Solute Adsorption at the Gas-Liquid Interface in Gas-Liquid Chromatography DANIEL E. MARTIRE Department of Chemistry and Institute for Advanced Analytical Chemistry, Georgetown University, Washington, D.C. Recent static measurements of partition coefficients for solubility and adsorption have verified that solute adsorption at the gas-liquid interface can significantly affect chromatographic retention behavior. GLC measurements are made for polar solutes (normal alcohols) on polar liquid phases (glycerol and P,P'-oxydipropionitrile). These data and existing data permit identification of two distinct classes of adsorption on the liquid phase. The conditions and types of systems for which adsorption becomes an important factor, and its effect on retention data, thermodynamic values, and column efficiency are discussed.
M
(12) was the first to observe that the elution order from gaschromatographic columns containing polar stationary liquid phases changed with the ratio of liquid phase to solid support and with the surface area of exposed liquid. He attributed these changes to adsorption of solute molecules a t the gas-liquid interface, a heretofore neglected factor. -4new retention equation was suggested to account for this adsorption effect: ARTIN
+
V R ~ '= ~ V Lk a A ~
(1)
where VRg0is the corrected net retention volume per gram of packing, k, is the gas phase-bulk liquid partition coefficient, V L is the volume of liquid phase per gram of packing, k , is the gas phase-liquid surface partition coefficient (adsorption partition coefficient), and A t is the exposed surface area of liquid phase per gram of packing. Equation 1 was found to represent accurately the observed retention behavior, the concept being that the retention time depends upon adsorption a t the liquid surface, as well as upon solution in the bulk liquid. Martin discovered that adsorption on the liquid is most important when the liquid is highly polar and a poor solvent, the solute is nonpolar, the surface area is high, and the ratio of liquid to support is low-Le., low-loaded columns. It became apparent that if his findings and hypothesis mere correct, partition coefficients and activity coefficients determined by gas-liquid chromatog244
ANALYTICAL CHEMISTRY
raphy (GLC) for polar liquid phases would be in error unless corrected for adsorption at the gas-liquid interface, Furthermore, retention volumes and retention indices would become more difficult to catalog; it would be necessary to specify the per cent and surface area of the liquid phases, and k , as well as k, would have to be evaluated. Otherwise, for instance, one might unhappily discover that a reported separation of a multicomponent mixture using a 2% column liquid loading could not be carried out a t 10%. On the positive side, however, the new variable-the extent of liquid phase adsorptionintroduces an extra measure of selectivity in separations. In view of the important and controversial nature of this concept, Pecsok (19) extended and amplified the types of measurements made by Martin (12) to cover a wider variety of solutes and lower loaded columns. He concurred with Martin's finding that a t low loadings the surface contribution may even predominate over solubility > ~ . V L ) .Both researchers took the precaution to prove that the observed effect was not explainable by adsorption of solute on the solid support. To corroborate his GLC measurements, Martin (IS) studied adsorption a t the gas-liquid interface by another method. He showed that the Gibbs adsorption equation, which describes solute concentrations a t the gas-liquid interface, could be used to relate the partition coefficients, k , and k,, to the limiting value (infinite dilution) of the rate of change of solution surface tension with solute mole fraction, ( b y l a x ) " Briefly, since
then,
where rdl)is the Gibbs solute surface excess concentration in moles per square centimeter, x is the mole fraction of solute in the bulk liquid, R is the gas constant, T is the temperature, and n L is the moles of liquid per unit volume of the liquid.
