Anal. Chem. 1992, 64: 25-32
detected with a 110-cm LCOF. The details of this experiment will be reported. However, from this development and literature sources (1-4),one concludes that a long column, especially LCOF system methods, offers many advantages to problems requiring the ultimate in sensitivity.
Table 11. Enhancement Factor of Absorbance as a Function of Analyte Concentration analyte abs in
length, m
conventional 1-cm cell
obsd abs
factor
25
0.0000002 0.000 01 0.000 02 0.0000002 0.00001 0.000 02
0.0053 0.112 0.172 0.0089 0.184 0.286
9.6 3.5 2.4 7.9 2.7 1.9
50
25
REFERENCES Lei, Wei; Fujlwara, K.; Fuwa, K. Spectrosc. Spectre1 Anal. (6e!/lng) 1988, 6, 22-31. Fuwa. Kellchlro; Lei, Wel. Anal. Chem. 1984, 56, 1640-1647. Lel, Wel; Fujhvara, Kltao. Anal. Chem. 1989, 55, 951-955. Dasgupta. Purnendu K. Anal. Chem. 1984, 56, 1401-1403. Degrandpre, Michael D.; Burgess, Lloyd W. Anal. Chem. 1988, 60, 2562-2586. Zaganlarls, Alclblade. Appl. M y s . Lett. 1974, 2 5 , 345-347. White, K. I. Opt. Quantum. Electron. 1978, 8 , 73-75. Miller,, S. E.; Chynoweth, A. 0. QItical F l k &"imlcatkms, 1st ed.; Post and Telecommunicatlon Press: Bel Jln, China, 1985; Chapter 3. Pelda, Ye. OpmCal Fiber Theory, 1st ed.; Knowledge Press: Shang Hal, Chlna, 1985; Chapter 3. Gloge, D. Bell S p t . Tedr. J. 1972, 51, 1767-1783. Dugas, Jacques; Sotom, Mlchel. Appl. Opt. 1987, 2 6 , 4198-4208. Marcuse, Dletrich. 8eIlSyst. Tech. J . 1970, 49, 1695-1702. StOne, F. T.; Appl. Opt. 1978, 17, 2625-2830. Olshanschy, Robert; Oaks, Susan M. Appl. Opt. 1978, 17, 1830-1835. Coleman. J. Todd: Eastham. Jerome F. Anal. Chem. 1984, 56, 2246-2249. Lewis, G. F. An Introduction to AnalyNcal Chemistry; BDH Chemicals Ltd.: Oreat Brltain, 1973; Section 12. Wakafen, 0. E.;Stone, J. Appl. Spectrosc. 1972, 26, 585-589. Gomer, H.; Maier, M. J . Raman Spectfosc. 1974, 2 , 363-371.
In higher concentration ranges the plot of absorbance of an analyte versus concentration of the analyte is not linear and involves negative deviation from the line plot obtained at lower concentration. The above results show that there is a limit for LCOF length and analyte concentrationin LCOF systems used for spectrophotometry. This development places LCOF analytical measurements on a better theoretical platform, and resolves some of the previously reported anomalies. There are, however, many other factors that can affect absorbance studies, such as stray light (15)and the chemical properties of the solution itself (16).This development has only considered linear scattering. There is much nonlinear scattering which can decrease absorbance (17,18).This approach has only included secondorder mode a b r p t i o n and scattering and zero-order coupling. Further work remains. A very simple LCOF spectrophotometer has been constructed. In this system 0.4 ng/mL of Ag at X = 566 nm and 2.5 ng/mL of Hg at X = 560 nm can be
RECEIVED for review December 11,1990. Revised manuscript received September 12,1990. Accepted September 23,1991. This work was supported by the National Natural Science Foundation of China.
Analysis of the Surface Heterogeneity of Different Samples of Aluminum Oxide Ceramic Powders Jeffry Roles,' Kevin McNerney? and Georges Guiochon* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600,Division of Analytical Chemistry, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6120,and Coors Ceramics Company, 600 Ninth Street, Golden, Colorado 80401-1099
-
The homogenelty of the surface of the partlcles of powders can be characterizedby the adsorptlon energy dlstrlbutlon of dlfferent probe compounds selected for exhlbltlng dlfferent types of Interaction energy wlth the chemlcal groups which can be found on the surface. We descrlbe the results obtalned by thls InvestJgatknon a rnwnber of dmerent alumina samples. Slgnlflcant dlfferences between the adsorption energy dlstrlbutlons of hlghquallty alumlna powders for ceramlcs have been found for lchlorobutane, although these samples had ldentlcal adsorptlon energy dlstrlbutlons for diethyl ether. The dltferences observed for the adsorptlon energy dlstrlbutlon of l-chlorobutane, a weak acld, are correlated wlth signlflcant dlfferences between the process performance of the samples. The performance degrades with lncreaslngfractbnal d a c e coverage of a hlgh-energy mode of the dlstrlbutlon.
