798
Anal. Chem. 1992, 6 4 , 798-801
D d* k
diffusion coefficient Stokes diameter Boltzmann constant M molecular weight T absolute temperature to void time tr retention time U crossflow velocity 0 flow rate of channel stream flow rate of crossflow stream void volume W channel thickness Greek Characters viscosity p:2 variance due to polydispersity x retention parameter Registry No. Isotactic polypropylene (homopolymer),
Giddings, J. C. Chem. Eng. News 1988, 66, 34-45. Caldweii. K. D.Anal. Chem. 1988, 60, 959A-971A. Glddings, J. C. J. Chromatogr. 1989, 470, 327-335. Giddings, J. C.; Lin, G. C.; Myers, M. N. J. L i q . Chromatogr. 1978, 1 , 1-20. ~~
Wahlund, K.-G.; Winegarner, H. S.;Caldwell, K. D.; Gwings, J. c. Anal. Chem. 1988. 58. 573-578. Sourirajan, S.; Matsuura, T. Reverse Osmosis/UtYra~~tion Process Principles ; National Research Countii Canada Publications: Ottawa, 1985.
Goddard, E. D.; Vincent, B. Polymer Adsorption and Dispersbn Stablllty;ACS Symposium Series 240; American Chemical Sockty: Washington, DC. 1984. Danieiii, J. F.; Rosenberg, M. D.; Cadenhead, D. A. Progress In Surface and Membrane Science; Academic Press: New York and London, 1971; Voi. 4. Napper, D. H. Polynieric Stabiiizatlon of Colloidal Dispersions ; Academic Press: New York, 1983. de Gennes. P.G. Scaling Concepts In Polymer Physics; Cornell Unlversity Press: Ithaca, NY, 1979. Caldweil, K. D.; Brimhail, S. L.; Gao, Y.; Gkklings, J. C. J. Appl. Polym. Sci. 1888, 36. 703-719. Ratanathanawongs, S. K.; W i n g s , J. C. Anal. Chem. 1982, 64,
$
25085-53-4.
I
6-15.
REFERENCES
I 1
(1) Bekturov, E. A.; Bakauova, 2. Zh. Synthetic Water-Soluble Polymers in Solutbn; Huethig & Wept Basei, 1986. (2) Molyneux. P. Water-Soluble Synthetlc Polymers: Propertles and Behavior; CRC Press: Boca Raton, FL, 1984; Vois. I and 11. (3) Dubin, P. L.; Principi, J. M. Macromolecules 1989, 22, 1891. (4) Rollings, J. E.; Bose, A.; Caruthers, J. M.; Tsao, G. T.; Okos, M. R. I n Polymer Charactekation; C r a w , C. D., Ed.; Advances in Chemistry Series 203; American Chemical Society: Washindon. DC, 1983; Chapter 18. (5) Sorb, V.; Campos, A.; Gar&, R.; Parets, M. J. J. L l q . Chromatogr. 1990. 73. 1785-1808. ..., (6) Gkklings, J. C: In-Advances in Chromatography; Glddings, J. C., Grushka, E.; Cazes, J., Brown, P. R., Eds.; Marcel Dekker: New York, 1982; Vol. 20, Chapter 6. (7) Glddings, J. C. J. Chromatogr. 1978, 125, 3-16. (8) Giddlngs, J. C.; Kucera, E.; Russell, C. P.; Myers, M. N. J. Phys. Chem. 1988, 72, 4397-4408. (9) Barth, H. G.; Carlin, F. J., Jr. J. L l q . Chromatogr. 1984, 7 , 1717-1 738. (10) GMdings, J. C. Anal. Chem. 1981, 53, 1170A-1175A. (11) Glddings, J. C. Sep. Scl. Technol. 1984, 79, 831-847
..
