2520
Anal. Chem. 1988,60,2520-2527
Investigation of the Retention Mechanism in Nonionic Micellar Liquid Chromatography Using an Alkylbenzene Homologous Series Michael F. Borgerding,’ F r a n k H. Quina, a n d Willie L. Hinze*
Department of Chemistry, Wake Forest University, P.O. Box 7486, Winston-Salem, North Carolina 27109 J e f f Bowermaster2 a n d Harold M. McNair
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
Micellar iiquld chromatography (MLC) of a homologous series of alkylbenrenes with two nonionic micellar mobile phases, i.e., poly[oxyethylene(10 or 23)ldodecanoi (Brij-22 or Brlj-35), indicates a linear relationship between the capacity factor, k’, and the homdogue carbon number, n,. This is in contrast to the usual log k’ vs n , relatlonshlp found for a conventional methanol-water mobile phase. I n order to Identify the origin of this difference, the various partition coefficients unique to MLC were determined as a function of carbon homologue number in the usual manner, employlng an Improved procedure for estimation of the chromatographic phase ratio based on measurement of the cumulatlve pore volumes of the stationary phase. Examination of the partition coefficient data, together with solute selectivity and water solublity data, leads to the conclusion that separate and dlstlnct retention mechansisms are operative in MLC, depending upon the degree of water solublllty or lnsdubllity of the solute. Thus, the retention of sparingly soluble or water-insoluble solutes is largely governed by the partition coefflcient associated with the direct transfer of the solute from the mobile phase micelle to the surfactant-modified (hemimicellar) stationary phase. A solubility limit equation, derived from the original Armstrong-Nome MLC theory, is shown to describe the observed linear k’ vs n , behavior adequately for the homologous serles with these mlcellar mobile phases.
surfactant, p is the ratio of the stationary phase volume V , and the mobile phase volume V,, C, is the concentration of the micellized surfactant (total surfactant concentration minus the critical micelle concentration), and P,, and P,, are the solute partition coefficients between the micellar pseudophase and water and the stationary phase and water, respectively (1,6). A third dependent partition coefficient, P,, describes the direct transfer of a micellar bound solute to the modified stationary phase. In all work reported to date, it has always been assumed that direct transfer of a solute between the micellar and stationary phases is minimal (5,8). The predicted linear relationship between l / k ’ and C, (eq 1) has been verified for all conceivable polarity combinations of surfactant and solute (6, 7, 9-12). Consistent with accepted RPLC theory, when there are no micelles present in the mobile phase, equation 1 reduces to
A number of workers have used homologous series to study both mobile and stationary phase contributions to the retention mechanism in reversed-phase liquid chromatography (13-20). In these studies, a linear relationship is generally found between log k ’, the log of the capacity factor, and n,, the number of repeat units in the homologous series, although some deviations from linearity have been noted (21,22). The relationship between k’ and n, is often expressed as
log k’ = (log a ) n , + log p The use of aqueous micellar solutions, i.e., solutions containing a surfactant at a concentration above its critical micelle concentration, as reversed-phase high-performance liquid chromatography (RPLC) mobile phases (1) has been an area of considerable interest over most of the past decade (for reviews, refer to ref 2-5). As in traditional RPLC, solute retention in micellar liquid chromatography (MLC) can be controlled by manipulation of the mobile phase composition. MLC is unique, however, in that three phases are present (i.e., a micellar pseudophase in addition to the usual bulk mobile phase and stationary phase), making multipartitioning possible. The relationship between solute retention and micellar mobile phase composition, first treated theoretically by Armstrong and Nome (6), is typically expressed as (1, 7)
where k’is the capacity factor, is the molar volume of the Present address: R. J. Reynolds Tobacco Co., Bowman-Gray Technical Center, Winston-Salem, NC 27102. 2Present address: Ciba Geigy Corp., P.O. Box 18300,410 Swing Rd., Greensboro, NC 27419.
(3)
where LY is the nonspecific selectivity of a methylene group and p is the retention contribution from the functional group within the homologous series. Although homologous series have been widely used in RPLC studies, there had been no reports of an analogous study in micellar liquid chromatography when this project was initiated. Recently, Khaledi and co-workers have reported the retention behavior of homologous series in RPLC using anionic sodium dodecyl sulfate (NaLS) and cationic hexadecyltrimethylammonium bromide (CTAB) micellar mobile phases (23). In contrast to the usual logarithmic dependence (eq 3), they noted a linear relationship between k ’and n, with the micellar mobile phases and stated that the reason(s) for this observation was unclear (23). During the past several years, observations in our laboratories have also consistently indicated that, for nonionic Brij micellar mobile phases, the relationship between k ’and n, is not logarithmic, but rather approximately linear (24). This linear dependence can be qualitatively rationalized by consideration of solvophobic theory and the inherent differences present in MLC compared to that of RPLC. In order to gain better insight into the unique k’vs n, behavior, the multipartition theory has been applied in this work to a homologous series of alkylbenzenes, using nonionic micellar Brij mobile phases to determine the partition coefficients associated with
0003-2700/88/0360-2520$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
the MLC phases. After initial development of a method for the accurate determination of the chromatographic phase ratio, the relationships between the various partition coefficients and n, are examined and interpreted in terms of the prevailing MLC retention theory. Resulting data are consistent with the existence of separate and distinct retention mechanisms for water-soluble and insoluble solutes. It is proposed that materials that are not water-soluble are transferred directly between the micellar pseudophase and the surfactant-modified stationary phase. A modified Armstrong-Nome equation (6) is proposed, which adequately describes the observed retention behavior of these solutes.
