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Langmuir 1988,4,594-598
4). T, is the critical temperature of the condensed phase, and above it the liquid collapses directly to multilayers. The values of T, obtained in this study are approximately C4.0, 32, 43, 50, and >50 "C for PP8 through PP16, respectively, and are plotted in Figure 5. For comparison, T, for trilaurin (C12)is 21 OC and for trimyristin is 43 O C . l Tc1is the critical temperature for the liquid phase. Above this temperature the monolayer does not liquify. TC1 for the current TGs was well above 50 OC and is not shown on the P-A-T surface (Figure 4). All isotherms were flat at large areas, indicating gas and liquid coexistence. T,, is lower in cases where the monolayer had a high affinity for the subphase (large work of adhesion) or a low work of cohesion as in shorter chain compounds. For example the c6 triglyceride has a TcIof less than 19 OC.' The mean height of the monolayer can be estimated at any point on the isotherm if the bulk density (or volume/molecule) is known. The volume per molecule divided by the area per molecule yields the average height. The problem lies in determining the volume per molecule. In order to estimate these volumes in the monolayer the following procedure was followed (1) The specific volumes at their freezing points (Dfp) were calculated for the Clot C12,CI4,C16,and C18monoacid triglycerides. These values were calculated from the tabulated values of specific volume at 80 "C and the coefficient of expansion.8 (2) These calculated Bfp values were plotted against the total number of carbons in each of the monoacid triglycerides, yielding a straight line. (3) It was assumed that a specific volume at the freezing point for each of the PP8-PP16 TGs could be estimated from the plot of Dfp vs total carbons. The volumes at a specific monolayer temperature were then calculated, assuming the monoacid TG coefficient of expansion. The average height at liftoff vs temperature for each TG is shown in Figure 6. In all cases except PP8 there was a sudden decrease in height at a temperature near T, or T,, corresponding to the Labrouste transformation. The mean height below T, was about 19-21 A. If we assume that the distance between carbons along the acyl chain is about 1.26 A, then the 20-A height is consistent with rigid
chains of palmitic acids extending perpendicular to the interface. At TIthe height is about 11 A. The height was 9 A for PP8 and did not vary significantly over the temperature range studied. The mean height decreased more rapidly with increasing temperature above TI as the sn-3 chain was lengthened. Previous authors have tried to equate monolayer and bulk phases by comparing surface and bulk melting points. The chain packing in the solid monolayer phase with an area of about 21A2 has been considered equivalent to the packing on the bulk a phase, which contains frozen chains hexagonally packed without specific chainlchain orientation and an area of about 20.5-21 A2/chain.8 However, there has been disagreement over the correspondence between the bulk a phase melting point, T,, and a characteristic surface temperature. Because the chain packing just below the surface melting temperature is similar to the packing in the CY bulk phase, one could suggest that T, should be similar to Tau. However, in this study T,, bore no consistent relationship to T, (Table I). Dervichianl and Merker and Daubert6 found that TI for monoacid triglycerides was very similar to the melting point of the bulk a phase. On the other hand, Lundquistg found for monoglycerides that the a form melting point coincided with Tee. Values for T,, TI,and T,, for the TGs in the present study (Table I) do not suggest a strict correspondence between any two of them, although for tripalmitin, T,, is within 2 "C of TI. For PPlO T, is intermediate in value between TI and Tcc. We conclude that the unique conformation of triacylglycerol at the interface with all three chains oriented up from the interface imparta unique properties to these systems which are not strictly analogous to triacylglycerol behavior in bulk systems, where the conformation around the glycerol is clearly different.8
(8) Small, D.M.In The Physical Chemistry of Lipids, From Alkanes to Phospholipids; Plenum: New York, 1986.
(9) Lundquist, M.In Surface Chemistry;Munksgaard Copenhagen, 1983; p 294.
Acknowledgment. We thank Dharma R. Kodali for synthesizing the triacylglycerols and Anne M. Gibbons and Irene L. Miller for preparation of the manuscript. Registry No. PP8, 92734-30-0; PP10, 92734-31-1; PP12, 5281-83-4; PP14, 57416-13-4;PP16, 555-44-2.
