Anal. Chem. 1993, 65, 2141-2144
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Side-Chain Liquid Crystalline Polysiloxane Containing Crown Ether Used as Stationary Phase for Capillary Gas Chromatography Ruonong Fu,’ Peng Jing, and Junling Gu Department of Chemical Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
Zaifu Huang and Yanfei Chen Department of Environmental Science, Wuhan University, Wuhan 430072, People’s Republic of China
A new stationary-phase, side-chain liquid crystalline polysiloxane containing crown ether was developed and used as a stationary phase in capillary gas chromatography. This phase can be easily coated on the fused-silica tubing and possesses a high efficiency and moderate polarity. It exhibits the retention properties of both liquid crystal and crown ether stationary phases and is suitable for the separation of a variety of isomeric compounds.
E-I
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t o 1uene
INTRODUCTION Side-chain liquid crystalline polysiloxanes have been developed and used as capillary gas chromatographic stationary phases since 1982 by Finkelmann.1 These kinds of stationary phase possess unique separation properties, especially for geometricalisomer separation. On the other hand, side-chain crown ether polysiloxanes have been used as stationary phases for capillary gas chromatography since 1988.= These stationary phases exhibit some excellent selectivity. In our laboratory both side-chain liquid crystalline siloxane and side-chain crown ether polysiloxane were synthesized and used as capillary gas chromatographic stationary phases.617 In our studies it was demonstrated that these stationary phases have unique selectivity and thermostability and are suitable for the separation of a variety of isomeric compounds. In 1989 Perceca synthesized some side-chain liquid crystalline polysiloxanes containing crown ether. What are the behaviors of this kind of material when used as a gas chromatographic stationary phase? This is an interesting problem. To date, to be best of our knowledge there is no report on using this material as a chromatographic stationary phase. The goal of this paper is to describe the synthesis of a novel side-chain liquid crystalline polysiloxane containing crown ether, which was used as a stationary phase for capillary gas chromatography. The structure and preparation of the side-chain liquid crystalline siloxane containing benzo-15crown-5 ether is shown in Figure 1. This material was used (1)Finkelmann, H.; et al. Polynuclear Aromatic Hydrocarbons: Physical and Biological Chemistry;Battelle Press: Columbus,OH, 1982; p 275. (2) R o w , C. A.; Finlinson, C.; Tarbet, B. J.; et al. Anal. Chem. 1988, 60, 901. (3) Wu, C. Y.; Wang, C.-M.; Zeng, Z. R.; Lu, X. R. Anal. Chem. 1990,
62, 968.
(4) Ge, J.; Fu, R.; Zhang, A., et al. J. Microcolumn Sep. 1991,3,121. (5) Zhang, A.; Ge, J.; Guan, Z.; et al. J. Chromatogr. 1990, 521, 128. (6) Jin, Y.; Fu, R.; Huang, Z. J. Chromatogr. 1989, 483, 394. (7) Wang, H.; Gen, T.; Fu, R.; et al. J . Chromatogr. 1992,609, 414. (8) Percec, V.; Rodenhouse, R. Macromolecules 1989, 22, 4408. 0003-2700/93/0365-2141$04.00/0
@
Psc-3
Q 9
c=o I
Figure 1. Scheme of preparation of PSC-3 from OC-1 and poly(hydromethy1)siloxane.
as a stationary phase and characterized by ita phase transition temperature, efficiency, selectivity, and polarity.
