Table 11. Mean Particle Diameter of Thoria Sols, A Small-angle scattering method Line-broadening method 80
79
81
82 112 37 67 91 35 65 46
100
46 67
112 44 66 49
irrespective of dilution over a concentration range of 5-50z in water. Outside of this range the intensity of the curve changed, but not the shape. Various changes in instrument conditions were tried in addition to those previously noted. None produced a change in the shape of the scattering curve; overall intensity, however, depended on the width of the receiving slit. Plots of log intensity cs. scattering angle squared produced curves that were concave upward. Figure 5 is an example based on the same data used for the plot of Figure 4. Systems of single size particles produce linear plots on this basis. The departure from linearity is usually interpreted as indicating the presence of a distribution of particle sizes. Resolution of the curve into its components was done using the graphical method of Jellinek, Solomon, and Fankuchen (6). Table I shows the result of particle size distribution analysis of five samples of thoria sols by this method. The third column shows the weighted average radius computed on a weight (or volume) per cent basis (not on a particle number basis). (6) M. H. Jellinek, E. Solomon, and I. Fankuchen, IND.ENG.CHEM. ANAL. ED., 18, 172 (1946).
The weight fractions as indicated in Table I should be interpreted as points on a continuous distribution, rather than completely separate fractions. Jellinek et a/., proved that such a series of points approximates the true distribution. Calculation of each distribution analysis to a weighted average radius on a volume per cent’basis seems justified, because it is reasonable to suppose that the small-angle scattering curve should be responsive to the volume fraction of material in each size range. The method used for calculating particle radii uses a value for particle radius of gyration that assumes a spherical shape. This assumption is arbitrary, as the shape of the thoria particle has not been determined. The crystallite size of thoria sols has been determined routinely using a modification (7) of a method proposed by Mazur (8) and Hall (9) to separate strain broadening from crystallite-size broadening. A liquid sample cell was developed for the purpose. A comparison was made of mean particle diameters of nine samples calculated from the size distribution analysis with the mean crystallite sizes as determined by line broadening. The results are shown in Table 11. The agreement appears good, particularly considering the number of assumptions involved in both methods of determination. The conclusion seems justified, however, that the particles, as measured by small-angle scattering, are composed of single crystallites. RECEIVED for review January 9: 1967. Accepted February 27, 1967. Work supported by the United States Atomic Energy Commission, Contract W-14-108-Eng.-8. (7) W. C. Stoecker and J. W. Starbuck, U.S.A t . Energy Comm. Rrpt. MCW-1493 (1965), pp. 15-25. (8) J . Mazur, Nature, 164, 358 (1949). (9) W. H. Hall, Proc. Phys. Soc. (Lotidou), 62A, 741 (1949).
Gas Chromatographic Comparisons of Selectivities of Some Alkali Metal Halides toward Certain Organic Isomers and Homologs Bohdan T. G u a n 1 and L. B. Rogers Chemistry Department, Purdue University, Lafayette, Ind. Six unsupported alkali metal halides were used as adsorbents i n gas-solid chromatography in an attempt to examine the roles played by the cation and anion. From salt to salt, adsorption behavior for homologous and isomeric pairs differed greatly in terms of distribution (capacity) ratios but differed much less in terms of separation factors. Changes in capacity ratios with conditioning of a given salt as well as differences in ratios between salts were attributed largely to differences in the amounts of adsorbed water. Separation of an isomeric pair appeared to depend primarily upon the molecular geometry of the isomer around its dipole.
NEGLECTED for many years, gas-solid chromatography (GSC) is coming into prominence as a useful supplement to gas-liquid chromatography (GLC) in the field of separations. The great potential selectivity of solids, together with their 1 Present address, Industrial Laboratory, East man Kodak Co., Rochester, N. Y.
632
ANALYTICAL CHEMISTRY
high thermal stabilities, makes GSC especially desirable for the separation of isomers and high molecular weight compounds. Small kinetic mass-transfer terms (/) and flat minima in van Deemter plots offer GSC potential advantages over G L C in speed and selectivity. GSC columns with plate heights as low as 0.035 cm, and analysis times within seconds have been reported ( 2 ) . Aside from the analytical applications, GSC is becoming increasingly useful for evaluating thermodynamic functions and kinetics of adsorption (3-10). Greene and Pust (6)first (1) J. C. Giddings, ANAL.CHEM., 36, 1170 (1964).
(2) I. Halasz and H. 0. Gerlach, Ibid.,38, 281 (1966). (3) P. E. Ekrly, J . Phys. Chern.,65, 68 (1961). (4) P. E. Eberly, J . Appl. Chem., 14, 330 (1964). (5) R. L. Gale and R. A. Beebe, J . Phys. Chem., 68, 555 (1964). (6) S. A. Green and H. Pust, Ibid.,62,55 (1358). (7) A. V. Kiselev, “Gas Chromatography-1962,” M. van Swaay, ed., Butterworths, Washington, 1962, p. xxxiv. (8) H. Knozinger and H. Spannheimer, 2. Phys. Chem. (Frankfurt), 45.
117 (1965).
used GSC to determine heats of adsorption for a variety of fixed gases and lowboiling hydrocarbons on a number of solids and compared them with calorimetric and isosteric heats obtained from static measurements. Detailed comparisons of gas chromatographic and isosteric heats of adsorption on graphite and carbon blacks were reported by Ross (9), Eberly (3),Gale and Beebe (9,and Beebe ef af. (11). Kiselev (7) studied extensively the adsorption of various organic compounds on geometrically modified silica gels, while Eberly (4and Scott (IO) have looked at the kinetics of adsorption by GSC. Alkali halides have been extensively investigated with the aim of describing their surface adsorption characteristics (7, 8, 12-15). For a thorough review see DeBoer (16), and Young and Crowell (17). These studies were concerned with theoretical calculations of adsorption energies for such gases OD2, and CHa adsorbed on the 100 and 1 1 1 as Ar, Kr, N2,02, faces of alkali halides. In most cases, the calculated total adsorption energies were smaller than the calorimetric or isosteric heats of aiisorption. The lower calculated values are to be expected because they are based on perfect crystals whose faces are all of one type, and they do not take into account adsorption ton edges, corners, and dislocations. Alkali halides have been used in GSC by coating Chromosorb, silica, or ahmina with these salts to separate high molecular weight 'someric species (13, 18, 19). Scott's investigation (10) of three sodium halides supported on alumina is the mosi. detailed study of alkali halides by GSC. His results indicate that the larger the halide ion, the relatively greater its interaction with unsaturated hydrocarbons, presumably because of greater polarizability of the larger halide ions. Because the alumina support probably influenced the adsorption properties of the halides just as Chromosorb did for thallium nitrate (20), his results may not agree with results obtained using pure, unsupported salts. In the present study, six alkali halides were studied by GSC to determine what factors influence the retentions and separations of selected organic compounds. Of special interest was the effect of cation and anion of the adsorbent and the effect of the structural differences in isomeric compounds on adsorbent-adsorbate interactions. When it was found that very small amounts of adsorbed water markedly influenced the retention behavior or adsorbates, two halides were studied to determine the effel-ts of conditioning on retention times and on separate factors.
