Langmuir 1994,10, 2399-2402
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Thermodynamics of Adsorption of C(CH&-,Cl,(n = 0-4) on Sterling FT Graphite at Zero Coverage Using Gas-Solid Chromatography Edwin F. Meyer* and Janine Feil Department of Chemistry, DePaul University, Chicago, Illinois 60614 Received March 14, 1994@ Zero coverage thermodynamic properties of adsorption on Sterling FT graphite at 60 “C obtained by using gas-solid chromatography are presented for the five adsorbates from neopentane through carbon tetrachloride resulting from sequential substitution of a chlorine atom for a methyl group. Enthalpies of adsorption(as well as of vaporizationof pure liquid)are reproduced by a simple model that ascribes a fixed attractive energy to each of the C-CH3 and C-C1 bonds, and a third contribution proportional to the square of the molecular dipole moment of the adsorbate. The role of the C-C1 dipole in the total attraction of adsorbate for the surface is compared with its role in mutual attraction in the liquid state, and it is found to be significantly weaker in the former. It is estimated that neopentane loses about 6% of its rotational freedom uDon adsomtion. while the adsorbates containing from one to four chlorine atoms lose from 11 to 15%.
Introduction The role of an electric dipole in the adsorbate in establishing the strength of attraction (reflected in the thermodynamic properties of adsorption) between it and a homogeneous graphite surface has not been widely investigated. Part of the reason for this is the fact that adsorbates with significant dipole moments perforce have lower vapor pressures, making direct measurement of their adsorption isotherms inconvenient. Furthermore, the presence of the dipole enhances mutual adsorbate interactions on the surface, complicating the process of isolating the role of adsorbent-adsorbate interactions in establishing the magnitudes of the thermodynamic properties measured. A gas-solid chromatographic (GSC) study of the retention times of very small samples of adsorbate on a column of highly graphitized carbon provides a way around these difficulties. It is possible to obtain standard enthalpies, entropies, and Gibbs energies for the adsorption process at zero coverage from the temperature dependence of retention time.l We have recently described an apparatus and method for doing so and provided illustrative results for neopentane on Sterling FT graphitea2 In that work we compared the entropy of adsorption of neopentane (NEOP)with literature values for carbon tetrachloride (CCL), to investigate any effect the C-C1 dipole might have in restricting adsorbate motion on the surface. The entropy of adsorption of carbon tetrachloride is significantly more negative than that of neopentane, but the precise value for the former is open to question,2 preventing any attempt at quantitative comparison. The present work was undertaken to provide consistent thermodynamic properties of adsorption for ccl4 and includes similar results for the three compounds that lie between these two, obtained by sequential replacement of a methyl group by a chlorine atom: 2-chloro-2methylpropane (PCMP), 2,2-dichloropropane (22DC), and l,l,l-trichloroethane (111T). These three intermediate adsorbates all differ from the first two mentioned in that they possess net electric dipole moments: 2.13,2.27, and Abstract published in Advance ACS Abstracts, May 15,1994. (1)Meyer, E.F. J.Chem. Educ. 1980,57, 120. (2) Meyer, E.F.; Mulvihill, G.; Feil, J. Langmuir 1993,9, 3239.
@
1.78 D, re~pectively.~Thus they will provide some measure of the effect of the overall moment relative to the individual bond moments in CC4. An obvious advantage to the study of this series of adsorbates is their possessing very similar shapes, minimizing the role of molecular geometry in establishing the observed properties of adsorption.