To evaluate the Gibbs equation, the decrease in surface tension was measured with a du Nouy tensiometer at a single mole fraction, 0.01. Then, by assuming that the measured value of the surface tension lowering was equal to the limiting value, good agreement with the GLC data was obtained, apparently confirming his adsorption hypothesis. The significance of Martin's work has never been fully appreciated. The lack of acceptance was partially justified because the evidence offered to substantiate his hypothesis was not entirely conclusive. The values of k , and k , were never determined by an independent method for compa,rision with the GLC data. Recently, however, Martire, Pecsok, and Purnell (15, 16) have made measurements in a single well-thermostated chamber containing a ILlcBain balance and du Souy tensiometer, Solute activity coefficients and solution surface tensions of highly dilute solutions were determined. These values were converted to the partition coefficients through the relationships :
RT n L k, = f P m PZ0
- -2fPrn P2O
(4)
(2)"
(5)
where j," is the solute activity coefficient a t infinite dilution determined by extrapolation, ( d y l b s ) " is the slope of the plot of solution surface tension us. solute mole fraction as x -+ 0, and p Z o is the saturated vapor pressure of the pure solute. A direct comparison of these static measurements with those previously made by gas chromatography shows good agreement if the proper value is used for the surface area of the liquid spread on a porous support. Thus, their work unequivocally substantiates the hypothesis that excess surface concentrations may play an important role in determining the magnitude of the retention volume, and hence the bulk partition coefficient and activity coefficient measured chromatographically. The present study was initiated because the importance of this new
of a soap film flowmeter. The flow rate (45 to 120 ml. per minute) was adjusted, by varying the irilet pressure, to give convenient elution times and near-optimum column efficiency. The outlet pressure was a t all times atmospheric. Individual samples were injected from a 10-111. Hamilton syringe, using the smallest detectable sample size; reproducibility of sample size was not important in this work. Solute retention times past the air peak were determined by taking an average of a t least three measurements. RESULTS
VL x 10'
,-
Specific retention volumes, V,", were calculated from the retention times and the column operating conditions by using the well-known expression developed by Phillips and coworkers (11) (Table 11). Only in the case of methanol was there a nonrandom variation of V," with the per cent liquid loading. By calculating V R ~ "(see ilppendix), rearranging Equation 1,
crn.
AL
Figure 1 .
Determination of partition coefficients, k, and k , Q
A
Methanol in ODPN at 60.3' C. Methanol in glycerol at 62.4' C.
concept is still being disregarded by many. Its purpose is threefold: To examine systems consisting of highly polar solute molecules and highly polar liquid phases, where solute orientation on the liquid surface could be significant. To attempt to define the conditions whereby adsorption at the gas-liquid interface becomes an important factor. To relate the effect of adsorption on retention volumes, Kovhts retention indices, activity coefficients, partition Coefficients, and column efficiency. EXPERIMENTAL
Employing the stationary liquid phases, glycerol (from Matheson, Coleman and Bell) and p,p'-oxydipropionitrile (ODPN) (from Eastman Organic Chemicals), retention volume data were obtained for four solutes: methanol, ethanol, 1-propanol, and 1-butanol. The general procedure has been described (14, l?'). Only the salient features are discussed here. Three glycerol columns and three ODPN columns were prepared in the usual manner. The packing materials consisted of 2, 5, and 10% by weight of liquid phase on 60- to BO-mesh Chromosorb-W which had been treated by the manufacturer (Analabs) by acid washing, base washing, and siliconizing. All materials were used without further purification or treatment. The columns were made from approximately 6 feet of l/4-inch O.D. copper tubing, subsequently coiled. Table I summarizes the pertinent column characteristics. The liquid phase densities
were measured by using a pycnometer and a constant temperature bath controlled to 1 0 . 0 5 " C. by a Techne Tempunit-8. The reported exposed liquid surface areas are Pecsok's (19) values for B,P'-thiodipropionitrile on 60- to BO-mesh Chromosorb-W. The glycerol columns were installed in an Aminco Research gas chromatograph, and the ODPN columns in a Barber-Colman Model 5000 gas chromatograph. Both units were equipped with thermistor-type thermal conductivity detectors and 0- to 1-mv. recorders. The column temperatures were measured by employing a Leeds & Northrup Precision potentiometer, Model 6882,in conjunction with copperconstantan thermocouples attached to various points on the column. The glycerol columns were maintained at 62.4" =k 0.2" C., and the ODPN a t 60.3" It 0.2" C. The carrier gas (helium) flow was measured by means
Table
I.