2521
g), CPV, is the cumulative pore volume (cm3/g) of unbonded Resolve silica, CPVbis the cumulative pore volume of the bonded C18 Resolve packing material before or after exposure to surfactant, and V , is the mobile phase volume. Cumulative pore volume measurements were made on aliquots of C18 Resolve packing material after exposure to different amounts of Brij surfactant. One-gram samples were agitated with 25 mL of the surfactant solution for ca. 3 h during a total surfactant exposure period of 24 h. The samples were vacuum-filtered onto a 0.45-wm Nylon 66 membrane filter, transferred to a sample vial, and dried in a vacuum oven at 60 “C for a minimum of 48 h. After drying, nitrogen sorption measurements were made with the Digisorb 2600. Standard adsorption and desorption procedures were performed under computer control to develop the nitrogen adsorption isotherm at 77 K. Pore volume distributions and the cumulative pore volume were calculated from the adsorption isotherm for pores between 20 and 600 A, assuming cylindrically shaped pores, by using the method of Barrett et al. (26). No significant differences were observed between cumulative pore volumes calculated from the adsorption isotherm and from the desorption isotherm.
EXPERIMENTAL SECTION Apparatus. The HPLC system, constructed of Waters (Waters Associates, Milford, MA) components, consisted of a M6000A pump, a Waters Intelligent Sample Processor (WISP), a Model 441 fixed-wavelength UV detector, and a Model 720 system controller. A Waters Resolve 5-pm C18 spherical packing, 3.9 mm X 15 cm stainless steel column was used. Data acquisition and integration were performed with Computer Automated Laboratory RESULTS AND DISCUSSION Systems (CALS) software and HP 1000 hardware (HewlettPackard, Avondale, PA). All data were acquired at a flow rate General Comparison of Micellar and Hydroorganic of 1 mL/min and a detector wavelength of 254 nm. Mobile Phase Separations. Elution of a homologous series All surface area and pore volume measurements were made with of alkylbenzenes using methanol/water (75/25) (v/v) and the a Digisorb 2600 (Micromeritics, Norcross, GA). aqueous Brij nonionic micellar mobile phases produces striking Reagents. Brij-35 (poly[oxyethylene(23)]dodecanol, CH3differences in separation characteristics. Comparison of the (CH2)11(0CH2CH2)230H), obtained from Fisher Scientific Co. MLC separations (Figure lA,C) with the methanol/water (Raleigh, NC) as a 30% (w/v) aqueous solution, and Brij-22 separation (Figure 1B) indicates differences in chromato(poly[oxyethylene(lO)]dodecanol, CH3(CH2)11(0CH2CH2)loOH), graphic efficiency, column capacity, and separation selectivity. obtained from Sigma Chemical Co. (St. Louis, MO), were used First, the MLC peaks are broader, indicative of poor sepaas received. Methanol and benzene were obtained from Burdick and Jackson (Muskegon,MI), and all alkylbenzenes were obtained ration efficiency and generally reduced resolution. Similar from Aldrich (Milwaukee,WI). All test solutes had stated purities reductions in chromatographic efficiency with the use of of 96 or greater and were used as received. HPLC water was surfactant mobile phases have been consistently observed and distilled and deionized with a Barnsted NANOpure System. discussed by other workers (2, 3, 8, 27, 28). Although less Procedures. Determination of Retention Data. The chroefficient, the symmetric MLC peak shapes (Figure 1C) suggest matographic column was equilibrated with several liters of neat that Brij micellar mobile phases may increase column capacity. methanol, followed by a methanol/water (75/25) (v/v) solution. Comparatively, the peak shapes with the conventional Retention data for alkylbenzenes, prepared as 1% (v/v) in methanol/water mobile phase (Figure 1B) are not symmetric, methanol, were determined alternately with the methanol/water with peak fronting indicating stationary phase overload. The (75/25) (v/v) mobile phase and a series of aqueous Brij surfactant observed difference in peak shape is not a mobile phase mobile phases. The column was reequilibrated between the use of Brij-22 and Brij-35 surfactants by washing with several liters phenomenon, but rather the result of stationary phase modof water, methanol, and finally methanol/water (75/25) (v/v). ification caused by surfactant sorption. Recent work in our Aqueous Brij-22 mobile phases evaluated included surfactant laboratories has demonstrated that, a t the mobile phase concentrations of 0.5, 1.0, 2.0, 3.0, and 4.0% (w/v) with benzene, surfactant concentrations typically used in MLC (IO,29), toluene, ethylbenzene, propylbenzene, and butylbenzene employed surfactant sorption onto the packing can occur in amounts as test solutes. Aqueous Brij-35 mobile phases evaluated were approximating that of the bonded stationary phase itself. 2, 3, 4, 5 , and 6% (w/v), and the alkylbenzene solute range was Finally, a larger number of homologues are eluted per unit expanded to include higher homologues (amylbenzene through time when the Brij micellar mobile phases are used, as comphenylundecane). Solutes were evaluated individually in the case pared to the traditional methanol/water mobile phase (e.g., of Brij-22 and, for comparison, both individually and as mixtures in the case of Brij-35. The injection volume in all cases was 15 between ca. 30 and 120 min in Figure 1,three peaks are eluted pL. The capacity factor was calculated as k’= (t,- to)/to,where with a 75/25 (v/v) methanol/water mobile phase (B), as t, is the elution time of the solute and to is the elution time of compared to six or more with an aqueous 6% Brij-35 micellar uracil. The chromatographic system volume was determined by mobile phase (A)) although the efficiency (i.e. ability to resolve replacing the column with a zero dead volume union and meassolutes) is less for the micellar phases. The observed selectivity uring the elution time of uracil. All times were corrected acdifference is not attributable to a difference in solvent cordingly. strength, as the alkylbenzenes begin to elute earlier with the Determination of Partition Coefficients. To determine parmethanol/water mobile phase. tition coefficients P,, P,,, and P,, regression analyses of 1/k’ The unique selectivity of nonionic micellar mobile phases vs the concentration of the micellized