2409
Anal. Chem. 1993, 65, 2489-2492
Migration Behavior of Phthalate Esters in Micellar Electrokinetic Chromatography with or without Added Methanol Sahori Takeda,' Shin-ichi Wakida, Masataka Yamane, Akinori Kawahara, and Kunishige Higashi Department of Material Chemistry, Government Industrial Research Institute, Osaka, Midorigaoka 1-8-31, Ikeda, Osaka 563, Japan
Selected phthalate esters including priority pollutants were analyzed by micellar electrokinetic chromatography (MEKC). Addition of methanol (20%, v/v) to aqueous migration buffer solution containing 0.05 M sodium dodecyl sulfate (SDS) improved the resolution of their separation. To investigatethe migration behavior of the phthalate esters, we calculated distribution coefficients in micellar solubilizationof the phthalate esters from the analytical data of MEKC. A linear relationship between the logarithms of the measured distribution coefficients and those of octanolwater partition coefficients reported in the literature was observed with both SDS solution and methanolmixed solution. Using this relationship, the migration times of phthalate esters were estimated from their octanol-water partition coefficients. Enthalpy and entropy changes resulting from their micellar solubilization were also evaluated in order to investigate the distribution mechanism of the phthalate esters. The enthalpy changes decreased with an increase in the alkyl chainlengthof thephthalateesters with both SDS solution and methanol mixed solution. On the other hand,the trends of theentropy changes with alkyl chain length were different for the two solutions. These results suggest that the solutesolvent interaction is different for the two solutions. INTRODUCTION Micellar electrokinetic chromatography (MEKC) is a new analytical method which can provide high resolution.1.2 This method is performed with the same apparatus as capillary electrophoresis although its separation principle is based on chromatography, that is, the difference in the distribution between solvent and micelles of ionic surfactants. In MEKC, it is possible to estimate distribution coefficients and thermodynamic parameters in micellar solubilization.2 For target analytes with MEKC, we chose phthalate esters. Phthalate esters are widely used as plasticizers in industry, so they are common contaminants in aquatic environment^.^ According to the US. Environmental Protection Agency (EPA) method 606,4 dimethyl phthalate (DMP), diethyl phthalate (DEP),di-n-butyl phthalate (DNBP), benzyl butyl phthalate (BBP), bis(2-ethylhexyl) phthalate (DEHP), and di-n-octyl phthalate (DNOP) are listed as the priority pollutants among phthalate esters. EPA method 606 specifies
analysis by gas chromatography with an electron capture detector; however, appropriate pretreatment is necessary. In some cases, phthalate esters are analyzed by high-performance liquid chromatography (HPLC) but the resolution is poor.6 The separation of some phthalate esters by MEKC with SDS solution has been reported.6 Six selected phthalate esters were separated, and it was pointed out that the migration order agreed with that of the logarithms of octanol-water partition coefficients. However, the resolution of phthalate esters was poor and the octanol-water partition coefficients were simply used to qualitatively explain the migration order without quantitative discussion. In this paper, we present the quantitative relationship between distribution coefficients in micellar solubilization determined by MEKC with SDS solution or methanol mixed solution and octanol-water partition coefficients reported in the literature.' Also a method of estimating the migration time of phthalate esters from this relationship is described. The thermodynamic parameters in micellar solubilization of phthalate esters are determined by MEKC with both SDS solution and methanol mixed solution. The trends of thermodynamic parameters in SDS solution or methanol mixed solutions are discussed.