and plotting (VRgo/AL) against (VL/AL), the values of k , and k , were determined from the slope and intercept, respectively, of the straight-line plot (see Figure 1). The values so determined are: For methanol in ODPN, k, = 106.2 and k , = 12 x 10-8 em. For methanol in glycerol, k , = 258.9 and k, = 96 X 10-6 em. Listed also in Table I1 are the values of the bulk partition coefficients for ethanol, propanol, and 1-butanol. For these compounds, k , was calculated from the average V," value and the well-known expression
(7) where p is the liquid density. The solute activity coefficients, (f,"), were calculated from Equation 4. The saturated vapor pressures of the pure
Column Characteristics
Glycerol, Molecular Weight, M = 92.09; Liquid Density at 62.4' C., p = 1.236 g./cc. Liquid weight % 1.961 5.039 10.103 Packing, grams 11.5682 11.5352 12.5537 Liquid phase, grams 0.2269 0.5813 1.2683 Surface area of exposed li uid, sq. meters per gram of pacPring 0.92 0.74 0.67 Oxydipropionitrile, Molecular Weight, M = 124.14; Liquid Density at 60.3" C., p = 1.018 g./cc. Liquid weight yo 2.018 5.016 10.014 Packing, grams 11.1250 12.2305 11.9418 Liquid phase, grams 0.2245 0.6135 1.1959 Surfacearea of exposed liquid, sq. meters per gram of packing 0.92 0.74 0.67
VOL. 38, NO. 2, FEBRUARY 1966
a
245
Table II. Specific Retention Volumes and Bulk Partition Coefficients
V,", liquid weight % Solute
1.961
5.039
Av. V,"
10.103
ka
Glycerol at 62.4" C. Methanol Ethanol 1-Propanol 1-Butanol
206.7 i61.3 187.4 242.8
182.7 161.1 185.6 237.6
175.7 163.8 187.3 238.5
170.6~ 162.1 186.8 239.6
2.5s D 246.0 283.5 363.6
85.5" 108.2 191.6 356.0
106.2 134.4 238.1 442.4
V,", liquid weight % 2.018
5.016
10.014
Oxydipropionitrile at 60.3" C. Methanol 89.8 87.1 86.0 Ethanol 108.9 107.6 108.1 1-Propanol 191.6 191.3 191.9 1-Butanol 355.5 357.4 356.1 a V," value if adsorption term (k,A L) were zero. Table 111.
Vapor Pressures, Second Virial Coefficients, Uncorrected Activity Coefficients, and Corrected Activity Coefficients
Solute
p 2 0 , mm.
B, cc./mole Glycerol a t 62.4' C.
Methanol Ethanol 1-Propanol 1-Butanol
695.0 395.7 161.2 67.71
- 1267 - 1651 - 1052 - 772
Methanol Ethanol 1-Propanol 1-Butanol
638.7 357.9 146.4 60.30
fP
fP
1.561 2.885 6.145 11.41
1.63 2.98 6.18 11.4
Oxydipropionitrile at 60.3" C.
