoff, B. M. E. J . Polym. Scl., Polym. Chem. Ed. 1979, 1 7 , 493. (24) Buriiie, P.; Bert, M.; Michei, A.; Guyot, A. J . Polym. Sci., Polym. Lett. Ed. 1978, 76,181. (25) O'Mara, M. M. J . Polym. Sci., Polym. Chem. Ed. 1970, 8 , 1887. (26) Chang, E. P.; Saiovey, R. J . Polym. Sci., Polym. Cbem. Ed. 1976, 12, 2927. (27) Ahlstrom, D. H.; Liebman, S. A.; Abbas, K. B. J . Polym. Sci., Polym. Chem. Ed. 1976, 14, 2479. (28) Aiajberg, A.; Arpino, P.; DeurSiftar, D.; Ginochow, G. J . Anal. Appl. Pyrol. 1980, 7 , 203. (29) Ballistreri, A.; Foti, S.; Montaudo, G.; Scamporrtno, E. J . Polym. Sci., Polym. Chem. Ed. 1960, 78, 1147. (30) Dougherty, R. Org. Mass Spectrom. 1972, 16, 1171. (31) Thinius, K.; Schroeder, E.; Guste, A. Plaste Kautsch. 1984, 1 7 , 67. (32) Guyot, A.; Benevise, J. P.; Trambouze, Y. J . Appl. Polym. Sci. 1962, 6, 103. (33) Taiamini, G.; Pezzin, G. Makromol. Chem 1960, 39, 26. (34) Stromberg, R. R.; Straus, S.;Achhammer, B.G. J , Polym. Sci. 1959,
.
35,355. (35) Straus, S.;Madorsky, S.L. Ind. Eng. Chem. 1956, 4 8 , 1212.
RECEIVED for review March 15, 1982. Accepted August 12, 1982. This work was supported by a grant from the National Institute of Allergy and Infectious Diseases (AI 16384). The MODCOMP 11/25 computer system was purchased with funds from the U.S. Environmental Protection Agency (R-806558), the National Institute of Allergy and Infectious Diseases (AI 16384),and the Division of Research Resources at the National Institutes of Health (RR-05445).
Selectivity in Liquid Chromatography with Micellar Mobile Phases Paul Yarmchuk, Robert Weinberger, FI. F. Hirsch, and L. J. Cline Love* Seton Hall University, Department of Chemistry, South Orange, N e w Jersey 07079
Mlcellar mobile phases are shown to offer control over selectlvity In liquld chromatography. While retentlon of all solutes decreases with increasing surfactant concentration, the rate of decrease varies considerably, producing lnverslons In retentlon orders. The retsntlon order reversals are the result of two competlng csqulllbrla, namely, solute-mlcelle assoclation characterized by K,, and solute-stetlonary phase interactlon Characterized by Kws. Increased micelle concentration can drlve the solute into the movlng mlcellar phase whlle havlng little or no effect on the stationary phase equillbrla. A second Important micellar effect on retention Is electrostatlc interactions between lonlc surfactant and ionlzable solutes In a manner analogous to Ion-lnteractlon chromatography. The nature of thls effect, whether repulsion or attractlon, depends on the lonlc character (anionlc or catIonic) of the surfactant and the correspondlng solute. Examples are given to show hew selectlvity can be enhanced by proper choke of surfactant type and moblWe phase concentratlon.
Ionic surfactants have been employed extensively in liquid chromatographic (LC) mobile phases as ion interaction reagents, typically in conjunction with conventional mobile phase modifiers (1-3). The benefit obtained via these secondary equilibria or side reactions is enhanced selectivity for separations of interactive solutes. More recently, aqueous solutions of similar ionic surfactants, but at concentrations above the critical micelle concentration (CMC), have been shown to have properties analogous to those of conventional mobile phases for reversed-phase LC (4). The fundamental properties of these micellar solutions which enhance the organization of reactants on a molecular level have been used to advantage in catalysis ( 5 ) ,room-temperature phosphorescence (6), and drug absorption (7), as well as in chromatographic separations. Recently, it has been shown that micellar solutions can be utilized in a total analytical scheme involving micellar LC with micelle-stabilized roomtemperature phosphorescence detection (8). A theoretical basis for the chromatographic resolving power
0003-2700/82/0354-2233$01.25/00 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
of micellar mobile phases has been described (9);however, this model does not account for electrostatic interactions between the ionic surfactant and ionized solutes. We w ill show that such effects can provide for selectivity in micellar chromatography in a manner comparable to ion interaction chromatography. Note, however, that a micellar mobile phase differs from a conventional one in a singularly important characteristic. Micellar solutions can be considered as microscopically heterogeneous, being composed of the micellar aggregate and the “bulk” surrounding medium, whereas a conventional mobile phase is homogeneous. This feature can be used to explain the electrostatic and hydrophobic effects that combine to form the basis of micellar chromatography. This paper describes the effect of micelle concentration on LC retention and explains the observed chromatographic selectivity by an extension of a multiple equilibrium theory (9). The profound effects of the surfactant are demonstrated by comparing selectivity obtained by using anionic and cationic micellar mobile phases.