* To whom correspondence should be addressed at the University of Tennessee. University of Tennessee. * Coors Ceramics Co. 0003-2700/92/0364-0025$03.00/0
INTRODUCTION We have previously discussed (1-3)a new procedure for the investigation of the surface properties of solids. This method suits well the study of the surface heterogeneity of fine, solid particles, such as those of the raw materials used to prepare ceramics. We present here results obtained in its applications to alumina samples of various origins. Advanced ceramic materials have achieved a great importance in several high technology industries (4).In many casea, ceramic engineers and material scientists have observed a common problem. The raw material used to prepare ceramic parts is a finely divided powder (e.g., carbide, oxide, or nitride of silicon or aluminum). In processing, this powder is mixed with a binder (usually a mixture of large molecular weight compounds, including paraffins and glycols), placed in a mold, and fired. Depending on the process, the mixture injected into the mold may be dry, or be in the form of a slurry or a gel. Thus, other materials (solvents, modifiers, wetting agents) may be present in addition to the binder. Different lots of a powder obtained from the same vendor and manufactured 0 I991 American Chemical Soclety
26
ANALYTICAL CHEMISTRY, VOL. 64, NO. 1, JANUARY 1, 1992
by the same process give products which behave differently during the firing process (5). The degree of heterogeneity of the surface of the ceramic particles is important because it controls to some extent the properties of the final prcduds. During the stages of molding, drying, and firing, the nonconsolidated powder, which has an approximately 40% void volume, is transformed into a solid part. The solid material is redistributed. The organic compounds (solvents, wetting agents, binder etc.) introduced to facilitate the molding of the green body are eliminated. Their fate (evaporation, partial or complete pyrolysis) depends largely on the composition of the surface, on the adsorption energy of the organic chemicals used, and on the distribution of this adsorption energy. So far, however, little success has been achieved in the determination of characteristics of ceramic powders which are correlated with processing behavior (6). The lot to lot variations which are observed remain unaccounted for by the traditional methods of analysis, such as titration curves, { potential measurements, etc. (7). The decision to use a particular lot of powder in a process must often be made after performing pilot scale processing experiments (3,an expensive proposition. High rejection rates result in a large fraction of the total cost of ceramic production (8). Clearly, the situation would be greatly improved by the implementation of methods of surface analysis which could provide more useful information when applied to the characterization of powders. Physical methods of surface analysis have not been successful because of the small size of the ceramic particles (of the order of 2000 A) and the fact that only the first few atomic layers of the particle contribute effectively to the adsorption energy potential above the surface. Physicochemical methods of adsorption studies are better suited for this type of investigation. The concept of adsorption energy distribution dates back to Langmuir (9). This distribution has been studied actively in the last 20 years (10).The characterization of surfaces by the determination of the adsorption energy distribution of a series of probes has been described by Jaroniec (11)and applied by Boudreau et al. (12,13). We have recently described a general solution to the problem of relating the experimental adsorption isotherm of a probe and its adsorption energy distribution on the studied surface ( I ) . This solution is free of many of the assumptions and restrictions of previous ones, especially as regards the nature of the local adsorption isotherm, on a homogeneous patch of the surface. We have also discussed the experimental problems associated with the need for a rapid and accurate determination of the adsorption isotherm of the probes selected for the characterization of the surface (2, 3). Alumina was chosen as the test material for several reasons. First, it is an important material, both in the ceramics community and in other important areas of technology including catalysis and separation science. Because of its importance, it has been widely studied. A great deal is known about its chromatographic behavior. We know, for example that certain compounds should not be used as probe solutes. Many alcohols dehydrate on the surface of alumina. Acetone chemisorbs on its surface (14). The most important reason for using alumina compared to many other high-performance ceramic materials, however, is that its surface is oxidized. This makes much easier the handling of the samples during the development of the method than those of aluminum nitride, for example. The material is also relatively stable a t conditions under which the experiment is performed. One may be reasonably certain that the experiment does not greatly change the surface chemistry of the material. It was desired that the method be developed without having to worry about problems related to surface oxidation due to the chromatographic
conditions. These problems may be very important when the method is used for other materials (e.g., nitrides and carbides) and must be dealt with at that time. The adsorption heterogeneity manifested by different samples of aluminum oxide may be due to a variety of factors. It exists in at least five thermodynamically stable phases and several metastable ones (15).It is known (16) that the alumina surface is covered with one type of proton-donating sites, the surface hydroxyls, and three typea of Lewis acid sites, corresponding to A13+ and two different electron-deficient oxygen species. Basicity may be due to oxides, 02vacancies, or to basic hydroxyls (15). Other atoms (e.g. magnesium) may also be present and concentrated a t some location on the surface. While it would be interesting to rationalize the surface heterogeneity of a particular sample of alumina (i.e. to explain it in terms of the chemical interactions involved), this will not be attempted here. Data from other source8 would be needed, including spectroscopic studies (10).The molecules used as probe solutes are relatively large and capable of adsorbing onto the surface in more than one way. The adsorption data measured or calculated must be regarded as a statistical average of all the possible modes of adsorption. Trying to interpret such data in terms of specific interactions is highly speculative unless independent data are available. Nor would such speculation be relevant a t this stage, since the focus of the research is on the development of an analytical technique for the characterization of solid surfaces. Our goal is to determine lot to lot differences in the adsorption energy distribution of probe compounds which correlate with differences in processing characteristics. We demonstrate here the ability of the method to measure those differences. We first establish the ability of the method to discriminate between materials which are expected to manifest fairly significant differences. Then we investigate series of materials whose process performances are quite similar or very different.
