Adsorption as a Function of Molecular Parameters in Gas-Solid Chromatography Judy P. Okamura and Donald T. Sawyer Department of Chemistry, University of California, Ricerside, Culif. 92502 The differential enthalpies, entropies, and f r e e energies of adsorption for sorbate molecules have been studied in relation to their molar refractions, dipole moments, and specific interactions. The contributions of these molecular parameters to adsorption on columns of Graphon, salt-modified alumina, and saltmodified porous silica beads have been determined. T h e resulting data allow prediction of retention volumes for other compounds, as well as the molecular conformation of adsorbed polar molecules. The relationship of the thermodynamics of absorption to the types of adsorptive surfaces also is discussed.
SALT MODIFICATION of aluminas and porous silica beads yields adsorbants of increased utility for gas chromatography ( I ) . Previous studies (2-5) have investigated the adsorption of sorbate molecules on salt-modified adsorbants in terms of specific and nonspecific interactions, with the latter considered to be the entire contribution of a functional group, The degree of interaction between a sorbate molecule and a n adsorbant has been studied in terms of specific retention volume and the enthalpy, entropy, and free energy of adsorption. The computation of these thermodynamic quantities has been discussed previously (3). However, in the case of aliphatic compounds on porous silica, anomalies occur for the specific carbon contribution when the carbon substituents are varied (4). Attempts to predict the specific retention volumes of multi-substituted molecules also fail when the specific thermodynamic parameters for halogens are used. Consequently, a more rigorous evaluation of adsorption in terms of the molecular parameters of the sorbate is necessary and has led to the present study. The contribution of a molecule’s molar refraction, dipole moment, and nonbonding electrons to specific interactions with the adsorbant has been investigated. The adsorbants include Graphon, 10% Na2S04 on acidwashed F-1 alumina, and 10% Na2S04 on porous silica (Porasil C) ; hydrocarbons and halogenated methanes have been used as the sorbate molecules. By relating adsorptive interactions to molecular parameters, the specific retention volumes of other compounds can be predicted. In cases where the molecular parameters change with changes in conformation, the conformation of the adsorbed molecule can be discerned. This approach to the thermodynamics of adsorption also provides a means of studying the nature of adsorbant surfaces.
EXPERIMENTAL The 10% wt/wt Na2S04 modified Porasil C adsorbant (Waters Associates, Framingham, Mass.) was prepared by a (1) C. S. G. Phillips and C. G. Scott in “Progress in Gas Chromatography,” Vol. 6, J. H. Purnell, Ed., Interscience Publishers, New York, N.Y., 1968. p 121. (2) D. J. Brookman and D. T. Sawyer, ANAL. CHEM.,40, 106 (1968). (3) D. T. Sawyer and D. J. Brookman, ihid., p 1847. (4) A. F. Isbell, Jr., and D. T. Sawyer, ihid., 41, 1381 (1969). (5) R. L. McCreery and D. T. Sawyer, J. Chromatogr. Sci., 8, 122 (1970). 1730
previously described procedure and the 10% wt/wt Na2S04 modified acid-washed F-1 alumina column was that used by Brookman (2). Graphon, which is a spherical, partially graphitized carbon black, was obtained from Godfrey L. Cabot, Boston, Mass. This material was sieved to 40/60 mesh because the smaller particle sizes appeared to be broken spheres. The material was easily crushed, which required a precoiled column and gentle packing under vatuum. The surface area of Graphon was 127 m2/gusing 19.3 A2/molecule as the cross-sectional area of N2 on graphite (6). The surface area of the modified Porasil C was 61 m2/g ( 4 ) and of the modified F-1 alumina 254 m2/g (2). Column preparation, sample handling, surface area determinations, and retention volume ( V R )measurements were the same as in an earlier study (3), except as noted with Graphon. The system dead-volume (void space) was initially assumed to be equal to the retention volume of methane a t high temperature (-275 “C). When plots of carbon number cs. log VR indicated adsorption of methane, the dead-volume was refined by a computer program. Instrumentation included a Varian Aerograph Model 1200 gas chromatograph equipped with a flame ionization detector. Modifications and accessories have been described previously (4). The molar refraction values used in the calculations were either obtained from the 40th Edition of the Handbook of Chemistry and Physics (7) or calculated from the additive contributions given in the 45th Edition (8). The dipole moment values were obtained from the tabulation by the National Bureau of Standards (9).