surfactant, C,, were peris apparent in Figure 2A, in which the capacity factor, k’, is formed for each solute/surfactant combination according to eq 1. P,, was determined from the resulting intercept, and P,, was plotted as a function of the alkylbenzene homologue number. determined from the slope/intercept ratio after correcting for D Instead of the logarithmic relationship generally observed and p as required. The partial molar volume of Brij-35 has been between k’and homologue number (as shown in Figure 2B previously determined (IO), and the partial molar volume of for the methanol/water mobile phase), a linear relationship Brij-22 (0 = 0.9542 cm3/g) was determined according to the is observed for the aqueous Brij-35 micellar mobile phase procedure of Mukerjee (25). Density measurements were made (Figure 2A), representing a significant departure from all other with a Mettler/Paar DMA-46 density meter. forms of chromatography. Likewise, a linear relationship The phase ratio was calculated as between k’ and homologue number was observed for the p = W(CPV, - CPVd / v, (4) aqueous Brij-22 mobile phase (not shown). This appears to be a general phenomenon in MLC, since a recent report also where: W is the weight of packing material in the column (1.4
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988 A
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Figure 1. Chromatograms showing the separation of alkylbenzene homologues with different mobile phases. Chromatographic conditions are described in the Experimental Section. Conditions: (A) 6% Brij-35 mobile phase, test mixture includes propylbenzene (ca. 50 min), butylbenzene, amylbenzene, phenylheptane, phenyloctane, phenylnonane, and phenylundecane; (B) methanoVwater (75/25) (v/v) mobile phase, test mix as in (A); (C) 2 % (w/v) Brij-22 mobile phase, composite of individual chromatograms for benzene, toluene, ethylbenzene, propylbenzene, and butylbenzene test solutes.
showed that similar linear relationships were obtained by using cationic CTAB or anionic NaLS aqueous micellar mobile phases (23). This linear relationship observed with the aqueous micellar mobile phases may result from energetics unique to the pseudophase environment. In conventional RPLC with hydroorganic mobile phases, the linear dependence between log k ‘ and n, has been attributed to the direct proportionality between log k’ and the free energy of retention, AGr, which is in turn a linear combination of the retention free energy increments associated with the constituent parts of the molecule (23-22). The predominant free energy contribution derives from the cavity formation within the mobile phase solvent structure that is required to accommodate the solute molecule transferred from the stationary phase. The free energy of cavity formation, AG,, is a function of the cavity surface area and, therefore, proportional to the regular increase in molecular volume upon addition of each homologous
0
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988 2523 .50
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Micellied Surfactant (M) Figure 3. Dependence of the phase ratio on the concentration of
micellized surfactant in the mobile phase: (+) curve describing the phase ratio for both Brij-22 and Brij-35 when the entire sorbed surfactant molecule is treated as part of the stationary phase, (A)Brij-22 curve when only the hydrocarbon tail of the molecule is treated as stationary phase, and (m) Brij-35 curve when only the hydrocarbon tail of the molecule is treated as stationary phase. volume or stationary phase surface area. For this reason, past efforts to determine MLC pseudophase partition coefficients have employed the crudest of approximations to estimate V,, taking V , to be the difference between the empty column volume and the packed column void volume (6, 7,23,31,32). This difference is clearly a poor estimate of V , because it includes the entire volume occupied by the silica solid support particles rather than just the true stationary phase. The use of such a V, value to estimate the phase ratio can be expected to result in MLC partition coefficients P,, and P,, that are significantly in error, although accurate P,, partition coefficients can still be derived (2, 6, 23). The approach we have chosen for the determination of the phase ratio (eq 4) completely excludes any volume associated with the base silica material. Estimation of V , involves measurement of the cumulative pore volumes (CPVs) of Resolve silica, Resolve C18 packing material, and surfactant-modified Resolve C18 packing material by using nitrogen porisimetry. V , is calculated as the difference between the CPVs of the C18 or surfactant-modified C18 packing material and that of unbonded Resolve silica material. Thus, the stationary phase volume is assumed to be that portion of the silica pore volume filled upon bonding the octadecylsilane chains and, in the presence of detergent, by sorption of surfactant to the C18 chains. The C18 phase volume determined by this approach (0.204 cm3/g) compares favorably with the C18 phase volume predicted from the density of octadecane and a 12% hydrocarbon loading (0.154 cm3/g). The larger measured value is to be expected because bonding of the C18 molecules to the silica support imposes chain spacing; hence, the limiting density of the neat liquid cannot be achieved. Figure 3 depicts the relationship observed between the phase ratio and the amount of micellized surfactant in solution. The phase ratio is logarithmically related to micellized surfactant over the surfactant concentration range studied and mimics the surfactant/C18 packing material adsorption profile. Somewhat surprisingly, similar amounts of both of the nonionic surfactants studied are sorbed at each concentration, yielding a single phase ratio curve for both Brij-35 and Brij-22. Regression analysis of cumulative pore volume data (cubic centimeters per gram) for both surfactants as a function of micellized surfactant yielded the following: CPV = 0.05-0.03 In [C,] (correlation coefficient 0.975, n = 9). Nonionic Brij-35 or -22 surfactant micelles contain a nonpolar dodecyl core region and a rather polar surface layer of 23 (or 10) oxyethylene moieties. Multiple modes of solubilization are thus possible with these micellized surfactants.
