Heterogeneity in solution adsorption: Edge carbon and oxide coverages

Heterogeneity in Solution Adsorption: Edge Carbon and Oxide Coverages 1 I. Methanol-Benzene M A R C E L L U S T. C O L T H A R P AND N O R M A N H A C K E R M A N 2 Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 Received December 13, 1971 ; accepted September 22, 1972 Adsorption from methanol-benzene system at 25 ° was investigated on two carbon black heterogeneity sequences. In one sequence the heterogeneity was edge carbons, the carbon atoms on the erystallite sheet edges, and in the other the heterogeneity was surface oxide groups. When edge carbons replaced basal carbon atoms adsorption changed from near total benzene preference to strong methanol preference. Covering the edge sites with oxide groups caused little change in adsorption preference except when significant amounts of surface COs complexes were present. The methanol preference increased with COs content and total preference is predicted for an all CO2 monolayer. The changes in preference caused by each heterogeneity type indicate qualitatively the relative strength of interaction between a surface group and the various solution monomers and hydrogen bonded species. Simple monolayer-mass balance theory shows multilayer development in several cases although this theory failed when applied to monolayers. The results suggest that surface heterogeneity must be included as a major factor in the more involved solution adsorption theories. INTRODUCTION

A change in surface from homogeneous to heterogeneous can completely alter adsorption at the solution/solid interface (1). Investigations of effects due to such an alteration between extremes in the nature of a surface are few, although there are a number of comparisons between general types of surfaces, i.e., differences in adsorption between active carbon and silica gel are due to the former being a nonpolar and the latter a polar surface, etc. 1 (a) Portions of this paper were presented at the 151st meeting of the American Chemical Society, Pittsburgh, Pennsylvania, March 1966. (b) Based in part on the dissertation submitted by M. T. C. to the Graduate School of the University of Texas at Austin, Austin, Texas, August, 1966. 2 Present Addresses: M. T. C., Department of Chemistry, Kentucky State University, Frankfort, Kentucky 40601. N. H., Department of Chemistry, Rice University, Houston, Texas, 77001.

(2). Most studies in nonaqueous solution adsorption have been concerned with the effects of solution component type where it has been considered satisfactory to state the general kind of solid. In order to better establish the influence of specific surface variables we have investigated the changes in adsorption as two types of surface heterogeneity, edge carbon atom and oxide coverage, are progressively varied on a nonporous carbon black. We report here the results for methanol-benzene, a system highly sensitive to both variables. In Part I I we report the effects of these variables on a less sensitive system, n-butanol-benzene. In earlier work on these and other alcohol-benzene solutions Gasser and Kipling examined behavior at only two of the several extremes--a bare carbon and a carbon assumed to have a chemisorbed oxide monolayer (1). More re-

176 Journal of Colloid and Interface Science, Vol. 43, No. 1, April 1973

Copyright ~ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.

HETEROGENEITY IN SOLUTION ADSORPTION cently, Puri, Kumar, and Sandle reported isotherms for methanol- and ethanol-benzene on porous carbons at several stages of oxide decomposition (3). They did not, however, establish the actual oxide coverages nor consider the effects of carbon substrate structure. The questions that our results resolve are: (a) which changes in adsorption are dependent on fraction of each carbon atom type present and which are dependent on oxide coverage, (b) when oxygen is influential is the total coverage or the fractional coverage of a particular functional group the major factor and (c) what are the limits in extent and range of preference on various completed monolayers. Each trend qualitatively indicates the relative strength of interaction between a specific surface group and a particular monomer or associated species prevalent in the solution phase. Our results provide a test of simple monolayer and more involved solution adsorption theories. EXPERIMENTAL Materials

The methanol used was Matheson, Coleman and Bell's "Chromatoquality" with 0.01 and 0.001% impurities. The benzene was the J. T. Baker Chemical Company's "Baker Analyzed." Refractive indices corrected to 20 ° were 1.3297 and 1.5008 for the methanol and benzene respectively, which compare favorably to literature values (4,5). To establish whether further purifications were needed, the effects of representative impurities were investigated at the isotherm maxima and minima for the coverage extremes (the graphitized carbon and the carbon with 68o-/0 oxygen). Cyclohexane and water at saturation or 1°-/o levels affected adsorption while exposure to powdered glass I-boron-methanol reaction (6)~ did not. Since nonpolar impurities were not present at a 1% level and since molecular sieving of uncontaminated liquids did not affect adsorption, no further purification was needed. The carbon blacks used were Spheron 6, a commercial channel black, and several of its