EXPERIMENTAL SECTION Apparatus. MEKC was performed with a Model 270A analyticalcapillary electrophoresis system (Applied Biosystems Inc.). A fused-silica capillary tube (720 mm X 50 pm i.d.) was used as a separation tube. Both ends of the tube were dipped into carrier solution with platinum electrodes and dc voltage (5-30 kV) was applied between them. The capillary was thermostated by air coolant. Migrating solute bands were detected at 500 mm from the positive end by on-column measurementof UVabsorption (210nm). A ChromatopacC-R6A (Shimadzu) was used for data processing. Reagents. Selected phthalate esters, DMP, DEP, DNBP, BBP, DEHP, DNOP, and diisobutyl phthalate (DIBP) were obtained from Wako Pure Chemicals. Di-n-propyl phthalate (DNPP),diisopropyl phthalate (DIPP),and di-n-amylphthalate (DNAP) were obtained from Tokyo Kasei Kogyo. Sodium dodecyl sulfate from Nacalai Tesque was used as a micelle-forming anionic surfactant. An SDS solution was prepared by dissolving SDS in a mixture of 0.02 M sodium dihydrogen phosphate solution and 0.02 M sodium tetraborate solution adjusted to pH 9.0. All reagents and solvents were of analyticalgrade and used without further purification. Procedure. Stock solutions of the phthalate esters were prepared to be 10 g/L in methanol. A standard solution of a mixture of 10 phthalate esters was made by diluting the stock solutionswith SDSsolution. The concentration of each phthalate ester was 50 mg/L, except for DEHP and DNOP, for which the concentration was 25 mg/L. Samples were injected by vacuum injection (5 in. of Hg, 0.2 8 ) throughout all experiments. The injection volume was about 1.5 nL.
~~~
(1) Terabe, S.;Otsuka, K.; Ichikawa, K.; Tsuchiya, A.; Ando, T. Anal. Chem. 1984,56, 111. (2) Terabe, S.;Otsuka, K.; Ando, T. Anal. Chem. 1986,57, 834. (3) Wolfe, N. L.; Burns, L. A.; Steen, W. C. Chemosphere 1980,9,393. (4) Keith, L. H.; Telliard, W. A. Enuiron. Sci. Technol. 1979,13,416. 0003-2700/93/0365-2489$04.00/0
(5) Mori, S. J. Chromutogr. 1976, 129, 53. (6) Ong, C. P.; Lee, H. K.; Li, S. F. Y. J. Chromatog. 1991,542,473. (7) Leyder, F.; Boulanger, P. Bull. Enuiron. Contam. Toxicol. 1983, 30, 152.
0 1993 Amerlcan Chemlcal Society
2490
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
mechanism of separation is almost maintained, as evidenced by the experimental data and from the following discussion. Relationshipof Distribution Coefficients to OctanolWater Partition Coefficients. To investigate the distribution of phthalate esters with both SDS solution and methanol mixed solution, the distribution coefficients in micellar solubilization were calculated from analytical data as follows.2 The capacity factor &' can be calculated from migration time of water to, solute t ~and , micelle t,, by
BBP
I
k'
I
0
tmc
\:
20
j10
30
I
160
50
40
,
Time/min 1
0
:
I
I
1
1
1
2
I
5
10
i?
2
I
t
I 1
0 / p 50 100
Figure 1. Chromatogramsof phthalate esters in MEKC wlth methanol mixed solution. Conditions: micellar solution, 0.05 M SDS in 0.02 M borate-phosphate buffer solution(pH9.O)-methanol(8020, v/v); applied voltage, 20 kV; temperature, 38.9 OC; detection wavelength, 210 nm.
RESULTS AND DISCUSSION Separation of the Phthalate Esters. When 0.05 M SDS solution was used, the phthalate esters migrated in the order DMP, DEP, DIPP, and DNPP, followed by the other phthalate esters, all within 20 min (applied voltage, 20 kV; temperature, 38.9 "C).8 This order is similar to that in GLC,Q since the separation mechanism in MEKC is based on distribution between the solvent and the micelle.lP2J0 Theoretical plate numbers of these peaks estimated from uncorrected peak widths were 100 000-180 000 in this MEKC separation. DMP, DEP, DIPP, and DNPP may be partially distributed into the micelle, but the other phthalate esters may be mostly distributed. The distribution must be better controlled in order to separate more phthalate esters. Accordingly, we used a mixture of methanol with SDS solution in order to reduce the distribution of phthalate esters into the micelle.10 Addition of methanol (20%,v/v) to 0.05 M SDS solution gave better resolution in the separation of the phthalate esters. The chromatogram is shown in Figure 1. In addition to DNPP and DIPP, the separation of another pair of alkyl isomers, DNBP and DIBP, was achieved. BBP and DNAP were also separated. This order is also similar to that in GLC.9 The theoretical plate numbers of these peaks estimated from uncorrected peak widths were 170 000-200 OOO in this MEKC separation, but the migration time became longer (ca. 60 min). This phenomenon is caused by a decrease in the electroosmotic velocity.1° The influence of organic solvents on the electroosmotic velocity has been studied in the literature;" that is, the electroosmotic velocity and p potential decrease with increasing content of organic solvent and this trend is explained by changes in the dielectric properties of the electric double layer and of the charge generation on the fused-silica surface. Also, the influence of methanol on SDS micelle formation has been studied by light scattering measurements.12 At a mole fraction of methanol of 0.12 (about 23% methanol (v/v)),the aggregation number of SDS micelles is lower than that in pure water. But the (8) Takeda, S.; Wakida, S.; Yamane, M.; Kawahara, A.; Higashi, K. Anal. Sci. 1991, 7 , 1113 (Suppl). (9) Ishida, M.; Suyama, K.; Adachi, S. J. Chromatogr. 1984,294,339. (10) Otsuka, K.; Terabe, S.; Ando, T. Nippon Kagaku Kaishi 1986, (7), 950. (11) Schwer, C.; Kenndler, E. Anal. Chem. 1991,63, 1801. (12) Parfitt, G. D.; Wood, J. A. Kolloid 2.2.Polym. 1969, 229, 55.