Experimental Section The apparatus and general method of obtaining data have been described.2 In the course of the present work we discovered that the time for elution of half of the injected sample was a slightly more robust datum for these adsorbates than the retention time determined from the Guiochon model4used in the earlier work. For example, for a series of five successive injections, the half time would typically provide a precision of 0.05% while that in the retention time of the Guiochon model would be more like 0.1%. Consequently, we used the half-times in calculating the specific retention volumes used for estimation of the standard thermodynamic properties of adsorption in this work. The adsorbates were used as received from Aldrich Chemical Co. Stated purities of the CCld and l l l T were 99+%,the 2CMP was 99%, and the 22DC was 98%. In no case was there a recognizableimpuritypeak in the GSC spectrum ofthe adsorbate, though one would appear in that of 2CMPwhen it was left exposed to the copper tubing ofthe apparatusfor extended periods. Fresh samples were always injected for this adsorbate. The results of the present study were obtained using the same sample of graphite used for the neopentane work.2 The current results were completed a little more than 1 year after the start of the original, and neopentane was run again as a check on the stability of the graphite. It was found that its specific retention volume had decreased by 2.4% over that interval, and that a significant change in the “leaningcoefficient”(a measure of the curvature of the adsorption isotherm at zero coverage) of the Guiochon model had occurred. The latter change was in the negative direction; i.e., that of apparently weaker adsorbateadsorbate attractions. These changeshave an insignificanteffect on the properties reported herein but throw a measure of doubt on the values reported for the change in heat capacity on adsorption and the two-dimensional vinal coefficients reported in the neopentane work.2 (3)Nelson, R. D., Jr.;Lide, D. R., Jr.; Maryott,A. A. Selected Values Gas Phase; NSRDSNBS 10; National Bureau of Standards: Washington, DC, 1967. (4)Jaulmes, A.;Vidal-Madjar,C.; Ladurelli, A.; Guiochon,G. J.Phys. Chem. 1984,88,5379. of Electric Dipole Moments for Molecules in the
0743-746319412410-2399$04.50/0 0 1994 American Chemical Society
2400 Lungmuir,Vol. 10, No. 7, 1994 Table 1. adsorbate NEOP 2CMP 22DC lllT CC14
Meyer and Feil
+
+
Best Fit Parameters for In VgT = A B/T C/!P A B C a v d S VETat600C -5.89486 2056.97 195525 0.022 7.70 -6.93158 2792.58 130706 0.013 13.85 -8.36749 3771.98 0 0.024 19.20 -6.59020 2644.17 198876 0.001 23.07 -8.49863 3924.44 26.61 0 0.062
Table 2. Standard Thermodynamic Properties of Adsorption at Zero Coverage and 60 "C ads -AH/(kJ/mol) -AS/(J/(mol K)) -AG/(kJ/mol) ppTorr NEOP 26.04 34.24 14.63 3.861 2CMP 29.74 16.26 2.142 40.45 22DC 31.36 42.60 17.17 1.545 lllT 31.91 42.72 17.68 1.286 CC14 32.63 43.69 18.07 1.115
-334 0
1
Discussion
3
4
y i
Reeults For 2CMP and 111T, a quadratic fit of the In VgTvs l/T data provided significantly better fits than linear ones; for 22DC and CCh linear correlations were used. Table 1presents the optimized parameters based on weighted least squares, the average difference between observed and calculated specific retention volumes for each adsorbate studied in the present work, and VgT at 60 "C. (Reference 2 may be consulted for the original neopentane data.) Table 2 presents standard thermodynamic properties of adsorption at zero coverage. The initial state is the ideal gas at 1 atm; the final state is the ideal twodimensionalgas at a pressure of 0.338dyn/cm (the deBoer standard state6). In addition, the pressure of adsorbate in equilibrium with the standard surface state at 60 "C is included as a more readily visualizable equivalent of the standard Gibbs function change on adsorption. The maximum imprecision in the enthalpies of adsorption is estimated to be 0.1 kJ/mol; in the Gibbs functions, 0.01 kJ/mol; in the entropies, 0.3J/(mol K);in the equilibrium pressures, 1 mTorr. The curvature in the plots of the logarithm of specific retention volume vs inverse temperature is in the direction of positive heat capacity change upon adsorption, but they are of questionable quantitative significance: for 2CMP we observe 20 f 17; and for 111T, 29 f 7 J/(mol K).
2 XofCIAl"
Figure 1. Enthalpies of adsorption as a function of number of chlorine atoms in the adsorbate. The plus symbols refer to the model described in the text.
