S
=
tc
=
U
=
F,'
=
K L
=
sample size temperature of the column
vi 1'0
= = =
mean linear velocity of the gas volumetric flow rate of the carrier gas at 20" C. and 1 atm. pernieability constant column length liquid volume of the column gas volume of the column
ACKNOWLEDGMENT
It is a pleasure to thank E. E. Wegner for the rewltq referred to in Figure 6, as well as for many helpful and fruitful discussions. The authors are grateful to DEGI-SSA, Frankfurt am Main,
R e r k Rheinfelden, Germany, for presenting us with highly dispersek metal oxides. LITERATURE CITED
(1) Brooks, V. T., Collins, G . A,, Chem. Ind. (London) 1956,p. 921. ( 2 ) Bruderreck, H., Schneider, W., Halitsz, I., ASAL. CHEM.36, 461 (1964). (3) Brunner, H., Pharm. Znd. 20, 581 (1958). (4) Desty, D. H., Goldup, A, Saanton, IT. T.,A\-atztre 183, 107 (1959). (5) Furuyania, S., Kwan, T., J . Phus. Chem. 65, 190 (1961). (6) Greene, S. -4.) Pust, H., ANAL.CHEM. 29, 1056 (1957). (7) Halitsz, I., Heine, E., Suture 194, 971 f 1962) ( S j Haibz, I., Horvitth, C., AKAL. CHEM.36, 1179 (1964).
(9) Halitsz, I., Horvitth, C., A'ature 197, 71 (1963). (10) Halitsz, I., FTegner, E. E., BrennstogC'hemie 42. 261 (1961). (11) Halhsz,' I., ivegner, E. E., Yature 189, 670 (1961). (12) Scott, C. G., J . Znst. Petrol. 45, 118 (1959). 13) Scott, C. G., Itoffell, D. A , , Sature 187, 143 (1960). 14) Scott. K . P. W..Benzol Producers. Ltd., Witford, England, Research Paper 8-1957. 15) IVegner, E. E., Dissertation, L-niversitat Frankfurt am lIain, 1961. 16) Zlatkis, A . , Ling, S., Kaufman, H. R., SAL. CHEM.31, 945 (1959). RECEIVED for review February 10, 1964. Accepted July 29, 1964. Presented at the International Symposium on Advances in Gas Chromatography, University of Houston, Houston, Texas, January 21-24, 1963.
Vapor Prlessure of Stationary Phases over Gas Chromatographic Supports STEPHEN J. HAWKES and J. CALVIN GlDDlNGS Department o f Chemistry, University o f Utah, Salt lake City, Utah Several methods have been explored for measuring the vapor pressure of the staiionary phase in GLC columns. The value is approximately equal to that of the bulk stationary phase when the latter is supported on Chromosorb P at loadings above 2%. At lower loadings the vapor pressure drops sharply, approximately in accordance with theory. The implications of these results are discussed. APOR PRESSURE O F STATIOKARY PHASES above the solid support in
gas-liquid columns has been the subject of considerable speculation, but no quantitative documented report has yet dealt with the subject. h considerable amount of indirect evidence, summarized elsewhere ( 2 , 3), indicates that vapor pressures are reduced very little below bulk liquid values. The present report is designed to lessen the present uncertainty by measuring the vapor concentrations and losses directly. X significant reduction of vapor pressure below bulk values would be important, in several ways, the most obvious of which is a decreased bleed rate and thus longer column life. However, it is probably more important that such a reduction would reflect an attraction between the support and the stationary phase, strong enough also to enhance the retention of solutes; thus retention parameters would depend on the nature' and loading of the support and would lose much of their value for identification. Moreover, so strong an
attraction would influence the configuration of the liquid on the solid and thus strongly affect the column efficiency ( 2 ) and probably the retention parameters also. Hence the effect of the support on the vapor pressure of a liquid is of considerable importance in gas-liquid chromatography. Such interactions are probably also important in liquid-liquid chromatography since the adsorption forces are much the same.
(the -CH2band) was deterinined in each case. Exactly 2 liters of nitrogen were passed through ethylbenzene columns a t room-temperature (ea. 20" C.) with various loadings, and then into a I,'*inch 0.d. copper column containing methylcyclohexane cooled in acetonesolid CO,. l h e liquid was washed out of the tube with methylcyclohexane into a 25-ml. volumetric flask, made up to the mark, diluted 2 to 25, and the absorbance was determined a t the maximum near 260 mp.
EXPERIMENTAL
The vapor pressures of ethylenediamine, ethylbenzene, m-cresol, and water supported on Chromosorb P were investigated by various methods. A m-cresol column w-as weighed, eluted: and reweighed, and the vapor pressure of the stationary phase was calculated from loss in weight, elution time, and flow rate. The retention time of toluene was measured on this column bpfore and after elution, and the fractional loss of stationary phase, hVI/'VL, was calculated from the change in the adjusted retention time (peak minus air time) according to the formula
A similar system n a s used n i t h pentane on an ethvlbenzene column, and ether on a water column. Pre,-.ure drop A as negligible. Sitrogen wab passed through ethylenediamine at room temperature (ea. 25' C.) on Chromosorb P a t variouq column loadings and was led into a gas cell, the absorbance a t 3 4 microns
RESULTS AND DISCUSSION
The results are shown in Tables I and I1 and Figures 1 and 2. In several cases it was not possible to reduce the precision to the desired level. These results show that at loadings above 1 or 2y0 the vapor pressure is approximately that of the bulk liquid. h t lower loadings the vapor pressure drops sharply. The specific surface area of Chromosorb 1' is given as 1.4 sq. meters per gram b>-I3randt (9) and as 4 sq. meters per gram by Ottenstein ( 6 ) and Pecsok ( 7 ) . Martin ( 5 ) givps the surface area at various column loadings, and reports 2.8 sq. meters per gram for the lowest loading tried, indicating a higher figure for the naked support. Accordingly, 4 sq. meters per gram has been assumed for the calculation of the number of monolayers shown in Figures 1 and 2. *4ssurning that the inolecnle is cubic, it appears that about five monolayers are necessary to approarh the bulk vapor pressure. VOL. 36, N O . 12, NOVEMBER 1964
2229
as
033
066 la
50
20
10
%LOADING
..