=
tR
-
(1) tO(1 - tR/tmc) Experimentally, to and ,& ,, were measured with methanol and the overlapping peak of DEHP and DNOP. Under the conditions in which the solution containing 20% methanol was used, it is difficult to choose a marker for the micelle. Sudan I11 gave more than one peak under these conditions, and the migration time of the last peak agreed with that of the peak of DEHP and DNOP. Therefore, as an approximation, the velocity of the micelle is equal to that of DEHP and DNOP. As described ref 2, the capacity factor & decreased almost linearly with an increase in the electroosmotic velocity urn, as the applied voltage was varied over the range of 5-30 kV. The cause of the phenomenon is the rise of temperature within the capillary _dueto joule heating. Consequently, each line of the plot of k' vs ueowas extrapolated to the intercept a t u, = 0, and the value of &' a t u, = 0 was assumed to be the &' a t the temperature of the air flowing around the capillary. These corrected capacity factors were used in the calculations that appear below. The capacity factor &' can be related to the distribution coefficient K,2 I
KD(c, - cmc) (2) where D is partial specific volume of micelle, c ~ is f the concentration of surfactant, and cmc is the critical micellar concentration. The plots of the corrected capacity factor vs SDS concentration showed good linearity from 0.03 to 0.15 M (Five concentrations were used, and the correlation coefficientswere over 0.99,). The distribution coefficientwas calculated from the slope of the plots of &' vs the concentration of SDS. The value of 0 was calculated from literature data13 at the experimental temperature. In environmental analysis, the octanol-water partition coefficient P is an important parameter used to estimate the behavior of hazardous chemicals in the environment. Therefore, we compared the calculated distribution coefficients in micellar solubilizationto octanol-water partition coefficients. The octanol-water partition coefficients in the literature' were measured a t 20 "C, so the distribution coefficients at 20 "C were estimated from the variation of K with temperature.2 The variation of K with temperature is as follows (the van't Hoff equation): k'
l n K = - - AHo ASo (3) RT+R where AHo and ASo are the thermodynamic parameters enthalpy change and entropy change in micellar solubilization of the phthalate esters. R is the gas constant and T is the absolute temperature. The van't Hoff plots of experimental data showed good linearity over the range of experimental temperatures (the setup temperatures were 26.7, 38.9,51.1, and 63.3 OC, and the correlation coefficients were over 0.98). The distribution coefficients at 20 OC were evaluated by extrapolating these plots. (13) Shinoda, K.; Soda, T. J. Phys. Chem. 1963,67,2072.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
12
Table I. Calculated and Observed Migration Times of Phthalate Esters with SDS Solution. tR(obs)/min error/ % solute tR(calc)/min
DNAPo
DMP DEP DIPP DNPP
9 DIPP,
14.6 19.9 21.2 21.8
14.5 19.3 21.5 21.7
0.7 3.1 -1.4 0.5
Solution, 0.05 M SDS (pH 9.0); applied voltage, 20 kV; tamperature, 26.7 "C. to(obs) = 5.76,t,,(obs) = 22.2.