EXPERIMENTAL SECTION Apparatus. The HPLC system consisted of a Constametric I11 pump and UV I11 monitor at 254 nm (Laboratory Data Control Co., Riviera Beach, FL) and a Model 7120 sample injector with a 20-pL loop (Rheodyne, Inc., Berkeley, CA). The column (15 cm long and 4.6 mm i.d.) was packed with 5 pm Supelcosil LC-1 (Supelco, Inc., Bellefonte, PA). Separate columns were used for anionic and cationic surfactants. The void volume of the system was calculated by using the displacement peak of an adsorbed mobile phase impurity caused by the actuation of the sampling valve. With sodium lauryl sulfate the void volume was 0.94 mL, and with dodecyltrimethylammonium bromide, it was 0.96 mL. A Fisher Recordall, Model 5000, strip chart recorder (Fisher Scientific Co., Springfield, NJ) was used to record the chromatogram. Column temperature was maintained at 298 K by a Model 73 circulating heater (Fisher Scientific Co.). Reagents. The sodium lauryl sulfate (SLS) was obtained from BDH Chemicals, Ltd., and was recrystallized twice from water/methanol. The dodecyltrimethylammonium bromide (DTAB) was obtained from Fisher Scientific Co. and was recrystallized twice from acetone/chloroform. The water was deionized and then distilled. Benzene, toluene, nitrobenzene, and phenol were obtained from Fisher Scientific Co. p-Nitrophenol and p-nitroaniline were obtained from MCB. These solutes were used as received. Benzylamine was obtained from Eastman and was redistilled. Procedure. Mobile phases were prepared by dissolving the appropriate quantity of surfactant in water and filtering through a 0.5-pm cellulosic membrane filter (Rainin Instrument Co., Inc., Woburn, MA). The pH for SLS was 6.0-6.4 and for DTAB 5.4-5.8, depending on surfactant concentration. Stock solutions of the test solutes were prepared in methanol and then diluted to the appropriate working concentration with 0.10 M SLS or DTAB. The working concentrations are as follows: phenol (52 pg/mL), p-nitrophenol (60 pg/mL), p-nitroaniline (17 pg/mL), benzene (380 gg/mL), nitrobenzene (9.0 gg/mL), 2-naphthol (31 pg/mL), toluene (330 pg/mL), and benzylamine (450 pg/mL). Individual solutes were chromatographed with a flow rate of 2.0 mL/min, and their retention times and peak widths were measured manually.
RESULTS AND DISCUSSION For this study two surfactants were chosen which would form comparable micelles difffering essentially only in the nature of the polar end group. The anionic surfactant, sodium lauryl sulfate, and the cationic surfactant, dodecyltrimethylammonium bromide, both contain a linear (2-12 chain as their primary hydrophobic portion and have similar critical micelle concentrations of 0.008 M (10) and 0.016 M ( I I ) , respectively. The test solutes chosen were selected in order to have a mixture of polar and nonpolar as well as acidic and basic compounds. 2-Naphthol was included as a fused ring compound with a reasonably short retention time.
