89 1
Anal. Chem. 1992, 64, 891-895
Regietry No. Lys-Trp-Lys, 38579-27-0;thymopoietin I1 fragment, 69558-55-0;adrenocorticotropic hormone fragment, 4037-01-8;bradykinin, 5882-2;angiotensin II,4474-91-3; lysozyme, 9001-63-2; cytochrome C, 9007-43-6; ribonuclease A,9001-99-4; a-chymotrypsinogen A, 9035-75-0;a-chymotrypsin, 9004-07-3; hemoglobin A, 9034-51-9.
REFERENCES (1) Huang, X.; (krdon, M. J.; a r e , R. N. Anel. Chem. 1988. 60,
(8) Welllngford. R. A.; Ewing, A. 0. A&.
chrome^.
1989, 29, 1-78.
Chem. 1966, 5. 82-90. (10) Richard, E.; Strohl. M.; Nblwn. R.; Farb, P. Thlrd International Sym(9) Skoog. 6.; Wlchman, A. T&Anal.
possium on Hi@ Performance CaplHary Electrophoresis 1991,Poster Paper PT-19. (11) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53,1298-1302. McCormlck, R. M. Anal. Chem. 3968, 60, 2322-2328. (12) (13) Lauer. H. H.; MchrbnigUI, D. Anel. (2”.1980, 58. 186-170. (14) Worthlngton, C. C. Worfhlngton Enzymo Manual; Worthlngton Bbchemlcal Corporatkn: Freehold, 1988. (15) Cobb, K. A.; Dolnik, V.; Novotny, M. Anal. Chem. 1990. 62.
1837-1838. (2) Tsuda, T.; Nomura, K.; Nakagawa, G. J . Chromtogr. 1982, 248, 241-247. (3) . . Lee, C. S.;Blanchard. W. C.; Wu. C. T. AMI. Chem. 1990. 62, 1550-1552. (4) Lee, C. S.;McManlglll, D.; Wu, C. T.; Patel, 8. Anal. Chem. 1991, 63, 1519-1523. (5) Lee, C. S.; Wu, C. T.; Lopes, T.; Patel. B. J . Chrometogr. 1991, 559, 133-140.
2478-2483, (16) Nelson, R. J.; Paulus, A.; Cohen, A. S.; Outtmen, A.; Karger, B. L. J . C h f O I M w . 1989, 480, 111-128. (17) McManiglll, D.; Swedberg, S. A. In Techniques In Protein ChCHnlpby: Hiall, T. E., Ed.; Academic Press: Sen Dlego, 1989 pp 488-478. (18) ahowski. K.; ale,R. In 8kmensw Technokgy; Buck, R. P., Hatfbkl,
Science: principles and Appll(8) Hunter, R. J. Zeta Potentlel In cW~&i caf&ms; Academic Press: New York. 1981. (7) Jorgenson. J. W.; Lukacs, K. D. Science 1983, 222, 286-272.
R~~~~~ for review October 1,1991. Accepted J~~~ 31,
W. E., Umana, M., Bowden, E. F., Eds.; Marcell Dekkar: New York, 1990 pp 55-82.
1992.
Comparison of Methods To Assess Surface Acidic Groups on Activated Carbons Teresa J. Bandosz, Jacek Jagiello,’ and Jamee A. Schwarz* Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244-1190
The effect of oxidation wlth nItrlc acld on activated carbons from dH.rent ortglns has been studled by Inverse gas chromatography at Inflnlte dlutlon, h h m tItratlon, and mass 11tratbn to determlne the polnt of zero charge (PZC) of carbons. l h e ~ a f o x l d a t b Is n to generate acldlcgrouw and thk becomes more pronounced as tho temperature of oxldatbn k Increased. We Ikrd that the Boehm tltratkn results and th.c a m ’ PZCs correlate wtth the severtty of the oxldatkn treatment. These methods “study” the surface of carbons by Its reponu to an aqueous environment. Inverse gas chromatography results show that Increasing acldlty Is reflected by an Increase In the speclflc lnteractlon of r electrons In n-akmm Wlth addlc centers on th8 surface of carbone. This method probes the donor/acceptor nature of the carbon’s surface and provldes lnfonnatlon compltmentary to that wpplied by the other methods.