was packed in coiled 6-mrn i.d. glass columns about 100 cm long. One end of the glass column was plugged with glass wool and attached to a water aspirator while the salt was poured in. Columns were vibrated by an electric vibrator during packing. Using this procedure, the weight of each salt in a packed column was reproducible to within 3%: NaCI, 13.6 grams; NaBr, 20.0 grams; NaI, 21.3 grams; KCI, 13.3 grams; KBr, 18.2 grams; KI, 20.1 grams. Each column was conditioned at 120" to 125" C, with nitrogen flowing at 25 ml per minute, for approximately 2 days unless otherwise stated. Samples of adsorbates were prepared by injecting 5 pl of each liquid into a 1-liter septum-capped bottle containing nitrogen. Ketones and esters were prepared as two mixtures of four isomers each. A Hamilton gas syringe was used to inject 30 pl of the gas mixture into the column. Using an average liquid density of 0.85, the gas samples injected into the columns were calculated to be about 1 X lo-' gram of each adsorbate. The orders of injection of samples and of column temperatures were randomized so as to minimize systematic errors. An Aerograph 660 gas chromatograph equipped with a flame ionization detector was used. The carrier gas was high-purity Airco nitrogen which had been passed through a trap, 30 cm long and 2.5 crn in diameter, containing 4A Linde Molecular Sieve. Flow rates measured at room temperature were 25 to 27 ml per minute and gave elution times of about 16 seconds for methane, which was used as the nonadsorbed gas. Retention data were recorded on a I-mV Esterline-Angus stripchart recorder, and retention times were determined to *0.2 second using a stop watch. Retention time was converted to distribution ratio or capacity ratio, k, by calculating ( t , - t,)/t, where f, was the uncorrected retention time of the adsorbate and t, was the elution time of methane. A separation factor was calculated by taking the quotient of two capacity ratios. Heats of adsorption, AHa, were determined a t a minimum of three temperatures over a span of about 20" C in the range 80" to 125" C from a plot of log k us. the reciprocal of the absolute temperature. Since each sample was injected at least three times at each temperature, nine points or more were used to calculate AHa values. This was done by a least-squares procedure using a computer program for regression analysis. The program was also designed to calculate from the slopes the capacity ratio at any temperature of interest. The values of k in Figure 1 were obtained in that manner.
EXPERIMENTAL
Retention Times. During preliminary studies, it became apparent that distribution ratios were dependent on column preparation, conditioning time, and conditioning temperature to such an extent that highly reproducible retention times could not be obtained from one column t o another. Variations in k were especially noticeable if a salt from a different bottle was used to prepare the second column. These variations in k may have resulted from the fact that two columns of a given salt may have held different amounts of adsorbed water even after the columns had been conditioned at the same temperature for the same length of time. T o determine the reasons for the variation in retention data, two halide columns were studied as a function of conditioning. Figure 1 shows the variation in k values on a KBr column as a function of conditioning. As the conditioning temperature was raised to about 125" C, retention times generally decreased for most adsorbates (steps A to C of Figure 1). However, further increases in the temperature of conditioning resulted in an appreciable increase in retention times (Figure 1, steps D and E).
Baker and Mallirickrodt alkali halide reagents were used without further purification. Each salt was ground in a mortar and sieved to obtain an 80/120-mesh fraction which (9) S . Ross, J. K. Saelens, and P. Olivier, J . Phys. Chem., 66, 696 ( 1962). (10) C . G. Scott, %;is Chromatography-1962," M. van Swaay, ed., Butterworths, Washington, 1962, p. 36. (11) R. A. Beebe, P. 1,. Evans, T. C . W. Kleinstenberg, and L. W. Richards, J . Phys. C'hem., 70, 1009 (1966). (12) T. Hayakava, Bur% Cliem. Soc. Japan, 30,343 (1957). (13) G. 8.Jamieson, Nature, 207, 358 (1965). (14) F. V. Lene1,Z.Phys. Chem., B23,379(1933). (15) D. M. Young, Trcms. Faraday Soc,, 50,838 (1954). (16) J. H. DeBoer, Ailuan. Catalysis, 8, 17 (1956). (17) D. M. Young and A. D. Crowell, "Physical Adsorption of Gases,'' Butterworttis, London, 1962. (18) J. A. Favre and L. R. Kallenbach, ANAL.CHEM., 36,63 (1964). (19) A. J. Moffat and P. W. Solomon, U.S. At. Energy Comm., Res. and Develop. liept. IDO-16732, 1961. (20) B. T. Guran and L. B. Rogers, J . Gas Chromatog., 3, 269 (1965).
RESULTS
VOL 39, NO. 6, M Y 1967
633
2o
t
*10 O.0' I 9.0 8.0
c.