Nonpolar solutes such as the alkylbenzenes tend to incorporate into the hydrocarbon-like regions of the micelle (33-35). Assuming analogous behavior for incorporation of these solutes into the surfactant sorbed onto the column packing material, it would be inappropriate to consider the entire sorbed surfactant volume as part of the stationary phase with alkylbenzene solutes. The additional curves shown in Figure 3 represent corrected phase ratio data, in which only the bonded C18 hydrocarbon of the Resolve packing and the C12 hydrocarbon chain portions of the sorbed surfactant are considered to contribute to the stationary phase volume. The slight offset between the two curves may be attributable to errors associated with the determination of the surfactant and hydrocarbon partial molar volumes used in these calculations. Regardless of precisely what portion(s) of the surfactant molecule should be included as stationary phase, it should be noted that the surfactant-modified phase ratios are significantly larger than those of unmodified column packing material. Additionally, net sorption of surfactant is greatest at the lower surfactant concentrations, such that the phase ratio remains essentially constant over the concentration range (0.03-0.17 M surfactant) examined. Pseudophase Partition Coefficients for the Alkylbenzene-Brij Systems. Previous studies of MLC partition coefficients have focused on investigation of the MLC retention mechanism. Good general agreement has been found between chromatographically determined partition coefficients or related binding constants and partition coefficients determined by alternate techniques (11,32,36,37),in support of the multipartition theory. Relative contributions of P,, and P,, to solute retention have been studied by variation of surfactant type and solute or stationary phase polarity (7,231, although poor phase ratio estimates have limited both the quality and quantity of P,, data. In this work, MLC partition coefficients have been determined for a series of alkylbenzene homologues in order to test (1) the role of water in MLC retention and (2) the relative performance of two different nonionic surfactants as MLC mobile phases. In RPLC, the energetics of cavity formation within the water structure upon partitioning of a solute from the stationary to the mobile phase is the key factor responsible for the linear log k’vs carbon number relationship. By analogy to RPLC theory, the individual partition processes within the MLC mechanism would also be expected to exhibit a logarithmic relationship for any partition process involving water as a phase. The data in Figure 4 confirm that such a linear relationship between log P,, or log P,, and carbon number indeed exists for the first few alkylbenzene homologues, consistent with both the multipartition and RPLC retention theories. It has been previously shown in the micellar literature that linear relationships between the log of the partition coefficient (or corresponding binding constants as determined by kinetic or spectroscopic techniques) and solute homologue number exist for the transfer of a series of solutes from water to the micellar pseudophase (38-40). In their recent study, Khaledi et al. also noted a linear dependence between log P,, (or log P,,) and homologue number (23). What is not obvious from Figure 4, and perhaps more significant, is that the linear relationship cannot be extended to the higher homologues because negative intercepts are consistently encountered in the llk’vs C, regression analysis (Table I). The significance of this finding is discussed below. The relative performance of Brij-35 and Brij-22 as micellar mobile phases can also be evaluated from Figure 4. Analysis of the data using the molar volume of the entire surfactant molecule produces separate P,, curves (Figure 4A) for the two surfactants, yet virtually identical P,, curves (Figure 4B). This would imply that, although the stationary phase inter-
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988 4 r
Table I. Results of Regression Analyses for Brij-35 Using Eq 1 compound
A
slope std error intercept std error corr coeff 3 -
benzene toluene ethylbenzene propylbenzene butylbenzene amylbenzene phenylhexane phenylheptane
0.849 0.694 0.583 0.467 0.391 0.328 0.277 0.233
0.012 0.007 0.007 0.003 0.002 0.002 0.002 0.002
0.024 80 0.072 30 0.002 00 0.000 50 -0.000 04 -0.000 16 -0.000 16 -0.000 06
--
0.000 50 0.000 30 0.000 30 0.000 10 0.000 08 0.000 08 0.000 07 0.000 07
0.998 0.999 0.999 0.999 0.999 0.999 0.999 0.999
actions are similar for the two surfactant systems, the chemical environments of the mobile phases are significantly different, an unreasonable result considering the comparable chemical compositions of the surfactants and the test solutes used in these experiments. However, if the respective Brij surfactant partial molar volumes are corrected to reflect only the hydrocarbon portion of each surfactant (in a manner analogous to that already described for correction of the phase ratio in the previous section), this apparent discrepancy is largely eliminated. Tanford’s equation for the core volume of a micelle yields a value of 0.211 L/mol for a C12 hydrocarbon chain (40). Algebraic manipulation of the Brij-22 (C12E10) and Brij-35 (C12E23) partial molar volumes determined in this work yields 0.225 L/mol, which agrees well with 0.228 L/mol reported for dodecane (41). Therefore, a value of 0.225 L/mol was used throughout this work as the molar volume of the C12 hydrocarbon chains in the mobile phase micelles and for surfactant sorbed to the column packing. If such corrected molar volumes and phase ratios are utilized to calculate P,, and P,, the resultant P,, curves are virtually coincident, while the P,, vs homologue number curves (Figure 4C) fall much closer together for the two Brij surfactant systems. The slight offset that still persists for the P,, curves after exclusion of the ethyleneoxide portion of the surfactants may be due to errors in the partial molar volumes or to small intrinsic differences in the chromatographic performance of the two surfactants. Based on surfactant composition and stationary phase adsorption data for Brij-35 and Brij-22, similar molar concentrations