Surface Chemical Properties of Zinc Sulfide W. Hertl Research & Development Division, Corning Glass Works, Corning, New York 14831 Received August 5, 1987. In Final Form: November 12, 1987
,
FTIR spectral studies of reagent grade ZnS consisting of sphalerite and wurtzite show (1) sulfate and free ZnOH groups are present, indicating partial oxidation of the surface (the ZnOH groups react readily with propanol), (2) pyridine coordinately bonds to Lewis acid sites (Zn+)and also sorbs on the surface, and (3) ammonia coordinately bonds to two seta of Lewis acid sites, with concurrent formation of NH4+ ions from the reaction of NH3 with ZnOH groups and/or sorbed water. Similar studies with electronic grade ZnS consisting of wurtzite only show (1) the surface has no OH groups but does contain SH and sulfite groups, (2) heating in air to 400 "C oxidizes the SH to OH and sulfite to sulfate but causes little additional oxidation of the ZnS surface, (3) NH3 adsorption shows the presence of only one set of Lewis acid sites on both reduced and 400 "C oxidized ZnS and concurrent formation of NH4+due to reaction with sorbed water. The two sets of Lewis acid sites are probably associated with the hexagonal wurtzite and cubic sphalerite structures.
Hot-pressed zinc sulfide has excellent infrared-transmitting properties with relatively low chemical reactivity and is often used as an optical window material. Zinc 0743-7463/88/2404-0594$01.50/0
sulfide powders are also widely used in the phosphor layer of cathode ray tubes. Although zinc oxide surfaces have been extensively studied, very few infrared spectroscopic 0 1988 American Chemical Society
Langmuir, Vol. 4, No. 3, 1988 595
Surface Chemical Properties of ZnS surface studies have been carried out on zinc sulfide. Several studies related to the mineral processing industry dealt with xanthate absorption on the surface of zinc sultide,l oxidized zinc sulfide: and lead- and copper-doped zinc sulfide? Other spectral studies have dealt with metal sulfide vapors at elevated temperature^.^ Far-infrared spectra have been used for bulk characterization of various sulfides (ref 5 and references therein). The preparation of ZnS by a precipitation reaction with incorporated pyridines was studied mainly for the effect of the additives on the semiconducting properties; IR spectroscopy was used only to verify the presence of the dopants.6 The adsorption of CO on high surface area ZnS evaporated films was studied spectroscopically but only in the lowtemperature range 77-100 K.' Thus, there is a paucity of published information on the surface chemical properties of zinc sulfide. This paper describes an infrared spectroscopic study of the surface chemistry of two zinc sulfide polytypes. The object was to identify the specific types of surface sites, both by direct observation and by the use of probe molecules. The effect of incipient atmospheric oxidation of the sulfide surface was also investigated. In addition, the identification of sites that can be used to chemically attach long-chain molecules is of interest for forming steric or entropic barriers between particles in order to obtain stable colloidal dispersions for use in processing.