177

heat treated forms. The parent and the 2700 ° graphitized form (Graphon) were obtained from Godfrey L. Cabot, Inc. Desired oxide coverages were generated by thermally decomposing the chemisorbed oxygen on Spheron 6 to CO, CO2, and H20 following previously determined conditions (7). Total time to generate a given coverage was minimized by increasing temperature intervals (AT) and shortening time at each temperature. Oxidefree carbons, other than Graphon, with different basal and edge carbon atom coverages were obtained by heating the parent to 900 and 1000 °. Since physically sorbed water affects adsorption (see following), the parent Spheron 6 and all other oxide forms exposed to the atmosphere were dried one hour at 150 ° and stored under nitrogen. Moisture uptake during transfer was avoided by using a dry bag. (The only exception here was during the 30 sec or less when solution was pipetted into a carbon-loaded tube.) Following a suggestion by Parfitt (8) checks were made for the presence of so-called tars (partially decomposed starting material leftover from the formation of the black). The usual test for tars, benzene extraction, was negative (only a colorless liquid was obtained). Benzene from the first extraction (26 hr) showed a slight increase in refractive index (interferometric--see below). This was spurious, however, since the benzene accumulated (81 hr total) from extraction of three different samples had no change in refractive index. Mass spectra of the volatile products from pyrolysis of three unextracted Spheron 6 forms had no peaks for decomposition of the supposed tars and the spectra of an extracted form had a benzene peak. Adsorption on extracted blacks at 16 indicator points changed from that of the unextracted blacks in only two cases (at the extrema for the parent and for the system in Part II on the carbon with 500-/o oxide). The changes were in the direction expected from having traces of benzene left on the surface Journal of Colloid and Interface Science, Vol. 43, No. 1, April 1973

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after a 150 ° heating for removal of extraction solvent. All of this evidence indicates that there were no tars on Spheron 6, as the manufacturer reports (9). Further, there are indications that extraction can contaminate a surface.

Adsorption System and Measurement Each adsorption tube held approximately 0.8 g of carbon in contact with 7 g of solution at 25 °. At least 10 hr were allowed for establishment of equilibrium. The tubes and stock solution bottles were fabricated from vacuumgrade cone and socket joints. They were sealed with electrician's tape to prevent evaporation. Concentration was determined with a Carl Ziess (Jena) portable interferometric refractometer using a 10-ram cell. All measurements were in the linear scale region below the first center fringe color shift (10). Corrections to zero the scale and to compensate for scattering were 16 and 14 scale units, suspended carbon in the left and right chambers, respectively. The 10% or less deviation found in the exploratory experiments describes the spread of the vast majority of points about the smoothed isotherms. No "best point" selection was used. RESULTS

Variation of Adsorption with Edge Carbon Coverage The effects of progressively replacing the basal carbon atoms of a homogeneous surface with edge carbon atoms are shown in Figs. 1-3. The adsorption variable is the surface excess, F, and the solution concentration is mole fraction benzene, x,. F is f~/A where f~ = AX,no/m and A is the surface area (BET) in m~/g. Here AX, is the difference between initial and final concentrations, no the millimoles of solution and m the mass of carbon in the adsorption system. The extent of heterogeneity for the oxidefree carbon series was established by means of a previously discussed model (7), modified by more recent results (11). In brief, a carbon black particle is made of approximately parallel