of either surfactant in the mobile phase should provide comparable amounts of both “hydrocarbon” mobile phase and “hydrophobic” stationary phase. Although not studied in detail, separation of an alkylbenzene mixture with an equimolar concentration of each of the two surfactant mobile phases yields similar retention times (capacity factors) for some of the solutes examined. Not shown in Figure 4 is the carbon number dependence of the partition coefficient, P,, (given by the ratio of P,, to P,, (2,6)),for the direct transfer of the alkylbenzene solutes between the surfactant-modified stationary phase and the Brij micelle. Analysis of the data obtained with both Brij mobile phases for the limited number of solutes reveals that almost equally good fits were obtained for plots of either P,, vs n, (correlation coefficient 0.999) or log P,, vs n, (correlation coefficient 1.OOO). Development of a Solubility Limit Theory. Obseruation of Negative Intercepts and Related Considerations. There have been few reports in the MLC literature of negative intercepts encountered during regression analyses of l / k ’ vs C, (eq 1). Prior to the recent work of Khaledi et al. (23),only a single negative intercept had been reported (11). In this latter work, a negative intercept was reported for perylene with a NaLS micellar mobile phase and a C18 column. Khaledi and co-workers reported negative intercepts for ethylbenzene through hexylbenzene with NaLS and CTAB micellar mobile phases and C8 and C18 stationary phases. After considering
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Alkylbenzene Homolog Number Flgure 4. Plots showing the logarithmic relationship between pseudophase partition coefficients and a limited number of alkylbenzene homologues for partiiion processes that include water as a phase: (A) log P,, curves for treatment of ( 6 )entire Brij-35 molecule, (0)entire Brij-22 molecule, (B) hydrocarbon portion of Brij-35 molecule, and (A) hydrocarbon portion of Brij-22 molecule; (6)log P,, curves for treatment of (B) entire Brij-35 molecule, (A)entire Brij-22 molecule, ( 6 ) hydrocarbon portion of Brij-35 molecule, and ( 0 )hydrocarbon portion of Brij-22 molecule; (C) composite of log P,, and log P,, curves when only the hydrocarbon portion of the surfactant is treated, which demonstrates the similarity of the surfactant-modified stationary phase and the micellar pseudophase.
several potential sources of error, without finding a clear reason for the negative intercepts, they finally concluded the measurement error was the probable cause (23). Development of an understanding of the negative intercepts encountered for butylbenzene and higher homologues with both nonionic surfactant mobile phases (Table I) has been the central focus of this work. Investigations of column void markers, system void volume (Le., connecting tubing, etc.), the dependence of the phase ratio on the micellized surfactant concentration, and the form used for the multipartition equation ( 2 ) produced no indication that the negative intercepts could be ascribed to experimental or procedural artifacts.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
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Alkylbenzene Homolog Number Flgure 5. Plot showing the chromatographic selectivity for alkylbenzenes with different mobile phases: (A)3% (w/v) Brij-35, (0)6% (w/v) Brij-35, (m) 6% (w/v) Brij-22, (*) methanoVwater (75/25)(v/v), and (+) (75/25) (v/v) methanol:20% (w/v) Brig-35 in water.
Having exhausted attempts to prove that the observed negative intercepts are artifactual, one is forced to consider whether the available chemical data are consistent with the observation of negative intercepts. Three types of information that help clarify the basis for “negative intercepts” are (1) water solubilities of alkylbenzene homologues, (2) predicted P,, and P,, values for the higher molecular homologues, and (3) separation selectivity data. If the “negative intercepts“ and associated errors in Table I are examined closely, it is evident that all of the intercepts for the higher molecular weight solutes are approximately zero within experimental error, rather than truly negative. The physical implications of zero intercepts are quite different from those of negative intercepts. Considering eq 1,a zero intercept requires either that the reciprocal of the phase ratio must be zero, which is not physically possible, or that P,,, the water/stationary phase partition coefficient (i.e., the reciprocal must be ”zero”. A near-“zero” P, (which means that of Paw) Paw is very, very large) is not only physically possible but also consistent with available solubility data for alkylbenzene homologues, which indicates that all homologues beyond butylbenzene are essentially insoluble in water (41,43). Thus, from regression analysis of the curves in Figure 4 for Brij-35, values for butylbenzene are ca. 1.3 X lo4 and the P,, and Paw 2.6 X lo4 per monomer, respectively, or ca. 5.2 X lo5 and 1.0 X lo6 per micelle, which underscores the marked affinity of this alkylbenzene for the organic phase! The very low water solubility of the higher alkylbenzene homologues raises the question as to whether separate mechanisms contribute to the retention of water-soluble and water-insoluble materials. Tchapla and co-workers (19)have demonstrated that plots of methylene group selectivity vs homologue number are useful for discerning subtle retention discontinuities. Selectivity, CY, is defined as
where n , and riel indicate k’values for two homologues differing by a single carbon unit. Inspection of separation selectivity data for aqueous Brij-35, aqueous Brij-22, methanol/water, and methanol-modified (nonmicellar) surfactant mobile phases (Figure 5) reveals several interesting points. Methanol/water and methanol-modified (nonmicellar) surfactant mobile phases exhibit essentially constant selectivity over the range studied, with a small positive slope that may be related to the non-Gaussian peak shape observed with these particular mobile phases. In contrast, with aqueous micellar Brij-35 and Brij-22, the selectivities decrease sharply for the early homologues, reaching approximately constant values at the “water solubility limit” of butylbenzene.