- 1267 - 1651 - 1052 ~~~
-772
solutes, p z o , were determined a t 60.3' and 62.4' C. by means of the Antoine equation : log p2' (mm. of Hg) = A
B -(8) t+C
where A , B , and C are the Antoine constants for the compound and t is the temperature in degrees centigrade. The constants were evaluated by an interpolation procedure on the published experimental vapor pressure data (23) known as Thomson's method (22). Table I11 lists the vapor pressures, in units of millimeters of mercury absolute, and the computed activity coefficients. Finally, to arrive a t the corrected or true activity coefficients, one must correct for the nonideality of the vapor phase. This is readily accomplished through the expression (4)
where B is the second virial coefficient of the solute vapor. The virial coefficients were determined at 61.4OC. For methanol, the value was obtained from the relation (6)
for ethanol, from the relation ( I ) ,
B
=
246
- 1534 - 7.0883 X lO-'exp ANALYTICAL CHEMISTRY
(63T28) --
2.514
3 . .544
4.894 6.394
2.61
3..8_._5
4.92 6.41
and for 1-propanol and 1-butanol, because of the lack of published data, the values were estimated from the corresponding states expression of Beattie and Bridgeman (21)
B
= V,[0.461
-
1.158
(2)-
0.503
preciable orientation on polar liquid surfaces. Partially because of this omission, the present study was undertaken. I n this discussion the ratio k,/k, is taken as the gauge in comparing the relative magnitudes of adsorption. The most pronounced case of adsorption was observed for 2,2,4-trimethylpentane (TMP) in @ ,@'-thiodipropionitirile (TDPN) a t 25" C. For this system Pecsok (19) found that k , = 246.7 x cm., and k, = 15.7; thus, k,/k, = 1.57 X lO-5cm. For comparison, some available data (not obtained by GLC methods) were examined for two polar-polar systems, propanol and butanol, in the liquid phase water at 25' C. For these systems, Burnett (2)has measured partition coefficients k,, by an entrainment method, and Meissner and Michaels (28) have measured solution surface tensions a t various solute mole fractions, Froin ~ calthe latter data, ( d y / d ~ ) was culated from the limiting value (z -+ 0 ) of their Szyszkowski-type equation for surface tension. Then, k , ivah determined from Equation 5 and k J k 8 calculated (Table IV). It is apparent from the k,/lc, values that the butanol-water system, if studied by GLC, 11ould have adsorption effects comparable in magnitude to the TMP-TDPS system. ;It this point it would be inteiesting to investigate whether such large effects could occur in more typical polar-polar GLC systems, especially those where hydrogen bonding might be taking place. An attempt to resolve this matter led to the GLC measurements described above. Surprisingly, in face of the large effects for the butanol-water sj.stem, only in the case of methanol mas there any effect; a t that, it was m a l l : for methanol in glycerol, k,/k, = 3.71 x 10-7 cm.; for methanol in ODPX, k,/k, = 1.13 X 10-7 cm. For the solutions containing ethanol, propanol, and butanol there was no discernible variation of specific retention volume with the weight per cent of liquid phase; hence zero values for k , and k,lk,. One is now confronted with this perplexing situation-why do foine systems involving polar solutes and polar solvents exhibit large surface effects, nhile others do not? One might also ask, ''Is there any general correlation between some theimodynaniic property of the solution and the niagnitude of the adsorption effect?" Martin (12) states that the preferential adsorption of solutes depends largely on the polarity of the liquid phase, being most likely to occur when the liquid phase is highly polar and a "poor solvent." Pecsok (19) points out that when the nature of the solute and liquid phase is such that the surface tension of the liquid layer decreases with
(31
where T , and Ti, the critical constants, can be found in the tables of Kobe and Lynn (9). The values for B are given in Table 111, with the values for fzm computed from Equation 9. DISCUSSION
While adsorption at the gas-liquid interface is negligible for all solutes, nonpolar and polar alike, on nonpolar liquid phases, it is an important effect when dealing with nonpolar solutes on polar liquid phases. Some question still remains as to whether polar solutepolar liquid phase systems could display adsorption effects of comparable magnitude. Pecsok (19) studied some polarpolar systems, concluding that liquid surface adsorption effects were small for these except at very low loadings. However, none of his systems contained solutes with both large dipole moment and strong hydrogen-bonding propensity, thus possibly capable of ap-
accurately predicting the value of (bylbz) for a solution from the surface tensions of the two pure components, it seems unlikely that any simple correlations with y 2 O can be found. For instance, if one attempts to correlate the "adsorbability" (the product, k,pzO) with the surface tension of the pure solute, as Martin ( l a ) did for some of his systems, one can immediately see that there is no strong correlation, mainly because of the large scatter of the oxy compounds. However, if one now plots k,/k, against f, (see Figure 2), one observes a definite correlation between these variables, with the relative magnitude of solute adsorption a t the gas-liquid interface (k,/k,) increasing with the increasing nonideality of the solution
15
d
10
6 Y
2 d
5
(f,").
