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Anal. Chem. 1981,
(11) Sawynok, J.; Dawborn, J. K. Clln. Exp. Pharmacol. Physioi. 1875, 2 , 1-15. (12) Lazdins, I.; Dawborn, J. K. Clin. Exp. Pharmacol. Physiol. 1978, 5, 75-80. (13) Bonas, J. E.; Cohen, B. D.; Natelson, S. Mlcrochem. J . 1863, 7, 63-77. (14) Natelson, S.; Stein, I.; Bonas, J. E. Microchem. J. 1984, 8, 371-382. (15) Kamoun, P. P.; Pleau, J. M.; Man, N. K, Ciin. Chem. (Winston-Salem, N . C . ) 1972, 18, 355-357. (16) Mori, A.; Hosotanl, M; Choong, T. L. Blochem. Med. 1974, 10, 8-14. (17) Durzan, D. J. Can. J . Blochem. 1988, 47, 657-864. (18) Menlchini, G. C.; Giovannettl, S.; Lupetti, S. Experientia 1973, 29, 508-507. (19) Gordon, H. T.; Thornburg, W. W.; Werum, L. N. J. Chromatogr. 1972, 9 , 44. (20) Lowery, J. A.; Cassidy, J. E. J. Chromafogr. 1864, 13, 487-474. (21) Jones, A. S.; Thompson, T. W. J. Chromatogr. 1983, 70, 248. (22) Mottale, M.; Stewart, C. J. J. Chromatogr. 1975, 100, 263-270. (23) Clncotta, J. J.; Feinland, R. Anal. Chem. 1982, 34, 774-776. (24) Stalling, D. L.; Gehrke, C. W. Biochem. Biophys. Res. Commun. 1966, 22, 329-335. (25) Malcolm, S. L.; Merten, T. R. Anal. Chem. 1878, 48, 807-809. (26) Erdtmansky, P.; Goehl, T. J. Anal. Chem. 1975, 47, 750-752. (27) Hengstmann, J. H.; Falkner, F. C.; Watson, J. T.; Oates, J. Anal. Chem. 1874, 46. 34-38. (28) Patel, H.; Cohen, B. D. Clin. Chem. (Wnston-Salem, N . C . ) 1975, 21, 830-043. (29) Eksborg, S.; Persson, B. A.; Allgen, L. G.; Bergstrom, J.; Zimmerman, L.; Furst, P. Ciin. Ch/m Acta 1978, 82, 141-150. (30) Yamada, S.; Itano, H. A. Blochlm. Biophys. Acta 1868, 130, 538-540.
53, 1662-1666 (31) Yamamoto, Y.; ManJi,T.; Saito, A.; Maeda, K.; Ohta, K. J. Chromator. 1978, 162, 327-340. (32) Yamamoto, Y.; Sako, A.; ManJl, T.; Maeda, K.; Ohta, K. J , Chromatogr. 1879, 162, 23-29. (33) Tomllnson, E.; Jeffries, T. M.; Riley, C. M. J. Chromatogr. 1978, 750, 315-358. (34) Karger, B. L.; LePage, J. N.; Tanaka, N. I n “High Performance Llquld Chromatography”; Horvath, C., Ed.; Academlc Press: New York, 1980; VOI. 1. pp 113-206. (35) Knudson, E. J.; Lau, Y. C.; Veenlng, H.; Dayton, D. A. Clln. Chem. ( Winston-Salem, N . C . ) 1878, 24, 686-691. (36) Senftleber, F. C.; Halline, A. 0.; Veenlng, H.; Dayton, D. A. Clln. Chem. ( Winston-Salem, N . C ) 1976, 22, 1522-1527. (37) Ratner, S.; Petrack, B.; Rochovansky, 0. J . Blol. Chem. 1953, 204, 95-113. (38) Falley, C. F.; Brand, E. J . Biol. Chem. 1833, 102, 787-771. (39) Snyder, L. R.; Kirkland, J. J. “Introduction to Modern Llquld Chromatography”, 2nd ed.; Wiley-Intersclence: New York, 1979; Chapter 11.