0
5 4.0 a
I .o
0.9 0.8 0.7 0 . 6 L A
CONDITIONING STEP
Figure 1. Effects on k of successive pretreatments of a potassium bromide column operated a t 86.6" C Pretreatment Steps A . Conditioned at B. Conditioned at C. Conditioned at D. Conditioned at E. Conditioned at
100" C for 20 hours 100" C for 4 days 125" C for 7 hours 180" C for 12 hours 250" C for 15 hours
Adsorbares 1. 2-Heptanone 2. 3-Ethyl-3-pentanol 3. 2,4-Dirnethyl-3-pentanol 4. 3-Heptanone 5. Butyl acetate 6. Ethyl butyrate 7. 2-Pentanone 8. tert-Butanol 9. n-Propanol 10. n-Dodecane 11. Ethyl propionate 12. 3-Pentanone
Because decreases in k observed up to 125" C were thought t o be due to the loss of adsorbed water, and because confirmation of the general trends in Figure 1 seemed desirable, a column of a second salt, sodium chloride, was conditioned until increases in k appeared (Figure 2: steps A to C). Then, 0.2 pi of water was injected onto the column, and there were larger increases in X-(step D). Subsequent conditioning at and above 153" C resulted first in a decrease and then an increase in X- (s!eps E and F). 634
0
ANALYTICAL CHEMISTRY
I/
0
/
I
I
C
D CONDlTlONlNG STEP 0
I
I
E
F
Figure 2. Effects on k of successive pretreatments of a sodium chloride column operated a t 83" C Pretreatment Steps A. Operated below 100" C to obtain AH, data B. Conditioned at 150" C for 1.5 hours C . Conditioned at 200" C for 1.5 hours D. 0.2 p1 water added E. Conditioned at 150" C for 2 days F. Conditioned at 300" C for 2 hours Adsorbates 1. n-Propanol 2. 3-Heptanone 3. 3-Ethyl-3-pentanol 4. Ethyl butyrate 5. Propyl acetate 6. 2-Pentanone 7. fert-Butanol 8. Ethyl acetate 9. 3-Pentanone The decrease in k followed by a n increase appears to result from a double role played by the surface water. In small quantitites, the adsorbed water blocks the more active sites on the surface and prevents a strong interaction of the adsorbate molecules with those sites. At higher coverages, the water appears to contribute to the longer retentions much like a partitioning liquid a; very low loading. There is evidence from contact angie i 2 I ) and surface conductivity (21) W. von Engelhardt, "Adsorption et Croissance Cris:al!ine, No. 152," ed. d u Centre Nationa! de i:i Recherche Scientifique. Paris, 1965. p. 416.
Ratios of k Values as a Function of Successive Conditioning Steps of a Potassium Bromide Column Operated at 86.6" C Conditioning Step Adsorbates A B C D E 0.42 0.42 0.41 0.41 0.40 3-Pentanone/2-pentarione 0.35 0.36 0.36 0.35 0.34 3-Heptanone/2-heptanone 0.14 0.14 0.14 0.12 0.095 3-Pentanone/3-heptarione 0.12 0.12 0.12 0.11 0.079 2-Pentanone/2-heptanone 0.63 0.63 0.60 0.66 0.65 Ethyl propionateipropyl acetate 0.61 0.59 0.61 0.61 0.61 Ethyl butyrate/butyl acetate 0.36 0.35 0.32 0.38 0.37 Ethyl propionate/eth:yl butyrate 0.35 0.33 0.32 0.35 0.35 Propyl acetate/butyl ,acetate 1 .oo 1.14 1.45 1.04 1.01 rert-Butanolln-propanol 3.66 3.55 3.41 4.26 6.62 2,4-Dimethyl-3-pentanol/n-propanol 4.64 4.51 4.39 5.25 8.30 3-Ethyl-3-pentanol/n-propanol 0.29 0.27 0.22 0.28 0.28 rerr-Butanol/2,4-dimethyl-3-pentanol 0.78 0.81 0.80 0.19 0.79 2,4-Dimethyl-3-pentanol/3-ethyl-3-pentanol Table I.
A . Conditioned at 100" C for 20 hours. B. Conditioned at 100" C for 4 days. C . Conditioned at 125" C for 7 hours. D . Conditioned at 180" C for 12 hours. E. Conditioned at 250" C for 15 hours.