INTRODUCTION During the past 20 years great demands have been placed on the development of carbon materials for diverse applications of practical importance. In particular, activated carbons are widely used as adsorbenta in gaseous, aqueous, and nonaqueous streams, as electrode materials in fuel cells, as catalyat supporta, and as fibers for structural reinforcementor fiiters. The demands placed on activated carbon based technologies have outpaced fundamental studies of the relationship between the properties of carbons and their performance. If these materials are to find continued successful use, then the properties of existing materials will require better under-
* Author to whom correspondence should be addressed. ‘Permanent address: Institute of Ener ochemistry of Coal and Physicochemistry of Sorbents,University of Mining and Metallurgy, 30-059 Krakcw, Poland. 0003-2700/92/0364-0891$03.00/0
standing so that new generations of these materials can be developed. A particularly desirable property of activated carbons in their use as an adsorbent is their high surface area, which is the result of their microporosity. The adsorption characteristics of activated carbon are affected by the type of carbon, and that is governed by the source of raw material and the preparation procedures used during carbonization and activation.14 Some adsorption properties can be explained by differences in the microporosities of carbons. However, another important considerationis the surface chemistry of the carbons. The carbon matrix is decorated with heteroatoms; the main heteroatom is oxygen. These can be connected with peripheral atoms of carbon (edges and corners of crystallites) as well as find location in intercrystalline spaces or in defected areas between the planes that create the crystallites. Of particular importance are the heteroatoms located on the carbon surface. Different functional groups can be derived from these chemical centers, and it is found that these groups are analogous to typical organic compounds. The most common are carboxyl, lactonic, carbonyl, and phenolic. The presence of these groups engenders an amphoteric property to the carbon when placed in an aqueous environment. The acidlbase properties can be determined using titration techniques employing specific titers. The most common is that described by Boehme5 However, acidlbase potentiometrictitration can be also used to yield an index of the carbon acidity denoted its point of zero charge (PZC). Another method which provides an estimate of a carbon’s PZC has been designated mass titration? We find that there are certain advantages to this method. In addition to the results obtained from optical spectroscopies? a very promising technique that can be used to study carbon functional groups without a surrounding aqueous environment is X-ray photoelectron spectroscopy (XPS). This method is very sensitive to the chemical speciation on the 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1992
sample, but the interpretation of the results in the case of carbonaceusmaterials is In addition, sophisticated and expensive equipment is required. Gaseous base adsorption using titers such as pyridine or ammonia can be used to study the chemical nature of a carbon's surface. However, in this case, an absolute determination of the "acidity" is not possible, because the adsorption properties depend upon the experimental conditions.6 It is evident from the above that there is a need for continuous improvement and development of experimental methods for characterization of surface groups on activated carbons. The use of different classes of adsorbates capable of probing a hierarchy of interactions with a given surface would be a better proposal than looking for several universal adsorbates. The use of polarizable molecules, like alkenes or aromatic hydrocarbons, allows for the screening of a large spectrum of specific interactions. For example, molecules with electron-donor character will permit characterization of acceptor properties of the most reactive surface sites. A description of surface properties of carbon in terms of acceptor/donor properties in relation to a homologous series of probes could be more useful than the usual approach of determining its acid/base character. Furthermore, it would desirable that the parameters derived from using different probe molecules could be quantified in terms of standard thermodynamic properties. The objective of this report is to introduce a convenient, realiable, and rapid technique for assessing chemical functionalities on a carbon surface when it not in contact with an aqueous phase. This feature of the technique can be important in those applications when the carbon is used in an anhydrous environment such as gas adsorption or structural reinforcement applications. We propose that inverse gas chromatography at infinite dilution can conveniently bridge the gap between those techniques that study a carbon surface under aqueous and anhydrous conditions. To test this assertion we present the data from four carbons derived from different sources and samples derived from them as a result of controlled treatments with nitric acid. These data were obtained from application of the Boehm titration, PZC measurements of the carbons, and the use of inverse gas chromatography. We find that results from the first two techniques correlate with each other as we might have expected since they measure quantities which reflect the response of the carbon's surface to an aqueous environment. The resulb of applying gas chromatography correlate with the treatment procedures given the carbon, and, furthermore, they correlate with Boehm titration and PZC results. Certain advantages inherent to the chromatography technique have been elucidated and are discussed and critically evaluated.
