1727
Langmuir 1991, 7, 1727-1733
Deuterium NMR Investigation of Benzene Adsorbed on Boehmite Glasses Jun-ichi Fukasawa,+Chi-Duen Poon, and Edward T. Samulski' Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received November 5,1990. In Final Form: February 19, 1991 The structure of boehmite glasses adsorbed with deuterated benzene was studied by deuterium nuclear magnetic resonance. Macroscopic, monodomain boehmite glasses with long-range structural order were prepared by two distinct routes: air-dryingand freeze-dryingsol-gelsof aluminum alkoxide to yield glassy plates and plugs, respectively. The deuterium NMR spectra of adsorbed benzene (- 10%c&3 by weight) is indicative of high adsorbate mobility in the glasses;the spectra show quadrupolar splittingswhich reflect the nature of the long-range structural order and morphology in the glasses. The angular dependence of the quadrupolar splitting for the air-dried boehmite plate indicates a uniplanar arrangement of the ultrafhe boehmite primary particles. The spectra also exhibit substantialanisotropic magnetic susceptibility influences. Quadrupolar splittings in the freeze-dried plug are consistent with a uniaxial arrangement of the ultrafine boehmite particles. Spectra having the plug symmetry axis at 90° to the spectrometer field showed an unusual 2-D powder pattern which, when simulated, gives information about the dynamics of the adsorbed benzene; Le., the rate of surface translational diffusion of the adsorbate influences the appearance of the NMR spectrum.
I. Introduction Inorganic surfaces are often active exposing sites for physical and chemical adsorption of organic molecules. An adsorbed molecule (adsorbate)may be orientationally ordered with respect to the surface and exhibit physicochemical properties that depend on the nature of the constraints it experiences on the surface. Additionally, the details of the surface-adsorbate interaction are important for realizing new applications such as catalysis; hence these interactions have been extensively investigated. The general utility of nuclear magnetic resonance (NMR) in this respect was recentlyreviewed by Mehring.' Deuterium nuclear magnetic resonance (2H NMR) is a particularly useful method for determining molecular orientation and dynamics of partially ordered, labeled adsorbates. It has been applied to a variety of adsorbates on a wide range of substrates, e.g., alumina>+ carbon and boron nitride: and titania (rutile).6 In this report we demonstrate that 2H NMR spectra of deuterated adsorbates (probe molecules) offer an opportunity to derive unique structural/dynamical information about glasses prepared from boehmite (7-AlOOHJ/zH20). The NMR results corroborate the morphology of the boehmite glass whose structure was independently elucidated with X-ray diffraction and polarizing microscopy. Investigations of deuterated benzene adsorbed on macroscopically oriented glasses give dynamical information about the adsorbate, i.e., the effective surface diffusion coefficient, Ds,of the adsorbate in conjunction with the morphology of the boehmite structure influences the adsorbate NMR lineshape. In section I1we review the known details of the structure of two distinct boehmite morphologies and present the NMR background relevant to thia study. The details of t Permanent address: Kao Corporation, Tokyo Research Laboratories, 2-1-3 Bunka, Sumida-ku, Tokyo 131, Japan. (1) