sorbed from solution (47). For esters such as phthalates, surface hydrolysis takes place to give products on which the ester does not spread. On the other hand, SF-96 like other polydimethylsiloxanes is nonautophobic (39); yc for a silicone monolayer is near 2.4 X 10-2 N m-1 (39). A hydrocarbon surface composed largely of phenyl groups has yc = 3.5 x 1 0 - 2 N m-1 (39); a thicker layer is found for dinonyl phthalate on phenyltrichlorosilanized columns (Table V). A surface on which dinonyl phthalate has 0 = 0 (measured both by capillary rise and tilting plate methods), however, is allyltrichlorosilanized glass, further heated in a current of oxygen (22). Columns so treated yielded films of dinonyl phthalate with mean d~ (8.9 X 10-8 m) in good agreement with d p calculated from Equation 3 (Table V). The value of R (5.0 X 10-4-10w because of the small of the coating solution) makes Equation 2 less applicable here. The conditions of zero 0 and “compatibility” are now evidently met and columns prepared in this way had high efficiencies of the order of
(47) R. J. Ruch and L. S.Bartell, J. Phys. Chem., 84, 513 (1960).
2000 theoretical plates m-I, enabling, for example, mand p-xylenes to be resolved in 10 min at room temperature for a column as short as 15 m. Less severe oxidation of the allyl group bonded to the glass, or use of other silanizing agents, gave surfaces slightly less “compatible” with dinonyl phthalate, and hence lower values of d~ (Table V). Preparation of Open-Tubular Columns. From these results a procedure for the preparation by the dynamic method of glass open-tubular columns for gas chromatography with known film thickness can be recommended and is outlined in Table VI. ACKNOWLEDGMENT The author is grateful to M. Novotliy for helpful discussion. This paper was read, in part, by the author before “Analysis 72,” Society for Analytical Chemistry/Chemical Society Symposium on Analytical Chemistry. London, Sept. 20,1972. Received for review February 20, 1973. Accepted March 29, 1973.
p-Azoxyanisole Liquid Crystal as a Stationary Phase for Capillary Column Gas Chromatography Eli Grushka and J. F. Solsky Department of Chemistry, State University of New York at Buffalo, Buffalo, N. Y. 74274
Liquid crystals have been used previously as stationary phases in gas chromatography using packed columns. In the present work, the liquid crystal 4,4‘-dimethoxyazoxybenzene (PAA) was used to coat a capillary column. The study indicates that at a given temperature in the nematic region of the liquid crystal, the retention time as well as the plate height of o-xylene can have two different values depending on whether the column is being heated or cooled. Upon cooling, the magnitude of these parameters is smaller. This indicates changes in the macrostructure of the mesophase (perhaps toward greater order) which are due to either supercooling or surface orientation effects. The resistance to mass transfer in the stationary phase is larger than that in the mobile phase. The liquid crystalline nature of the mesophase can be maintained at temperatures up to 16 “C below the melting point. This fact allows an almost complete separation of m- and p-xylene, which was not accomplished previously with PAA on packed columns. The importance of increasing relative retention as compared with increasing plate height while decreasing the temperature is clearly demonstrated.