RESULTS The gas chromatography of halogenated methanes and pentane have been studied on a Graphon column. From the corrected retention volumes at four different column temperatures, the standard state enthalpies, A H ” , and entropies, AS”, of the compounds have been evaluated. The relationship between experimental retention volumes, V,, and thermodynamic quantities is given by (3) R T l n ( V R / A )= -AG
=
-AH - (-AS)T
(11
where R is the gas constant, T the column temperature, and A the surface area of the adsorbent in the column. The standard state entropy, -AS”, is obtained by the relation (3)
-ASo = - A S - 11.33
(2)
and - A H ” is equal to - A H . (6) C. Pierce, J . Phys. Clzem., 72, 3673 (1968). (7) “Handbook of Chemistry and Physics,” C. D. Hodgman, Ed., 40th ed.. Chemical Rubber Publishing Co., Cleveland, Ohio, 1958, pp 253C2537. (8) “Handbook of Chemistrv and Physics.” R. C. Weast. Ed., ~ , 48th ed., The Chemical Rubber Co., Cleveland, Ohio, 1967, p. E-159. (9) “Selected Values of Electric Dipole Moments in the Gas Phase.” National Standard Reference Data Series-National Bureau of Standards 10, U. S. Govt. Printing Office, Washington, D.C., 1967.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
15
I
1
1
H
I
5 7 /
-0
'IO
10
u
3
Y
C H 2 CI2
a,
0 -
I
Lc
a
a
I
I 5
IO
C H CI
//CH
-
C l F2
* 5
I3
MOLAR
I5
23
25
REFRACTION
Figure 1. Standard state enthalpies and entropies of adsorption on Graphon for a series of molecules as a function of their molar refraction Kiselev has shown that heats of adsorption for other molecules are proportional to their polarizabilities (IO), and a similar relationship may exist for the present set of compounds on Graphon. Because the molar refraction of a molecule is directly proportional to its polarizability and is a more readily available figure in the literature, molar refraction has been used throughout the present study. Plots of -AH" and -AS" us. molar refraction ( R ) for a Graphon column are presented in Figure 1. A least squares analysis of the resulting straight lines yields
-AH"
Figure 2. Proposed conformation for 1,Zdichloroethane when adsorbed on a Porasil C surface
=
(0.67 X R - 3.67)
(3)
-AS" = (0.96 X R - 4.21)
(4) On the basis of the Graphon results, a series of compounds have been studied on a column packed with acid-washed F-1 alumina coated with 10% wt/wt Na2S04. Using this column with a group of normal alkanes, a plot of log V, us. molar refraction yields a straight line, which indicates that the adsorptive interactions of the compounds are directly related t o their polarizability. Studies of CC14 on the alumina column establish that it is adsorbed more than is predicted on the basis of its molar refraction. Therefore, a specific chlorine interaction also must contribute to the adsorption of chlorinated compounds. As a first approximation CC14 is assumed to have three chlorine interactions, and CH2C12 to have two specific chlorine interactions. With fluorine-substituted chloro compounds, the retention volume data indicate that there is not a specific chlorine interaction because of the electron withdrawing effect of the fluorine on the chlorine. Because CH2C12 has a larger retention volume than pentane or CC14, the dipole moment of a molecule also must have an important influence on the extent of adsorption. The coefficients of these three molecular parameters, in terms of adsorption thermodynamics, have been obtained by analyzing the retention volume data for a set of model compounds with a least squares program (11) translated (10) A. V. Kiselev, J. Chromarogr., 49, 84 (1970). (11) K. B. Wiherg, "Computer Programming for Chemists," W. A. Benjamin, New York, N.Y., 1965, pp 44-47.