graphitic sheets which contribute two main types of carbon atom to the surface. One type, the basal, is the atom on the inside of a sheet. The hexagonal arrangement of basal atoms is a group of homogeneous sites. The other type of carbon atom is the atom on the sheet edge or around vacancies in the sheet.. These edge atoms are heterogeneous sites to which oxide groups (-- COOH, --OH, etc.) can be attached. Thus the amount of chemisorbed oxygen is a measure of the edges present. The edge coverages available were: 0.2°~o-Graphon (12), 47~o--1000-1100 ° carbon 3 and 68%--900 ° carbon. (Details on coverage possibilities and calculations are given in Ref. 7. The BET-N2 surface areas of our samples were reported by Wade (15). He assumed a nitrogen molecular area of 16.2 A s for all but Graphon for which 20.0 A 2 was assumed.) Our isotherm on the most homogeneous surface, Fig. l, shows a general preference for benzene in the absorbed phase. The slight methanol preference at the highest concentrations is the main disagreement of our results with the earlier isotherm of Gasser and Kipling. This difference could be due to their carbon being benzene extracted, but is more likely due to our carbon having more edges (a lower degree of graphitization) since there is a preference for methanol on edges. This strong preference for methanol on edges is most obvious in Fig. 2. Here the magnitude of preference for methanol is twice that for benzene, judging from the depth of the minim u m in comparison the height of the maximum, and the range of methanol preference is now about half the total concentration range. The alteration of preference by edges is summed up in Fig. 3 where the isotherm characteristics (extrema, their location, and the point of no apparent adsorption) vs edge coverage for our three isotherms are given. All characteristics indicate a linear increase in methanol preference with edge fraction. Extra-

Journal of Colloid and Interface Science, Vol. 43, No. 1, April 1973

a This was estimated by assuming that the drop ill edge fraction between 900 and 1000 ° was identical with the reported surface area drop in this range (13,14).

HETEROGENEITY IN SOLUTION ADSORPTION

179

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as the oxide consists solely of CO complexes. When the number of CO2 complexes first become appreciable, about 40% coverage, the methanol preference increases. This preference continues to increase as CO2 content increases. The only linear trend with total coverage is that of the maximum. The curves for the other characteristics separate into a part each for CO and CO2 complexes. The isotherms at 68% oxide coverage (maximum available), Fig. 5, are of special interest because they illustrate the sensitivity of the oxide surface to contamination by water. Our first results on the untreated Spheron 6 are indicated by the partial isotherm (---) in Fig. 5. The erratic spread of points and the appearance of a new liquid phase (droplets in the adsorption tube) shown by diagonal hatching led to the drying and other precautions noted

Variation of Adsorption with Oxide Coverage The results of varying the amount of chemisorbed oxygen on the carbon with 68% edges can be seen in Fig. 41 where the characteristics of five isotherms are plotted vs oxide coverage. The adsorption variable is now t2 since these carbons have identical surface areas. The overall effect of increasing oxide coverage is an increase in methanol preference. The change in preference is slight in general, however, as long

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180

COLTHARP AND HACKERMAN ~

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faces were constructed from extrapolations of the trends for the isotherm characteristic points. The lowest methanol preference, judging by the range and extent of preference (depth of minimum) would be found on a surface of all CO complexes (curve a). Overall, concentration changes due to adsorption would be smaller on a complete CO complex monolayer than on the surface with 68% coverage of mixed oxide complexes (Fig. 5). Replacing half the CO complexes with CO2 complexes (curve c) would make methanol preference greater than the highest found experimentally, and a complete CO2 monolayer would give methanol preference at all concentrations (curve b, a " U " shaped isotherm). - 0.15

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HETEROGENEITY IN SOLUTION ADSORPTION

181

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Combination of both heterogeneity series gives the most complete possible alteration in adsorption since the isotherms go from a near U shape for one component through intermediate "S" shapes to a U shape for the other component. Thus nature and number of surface groups can control the type, range and extent of adsorption. Several tendencies found agree with those Purl et el. (3) found on heat-treated charcoals. This comparison is qualified, however, by the effects mentioned earlier (see Introduction) as well as uncertainties in their data due to porosity and possibly an undetermined amount of physically bound water (no mention was 0.40

DISCUSSION

Trends in Preference: Solution Species and Properties Interpretation of the adsorption changes with coverage is complicated by the variety of