Liquid crystals are commonly used as selective stationary phases for the chromatographic separation of structural isomers, in particular p - and m-substituted benzenes (1-12). Several recent comprehensive reviews have been devoted to the usage of liquid crystals in chromatography 1836
(13, 14). In addition, gas chromatography is becoming an important tool in studying the thermodynamic parameters of mesophases (9, 15-22). To date, however, all but one of the pieces of work done with liquid crystals employed
( 1 ) H. Kelker, Ber. Bunsenges. Phys. Chern., 67,698(1963) (2) H. Kelker, Z.Anal. Chem., 198,254 (1963). (3) M. J. S. Dewar and J. P. Schroeder, J. Amer. Chem. Soc.. 86,5235 (1964). (4) M. J. S. Dewar and J. P. Schroeder, J. Org. Chem., 30, 3485 (1965). (5) E. M. Barrall 1 1 , R. S. Porter, and J . F. Johnson, J. Chromatogr.. 21, 397 (1966). (6) H. Kelker and H. Winterscheidt, Z.Anal. Chem., 220,1 (1966). (7) M. J. Dewar, J. P. Schroeder, and D. C . Schroeder, J. Org. Chem., 32, 1692 (1967). (8) H. Kelker, B. Scheurke, and W. Winterscheidt, Anal. Chem. Acta, 38,17 (1967). (9) W. Zielinski, D. H. Freeman, D. E. Martire, and L. C. Chow, Anal. Chem., 42, 176 (1970). (IO)J. P. Schroeder. D. C. Schroeder, and M. Katiskas, “Liquid Crystals and Ordered Fluids,” J. F. Johnson and R. S. Porter, Ed., Plenum Press, New York, N.Y., 1970,p 169. (11) P. J. Porcaroand P. Shubick, J. Chromatogr. Sci., 9,690 (1971). (12) M. A. Andrews. D. C. Schroeder, and J. P. Schroeder, J. Chromatogr., 71, 233 (1972). (13) H. Kelker and E. Von-Schivizhoffen. Advan. Chromatogr.. 6, 247 (1968). (14) G.H.Brown,Anal. Chem., 41 (13) 26A (1969). (15) H. Kelker and A. Verhelst, J. Chromatogr. Sci., 7,79 (1969). (16) D. E. Martire. P. A. Blasco, P. F. Carone, L. C. Chow, and H. Vicini, J. Phys. Chem., 72,3489 (1968) (17) L. C.Chowand D. E. Martire, J. Phys. Chem.. 73,1127 (1969). (18) L. C. Chow and D. E. Martire, J. Phys. Chem., 75,2005 (1971). (19) L. C. Chow and D. E. Martire. M o l . Cryst. Liquid Cryst., 14, 293 (1971). (20)J. M. Schnur and D. E. Martire, Anal. Chem.. 43, 1201 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
packed columns. The sole and notable exception is a paper by Taylor, Culp, Lochmuller, Rogers, and Barrall (23).They investigated the dependence of the peak shape and retention times on an electric field, using short pieces of glass capillaries coated with some mesophases. I t also seems that none of the workers in the field'were interested in investigating the chromatographic processes occuring when the stationary phases are liquid crystalline in nature. T o our knowledge, mass transfer phenomena and plate heights were not measured and analyzed. This is surprising since the mass transfer of the solute in the mesophase can be indicative of the latter's macrostructure. Similarly, the relative retention, a , of the closely eluted solutes can be indicative of the magnitude of the ordering factor, S. In addition, supercooling effects and surface orientation phenomena were not carefully investigated in the case of gas chromatography. We wish to report here a study designed to evaluate several factors: the feasibility of using capillary columns coated with liquid cryst.als. possible ways of studying mass transports in mesophases, and possible hysteresis behavior in the retention and mass transfer data as the column is first heated and then cooled. For these studies, we used 4.4'-dimethoxyazoxybenzene (PAA) as the stationary phase and the three xylene isomers as the solutes. We chose this system since previous reports in the literature indicate that PAA is not an effective stationary phase for the separation of the para and meta isomers (3, 12), and since PAA had already been analyzed, characterized, and studied using nonchromatographic methods. Because of the nature of this investigation. no other solutes or mesophases were studied. Further work is now in progress with other liquid crystals. The advantages of' gas chromatography in studying the properties of solvents are well known. Briefly, the attractive features are instrumental simplicity, good temperature control, capability of operating over a wide range of temperatures, and the possibility of measuring thermodynamic quantities a t infinite dilution. The last