8
Wahlund, K.-G.; Giddings, J. C. Anal. Chem. 1987, 59, 1332-1339. Glddings, J. C.; Yang, F. J.; Myers, M. N. Anal. Chem. 1978, 48, 1126-1132.
Tanford, C. Physical Chemistry of Macromolecules; Wiley: New York,
1961. Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. Morawetz, H. Macromolecules in Solution; John Wiiey 8 Sons: New York, 1965. Giiings, J. C.; Chen, X.; Wahlund, K.-G. Anal. Chem. 1887, 59, 1957-1962. Yang, F. J.; Myers, M. N.; W i n g s , J. C. Anal. Chem. 1977, 49, 659-662. Schimpf, M. E.; W i n g s , J. C. Macron7o/ecu/es 1987, 20, 1561-1563. Schimpf, M. E.; Myers, M. N.; W i n g s , J. C. J. Appl. Polym. Sci. 1887, 33, 1170. Hansen, M. E.; Gddings, J. C.; Beckett, R. J. Colloid Interface Sci. 1989, 132, 300-312.
RECEIVED for review August 5, 1991. Accepted January 2, 1992.
Migration Behavior of Inorganic Anions in Micellar Electrokinetic Capillary Chromatography Using a Cationic Surfactant Takashi Kaneta, Shunitz Tanaka, Mitsuhiko Taga,* and Hitoshi Yoshida Department of Chemistry, Faculty of Science, Hokkaido Uniuersity, Kita-ku, Sapporo 060, Japan The Interactions between lnorganlc anions and a catlonlc surfactant were lnvestlgated uslng mlcellar electroklnetlc caplllary chromatography (MECC). Uslng cetyltrlmethylammonium chlorlde (CTAC) as a catlonlc surfactant, the effectlve electrophoretlc mobllltles of lnorganlc anlons decreased due to two processes. The first was the Ion assoclatlon equlllbrla wlth a monomerlc surfactant below the crltlcal micellar concentration (cmc) and the other was partltlonlng Into the mlcelle above the cmc. Fundamental equatlons for mlgratlon of knlc specks In MECC were dehed uslng the effectlve electrophoretlc moblllty as an electrochemical parameter. Ion assoclatlon constants of lnorganlc anlons wlth catlonlc surfactant and dlstrlbutlon coefflclents Into the mlcelle were estlmated on the bask of these equatlons.
INTRODUCTION Micellar electrokinetic capillary chromatography (MECC),
which was first introduced by Terabe et al.,1,2is an electrokinetic separation technique for electrically neutral solutes. The separation mechanism is based upon the differential partitioning of analytes between an electroosmotically pumped aqueous mobile phase and an electrophoretically retarded micellar pseudophase. Because of its high efficiency and resolving power, MECC provides excellent separationsof many neutral substances, for example, the separation of phenylthiohydantoin-amino acids: aromatic sulfides? and so ~ n . ~ , ~ Also, there are some applications of MECC to ionic species, for example, catecholamine^^,^ and water-solublev i t a m i n ~ . ~ J ~ Terabe et al. investigated the migrating behavior of anionic species in MECC using an anionic surfactant, sodium dodecylsulfate (SDS), and reported that anionic species were rarely retained in the anionic micelle. However, ionic species exhibit a strong interaction with a surfactant having the opposite charge and are retained in its micelle.1° Although the unique separation selectivity for anionic species can be expected to be achieved by MECC using a cationic surfactant, there are no reports an the migration behavior of anions in a cationic surfactant system.