Figure 1 . Volatility of ethylbenzene over Chromosorb P (arbitrary units)
The low apparent vapor pressure of ethylbenzene at lY0 load, when determined by retention time, was caused by the adsorption of the sample on the Chromosorb, the retention time being the same with 1% load as when naked. Hence this method is unreliable. The use of water as stationary phase with ether as sample proved unsatisfactory also, because the retention time increased with the elution of water, presumably because of increased adsorption on the support. It is interesting to compare the reduction in vapor pressure a t low loads with that predicted theoretically. An earlier treatment (1) showed that the thickness, d,, of a liquid adsorbed on a flat surface should be given by d,3
Table 1.
=
V,AE NRT In p 0 / p
(2)
Relative Volatilities by Absorbance Method
Stat. phase, ilbsorbg./100g. ance, Chromosorb P mM Monolayers ETHYLENEDIAMINE 0 336 600 100 0 331 450 75 0 333 240 40 0 359 120 20 60 0 337 10 30 0 290 5 6 0 274 1 6 0 204" 1 4 0 130a '/3 2 0 025" 1/3 ETHYLBEKZENE m b 0 568 0 468 350 75 0 568 100 20 0 505 50 10 0 538 25 5 0 530 10 2 0 576 5 1 2 0 184 0.4 0 063 1 0.2 0 025 0.5 0.1 0.25 0 025 0.05 a Determined at a later date, probably with a slight teniperature difference. b Unsupported free liquid. mb
2230
ANALYTICAL CHEMISTRY
X LOADING
Figure 2. Volatility of ethylenediamine over Chromosorb P (arbitrary units)
if there are several (>3) monolawrs. If the number of monolayers is given by n = d,/6 where 6 is the thickness of a mondayer, calculated as above, then 1
This curve has been plotted to yield the theoretical curves in Figures 1 and 2, taking the value of AE as 1000 cal. per mole as estimated in the original paper. I t has been extended to lower loadings than those to which the equation is properly applicable, but indicates the general shape and is in reasonable agreement with the experimental data. The result is similar to the experimental plots although the predicted drop-off in vapor pressure occurs with several times less than the observed liquid load. This may be due in part to liquid tied up in very fine capillaries. The agreement is on the whole satisfactory and confirms the deduction ( 1 ) that stationary phase liquid approaches the bulk form, with attendant bulk properties, a t most practical loadings.
NOMENCLATURE
d,
=
AE
=
II'
= =
p p,
R t'
0
= = =
T
=
VI
=
V,
=
6
=
thickness of liquid film difference in interaction energy per gram-atom between liquidliquid and liquid-solid pairs Avogadro number vapor pressure of supported liquid vapor pressure of bulk liquid gas constant adjusted retention time (peak minus air) absolute temperature volume of stationary phase in column molecular volume of stationary phase thickness of one monolayer
Table II. Vapor Pressures by Weighing and Retention Time
Stat. hase Toad, g./100 g. Chromosorb T , P "C.
Literature EXPt. P, p, ' mm. hlonomm. ( 4 , 8) layers m-CRESOL B Y WEIGHING 12.5 52 1.4 0.9 51 12.5 84 6.lY 7.9 51
m-CRESOL
(FROM
RETENTION TIME
'YOLUESE
6 4.5
85
7.7
85
7.6
OF
)
8.2 8.2
25 18
ETHYLBENXENE (FROM RETENTION TIME OF WPEKTANE) 36 30b 14 11 180 4 32b 14 14 20 8 12 10 2 31b 1 30b 0 11 5 a Mean of six readings, u = 0.2. There is some uncertainty in these temperatures. LITERATURE CITED
(1) Giddings, J. C., ANAL. CHEM. 34, 458 (1962). (2)Ibid., 35, 439 (1963). (3) Hftwkes, S.J., Mooney, E . F., AKAL. CHEM. 36, 1473 (1964). (4)Jordan, T. E., Vapor Pressure of Organic Compounds," Interscience, 1954. (5) Martin, R. L.,AKAL. CHEM.33, 347 Il!Xl) \ - - - - I
(6) Ottenstein, 1). >I., J . Gas Chromalog. 1, S o . 4, 11 (1963). (7) Pecsok, R. L., de Yllana, A . , AbdulKarim, A,, ANAL. CHEM. 36, 465 (1964): (8) Rossini, F. D., "Selected 1.alues of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds," p. 362, Carneyie Press, for American Petroleum Inst,itute, 1953. (9) Scholz, 12. G., Brandt, W. \V., in "Gas Chromatography," ?u'. Brenner, et al., eds., Chap. 11, Arademic Press, S e w York, 1962. RECEIVEDfor review J u n e 29, 1964. Arcepted September 2, 1964. Work supported by the I-. s. Atomir Energy Commission under Contract To. AT( 11-1)748, A.E.C. COO-748-37, and by a grant from the Xational Science Foundation.