RECEIVED for review March 17,1981. Accepted June 23,1981. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Institute of Arthritis, Metabolism and Digestive Diseases (Grant AM 25785) for financial support. This paper was presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1981.
Partitioning Behavior of Solutes Eluted with Micellar Mobile Phases in Liquid Chromatography Danlel W. Armstrong” and Faruk Nome Department of Chemistry, Georgefo wn Universjfy, Washington, 0. C, 20057
Equatlons are derived whlch account for the liquid chromatographlc behavlor of solutes eluted wlth a mlcellar mobile phase In HPLC. With this treatment one can calculate the partltlon coefficients of compounds between water and micelles, between the stationary phase and water, and between the statlonary phase and mlcelles. The treatment Is experimentally verified wlth flve compounds: hydroquinone, resorclnol, p-nltrophenol, p-nitroaniline, and naphthalene. Mlcelle concentratlons In the mobile phase and surfactant bindlng to the statlonary phase affect solute elutlon volumes and capaclty factors. The benefits as well as the llmltations of thls technique are discussed.
It has become apparent that the ability to engineer “specificity” or “selectivity” into the mobile phase of a liquid chromatographic technique (e.g., HPLC and TLC) results in a degree of flexibility (and therefore advantage) not shared by gas chromatography. Liquid chromatographic separations involving micelle or cyclodextrin mobile phases are often highly specific (1-6). Indeed, the complex combination of hydrophobic, electrostatic, and steric interactions of a solute with a micelle or cyclodextrin molecule cannot be duplicated by any traditional pure or mixed solvent system (7-13). The term pseudophase liquid chromatography (PLC) was coined to describe separations where significant partitioning of a solute occurs to discrete aggregates (or other species) dissolved in the mobile phase rather than to the bulk solvent of the mobile phase (4). Numerous other advantages of PLC have been cited such as: the low cost of micellar HPLC solvents, 0003-2700/81/0353-1662$0 1.25/0
the ability to simultaneously chromatograph hydrophilic and hydrophobic solutes, and greater safety, as aqueous pseudophases are nontoxic and nonflammable (1-4)In this work, a basic partition treatment is developed which can account for the chromatographic behavior (in HPLC) of many solutes eluted with aqueous micellar mobile phases. The compounds used to experimentally test the efficacy and limitations of this treatment were hydroquinone, resorcinol, p-nitrophenol, p-nitroaniline, and naphthalene. The classic work of Martin and Synge (14) and Herries et al. (15) are taken as the point of departure for the derivation of this treatment. A chromatographic treatment involving micellar mobile phases must explain the elution behavior of a solute in terms of certain column parameters, certain micelle characteristics, and three partition coefficients (Figure 1). The partition coefficients involved include that of a solute between the stationary phase and water (Ksw), between the stationary phase and the micelle (KsM),and between the micelle and water ( K M W ) , Such a treatment would allow greater understanding and utilization of this chromatographic technique. In addition, a successful treatment would have important implications outside the field of chromatography. For example, the ability to quickly, accurately, and unambiguously evaluate the partition coefficients of a wide variety of solutes between micelles and water (KMw)is of great importance in the fields of micellar catalysis (7-9), tertiary oil recovery (16), enzyme and membrane modeling (7-9), and so forth. This technique may be used to gather information about the physical and chemical properties of the micelle itself. As an interesting analogue, it may also be possible to treat micellar HPLC according to previously derived (for gas chromatog0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
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sorbed in the stationary phase, M , = fractional mass of solute initially present in the first theoretical plate absorbed in the micelles, Map= fractional mass of solute initially present in the first theoretical plate absorbed in the nonmicellar mobile phase (i.e. the bulk water) By definition Maq+ M , + M, = 1 (1) If
i l l STATION4RY PHASE
Figure 1. A schematic representation of the three “phase” model for micellar chromatography. The elution behavior of a solute would be dependent on the combined affects of three partition coefficients (KW
ME hAm(1 -PI Ksw = - X hA, Mag and
Then
= partition coefficient between micelle and water, Ksw = partition Coefficient between statioinary phase and water, K,, = partition coefficient between stationinry phase and micelle.
raphy) microscopic partition theory (1 7, 18).
