The Journal of Physical Chemistry, Vol. 82, No. 23, 1978 2409
Quasi-Crystals
Quasi-Crystals. Growth from Photochromic Spiropyrans on Irradiation in a Constant Electric Field V. A. Krongaur,* S. N. Fishman, and E. S. Goldburt Depadrnent of Structural Chemistty, The Weizmann Institute of Science, Rehovot, Israel (Received February 2, 1978; Revised Manuscript Received May 24, 1978)
A linear quasi-crystalline material is formed during the photochemicaltransformation of solutions of photochromic spiropyrans in nonpolar solvents in a constant external electric field. The quasi-crystalsare composed of globules -0.1-0.4 pm in diameter joined together like straight strings of beads aligned along the electric lines of force. The globules in turn consist of highly dipolar crystalline nuclei coated by less polar amorphous envelopes. All stages of the quasi-crystal growth were followed in an electron microscope: formation of microcrystals, their consolidation in globular nuclei, coating of the nuclei by amorphous material, and finally formation of quasi-crystalline threads. Quasi-crystals exhibit features common to both crystalline (optical anisotropy, X-ray diffraction) and amorphous material (droplike form of globules, fluidity). Moreover they exhibit some new features: (a) Every quasi-crystalline thread consists of globules of equal size. (b) The globule radii increase discretely during irradiation, resulting in the appearance of several generations of threads. These features were explained by considerations of the thermodynamics and kinetics of quasi-crystal formation. A possible analogy between quasi-crystals and biological structures is considered.
Introduction Previous ~tudiesl-~ of the photochemical transformation of photochromic spiropyrans in nonpolar solvents have led to the conclusion that molecules of the photoproduced
B colored merocyanine form B interact with the uncolored molecules A in solution to give two kinds of complexes: AB and A,B, where n 2-3. These complexes combine in turn to form giant aggregates of >lo6 molecules. The absorption spectrum of the AnB complexes (regarded as charge transfer complexes A,+B- in previous papers) in the aggregates is red-shifted by about 100 nm with respect to the absorption of the AB dimers. The proportion between AB and A,B in the aggregates depends on light intensity and on temperature, since the formation of the A,B complexes from A and AB involves an activation energy of about 5 kcal/mol. When the irradiation was performed in an external constant electric field (>5 kV/cm), a new phenomenon was observed: quasi-crystalline threads are formed which extend from one electrode to the other along the electric lines of force. The absorption spectrum of the threads is identical with that of the aggregates formed in the absence of an electric field, indicating that the threads are also composed of AB dimers and A,B complexes. Linear dichroism measurements showed that the AnB complexes are oriented, but the dimers are arranged at random. The threads were formed only in the temperature range in 0022-365417812082-2469$01.OO/O
which A,B was observed in the absence of an electric field, Le., the existence of A,B is essential for formation of the threads. Studies of the precipitated aggregates and of the quasi-crystalline threads in the electron microscope showed that they both consist of globules about 0.2-0.4 pm in diameter. In the aggregates the globules are stuck together at random; in the quasi-crystals they are aligned beadlike in rather straight chains oriented along the electric field. The X-ray powder patterns of the quasi-crystals are identical with those of the precipitates obtained by the coagulation of the aggregates formed in the absence of an electric field and consist of sharp reflections superimposed on a diffuse pattern. The intensity of the diffuse pattern rises with increasing dimer content. It was suggested3that the globules are composed of crystalline nuclei coated by amorphous envelopes, and that the nuclei consist of A,B, while the envelopes consist of AB. The crystallites in the nuclei (and therefore the A,B in the crystallites) have apparently net dipole moments which cause the alignment of the globules in the electric field. An orientation