into APL and run on an IBM 360150 computer. These coefficients are summarized in Table IA. The compounds, their molar refractions (R),dipole moments ( p ) , and number of specific chlorine interactions, and the calculated -AH", - A S o values in comparison to those obtained experimentally are summarized in Table IB. The same type of results for a column of Porasil C coated with 10% wt/wt Na2S04 are presented in Table 11. The specific bromine interaction of 1.5 times chlorine is a rough estimate of its hydrogen bonding ability. Thermodynamic data for CH21 ( R , 19.3; p , 1.62) also have been obtained, but a good fit has required that a molar refraction value of 16.5 be used (obtained with a Graphon column). The calculated values then are -AH", 8.06 Kcals; - A S " , 15.36 cals-degree-' as compared to experimental values of - A H " , 7.94 Kcals, and -ASo,14.65 cals-degree-'. Experiments with the Porasil C column for multi-halogenated ethanes, propanes, and longer carbon chains yield satisfactory agreement between the experimental values and those predicted from the coeficients in Table I1 in all cases except where the dipole moment is strongly dependent on molecular conformation. In those cases, the calculated retention volumes are low. Calculations for the alternative conformers indicate that the energy gained from the increased interaction of the unfavorable conformer is greater than the energy difference between the favorable and unfavorable conformers. An illustrative example is 1,2-dichloroethane. At 412 "K in the gaseous phase, its dipole moment is 1.46 D, which for the Porasil C column gives calculated values for -AH" of 9.43 Kcals and for - A S o of 17.05 cals-degree-'. These quantities predict a retention volume at 400 OK of 10.0 ml. The experimental results are - A H " , 10.60 Kcals, and -ASo, 17.98 cals-degree-', and a retention volume of 25.6 ml. However, by assuming that the molecule adsorbs on the surface as the gauche conformer (Figure 2) new values are obtained for the dipole interaction. Because C1( 1) is approximately 40" from perpendicular with the surface, its dipole interaction is 1.87 D X cos 40" = 1.43 D. Cl(2) is about 60" from perpendicular to the surface which gives an interaction of 1.87 D X cos 60" = 0.94 D; when added to C1(1), this gives a total dipole interaction of 2.37 D. If the original gas phase dipole of 1.46 D is replaced with this number and the gauche C1-Cl repulsive interaction of 1.1 Kcals (12) is subtracted, the result is -AH" = 10.27 Kcals. Similarly, a new calculated entropy is obtained. However, because there are four equivalent ways the gauche conformer can sit on the surface, a factor of R In 4 (equal to 2.75) must be subtracted to give a -AS" value of 17.47 (12) E. L. Eliel, N. L. Allinger, S. J. Angyal, and G. A. Morrison, "Conformational Analysis," Interscience New York, N.Y., 1967, p 13.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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Table I. Contributions of Molecular Parameters to Standard State Enthalpies and Entropies of Adsorption on 10 Na2S04on Acid-Washed F-1 Alumina A. Parameter Dependence Molecular parameter -AH" - ASo Zero point 0 Kcal 4,28e m Molar refraction (R) 0.279 Kcal/R 0.338 e.u./R Dipole moment (q) 2.654 Kcal/debeye 3.872 e.u./debeye Chlorines capable of specific interactions 0.629 Kcal/CI 0.870 e.u./CI B. Comparison of Calculated and Experimental Thermodynamic Quantities Calcd Exptl Compound R P CI -AH" -ASo -AH" -ASo 0 C:, 24.3 0 6.78 12.51 7.27 13.30 CClnFz 15.90 0.51 0 5.79 11.64 5.03 10.40 CHClFz 11.08 1.41 