EXPERIMENTAL SECTION Materials. Electrophoresis purity sodium dodecylsulfate (SDS) was obtained from Bio Raid Laboratories and recrystallized from HPLC-grade methanol to remove a small amount of ultraviolet light aborbing impurity. Hydroquinone, resorcinol, p-nitrophenol, p-nitroaniline, and naphthalene were obtained from Aldrich and used as received. HPLC grade water from Baker was used as the bulk solvent for the micellar mobile phase. Methods. A Varian Model 5020 liquid chromatograph equipped with a UV 254-nm- detector was used for all runs. The elution volumes of hydroquinone, resorcinol, p-nitrophenol, p-nitroaniline, and naphthalene were determined at 22 “C at LL variety of SDS concentrations (see Results and Discussion). Identical measurements on the same compounds using the same SDS,, mobile phase were made using two different types of columns (i.e., stationary phases). One column contained Varian MicroPak MCH-10,lO-pm octadecylsilane packing and the other a Varian MicroPak CN, 10-pm alkyl nitrile bonded-phase packing material. Both columns were 30 cm long and were 4 mm i.d. Samples were prepared by dissolving in 0.1 M SDS,. All com pounds except hydroquinone were stable in solution for the duration of the experiment. Fresh hydroquinone samples were made as often as needed. An aqueous sodium iodide solution was used to measure the void volume of the column. The elution volume of NaI was not significantly different when it was eluted either with pure water or with aqueous SDS solutions. For the CIB reversed-phase column, the total column volume (V,) was 3.77 mL, the void volume (V,) was 1.80 mL, and the volume of the stationary phase (V,) was taken as the difference between Vi and V, 1.97 mol. For the alk,yl nitrile column, Vt was 3.77 mL, V, was 2.56 mL, and V, was 3.21 mL. Derivation of Partition Equations. For the sake of continuity and clarity, the follovving derivation and symbols will follow (as closely as possible) the {exampleof Martin and Synge (14) and Herries et al. (15). For an HPLC column (containing bonded stationary phase) of many theoretical plates, the following parameters are defined: h = the height equivalent to a theoretical plate, A, = cross sectional area of the column, A, = (crosssectional area of the stationary phase, A, = cross sectionall area of the mobile phase, V, = total volume of the column, V, = volume of the stationary phase, V, = volume of the mobile phase (i.e., void volume of the column), V, = total volume of eluent needed to elute a given solute from the column (i.e., elution volume of a solute), Ksw = partition coefficient of a solute betvveen the stationary phase and water, KSM= partition coefficient of a solute between the stationary phase and micelle, K w = piartition coefficient of a solute between micelle and water, 0 = partial specific volume of the surfactant in the micelle, C , = concentration of surfactant in the micelle in g/mL (C, = total surfactant concentration in the mobile phase - CMC), CMC = critical micelle concentration, P = oC, = volume fraction occupied by micellen in the mobile phase, M, = fractional mass of solute initially present in the first theoretical plate ab-
M,=
KswhAaMaq hArn(1- 8)
and PhAmKMWMaq hAm(1 - P) Substituting the quantities of M, and M , back into eq 1 M, =
Which can be rearranged to the following expression:
By setting the numerator in the above expression equal to V (i.e., V = hA,(l- 0) + KswhA, + @hA,Kw) and rearranging one obtains
The expression for V derived above is analogous to those defined by Martin and Synge (14) and Herries et al. (15). In the same manner one can obtain
Thus the fraction of solute in the moving liquid of the mobile phase is
If one defines a term W = KMwP+ (1- P) then WA,h M , + Maq = (4) V Equation 4 is equivalent to that of Herries et al. (15), except that the quantity, V, is different. Consequently, one can use the previously developed treatment from this point (14, 15). When an infinitesimallys m d volume of mobile phase Wpasses through the column, the fraction of solute reaching the next theoretical plate is W(SV/V). If a solute is eluted from the column then V, = ndV. After the passage with “n” successivevolumes (W) of “n” successivevolumes of mobile phase, the location of the band center of the solute on the column can be expressed as the serial number of the maximally occupied theoretical plate (i.e., nW6V/ V = 1). Substituting V, for n6V one obtains