is calculated from this quantity A G O
=
VN
-RT In mS + C
where R and T a r e the gas constant and temperature, m and S are mass and specific surface area of the adsorbent, and C is a constant related to the standard states of gas and adsorbed phases. Analysis of AGO values obtained under the conditions of infinite dilution provide information about the interaction of probe molecules with the surface only, since the interaction between adsorbed molecules can be neglected. In the case of energetically uniform surfaces, these values are directly related to the adsorption energy while for energetically heterogeneous surfaces such as activated carbons, they should be considered as related to some average value of the adsorption energy. In general, it can be assumed that adsorbate-adsorbent interactions can be classified as dispersive (non specific) or polar (specific). Several empirical methods were proposed to estimate dispersivelo and specific interaction^^^-'^ using different polar probes such as alcohols, chlorides, nitriles, ethers, aromatics, etc. and usually alkanes as dispersive interacting molecules. Application of Alkanes and Alkenes. It is well-known from the chromatographic literaturegJOthat the logarithms of VNfor n-alkanes vary linearly with their number of carbon atoms. The value of the difference in the AGO of two subsequent n-alkanes represents the free energy of adsorption of a CH2 group. Thus it is not related to any particular alkane molecule and, due to its incremental character, is no longer dependent on the choice of the reference state. Alkanes have also been used in conjunction with unsaturated and aromatic hydrocarbons to study specific interactions of different surface^'^^^ in terms of enthalpy and free energy of adsorption. The comparison of AGO values of n-alkenes and n-alkanes was used to study the effect of the r-bond interactions with electron acceptor sites on the The specific interaction parameter c, was defined by
(3) Based on the well-known electronic structure of alkanes and alkenes, this parameter can be taken as a measure of the specific (electron acceptor) interaction capacity of the surface. It is worthwhde to note that when the experimentalconditions are maintained constant, we can write the definition of the specific interaction parameter, c,, with the aid of eqs 1 and 3 in terms of net retention times, t N
INVERSE GAS CHROMATOGRAPHY AT INFINITE DILUTION This method is based on the study of physical adsorption of appropriate molecular probes by means of chromatographic (dynamic) experiments. The general theory of gas chromatography is given e l s e ~ h e r e .Here ~ we only present briefly the major assumptions and concepts used in our approach. The amounts of injected solutes are very small, and it is assumed that the adsorption is described by Henry's law. This assumption is fulfilled when the measured net retention volume, VN, is independent of amount injected, and this can be easily verified experimentally. The quantity VN is calculated from the measured net retention time, tN,by the formula
V , = Fjt, where F is the flow rate and j is the James-Martin compressibility factor which is dependent on the pressure drop along the chromatographic column. The free energy, AGO,
(4)
It is seen from this equation that, due to the cancelation of the corrected flow, cr depends only on temperature and the net retention time.
EXPERIMENTAL SECTION Oxidation. Four activated carbons from different sources (Westvaco, Norit, North American, Calgon) were used in this study. Table I gives their measured BET surface area and total elemental impurity content as determined by electron spectroscopy for chemical analysis (ESCA) at Brookhaven National Laboratories. They were oxidized with 15 N HNOBfor 2 h while continuously stirred. The temperatures selected were 25,50, and 78 "C. A vigorous oxidation reaction wm indicated by the release of brown fumes (nitric oxides). At higher temperatures the density of fumes was greater. This experiment was performed under a hood to eliminate the toxic influence of nitric oxides. After treatment, the samples were washed with distilled water in a
ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1992
893
Table I. General Characteristics of Carbon Samples carbon sample
origin
BET surface area, m2/g
total metal content, %
Westvaco Norit
wood peat moss coal coconut
1700 1300 1300 1500
0.038 0.48 0.041 0.079
Calgon
North Am.
10
-.