0003-2700/92/0364-0798$03.00/00 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 7, APRIL 1, 1992
On the other hand, in order to investigate the interaction between ionic species and a charged surfactant in detail, it is easential to derive fundamental equations which represent the migration of ionic species in MECC. Recently, Ghowsi et al. reported a theoretical study for MECC based on electrochemical parameters." However, only neutral species were discussed in their work. Terabe et al. investigated the migration behavior of ionic species in an anionic micellar solut i ~ n , but ~ J ~only the capacity factors for ionic species were reported. In this paper, fundamental study of the migration behavior of inorganic anions in the presence of a cationic surfactant was carried out by MECC using a cationic surfactant, cetyltrimethylammonium chloride (CTAC). An interesting separation selectivity and a high resolution based on the interaction between anionic species and a cationic surfactant were observed in this migrating system. To investigate the interaction with cationic surfactant in detail, fundamental equations representing the migration of anionic species were derived. By using an effective electrophoretic mobility as an electrochemical parameter, the influences of electroosmotic velocity could be eliminated. On the basis of the derived equations, the capacity factors for inorganic anions were calculated and the ion association constants with monomeric surfactant and the distribution coefficients into the micelle were estimated.
THEORY The net electrophoretic velocity of ionic species, uep, and the electroosmotic velocity, u,, in an electric field, E, are given
as uep
=p
e p
(1)
ueo
=p
e J
(2)
where pepand peo are the electrophoretic and the electroosmotic mobilities, respectively. The observed migration velocity of an ionic specie, Uob, is the summation of them uob
= uep
+ ueo = (pep + peo)E
(3)
In the presence of a cationic surfactant, the electrophoretic velocity of an anion will be influenced either by ion association below the critical micellar concentration (cmc) or by partitioning into the micelle above the cmc. The ion association constant between a monovalent anion and a cationic surfactant, KIA,is defined as (4)
where [A-] is the concentration of the free anion, [C'] is the concentration of the free cationic surfactant, and [AC] is the concentration of the associated anion with the cationic surfactant. In light of the fact that the free anion migrates at uep and the associated anion migrates at u, because it has no charge, the observed migration velocity below the cmc, uob, is represented with uep and u, as uob
nA-
= nA-
+ nAC
(uep
+ ueo) +
nAC nA-
+ nAC
ueo
(5)
where n A - and nACare the total moles of the free and the associated anions, respectively. Under the approximate assumption that [C+] is equal to the bulk concentration of the cationic surfactant, Csf, eq 4 is rewritten as
By substituting eq 6 into eq 5, we obtain
700
The effective electrophoretic velocity, ueff, and the effective electrophoreticmobility, p&, for an ionic species canbe defined as
The observed migration velocity, uob, can also be represented with ueff and peff as
= ueff + ueo = b e f f + peo)E By substituting eq 10 into eq 7, we can obtain uob
(10)
Equation 11 can also be rewritten by using the effective electrophoretic mobility as IA 1 =K -c,,+ -
1 -
Weff
Pep
pep
Equation 12 shows that the plots of l/peff vs Csf give linear relationships below the cmc, and the ion association constant between the anion and the surfactant can be obtained from the slope and the intercept. The concentration of the monomeric surfactant above the cmc is constant and is the same as that of the solution at the cmc,13 Therefore, the effective electrophoretic velocity and mobility of anion in the aqueous solution above the cmc will be also constant and the same as the electrophoretic velocity, ueff(cmc),and mobility, P ~ ~ (at~the ~ cmc, ) , respectively. On the other hand, the electrophoretic velocity of the anion incorporated in the micelle can be assumed to be the same as that of the micelle, I&. Then, uob in the solution above the cmc will be presented as
(13)
where naqand nmcare the total moles of the anions in the aqueous phase and in the micelle, respectively. In the case of a neutral solute the electrophoretic velocity is zero, and uob is given by
By using the capacity factor, k' = nmc/naq, eq 13 can be shown as