0
Activity Coefficient, fp"
Figure 2.
k,/k, vs. activity coefficient, fpm,for solutes in TDPN at 25' C.
adsorption of the solute, this phenomenon will occur. Both statements are essentially correct, since they are based on experimental observations. However, neither gives tlic criterion for predicting the types of systems that display large adsorption effects and the extent to which they will take place. Let us consider in more detail Pecsok's (19) data on the liquid phase p,P'thiodipropionitrile. From his k , values and Equation 4, the activity coefficients, f p m , were calculated. The vapor pressures, pz", a t 25' C. were obtained directly for the pure hydrocarbons from Dreisbach's compilation (3). However, vapor pressure data were available for only three of the oxy compounds. For these, p z o values were determined a t 25' C. by applying Thornson's interpolation procedure (22) on the published experimental data (23). In Table V are summarized the values for k , and k , [from Pecsok's (19) Table 1111, k,/k,, pz", f p m , and the surface tension, y~', of the pure solutes (3,23). Each solute has a much lower surface tension than the solvent; thus the surface tension of the solution will be less than that of the pure solvent. Furthermore, a cursory sight inspection will verify that there is no simple correlation between k,/k, and y2', or between k , and y z o . In fact, recalling the data for alcohol-glycerol systems, the k. values are zero (or so near to zero as to be inseparable from the experimental error) for ethanol, propanol, and butanol. For these solutes the surface tensions a t 20' C. are, respectively, 22.3, 23.8, and 24.6 dynes per cm., while that of glycerol is 63.4. Thus, while solutions which exhibit appreciable adsorption
effectshave solute surface tensions much lower than the solvent value, this is also true of solutions which have negligible adsorption effects. In addition, since no general method exists for
All of the solutions with TDPN exhibit positive deviations from Raoult's law (f, > 1); the greater the activity coefficient, the more nonideal the solution. Large positive deviations indicate that the solute molecules are being forced out of the bulk liquid onto the liquid surface or into the vapor phase, primarily because the solvent-solvent interactions are so much stronger than the solute-solvent interactions that the solute molecule cannot begin to compete with a solvent molecule for the
Table IV. k,/k, Values for Various Systems Liquid phase Solute k, (dYlaz)ka Water Propanol 4000 -4.997 X lo3 1.451 X Water Butanol 3390 -18.560 X lo3 4.563 X 10-2 TDPN TMP 15.7 ... 2.467 X 10-4 Glycerol Methanol 258.9 ... 9 . 6 X 10+ ... 1 . 2 x 10-5 ODPN Methanol 106.2 ( k J k J in cm. (dy/dz)- in dynes/cm.
T:mp., C. 25.0 25.0 25.0 62.4 60.3 k, and
Table V.
ka/ka
3.63 X 1.35 x 1.57 X 3.71 X 1.13 x
10-8 10-6
10-6 10-7 10-7
Data for Solutes in @,@'-Thiodipropionitrile at 25" C.
(Based on Pecsok's (19) k , and k , values) Solute 2,3-Dimethylpentane 51.5 7.4 6.97 2-Methylpentane 54.5 6.8 8.01 3-Methylpentane 56.9 8.6 6.62 n-Hexane 66.3 9.3 7.13 n-Heptane 196.2 20.7 9.47 2,2,4Trimethylpentane 246.7 15.7 15.7 Cyclopentane 23.3 15.6 1.49 Methylcyclopentane 54.8 21.8 2.51 Cyclohexane 58.9 34.0 1.73 1-Hexene 76.2 17.8 4.28 Cyclohexene 78.4 93.6 0.836 Benzene 105.0 451.6 0.232 Diethyl ether 54.0 30.7 1.76 Ethyl acetate 495.7 469.1 1.06 Acetone 227.4 454.1 0.501 Molecular weight of TDPN, M = 140.2. Density at 25" C., c = 1.111 g./cc. Surface tension at 30' C., (16) y l 0 = 49.92 dynes/cm.