EXPERIMENTAL SECTION The experimental protocol has been reported elsewhere (2,3). The sources of experimental errors, their contribution, and the experimental procedures used to minimize them have been discussed in detail (3). We provide here only a succinct description of the main steps of the determination. Equipment. Chromatographic data were obtained using a Perkin-Elmer 8500 (Norwalk,CT) gas chromatograph. Two minor instrumental modifications were made to improve the accuracy of the data. First, a correction to the data is needed to account for the effect of the carrier gas compressibility. Although this correction is minimized here by the use of open tubular columns, it is important to know accurately the column inlet pressure. The inlet pressure transducer was moved to a position on the gas line close to the injection port, to reduce the pressure drop between the transducer and the column inlet and obtain a more accurate measure of the column inlet pressure. Secondly, to permit accurate calibration of the detector and quantitative analysis, two parallel columns are used, a calibration column on which the probe is retained without experiencing irreversible adsorption and the wall-coated open tubular column made with the ceramic material studied. The analog output from the flame ionization detector was digitized and recorded on an IBM PC @oca Raton, FL). The computer was interfaced to the chromatograph by an 1/0board (Data Translation, Marlboro, MA) which used 12 bit A/D conversion and was controlled by in-house written software. Materials. Two classes of aluminum oxide were studied. Those classes are referred to as 0.8- and 1.5-pm aluminas. These dimensions refer to the average spherically equivalent particle diameter of the powder. The two classes are given those designations for convenience only. It should not be inferred that the average particle diameter is the only nor even an important difference between the samples. They are ground to different dimensions because they are intended for use in different ceramic processes.
ANALYTICAL CHEMISTRY, VOL. 04, NO. 1, JANUARY 1, 1992
(a) 0.8-pm Alumina. Three samples of different origins were All three have average particle diameters of approximately 0.8 pm and have the same nominal characteristics. They are manufactured by different processes, however, and may have different impurities, although no significant differences in ceramic processing among the three powders have been reported. RCHP Alumina. High-Fkrity a alumina (product identification number RCHP-DBM (w/o MgO), Lot Number BM-2216) was obtained from Malakoff Industries (Malakoff, TX).The powder has been dry ball milled by the manufacturer to an average particle diameter of 0.5-0.8 pm. The surface measured by the BET method was 8-10 m2/g. The geometrical surface area of spherical particles having a diameter of 0.5- and 0.8-pm, and a density of 4 g/cm3 would be approximately 3 and 2 m2/g, respectively. Sumitomo Alumina. High-purity alumina (Product Identification Number, AKP-50; Lot Number HD-9YO1) was obtained from Sumitomo Corp. of America (New York, NY). Baikowski Alumina. High-purity alumina (Product Identification Number, CR 10; Lot Number, 290) was obtained from Baikowski International Corp. (Charlotte, NC). Modified 0.8-pm Alumina. Modified high-purity alumina (Product Description Number HPA-0.5 (w/MgO),Lot Number, BMP 88004-03) was obtained from Ceralox Corp. (Tucson,AZ). The MgO concentration was 500 ppm and was added by introducing magnesium aluminate (normal spinel form) during the dry ball milling process. This powder will be referred to as modified alumina. (b) 1.5-pm Alumina. Aliquots from three lots of alumina were obtained from Coors Ceramics Co. (Golden, CO). The three samples are referred to as good, medium, and bad. These names are derived from their performance in ceramic processing. For each lot, the calcined alumina (Relox grade 152) was obtained from Alcoa (Aluminum Co. of America, Pittsburgh, PA). It was dry ball milled (at Coors) to an average spherically equivalent particle diameter of 1.5 pm. This class of material is a less pure a alumina than the 0.8-pm material described above. During the ceramic processing undergone at Coors, it was observed that the good lot resulted in the desired relatively dense ceramic product. The density of the ceramic product resulting from the medium lot was somewhat reduced, and the density of the ceramic product prepared with the bad lot was even less (compared to the good lot). It is assumed that the reduced performance of the medium and bad lots was a result of an increase in the degree of interaction between the more polar components of the binder (e.g., glycols) and the surface of the particles. In processing, as the material in the mold is heated to an intermediate temperature (in order to desorb and boil off the binder) some molecules may remain adsorbed. Later, when the material is fired (i.e., heated to a very high temperature, to cause the particles to fuse together) the residual pyrolyzate of the organic chemicals results in structural deformities which reduce density and strength. (c) P r o b e Compounds. We used diethyl ether and 1chlorobutane, both from Aldrich Chemicals (Milwaukee, WI). They were used as received. Experimental P r o c e d u r e . The carefully dried sample is suspended by sonication in a solvent. Dimethyl sulfoxide was found suitable for all alumina samples. A 15 m long, 0.53 mm i.d. cleansed quartz tubing (Alltech Europe, Nazareth, Belgium) is filled with the 1% slurry. One end is closed, and the tubing is slowly coiled into an oven heated at 250 "C, through a long metal tube in which a shallow thermal gradient is established. After coiling is achieved, the column is conditioned at 320 OC for 72 h. The equilibrium adsorption isotherms of the probe compounds are determined using the ECP method (In,by injecting a large size sample in the column, recording the profile of the elution band, and transforming the profile of the detector signal into a profie of probe partial pressure. The distribution of adsorption energy is determined numerically, from the experimental adsorption isotherm (1).