Table I. Variation of Capacity Factors with SLS Concentration capacity factors ( k ’ = (V, - V,)/V,) at the following SLS concns in the mobile phase 0.02 0.05 0.10 0.15 0.20 compd M M M M M phenol 7.40 p-nitrophenol 13.2 p-nitroaniline 13.5 benzene 18.6 nitrobenzene 29.7 toluene 44.5 2-naphthol 57.8 benzylamine 149
5.46 7.64 7.74 12.5 18.2 20.4 19.0 42.3
4.49 4.93 5.55 8.31
3.47 3.91 3.91 6.15 11.0 8.37 11.5 8.04 9.60 6.60 21.2 13.0
2.94 3.17 3.21
4.96 6.94 6.17 5.11 10.4
-
Table 11. Variation of Capacity Factors with DTAB Concentration
capacity factors ( h ’ = (V, - V,)/V,) at the following DTAB concns in mobile phase 0.03M 0.05 M 0.10M 0.15 M
compound benzene phenol nitrobenzene p-nitroaniline toluene p-nitrophenol 2-naphthol
1.8
-
1.4
-
22.1 25.7 27.4 36.5 43.8 79.0 87.7
16.2 17.6 19.7
9.75 9.73 11.6 11.0 13.4 17.5 16.6
21.9
26.7 41.3 39.6
7.19 7.02 8.42 7.60 9.27 13.0 11.0
x m
0I
-
1.0
0.6
-
I
I
I
-1.8
-1.4
Log
I
-1.0
I
-0.6
Bodium Lauryl S u i f a t a ( M )
Flgure 1. Effect of SLS concentration on retention: ( 0 )phenol, (0) p-nitroaniline, (0) benzene, (+) nitrobenzene, (A)toluene, (m) 2naphthol.
Effect of Surfactant Concentration. If a micellar solution mimics the behavior of a conventional reversed-phase mobile phase, then increasing the surfactant concentration and, thus, the number of micelles should result in a decrease in retention. This is in contrast to reversed-phase ion-interaction chromatography where the surfactant concentration is below the CMC, i.e., no micelles exist, and the addition of an ionic surfactant will increase retention for compounds which interact electrostatically with it. Indeed, the decrease in retention with increasing micelle concentration is found and shown in Tables I and 11. However, when log capacity factor is plotted against log surfactant concentration, as in Figures 1 and 2, an important feature of micellar chromatography becomes apparent. The linear plots are not parallel, but intersect one another. Thus, not only the capacity factor, k’,but also the separation factor, a , is changing. This sur-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER
Table 111. Variation of V,/(V,
1982
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- V,) with SLS Concentration
V,](V,
compound
0.02 M (0.0035 g/mL)
at the following SLS concns in the mobile phase (SLS concns in micelles a ) 0.20 M 0.05 M 0.10 M 0.15 M (0.0553 (0.0265 (0.0386 (0.01 21 g/mL) g/mL) g/mL) g/mL)
phenol p-nitro phenol p-nitroaniline benzene nitro benzene toluene 2-naphthol
0.223 0.125 0.122 0.0888 0.0535 0.0370 0.0285
0.292 0.216 0.213 0.132 0.0876 0.0810 0.0918
-
V,)
0.355 0.323 0.287 0.192 0.135 0.1 39 0.166
0.562 0.520 0.513 0.333 0.230 0.267 0.323
0.475 0.421 0.421 0.268 0.190 0.205 0.250
slope
intercept 0.202 0.115 0.106 0.0726 0.0447 0.0242 0.0168
6.59 7.58 7.55 4.78 3.48 4.47 5.70
a The concentration of surfactant residing in micelles (excluding free monomers) is C , = C - CMC, where C , is surfactant concentration in micelles, C is the total surfactant concentration, and CMC is the critical micelle concentration. All values are converted to g/mL since the partial specific volume u used to calculate the equilibrium constants is in mL/g and the units must cancel. The CMC of SLS is 0.008 M (10).
2.
B
1.
31
1. Y
m A 0
1.
!+
1.