234.6 211.8 189.8 151.3 45.81
84.6 102 90.1 105 155
16.87 16.87 17.60 17.90 18.82
49.34 317.5 137.5 97.58 187.2 88.83 95.18 537.0 93.40 229.4
190 29.7 49.1 44.3 44.1 17.7 3.43 8.94 3.36 1.42
18.33 21.18 21.05 23.71 17.37 25.88 28.19 16.50 23.15 22.67
VOL. 38, NO. 2, FEBRUARY 1966
247
Table VI.“ Relative Retention Volumes (n-Pentane = 1 .OO)and K o v h Retention Indices (n-Pentane = 500.0, n-Heptane = 700.0) for Various Compounds in TDPN at 25” C.
Relative retention volumes, liquid weight yo 1.00 6.00 12.00 1.00 1.00 1.00 2.89 2.76 2.46 8.28 7.90 6.99 1.10 1.80 2.41
Kovkts retention indices, liquid weight 70 1.00 6.00 12.00 500.0 500.0 500.0 600.4 598.4 592.6 700.0 700.0 700.0 508.8 557.0 590.8
Solute n-Pent ane n-Hexane n-Heptane Cyclo entane 3-Met:ylpentane 2.36 2.37 2.15 581.3 Diethyl ether 2.72 3.99 4.70 594.8 1-Hexene 3.46 3.72 3.68 617.4 Cyclohexene 4.38 8.98 12.53 639.7 Based on experimental data taken from Pecsok’s (19) Table
interaction of a neighboring solvent molecule. Thus, the solute has minimal solubility in the bulk liquid-i.e. , the solvent is a “poor solvent”; the “poorer” the solvent, the higher the activity coefficient and (from Figure 2) the larger the value of k./k.. The buccess of this correlation helps us to anbwer the question as to why some polar-polar systems exhibit large surface eflccts, while others do not. Consider the propanol-water and butanolwater aystems. Tlie p Z o values a t 25” C. ale computed by accurate interpolatioii (22) of the published experimeiilal data ( 2 3 ) : p o = 19.92 mm. (piopanol) and p20 = 6.42 mm. (butanol). With these data and the k, values from Table IV, Equation 4 yields the solute activity coefficients: f,” = 13.0 (propanol) and f,” = 47.5 (butanol). Thus for the butanol-water system as well, the high value of k,/kb (13.5 x 10-6cm.) is compatible with the large activity coefficient. The result is consistent with the concepts previously discussed, because, although butanol is usually considered a polar solute, it is relatively nonpolar to thz eyes of the water molecules; hence it is not appreciably soluble in bulk water. It is not the absolute polarity which counts, but rather the relative polarity of the solute with respect to the solvent. Adsorption a t the gas-liquid interface is most likely to occur when the solute is least polar with respect to the solvent, and when the solvent is a “poor solvent.” In contrast to the butanol-water system, consider the solutions containing the solutes, ethanol, propanol, and butanol in the solvents, glycerol and ODPN-polar-all polar systems with negligible surface effects and, as would be expected, low activity coefficients (see Table 111). Here the solutes are sufficiently polar with respect to the solvents (which are less polar than water) and are sufficiently soluble in the bulk liquid (fpm is greater than unity, but not extraordinarily large). On the 248
o
ANALYTICAL CHEMISTRY
583.5 633.9 627.3 712.4
578.7 659.4 634.1 760.3
11.