of single A,B complexes in weak electric fields (5-25 kV/cm) for any reasonable molecular dipole is very unlikely, in view of the thermal molecular motion at room temperature. We therefore concluded that A,B’s form dipolar molecular stacks (similar to the J- or Scheibe-stacks inherent in many cyanine dyes4),which in turn form highly dipolar crystals and then the nuclei of globules . We now report the results of a detailed study of the growth and structure of quasi-crystals and aggregates and discuss the underlying rules. The quasi-crystals produced from the usual spiropyrans are rather unstable mechanically and the early stages of their formation are therefore difficult to study. Preliminary experiments showed that the derivative where R1 = -C2H4*OC0.C(CH3)=CH2 gives more stable and mechanically stronger quasi-crystals, probably because of fractional photopolymerization of the amorphous envelopes of the globules. Comparative studies have shown that other properties as well as the structure of the quasicrystals formed from this spiropyran are practically identical with those formed from the usual spiropyrans. 0 1978 American Chemical Society
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The Journal of Myskal Chemistry, vol. 82. No. 23. 1978
-
V. A. Krongauz. S. N. Fishman. and E. S. Goldburl
Fbura 1. Mlcrographs 01 the quasi-crystals at different stages of formation. (a.b) Mlcrocrystals surrounded by amorphous material and iyplcal electron dlffractlon from the mlcrocrystals. Concentratlon of orlglnal solution c = 5 X IO-' M. exposure t = 15 s. field V = 25 kVlcm. (c.d) A nucleus and Its diffraction pattern, c = 5 X to-' M. t = 2 s. V = 25 kV1cm. (e) A globule coated by an incomplete envelope. c = 5 X to4 M. t = 3 8. V = 25 kV1cm. (1) c)uasi-crystals obtalned by short UV exposure. c = 5 X lo-' M, t = 3 s. V = 25 kVlcm. (g) Quasi-crystals obtalned by b w UV exposure. c = 5 X to4 M. t = 10 s. V = 25 kVlcm. (h) A fragment of a stretched quaskrystalllne thread (thkd generation). (I)Threads washed by methylcyclohexane for 20 mln. c = 5 X IO-' M. t = 30 8. V = 25 kV1cm. U.k) Micrographs of prgclpitated aggregates formed in the absence of an electric fleld. E = 5 X lo4 M, t = 10 s. (I) At roam temperature. (k) AI 150 K.
high-pressure mercury arc in a Wild housing was filtered by a Corning 5840 glass filter transmitting roughly in the l-~~-Methacryloxyethyl)-3,3-dimethyl-6'-nitrospiro(in~ range 300-400 nm. The light intensity at the target was doline-2,2'-[2H-l]benzopyran),i.e., R, = -C2H,.0C0.Cabout lo-' einstein cm-* s-'. The exposure time waa fairly (CH3)=CH2, & = H, waa prepareds by Dr. R. Bertelson short (530s) so the degree of the photochemical transof Chroma Chemicals, Dayton, Ohio. It was purified by formation of A to B did not exceed 25%. chromatography on silicic acid and recrystallized from For electron microscope studies of the quasi-crystals a several solvents. The compound was identified by mass small amount of solution was placed between two 1-mm and IR spectroscopy. It had a melting point of 87-89 "C. brass electrodes placed 1 mm apart on a quartz plate Spectrograde methylcyclohexane was used aa a solvent. covered by a parlodion film. The solution was kept beIt was dried by passing through a column of neutral tween the electrodes due to capillary action. The quartz alumina (Woelm). Irradiation from an Osram HBO 200 plate waa placed on the stand of an optical microscope and Experimental Section
Quasi-Crystals
The Journal of Physical Chemistry, Vol. 82, No. 23, 1978 2471
irradiated through the illuminating condenser of the microscope, applying a dc high voltage at the same time. The growth of the quasi-crystals was observed through the microscope. Irradiation and the electric field were switched off and the solvent evaporated in situ. The parlodion film covered with the quasi-crystalline threads was transferred to grids and washed with methylcyclohexane for a few minutes to dissolved unreacted spiropyran A. Sometimes the washing was prolonged to 30 min in order to partly dissolve the amorphous envelopes of the globules. An essentially similar technique was used to obtain a precipitate of aggregates in the absence of an electric field. A Philips EM 300 electron microscope was used.