Experimental Section Materials and Methods. Zinc sulfide, purified grade, was obtained from Fisher Scientific Co. X-ray analysis showed that the material consists of major amounts of both sphalerite and wurtzite and contains trace ZnO and unidentified minor phases. The BET surface area was 8 m2/g. Zinc Sulfide,Gold Label electronic grade 99.99%, was obtained from Aldrich Chemical Co. This material was packed in nitrogen and also handled in the laboratory in a nitrogen atmosphere. X-ray analysis showed that the major crystalline phase was wurtzite with no other phases detected. The BET surface area was 21.9 m2/g. Pyridine was reagent grade. The sulfide sampleswere generally evacuated at 150 "C to remove sorbed water, exposed to pyridine vapor, generally evacuated at room temperature, and then evacuated again at 150 OC to remove any loosely sorbed pyridine, leaving only chemisorbed pyridine. Ammonia was lecture bottle grade. The ZnS was initially evacuated at 150 OC to remove the loosely sorbed water. The ZnS was exposed to ammonia, at room temperature or at 150 "C, followed by evacuation at 150 "C. D20,from Mallinckrodt Chemical Works, had 99.8% isotopic purity. The samples were exposed to DzO vapor, generally at ambient temperature,and evacuatedat 200 OC after each exchange to remove any weakly sorbed D20. The exchanges were essentially completed within 20 min, with further exposureto D20 at ambient temperature or 200 "C resulting only in very small changes. Spectra. All spectra were recorded with a Perkin-Elmer Model 1800 double-beam Fourier transform infrared spectrophotometer, using a Harrick Scientific Co. diffuse reflectance accessory (DRIFT). This accessory was equipped with a heatable vacuum cell with ZnS windows, which was used to hold the zinc sulfide powders. ~
(1) Yamasaki, T.;Usui, S.; Sasaki, H. Chem. Abstr. l971,75,53524d. ( 2 ) Yamaaaki, T.;Usui, S.; Sasaki, H. Chem. Abetr. 1971, 75,53566~. (3) Mielczarski, J.; Nowak, P.;Strojek, J. W.; Pomianowski,A. In 13th International Mineral Processing Congress, Warsaw, 1979; Elsevier/ North Holland New York p 110. (4) Martin, T. P.; Schaber, H. Spectrochim. Acta, Part A 1982,38A, 6.5.5
(5) Soong, R.; Farmer, V. C. Mineral. Mag. 1978,42(322), 277. ( 6 ) Yamamoto,T.; Taniguchi, A. Znorg. Chim. Acta 1986, 97, L11. (7) Lubezky, A,; Folman, M. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 791.
W
t
v -
z
2-
FREQUENCY (cm-')
Figure 1. Spectrum of ZnS (purified grade). Full vertical scale = 0.871 absorbance units.
t
,t-\
I !
\\INITIAL '\
Figure 2. Spectra of ZnS (purified grade) before and after D20 exchange. Sample was evacuated at 200 "C before and after the exchanges. Full vertical scale = 0.194 absorbance units. The spectra were acquired by using a DTGS detector operating at ambienttemperature. Qpical instrument operating parameters were 20 sample scans at 4-cm-' resolution with 0.10 cm/s interferometer mirror speed. The peak to peak signal to noise ratio, measured on the background of qll the spectra over a 100-cm-' interval between 1870 and 2000 or between 2400 and 2640 cm-', was 9OO:lor better. Figures were traced from the original spectra. Spectral changes are often more easily visualized by using difference spectra. These were obtained by taking spectra of the same sulfide in the cell immediately before and after a given treatment. No reference beam compensation was used. A 1:l subtraction of the after treatment spectrum minus the before treatment spectrum was carried out mathematically using the software provided with the spectrophotometer. The frequency repeatability of FTIR machines permits accurate mathematical subtraction of spectra to be carried out. Treatments. The powder treatments were carried out in the heatable vacuum cell by attachingthe cell to a vacuum rack, which attained a vacuum better than lod Torr. The desired gas or vapor was then metered into the cell via the vacuum rack manifold. At the completion of the treatment the cell was evacuated, closed off, and placed in the reflectance accessory. The spectrum was then recorded.
Results and Discussion The results obtained with the purified zinc sulfide will be given first followed by the studies using the electronic grade zinc sulfide. Purified Zinc Sulfide. In Figure 1 is given a typical spectrum of the purified zinc sulfide. The specific features observed are those of hydroxyl groups (3691 cm-'1, water (3410, 1640 cm-l; some of which remains sorbed after evacuation at 400 "C),and sulfate groups (1086,1132,1191 cm-'). On a pure ZnS surface one would not expect to see ZnOH groups. This material is partially oxidized on the surface as shown by the presence of sulfate groups. X-ray
596 Langmuir, Vol. 4, No. 3, 1988
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3 2400
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Figure 3. Spectra of ZnS (purified grade) before and after exposure to l-propanol vapor. Sample was evacuated at 150 OC before and after exposure. Full vertical scale = 0.260 absorbance units.