Table 11. Ratios of k Values of Alkali Halide Columns Operated a t 86.6' C Alkali halide column Adsorbates NaCl NaBr NaI KCI 0.64 0.43 3-Pentanone/2-pentatione 0.54 0.55 0.58 0.38 0.48 0.48 3-Heptanone/Z-heptanone 0.14 0.15 0.18 0.15 3-Pentanone/3-heptatione 0.16 0.13 0.13 0.13 2-Pentanone/2-heptarione Ethyl propionate/propyl acetate 0.62 0.61 0.67 0.66 0.61 0.61 Ethyl butyrate/butyl acetate 0.61 0.57 0.39 0.39 0.41 0.38 Ethyl propionate/ethyl butyrate 0.36 0.37 Propyl acetate/butyl itcetate 0.41 0.36 0.62 1.G rert-Butanolln-propanol 0.24 0.43 0.35 0.28 rert-Butanol/2,4-dimethyl-3-pentanol 0.34 0.37 0.65 0.79 2,4-Dimethyl-3-pentanol/3-et hyl-3-pentanol 1.oo 0.63
(22) measurements that water adsorbs on alkali halide surfaces in discrete domains even a t very low vapor pressures. It is possible that those domains take part in the partitioning process, leaving the remaining surface free to take part in adsorption. To explain the results in more detail, the following additional interadions may be assumed: hydrogen bonding between the adsorbate molecules and surface water, interactions of the dipoles of the adsorbate with the electrostatic field above the lattice ions, and dispersion and induced dipole interactions of the adsorbate with the halide ions. The above interactions have been previously postulated for inorganic surfaces and polar adsorbates (16, 17, 23) and are reasonable to expect here. There are other types of interactions but their contributions lo the total interaction appear to be small enough to be ignored. The behavior of each adsorbate in Figure 1 and Table I can be rationalized as follows. During conditioning steps A through C, loosely bound water was lost from the surface. This produced a drop in k for all oxygenated adsorbates because of a decrease in li from the partitioning process. (22) A. Oberlin and h l . Hucher, "Adsorption et Croissance Cristalh e , No. 152," ed. d u Centre National de la Recherche Scientifique, Paris, 1965, p. 407. (23) W. J. C. Orr, Tram Faraday Soc., 35,1247 (1939).
__
KBr 0.42 0.36 0.14 0.12 0.66 0.61 0.38 0.35 1.04 0.28 0.79
KI 0.64 0.57 0.17 0.15 0.75 0.70 0.42 0.39 0.97 0.25 0.74
n-Dodecane probably did not take part in this process to any significant degree and, as a result, showed no appreciable change ink. Conditioning the column above 150" C (Figure 1, steps D and E) appears to have exposed more energetic sites on the surface and caused an increase in k for all adsorbates. The relatively greater increase in k for n-dodecane, compared t o ketones and esters, suggests that most of the uncovered sites must have been the more polarizable halide ions, especially since the ratios of k of two homologs also started to change a t that point. During that same conditioning interval, there were much smaller increases in k for alcohols which can be rationalized as follows. Since n-propanol had the greatest hydrogen-bonding ability, the contribution to k from hydrogen bonding was very large, even on a moderately dry surface. When the surface was dried further, the loss of water increased substantially the interaction of n-propanol with the more energetic sites, but the simultaneous decrease in its hydrogen-bonding to adsorbed water limited the increase in k t o a small net change. The behavior of teri-butanol, which showed a larger net change in k as surface drying progressed, is in line with its weaker hydrogen-bonding ability. Separation Factors. Because values of k were very sensitive t o column conditioning, they did not represent a reliable basis for comparing the adsorption behavior of different columns. For that reason, the use of separation factors VOL 39, NO. 6, MAY 1967
635
Table In. values of dAGE, for Isomeric Pairs (cal/mole) Alkali halide column Isomericpair NaCl NaBr NaI KCI KBr KI Ketones 43 43 42 39 49 36 Esters 0 22 22 20 25 21
was explored. When separation factors were calculated for isomeric pairs, the values for a single salt were almost independent of conditioning as shown in Table I. Hence, it seemed reasonable to use separation factors as a basis for comparing columns of different sodium and potassium halides with less concern for a n absolute value for the amount of adsorbed water remaining after a n identical pretreatment of each column. Table I1 lists separation factors for the six halide columns. Generally, the values obtained under a given set of conditions were reproducible t o within +0.02 and often within +0.01. The ratios for isomeric ketones were surprisingly similar on the chloride and bromide salts of each cation, but were significantly different for each cation. Thus, the isomeric ketones were better separated on the potassium chloride and bromide columns than on the corresponding sodium halides. In contrast, the ratios taken from the two iodide salts appeared to be independent of the alkali metal ion. The data indicate that, if the anion was not disproportionately larger than the cation, the interaction of the carbonyl dipole in ketones depended not only on the steric configuration of that dipole but also on the cation of the solid. In the case of the iodides, however, the greater polarizability of the anion appears to have been the governing factor. Separation