-444 0
1
2
3
X of CI Atoms
Figure 2. Entropies of adsorption as a function of number of chlorine atoms in the adsorbate. Table 3. Contributions to Enthalpies of Adsorption and Vaporization (kJ/mol) E, Ed E.. adsorption 6.53 8.10 0.435 vaporization 5.28 7.67 0.891 addvap 1.24 1.06 0.49
The thermodynamic properties of adsorption change monotonically from neopentane through carbon tetrachloride, but not in a smooth way. Figures 1and 2 show the variation in enthalpy and entropy of adsorption, respectively, with the number of chlorine atoms in the adsorbate. A straight line between neopentaneand carbon tetrachloride has been added for visual emphasis of the fact that replacement of the first methyl group by a single chlorine atom has a greater effect on these properties than the combined effect of replacing the remaining three. This observation is even more pronounced in the entropy,where 88% of the difference between neopentane and carbon tetrachloride occurs upon replacement of the first methyl group, and relatively little change occurs as each successive methyl group is replaced by a chlorine atom. The following simple model is able to reproduce the observed enthalpies of adsorption with an average difference of 0.2 kJ/mol: Suppose there are but three M e r e n t modes of attraction between adsorbate and adsorbent (1)Ec, the dispersion energy due to each C-CHs bond; (2) Ecl, a combination of dispersion and induction energy due
where n represents the number of CH3 or C1 groups in the adsorbate and p is ita dipole moment. Simple leastsquares fitting of the data provides the parameters presented in Table 3,which result in calculated enthalpies represented by the "plus" symbols in Figure 1. The average error in the calculated enthalpies of adsorption is 0.2 kJ/mol. The values of the parameters produced by the model are quite consistent with earlier work on the influence of the C-Cl dipole on molecular attraction in organic liquids.6 The induction energy between the C-C1 dipole and a sea of CH2 groups was estimated at 3 kJ/mol; this value is
(5) deBoer, J. H. The Dynamical Character of Adsorption, 2nd ed.; Clarendon Press: Oxford, 1968.
642.
to each (polar) C-C1 bond; (3) E,, a separate induction energy due to the overall molecular dipole moment. Thus -AH = nGc + nc&,
+ p2EG
(6)Meyer, E. F.; Renner, T. A.; Stec, K. S.J.Phys. Chem. 1971,75,
Langmuir, Vol. 10,No. 7, 1994 2401
Adsorption of C(CH&-nCln on Graphite expected to be proportional to the polarizability of the matter into which the dipole is induced. The polarizability ofthe CH2 group is 12.5 x cm3/molecule,’ while that of the graphite surface is essentially zero in the perpendicular direction, and 3.42 x cm3/molecule along the C-C bond in the surface.* Thus we would expect an induction energy between the C-Cl bond and the graphite surface smaller than that between the C-C1 bond and a sea of methylene groups by the factor 3.42l12.5, or roughly, 0.8 kJ/mol. If this is subtracted from Ecl, the remainder represents the contribution of pure dispersion energy to the attraction of the C-C1 bond to the surface: 7.3 kJ/ mol. The same work6used the Slater-Kirkwood equation to estimate the “dispersion equivalence” of the C-C1 bond: its dispersion attraction (assuming equal intermolecular distances) is equivalent to that of 3.1 C-H bonds, or just slightly more than that of one methyl group. This is nicely in accord with the values 6.8 and 7.3 for C-CHs and C-C1, respectively, provided by the very simple model under discussion. The success of this model of the adsorption process led us to apply it to the enthalpies of vaporization to the ideal gas state of the pure liquid adsorbates (see below). The average error in the calculated enthalpies is 0.1 kJ/mol using values for the cohesion parameters presented in Table 3. The ratios for adsorptiodvaporization indicate that pure dispersion attraction is enhanced by about 25% on the graphite surface but that this is essentially cancelled out for the chlorine-containing adsorbates, due to the greater efficiency of the bond dipole’s attractive interactions in the three-dimensional liquid state relative to the two-dimensional surface state. Furthermore, the role of the molecular dipole moment is apparently twice as important in three as in two dimensions. We conclude that the observed enthalpies of adsorption at zero coverage are quite consistent with the magnitudes of cohesive energies which arose out of studies of the liquid state, and that there is, for the adsorbates under study, nothing unique about the energetics of their interaction with the graphite surface. The model used suggests that a polar bond in the adsorbate may contribute to cohesion with the surface in two approximately additive ways: directly through its own moment and indirectly through its contribution to a net molecular moment, but that