point is a crucial one, since, if for no other reasons, the mesophases's properties are affected by impurities. Capillary columns have an added advantage: the complexities of packed column geometry are all but nonexistent. Wall effects, if such exist, can be studied. The theoretical expressions describing the processes in the column are better developed and better understood than in the case of a packed column. Thus, mass transfer calculations can be obtained with relative ease from experimental data. As an example, we shall demonstrate how the diffusion coefficients of o-xylene in PAA can be obtained. EXPERIMENTAL A p p a r a t u s . A homemade gas chromatographic system was designed and built to have a minimum of dead volume. A Hotpack drying oven was converted for use a s t h e GC oven, A fan and several baffles were installed to eliminate temperature gradients. T h e heating system consisted of two 750-W electric heaters with simple on-off switches, one 750-W heater under Variac control, and one 750-W heater under direct control of a Fisher Proportional Temperature Control. Temperatures a t any point in the oven could be held to k0.02 "C. Temperature gradients were held t o within fO.l t o 0.20 "C. Temperatures were measured a t four different points in the oven close to the column. Temperatures were measured with copper-constantan thermocouples in conjunction with a Leeds and Northrup Model 8686 Millivolt Potentiometer. In general. for oven temperatures between 100 and (21) D. G . Willey and D. E. Martire, Mol. Cryst. Liquid Crysf., 18, 55 (1972). (22) D. G .Willey and G . H. Brown, J. Phys. Chem., 76, 99 (1972) (23) P. J. Taylor, R . A . Culp, G. H. Lochrnuller, L. B. Rogers, and E. M . Barrall 1 1 % Separ. Sci., 6 , 8 4 1 (1971).
140 "C, t h e Variac controlled heater a n d t h e one controlled by the Fisher Controller were used. T h e injection system consisted of a Seisior Gas Sampling Valve Model VIII, which was actuated by a solenoid valve. Samples were injected by allowing the sample carrier gas (helium) t o bubble through a liquid sample chamber, become saturated with t h e vapor of the liquid sample, and then pass through t h e sampling valve. T h e valve was thermostated inside t h e oven while the sample chamber was located outside t h e oven. T h e detector was a Beckman GC-4 FID. T h e potential across the plates was held a t 300 V by a battery. T h e signal from t h e detector was amplified by a Keithley Model 417K chromatograph electrometer. T h e output from the electrometer was displayed on a Esterline-Angus Model S-601-S Speedservo 5-in. strip chart recorder. Reagents. Commercially available helium was used a s the sample carrier and as the carrier gas. T h e carrier gas flow was regulated with a Brooks Model 8601 pressure regulator. T h e s a m ple carrier flow was regulated by a Nupro needle valve. T h e p azoxyanisole (PAA), purchased from Eastman, was recrystallized three times from 95% ethanol. T h e melting point of PAA was 118 "C a n d the nematic isotropic transition occurred a t 135 "C. T h e xylenes used were purchased from Eastman. Methane was used as the unretained peak in all cases. Procedure. A 9190-cm (300-ft) stainless steel capillary column was used for all of t h e experiments. Its radius was 0.0370 cm. T h e capillary column was cleaned several times with 10-ml portions of various organic and inorganic solvents. These were forced through the column under pressure using nitrogen. The last solvent forced through the column was methylene chloride. The coating solution was made by dissolving 2 . 5 g of PAA in methylene chloride and diluting t o a total volume of 25 ml (10% w/v). T h e plug method was then used to coat the column. T e n milliliters of this solution was piped into the reservoir and forced through t h e column with He a t a speed of 6.6 cm/sec. The column was conditioned overnight a t 145 "C. At least five runs were made at each temperature. while a t least 60 min were allowed between runs a t different temperatures t o achieve equilibration. The peak widths at half-heights were measured from the chromatograms by hand. Retention times were obtained by stopping a n electric timer a t the peak maximum. Peak widths were always much greater t h a n 1 cm.