TABLE I: Size of Globules in the Quasi-Crystals of Different Generationsa av globule calcd radius,c generation radius,b pm pm __ I 0.04 0.04 I1 0.07 0.06 I11 0.10 0.10 IVd 0.18 0.18 a Original solution concentration 5 X M, light intensity 10-7-10-8 einstein cm-* s-’. The maximum dispersion is f 15%. The radius R of the original nucleus and the thickness h of the envelope were both taken as 0.02 pm. Obtained only for a light intensity of l o m E einstein cm-* s-’. ‘
Results We succeeded in observing the early stages of quasicrystal formation either by using very dilute solutions of spiropyran or by applying very short light exposures. In Figure 1, micrograph a, such a material occurring prior to globule formation is shown. One can see microcrystals about 0.01-0.04 pm across with unorganized, probably amorphous, material around them. The microcrystals have a streaky texture. The streaks, ca. 0.5-1 nm thick, are parallel and evenly spaced. Sometimes the streaks stick together and form stripes about 1.5-2 nm wide. The microcrystals exhibit distinct discrete electron diffraction (photograph b). Micrograph c shows a nucleus which is either uncoated or covered by a very thin amorphous layer. Micrograph e is seen rather seldom and shows an incomplete or accidentally partially damaged globule with a nucleus. From nuclei in micrographs c and e distinct and discrete electron diffraction patterns were obtained (d). Micrographs f and g show quasi-crystals produced, respectively, with a short and with a longer exposure to UV irradiation. The globules usually do not exhibit discrete electron diffraction. The fragment of a thread stretched by increasing the field (micrograph h) spectacularly demonstrates the droplike form and fluidity of the globules. There are three very remarkable facts concerning the growth of the quasi-crystals: (a) As a rule each thread is composed of globules of equal size. (b) The shorter the exposure, the smaller the globules which constitute the quasi-crystalline threads. Thus in micrograph f only the first generation and in micrograph g three generations of threads are shown. (c) The globule size increases discretely from one generation to another. In our experiments the diameters of the globules comprising threads of the first, second, and third generations are in the ratio -4:7:10 (see Table I). Experiments were carried out at lower light intensity as well. When intensity was decreased by about one order of magnitude the exposure time was proportionally longer, so that the total incident energy was the same as in experiment g. In this case strings of even larger globules (R = 0.18 pm) were observed, so-called “fourth-generation” globules. Also, the overall fraction of higher-generation globules was greatly enhanced under these conditions. The diameters of the globules of each generation was not affected by variations in the light intensity. Attempts were made to dissolve the outer layers of the globules in order to obtain bare nuclei. By washing for 20-30 min with methylcyclohexane it was possible to partly remove the amorphous envelopes of the globules and to bare the bumpy surface (micrograph i). The threads of the first generation, composed of small globules, usually change much less by washing, probably because of the greater degree of polymerization of the amorphous en-
velopes or their relatively greater thickness. Some of the larger globules are almQst completely destroyed and it is possible to see that they include a number of small units. For instance, the second generation globules include 4-10 units 0.03-0.04 pm in diameter, Le., similar in size to the microcrystals. In micrographs j and k aggregates are shown which are produced in the absence of a field, at room temperature and at 150 K respectively. At 150 K no A,B complexes, and therefore no microcrystals, are formed3 and consequently aggregates do not exhibit a globular structure.
Discussion I. Energetics of Quasi-Crystal Formation. We can distinguish several levels of organization of the material: (1) closely packed one-dimensional stacks similar to Scheibe-structures of cyanine dye molecules; (2) a number of such stacks join in three-dimensional microcrystals; (3) the microcrystals become coated by an amorphous phase, giving globules; (4) these globules form long threads in an electric field, giving once again a linear organization which looks in the electron microscope like a string of beads. We will discuss here the nature of the organization at each level. (1)It is reasonable to assume that the thin lines seen in the texture of microcrystals in micrographs a, c, and e (Figure 1) are the Scheibe stacks with a tilted “deck-ofcards” structure. In the first simple theoretical models the interactions of dye molecules in stacks were treated using a classical dipole-dipole potential? Exciton theory predicts a blue spectral shift, when the molecular axes are tilted more than 54’40’ from the line connecting the centers of molecules (Haggregate) and a red shift, when this angle is less than 54’40’ (J aggregate). Let us consider in a very simple manner the influence of the dipole-dipole interactions on the polarity of the stacks. If the molecular axes are placed in one plane, we may use the familiar formula for the dipole-dipole interaction P2 u = -[cos (a1 - a2) - 3 cos r3
a1
cos a21
(1)
where p’ is the dipole moment, r is the distance between centers of the dipoles, and a1and a2 are the angles between the dipole moment vectors and the line connecting their centers. The energy of two parallel dipoles (a1= a2 = a ) is P2 u ( t t ) = -(1 - 3 cos2 a ) (2) r3 For antiparallel dipoles (a1= a2 + 180’ = a) we obtain