of isomeric esters was not influenced by a change in cation, and was poorer compared to the ketone isomers, even though the esters differed much more in their boiling points. This was probably due to the more open molecular structure around the dipole in the esters and, consequently, less steric influence on the dipole interaction. If a change in free energy, AGu, for the addition of one CHZ group to a ketone or a n ester is calculated from the ratios in Table 11, a value of about 7 kcal per mole per CHz group is obtained. This value is independent of the column material and the adsorbate molecule t o which the group is added. Table I1 also shows that the higher isomeric pairs of ketones and esters were better separated than the lower pairs on each column-Le., the ratios are smaller. It can be shown that this difference is due mainly to the greater contribution to k from the added CHz groups in the higher homologs rather than t o a difference between the homologs in the steric environment of their dipoles. Consider the change in free energy of adsorption between the isomeric homologs, dAC. For the pentanone isomers this becomes: dAGp =
AGz-pentanone
- AGa-pentpnone
Although k is not a thermodynamic constant and cannot be used easily t o calculate free energy of adsorption, ACu, ratios of k of two compounds can be used to calculate the difference in their ACuvalues. The above equation can then be written as: dAGp = --RT In (k2-pntpoonejk~-pentanone).
Since dAGp and dAGH are directly related to the separation of each isomeric pair, the difference in separation, (dAcE,),
is then a measure of the increase in separation of the higher isomeric pair. A similar dAGE, can be determined for the homologous isomers of esters. Using the values for the ratios of k from Table ,II, dAGE, values were calculated for the ketone homologs (addition of two CHZ groups), and ester homologs (addition of one CH, group). The calculated values are listed in Table 111. From these values it is apparent that dAGE, for ketones was about twice as large as for esters, so that the difference was about 21 cal per mole for each added CHz group for both classes ,of compounds. If the increased separations of the higher homologous isomers were due to differences in the steric environments of their dipoles, one would not have expected these results. The chief exception was that of the esters on sodium chloride where the value for dAGE, was zero. Rationalizing the behavior of alcohols on the six columns is more difficult because, as discussed earlier, their retentions depended to a significant degree on their abilities to hydrogen bond. The most obvious difference for the alcohols in Table I1 was the fact that, on the sodium halide columns, n-propanol was held up much longer than terr-butanol and eluted about the same time as diisopropyl carbinol. In contrast, on the potassium halide columns, n-propanol eluted with fert-butanol but was very nicely separated from diisopropyl carbinol. The remaining two ratios for alcohols in Table I1 also differed significantly between the sodium and potassium salts but showed little change with halide ion. The ratio changes for alcohols between the sodium and potassium halides may be attributed to a greater hydrogenbonding contribution on the sodium halide columns. This supposition is supported by the work of Whetten (24) who analyzed bursts of gases released during cleavage of alkali halide crystals. The mass spectrum obtained from sodium chloride showed a large peak a t mass 18 (water) but only a small peak a t mass 28 (carbon monoxide). For the potassium halide crystals, the intensities at the two mass units were reversed. As expected, the hydrogen-bonding effect was felt most strongly by the primary alcohol, n-propanol, and consequently it was held up relatively longer on the sodium halides. It is interesting to note that one can differentiate between the hydrogen-bonding abilities of diisopropyl and triethyl carbinols from the ratios of k . The larger ratios on the potassium halides suggests that triethyl carbinol hydrogen bonded more strongly. Other Studies. AHu values for each adsorbate were obtained o n each of the six columns within 6 hours after completing the conditioning at 110" t o 115" C for 2 days with carrier gas flowing. The values were obtained between 80" and 125" C and ranged from 10 to 18 kcal per mole depending upon the conditioning, but were surprisingly constant, usually within 375, for a given compound following any given type of conditioning. The values appeared to depend little on the column material, considering that changes up to 5 kcal per mole were found for a given adsorbate after differen1 pretreatment of potassium bromide alone and that one had no assurance that the same pretreatment of columns of ditTerent salts led to a corresponding condition of dryness. 4 ::hange
A similar expression can be written for the trvo heptanones
dAGa
636
c
=
-
R T l n (k2-leptanonejkj-heptanone)
ANALYTICAL CHEMISTRY
(24) N. R. Whetten, J. Vacuum Sci. Tech., 2, 84 !!31.,
in the sample size by a factor of lo0 sometimes changed retention times by as much as 30z,but rarely ever changed AH, by as much as 5 (usually 1 t o 2
z).