the role of the electric dipole is significantly less efficient on the surface of graphite than in the bulk liquid adsorbate. The entropy results in Table 1and Figure 2 support the general conclusion of our earlier work that neopentane suffers a significantly smaller loss of entropy upon adsorption than does carbon tetrachloride. Thus the overall symmetry of the carbon tetrachloride molecule does not allow it the freedom of motion on the surface that neopentane enjoys. An interesting further result of the present study is that each of the intermediate adsorbates, in spite ofthe presence of a permanent dipole, also suffers less loss of entropy than does carbon tetrachloride. The magnitude of the molecular dipole moment seems not to play a significant role in establishing the magnitude of the entropy of adsorption of these adsorbates. The presence of a single chlorine atom essentially establishes the entropic character of the adsorbed chloro species, the change from 1 through 4 chlorine atoms therein being relatively minor. Furthermore, a permanent dipole might be expected to lower the entropy of the adsorbed species, yet the entropy of adsorption becomes slightly more (7)Moelwyn-Hughes, E.A.Physical Chemistry, 2nd ed.; The Macmillan Co: New York, 1961. (8)Lippincott, E. R.; Stutman, J. M. J.Phys. Chem. 1964,68,2926.
Table 4. Rotational Contribution to Ideal Gaseous Entropy and Estimates of the Loss of Rotational Entropy (J/(molK))upon Adsorption rigid CH3 hindered CH3 ads -Asrot Smt %loss Smt %loss NEOP 2CMP 22DC lllT CC14
6.1 12.7 14.0 14.1 15.0
86 102 109 109 100
7.1 12.5 12.8 12.9 15.0
117 124 124 116 100
5.2 10.2 11.3 12.1 15.0
negative from 22DC through CC4, as the permanent moment decreases from 2.27 D to 1.78 D to 0. It would be of interest to be able to split the observed loss of entropy upon adsorption into its component parts. Myers and Prausnitzghave suggested a way of doing this which is relatively simple but should be of value in the comparison of the quite similar adsorbates under discussion. One degree of translational freedom is assumed lost, and the gain in vibrational entropy is calculated from an algorithm that estimates the overall frequency of vibration normal to the surface from the critical temperature of the adsorbate. The difference between the observed loss in entropy and the combined translational loss and vibrational gain is assumed to be the loss in rotational entropy. The results of this calculation are presented in Table 4. Given estimates of the loss in rotational entropy upon adsorption, it becomes of interest to obtain estimates of the rotational entropies of these adsorbates in the ideal gas state before transfer to the surface. Rotational entropies for all five adsorbates were estimated using statistical thermodynamics by calculating their moments ofinertia. The methyl groups were assumed point masses for the calculation. C-C and C-C1 bond distances were taken as 1.54 and 1.77 A, respectively, and tetrahedral angles were assumed. (The C-C-C1 angle in 2CMP is closer to 207”,1°but this was considered negligiblydifferent for the present purpose.) Two calculations were performed, one with rigid methyl groups and one with contributions of hindered rotation thereof. If there is no change in the rotational freedom of the methyl groups upon adsorption, the former is of greater relevance. If some change in rotational freedom of the methyl groups occurs, then the latter is preferred. The contribution of free methyl group rotation was included in the calculation of rotational entropy by assigning it a moment of inertia of 5.26 x g cm2.11 This led to an additional contribution of 11J/(mol K) per methyl group. The effect of hindrance of this rotation was estimated as follows: Pitzer12calculated thermodynamic properties of neopentane assuming rigid methyl groups and had to add 31 J/(mol K) to obtain agreement with experiment, Le., about 7.8 J/(mol K) per methyl group. Later work by Rubin et a1.l1provides a similar calculation for 111T, but including free rotation of the single methyl group. These workers had to subtract 5.2 J/(mol K) to obtain agreement with calorimetric results. (The sum ofthe two corrections, 7.8 J/(mol K) added if methyl rotation is neglected, plus 5.2 J/(mol K) subtracted if it is assumed totally free, is sufficiently close to the difference in rotational entropies in our two calculations (Le., 13 compared t o 11)to lend support thereto.) In fact, we might expect the hindrance to methyl rotation to decrease slightly as chlorine atoms replace methyl groups in these adsorbates. It was decided (9)Myers, A.L.; Prausnitz, J. M. Trans. Faraday SOC.1965,61,755. (10)Coutts, J. W.;Livingston, R. L. J. Am. Chem. SOC.1953, 75, 1542. (11)Rubin, T.R.;Levedahl, B. H.; Yost, D. H. J. Am. Chem. SOC. 1944,66,279. (12)Pitzer, K.S.J. Chem. Phys. 1937,5 , 473.