u(t1) = --(1P2 r3
3 cos2 a)
(3)
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The Journal of Physical Chemistry, Vol. 82, No. 23, 1978
If cos2a > 1/3, u ( f f ) u ( f J ) if, cos2a 1/3, u ( t t ) > u(t4). Therefore if 0 < a < 54’40’ the parallel configuration is energetically more favorable, and if 54’40‘ < a < 90’ the antiparallel configuration becomes preferable. So the configuration giving the red-shifted absorption has to be of high polarity, and it is the configuration that forms stacks in our microcrystals. The nonpolar amorphous phase can be supposed to consist of dimers or oligomers with anti-parallel dipoles. As has been shown in previous experiments2 the formation of the highly polar configuration is temperature dependent and demands an activation energy of -5 kcal/mol. As stated above it has the formula A,B. Perhaps the additional solvation of molecule B by A creates the steric conditions promoting the tilted J-type arrangement. The degree of charge separation in the parallel dipole structures is very large; that is why they have been considered in previous discussion^^-^ as charge transfer complexes. ( 2 ) As calculated below the microcrystals formed in an electric field from the highly polar stacks also exhibit a high polarity. We have not yet demonstrated a direct proof of the polarity of the microcrystals formed in zero-field conditions, but in view of the identical X-ray and electron diffraction patterns obtained from microcrystals produced both in the presence and absence of an external field we suggest that the field does not influence the packing of the stacks in microcrystals. (3) The high polarity of microcrystals must make their lifetime as a dispersion in nonpolar solutions very short. They have to coagulate and sediment if they are not coated by the slightly polar amorphous phase. The amorphous envelope has a spherical form which corresponds to the minimum surface energy. So the globules have highly polar crystalline nuclei and less polar amorphous covers. (4) The set of globules in solution can be regarded as a system of strong dipoles. These dipoles interact with an external field E and with one another. Thermal motion destroys the orientation caused by electrostatic interactions. If the field is strong enough p E > kT
(4)
and the dipoles become oriented along the field. From this inequality the minimum dipole moment can be estimated which makes dipoles oriented in a given field. For E = 5 kV/cm, p > 2.5 X lo3 D. Now let us consider the conditions of the arrangement of globules in threads and estimate the value of p necessary for a transition from the random arrangement of parallel dipoles to the chain structure. This transformation can be treated as a first-order phase transition. The difference in free energy of the two configurations is A F = AU - TAS
(5)
where AU is the energy difference between the “random” and quasi-crystalline thread phases, and A S is the entropy difference, In the random conformation the probability of finding a dipole in an elementary domain dV is proportional to the concentration C. The energy of interaction of a dipole with all randomly placed parallel dipoles in a volume V is
Inserting eq 2 with a = 0 into (6) and calculating the integral we obtain u, = 0. Therefore the energy difference AU 2 Uq - U, is equal to the energy of the electrostatic interactions in the string of beads. It can be approximated by the energy of the nearest neighbor interaction^.^ Using
V. A. Krongauz, S. N. Fishman, and E. S. Goldburt
eq 2 with a = 0 we have
AU = uq
N
2P2 --n rO3
(7)
where n is the number of globules and ro is the distance between their centers. The arrangement of globules in the threads is unambiguous, so the entropy of the system is zero and the entropy difference A S = S,- S, is determined by the entropy of the random phase which can be approximated as the entropy of an ideal dilute solution.8
A S = -SI
N
- nk In C
(8)
Inserting eq 7 and 8 into eq 5 we have
The condition for a transition from the random to the thread phase is AF < 0 or 2p2/ro3> -kT In C
-
(10)
From eq 10 we can estimate the dipole moment necessary for the transition “random” quasi-crystals, when T N 300 K, ro N 8 X cm, C = C’r? is the dimensionless concentration, and C’ N 10l2 cm-3 is the volume concentration of globules. The dipole moment has to be p > 104 D. 11. Kinetic Aspects of Phenomenon. One of the most interesting features of the system under investigation is the homogeneity of the strings of beads and the discrete size of the beads in different strings. Any kinetic treatment has to answer the three following questions: (1)Why is it that only globules, and never stacks or microcrystals constitute the quasi-crystalline threads? ( 2 ) Why are the threads composed of identical globules? (3) Why are the radii of the globules in the threads of different generations in a discrete ratio? (1) As stated above the microcrystals are highly polar particles. Obviously they can exist as a more or less stable dispersion in a nonpolar solvent only if they are separated from it by a less polar amorphous phase. If the lifetime of the microcrystals, rcr,was of the order of the characteristic time of the thread formation, 7,we would obtain threads of crystals covered by an amorphous cylinder-like envelope. However the experiment shows that T ~ vk,, but if k , < vk,, the excess becomes smaller and a deficiency begins at time t = ao(vk, - kJ1. This is approximately the time when globules of the second generation start to form. The results obtained upon irradiation with light of low intensity qualitatively confirm the above treatment. It has been shown2 that the relative