0 6.83 13.49 7.18 13.89 CHnClz 16.3 1.60 2 10.05 17.73 10.48 18.31 CFSH 6.8 1.65 0 6.28 12.97 5.80 12.45 CCI4 25.9 0 3 9.11 15.66 8.83 15.27 Table 11. Contributions of Molecular Parameters to Standard State Enthalpies and Entropies of Adsorption on 10 Na2S04on Porasil C A. Parameter Dependence Molecular parameter -AH" -AS" Zero point 0 Kcal 2.48 e.u. Molar refraction (R) 0.276 KcaliR 0.443 e.u./R Dipole moment (w) 2.1 31 Kcalldebeye 3.478 e.u./debeye Chlorines capable of specific interactions 0.263 Kcal/CI 0.085 e.u./CI B. Comparison of Calculated and Experimental Thermodynamic Quantities Calcd Exptl Compound R !J CI -AH" -ASo -AH" - ASo 24.3 16.3 21.2 25.9 21.4 7.12 15.9 11.08
0 1.60 1.01 0 1.43 0 0.51 1.42
0 2 3 3 -3 0 0 0
cals-degree-'. These new values predict a retention volume of 22.8 ml at 400 OK, which is in good agreement with the experimental value. Examples of other molecules which appear to undergo conformational change upon adsorption are 1,2-dichloropropane, 1,1,2-trichIoroethane, 1,2,3-trichloropropane, and 1,3-dichloropropane. This conclusion is based on models of adsorptive conformers which yield the calculated enthalpies and entropies of adsorption in agreement with the experimental values.
DISCUSSION AND CONCLUSIONS The data presented in Figure 1 and Tables I and I1 in'dicate good agreement between the predicted and experimental thermodynamics of adsorption for all three columns. The Graphon column has a nonzero intercept for - A H " , which is not typical. One explanation is that most of the compounds studied contain chlorine which has a higher van der Waals' radius (1.80 A) than hydrogen (1.20 A), possibly preventing the molecule from approaching close enough for maximum adsorption. Kiselev (10) has noted this type of behavior in the cases of fluorine-substituted alkanes. Figure 1 also indicates that bromine (1.95 A) containing molecules exhibit smaller interactions than chlorine 1732
6.70 8.43 8.79 7.94 9.74 1.96 5.47 6.06
13.25 15.44 15.65 l4,22 17.20 5.63 11 , ? 0 12.30
6.81 8.34 8.75 8.02 9.76 1.77 5.25 6.26
13.20 15.19 15,75 14.48 17.00 5.53 11.04 12.80
containing ones with similar molar refractions. At lower temperatures CHJ (2.15 A) has a retention volume equivalent to a molar refraction of 16.5 compared to the literature value of 19.3. Thus, the data of Figure 1 are in general agreement with Kiselev's proposals of a direct relation between the polarizability and the adsorption of a molecule. However, the results also indicate that the size of a substituent has a secondary effect. Although there is reasonable agreement between the calculated and experimental values for the salt-modified alumina column, the discrepancies imply that the factors may not be strictly additive. Snyder (13) has stated that most molecules adsorb on alumina at acidic sites which have strong positive fields. Consequently, the most favorable position for a halogenated molecule to adsorb is one in which the negative end of the dipole is adjacent to the surface. For a molecule such as CC12F2,this dipole orientation probably places the chlorines away from the surface such that they cannot interact with the surface. Thus a greater heat of adsorption is calculated than is observed, At the opposite extreme, all possible effects favor one orientation for C H C l , with the chlorines at the surface to give an unusually strong interaction. ~~~~
(1 3) L . R. Snyder, "Principle? of Adsorption Chrcmatography," Marcel Dekker, New York, N.Y., 1968, p 271.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