1.1
I; L o g m o d e c y It r 1 met h y la in in 0 n Iu m B r o m I d
a
(M)
Flgure 2. Effect of IDTAB concentration on retention: (0)benzene, ( 0 )phenol, (e) nitrobenzene, (0)p-nitroaniline, (A)toluene, (V)p -
nitrophenol, (m) 2-naphthol.
factant concentration effect gives us the first of two mechanisms for adjusting retention times with (a micellar system. In Figure 1 the plots for 2-naphthol, toluene and nitrobenzene cross. This point is amplified in Figure 3 where with 0.20 M SLS the elution order is 2-naphthol, toluene, nitrobenzene, while with 0.02 Pvl SLS, the elution order is reversed. The same phenomenon is seen in Figure 2 with DTAB, where p-nitroaniline and nitrobenzene have crossing plots. In Figure 4, using 0.15 M TITAB, p-nitroaniline elutes before nitrobenzene, whereas with 0.05 M DTAB, nitrobenzene elutes first. Separation can therefore lbe controlled by tlhe simple measure of changing the surfactant concentration. A micellar mobile phase is not homogeneous as a conventional mobile phane but in comprised of at least two distinct media, the micelle itself (and the surrounding bulk aqueous phase. With a micellar mlobile phase there are two partition the water to stationary phase and coefficients, K,, and K, water to micelle equilibrium constants, respectively, both of which can affect retention. It is the second partition coefthat imparts uniqueness to micellar chromaficient, K,
TIME (mln )
Flgure 3. Micellar chromatograms of (1) nitrobenzene, (2) toluene, and (3)2-naphthol; mobile phases, (A) 0.02 M SLS, (B) 0.20 M SLS; detector sensitivity,(A) 0.004 absorbance unit full scale (AUFS), (B) 0.020 absorbance unit full scale (AUFS).
tography since K, and K,, have opposing effects on retention. As K,, increases, retention increases, but as K,, increases, retention decreases due to increased partitioning into the micelle. The equilibrium theory proposed in ref 9 was used to quantify the change in separation with surfactant concentration. From the retention data, the ratio V,/(V, - V,) was calculated for each solute (Tables I11 and IV) where V, is the volume of stationary phase, V , is the elution volume of the solute, and V , is the volume of the mobile phase. V , was taken to be the void volume, and V , was taken to be the total column volume minus V,. This ratio was plotted vs. the concentration of surfactant in micelles (total surfactant concentration minus CMC) in order to obtain the intercept where the micelle concentration is zero. For each solute, K,, and Kwmwere calculated from the slope and intercept obtained from the plot by linear regression (Table V). The relative standard deviations of the slopes were all less than 7.5%. The principal source of error, however, is the relative standard deviation of the intercept which can be large when the intercept is very small. This occurs with later eluting compounds
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
Table IV. Variation of V s / ( V e- V,)
with DTAB Concentration
V,/( Ve - V,) at the following DTAB concns in the mobile Dhase (DTAB concns in micelles a )
compound benzene phenol nitrobenzene p-nitroaniline toluene p-nitrophenol 2-naphthol a
0.05 M (0.0105 g/mL) 0.0981 0.0906 0.0810 0.0729 0.0597 0.0386 0.0402
0.03 M (0.0043 g/mL) 0.0720 0.0621 0.0581 0.0437 0.0364 0.0202 0.0182
0.10 M (0.0259 g/mL) 0.163 0.164 0.138 0.145 0.119 0.0910 0.0960
0.15 M (0.0413 dmL)
slope
intercept
0.222 0.227 0.189 0.210 0.172 0.123 0.145
3.97 4.38 3.46 4.38 3.58 2.73 3.34
0.0587 0.0474 0.0468 0.0298 0.0248 0.0137 0.0076
Same as given in Table 111; the CMC of DTAB is 0.016 M (11). Table V. Calculated Equilibrium Constants of SLS and DTAB Micellar Systems SLS
phenol p-nitrophenol p-nitroaniline benzene nitrobenzene toluene 2-naphthol
2
B
1
1
1
I
I
0
I
5
Lmin.’I
Figure 4. Micellar chromatograms of (1) nitrobenzene and (2) p nitroaniline: mobile phases, (A) 0.05 M DTAB, (B) 0.15 M DTAB; detector sensitivity, (A) 0.01 AUFS, (B) 0.02 AUFS.