-
RESULTS AND DISCUSSION Initial experiments were attempted with two 50-ft segments of 0.Ol-in. i.d. columns connected with a Swagelok union, which was bored through so that the column ends could touch one another. Frequently, however, this column was plugged with solid PAA crystals. We, thus, decided to utilize a capillary with a larger diameter in order to avoid this problem. In this study, 300 ft of 0.03-in. i.d. column was used. Efficiency Studies. In trying to characterize the mesophase, we first obtained the van Deemter plots for o-xylene a t three temperatures: 120, 110, and 90 "C. The experimental order in which these plots were done was high to low temperatures. The results are shown in Figure 1. Several interesting points can be made regarding Figure 1. PAA is in its nematic form a t 120 "C (its K N transition temperature is 118 "C). At 110 "C, the PAA should be in its solid form. However, the van Deemter plot a t that temperature indicates that PAA is still in a liquid crystalline form. This fact may be attributed to supercooling. With packed columns, supercooling was also observed by Dewar and Schroeder ( 4 ) . by Barrall e t a1 ( 5 ) , and by Kelker e t al (6). We shall have more to say about that phenomena later on since it is not clear to us whether the mesophase is, indeed, supercooled or is oriented by the wall surface. At 90 "C, it seems that the liquid crystal solidified (no smectic phase was ever observed with PAA). At 90 "C, the data were taken from two directions; i . e . , the temperature of the column was lowered a t first from 110 "C. When the data were analyzed, it was found that the maximum efficiency was not yet achieved. We then raised the temperature from ambient to 90 "C in order to
-
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
1837
03
01
1 9
I
I
20
I
I
60
I
I
100
fi ( C M / S E C l
I
I 140
I
I
Figure 1. HETP vs. carrier velocity 100
In the 90 "C run 0 indicates data obtained upon cooling and A indicates data upon heating
110
120
130
140
T 'C
Figure 2. Capacity ratio vs. temperature
complete the van Deemter plot. As Figure 1 shows, the two portions of the plot do not quite merge although the general trend is obvious. Chow and Martire (17) have observed two solid structures of PAA. This may explain the slight differences in the HETP values obtained at 90 "C upon cooling and heating. Figure 1 also shows that the minimum H values in the mesophase region are shifted to lower carrier velocities. This is indicative of the prominence of the resistance to mass transfer terms in the liquid crystalline range. The velocity a t which the minimum in the van Deemter plot occurs is given by
where 0 is the average linear velocity, I3 is the molecular diffusion term, and C m and C, are, respectively, the resistance to mass transfer terms in the mobile and stationary phases. From Equation 1, it is clear that as the resistance to mass transfer increases, U m i n will decrease. The increase in C m and C, is not surprising since the mesophase is a viscous liquid in which solute diffusion is relatively slow. The fact that the HETP curve at 110 " C lies above the 120 "C curve is due to an increase in resistance to mass transfer in the liquid crystal, which at this temperature is either supercooled or is in a geometry-induced orientation. At 90 "C, the minimum is shifted to a faster velocity and Hminhas roughly the same value as H m i n a t 120 "C. At 90 "C, the system is essentially an open tubular one with some adsorption (small partition coefficient) on the PAA solid crystals. In an attempt to gain further insight into the mass transfer behavior in PAA, we have decided to obtain the plate height and the capacity ratio, k , of o-xylene as a function of the temperature a t a constant velocity. The capacity ratio is defined as the retention time of the solute minus the retention time of a nonretained peak divided by the latter quantity. We chose the carrier velocity of 53.2 cm/sec since a t that velocity, the plate height is dominated by the resistance to mass transfer in the mesophase region. Figure 2 shows the behavior of the capacity ratio k as a function of the temperature. The data were collected by starting at about 95 "C, increasing the temperature to about 140.5 "C, and then decreasing it until the solidification of the mesophase occurred at about 102 "C. Figure 2 shows several features, some of which were observed before by other workers and some of which have not previously been described. From 95 to about 116 "C, 1838
(;I = 53.2 cm/sec: (0) heating, ( A )cooling