4000
V I 3500
I 3ooo
I
I
2500 2000 FREQUENCY (em-')
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I
i
,
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,
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Figure 5. ZnS (purified grade) difference spectrum. Spectrum of ZnS exposed to NH3 minus initial ZnS spectrum. Band assignments are given in Table I. Full vertical scale = 0.210 absorbance units. Table I. Band Assignments (om-') for Ammonia-Treated ZnS"
!1800
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Figure 4. Spectra of ZnS (purified grade) before and after exposure to pyridine vapor. Sample was evacuated at 150 O C before and after exposure. LA, band due to pyridine coordinately bound to a Lewis acid site; BA, band due to pyridine bound to Br0nsted acid site. Full vertical scale = 0.451 absorbance units. analysis also showed the presence of some ZnO. In addition to these, there are other surface sites detectable by pyridine and ammonia probes. These will be considered in turn. 1. D20 Exchanges. Exposing a sample to D20 vapor can be useful for indicating OH species. If exchange takes place with a given species, it must be accessible to the vapor phase. D 2 0 exchange with OH to give OD results in a decrease in the observed stretching vibration frequencies. In Figure 2 are given spectra of ZnS before and after exposure to D20 vapor at room temperature. The OH band at 3650 cm-l due to ZnOH groups and the water stretching band near 3400 cm-l are greatly reduced in intensity, and bands appear near 2690 and 2560 cm-'. The frequency shifts are close to the factor 0.739 calculated from the harmonic oscillator model. In addition, the H 2 0 bending vibration at 1638 cm-' shifts down to 1437 cm-'. About 60-70% of the hydroxyl groups was exchanged and about 68% of the water. 2. Hydroxyl Groups. Figure 3 shows spectra of ZnS before and after exposure to l-propanol. After exposure the ZnOH (3650 cm-l) groups disappeared and the typical CH2/CH3bands from alcohol appeared. This is a common reaction with most surface hydroxyl groups: Zn-OH + PrOH = Zn-OPr
+ HOH
purified ZnS assignment electronic grade ZnS 3660 (4) ZnOH 3485 NH,:Lewis acid 2 3395 3434 (vw) 3334 NH,:Lewis acid 1 3308 3230 3225 NH4+ 3153 3156 3078 NHS:Lewis acid 1618 1606 NH,:Lewis acid 1600 1489 NH4+ 1430 1448 "Samples were evacuated initially at 25, 150, or 400 O C . After ammonia addition samples were evacuated at 150 O C .
3. Acid Sites. The presence of a Lewis acid (surface cation) can be observed by exposing the surface to an electron-donating base, such as pyridine or ammonia, which form coordinate bonds with the site. Proton-donating Brernsted acids react with the bases to form the pyridinium or ammonium ion. (a) Pyridine Probe. Figure 4 shows spectra of the ZnS before treatment and after exposure to pyridine. The bands are identified in Figure 4. Bands due to both Brernsted (proton-donating) and Lewis acid (electron-accepting surface cation) sites are present. On exposure to pyridine, the ZnOH band disappears, and it is probable that this OH is the Bransted acid. However, reaction with the strongly chemisorbed water could also give rise to the band ascribed to the Brernsted acid. (b) Ammonia Probe. Pyridine (pK, = 5.21) is a weaker base than ammonia (pK, = 9.27) and will only react with the stronger Lewis acid sites.8 To determine if weaker Lewis acid sites exist, we probed the ZnS surface with ammonia. In Figure 5 is given a difference spectrum of ZnS, before and after exposure to ammonia, to show the peak positions more clearly. The observed bands and their assignmenta are given in Table I. These band assignmenta were taken from a review of the literature: which indicates that when ammonia is coordinated by its lone-pair electrons to a Lewis acid site the NH stretching bands can vary from about 3260 to about 3430 cm-'. The 3480-cm-l band is somewhat higher than usually observed on oxides, but (8) Little, L. H. Infrared Spectra of Adsorbed Species; Academic: New York, 1966;p 193. (9) Tsuganenko, A. A.: Pozdnhakov, D. V.; Filimonov, V. N. J. Mol. Struct. 19?5,29, 299.