27
used.
RESULTS AND DISCUSSION I. Comparison b e t w e e n t h e RCHP Alumina and t h e Modified Alumina. The modified alumina is similar to the RCHP alumina, but contains 500 ppm of magnesia. Because
00
0.55
1.10
1.65
20
Retention Time (Minutes) Figure 1. Elution profiles of diethyl ether on modified alumina at 00 OC. Experknental profiles were derhred from the chromatograms (miid #ne)and profiles calc~letedwith the semiideal model (dashed line) from the adsorption Isotherm derhred by ECP from the experimental profile (Figure 2). Chromatographic conditions: P,, 1.32 atm; L , 1500 cm; to, 0.413 mln; m,, 43 mg; sample sizes, 0.53,0.13,and 0.017 pg, respecthrely; N (number of theoretical plates of the column), 8000. Difference parameters (between the simulated and the experimental peaks): tRW, 0.87%; A, 1.5 X lo-' (3).
of the procedure followed to perform the addition, it is unclear which proportion of the magnesia is coated on the surface of the alumina particles and which proportion is unavailable for gas-solid adsorption. The experimental elution profile of diethyl ether on a column made with the modified alumina is shown in Figure 1(solid line). This profiie is the true profile, giving the partial pressure of diethyl ether in the carrier gas as a function of time. It was derived from the chromatogram (flame ionization detector signal versus time) by calibration. The isotherm obtained from this elution profile, using the ECP method, is shown in Figure 2 (solid line). From the isotherm and using a classical procedure of nonlinear chromatography (18,19), the elution band profiie was calculated. This profile is plotted in Figure 1 (dashed line) where it can be compared to the experimental profile (solid line). The agreement between calculated and experimental profiles validates the whole procedure of isotherm determination (3). The band profiles obtained in nonlinear chromatography are very sensitive to minor changes in the slope of the isotherm (19). The distributions of the adsorption energy of diethyl ether on the two adsorbents are compared in Figure 3. The thermodynamic parameters calculated for the adsorption of diethyl ether on modified alumina are compared with those obtained with the same probe on RCHP alumina in Table I. Enamination of thistable reveals that the differences between the characteristics of the adsorption energy distribution of diethyl ether for the two adsorbents are smaller than the 95% confidence interval for the reproducibility (3)of these data on the RCHP alumina (different columns, different days; see Table I). Thus they are not statistically significant. From the point of view of diethyl ether adsorption, the two surfaces are not different. The adsorption isotherm can be calculated easily from the adsorption energy distribution. This direct problem is much easier to solve than the inverse problem, the derivation of the adsorption energy distribution from the experimental isotherm. The measured isotherm (solid line) and the isotherm recalculated from the adsorption energy distribution (dashed
28
ANALYTICAL CHEMISTRY, VOL. 64, NO. 1, JANUARY 1, 1992
Table I. Characteristics of the Adsorption Energy Distribution of Diethyl Ether on Different Alumina Samples Qm*
adsorbent f
pmol/g
+
RCHP modified alumina Sumitomo Baikowski Coors good medium bad RCHP modified alumina Sumitomo Baikowski Coors
+
medium bad 1.15
2.30
3.45
variance, ca12/mo12
Low-Energy Peak 5.36 11.04 5.12 10.96 5.09 10.95 5.24 10.95 1.77 1.67 1.72
16.6 10 40 13
10.85 10.89 10.87
10 4.2 8.3
High-Energy Peak 2.53 12.68 2.42 12.54 2.35 12.53 2.37 12.53
1.0 1.4 1.4 1.0
12.49 12.52 12.50
1.0 1.0 1.4
0.86 0.80 0.82
good
0.00
Ea,,, kcal/mol
4.60
Solute Partial Pressure (Atm.)
RHCP
Figure 2. Adsorption isotherm of diethyl ether on modified alumina, measured by ECP (solid line, left ordinate scale). The isotherm cab culated from the optimized adsorption energy distribution (Figure 3) could not be distinguished. The difference between the two isotherms is shown by the symbols (right ordinate scale).
RHCP
0
k N
Modified 0
m
0 00
h
w o
105
2 10
Retention Time
w(D L
0 -?