the k values pass through a shallow minimum (0.0149 at 95 "C, 0.0123 a t 107.4 "C, and 0.0191 at 115.8 "C). Because of the scale of the plot, this is not obvious in Figure 2. At 118 "C, the melting point, k increases sharply to 0.325. Upon further increase in T, k drops until about 134 "C. Through the clarification transition, k at first increases, reaching a maximum at about 135.0, and then decreases with increasing T. The increase in k (or in retention time for that matter) a t the nematic-isotropic transition is a well-known phenomenon observed by many (uit.,15, 18). Dewar and Schroeder attributed it to a decrease in the free energy of solubility of the solute during the pretransition range. Price and Wendorff ( 2 4 ) have recently studied the transformation behavior of PAA. In investigating the changes in the density of PAA as a function of the temperature, they have found that a linear relation exists between the two parameters up to 130.2 "C. From there to the transition point, there was a marked curvature in the density-temperature relationship. The density decreased a t a rate greater than that predicted by the van't Hoff equation. Kelker, in his review (13), summarizes other data showing peculiar pretransition behavior of PAA. Thus, the structure of the nematic phase a t the preclarification point is altered, perhaps becoming less ordered. Once in the isotropic region, we began to lower the temperature of the oven. Figure 2 shows a remarkable hysteresis effect in the behavior of k . The "cooling down" curve nematic transition exhibiting goes through the isotropic the same behavior as the "warming up" curves with the noticeable exception of lower k values for equal temperatures. In the mesophase range, again, the decreasing temperature curve is below the increasing temperature curve. In changing from one temperature to another, we waited for about an hour for equilibration. Moreover, obtaining the data for Figure 2 took about a week's time and frequently the oven was left overnight at a particular temperature. No change in the chromatographic behavior was observed even after such a long equilibration period. Price and Wendoff ( 2 4 ) did not find any difference in the density of PAA upon heating and then cooling. Our findings are in marked disagreement. Also relevant, perhaps, is the work of Barrall, Porter, and Johnson ( 2 5 ) who have investigated, among other things, the specific heat of PAA. In
-
(24) F. P. Price and J. H. Wendoff, J. Phys. Chem., 76, 2605 (1972). (25) E. M. Barrall II, R. S. Porter, and J. F. Johnson, J. Phys. Chern., 71, 895 ( 1 967).
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
one experiment, they cooled the melt to 107 "C and then heated it to 121 "C. In a second experiment, the solid phase was heated into the nematic phase, cooled down to 107 "C, and then heated again to 121 "C. The rationale behind these experiments was to see if the specific heat of the mesophase was identical whether formed by cooling the isotropic liquid or by melting the solid. However, in both experiments the starting point was the supercooled PAA a t 107 "C. It is, thus, not surprising that not much difference was found in the heat capacity obtained from both experiments. Upon decreasing the temperature below 118 "C,k increased until about 102 "C where solidification occurs, 16 "C below the melting point. In the mesophase region, both k us. T curves are almost parallel. Around 105 "C, the slope of the curve seems to change, perhaps indicating structural differences of PAA in the two temperature regions. The duality of k values a t a given temperature in the mesophase range has an important implication. As previously mentioned, gas chromatography is used in studying thermodynamic parameters of liquid crystals. These studies involve measurements of specific retention volume, V,. Figure 2 indicates that two different V , values can be obtained at the same temperature, depending on whether the data are obtained while heating the column or cooling it. Although the trend in calculated enthalpies and entropies will be the same, their absolute magnitude will change. As to the explanation of the behavior of k with temperature, three alternatives might exist. One of them is that thermal equilibrium in the system was not achieved. This, however, was discussed above and we feel confident that we, indeed, operated under the condition of thermal equilibrium. Other explanations might lie in supercooling and surface orientation. Both these phenomena are well known in liquid crystals. The effect of surface walls was discussed by Brown, Doane, and Neff in their review on liquid crystals (26) and more specifically analyzed recently by Rapini, Papoular, and Pincus (27), De Gennes (28), Meyerhofer, Sussman, and Williams (29), and by Uchida, Watanabe, and Wada (30), to name just a few. Most orientation studies were done on glass surfaces where it was observed that, depending on the surface treatment, the liquid crystal molecules can be parallel or perpendicular to the surface. Wada (30) in one study was looking for the orientation of PAA molecules between a stainless