Table 111. Predicted Thermodynamic and Log V RValues for a Series of Fumigants Compared to Experimental Log VRValues on 10% NazSOaon Porasil C at 125 "C Calculated Experimental -AH' -AS" Compound R fi CI Log VRa Log VR 11 2 1 87 1 CHKl 7.33 14.05 0.51 0.33 8.43 CHzCl, 16 3 1 60 2 15.44 0.81 0.77 CCli 7.94 25 9 0 3 14.22 0.77 0.81 CHC13 21 2 1 01 3 8.79 15.65 0.97 0.88 CC1ECH 3 26 1 1 78 3 11.78 20.49 1.55 1.02 BrCH2CH2CH1 23 7 1 7Xb 15 10.73 19.31 1.23 1.21 CH2ClCHCl 21 0 2 37h 2 17.47 10.27 1.38 1.40 BrCH?(CH&CH3 28 3 1 70h 15 11.83 21.07 1.45 1.49 ClCH2CHCICHI 25 6 2 37h 2 11.54 19.48 1.64 1.62 BrCH2(CH2)3CH3 33 1 1 80b 15 13.36 23.54 1.75 1.76 CHCI2CHC1? 30 6 1 60h 3 12.64 20,49 2.02 1.85 BrCH2CH2CH2Br 31 4 2 80b 3 15.43 26.38 2.27 2.18 2.3 RT log A ) / 2 . 3 RT. Log V R (-ACT - 11.33 T Effective dipole moment. ~~
+
The electric fields on silicas, a similar surface to porous glass beads, occupy less rigidly fixed positions (13). This is confirmed by the data in Table 11; errors due to different possible combinations of preferable orientations are minimized. Another approach t o the study of the heterogeneity of surfaces may be available by utilizing the standard state entropy of adsorption data, ASo. These frequently are a function of the heats of adsorption, because the more tightly a molecule is adsorbed, the less freedom it has relative to the gas phase. However, for gases with zero values for their heats of adsorption, the entropy of adsorption should depend solely on the increased ordering due to the heterogeneity of the surface. The A S " values a t zero heat of adsorption are -1.06 e.u. for Graphon, -2.48 e.u. for modified Porasil C, and -4.28 e.u. for modified alumina. These values are consistent with the increasing order of the adsorptive surface. The most practical aspect of the results is the ability to predict retention volumes on the bask of molecular structure after running a small series of model compounds. Table I11 give3 the predicted logarithms of the corrected retention volumes of a series of fumigants compared to those found experimentally. Figure 3 illustrates the resulting chromatogram. A significant factor in the accuracy of the predicted retention volumes is the uncertainty of the values for tne molecular parameters. Where the use of "effective" dipole moments is noted, the compounds either contain bromine (which models indicate is forced about 35" from perpendicular t o the surface because of its large size) or the molecule requires a conformational change in its adsorbed state (as previously exemplified by 1,2-dichloroethane). The converse also should be possible. With the accumulation of enough data to pick the best columns, the structure
6
i
L - a - 1
15
I
10
1
'
I
'
5
'
'
'
'
'
0
M I NUTES Figure 3. Gas chromatogram for a series of fumigants on a 10% wt/wt Na2S04-Porasil C column at 125 "C. Sample components: 1, CHKl; 2, CH2C12; 3, CC14; 4, CHC13; 5, CC13CHz; 6, BrCH2CH2CH3;7, CICH2CH2CI; 8, BrCH&CH2)2CHa; 9 , CICH2CHCICH3; 10, BrCH2(CH?),C&; 11, C12CHCHC12; 12, BrCHzCHzCHzBr
of a compound could be determined by its retention on a series of columns. This should be feasible in gas-solid chromatography because of the long term column stability relative to gas-liquid columns as well as the greater ease of reproducibility in making duplicate columns.
RECEIVED for review April 19, 1971. Accepted August 2,1971. We are grateful for the support of a n NDEA Title IV Fellowship and a US.Public Health Service Environmental Sciences Traineeship t o J.P.O.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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