C C
0 80
11.35
11.90
12 45
Energy (KCol/Mol)
Flgure 3. Comparison between the distributions of adsorption energy for diethyl ether on RCHP alumina (top) and on modified alumina (bottom).
line) are compared in Figure 2. They cannot be distinguished. The differences between the two curves are also shown, with an amplified scale. They are small (of the order of 1 ppm) and exhibit no trends. This result validates the procedure of calculation of the adsorption energy distribution, because the two calculation process are independent (1). Contrary to the results obtained with diethyl ether, significant differences in the adsorption energy distributions of the two samples were seen when 1-chlorobutanewas used as probe. The experimental and simulated elution profiles are compared in Figure 4 for the two alumina samples. As in the
(Mid
3 15
20
Figure 4. Elution profiles for l-chlorobutane on RCHP alumina (top) and on rrdfied a i h (bottom)at 40 'C. Experhnentaiprofiles were derived from the chromatograms (solid lines) and proflies calculated from the lsottrerm (not shown), using the semydeai model (dashed lines). Chromatographic conditions: L , 1500 cm.; N, 8000. olp () PI, 1.32 atm; to, 0.358 min; m,, 44 mg; sample sires 0.18, 0.037, and 0.010 pg, re9pecthrely; fwS, 0.71%; A, 8.5 X lo-'. (Bottom) PI, 1.34 atm; to. 0.396 min; m,, 43 mg; sample sizes, 0.44, 0.086, and 0.026 bug, respectively; fRm, 0.99%; A, 4.7 X IO-'.
case of diethyl ether they agree very well, validating the adsorption isotherm. This isotherm, which is very similar to the one shown in Figure 2 (20),is not informative enough to be reported here. The distributions of the adsorption energy of 1-chlorobutane on the two alumina samples are compared in Figure 5. The thermodynamic parameters characterizing thew distributions are compared in Table II. The high-energy peak for the modified alumina is significantly shifted toward a higher energy and very much broadened, as illustrated in the inset. We attempted to deconvolute the broad high-energy band in the adsorption energy distribution for 1-chlorobutane on modified alumina into two Gaussian peaks. To that effect,
ANALYTICAL CHEMISTRY, VOL. 04, NO. 1, JANUARY 1,
I
RHCP
RHCP
2
Sumitomo
Modified
1992 20
I
I
n
W
W c
I 2 11.8
10.9
12.7
13.6
2q
W
W
12.7
13.6
Energy (KCal/Mol) Figure 5. Comparison between the adsorption energy distributions for lchbrobutane on RCHP alumina (top) and on modified alumina (bot-
tom). Table 11. Characteristics of the Adsorption Energy Distribution of 1-Chlorobutane on Different 0.8-pm Alumina Samples Qm9
adsorbent
pmol/g
E,, kcal/mol
12.8
Sumitomo alumina; (bottom) Baikowski alumina.
n
10.9
11.8
Figure 6. Comparison between the adsorption energy distributions for dethyi ether on 0.6-pm a l u " samples: (top)RCHP alumina; (middle)
I\
Modified
10
10.8
Adsorption Energy (KCol/Mole)
Energy (KCol/Moi)
variance, ca12/mo12
RCHP modified alumina Sumitomo Baikowski
Low-Energy Peak 11.0 10.2 11.9 10.2 10.5 10.15 12.1 10.21
1970 2050 1875 2114
RCHP modified alumina Sumitomo Baikowski
High-Energy Peak 0.86 12.56 1.40 12.97 0.82 12.52 0.92 12.56
1.0 13900 1.1 1.0
the three Gaussian parameters corresponding to the low-energy peak were constrained to the values shown in Table 11. The optimization was redone, with the variance and the average energy of the two high-energy Gaussian peaks set to the values given for the single high-energy Gaussian peak (Table II). The monolayer capacity was divided equally between the two peaks. This new optimization failed to give a better fit than the first, twepeak optimization. If two high-energy peaks are actually present in the high-energy mode of the distribution, they are closer in energy than the ability of the method to resolve two adsorption energy peaks. This comparison demonstrates that the method is capable of measuring differences between modified and unmodified
aluminas. There is no doubt that the incorporation of a few thousand ppm of MgO only would be easily detected. The results are not entirely conclusive for our purpose, however, since we want to distinguish between powders which differ in their processing qualities but not in their chemical composition (within experimental errors) and for which the impurity concentrations are 2 orders of magnitude below the concentration of MgO in the modified alumina. We compare now materials considered as equally good for their processing performance (Section 11) and a series of samples having significantly different performance (Section 111). 11. Comparison between the Adsorption Energy Distributions of 0.8-pm Alumina Samples from Different Origins. In this study, no statistically significant differences were found between the adsorption energy distributions of either diethyl ether or 1-chlorobutaneon the samples studied. The experimental elution profiles of diethyl ether and 1chlorobutane on the Sumitomo and the Baikowski aluminas, and thus also the corresponding adsorption isotherms, are very similar to those observed on the RHCP alumina (20) and are not shown. The energy distributions correspondingto all three samples (RCHP, Sumitomo, and Baikowski) are compared in Figure 6 for diethyl ether and in Figure 7 for 1-chlorobutane. The thermodynamic parameters for both solutes on all three materials are given in Tables I and 11. It is not surprising, but quite satisfactory, that no differences were measured in this study, since all three materials are of a very high purity and that no processing differences had been observed. It is interesting to note that the energy distributions and the thermodynamic parameters measured for these three materials are closer than the three sets of data obtained in the study of different column, different day reproducibility. This may be explained by the different position of the two series of experiments on the learning curve. Much experience has been acquired in performing the experimental procedures (e.g., in manufacturing columns) during the investigations which lead to the validation of the experimental protocol (2, 3). 111. Comparison between the Adsorption Energy Distribution of Different Lots of 1.5-pm Alumina Manufactured by the Same Process. As was the case in the