Y
-C
Table 11. Calculated and Observed Migration Times of Phthalate Esters with Methanol Mixed Solution. solute tR(calc)lmin tR(obs)/min error/ %
6
D
3
2491
w
M
v
DMP DEP DIPP DNPP DIBP DNBP BBP
~ I
I 1
I
3
5
log P Figure 2. Plots of logarithmsof the distributlon coefficients In micellar solublllzation vs logarithms of octanol-water partklon coefficients of phthalate esters at 20 "C with SDS solution (a) and methanol mlxed solution (20%, v/v) (b).
Logarithmicplots of the distribution coefficientsvs octanolwater partition coefficients for both SDS solution and methanol mixed solution are shown in Figure 2. These plots for both solutions show good linearity except for DNAP. This result suggests that the mechanism of separation is almost the same in both two solutions. With regard to DNAP, it migrates at almost the same speed as the micelle, so the calculated value of K for DNAP may include a relatively larger error than the values of K for other phthalate esters. Also, according to ref 7, the reported value of the octanol-water partition coefficient of DNAP is doubtful because of the impurity of the reagents. By use of these linear relationships between the distribution coefficients and octanol-water partition coefficients, the migration time of phthalate esters may be estimated from their octanol-water partition coefficients, P. These linear relationships can be expressed as In K = a log P + b (4) Where a and b are constants that can be determined from the observed data. Substitution of eqs 1 and 2 into eq 4 gives
t, =
t,
exp(a log P + b + c ) + to 1 + exp(a l o g P + b + c )
(5)
where
+
c = ln@(c,, - cmc)} ln(to/tmc)
The dependence of k' on u, is not considered here. Actually, the migration orders of other substances sometimes change with electroosmotic velocity.2 To determine the values of a and b, at least two data sets of log P and t R are necessary. In the following estimation, we have provisionally used the values calculated from the plots shown in Figure 2. The calculated values of t R from the literature values of log P for the phthalate esters are shown in Tables I (with SDS solution) and I1 (with methanol mixed solution). They are nearly equal to the observed values. The octanol-water partition coefficients were measured at 20 "C, and the distribution coefficientsat 20 "Cwere estimated from the dependence of K on the temperature as described above. However, to, tm,, and observed values of migration time at 26.7 "C were used because the Model 270A cannot control
17.2 30.1 42.0 53.8 70.5 75.3 77.4
16.5 26.5 44.3 49.2 68.7 71.3 73.9
4.2 12.0 -5.5 8.6 2.5 5.3 4.5
Solution, 0.05 M SDS (pH 9.0)-methanol (80:20,vlv); applied voltage, 20 kV; temperature, 26.7 "C. to(obs) = 9.96,t,(obs) = 80.8.
Table 111. Enthalpy and Entropy Changes in Micellar Solubilization with SDS Solution AH"/kJ ASo/J AH"/kJ As"/J solute mol-' mol-' K-l solute mol-' mol-' K-' DMP DEP
-18.7 -19.7
-21.4 -12.9
DIPP DNPP
-20.6 -21.8
-3.3 -4.2
Table IV. Enthalpy and Entropy Changes in Micellar Solubilization with Methanol Mixed Solution (20%, v/v) AH"/kJ ASo/J AHolkJ Aso/J solute mol-l mol-' K-l solute mol-' mol-' K-' DMP DEP DIPP DNPP
-13.0 -19.4 -21.9 -24.1
-17.1 -28.4 -26.9 -31.3
DIBP DNBP BBP DNAP
-33.4 -37.5 -45.1 -77.7
-49.3 -60.1 -81.9 -168.3
the internal temperature at values below the room temperature + 5 "C.If this calculation is applied to other conditions or substances, it should be noted that the constants a and b may be different from those of this case. Thermodynamic Parameters. The thermodynamic parameters, enthalpy change AH" and entropy change AS", in micellar solubilizationof the phthalate esters can be estimated from the dependence of K on the temperature as described above. The values of enthalpy and entropy changescalculated from eq 3 are listed in Tables I11 (for SDS solution) and IV (for methanol mixed solution). All of the enthalpy changes in Tables I11and IV are negative and decrease with an increase in the alkyl chain length of phthalate esters. The value of the n isomer is smaller than that of corresponding i isomer for the two pairs of isomers. Therefore, in both solutions, the phthalate esters distributed in the micelle are more advantageous with regard to enthalpy than those distributed in the solvent. The phthalate esters having longer alkyl chains or n isomer are more advantageous with regard to enthalpy in the distribution of the micelle. All of the entropy changes are also negative with both solutions. The phthalate esters distributed in the micelle are less advantageous with regard to entropy than those distributed in the solvent. While the entropy changes increased with an increase in alkyl chain length with SDS solution, they decreased with an increase in alkyl chain length with methanol mixed solution. However, the value of the n