PP
Flgure 6. Artistic representation of the direct transfer process for the distribution of a solute between the micelle pseudophase and the surfactant-modified (hemimicellar) stationary phase. Note that this representation is not meant to imply that the same micellar entity sorbs, merges, and desorbs the stationary phase since a dynamic equilibrium situation exists with surfactant monomer molecules in equilibrium with both the micellar species in the mobile phase and the surfactantcoated stationary phase.
Solubility Limit Model and Equation. The negative intercept and selectivity data strongly suggest that a separate MLC retention mechanism exists for the higher molecular weight homologues that are essentially insoluble in water. One model, consistent with the observed data, is the direct transfer of these higher homologues from the micellar pseudophase to the surfactant-modified stationary phase via reversible sorption of the solute-occupied micelle onto the “hemimicellar” surfactant-modified stationary phase (28,29) (Figure 6). This type of direct transfer process finds analogy in the molecular mechanism for solubilization of water-insoluble solids by micellar solutions, which is thought to involve direct micelle diffusion to and from the surfactant-modified solid surface, in series with interfacial steps including adsorption and desorption of the micellar-solute species (44, 45). Such a mechanism is consistent not only with the observed selectivity data, but also with pertinent solubility parameters, observed reduction of chromatographic efficiency in MLC, and the almost linear dependence of P, (i.e. the partition coefficient for the direct transfer) on carbon homologue number. A modified multipartition equation that describes retention behavior in the limit of water insolubility and is consistent with sorption of a solute-occupied micelle can be derived beginning with eq 1 (2, 6, 7). Thus, assuming that (P,, - 1) i= P,, and recalling that P,, = P,,/P,,, one obtains eq 6, a reexpressed version of eq 1 (2, 6):
which, in the limit of water insolubility, becomes (after inverting)
k’ = Pwn(CP/uCrn)
(7)
The resulting equation describes retention as the product of a partition coefficient term and a “total phase ratio” term which is analogous to eq 2 for RPLC with one important difference, viz., the capacity factor is related to the concentration of micellized surfactant. Since the equation was derived for the limit of water insolubility, for eq 7 to be valid, solute containing micelles must sorb/desorb directly to/from the stationary phase (Figure 6). Such a process is consistent with the significant amount of surfactant sorbed onto the stationary phase found in this and other work (i.e., ca. 10% of the total column packing material weight) (29). The solubility limit theory can be tested in a number of ways. First, the ”complete phase ratio” term, cp/~X,,predicted from eq 7 can be compared to the measured value, given by the slope of k’vs P,, curves for the higher molecular weight alkylbenzene homologue range. Applying this approach to propylbenzene through phenylheptane, one finds that mea-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
2526
T a b l e 11. C o m p a r i s o n of P r e d i c t e d and M e a s u r e d Phase Ratios
Brij-35, %
predicted phase ratiou
graphically measured phase ratiob
2
76.6
92.6
3 4 5
51.5
55.1 41.0 31.9
38.9 31.3 26.2 22.5
6
7
41
26.9 21.6
aTotal phase ratio term calculated as V,/(V,tX,). *Total phase ratio term determined from the slope of k’ vs P,, plots (eq 7) for orowlbenzene through Dhenvlheptane.
125
-
100
-
75
-
50
-
25
-
175
150
4
0
I
1
I
I
1
1
I
1
2
3
4
5
6
7
Alkylbenzene Homolog Number Figure 8. Comparison of P, values predicted from (D) the multipartition theory (eq 1) and ( + ) the solubility limit theory (eq 7).
carbon homologue number (Figure 2A).
ACKNOWLEDGMENT We thank G. W. Fulp, Jr., and L. D. Stafford (R. J. Reynolds Tobacco Co.) for some surface area and C, H, N analyses, D. W. Armstrong (University of Missouri), A. Berthod (University Claude Bernard), C. J. Jackels (WFU), E. Ruckenstein (State University of New York-Buffalo), and s. G. Weber (University of Pittsburgh) for helpful comments with regard to this work, and M. G. Khaledi (University of New Orleans) for sending a preprint of his work in this area. 0
1
2
3
4
5
6
LITERATURE CITED
7
Homolog Number Figure 7. Comparison of k’ predicted by the limiting solubility theory (using P,, values determined from the multipartition theory) with measured values for alkylbenzenes: (e) k’ predicted for 3 % (w/v) Brij-35 mobile phase, (D) k’ measured for 3 % (w/v) Brij-35 mobile phase, (0)k‘ predicted for 7 % (w/v) Brij-35 mobile phase, (A)k’ measured for 7 % (w/v) Brij-35 mobile phase.