such as 2-naphthol which have large partition coefficients. Since K,, is the reciprocal of the intercept and K,, is used to calculate K,, both partition coefficients can have substantial error for those late eluters. In Table V the solutes are listed in their elution order by use of 0.02 M SLS. At this low micelle concentration the system resembles reversed-phase chromatography and K,, controls retention as seen by the increasing retention with increasing K,, values. However, as the concentration of surfactant is increased, K,, has an increasing effect due to the larger number of micelles present in the mobile phase. The larger the value of K-, the greater the effect of increasing concentration as seen by steeper slopes in the log-log plots of k’vs. surfactant concentration (Figures 1 and 2). K,, might also be expected to change since surfactant molecules can adsorb onto nonpolar stationary phases, especially when the surfactant contains a large hydrophobic chain. However, the effect of K,, on retention is independent of micelle concentration since, as reported by Hung and Taylor (12),the amount of surfactant adsorbed on the stationary phase remains essentially constant after equilibration once the concentration is above the CMC. 2-Naphthol has a larger K,, value than
Kws 5.0 8.1 9.4 13.8 22.4 41.3 59.5
Kwma 39 77 84 77 90 215 393
DTAB Kws
Kwma
23 99 39 18 23 47 235
111
305 192 80 89 188 875
a In order to calculate the Kw, values, the partial specific volume ( u ) must be known. For SLS u = 0.862 mL/g as determined by Mukerjee (13). For DTAB u = 0.929 mL/g as determined by Guveli et al. ( 2 1 ). The Kw, values are per surfactant molecule.
the other solutes and, consequently, the steepest slope in Figure 1. As the SLS concentration increases, the retention of 2-naphthol decreases faster than the retention of nitrobenzene or toluene so that elution order reversal occurs. Since the K- value for toluene is also higher than that of nitrobenzene, complete reversal of elution order takes place for 2-naphthol, toluene, and nitrobenzene going from 0.02 M SLS to 0.20 M SLS. While K,, for 2-naphthol is much higher than the Km’s of the early eluting solutes such as phenol, the difference in K,, values is so large that very high SLS concentrations would be needed to show a change in elution order. These calculated K,, values are consistent with the lipophilicity of the compounds. With the cationic micelles the same relationship between K,, and K,, is seen. The Km value of p-nitroaniline is about twice that of nitrobenzene. At low DTAB concentrations nitrobenzene elutes before p-nitroaniline since it has the lower K, value. At high DTAB concentrations, p-nitroaniline elutes first since the effect of Km has offset the retention obtained from K,, equilibria. Effect of Electrostatic Interactions. A second mode of selectivity can be employed via the selection of the surfactant, e.g., cationic or anionic, since polar solutes can interact electrostatically as well as hydrophobically with the surfactant. As previously stated, there is considerable adsorption of surfactant onto a nonpolar stationary phase, thereby providing for electrostatic interactions of ionizable solutes with the stationary phase as well as with the charged micelle. A proposed equilibrium model does not account for the type of surfactant and its interactions with the stationary phase although the equilibrium constants obtained are a useful measure of the overall processes. When an acidic solute is chromatographed with an anionic surfactant, electrostatic repulsion from both the micelle and the surfactant-modified stationary phase can occur. Repulsion
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
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1
A
B
1 1
8l 2
w
0 2
3a
wma
I
p+- * TIME fmin.!
b
A
io
i:
$0
TIME fmin.)
I Flgure 5. Anionic and cationic micellar chromatograms of (1) phenol and (2) p-nitrophenol: mobnle phases, (A) 0.05 M SLS, (B) 0.05 M DTAB; detector sensitivity, (A) 0.02 AUFS, (B) 0.01 AUFS.
from the stationary phase should cause a decrease in retention. Repulsion from the micelle should not affect retention since a solute would still reside in the bulk mobile phase and, thus, still move down the column. However, EL hydrophobic interaction with the stationary phase is still possible although its effect will be reduced by the electrostatic repulsion. For example, two related aciclic solutes, phenol and 2-naphthol, are well separated with anionic SLS due to retention from the hydrophobic interaction. As expected, the electrostatic repulsion model holds equally true for cationic solutes chromatographed with cationic surfactants. Benzylamine, a strong base, is unretained with a cationic DTAB mobile phase. When a solute is chromatographed with an oppositely charged surfactant, an electrostatic attraction between the solute and the surfactant occurs. Considering the micelle first, the electrostatic attraction would complement the hydrophobic interaction and decreased retention would be expected. However, an increase in retention is found as shown in Figure 5. The retention time of phenol increases jfrom 3.1 min with SLS to 8.9 min with DTAB. p-Nitrophenol, a stronger acid, shows a larger increase from 4.1 min with SLS to 18.0 min with DTAB. The increases are, in all probability, due to electrostatic interactions with the stationary phase. Analogous to ion-interaction chromatography below the CMC, two types of interactions are possible. Since the stationary phase is modified by adsorbed surfactant, an ion-exchange interaction can occur where a solute in the bulk mobile phase would be attracted to the oppositely charged stationary phase. However, since the solute molecule upon exit from a micelle is in close proximity to a large number of surfactant molecules, an ion pair may be formed which can interact hydrophobically with the stationary phase. Both models would increase retention, and both probably have some influence. It is not known which model predominates, but the combined effect of the electrostatic attractions and the hydrophobic interaction of the stationary phase apparently are large enough to offset the increase in micellar attraction. This is shown in Table V where K,, for phenol and p-nitrophenol increases going from SLS
Flgure 6. Anionic and cationic micellar chromatograms of (1) p nitroaniline and (2) p-nitrophenol: mobile phases, (A) 0.05 M SLS, (B) 0.05 M DTAB; detector sensitivity, (A) 0.02 AUFS, (B) 0.004 AUFS.