wv,/v=
1
(5)
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
rearranging ( Ve -
Vm) W
= V&SW
0 8
substituting the expression for W and rearranging
--VB - KMWP + (1 - @)
Ksw substituting the expression for fl and rearranging Ve
- Vrn
05
L "E
By plotting V,/(Ve - V,) (the terms of which can be measured) versus C, (which is known), one should obtain a straight line in which K ~ can w be calculated from the intercept and Kw can be calculated from the ratio of the slope over intercept (provided 0 is known). Technically the value of KMW is the partition coefficient between the micelle and water per monomer of surfactant. To get the true partition coefficient (per micelle), one simply multiplies Km times the aggregation number of the micelle. The quantity KSM can be obtained from the ratio of the other two partition coefficients
One should note that when micelles are not present in the mobile phase (i.e., as in traditional reversed-phase HPLC with homogeneous solvents), eq 6 reduces to the normal partition equation V, = V,
+ KswV,
(7)
RESULTS AND DISCUSSION It is apparent from this treatment (see Figure l),that the evaluation of Ksw, K S M , and K m for two different stationary phases (using the same micellar mobile phase) should produce different values of Ksw and KSM but identical values of K-. T o test the veracity of eq 6 and of the above assertion (concerning K m ) ,we measured the elution or retention volumes (V$) of a series of five compounds (hydroquinone, resorcinol, p-nitrophenol, p-nitroaniline, and naphthalene) on two different columns using an aqueous SDS mobile phase (see Experimental Section). The V, values and corresponding capacity factors obtained by using the C18reversed-phase column are given in Table I. Analogous data obtained by using the alkyl nitrile column are given in Table 11. According to theory, the elution of a solute with a surfactant solution below the CMC should be the same as that with pure water. This, of course, is due to the fact that there are no micelles below the CMC of a surfactant with which a solute can partition. In some cases (see the first two columns of Tables I and 11) this ideal behavior is followed, but in others (Le., hydroquinone in Table I and resorcinol or p-nitrophenol in Table 11) there is a small but real change in the retention volume even a t surfactant concentrations below the CMC. Further studies of these small changes indicated that any minute amount of surfactant could cause them but that further increase in surfactant concentration (in the mobile phase) caused little or no difference until the CMC was reached. It is likely that these small changes are simply due to modification of the stationary phase by adsorption of some of the surfactant from solution. The adsorption of an ionic surfactant to the stationary phase could alter the elution behavior in a t least three ways: (a) If the hydrophobic tail is strongly adsorbed (as one might expect with the CIS reversed-phase column) the ionic head group would be in contact with the solution and impart an ion exchange capacity to the stationary phase. (b) If the ionic head group is strongly adsorbed, the stationary phase could become more hydrophobic. (c) The surfactant might compete with a solute for adsorption sites on the stationary phase.
G'
n,4
02
0 0
0 02
0 04 CONCENTPATIGN
OF
SDS
0 08
0 06 IN
'~ICELLES, G / Y L
-
Flgure 2. Plots of the chromatographic parameters V.J V, V,) (for a C18reversed-phase column) vs. the concentration of surfactant in the micelle: hydroquinone = 0,resorcinol = 0,p-nitrophenol = 8 , and p-nitroaniline = A. According to eq 6 the intercept of such a plot is the inverse of K,, while KMWcan be calulated from the ratio of the slope over intercept.