concentration of the amorphous phase is proportional to the square root of the light intensity. This accounts for the increase in the population of strings with larger globules and for the production of globules of the fourth generation, when the intensity is diminished by an order of magnitude. (3) The size of globules of the second generation can be predicted from rather simple considerations. For simplicity let us suppose that the microcrystals have a spherical form. In fact the primary nuclei do have a form close to spherical as one can see in micrograph IC. If n of them join, the minimum radius of the globules of the second generation will be R1 = 2R A, where R is the radius of the primary nucleus and X is the thickness of the envelope, which is supposed to be equal for globules of all generations. The volume of amorphous matter in a globule
+
R nli3 - 1 Obviously the most favorable situation will occur when a sphere of radius 2R (this is the minimal radius for a sphere containing an integer number of primary nuclei n > 1) is closely packed with n nuclei of radius R. It is known from crystallography that the closest face-centered packing of a volume with spheres fills approximately 0.75 of the volume Therefore for a volume 4/,.rr(2R)3closely packed with spheres of radius R we have n N 6. Inserting n = 6 into inequality 15 we obtain AIR > 0.2. The ratio AIR in the experimental system ( R N 0.02 pm, X N 0.02 ym) fits into the interval restricted by the inequality. The value n = 6 is also in good agreement with experiment, which gives values between 4 and 10. It is reasonable to assume a similar mechanism for the formation of globules of following generations, Le., the radius of the nucleus of a globule of the third generation is equal to twice the radius of a nucleus of the second generation and so on. This permits us to calculate the sizes of globules of each generations, taking from direct measurement 0.02 ym for the radius of the primary nucleus (Figure IC),and X = 0.02 pm is constant for all generations. The calculated and experimental numbers are in excellent agreement (Table I). Conclusion This work provides further insight into the nature of a new form of organization of organic material discovered The recently and reported in previous quasi-crystals under investigation exhibit some features inherent in crystals (optical anisotropy, discrete X-ray diffraction) and some feature typical for liquids (droplike form and fluidity of the globules forming beadlike threads). However the discrete growth of the globules and their uniformity within a given chain are completely new features which have never been observed in other materials. The electric field probably determines the quasi-crystalline structure only in the last stage of its formation from globules. The sophisticated structure of globules composed of nuclei and covers arises spontaneously. It is controlled mainly by thermodynamic factors. The globules are selected for the construction of the quasi-crystalline strings by kinetic factors. The set of thermodynamic and kinetic parameters of the system is so favorable that it results in behavior similar to the self-organization of biological materials. The comparison is speculative but it is worthwhile to recall the following facts. A common feature of nearly all living cells is the existence of a potential inside the cell as compared with the surroundings (the so-called “resting potential”). This potential is, as a rule, of the order of -50 mV. The thickness of the cell membrane is of the order of 0.01 ym and therefore the field across the membrane is of the order of E N 50 kV/cm. (In our system the fields are of the same order of magnitude.) Depolarization of the membrane of excitable tissues gives rise to some membrane
2474
The Journal of Physical Chemistry, Vol, 82, No. 23, 1978
R. Fiedorow, I. G. Dalla
charge (or dipole) displacement with the resulting dipole moment of the order of lo3 D or more (the globules also have p Z lo4D). Lastly, the kinetic separation of coupled processes exhibited by our system is one of the intrinsic features of biological systems as well. Though there is some-similarity in behavior between our system and liquid crystals in an electric field, the nature of the phenomena is quite different. That is why we hope new practical applications for the discovered material will be found. Acknowledgment. The authors thank Professors E. Fischer and M. D. Cohen for valuable discussions. References a n d Notes
Lana, and S. E. Wanke
(2) A. A. Parshutkin and V. A. Krongauz, Mol. Phofochem., 6,437 (1974). (3) V. A. Krongauz and E. S. Goidburt, Nature(London), 271,43 (1978). (4) C. E. Mees and T. H. James, "Theory of Photographic Process", Macmillan, New York, N.Y., 1966, Chapters 11 and 12, (5) E. L. Zajtseva, A. L. Prokhoda, L. H. Kurkovskaya, R. R. Shifrina, N. S. Kardash, D. A. Drapkina, and V. A. Krongauz, Khlm. Geferotsikl. Soedln. no. 10, 1362 (1973). We could not obtain the melting point of 111-1 12 "C given in that PaDer, despite the fact that the other properties of ou? present compound are identical. (6) E. G. McRae and M. Kasha, J. Chem. phys., 28,721 (1958); "Physical Processes In Radiation Biology", Academic Press, New York, N.Y., 1964, pp 23-42. (7) The next neighbor interactlonenergy (-p2/4r3) is about an order of magnitude smaller than that of neighbor dipoles (eq 7). Therefore it can be neglected for the estimation of the order of magnitude of the dipole moment. (8) The nonuniformspacing of globules along a chain Is of no slgniflcance in our case. I t is important that the globules occupy fixed positlons. Therefore we have assumed that S , = 0.