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No. 1, April 1973

182

COLTHARP AND HACKERMAN

solution species present. Both solution components hydrogen bond; methanol acts as both donor and acceptor and benzene acts as a donor. Consequently there can be methanol dimers, trimers, etc., present as well as a methanol-benzene complex (16). Only at the concentration extremes is it clear which particular species prevails. At low mole fraction the high methanol concentration favors selfassociated alcohol complexes and the benzene monomer. At high mole fraction the monomers and the alcohol benzene complex are favored. The situation would be better defined if the composition and concentration of all inter and intramolecular complexes and the exact hydrogen bonding nature of the surface were known. The preferential adsorption found in a given concentration range indicates which particular solution species-surface group interaction is strongest. Thus the benzene preference found at low concentrations on all surfaces reveals that the interaction of the benzene monomer with any surface group is stronger than the interaction of the alcohol dimer, etc. with any surface group. The development of methanol preference at high mole fractions, when edges replace basal carbons, shows that the methanol monomer-edge interaction is stronger than the benzene monomer-edge (or basal) interaction. When CO groups occupy the edges, alcohol preference decreases at high concentration, consequently the methanol monomer-CO group interaction is weaker than the monomer interaction with edges. With CO2 groups on the edges the deepest minimum develops. This indicates that the methanol monomer-CO2 group interaction is the strongest interaction of all. Because of the high relative acidity of methanol compared to other alcohols (17), the interaction with CO2 groups [-most likely carboxylic functionalities (7)7 is a chemical reaction, an esterification. In this reaction acidity of the alcohol is very important. Esterification is also supported by the report of gas phase methanol chemisorption on Spheron 6 (1), where the coverage (20%) is Journal of Colloid and Interface Science, Vol. 43, No. 1, April 1973

close to the surface COs content, 16% (7). The high acidity also appears to account for the displacement of water by methanol on the undried black. The multilayers which develop according to simple adsorption theory (see following) can be explained by incipient phase separation. This concept was used earlier to explain a benzene multilayer on Graphon (1). The kind of deviation from ideality which methanolbenzene follows (higher vapor pressures than pure components) suggests that the system is close to phase separation, although there is no evidence that phase separation occurs near 25 ° (18,19). At low to medium mole fractions we found a benzene multilayer on surfaces with mainly basal carbons. This multilayer development appears to be an incipient phase separation induced by the similarity between the basal faces and benzene, although there is a considerable difference in the hexagon sizes. This tendency for benzene multilayer formation, however, is easily overcome since methanol multilayers form at the higher oxide coverage. In this case the polar sites nucleate separation of what can be thought of as a "methanol-rich" phase. Although methanol chemisorption plays a part, this is in accordance with the vapor pressure curve. Thus for this system at least, the surface groups can induce phase separation ; the particular surface type controls which of the two phases ends up at the surface. Solution Adsorption Theories: Application and Evaluation

Simple monolayer-mass balance theory can determine whether a multilayer develops and permits evaluation of individual component isotherms in monolayer cases (2). The basic relations of this theory are: = nl~X2 - n2~X1

(nl"/(nl") ~) + (n2'/(n~') ~) = 1

EI~

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Terms introduced here are n~*, the number of moles of component i in the adsorbed phase,

HETEROGENEITY IN SOLUTION ADSORPTION

and (hiS)m, the number of moles in a filled monolayer of component i. Table I summarizes multilayer development on our surfaces according to the conditions: f~ > (nl*) and f~ < (n2S)m where literature values of (nis) "~ were used (1). Multilayers of methanol occurred at high mole fractions on the higher oxide coverages. Multilayers of benzene were found at the lower edge coverages, but occurred over a wider solution concentration range than the methanol multilayers. From the mosaic or patch-like nature of the surfaces it appears that multilayer islands formed in both cases. When Eqs. [-1 and 2~ were solved for nd values under monolayer conditions all the resulting individual isotherms (plots of ni ~ vs X) had very pronounced maxima and minima. Similar results were found on the two surfaces reported earlier (1). A decrease in the adsorbedphase concentration of a component with an increase of bulk-phase concentration, as occurs here, is unreasonable and is contrary to general experience (2). Consequently, the simple theory is inadequate for our systems. This anomalous behavior is probably due to the varieties of monomer, dimer, etc. equilibria involved. In the more general theory of Siskova and Erdos the explicit influence of several factors on isotherm shape has been investigated (20). Their basic isotherm relation is

183

TABLE I MULTILAYER S

Component forming multilayer

Coverage %0i

Multilayer range

TM

J~l -~- (1 -- X1)e I where n ~ is the number of moles in the adsorbed phase and f a function of the adsorption potential for each component. S-shape isotherms develop when f passes through zero as : (a) it varies with the distance from the surface# (b) it is modified by the presence of the 4 This appears to be a surface heterogeneity effect in that one component is preferred at one position and the other at another. However the variation in preference with distance above, i.e. perpendicular to the surface differs from the more usual concept of heterogeneity where preference changes in moving along a plane parallel to the surface.