By rearrangement of eq 15 using the effective electrophoretic velocity and mobility, k'can be expressed as
k' =
Ueff(cmc) ueff
- "eff
- Vmc
- Peff(cmc) - Peff Neff
- Pmc
(16)
Equation 16 also shows that the relation between the effective electrophoretic velocity and capacity factor is not affected by the electroosmotic velocity. The capacity factor for a neutral species can be given as follows because the electrophoretic velocity for neutral species in aqueous phase is zero
800
a
ANALYTICAL CHEMISTRY, VOL. 64, NO. 7, APRIL 1, 1992
The relationship between k ' and the distribution coefficient, K , can be approximated as follows2 k ' = KO(CSf- cmc) (18) where is the partial specific volume. Then the distribution coefficient for an inorganic anion can be determined from plotting k' obtained by eq 16 as a function of CSf. EXPERIMENTAL SECTION Apparatus. An experimental equipment for capillary electrophoresis was build according to the literature.'J4 Fused-silica capillary tubes with 50 fim i.d. X 500 mm, were obtained from Gasukuro Kogyo (Tokyo, Japan). A high-voltage power supply, a Model HCZE-30PN0.25 (Matsusada Precision Devices, Shiga, Japan) was used for applying the voltage. A variable-wavelength absorbance detector, a Model CV4(ISCO,Inc., Lincoln, NE), was utilized to measure the absorbance at 210 nm. The detection was carried out by measuring the absorbance on the column at a position 20 cm from the positive end of the capillary tube. A new capillary was flushed with 0.1 M potassium hydroxide solution overnight before use. After rinsing with water, the capillary was filled with a buffer solution without CTAC and stood for 1 h. Then the capillary was filled with an operating buffer containing CTAC. A sample was injected by moving the injection end of the capillary to the sample reservoir and raising it above the other end for 5 s. The injection volume was about 0.6 nL for the injection time of 5 s. The electroosmotic velocities of the bulk solution and the electrophoretic velocities of micelles were evaluated from the measurement of the methanol peak, which is insoluble in the micellar phase, and the Sudan I11 peak, which is completely soluble in it, re~pectively.~J~ The mixture of 4 mM bromide and bromate, 2 mM iodide and iodate, and 1mM nitrate in Sudan 111-saturated methanol was used as a sample to investigate the migration behavior of these anions. The directions of both electroosmotic flow and electrophoretic migration of the CTAC micelle were the opposite of those in SDS solution. The direction of the electroosmotic flow was from the cathode to the anode, and that of the electrophoretic migration of the CTAC micelle was from anode to cathode. The direction migrating from cathode to anode was defined to be positive in this paper. Materials. All reagents were of analytical grade and used without further purification. Cetyltrimethylammonium chloride (CTAC) was obtained from Wako pure chemicals (Osaka, Japan). Standard solutions of inorganic anions were prepared by dissolving potassium salts with water. The background electrolyte solution was prepared as follows: the required amounts of CTAC were dissolved with water, and then 1.0 mL of 1 M potassium dihydrogen phosphate was added to the solution. The pH of the solution was then adjusted to 7.0 by adding 1M tris(hydroxymethy1)aminomethane(Tris);finally, the solution was diluted to 50 mL. RESULTS AND DISCUSSION If the inorganic anions have some interaction with a cationic surfactant, the migration behavior of ionic species in the solution containing surfactant will be different from that in a buffer solution which is used in capillary zone electrophoresis. Chromatograms of the mixture of several inorganic anions in the CTAC solution at the various concentrations are shown in Figure 1. The elution order in Figure lA, where the CTAC concentration is below the cmc (cmc = 1.0 mM), is bromide < nitrate < bromate = iodide < iodate. This order is in accord with the order of their molar conductivities (bromide (78.4 S cm2mol-'), nitrate (71.4 S cm2mol-'), bromate (55.8 S cm2mol-'), iodide (76.9 S cm2mol-'), iodate (41.0 S cm2 mol-') a t 25 OC16), that is, their absolute mobilities except for iodide ion. Iodide ion is retarded largely by ion association equilibria with a monomeric surfactant even below the cmc. The unique separation selectivity was observed in the micellar solution as shown in Figure 1C. The elution order, iodate < bromate < bromide < nitrate < iodide, is completely
4
B
?