SO
ANALYTICAL CHEMISTRY, VOL. 64,NO. 1, JANUARY 1, 1992
I
RHCP
Table IV. Comparison of the Monolayer Capacities and the Specific Surface Area for RCHP and Coors ‘Good” Alumina param compared
ratio (f24%) 3.0 2.9
Bad
I
r-l 10 8
98
I ’2 8
1’ 8
Adsorption Energy (KCol/Mole)
Figure 7. Comparlson between the adsorptkn energy distributions for 1-chlorobutane on 0.8-pm alumina samples: (top) RCHP alumina;
(middle)Sumltomo alumina; (bottom) Baikowski alumina. Table 111. Characteristics of the Adsorption Energy Distribution for 1-Chlorobutaneon Different 1.6-um Alumina Samples qm,
adsorbent
rmol/g
E,”,, kcal/mol
variance, ca12/mo12
Low-Energy Peak Coors good medium bad
3.82 4.06 3.94
10.21 10.18 10.20
lo00 1010 1040
Medium-Energy Peak Coors good medium bad
0.29 0.31 0.92
12.57 12.55 12.56
1.0 1.0 1.0
High-Energy Peak Coors good medium bad
0.0133 0.0722 0.290
14.00 14.01 14.03
1.0 1.0 1.0
other two series of experiments, no significant differences were observed between the adsorption energy distributions of diethyl ether on the samples studied. The results obtained for the elution profiles, the adsorption isotherms and the energy distributions of diethyl ether on the three samples are very similar to those already described (20) and are not reported here. Only the thermodynamic parameters characterizing the adsorption energy distributions are reported in Table I. The monolayer capacities of the three samples are quite lower than that of the previous samples (e.g., RCHP). This is a result of the greater particle diameter (thus the reduced specific surface area) of the 1.5-pm alumina samples. As shown in Table IV, the ratios of the monolayer capacities of either the high-energy or the low-energy peaks of the adsorption energy distribution of the good and the RCHP samples are equal to the ratio of their BET surface areas, indicating that the effect is simply due to the difference in the surface areas of the samples. In contrast, significant differences between the three lots were observed when 1-chlorobutane was used as the probe
0.000
0.415
0.030
1.245
50
Retention Time ( M i d
Figure 8. Elution Profiles of l-chlorobutane on three 1.5-pm alumina samples at 40 O C . The experlmental proflles are derived from the chromatogam (mild lines) and proflles calculated uslng the semiideal model and the experlmental Isotherm (Figure 1). Chromatographic conditions: L , 1500 cm;N , 8000. (Top) “bad”sample: PI, 1.33 am; to, 0.399 min; m,, 45 mg; sample size, 0.18 pg; t,, 0.77%; A,6.1 X lo-‘. (Middle) “medium” sample: PI, 1.32atm; to, 0.398mln; m,, 43 mg; sample size, 0.19 pg; t,,,, 0.63%; A, 1.5 X lo4. (Bottom) “good”sample: PI, 1.35 atm; to, 0.401 mln; m,, 46 mg; sample size, 0.18 gg; t,,, 1.0%; A, 9.9 x 10-4. solute. The experimental and simulated elution profiles of 1-chlorobutane on the 1.5-pm alumina samples are shown in Figure 8. Clearly, the band tails longer on the bad sample than on the good one. The energy distributions of the three samples are compared in Figure 9. These distributions are now trimodal. Important differences are observed for the highest energy peak of the distribution. This peak was nearly undetectable in the good sample, but it is large for the other two samples. This peak was absolutely not detectable for any of the 0.8-pm alumina samples tested. The thermodynamic parameters are given in Table 111. Since in the case of these three samples we have a trimodal distribution function, the calculation process is slightly different from the one previously reported (I) and needs some further explanations. The optimization involves now the calculation of the parameters for three Gaussian peaks. It is necessary to prove first that the energy distribution does in fact have three distinct modes and that one of them, at least, is not due to experimental noise. In order to establish the number of modes, we performed again the quasi-Adamson stage of the calculation replacing the measured portion of the isotherm by ita Akima spline representation (ZI),rather than its multi-Langmuir representation. The akima spline representation helps to prevent oscillatory behavior but does not influence the number of modes in the energy distribution. A bi Langmuir representation of this isotherm is used for the
ANALYTICAL CHEMISTRY, VOL. 64, NO. 1, JANUARY 1, 1992
31
ID
I 1 I 1.0
11.1
Bad
I
Medium
Good
12.2
13.3
4
Energy (KCal) Figure 9. Comparison between the adsorption energy distributions for l-chlorobutane on the three 1.5-pm alumina samples: (top) “bad” sample; (middle) “medium” sample; (bottom) “good” sample.
high-pressure region of the isotherm. The calculation revealed a three-peak energy distribution. In order to further ensure that the third peak was not spurious, the same optimization procedure was performed on a duplicate elution profile obtained on the same day. The two energy distributions obtained are shown in Figure 10. They demonstrate a trimodal energy distribution. Having established the number of modes of the energy distribution function, we performed the optimization in the conventional manner. The ECP isotherm was replaced by its tri Langmuir representation. The resulting quasi-Adamson energy distribution was used as the starting point for the distribution function substitution (DFS) method optimization. In order to reduce the number of simplex dimensions in the DFS optimization, the variance of the two highest energy peaks were constrained to the minimum variance allowed by the subroutine, i.e., 10 X lo+?kca12/mo12.