steel plate and various glass plates. He found no molecular alignment. Our system, however, is essentially a one boundary system with one side of the liquid crystal being exposed to the flowing stream of He. The inner wall of the stainless steel capillary is not as smooth as glass. However, the surface has high energy adsorption sites, which might induce alignment of the liquid crystal molecules. (We might add here that the PAA film thickness in our case is about 0.88 pm, as will be demonstrated shortly.) Supercooling can also explain the observed phenomena. The fact that upon decreasing the temperature below the melting point, the stationary phase is still a mesophase (as judged by the retention data) points to a supercooling (26) G. H. Brown, J. W. Doane, and V. D. Neff. "A Review of the Structure and Physicai Properties of Liquid Crystals," Chemical Rubber Publishing Co., Cleveland, Ohio, 1971. (27) A . Rapini, M. Papoular, and P. Pincus, C. R. Acad. Sci., Ser. B , 267, 1230 (1968). (28) P. G. De Gennes, Mol. Crysf. Liquid Cryst., 7,325 (1969). (29) D. Meyerhofer, A. Sussman. and R . Williams, J. Appl. Phys., 43, 3635 (1972). (30) T Uchida. H Watanabe, and M. Wada, Jap J. Appl. Phys., 11, 1559 ( 1972)
H
(cm)
.10
100
110
120
130
140
T "C
Figure 3. HETP vs. t e m p e r a t u r e
6 = 53.2cm/sec: (0) heating, ( A )cooling
effect. We found, however, that no matter how many times we repeated the experiments, or how the carrier flow rate was changed, or how long it took us to go through the cooling stages, the mesophase solidified at a temperature between 102.7 and 101.5 "C. This is somewhat surprising. If supercooling is occurring, the solidification point should not be as well defined as we found it to be (see, for example, Ref. 24). More extensive experimentation is needed in order to understand fully the behavior depicted in Figure 2. Work should be done on packed columns with different solid supports as well as on capillaries made of different materials such as glass or Teflon. Also, other liquid crystals should be investigated. We are now beginning to carry out some of these experiments. We would like to mention again the work of Chow and Martire (17). In investigating the melting temperature of PAA on two different solid supports (Chromosorb W and P) and of bulk PAA, no appreciable changes were found. No data were given, however, on the solidification of PAA on the two supports. The fact that k is lower in the 118-135 "C region when the system is cooled, indicates that the activity coefficient of o-xylene is larger than during the heating cycle. Using the arguments advanced by Kelker ( 1 5 ) , this might indicate that the difference in the solubility parameters of PAA and the solute is larger, thus pointing out a more ordered phase. Qualitatively, the mesophase "squeezes out" the solute. This, again, needs to be studied in more detail. In particular, the heats and entropies of solution should be investigated since they can provide the magnitude of S (19). Figure 3 shows the behavior of the plate height as a function of the temperature a t a constant velocity of 53.2 cm/sec. In general, the plot shows the same characteristic points as that of k us. T. The decrease in H right before the melting point is much more pronounced here, since at low k values, H is a strong function of k . The hysteresis behavior in the mesophase region is, again, evident. This fact is quite important as it indicates different resistance to mass transfer values at the same temperature. In the cooling cycle, the plate height is lower than in the heating part. This indicates a faster rate of mass transfer, possibly due to a decrease in the viscosity of the mesophase. It is interesting, consequently, to calculate the relative importance of each of the resistance to mass transfer terms in the H E T P values. In order to obtain these terms, the diffusion coefficient of o-xylene in He was estimated
A N A L Y T I C A L CHEMISTRY, VOL. 45, N O . 11, SEPTEMBER 1973
1839
Table I Mass Transfer Terms of o-Xylene in PAA
r, "c
C,U
118.0 119.5 121.7 125.0 129.7 132.5 133.2 134.8 135.2 136.3 138.5 140.4 138.5 136.5 135.4 134.0 132.2 127.9 123.1 118.7 11 7.2 116.2 113.5 111.3 109.2 106.9 104.9 103.4
0.0247 0.0242 0.0238 0.0224 0.0213 0.0212 0.0212 0.0212 0.0220 0.0215 0.0209 0.0199 0.0204 0.0208 0.021 1 0.0200 0.0197 0.0204 0.0213 0.0225 0.0229 0.0230 0.0238 0.0246 0.0254 0.0262 0.0270 0.0276
C,U
CsICm
0.105 0.1 00 0.0901 0.0844 0.0754 0.0700 0.0693 0.0770 0.0710 0.0677 0.0636 0.0577 0.0605 0.0643 0.0656 0.0642 0.0646 0.0698 0.0789 0.0887 0.091 7 0.0926 0.101 0.109 0.1 16 0.125 0.134 0.139
4.24 4.14 3.70 3.77 3.53 3.30 3.27 3.62 3.23 3.15 3.05 2.90 2.97 3.09 3.1 1 3.21 3.28 3.42 3.70 3.95 4.01 4.03 4.22 4.44 4.56 4.79 4.97
.. 700-
600-
-
DS xi07 500L
i
c
400r
I
100
=
C,L'
=
(1
+ 6k
-I-llk')r*D k)'D, 24(1
u
+
Figure 4.