Y 1
0
1
1
2
-
1
1
4
'
I
0
2 4 0 2 4 Timelmin Figure 1. Chromatograms of inorganic anions at various concentrations of CTAC: (A) 0.2 mM; (B) 1.0 mM; (C) 25 mM. Key: (1) bromide, (2) nitrate, (3) bromate, (4) iodide, (5) iodate. Conditions: buffer, 20 mM phosphate-Trls buffer, pH 7.0; applied voltage, 15 kV; temperature, 22 OC. The apparent migration velocities (mm s-') are (A) vR = 3.23, vw = 2.98, vMa = 2.62, vI = 2.62, vI4 = 2.20, v, = 1.07, (B) vBr= 3.13, vNo, = 2.86, vm0, = 2.63, v I = 2.14, vI0, = 2.27, v, = 1.14, and (C) vB = 1.91, vWs = 1.57, vM, = 2.12, v I = 1.00, vIo, = 2.48, v, = 1.51.
* I y j 7
/
P I
-1
0
1 2 [CTAC] 1 mM
I i
3
Flgure 2. Dependence of the reciprocal of the effective mobility of iodide and nbate on the concentrations of CTAC. Conditions are given in Figure 1.
different from that below the cmc. In order to investigate the interaction between inorganic anions and a cationic surfactant by MECC, the effect of CTAC concentrations on the mobilities of inorganic anions should be examined. However, the electroosmotic mobility varied depending on the CTAC concentration and preconditioning of capillary, and it has a poor reproducibility. Then the observed migration velocity cannot be used to evaluate a true influence due to interaction between anionic species and a cationic surfactant. On the other hand, the use of effective electrophoretic mobilities of inorganic anions enabled the evaluation of the interaction without influences of the electroosmotic mobility as described above. I t is predicted from eq 12 that l/Meff is in proportion to CBfand the slope and intercept give the association constant. Actually, the plots of l/Meff vs CSfbelow the cmc show the linearity as shown in Figure 2. The intercepts indicate the mobilities in the buffer solution without CTAC. The order of the values is in accord with the absolute mobilities except for iodide. The ion association constants for various inorganic anions calculated from the slopes and the intercepts are summarized in Table 1. The ion association constant of iodate is not shown in this table because the slope was a negative value. This is due to the very weak interaction of iodate with CTAC. The slope
ANALYTICAL CHEMISTRY, VOL. 64, NO. 7, APRIL 1, 1992
Table I. Ion Association Constants of Inorganic Anions
solute
ion assocn c0nstlM-l
solute
ion assocn const/M-’
bromide bromate
93 38
nitrate iodide
140 760
for iodate is really very small, and the value is near zero. The anion selectivity order found on this study is iodide > nitrate > bromide > bromate > iodate. This order is similar to the selectivity order of typical ion interaction chromatography (IIC) with tetraalkylammonium ~ a l t s . ~ ’Though J~ the separation mechanism of IIC is not simple, the anion selectivity order can be considered the order of the interaction between the tetraalkylammonium ions and anions. Ito et al. have reported the separation of inorganic anions by IIC with cetyltrimethylammonium, and their results are also in accordance with ours.19 The plot of l/peffvs Csf for the iodide ion in Figure 2 can be divided into two lines below and above 1 mM. The inflection point seems to give the cmc of CTAC under this experimental condition. The CTAC concentration at the reflection point agrees with a literature value of the cmc.20 Above the cmc, the effective mobilities of inorganic anions decreases due to partitioning to the micelle according to the eq 16. If eq 16 is solved for l/peff,the resulting equation is 1+k’ -1- (19) peff kbmc + peff(cmc) The linearity for iodide above the cmc can be explained as follows. At near the cmc, k’ is much smaller than unity so that we can consider k ’ h c nitrate > bromide > bromate > iodate. These values are much smaller than the values determined by micelle exclusion chromatography,22and the order bromide > nitrate differs
801
Table 1 1 Distribution Coefficients of Inorganic Anions”
solute
distrib coeff
bromide bromate
180 86
solute
distrib coeff
nitrate 270 iodide 880 iodate 11 “The partial specific volume of CTAC used is the value determined by Armstrong and Stine, 0.977 mL &.*l from our results. These differences seem to be ascribed to the coexisting ions because the values estimated in this study are virtually the conditional constants in the presence of background ions, such as phosphate and chloride. Okada also reported that foreign ions influence the retention behavior of inorganic anions in micelle exclusion chromatography.22 Mullins et al. have reported that the order of distribution Coefficients in a 10 mM phosphate buffer solution, whose experimental condition is relatively similar to ours, is nitrate > bromide in micelle ~hromatography.~~ This order is in agreement with our results. MECC using cationic surfactant not only shows the interesting separation selectivity for monovalent inorganic anions but also provides information on the ion association of inorganic anions with a surfadant monomer and the distribution into the cationic micellar phase. The method should be applicable to separation of many organic anions.