Although it is not the purpose of this study to evaluate these factors, it is apparent that in this particular case they are small (especially when compared to the large effects above the CMC) and will not interfere with the aforementioned treatment. It is likely, however, that when chromatographing charged solutes, with surfactant containing mobile phases, the imparted ion exchange capacity (factor a above) could play a significant role. At surfactant concentrations above the CMC there are dramatic decreases in the retention volumes and therefore capacity factors of all solutbs (Tables I and 11). Obviously one can control the capacity factors of many compounds by varying the surfactant concentration. Equation 6 would predict a decreasing retention volume with increasing micelle concentration, if a solute partitions to the micelle. The decrease in retention volume is of course limited to the void volume of the column. Experimental points taken near this region might deviate from theory and should be avoided. If a solute does not partition to the micelle, then its elution volume would be slightly more (according to eq 6) than that in the absence of micelles. The data in Table I, treated according to eq 6, are shown in Figure 2. Likewise, Figure 3 illustrates the treated data of Table 11. It is apparent that this treatment appears to be valid in these particular cases. A statistical evaluation of these data are given in Table 111. The values of K m (for resorcinol, p-nitrophenol, and p-nitroaniline) obtained by using the CIS reversed-phase column are the same as those obtained with the alkyl nitrile column within the experimental error of this method. As expected, the values of K ~ (the w reciprocal of the intercepts, Table 111) obtained by using the C18 column are very different from those obtained with the alkyl nitrile column. Assumptions and Error Analysis of the Partition Treatment, Equation 6 implies that certain assumptions have been made regarding the micellar and chromatographic system. For example, it is assumed that the CMC and the aggregation number of the micelle are not affected by the small amounts of solute that one is chromatographing. One assumes that increasing the concentration of micelles does not alter the aggregation number or geometry of the micelles. This assumption is only reasonable a t lower surfactant concentrations. Some surfactants, such as cetyltrimethylammonium
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
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Table I. Variation of the Retention Volumes and Capacity Factors of Compounds, from a C,, Reversed-Phase Column as a Function of Micelle Concentration in the Mobile Phase total SDS concentration in the mobile phase, M X 10 0
compound
0
hydroquinone resorcinol p-nitrophenol p-nitroaniline naphthalene
5.37 11.78
hydroquinone resorcinol p-nitrophenol p-nitroaniline naphthalene
2.0 5.5
0.03 0.12 0.51 1.00 2.00 concentration of SDS in the micelle" (Le., C,), g/mL x lo2 0
0.112 1.237 Retention Volume, mL, C,, Column 5.13 5.00 4.74 11.82 10.89 8.94 19.51 26.80 Capacity Factors ( k ' = ( V e - V,)/V,) 1.9 1.8 1.6 4.0 5.6 5.1 9.8 13.9
3.00
2.650
5.534
8.418
4.23 7.36 17.16 16.09
3.81 6.00 11.44 11.80 31.50
3.45 4.93 8.76 9.49 21.51
1.1 2.3 5.4 5.6 16.5
0.9 1.7 3.9 4.3 11.0
1.4 3.1 8.5 7.9
a The concentration of surfactant which resides in micelles (excluding free monomers) is C , = C - CMC where C , is the concentration of surfactant in the micelle, C is the total concentration of surfactant, and CMC is the critical micelle concentration. The units of C , , , C, and CMC are all converted to g/mL. The reason for this is that the partial specific volume (V)in eq 6 has units of mL/g and it is necessary that these units cancel so as to obtain dimensionless partial coefficients. M (19). The CMC of SDS is 8.1 x