sured values agree well with predicted values, except a t the lowest mobile phase surfactant concentration (Table 11). The discrepancy a t 2% (w/v) Brij-35 is consistent with the selectivity data in Figure 5, which shows that selectivity is still changing with propylbenzene. Comparison of k’values predicted by eq 7 with measured values provides a second test of the solubility limit theory. Comparisons for 3% (w/v) and 7% (w/v) Brij-35 mobile phases (Figure 7) provide good agreement for propylbenzene through phenylheptane. The divergence between predicted and measure k’values for the low molecular weight homologues reflects the selectivity bias observed in Figure 5, which is indicative of the predominance of other retention mechanisms for these compounds. It should be noted that the predicted k ’values in Figure 7 are based on P, values derived from eq 1. When P, values predicted by the multipartitioning theory (eq 1)and the solubility limit theory (eq 7) are compared (Figure 8),a pattern similar to that in Figure 7 emerges, i.e., good agreement is found with longer chain homologues, but not with the first members of the series. Interestingly, the solubility limit model appears to provide the best prediction of retention for the entire alkylbenzene homologous series. The error bars for the P, values from the limiting solubility equation (Figure 8) represent the standard deviation for the entire surfactant range studied (i.e., 2%-7% (w/v) Brij-35). Thus, with eq 7, P, values determined at any given micellar mobile phase composition can be employed to predict solute retention times for other mobile phase surfactant concentrations with little error over the entire alkylbenzene range! Additionally, the p,, curve is approximately linear, which is consistent with the linearity of k’with
Armstrong, D. W.; Henry, S. J. J. Liq. Chromatogr. 1980, 3 , 657. Armstrong, D. W. Sep. Purif. Methods 1985, 14, 212. Hinze, W. L. Ordered Media in Chemical Separations; Hinze, W. L., Armstrong, D. W., Eds.; American Chemical Society: Washington, DC, 1987;Voi. 342,pp 2-82. Dorsey, J. G. Advances in Chromatography; Giddings, J. C., Grushka E., Brown, P. R., Eds.; Dekker: New York, 1987; Voi. 27, p 167. Khaledi, M. G. Biochromatogr. 1988, 3 , 20. Armstrong, D. W.; Nome. F. Anal. Chem. 1981, 5 3 , 1662. Berthod, A.; Girard, I.; Gonnet, C. Anal. Chem. 1986, 58, 1359,
1362. Yarmchuk, P.; Weinberger, R.; Hirsch, R . F.; Cline Love, L. J. J. Chromatogr. 1984, 283, 47. Berry, J. P.: Weber, S. G. J. Chromatogr. Sci. 1987, 2 5 , 307. Borgerding, M. F.; Hinze. W. L. Anal. Chem. 1985, 5 7 , 2183. Arunyanart, M.; Cline Love, L. J. Anal. Chem. 1984, 56, 1557. Armstrong, D. W.; Stine, G. Y. J. Am. Chem. SOC.1983, 105, 2962,
6220. Horvath, C.; Melander, W.; Moinar, I. J. Chromatogr. 1976, 125, 129. Melander, W. R.; Nahum, A.; Horvath, C. J. Chromatogr. 1979, 185,
125. Melander, W. R.; Stoveken, J.; Horvath, C. J. Chromatogr. 1980, 199, 35.
Melander, W. R.; Horvath, C. Chromatographia 1982. 15, 86. Colin, H.; Krstulovic, A. M.; Gonnord, M. F.; Guiochon, G. Chromatographia 1983, 17, 9. Jandera, P. J. Chromatogr. 1984, 314, 13. Tchapla, A.; Colin, H.; Guiochon, G. Anal. Chem. 1984, 56, 621. Jandera, P. Chromatographia 1985, 19, 101. Melander, W. R.: Stoveken. J.; Horvath, C. J. Chromatogr. 1979, 185, 111. Kaiiszan. R. Quantitative Structure-Chromatographic Retention Reia tionships; Wiiey: New York. 1987;Chapter, 8. p 80. Khaiedi, M. G.; Peuier, E.; Ngeh-Ngwainbi, J. Anal. Chem. 1987, 5 9 ,
2738. Bowermaster, J., personal communication to M. F. Borgerding, June
15, 1984. Mukerjee, P. J. Phys. Chem. 1962, 66, 1733. Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. SOC.1951, 73, 373.
Dorsey. J. G.;DeEchegaray, M. T.; Landy, J. S. Anal. Chem. 1983. 5 5 , 924.
Armstrong, D. W.; Ward, T. J.; Berthod, A. Anal. Chem. 1986, 58,
579. Borgerding, M. F.; Hinze, W. L.; Stafford, L.; Fuip, G.; Hamiin, W. C., submitted for publication in Anal. Chem. Sadek, P. C.; Carr, P. W.; Bowers, L. D. LC Mag. 1985, 3 , 590. Yarmchuk, P.; Weinberger, R.; Hirsch, R . F.; Cline Love, L. J. Anal. Chem. 1982, 54, 2233Pramauro, E.; Peiizzetti, E. Anal. Chim. Acta 1983, 154, 153. Attwood. D.: Florence. A. T. Surfactant Svstems:. Chaoman & Hail: .
New York, 1983;p 241. Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York. 1978.