Langmuir, Vol. 4, No. 3, 1988 597
Surface Chemical Properties of ZnS
w
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m
=I-
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l m . n a l a n . . l
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Figure 6. Spectrum of ZnS (electronicgrade). Full vertical scale = 0.732 absorbance units. the bands do fit the pattern of about 90-cm-l separation for the symmetric and asymmetric NH bands. The reaction of NH3 with ZnOH to form ZnNH, would give rise to stretching bands in the regions 3400-3470 and 3330-3390 cm-' as well as a bending vibration near 1550 cm-'. The absence of the 1550-cm-' bending mode suggests this reaction does not occur. The presence of the band a t 3160 cm-l along with the band at 1448 cm-' indicates the formation of NH4+. Since the samples were generally evacuated at 150 "C after the ammonia exposure, none of the bands should be due to physical adsorption or H-bonding to OH. The disappearance of the ZnOH band at 3660 cm-l could be due to its behaving like a Brernsted acid, which results in the observed NH4+. Part of the NH4+could also result from interaction with the strongly chemisorbed water. Thus, the ammonia is present on the surface in three distinct forms, viz., chemisorbed to two discrete Lewis acid sites and present as NH4+due either to reaction with OH or to sorbed water. Samples exposed to air for 16 h and then evacuated at 150 "C showed that about 25% of the bonded ammonia hydrolyzed. 4. Zinc Oxide and Ammonia Interactions. To determine if any of the observed Lewis acid sites arise due to the ZnO on the ZnS surface, experiments were carried out with pure ZnO. The ZnO samples were preheated at 150 OC, exposed to NH,, and then evacuated at various temperatures. Some very weak ammonia bands were observed at room temperature, but on evacuating at 150 OC these were removed. These results are in agreement with published studies. Dehydroxylation at 500 OC and exposure to ammonia give rise to sorbed ammonia. The ammonia is greatly reduced by evacuating at 150 OC and completely removed by evacuation a t 250 O c a 9 Preheating at 450 OC followed by ammonia exposure for 8 h at 20 "C showed the presence of sorbed ammonia, but this is also removed by evacuating between 30 and 300 OC.l0 If any hydroxyl groups are present, NH, can H-bond to these.'" Under the conditions used here with the ZnO and the ZnS (150 "C evacuation) one would not expect to see any reactions between NH3 and the ZnO surface other than interactions with the hydroxyl groups and sorbed water. Thus the ammonia bands observed with the ZnS surface arise from the ZnS and not from any ZnO which might be present on the surface. (10) Morimoto, T.;Yanai, H.;Nagao, M. J.Phys. Chm. 1976,80,471. (11) Infrared Spectra of Mmerals; Farmer, V. C., Ed.; Mineralogical Society Monograph 4; Mineralogical Society: London, 1974; p 428. (12) Nakamoto, K.Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley-Interscience: New York, 1978; p 238.
I
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,
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Figure 7. Spectra of ZnS (electronic grade) before and after D20 exchange. Sample was evacuated at 150 O C before and after the exchanges. Full vertical scale = 0.541 absorbance units.
t
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P dJ 3600
I I 3200 2800 FREQUENCY (cm-I)
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J
2400
xxx)
Figure 8. Difference spectrum (spectrum after ammonia treatment minus initial spectrum)of purified and electronicgrade ZnS. Samples were evacuated at 150 "C before and after treatment. LA, bands due to NH3 coordinately bound to Lewis acid sites (surface cations). Full vertical scale = 0.116absorbance units for purified grade and 0.206 absorbance units for electronic grade.