DISCUSSION
It is important to note that separation factors for isomeric pairs of ketones and esters underwent smaller relative changes with changes in conditions than did homologous pairs. However, the isomeric pairs were more sensitive to changes in cation or anion of the adsorbent. This behavior helps to reinforce the conclusion that use of separation factors of isomers provides a reisonable basis for comparisons of adsorbent selectivitites. The fact that separation factors for homologs show greater differences than for isomers is not surprising considering that homologs have: a different number of CH, groups. Thus comparing homologs (of which the alcohols were the most extreme cases studied) represents the first step in the direction of comparing compounds having different compositions and structures. In addition, the separation factors for the alcohols reflected the fact that the OH group in an alcohol will itself exhibit greatly different hydrogen-bonding characteristics depending on the structure of the alcohol and the amount of moisture on the sarface of the adsorbent. Hence, comparisons of different adsorbents using alcohols would be least reliable. even i!r isomers were used. From the analytical standpoint, the present study has raised several points. First, how should one select a column for, as an example, a separation of ketones? It is clear from Table 11, potassium chloride and potassium bromide have separation factors which are most favorable for separating isomers and are equal, or better, than others for separating homologs. However, one must not overlook the fact that retention times are also important. For a given separation factor, an analysis may be unsatisfactory if the retention times are so short that the peaks overlap badly or if the times are so long that much timi: is wasted on the base line between peaks. In other words, the resolution is a function of retention times for a given separation factor. In such a case, the choice between potassium chloride and bromide may depend only on the ease and reproducibility of conditioning the salt so as to attain the optimum retention times.
A second related factor is the desirability of using partly deactivated columns. Examination of Table I shows that separation factors, even those for the alcohols, were most nearly constant when the column was “dried” under mild conditions (125” C or less). Presumably, all of the halides would be similar, though the upper temperature at which the changes became more marked would probably differ. Another reason for using partly deactivated columns is that tailing becomes more serious as the activity-ie., the heterogeneity of the energies of surface sites-of the column is increased. In the present study, the front-to-back ratio at the base line of a peak was sometimes much less than unity (-0.2) (especially for alcohols which tailed badly) when columns were conditioned at 250” C. At that temperature, the potassium bromide column still showed improved resolution as a result of the increased retention time from the higher drying times. However, it is conceivable that at some higher temperature, improved separation of peak maxima might not result in improved resolution at the base line. The final point to recognize in connection with the present study is the practical limitation on sample size that results from the use of pure salts rather than salts coated on a support. In our preliminary experiments, we found that coated columns produced much longer retention times per unit of column length, and that the retention times were much less sensitive to changes in sample size. The chief reason for studying the unsupported salts was to eliminate the possible influence of the support on,the observed selectivities (20). A study of supported salts parallel to the present one, but more extensive than the one by Scott (IO),is in order. ACKNOWLEDGMENT
The writers thank James E. Oberholtzer for developing the computer program to calculate heats of adsorption.
RECEIVED for review July 1, 1966. Accepted March 3. 1967. Research supported in part by the U.S. Atomic Energy Commission under Contract AT(11-1)-1222.
Vc),.
3P, NO. 5. iirAY i 9 6 T
637