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bulk phase, or (c) it is altered by nonideality in the bulk or adsorbed phase. Other theories, Everett's (21) being a recent example, suggest only the last factor as cause of S-shaped isotherms. Our results provide a means for qualitatively evaluating these factors, somewhat along the line suggested previously (21). Because methanol-benzene is so extremely nonideal in the bulk phase and thus also likely to be extremely nonideal in the adsorbed phase, it would be expected that only S-shaped isotherms would be found. The same prediction also follows from the other solution factor, i.e., the bulkphase effect on adsorption potentials would be strongly influenced by hydrogen bonding. However, a near U-shape was found experimentally at one surface extreme and a U-shape is predicted for this solution system at another surface extreme. When the remaining factor, surface potential or heterogeneity was varied in type and degree, the shape of both these U-shaped isotherms is altered through a progression of S-shapes. This factor, then can be the major cause of S-shape development. ACKNOWLEDGMENTS We are grateful to the Robert A. Welch Foundation of Houston, Texas, whose support made this work possible. We wish to thank the Cabot Corporation for the Spheron 6 and Graphon. Fellowship support for M.T.C. by the Proctor and Gamble Company is gratefully acknowledged. We are indebted to Professor W. H. Wade for the surface area measurements. Journal of Colloid and Interface Science,

Vol. 43, No. 1, April 19Y3

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COLTHARP AND HACKERMAN REFERENCES

1. GASSER,C. G., AND KIPLING, J. J., "Proceedings of the Fourth Conference on Carbon," p. 55. Pergamon Press, New York, 1960. 2. KIPLING, J. J., "Adsorption from Solution of NonElectrolytes," pp. 32-35, 59, 165-168, 311-317, 322-324. Academic Press, New York, 1965. 3. PuRJ, B. R., KUMAR, S., AND SANDLE, 1~. K., Ind. J. Chem. 1, 418 (1961). 4. KIPLING, J. J., AND TESTER, D. A., J. Chem. Soc. 1952, 4123 (1952). 5. KIPLING, J. J., AND PEAKALL,D. B., J. Chem. Soc. 1956, 4828 (1956). 6. CRATIN,P. D., AND GLADDEN,~[. K., J. Phys. Chem. 67, 1665 (1963). 7. COLTHARP, M. T., AND HACKERMAN,~Y., jr. Phys. Chem. 72, 1171 (1968). 8. PARFITT, G. D.~ personal communication. 9. Anonymous, "Carbon Black Pigments."Godfrey L. Cabot. Inc., Boston, Mass., Vol. 11, No. 1 --CBgen ID, July 1958. 10. CRIST, R. H., MURPHY, G. M., ANDUREY, H. C., J. Chem. Phys. 2, 112 (1934).

Journal of Colloid and Interface Science. Vol. 43. No. 1. April 1973

11. RIVIN, D., Rubber Chem. Technol., 44, 307 (1971). 12. LAME, N. R. VASTGLA, F. J., AND WALKER, P. L., JR., J. Phys. Chem. 67, 2030 (1963). 13. BEEBE, R. A., BISCGE, J., SmTH, W. R., AND WENDELL, C. B., J. Amer. Chem. Soc. 69, 95 (1947). 14. SCHAEF~'ER, W. D., AND SmTH, W. R., J. Phys. Chem. 57, 469 (1953). 15. WADE, W. H., J. Colloid Interface Sei. 31, 111 (1969). 16. PIMENTEL, G. C., AND McCLELLAN, A. L., "The Hydrogen Bond," pp. 40, 99, 202. W. H. Freeman and Co., San Francisco, 1960. 17. MORRISON, R. T., AND BOYD, R. N., "Organic Chemistry," pp. 335, 344. Allyn and Bacon, Boston, 1959. 18. PERgAKIS, N., J. Chim. Phys. 22, 280 (1925). 19. ROWI2NSGN,J. S., "Liquids and Liquid Mixtures," p. 198. Butterworths, London, 1959. 20. SISKOVA, M., AND ERDOS, E., Collection of Czech. Chem. Commun. 25, 1729, 2599 (1960). 21. EVERET% D. H., Trans. Faraday Soc. 60, 1803 (1964) ; 61, 2478 (1965).