I:
1
!
+
Figure 7. Anionic and cationic micellar chromatograms of (1) phenol and (2) benzene: mobile phases, (A) 0.05 M SLS, (B) 0.05 M DTAB; detection sensitivity, (A) 0.02 AUFS, (B) 0.02 AUFS.
to DTAB, but K,, increases to an even greater extent. This mode of selectivity can be used with p-nitrophenol and pnitroaniline which are not separated at any concentration of SLS. As seen in Figure 6 they are completely resolved on the cationic system. Accordingly, benzylamine, which was unretained with DTAB, interacts strongly with SLS and is retained (k’ = 21 with 0.10 M SLS). Nonpolar solutes such as benzene and toluene should not be affected by electrostatic effects, but only by hydrophobic
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
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1
/1
A
Y l
B
2
nitrobenzene and toluene so that base line resolution of the three is obtained. As evidenced by several broad peaks in the foregoing examples, the efficiency of micellar chromatography is somewhat less than that of conventional reversed-phase chromatography. This is due primarily to a large mass transfer term involving the micelles. This effect and means for minimizing it will be discussed in a future paper. It appears that micellar chromatography can offer several mechanisms for manipulating retention times and, as a consequence, controlling selectivity. We have demonstrated the versatility of micellar mobile phases for performing and controlling the separation of cationic, anionic, and neutral solutes without the inclusion of additional mobile phase modifiers. A better understanding of these mechanisms should be forthcoming as increased attention is given to this fledgling chromatographic technique. With the wide variety of surfactants available including ionic, nonionic and zwitterionic, it is likely many unique separations and applications will be developed.
ACKNOWLEDGMENT
+
+
I
TIME
4
(min.)
4
The authors thank Supelco, Inc., for aid in obtaining some of the columns.
LITERATURE CITED Figure 8. Anionic and cationic micellar chromatograms of (1) nitro-
benzene, (2) 2-naphtho1, and (3) toluene: mobile phases, (A) 0.05 M SLS, (B) 0.05 M DTAB; detector sensitlvlty, (A) 0.02 AUFS, (B) 0.01 AUFS.
effects. Since both surfactants have C-12 hydrocarbon chains, benzene and toluene should have approximately the same retention with SLS as with DTAB. In fact, their retention is slightly longer with DTAB, partly due to a lower concentration of micelles in the DTAl3 mobile phase since the CMC of DTAB is 0.008 M higher than that of SLS. Their values for K , and K , with both surfactants are virtually the same, verifying that they are not affected by the nature of the head groups. Since nonpolar compounds are only minimally affected by electrostatic effects, they can often be separated from polar compounds by the correct choice of the surfactant. With DTAB, phenol and benzene are poorly separated (Figure 7 ) , whereas, with the anionic system they are well resolved. Likewise, 2-naphthol elutes between nitrobenzene and toluene at 0.05 M SLS so that poor resolution of the three is obtained (Figure 8). In 0.05 M DTAB, 2-naphthol is eluted well after
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RECEIVED for review May 25, 1982.
Accepted July 19, 1982. This work was supported in part by the National Institutes of Health, Grant No. GM-27350. This work was presented in part at the 33rd Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 8, 1982, Abstract No. 82.