Anal. Chem. 1988,60,2527-2531 (35) Nakagawa, T. Nonionic Surfactants; Shick, M. J., Ed.; Dekker: New York. 1966. (36) Pelizzetti, E.; Pramauro. E. J . Phys. Chem. 1984, 88, 990. (37) Pramauro, E.; Saini, G.; Pelizzetti, E. Anal. Chlm. Acta 1984, 166, 233. (38) Bunton, C. A.; Sepulveda, L. J . Phys. Chem. 1979, 8 3 , 680. (39) Sepulveda, L.; Lissi, E.; Quina, F. Adv. ColloM Interface Sci. 1986, 25, 1-57 and references therein. (40) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biologlcal Membranes; Wiley: New York, 1980. (41) CRC Handbook of Chemistryand Physics, 59th ed. Weast, R. C., Ed.; CRC: Boca Raton, FL 1978.
2527
(42) The Merck Index, 9th ed. Windholz, M., Ed.; Merck: Rahway, NJ, 1976. (43) Sutton, C.; Calder, J. A. J . Chem. Eng. Data 1975, 2 0 , 320. (44) Chan, A. F.; Evans, D. F.; Cussler, E. L. AIChEJ. 1976, 22, 1006. (45) Shaeiwitz, J. A. Chem. Eng. Commun. 1987, 55, 225.
RECEIVED for review April 11, 1988. Accepted August 1, 1988. The authors thank the R. J. Tobacco cO.$ Winston-Salem, NC, for their generous support of this research.
Solvent Extraction of Lanthanoid Picrates with Crown Ethers: Preferential Sandwich Complexation and Unique Cation Selectivities Kazuharu Nakagawa and Shouhichi Okada Leather Research Institute of Hyogo Prefecture, 3 Higashikawara, Nozato, Himeji, Hyogo 670, Japan Yoshihisa Inoue* and Akira Tai Basic Research Laboratory, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671-22, Japan Tadao Hakushi Department of Applied Chemistry, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671 -22, Japan
Quantltatlve solvent extractions of aqueous lanthanoid plcrates with 15-crown-5 and 18crown-6 were conducted at low ionic strength In the absence of background salts. An overwheiming preference for the sandwlch complexation and unique cation selectivities were observed. The peak extraction constants were found for samarium with 15-crown-5 (1:2 stoichiometry) and for cerlum and praseodymium with 18crown-6 (1:l and 1:2 stolchiometrles, respectively). The facile sandwich complexation and unique cation seiectivlties are interpreted in terms of the heavy hydration of ianthanoid ions of high charge density.
Possessing close similarity in the chemical properties, the elements of the lanthanoid family are difficult in general to separate from each other (1). The crown ethers and the related macro(bi)cyclic ligands are known to recognize fairly strictly the size of the guest cation accommodated in the cavity (2), and are expected to discriminate the lanthanoid cations through the minimal difference in cation size owing to the lanthanoid contraction. Besides the earlier studies on the isolated lanthanoid complexes of the macrocyclic ligands (3), works on solvent extraction (4-14), as well as homogeneousphase complexation (15-20), have recently been conducted under a wide variety of conditions including varied solvents, ligands, counteranions, and ionic strengths. Some solvent extraction studies on the lanthanoids show apparent contradictions about the complex stoichiometry and the cation-selectivity sequence. In all preceding experiments, the lanthanoid picrates were prepared in situ and the aqueous phase contained a large excess of lithium salt as a background salt maintaining constant ionic strength. It has been demonstrated however that not only the concentration but also the counteranion of the background salt affect significantly 0003-2700/88/0360-2527$0 1.50/0
the extractability of aqueous metal picrates (21, 22). Furthermore our recent investigation (23) reveals that simple crown ethers extract aqueous lithium picrate in a comparable order of magnitude as lanthanoid picrates. In order to investigate the solvent extraction of lanthanoid picrates unaffected by the dense background salt, we first synthesized and isolated a series of lanthanoid picrates as crystal (24). In the present paper, we report our results of a quantitative solvent extraction study that uses simple crown ethers and aqueous lanthanoid picrates of low ionic strength and shows unique extraction behavior different from the previous reports.
EXPERIMENTAL SECTION Reagents and Instruments. Commercially available picric acid (Nakarai), lanthanoid carbonates and oxides (Mitsuwa, Nakarai, Wako, or Rare Metallic Co.), 15-crown-5(Nisso), and 18-crown-6(Nisso) were used without further purification. The lanthanoid picrates of La-Gd (undecahydrate) and of Dy and Yb (octahydrate) were prepared as reported previously (24). Similar procedures gave terbium and holmium picrate octahydrates in 84% and 73% yield, respectively. Tb(Pic),-8HzO:decomposition point, 295 "C (explosion point (ep) 351 "C). Anal. Calcd for TbCl8HZ2N9Oz9: C, 21.90; H, 2.25; N, 12.77. Found: C, 22.24; H, 2.16; N, 12.76. Ho(Pic),: decomposition point, 291 "C (ep 357 "C). Anal. Calcd for C, 21.76; H, 2.23; N, 12.69. Found: C, 22.34; H, 1.99; N, 13.67. Deionized water and distilled dichloromethane were used throughout the study. Electronic spectra were recorded on a Hitachi 228 spectrophotometer. Inductively coupled plasma (ICP) atomic emission analyses were performed on a Shimadzu GVM lOOP instrument, which was calibrated for each lanthanoid as reported (24). Proton NMR spectra were recorded on a Jeol JNM-GX400 instrument in dichloromethane-d, (Merck, 99.5% deuteriated) solution containing 2 % chloroform added as an internal standard for field calibration and intensity normalization. Extraction. The general procedures were analogous to those employed in the previous papers (25, 26). The solvents, dichloromethane and water, were saturated with each other prior 0 1988 American Chemical Society