Table 11. Variation of the Retention Volumes and Capacity Factors of a Series of Compounds, from an Alkyl Nitrile Column, as a Function of Micelle Concentration in the Mobile Phase total SDS concentration in the mobile phase, M X 10 0 0.03 0.12 0.24 0.51 0.75 1.00 concentration of SDS in the micelle" (C), g/mL x lo2 compound 0 0 0.112 0.459 1.237 1.929 2.650 Retention Volume, mL, Alkyl Nitrile Column 5.06 4.83 4.72 4.64 4.40 4.17 4.05 resorcinol pnitrophenol 6.56 6.91 6.26 5.80 5.34 5.00 4.80 p -nitroaniline 12.56 12.57 10.89 8.44 7.32 6.26 5.69 naphthalene 15.06 8.44 5.44 4.78 4.20 Capacity Factors ( k ' = ( V e - Vm)/Vm) resorcinol 1.00 0.9 0.8 0.8 0.7 0.6 0.6 p-nitrophenol 1.6 1.7 1.4 1.3 1.1 1.0 0.9 p-nitroaniline 3.9 3.9 3.3 2.3 1.9 1.5 1.2 naphthalene 4.9 2.3 1.1 0.6 0.4 " The concentration of surfactant which resides in micelles (excluding €ree monomers) is C, = C - CMC where C, is the concentration of surfactant in the micelle, C is the total concentration of surfactant, and CMC is the critical micelle concentration. The units of C,, C, and CMC are all converted to g/mL. The reason for this is that the partial specific volume ( u ) in eq 6 has units of mL/g and it is necessary that these units cancel so as t o obtain dimensionless partition coefficients. The CMC of SDS_- is 8.1X M (19). Table 111. Least-Squares Analysis and Calculated Partition Coefficients from the Treated Data of Tables I and I1 std, KMW, KMW,, intercept covariance deviation corr coef monomers" micelle C,, Reversed-Phase Column hydroquinone 7.35 0.59 0.051 0.25 0.998 14.5 900 4.92 0.21 0.023 0.17 0.998 27.1 1680 resorcinol p-nitrophenol 2.24 0.08 0.004 0.07 0.994 32.5 2020 p-nitroaniline 2.57 0.04 0.05 0.08 0.996 72.3 4620 Alkyl Nitrile Column 11.36 0.52 0.012 0.12 0.994 25.3 1560 resorcinol p-nitro phenol 9.08 0.32 0.08 0.10 0.986 32.9 2030 p -nitroaniline 9.63 0.14 0.009 0.10 0.989 19.8 4930 naphthalene 24.49 0.09 0.052 0.26 0.997 315.7 19050 " To calculate these values of KMW one must know the partial specific volume (T) of the surfactant in the micelle, For SDS,U= 0.862mL/g as (determinedby Mukerjee (20). The KMw per micelle is obtained by multiplying the KMw per monomer by the aggregation number of SDS (62in this particular case) (19). compound
slope
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
in the presence or absence of micelles in the mobile phase. In this particular case, it was difficult to collect adequate data for proper treatment according to eq 6. Limitations, such as those described for naphthalene (with the C18column) and hydroquinone (with the alkyl nitrile column) would be more serious if one were limited to a single stationary phase. Fortunately, one has several different polarity bonded stationary phases to choose from. As a result, a very wide variety of compounds can be chromatographed using micellar mobile phases and treated accordingly.
CONCLUSIONS
0
~
2
bromide, undergo "sphere to rod" transitions at higher aqueous surfactant concentrations. Furthermore, it must be assumed either that the binding of the surfactant to the stationary phase does not appreciably affect the retention time of the solute or (if the retention time is affected) that this binding reaches a level of saturation at or before the CMC of the surfactant (in the column) is reached. The stationary phase is treated as if it were a liquid. For convenience, the partition coefficients Ksw and KSMare treated by using the entire volume of the stationary phase. One should keep in mind, however, that only the surface bonded layer is actually involved. The statistical treatment of the plots in Figures 2 and 3 (see Table 111) indicates that the error in the slopes of these plots tends to increase when one is working with a compound with either very small or very large partition coefficients. Indeed, this is the case. There are certain limits to the applicability of this treatment with a given stationary phase. Consider, for example, the data (Table I) obtained for naphthalene with the CI8 reversed-phase column. Because of naphthalene's great affinity for the stationary phase and low affinity for water, one needs an appreciable concentration of SDS micelles in the mobile phase to achieve elution in a reasonable amount of time. In fact, it is difficult to obtain a sufficient amount of data at low enough surfactant concentrations to calculate an accurate partition coefficient for naphthalene using this stationary phase. On the other hand, naphthalene does not interact as strongly with the alkyl nitrile stationary phase (Tables I1 and 111). In this case one can collect sufficient data to calculate the various partition coefficients. However, even the error in these K values would be expected to be larger than those of p-nitrophenol and p-nitroaniline. A t least part