REFERENCES (1) Terabe, S.; Otsuka, K.; Ichlkawa, K.; Tsuchlya, A.; Ando, T. Anal. Chem. 1084. 56, 111-113. (2) Terabe, S.;Otsuka, K.; Ando, T. Anal. Chem. 1085, 57, 834-841. (3) Otsuka, K.; Terabe. S.; Ando, T. J. C h r m t o g r . 1085, 332,219-226. (4) Otsuka, K.; Terabe, S.; Ando, T. Nlppon Kageku Kabhl 1087, 7 , 950-955. (5) Eialchunas, A. T.; Sepaniak, M. J. Anal. Chem. 1088, 60, 617-621. (6) Bushey. M. M.; Jorgenson, J. W. Anal. Chem. 1080, 67,491-493. (7) Wallingford, R. A,; Ewing, A. G. Anal. Chem. 1088, 60, 258-263. (8) Wallingford, R. A.; Ewlng. A. G. J. Chromatogr. 1088, 441, 299-309. (9) Fujiwara, S.; Iwase, S.; Honda, S. J. Chromatogr. 1988, 447, 133-140. (10) Nishi. H.; Tsumagarl, N.; Terabe, S. Anal. Chem. 1089, 61. 2434-2439. (11) Ghowsl, K.; Foley, J. P.; Gale. R. J. Anal. Chem. 1900, 62, 27 14-2721. (12) Otsuka, K.; Terabe, S.; Ando, T. J. Chromatogr. 1085, 348,39-47. (13) Berthod. A.; Girard. I.; Gonnet. C. Anal. Chem. 1088. 58, 1356- 1358. (14) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53, 1298-1302. (15) Burton, D. E.; Sepaniak, M. J.; Maskarlnec, M. P. J. Chromatogr. Sc/. 1087,25,514-518. (16) Dobos, D. Electrochemical Data; Elsevlor: Amsterdam, Oxford, New York, 1975. (17) Cassidy, R. M.; Elchuk, S. Anal. Chem. 1982, 54, 1558-1563. (18) Iskandaranl, 2.; Pletrzyk, D. J. Anal. Chem. 1082, 5 4 , 2427-2431. (19) Ito, K.; Arlyoshi, Y.; Tanabikl. F.; Sunahara, H. Anal. C t ” . 1091, 63,273-276. (20) Rosen, M. J. Sutfactants and Interfacial Phenomena; Wlley: New York, 1978. (21) Armstrong, D. W.; Stine, G. Y. J. Am. Chem. SOC. 1083, 705, 2962-2964. (22) Okada. T. Anal. Chem. 1088, 60, 1511-1516. (23) Mullins, F. G. P.; Klrkbrlght, G. F. Analyst (London) 1084, 109, 1217-1221.
RECEIVED for review August 5, 1991. Accepted January 15, 1992.