EXPERIMENTAL SECTION Synthesis of Poly(methylsi1oxane) Containing 4-(3-Propan-l-lyoxy)-4’-(4’-carboxybenzo15-crown-5)-biphenylSide Chain (PSC-3). The PSC-3 was prepared by a hydrosilylation reaction between poly(methylhydrosi1oxane) and 4-(3-propanl-lyoxy)-4’-(4’-carboxybenzobiphenyl(OC-1). The procedure used was similar to the preparation of side-chainliquid crystalline siloxanes.6 A 0.3092-g(ca. 0.0006mol) sample of OC-1 (obtained from the Department of Environmental Science, Wuhan University) and 0.0392 g (ca. 0.000 017 3 mol) of poly(methylhydrosiloxane) [Merck-Schuchardt(Schuchardt,8011Hohenbrunn bei Munichen),the polymerization degree of the polymer is 351 were placed in 25 mL of dry, freshly distilled toluene in a dry three-neck round-bottom flask. The mixture was stirred and heated to 110 “C under reflux in a nitrogen atmospherefor some time until the solid reactant was dissolved in toluene; 40 p L of fresh catalyst solution (0.0539 g of chloroplatinic acid dissolved in 10mL of isopropylalcohol) was added to the reactant solution and continuously heated and stirred for 20-30 h until the Si-H 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
Table I. Liquid Crystalline Transition Temperatures of OC-1 and PSC-3 Using Hot-Stage Light-Polarized Microscopy transition temp, O C stationary phase heating cooling c-1 PSC-3
k 110 n 160 i k 105 n 177 i
k 109 n 161 i k 80 n 174 i
Table 11. Phase Transition Temperature of OC-1 and PSC-3 Using DSC-7 transition temp, O C stationary phase heating cooling oc-1 PSC-3 OC-6 PSC-8
k 116 n 161 i g 68 s 101 n 172 i k 147 i g 61 s 152 n 219 i
Table 111. Characteristics of PSC-3 Capillary Columns film flow column thickcolumn rate, effic, column stationary ness, temp, cm/ plates/ compd no. phase wm k "C s m tested0 1 2 3 4 5
PSC-3 PSC-3 PSC-3 PSO-B-15C5' PLC-IVC
0.29 0.29 0.29 0.28
3.85 3.91 4.18 1.58
220 220 220 140 200
12.7 12.9 13.6 13.6 12.8
4660 3440 4482 4240 2880
E E E 0 E
E, anthracene; 0, 1-octanol. b Poly(crown ether)siloxane, from ref 4. Liquid crystallinepolysiloxane, from Fenxi Huaxue (Chinese Anal. Chem.) 1991, 19 (ll),1238.
k 10 3 n 151 i n 162 i k 107 n 132 i k 52 s 147 n 263 i
bond was not detectable by IR spectroscopy (2140 cm-I). Generally, after 26 h the Si-H bond almost disappeared and ethylenewas then bubbled through the mixture for 2.5 h to react with all of the residual Si-H units. After the mixture was cooled, a kind of white precipitate was taken out and dissolved in a small amount of methylene dichloride. After 4 times the volume of methanol was added to the solution of polymer, a white product was obtained. The process of dissolutionin methylenedichloride and precipitation with methanolwas repeated two or more times. The product was centrifuged at ca. 11500g and 5 O C for 30 min and dried in air for 48 h removing the solvent. Some white powder (0.2044g) was obtained. Measurement of Phase Transition Temperature of OC-1 and PSC-3. For the determination of phase transition temperature of the stationary phase, a hot-stage light-polarized microscope and a DSC-7 differential scanning calorimeter (Perkin-Elmer Co.) were used. Column Preparation. Fused-silicacapillaries (0.25" i.d.; Yongnian Optical Fibre Factory, Hebei, China) were used. Capillaries were purged with nitrogen at 220 "C for 6 h before coating. The PSC-3 was dissolved in methylene dichloride at a concentration of about 0.46 g/mL using the static coating procedure. The columns coated with PSC-3 were conditioned under nitrogen at 140, 160, 180,and 200 "C for 2 h at each temperature and finally were purged at 220 "C for 4 h. Column Evaluation. Column evaluationwas carriedout with a HP-5890-11gas chromatograph (ChineseHewlett-PackardCo.) equipped with a FID using nitrogen as carrier gas. Solutes were injected using the split mode (the split ratio was 801). The polarity of the columns was determined using McReynolds ~0nstant-s.~ The retention time measurements were performed by a HP-3390 A integrator.
RESULTS AND DISCUSSION The transition temperature of the monomer (OC-1) and the polymer (PSC-3) were measured using a hot-stage lightpolarized microscope. The results are listed in Table I, and the phase transition temperatures, performed with a Model DSC-7apparatus, are shown in Table 11. For comparison, the data of similar compounds, OC-6 and PSC-8, which were studied by Percec and Redenhouse? are also listed in Table 11. The data show that this material meets the stationaryphase demand. The fused-silica capillary columns coated with PSC-3 possesses a higher column efficiency (Table 1111, similar to that obtained on columns coated with poly(crown ether)siloxane," which is easily coated on fused-silica capillary columns and has high efficiency. On the other hand, it is higher than those for the columns coated with side-chain liquid crystalline polysiloxane stationary phases: which is difficult to coat on the capillary columns and difficult to obtain a high (9) McRaynolde, W.0.J. Chromotogr. Sci. 1970,8, 685.