of the reason is due to the fact that the slope of the naphthalene plot (Figure 3) is large and the intercept is close to zero. Thus, small changes in the slope of the line would result in relatively large changes in the intercept. Hydroquinone and resorcinol are analogous examples. From Table I11 one can see that resorcinol and particularly hydroquinone have both the lowest K W values and the greatest amount of uncertainty in the slopes. Even more important, is the relative lack of interaction of hydroquinone with the C18-bonded stationary phase (Ksw = reciprocal of the intercept). This lack of interaction with the stationary was even more pronounced with the alkyl nitrile column. Consequently the retention volumes for hydroquinone on the alkyl nitrile column were not sufficiently different or consistent
The liquid chromatographic behavior of many compounds eluted with aqueous micellar mobile phases can be described by a basic partitioning treatment. This treatment involves three partition coefficients, that beween the stationary phase and water (Ksw),that between the stationary phase and the micelle (&MI, and that between the micelle and water ( K m ) . If a solute partitions to a micelle, then one observes a decreasing retention volume (V,) with increasing surfactant concentration in the mobile phase &e., C > CMC). An obvious consequence of this treatment is that the measured values of KSW,K ~ Mand , K m for a compound using the same micellar mobile phase, but two different stationary phases, will result in different values of K8w and KSMbut identical values of KMW.By the same token, the measured K values of a compound on a single stationary phase but with two different micellar mobile phases would result in different values of KsM and KMw but identical values of Ksw. Surfactants in the mobile phase can bind to the stationary phase sometimes causing changes in the V, and k'of a compound. These effects can be observed below the CMC of a surfactant and were small in this particular study. However, if an ionic surfactant binds to a stationary phase via its hydrophobic tail,leaving its polar, charged head group in contact with the bulk mobile phase, the stationary phase may acquire a capacity for ion exchange. There would be an expected effect on the V, and k'of some charged solutes in this case. The error involved in using this partition treatment increases if any of the partition coefficients are very large or very small. Fortunately, this error can be minimized and the applicability of the technique extended by using different stationary phases.
LITERATURE CITED (1) Armstrong, D. W.; Terrill, R. Q. Anal. Chem. 1979, 51, 2160-2163. (2) Armstrong, D. W.; McNeely, M. Anal. Lett. 1979, 12, 1285-1291. (3) Armstrong, D. W.; Henry, S. J. J. Li9. Chromatogf. 1980, 3 , 657-662. (4) Armstrong, D. W. J. Ll9. Chromatogr. 1980, 3 , 895-900. (5) Armstrong, D. W.; Fendler, J. H. Blochim. Blophys. Acta 1977, 478, 75-80. (6) Hinze, W. L.; Armstrong, D. W. Anal. Leff. 1880, 13, 1093-1104. (7) Fendler, J. H.; Fendler, E. J. "Catalysis in Micellar and Macromolecular Systems": Academic Press: New York, 1975. (8) Bunton, C. A. Pmg. Solid State Chem. 1973, 8 , 239-281. (9) Cordes, E. H.; Gitler. C. Prog. Biorg. Chem. 1973, 2 , 1-53. 10) Bender, M. L.; Kolyama, M. "CyclodextrlnChemistry"; Springer-Verlag: Berlin, 1978. 11) Grlffiths, D. W.; Bender. M. L. Adv. Catal. 1973, 209. 12) Hinze, W. L. "Solution Chemistry of Surfactants"; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 79, 13) Hinze, W. L. "Colloid and Interface Sclence", Kerker, M., Ed.; Academlc Press: New York, 1976; p 425. 14) Martin, A. J. P.; Synge, R. L. M. J. Biochem. 1941, 35, 1358-1368. 15) Herries, D. G.: BishoD. W.; Rlchards, F. M. J. Phvs. Chem. 1964, 68 (7), 1842-1852. (16) Doe, P. H.; Wade, W. H.; Schechter, R. S. J. Colloid Interface Scl. 1977, 59, 525-531. (171 Purnell. J. H.: Varaas de Andrade. J. M. J. Am. Chem. SOC. 1975. 97, 3590-3593. " (18) Laub, R. J.; Purnell, J. H. J. Am. Chem. SOC.1976, 98, 35-40. (19) Mysels, K. J.; Prlncen, L. H. J. Phys. Chem. 1959, 63, 1696-1701. (20) Mukerjee, P. J. Phys. Chem. 1962, 66, 1733-1735. .
I
RECEIVED for review April 2, 1981. Accepted June 22, 1981. This work was supported by a grant from the Research Corporation and we gratefully acknowledge their assistance.