Soxhlet apparatus to zero acid removal and oven-dried at 100 "C. For easier description, we denote these samples as NO, WO, NA, and CO where the symbols indicate Norit, Westvaco, North American, and Calgon, respectively. According to the temperature treatment we use the symbols A, B, and C; these symbols represent temperatures: A, carbon treated at 25 "C; B, carbon treated at 50 "C; and C, carbon treated at 78 "C. Baehm Titration Method. To estimate the acidic and basic properties of each of the surface modified activated carbons, the method proposed by Boehm5 was used. This method is based on acid/base titration of carbon acidic or basic centers. One gram of carbon sample (after washing) was placed in 50-mL vials of the following 0.05 N solutions: sodium hydroxide (NaOH),sodium carbonate (Na2C03),sodium bicarbonate (NaHN03),and hydrochloric acid (HC1). The vials were sealed and agitated for 24 h, 5 mL of each filtrate was pipetted, and the excess of base and acid was titrated with HC1 and NaOH, respectively. The number of acidic sites of various types was calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups; NazC03, carboxyl and lactonic; and NaHC03, only carboxyl groups. The number of surface basic sites was calculated from the amount of hydrochloric acid which reacted with the carbon. Mass Titration Method. This method is based on estimating the point of zero charge (PZC) of the carbon surface by means of pH measurements6when the mass percent of carbon in solution is increased. To measure the PZC of the carbon samples, three solutions of different ionic strengths of NaN03 were used. The concentrations of NaN03 were 0.1, 0.01, and 0.001 M. For each ionic strength, six bottles were filled with 20 mL of the solution and different amounts of carbon samples were added (0.05%, 0.1%, 0.5%, 1%,5 % , and 10% by weight). Following this the samples were agitated for 24 h, and the equilibrium pH was measured by a Corning pH-meter Model 145. Inverse Gas Chromatography. The chromatographic experiments were performed with an ANTEK 3000 gas chromatograph (from Antek Instruments Inc.). This chromatograph was equipped with a flame ionization detector. Samples of carbon were placed in stainless steel columns (20 cm long, 2.17 mm in diameter). Before filling, the carbon was separated into particles of size ranging from 0.2 to 0.4 pm; helium was used as a carrier gas with a flow rate of about 30 cm3/min. Injector temperature was 150 "C and detector temperature was 220 "C. The samples were conditioned at 350 "C in the chromatographiccolumn under helium gas flow for 15 h prior to the measurements. The hydrocarbons used for injections were HPLC grade (Aldrich
$ I\
I
04 0
100
200
300
400
number of carboxyl groups [meq/lOOg] Figure 1. Dependence of PZC on number of carboxyl groups. Chemical Co.). A 50-pL Hamilton syringe was used for injecting very small volumes of gaseous solutes. The experiments were done in the range of temperatures 200-350 "C. Under these conditions all chromatographic peaks were symmetrical, and retention times did not depend on amount injected (Henry's law region). Retention volumes were corrected for the gas compressibility. The error of the measurement of retention time was 5%, and the temperature was stabilized with an accuracy of fO.l "C.
RESULTS AND DISCUSSION Nitric acid is a very strong oxidant of the carbon ~ u r f a c e ; ~ , ~ nitric oxides are oxidizing agents in this reaction. The results of chemical oxidation of our carbons are collected in Table 11. The number of each kind of group was calculated according to the Boehm method. It is well-known that the surface chemical groups are much more complicated than we can "see" from the results of a Boehm titration,' but this method gives a semiquantitative measure of the surface functionalities. The results indicate that the oxidation treatment increases the number of acidic sites; the temperature enhances the oxidation effect. During the oxidation process we see the decrease of basic centers. This disappearance is connected with the very aggressive influence of nitric acid. This process is most apparent in the case of Westvaco. However, the most significant increase of total acidity occurs in the case of North American (about 13 times). Generally, if we take into account the kind of groups and their strength (carboxylic groups have the strongest character) we can notice that the most acidic carbon is Westvaco and after it Norit, Calgon, and North American. The total number of acidic groups and the number of carboxylic groups was correlated with PZC values (Table 11). The PZC of activated carbons can be estimated from the mass titration resulb within an error of k0.2 pH units,if the average