0.5 150
/@
/10
f70
t
190
200
210
C'C,
Figure 2. Plot of HETP against temperature for naphthalene on column 2.
efficiency. It is demonstrated that the PSC-3 can be easily coated on the unpretreated fused-silica capillary tubing and has much better film-formingability than the liquid crystalline polymer studied previously.' The effect of column temperature on the PSC-3 capillary column efficiency is notable. Evidently the efficiency of the PSC-3 columns will reduce when the column temperature is below the isotropic-state temperature, as depicted in figure 2. This phenomenon generally occurs in side-chain liquid crystalline siloxane and poly(crownether) siloxane stationary phases. Therefore, the PSC-3 columns exhibit arelativelower efficiency in the liquid crystalline state, but there is high selectivity for the separation of isomeric compounds. This property also appears in the case of side-chain liquid crystalline polysiloxane or poly(crown ether)siloxane stationary phases.4.5~7 The polarity of PSC-3columns was measured and expressed by McRenolds constants9 a t 120 "C. The results are listed in Table IV. In order to obtain reliable data, the measurement was carried out on the three PSC-3 columns. The polarity of PSC-3 is lower than that of PEG-POM, but higher than that ,of liquid crystalline polymer or poly(crown ether); therefore, this stationary phase is more suitable for separation of polar and easily polarizable compounds than the two others. The PSC-3 capillary columns show a unique selectivity for aromatic hydrocarbon isomeric derivatives. Table V shows the relative retention time (a)of isomeric chlorotoluene, bromotoluene, and xylene. It is very difficult to separate the para and meta isomers of these compounds on commonly used columns. Only some selective stationary phases, such as liquid crystals or cyclodextrin, have the ability to separate the geometrical isomers. On the other hand, this stationary phase has the capacity to separate the cresol and dimethylphenol isomers, which is similar to the behavior of sidechain crown ether polysiloxane (shown in Table VI and VII). Therefore, this stationary phase has the property of both liquid crystalline and poly(crown ether)siloxane depending on the column temperature as shown in Tables V-VII. The
ANALYTICAL CHEMISTRY, VOL. 85,NO. 15, AUGUST 1, 1993 2143
Table IV. McReynolds Constants of the PSC-3 Columns. McReynolda constantab stationaryphase X’ Y’ Z’ U’ S’ avpolarity b 258 266 254 259 Carbowax-2OM 322 PSO-B-15C50 121 liquidcryst 251
410 410 395 405 536 254 264
PSC-3~011 PSC-3~012 PSC-3~013 PSC-3(av)
338 324 324 329 368 192 330
508 478 475 487 572 293 429
461 453 444 453 510 284 420
395 386 378 387 462 229 339
0.2386 0.2386 0.2319 0.2364 0.2235
copolymep
-
1 1 1 1
a From our previous work.‘ b X’, benzene; Y’, 1-butanol; Z’, 2-pentanone;U’, nitropropane; S’, pyridine; b, the slope of the curve obtained when the logarithm of the adjusted retention times of n-alkane are plotted as a function of the number of carbon numbers. From our work J. Chromatogr. 1992,609 (1/2), 414. Carbowax20M data from: J. Chromatogr. Sci. 1970,8,685.