(1) V. A. Krongauz and A. A. Parshutkin, Photochern. Phofobiol., 15,
503 (1972).
Adsorption of Sulfur Dioxide on Heat Treated y-Aluminas at Room Temperature R. Fiedorow,+ I. G. Dalla Lana,* and S. E. Wanke Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada (Received September 6, 1977; Revised Manuscript Received August 11, 1978)
The adsorption of SO2at room temperature on y-Alz03calcined at 400,500,700, and 900 "C has been investigated using dynamic pulse adsorption and in situ infrared spectroscopy techniques. Interactions of SOz occurred with hydroxyl sites for all samples and the strength of these interactions increased with decreasing acidity of the hydroxyl sites. Strong adsorption occurred on the samples calcined at 700 and 900 "C and it is believed that this strong adsorption occurs on Lewis-acid sites.
Introduction Aluminas (generally y-A1203) are employed as adsorbents for SO2 and as catalysts for reactions involving SOz, hence, it is of interest to examine the interactions between SOz and y-Alz03. Most of the previous investigations have studied the adsorption of SOz on aluminas at elevated temperatures (150-600 0C).1-4 The results of these studied clearly show that SOz chemisorbs on y-A1203at elevated temperatures. The nature of the interactions of SOz with yA1203at room temperature is less certain. Grillet et a1.6 concluded that SO2is physically adsorbed on alumina and other oxides at 0-30 "C. Rosynek and c o - w o r k e r ~in, ~ ~ ~ their poisoning studies of y-A1203with SOz, observed no poisoning if SOz was adsorbed at room temperature, but observed a monatomic increase in the degree of poisoning with increasing SOz adsorption temperature. The present work presents results for the adsorption of SO2 a t 20-22 "C on various thermally treated y-Alz03 samples. Experimental Procedure a n d Results Materials. In all experiments, Alon (a high purity y-alumina manufactured by Cabot Corp.) was used. This alumina consists of very small particles (-30 nm in diameter) and its surface area is not significantly affected by treatment at elevated temperatures. Compacted samples of Alon were heated in air for 5 h at 400,500,700, and 900 OC. The resulting thermally treated Alon samples will be referred to as Alon-4, -5, -7, and -9. X-ray diffraction analysis of the thermally treated samples only showed the presence of y-alumina lines. 'Institut of Chemistry, A. Mickiewicz University, Poznan, Poland. 0022-3654/78/2082-2474$0 1.OO/O
TABLE I: Surface Areas and SO, Adsorption Uptakes of Alon Samples amount of strongly fractional surface adsorbed surface treatment area, SO,, coverage sample temp, "C m'/g mmol/g of SO, Alon-4 Nan-5 Alon-I Alon-9
400 500 IO0 900
100 100 93
88
0.01
0.00 0.01
0.15 0.21
0.34
0.00
0.23
The thermally treated samples were sieved and the -50 to +80 fraction was used for adsorption measurements. For the in situ IR studies, wafers were prepared by compacting 100-mg samples of the treated Alon a t 10 ton/in.2 in a l-in. diameter die. The SOz was purchased in cylinders from Canadian Industries Limited and was used after an initial isothermal withdrawal of vapor to eliminate possible volatile impurities. Since the infrared spectrum of the resulting SO2 vapor and published band assignments exhibited no disparities, it was used without further treatment. Adsorption of SOz. About 5 g of the sieved Alon were packed into a 0.25-in. stainless steel column. The column was placed into the oven of a gas chromatograph and maintained at room temperature. Helium was passed through the column at 20 mL/min and small pulses (0.5 cm3) of SO2were injected into the helium stream at 3-min intervals via a GC gas sampling valve. Injection of the SO2 pulses was continued until SOz pulses were eluted from the alumina sample. (Detection of eluted SO2 was done by means of a thermal conductivity detector.) The pulse technique for measuring adsorption uptakes only measures
0 1978 American Chemical Society