Table 11. Results of Boehm Titration and Mass Titration (PZC) number of basic number of acidic number of carboxyl number of lactonic number of phenolic sample groups, mequiv/100 g groups, mequiv/100 g groups, mequiv/100 g groups, mequiv/100 g groups, mequiv/100 g PZC
wo
WO-A
WO-B
wo-c NO NO-A NO-B NO-C NA NA-A
NA-B NA-C
co
CO-A
CO-B
co-c
13.8 0.0 0.0 0.0 68.2 27.0 31.3 10.0 36.2 25.0 17.5 12.5 30.0 19.4 22.5 7.5
75.0 121.0 236.2 440.0 23.7 75.0 97.5 205.0 18.0 72.5 102.5 230.0 22.0 71.9 100.0 240.0
27.5 86.2 135.0 350.0 2.5 19.0 27.5 102.5 1.6 17.5 40.0 150.0 5.0 17.5 56.3 160
14.5 5.70 30.0 12.0 7.5 15.0 23.7 15 4.6 20.0 10.0 25.0 6.0 18.7 5.7 5.0
33.0 29.1 71.2 78.0 13.7 41.0 46.3 87.5 11.8 35 52.5 55.0 11.0 35.7 38.0 75.0
4.31 2.40 2.25 1.86 10.26 4.71 2.83 10.32 4.63 3.43 2.17 9.7 6.2 4.16 2.31
804
ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1992
Table 111. Inverse Gas Chromatography Results 4 3 ) DO0 "CI,
d6) [350 "Cl,
kJ/mol
kJ/mol
-0.50
-0.35 0.55 1.02
sample
wo
0.00
WO-A WO-B
0.16
wo-c NO NO-A NO-B NO-C NA NA-A NA-B NA-C
co
CO-A CO-B
co-c
-
-
-0.94 -0.78 -0.43 -0.06 -0.75 -0.54 -0.45 0.43 -0.80 -0.27 -0.26 0.35
-1.84 -0.27 -0.25
0.10 -1.00 -0.25
-
2
- 1
0.23 -1.70
1
0
E ,46) [kJ/mol] Flgure 3. Conelation between specific interadon energy of propene,
-
t,(3), and hexene, e#).
-0.24
0.38 1
1,
.1
J. 100
0 1
2
2
3
4
5
number of carbon atoms Flgurr 2. Dependence of AGO of alkane (-)
and alkene (---) adsorption versus number of carbon atoms measured for Nork (NO) (0, 0) and Westvaco (WOB) (m, 0)at 200 O C .
value of the three asymptotic pH values is taken as the PZCa2 Figure 1presents the dependence of PZC upon the number of carboxylicgroups. It is apparent that this method is sensitive only up to about 100 carboxylic groups present on the surface of 100 g of carbon. If the number of carboxylic groups is higher (up to 350 groups/lOO g of carbon), the PZC values are almost the same. We assume that this value is the value of the [HI+ ion concentration after all acid groups, which are present on the surface, reach their dissociative/associative equilibria. Noh and Schwarz6 presented this method as a standard method for estimating this property of the surface. Indeed, we find that this method is sensitive for low concentration of acidic groups; however, if the concentration of acidic centers on the surface is high, this method appears to be less sensitive. Inverse gas chromatography results are reported in Table 111. Typical results depicting evaluation of t, values for selected carbon samples are shown in Figure 2. Almost parallel lines were obtained for alkanes and alkenes for all samples; the numerical values oft, were calculated from the difference between AGO of propane/ propene and hexane/ hexene. In our previous work we reported t, values only for hexane/hexenelg measured at 350 "C. For strongly oxidized samples it was impossible to do experiments under these conditions because of cracking reactions that occurred for strongly oxidized carbons during hexene adsorption. On the other hand, experiments for propane/propene at 200 OC were found to be free of the cracking effect. It is important to note that the results we were able to obtain for hexanelhexene correlated quite well (correlation coefficient, r2 = 0.80) with the propane/propene results (Figure 3). This feature can be
200
number of carboxyl groups [meq/lOOg] Flgurr 4. Dependence of ~ ~ (versus 3 ) number of carboxylic groups for different carbons: (0)Westvaco, (0) NorR, North American, (0)Calgon.
m)
0
100
200
300
total number of acidic groups [meq/100gl Flgurs 5. Dependence of tJ3) versus total number of acidic groups for all carbons. ( 0 )Westvaco, (0) NorR, North American, (0) Calgon.