5.05
(2,
where df is the mesophase film thickness and D, is the diffusion coefficient of o-xylene in the liquid crystal. The values of C, are obtained by subtracting from the experimental H values the molecular diffusion terms ( 2 D , / a and C,U. The specific retention volume V , a t a given temperature allows calculation of &. -
KP
kr
=
%P
~
(4)
where p is the density of the stationary phase and K is the partition coefficient of the xylene. Using the data of Kelker (15) for specific retention volumes and Price's density values (241, we have calculated d f at two temperatures to check the consistency of the data. At 119.5 "C, df was 0.87 pm. At 129.7 "C, we obtained df = 0.89 pm. Within experimental error, these values are in good agreement. In the calculation, we took df to be the average of the two, i.e., 0.88 pm. One other point must be made here. We used, in Equation 4, our k values from the heating cycle range. That is, we used the higher of the two possible k values. (31) E. N . Fuller, P. C . Schettler, and J. C. Giddings, Ind. Eng. Chem., 58, 19 (1966).
1840
I I I10
l
I
1
1
I
I
1
120
l
I 1 130
I
l
I
1 1 140
1
l
"C
Diffusion coefficient of o-xylene in
PAA vs.
ternpera-
(0) Heating, ( A )cooling
(3)
-
I
ture
where r is the capillary radius, is the carrier average velocity, and D , is the diffusion coefficient of the solute in He. The expression for the resistance to mass transfer in the liquid phase is
ve
I
T
using the FSG equation (31) at, each column temperature and mean pressure. The calculation of the resistance to mass transfer in the mobile phase, ,C, is thus feasible
H,
1
We did not know whether Kelker heated or cooled his columns. The values of df obtained, consequently, are "order of magnitude" values only, which are sufficient for our purposes of comparing C, and C,. From the C, term, making use of Equations 3 and 4, the diffusion coefficient D , of the o-xylene in PAA was calculated. Table I shows C,u and C,u values in the mesophase region. Also shown is the ratio C,/C,. In general, they follow the same pattern as the total HETP plot in Figure 3. As expected, C, is always larger than C,, indicating the importance of the mass transport in the stationary phase. The ratio of C,/C, has a minimum of 2.9 in the isotropic region and a maximum value of 5.05 a t 103.7 " C . The rate of change in the magnitude of C, is much more pronounced than in C ., I t would be of some interest to see the behavior of the diffusion coefficient in the mesophase since the diffusion is related to the viscosity and hence to the order in the liquid crystal. The dependence of D, on T in the ordered region is shown in Figure 4. The diffusion coefficients are slightly higher upon cooling as compared with the values when heating. The diffusion coefficients drop sharply as the liquid crystal approaches the clarification point. Upon cooling, the diffusion coefficients do not vary by much around that point although a small dip is noticeable. Around the melting point, in the cooling cycle, the diffusion coefficient line shows a change in slope. By ploting In D , us. 1 / R T we have obtained the activation energies, E, of diffusion in each region in the plot. It must be emphasized that because of experimental scattering of HETP points, and because of the method by which df was determined, the activation energies are accurate only to about 10-15%. In the nematic region and upon heating (118-133 "C), E was 6.26 kcal. Upon cooling, in the same range of temperature, E was 5.37 kcal. In the supercooled or oriented region (113-103 "C), E was 6.64 kcal. These E values, together with Figiire 4, again indicate that in the nematic range (118-135 " C ) mass transfer in the liquid phase is faster in the cooling cycle; hence, H is smaller. Upon cooling below the melting point, the viscosity of the stationary phase increases, thus increasing the mass transfer terms and the plate height value. Resolution of p - and rn-Xylene. The separation of these two compounds is a classic problem in chromatography. Although p - and m-xylenes were separated with liquid crystals, the usage of PAA was not previously attempted, perhaps because of the comments of Dewar and