CONCLUSION The results presented here show that the method can successfully measure lot to lot differences in the adsorption energy distribution of some probe solutes. These differences, which are strong for some probes and negligible for others, permit the chmacterization of the surface heterogeneity. They correlate with differences in the processing behavior of the powders studied. At this stage, however, one cannot make conclusions regarding the extent to which two surfaces must differ in order for the method to be able to measure differences in their heterogeneity. It was hoped that minor differences in the adsorption energy distribution for powders which manifested no processing differences (e.g., the powders coming from different vendors) could be seen but this was not the case. Either the surfaces of these three powders were very similar or, if their differences were significant, they did not manifest themselves in the interaction of either diethyl ether or 1chlorobutane. The method can be successful only provided the probes selected are suitable to reveal the differences. The probea must be selected for their ability to interact with specific chemical groups or surface defects expected to be found. Diethyl ether is a weakly basic compound; l-chlorobutane, a weak acid. Compounds which interact strongly with
7.3
9.3
11.3
13.3
15.3
Energy (KCal/Mold Figure 10. Reproduciblltty of the energy distribution functions. The functions were calculated by the quasCAdamson method, using the Aklma spline representation of the isotherm in the measured region. Duplicate measurements were made for the adsorption of l-chiorobutane on the “good” sample of 1.5-pm alumina.
the surface give results which are difficult to account for. For example, samples of increasing amounts of chloroform, benzene, and pyridine gave band profiles which do not have a common tail, as is the case for diethyl ether and l-chlorobutane (see Figures 1,4,and 8). Adsorption isotherms cannot be derived from these elution profiles by the ECP method without further investigation of the nature and influence of the discrepancy observed between the profiles of samples of different sizes. At any rate, our goal, which was the development of an analytical method capable of determining differences in the adsorption energy distributions of probe compounds which correlate with the processing behavior of the ceramic, has been realized. Systematic application will permit a better understanding of the behavior of the material.
ACKNOWLEDGMENT We acknowledge Eric Dose, University of Tennessee, for fruitful discussions and for his help in developing the data acquisition software and in using the Simplex program. We thank Marc Janney, Division of Metals and Ceramics at the Oak Ridge National Laboratory, Oak Ridge, TN, for supplying us with the samples of 0.8-pm alumina. REFERENCES (1) Roles, J.; Gubchon, G. J . Phys. Chem. 1991, 96, 4098. (2) Roles. J.; Gulochon, G. J . Chmmatogr., in press. (3) Roles, J.; Gulochon, G. J . Chromatogr.,in press. (4) Katz, N. R. I n Treatise on Materhl Science and Technology;Wachtman, J. B., Ed.; Academic Press: New York, 1989; p 1. (5) Perduljn, D. J. I n Encyclopedle of Materiais Science and Engineering; Bever, M. B., Ed.; MIT Press: Cambridge, MA, 1986. (6) Wang, F. F. Y. I n Advances in Powder Technology;Chin, 0. Y., Ed.; Amerlcan Society for Metals: Metals Park, OH, 1982. (7) Johnson, D. R.; Janney. M. A.; Mclung, R. W. Rocsedin@ of the Jolnt Conference On Nondesbuctive Testing of H e Performance &ramICs, Vaty, A., Snyder, J., Eds.; American Ceramic Society: Columbus. OH, 1987; p 19. (8) McCauiey, J. W.; I n Roc86dings of the Joint Conference On Nondestructfve Testing of Mgh Performance Ceramics; Vary, A,, Synder, J., Eds.; Amerlcan Ceramic Society: Columbus, OH, 1987; p 1. (9) Langmulr, I. J . Am. Chem. SOC.1918, 40, 1361. s (10) Jaronlec, M.; Madey. R. Physical Adsorption on M t e r ~ n e w Solkls; Elsevler, Amsterdam, 1988, Chapters 2 and 3. (1 1) Jaronlec. M. TMn Solid Films 1883. 100, 325.