2492
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
isomer is smaller than that of the i isomer with both solutions, that is, the n isomer is less advantageous with regard to entropy. This difference in the variation of entropy change with alkyl chain length for the two solutions can be explained as follows. The entropy changes obtained from these experiments are the values of the total system and can be divided into the entropy changes of the phthalate esters themselves and that of the surrounding solvents.14 The entropy changes of phthalate esters themselves decreased with an increase in alkyl chain length because the longer the alkyl chains they have, the more limited their motions are in the micelle. The entropy changes of surrounding solvents increased with an increase in alkyl chain length because the longer the alkyl chains the phthalate esters have, that is, the stronger their hydrophobicity is, the further the %ystalline" structure made by the solvent molecules extends when the phthalate esters are in the solvent.16Jg In the case of SDS solution,the entropy changes of solvents contribute more than those of the phthalate esters themselves to the entropy changes of the total system. On the other hand, in the case of methanol mixed solution, the entropy contribution of the "crystalline" structure of the solvent molecules is weakened by the presence of methanol. Therefore the entropy changes of the phthalate esters themselves contribute more than those of the solvent to the entropy changes of the totalsystem. As for the isomers, it is believed that the entropy changes of solvents are about the same but the entropy changes of phthalate esters themselves are different from each other. The reason may be that both the n isomer and the SDS molecule have straight alkyl chains. The thermodynamic data for homologous seriessometimes suggest that variations in enthalpies are opposed to variations in entropies in such a way that they compensate for each other on the free energy. In other words, plots of enthalpy changes vs entropy changes often form straight lines with a positiveslope. This is sometimes called the enthalpy-entropy compensation effect.17J8 This effect has been found in a wide variety of processes and reaction equilibria, and it has been suggested that the cause of the phenomenon is solute-solvent (14) Terabe, S., personal communication. (15) Tamaki, K. Hyoumen 1966,3,527. (16) Frank, H.S.; Evans, M.W . J . Chem. PhYS. 1946,13,507. (17) Krug, R. R.Znd. Eng. Chem. Fundom. 1980,19,50. (18) Takeyama, N.;Nakashima, K. Nippon Kagaku Kaishi 1987, (4), 610.
r
Y
-
r
E"
-J
2-
\
-1801
-80
-60
-40 AH"/ kJ mol-'
-20
Flguro 3. Plots of enthalpy changes vs entropy changes In mlcellar solubilkatlon with SDS soluHon (a) and methanol mlxed solution (20 %, vlv) (b).
interaction in many cases.17 According to the solutesolvent interaction theory, the enthalpy-entropy compensation effect is explained as follows. When the enthalpy of a solute is decreased by solvation, simultaneously the freedom of the solute is reduced; that is, the entropy of the solute is also decreased. Plots of enthalpy changesvs entropy changes obtained from the analytical data of present experimenta are shown in Figure 3. While the compensation effect is found with methanol mixed solution, it is not found with SDS solution. The explanation of the enthalpy-entropy compensation effect described above does not take intoaccountthe entropy change of the solvent. Therefore, the experimental data suggest that the enthalpy-entropy compensationeffect is valid only under the condition that the parameters of the solvent are negligible. In the case of SDS solution, the entropy changes of solvents contribute to a greater extent than when solutions containing methanol are employed, as mentioned above. Therefore the enthalpy-entropy compensation effect is not observed. These considerations indicate that the entropy changes in the case of SDS solution can be explained by the entropy changes of the solvents.
RECEIVEDfor review August 11, 1992. Accepted June 5, 1993.