2402 Langmuir, Vol. 10, No. 7, 1994
to obtain the total gaseous rotational entropy by adding Pitzer’s 31 J/(mol K)to the rigid value for neopentane and to add 7.5 J/mol per methyl group to the rigid values for the remaining adsorbates. These results are included in Table 4, as well as the percentageloss in rotational entropy upon adsorption for both calculations. The difference in the two is not outside ofthe uncertainty limits of the calculations, precluding any speculation about the role of methyl group rotation in establishing the observed entropies of adsorption. There would appear to be little doubt, however, that the carbon tetrachloride molecule suffers a significantly greater loss of rotational freedom on the surface than does neopentane. The question remains, is this due to an orienting force due to the surface or simply the result of greater steric hindrance of rotation due to a slightly closer equilibrium distance of the more strongly held carbon tetrachloride? It was decided to compare the “zero coverage”standard state on graphite with the pure bulk liquid for these adsorbates, by calculating the thermodynamic properties of transfer from the liquid to the surface at 60 “C. Second (and third, for all but carbon tetrachloride) vinal coefficients were taken from Dymond and Smith,13and their temperature dependence was estimated, in order to evaluate the enthalpy of vaporization to the ideal gas state and to provide fugacities for the entropy calculations. Vapor pressures and their temperature dependence were taken from Timmermans14 for neopentane, from Ambrose16 for 111T, and from Dreisbach16for the remaining compounds. Enthalpies and entropies of transfer as well (13) Dymond, J. H.;Smith, E. B. The Virial Coefficients of Gases; Clarendon Press: Oxford, 1969. (14) Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds; Elsevier Publishing Co.: New York, 1965; Vol. 2. (15)Ambrose, D. J. Chem. Sac., Faraday Trans. 1 1973,69,839. (16) Dreisbach, R. R. Physical Properties of Chemical CompoundsIII; Adv. Chem. Ser. 29; American Chemical Society: Washington, DC, 1961.
Meyer and Feil Table 5. Thermodynamics of Transfer from Pure Liquid to Standard Surface State at 60 “C
mop 2CMP 22DC lllT CCL
4.9 2.5 0.7 1.1 1.8
40.8 43.5 46.5 45.6 44.4
1.27 2.09 2.76 2.67 2.47
a s the ratios of equilibrium pressures are presented in Table 5. The energetic advantage of the surface is very slight, but the entropic advantage is overwhelming, leading to a lowering of escaping tendency by factors around 500. The observation that the enthalpy of transfer of neopentane from bulk liquid to surface is more negative than those of the chlorine-containing adsorbates, while its enthalpy of adsorption is more positive, is in accord with the conclusion that the extra cohesion due to the chlorine atom is more effective in the liquid state than on the surface. This is reflected also in the entropies of transfer, where it is seen that the chlorine-containing adsorbates gain significantly more entropy than does neopentane in going from the liquid to the surface.
Conclusion The presence of the C-Cl dipole in the adsorbates studied appears to enhance their strengths of adsorption in two ways: through its individual moment, and through its contribution to the overall moment of the molecule in question. Its role in surface attraction, however, seems significantly less than in mutual attraction in the bulk liquid state. The entropies of adsorption of the chlorinecontaining adsorbates are significantlymore negative than that of neopentane, but not strongly dependent on the number of chlorine atoms per molecule, nor on the overall adsorbate dipole moment.