other hand, methanol displays a small surface effectin ODPK, and a moderate effect in glycerol, although it has low activity coefficients in both solvents (f,” = 2.61 and f,” = 1.63, respectively). This is not an inconsistency; a different type of phenomenon is governing the adsorption. It is believed that the methanol molecules are being preferentially adsorbed on the surface of the liquid by means of hydrogen bonding, to a greater extent on the glycerol than on the ODPN. To summarize, there appear to be two distinct classes of adsorption a t the gas-liquid interface. The first type is characterized by large solute activity coefficients-Le., the bulk liquid is a poor solvent. Here, the less polar the solute is with respect to the solvent and the less soluble it is in the bulk liquid, the larger the solute surface excess concentration-Le., “adsorption effect.” The second type is characterized by relatively low solute activity coefficients, significant solute solubility in the bulk liquid, and a solute and solvent molecule of comparable high polarity, Here, the solute molecules are preferentially adsorbed because of strong dipoledipole interactions or hydrogen bonding between the solute and thc liquid surface. Present information indicates that the first type produces by far the larger surface effects, and that both types are favored by highly polar liquid phases. Also, it may be that some polar-polar systems with intermediate activity coefficients could exhibit a small amount of both types. Such might be the case for the three polar solutes in TDPN (see Table V).
CONCLUSIONS
A thorough understanding of the thermodynamics of the gas-liquid interface would involve a complete exposition of the Gibbs adsorption equation
(6, 7 ) . What we are aiming for here is not thermodynamic rigor [in fact, the common abusc of the term “polar solvent” has been criticized by this writer ( 1 7 ) ] ,but an appreciation of the conditions whereby solute adsorption at the gas-liquid interface becomes a significant factor in GLC, and the consequences thereof. In bulk solutions surface factors can be ignored because the ratio of liquid surface area to liquid volume is small. However, when the liquid is spread upon the solid support, the surfacevolume ratio becomes high and any difference in the concentration between the bulk and the surface becomes significant. Only if the solute concentration is uniform throughout the liquid and the surface excess concentration is negligible, can one regard the stationary liquid phase as possessing the properties of the bulk liquid. Retention Volumes, Relative Retention Volumes, and Kovlts Retention Indices. First, consider the methanol - glycerol system, which shows moderate absorption a t the gas-liquid interface. From the data in Table I1 one can readily see the marked variation of specific retention volume with the weight per cent of liquid phase on the column. The TMP-TDPN system (Table IV) would have an even more pronounced variation. It might be erroneously argued that one can ignore these surface effects when dealing with relative retention volumes, because these effects would cancel out each other. However, as Martin (12) first pointed out, normal paraffins, branched paraffins, cycloparaffins, olefins, etc., behave differently with a change in liquid phase percentage because they have widely different values of k, and k,. A numerical example illustrates this point nicely. From Pecsok’s (19) data on T D P S at 25OC. retention volumes relative to npentane were calculated for a number of typical compounds a t three liquid weight per cents (1.00, 6.00, and 12.00). From Table VI one can see the large variation and observe how even the elution order of the compounds changes with liquid loading. The Kovits retention indices are not foolproof either. Csing n-pentane and n-heptane as the standards, the retention indices were calculated from the same experimental data, using the expression proposed by KovAts (IO):
where I is the retention index of the substance, VN(substance) is its retention volume, V,(n-C,) is the retention volume of the normal paraffin chosen as the lower standard (in this case, n-
pentane with z = 5 ) , and VN(~-C,+Jis the retention volume of the normal paraffin chosen as the upper standard (in this case n-heptane with z 2 = 7). Again, a very significant variation with liquid phase percentage takes place (see Table VI). These examples emphasize the importance of reporting the percentage and surface area of the liquid phase with all GLC data taken on polar columns. Since it is not always possible to measure surface areas for every column, one should a t least report the type of solid support used and its mesh size, since these two factors largely determine the surface area. Partition Coefficients, Activity Coefficients, and Thermodynamic Quantities. Considerable caution is necessary in the interpretation of chromatographic data for the study of the thermodynamics of solutions. Large errors could result from improper handling of the data. The significant quantity is the bulk partition coefficient k,, which must be determined by the graphical method previously described, to ensure that all surface contributions to the retention volume are corrected for. A semi-empirical equation which expresses the limiting condition above which failurc to use a surface correction introduces an crror of greater than 1% in the evalunlion of the bulk partition vocflicient is:
+
V L -k ,< - 100Ar.
ka
For a column packing of 5% by weight TDPN on firebrick, k./k,