Electronic Grade Zinc Sulfide. A spectrum of the electronic grade ZnS is shown in Figure 6, and the principal bands are identified. The presence of water is common to both types of zinc sulfide. This grade ZnS contains no hydroxyl groups but does show the presence of SH groups. The presence of sulfite rather than sulfate groups indicates a less highly oxidized surface. Thus it is of interest to determine if all the observed surface sites are an inherent feature of ZnS or if some of these arise due to the presence of SH or oxidized species present on the surface. A similar set of experiments to those described above was carried out with this ZnS. The observed differences will be discussed below. 1. DzO Exchange. In Figure 7 are given ZnS spectra before and after DzO exchange. Under the conditions used here about 25% of the H20 (343013300 cm-') is exchanged. The 242012530-cm-' bands due to SH groups overlap the DOD bands. It is clear that at least some of these SH groups have exchanged since the SD bands (176811841 cm-l) are observed. 2. Ammonia Probe. The observed bands due to the sorbed ammonia are given in Table I. The band at 3434 cm-l is very weak and only barely detectable. This ZnS, then, has only one set of Lewis acid (surface cation) sites. The formation of NH4+is probably due to the interaction of the ammonia with strongly bound water, since the HOH stretchinglbending bands decrease about 15% in intensity after exposure to NH,. On exposing the sample to the atmosphere for 60 h, the sorbed water increased by 9%
598 Langmuir, Vol. 4, No. 3, 1988
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4obO
Figure 9. Spectra of ZnS (electronic grade) after various thermal treatments in air. Full vertical scale = 0.975absorbance units. Table 11. Observed and Literature Values for Frequencies (cm-') of Sulfate/Sulfite Bands grade bridging unidenoxidized 8bidentate tate purified electronic electronic ZnSOJl sulfate12 sulfite" 1070 10301086 1060 1050 1147 10901132 1123 1101 1191 1160 1197 11661195 1060 10501117 1114 10501117
and the NH4+ increased by 37%. For comparison purposea, difference spectra (i.e., sample after ammonia exposure minus sample before ammonia exposure) for both the purified and the electronic grade ZnS are given in Figure 8. The decrease in hydroxyl groups and the presence of ammonia coordinated to the two seta of Lewis acid sites on the purified ZnS are readily apparent, as is the water decrease and ammonia coordinated principally to one set of Lewis acid sites on the electronic grade ZnS. 3. ZnS Oxidation. The electronic grade ZnS was air-oxidized under various conditions in order to aid in identifying some of the bands and to determine if any reactive sites are produced by superficial oxidation. A typical series of spectra is shown in Figure 9. In general, it is seen that the water (3400/1600 cm-l) content decreases due to desorption, the SH (2417 cm-I) decreases, and shifts occur in the llOO-cm-l region due to the oxidation of sulf~te to sulfate groups. The observed bands in the llOO-cm-l region for both types of ZnS as well as the oxidized ZnS are listed in Table II along with published values for sulfite and various sulfate groups. There was little change in the intensities of these bands before and after oxidation. On standing in air at room temperature, the SH groups slowly
3800 I
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Figure 10. Difference spectrum (spectrum after ammonia treatment minus initial spectrum)of electronic grade ZnS which had been air-oxidized at 400 O C . Full vertical scale = 0.101 absorbance units. Table 111. Spectral Bands (cm-') Observed Due to Ammonia Adsorption on Reduced and Oxidized Electronic Grade Zinc Sulfide oxidized assignment reduced 3665 (1) ZnOH -3500 3500 HzO 3320 NH3:Lewisacid 1 3303 3235 3225 3168 NH4+ 3149 1650 (4) HzO 1620 (1) 1610 NH3:Lewisacid 1600 NH4+ 1440 1450 1283 1261
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hydrolyze, with 40% remaining after 72 h of exposure. Ammonia absorption was carried out on the 400 "C oxidized ZnS to see if this thermal treatment affected the Lewis acid sites. The spectral changes observed are shown in the difference spectrum in Figure 10 and are listed in Table I11 along with those changes observed with the unoxidized ZnS. Except for the hydroxyl band change, the spectra obtained with ammonia absorption on the oxidized and unoxidized ZnS are quite similar. They both show the presence of NH4+due to the interaction between the ammonia and the H 2 0 and/or OH groups. Both samples appear to show the presence of only one set of Lewis acid sites. The second set of Lewis acid sites (surface cations) observed on the purified ZnS does not arise from the surface oxidation. The difference between the two zinc sulfide types, other than surface oxidation, is that the electronic grade ZnSconsists of hexagonal closepacked wurtzite only, whereas the purified ZnS is a mixture of the hexagonal close-packed wurtzite and cubic close-packed sphalerite polytypes. It seems likely that one set of sites is associated with the wurtzite structure and a different set of sites is associated with the sphalerite structure. Registry No. ZnS, 1314-98-3;NH3, 7664-41-7;pyridine, 110-86-1.