0
1
2
9
4 3 itin min Figure 3. Chromatograms of dlmethoxybenzeneIsomers on column 1 at 180 O C (a) and on capillary column coated with small molecular weight liquidcrystal( M O B ) , CaHlPceH~CooCeHdBr~oCoCeH4CaH11. Peaks: (1) dlmethoxybenzene; (2) dlmethoxybenzene; (3)p dlmethoxybenzene. 0
Table V. Relative Retention (a)of Disubstituted Benzenes on the PSC-3 Column column a value compound temp, OC isomer on PSC-3 a @/m) chlorotoluene
100
0
m P
bromotoluene
100
0
m
P xvlene
80
m
P
1.00 1.07 1.15 1.00 1.08 1.16 1.00 1.06
1.07 1.07 1.06 I
Table VI. Relative Retention (a)of Cresol Isomers on PSC-3 and PSO-B-1SCS Columns a value column flow
t”,”cp’ %; 150
16.8
120
16.8
PSC-3
isomer o m p o
m p
PSO-B-15C5
Loo0 a ( m / p )= 1.008 1.oo0 a(m/p) = 1.038 1.373 1.370 1.362 1.320 1.OOO a@/m)=1.001 1.427 1.429
0 ’ 4
I
8
mi n
I
1 2 i 6
L
0
I
I
16
8
I
24
mi n
Figure 4. Chromatogram of dlnltrotolueneIsomers on column 1at 170 O C (A) and on side ligand crystalllne siloxane [(CH&SI[(CH3)SIO]lc caplllary column (24 Sl(CH3k](CH2CH2CH20CsH4COOCsH,CeH40CH3) m X 0.23 mm 1.d.) at 190 O C (e). Peaks: (1)2,WNT; (2)2,5-DNT;
(3)2,3DNT;(4)2,4-DNT;(5)3,5-DNT;(8)3,4DKT;(7-1l)trlnltrotoluene isomers.
Table VII. Relative Retention (a)of Dimethylphenol Isomers on PSC-3 and PSO-B-1SC5 a value column PSC-3 PSO-B-15C5 temp, OC isomer 150
100
2,6 2,5 2,4 2,3 3,5 3,4 2,6 2,5 2,4 2,3 3,5 3,4
1.00 1.69 1.69 2.14 2.34 2.88 1.00 2.18 2.23 2.96 3.29 4.33
1.00
a (2,5/2,6) = 1.69 1.65 a (2,5/2,6) = 1.65
(2,4/2,5) = 1.00 a (2,3/2,4) = 1.27 a (3,5/2,3) = 1.09 a (3,4/3,5) = 1.23 a!
1.65 a! (2,4/2,5) = 1.00 2.11 a (2,3/2,4) = 1.28 2.31 a (2,5/2,3) = 1.09 2.73 a (3,4/3,5) = 1.18
a (2,5/2,6) = 2.18 a (2,4/2,5) = 1.02 a (2,3/2,4) = 1.33
a (3,5/2,3) = 1.11 a (3,4/3,5) = 1.32
m-cresol isomer is eluted earlier than the p-cresol a t 150 “C, which brings the crown ether functional group ability into play; on the other hand, at 120OC the eluting order is reversed, implying that the para isomer of cresol is inlaid in the liquid crystalline lattice, which plays an important role for isomer separation using liquid crystalline stationary phases. The data in Table VI also demonstrate the mechanism of separation of isomers. The meta isomer of cresol is eluted
min mi n Figure 5. Chromatogram of three-rlng PAH Isomers on column 3 at 220 O C (a)andon a mesomorphiccopolyslloxanecaplllary column (10 m X 0.28 mm 1.d.) at 225 O C (from J. Chrmtogr. 1992, 609 (1/2), 414) (b). Peaks: (1) fluorene; (2) phenanthrene; (3) anthracene.
after the para isomer at 150 O C , showing the crown ether property; on the other hand, the para isomer of cresol is eluted after the meta isomer at 120 “C,impling the liquid crystalline stationary-phase property. Figures 3-5 illustrate the selectivity of this stationary phase for separation of the substituted benzene and three-ring polyaromatic hydrocarbon isomers. It is surprising that in Figures 3-5 the separation can be achieved above the nematic transition temperature while these isomers can be separated on the other liquid crystalline stationary phase below the liquid crystalline nematic transition temperature. This can be seen in the chromatograms in Figures 3-5b. This problem is still being studied in our laboratory. For comparison, the
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chromatograms of the same isomers separated on the other liquid crystal stationary phases that we have investigated are illustrated in Figure 3-5b. In any case, this kind of material is a fine stationary phase for solving practical problems.
phase possesses the behaviors of both liquid crystalline siloxane and poly(crown ether)siloxane and its polarity is also greater than both of them.
ACKNOWLEDGMENT CONCLUSION PSC-3 is a new polysiloxane stationary phase, which contains liquid crystalline side chains containing crown ether. The capillary column can be easily coated with this stationary phase and exhibits high efficiency, moderate polarity, and unique selectivity for positional isomers. This stationary
This work was kindly supported by the National Science Foundation of China.
RECEIVEDfor review July 21, 1992. Accepted December 31, 1992.