m)
very useful for estimating the carbon acidity levels by adsorption of long-chain hydrocarbons. In those cases were cracking effects preclude direct measurement it is possible to substitute longer hydrocarbons by shorter ones and doing the chromatographic experimentsat a lower temperature. The dependence of e,(3) upon the carboxylic group density gives a good correlation (r2 = 0.95) (Figure 4). If the oxidation degree is higher the 4 3 ) value is higher and eventually becomes a positive value. The differences in these values are small but they are very sensitive to the number of carboxylic groups (Figure 4). The 4 3 ) values of untreated carbons are negative, indicating that the configuration of the alkane on the carbon surface is more favorable for interactions than the
ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1992
configuration of the alkene. After oxidation tJ3) values are bigger and they became positive values, which indicates the dominance of specific interactions with electron acceptor surface s i t e induced by the oxidation process. The correlation between 4 3 ) and total number of acidic groups (Figure 5 ) was found to be not as good (r2 = 0.86) as that with the carboxylic group density. This demonstrates that carboxylic groups have the greatest influence on the acidic properties of activated carbons. The Boehm titration method is carried out in aqueous solutions and provide results which "see" the surface from the perspective of chemical reaction phenomena. On the other hand, physical adsorption of alkenes from the gas phase under conditions of infinite dilution is determined by the existence of highly energetic centers on the surface. The same acidic centers visible to the Boehm method may be ineffective in adsorption either because of their chemical strength or due to steric hindrance. However, our results are consistent with the results of the Boehm titration. Using the Boehm method it is possible to qualitatively separate and identify at least three acid strengths on the surface. Using chromatography we can estimate only a general trend. However, the advantage of the inverse gas chromatography method is that it identifies quantitatively the acidic centers that play an important role in gas-phase adsorption processes or any other anhydrous interaction with these heterogeneous surfaces.
CONCLUSION The threemethods that we used for estimating the chemical characteristics of a series of carbon surfaces are very useful for determining their degree of oxidation. However, estimating the PZC by mass titration appears to be less sensitive for highly oxidized carbons. The Boehm titration method is not limited to this constraint but is sensitive only to different centers when in aqueous solutions. The method of estimating the acidity by inverse gas chromatography is very useful in the case of carbons. It provides, with high sensitivity, a
895
measure of specific interaction sites that act as adsorption centers in anhydrous environments. The fact that the specific interaction parameter derived from this method also correlates with Boehm titration results makes the chromatographic method equally useful for characterization of activated carbon surface chemistry in aqueous applications.
ACKNOWLEDGMENT The work was supported by the New York State Energy Research and Development Authority under Contract 139ERER-POP-90.
REFERENCES Smlsek. M.; Cerny, S. Active Carbon; Elsevier: Amsterdam, 1970. Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1968. Ignask, 8. S.; Clugston. D. M.; Montgomery. D. S. Fuel 1972, 51, 76. von Fredersdorff, G. G.; Elliot, M. A. Coal Gazlfication, Chemistry of Coal lJti//zatiOn; H. H. Lowry: New York, 1963. Boehm, H. P.; Voll, M. Carbon 1970, 8. 227. Noh, J. S.; Schwarz, J. A. J. COlloM Interface Sci. 1988. 27, 531. Zawadzki, J. Chemistry and phvsics of Carbon; Marcel Dekker: New York, 1989; Vol. 21. Nakayama, Y.; Soeda,F.; Ishitanl, A. Carbon 1090, 28. 21. Klselev, A. V.; Yashln, Y. I. Gas Adsorption Chromatography; Plenum Press: New York, 1969. Dorris. G. M.; Gray, 0.G. J. CoIbkI Interface Sci. 1970, 71, 93. Klselev. A. V.; Koletnikova. T. A.; Nitkin, Y.; Tsllipotklna, M. V. KO//. Zh. 1087, 40. 865. Saint-Flour, C.; Paplrer. E. J. Collod Interface Scl. 1083, 91, 69. Schub, J.; Lavielie, L.; Martin, C. J. Adhesion 1087, 23, 45. Dong, S.; Brendle, M.; Donnet, J. B. Chromatographie 1080, 28, 469. Ahsan, T.; Colenutt, B. A.; Sing, K. S. W. J . Chromatogr. 1980, 464, 416. Ahsan, T.; Colenutt, B. A.; Sing, K. S. W. J . Chromatogr. 1980, 479, 17. SMql, M.; Llgner, G.; Jaglello, J.; Balard, H.; Paplrer, E. Chromatogrephle 1989, 28, 588. Contescu, C.; Jagiello, J.; Schwarz, J. A. J. Catal. 1091, 737, 433. JagieWo, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1902, 30, 63. Puri, B. R. Chemlsby and phvsics of Carbon; Marcel Dekker: New York, 1970; Vol. 6. Havard. D. C.; Wilson, R. J. Collod Interface Sci. 1075, 57, 276.
RECEIVED for review October 4, 1991. Accepted January 21, 1992.