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
Schroeder ( 3 ) . In our system, however, we can decrease the temperature to about 102 "C and still have a liquid crystalline type stationary phase. Figure 5 shows the separation of the two solutes a t 102.7"C. The separation is almost complete and the resolution is about 0.96. This separation was obtained a t a carrier velocity of about 30 cm/sec. The relative retention a plays an important role in the resolution
The subscript 2 indicates quantities measured from the more retained compound, which in this case is, as expected, p-xylene. Figure 6 shows a plot of a us. the temperature. From a value of zero, cy increases to about 1.035 a t 118 "C. Upon further increase in T,a decreases and approaches zero again a t about 125-126 "C. The resolution in this region is only slight (Rs = 0.2), just barely indicating the existence of two components. During the decrease in T, a steadily increases. In the nematic range, the a values are roughly the same whether heating or cooling. However, further decrease in the temperature increases a and the resolution. Note that although H also increases in this range, the increase in a is much more important because of the (a - l)/a terms in Equation 5. The resolution a t the last three a points is shown in an insert in Figure 6. Andrews et al. (12) indicated a noticeable scatter in a when plotted as a function of the temperature. We did not encounter this problem. The interpretation of the plot in Figure 6 is simple. Although the mass transfer increases as T decreases, the order in the molecular orientation of the stationary phase increases, thus allowing the separation of the two structural isomers according to the mechanism discussed by Zielinski, Freeman, Martire, and Chow (9). a then can be related to the ordering parameter S. The relative retention can be rewritten as
a = -Y,'P,O
??"p?(i
where y indicates activity coefficients a t infinite dilution and po is the vapor pressure of the pure solute. As discussed by Karger (32), a can be related to the difference in the partial molar excess free energies A € of the two solutes when their po values are close in magnitude (as is the case here).
lna
Til
AC
--
AS - AR - R ~
RT-RT
AH and AS are the differences in the infinite dilution solute partial molar excess enthalpy and entropy of the two solutes. Chow and Martire (19) have proposed a method by which the activity coefficient, A H , and AS of a solute can be utilized. to measure the fraction of liquid phase
(32) B. L. Karger, Ana/. Chem., 39 (8).24A (1967)
3 9 4.1' '38 9.2
t, ( S E C ) Figure 5. Separation of m- and p-xylene at 102.7 "C'
c\
I
100 120
T "C
Figure 6. Relative retention vs. temperature
molecules which are ordered, i.e., the parameter S. Their procedure fails when a is taken as the relevant quantity rather than the activity coefficient of one solute. Intuitively, however, one would expect that a is proportional to the ordering parameter S , since as the latter increases so does a. Subsequent publications will be devoted to the correlation of S and a,both empirically and theoretically. In conclusion, capillary columns can be used to obtain useful separations with liquid crystals as substrates. One must be careful, however, when interpreting the results because of different behavior of the system upon heating or cooling. The liquid crystalline structure can be maintained at temperatures below the melting point. Consequently, separations such as p- and m-xylene on PAA are feasible. Capillary columns can also be useful in obtaining diffusion data of various solutes in liquid crystals. Mass transfer properties of solutes in mesophases can be utilized in studying the macrostructure of these compounds. Received for review February 12, 1973. Accepted April 5 , 1973.
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