Anal. Chem. 1992, 64, 32-35
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(12) Boudrsau, S. P.; Cooper, W. T. Anal. Chem. 1987, 59, 353. (13) Boudreau. S. P.: Coow. W. T. Anal. Chem. 1g8g. 61. 41. (1.4 Todorovlc. M.; iaub. k. j. J . chrometogr. 1988, 442. io5. (15) Gocdboy, K. P. Chem. €ng. Rog. 1984, 80, 63. (16) Dewing, J.; Monks, G. T.; Youii, B. J. Cata/ysis 1978, 4 4 , 226. (17) Cremer, E.; Huber, J. F. K. Angew. C k m . 1981, 73. 461. (18) Rouchon, P.; Schonauer, M.; Valentin, P.; VidaCMadjar. C.; Guiochon, G. J . phys. Chetn. 1985, 89, 2076. (19) W i o n . G.; Golshan-Shirazi, S.; Jaulmes, A. Anal. Chem. 1988.60, 1856. (20) Roles, J. Ph.D. Dissertation, University of Tennessee, Knoxville, TN, 1991.
(21) Aklm. H. J . Am.
ef8ff SOC. l.
1970, 17, 589.
RECEIVED for review June 17,1991. Accepted October 2,1991. This work has been supported in part by Grant DE-FGOb 88EFt13859 of the U.S. Department of Energy, Office of Basic Energy Research, and by the cooperative agreement between the University of Tennessee and the Oak Ridge National We of our computational effort by the University of Tennessee Computing Center.
Study of the Surface Heterogeneity of Chromatographic Alumina Jeffry Roles and G o r g e s Guiochon*
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1501,and Division of Analytical Chemistry, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6120
The adsorption energy distribution of diethyl ether and 1chlorobutane on a sample of porous alumina for chromatography have been determined from their absorption rsothenns measured by the ECP method. The changes in these distributions with the degree of hydration of the surface have been studled. For both probes, the fully hydrated surface exhlblts a moderate retention factor at infinite dilution, a bi Langnuir isotherm and a bimodal energy distribution with two narrow modes, the higher one having an energy below 13 kcai/moi. For the starting materialor the M a c e dehydrated at 320 OC under a helium stream, the energy distribution is bimodal for diethyl ether and trimodai for l-chiorobutane. I n all cases, these high-energy modes are much wider than the energy modes observed with the hydrated cdumn or the iow-energy mode. They are wider and have a slightly higher average energy for the dehydrated column than for the starting material. These resuits demonstrate that much information on surface properties can be extracted from isotherm data.
INTRODUCTION In previous papers (1-51,we have described a procedure for the determination of the adsorption energy distribution of vapors on the surface of an adsorbent. This method is based on a new solution (1)of the linear Fredholm equation of the first kind which relates the experimental or global isotherm, the surface energy distribution, and the local isotherm, generally assumed to be Langmuirian (6, 7). This work extends previous results of Boudreau and Cooper (8)who simplified greatly the problem by assuming a local isotherm linear up to a saturation level. We have discussed the experimental problems associated with the measurements of accurate equilibrium isotherms (2). A porous-layer open tubular column is used. These columns can be prepared with a variety of materials. Their pressure drop is small, which makes easier the collection of accurate experimental data (2,3).The ECP method (9)is the preferred procedure for the determination of gas-solid isotherms by gas chromatography (3,5,9).The validations of the methods used *To whom correspondenceshould be addressed at the University
of Tennessee.
to derive the isotherm from the elution band profiie by ECP (3)and to derive the energy distribution from the isotherm data (4) have been presented elsewhere. In a sepmate paper (51,we show that the method developed permits a differentiation between lots of high-purity alumina for ceramics which have performed differently in pilot tests and can be used to sort out “good” from “medium” or “bad” samples (5). While the adsorption energy distributions of diethyl ether for all the samples are very similar, marked differences between the adsorption energy distributions of 1-chlorobutane are observed. This latter distribution exhibits three modes. The high-energy mode has a very small monolayer capacity for the good samples and a much larger one for the bad samples. Ceramic alumina is a powder of small, solid, crystalline particles of a alumina, with a very small specific surface are4 barely a few times as high as the geometrical surface area (5). We present here results obtained by applying the same method to a sample of chromatographicalumina, made of porous particlea having a much higher specific surface area and quite a different chemical structure. EXPERIMENTAL SECTION We used in this work the same instruments and procedures as for our investigation of the surface heterogeneity of ceramic grade alumina samples (5).The procedure for the preparation of the porous layer open tubular columns (2),the gas chromatograph (2),theprocedure for the determination of the equilibrium and the calculation of the isotherms by the ECP method (2,3), energy distribution (4) have been described in detail in earlier publications. The procedures used have been validated. Unless changes have been made in these procedures to adjust to a different material, only the most basic information is reported here. All other relevant information can be found in our referenced papers (1-5). I. Materials. The probe solutes used were diethyl ether and 1-chlorobutane(Aldrich, Milwaukee, WI). The chemicals were used as received. The silica tubing (i.d. 530 pm) used to prepare the open tubular columns was obtained from Alltech Europe (Nazareth, Belgium). 11. Samples. Aluminum oxide, of the type used as an LC stationary phase (Stock No. 02142), was obtained from Universal Scientific Inc. (Atlanta, GA). This porous alumina is predominantly y alumina. It was ground by the manufacturer to an average spherically equivalent particle diameter of 3-6 pm. The BET specific surface area reported by the manufacturer is 200 m2/g.
0003-2700/92/0364-0032$03.00/00 1991 American Chemical Soclety
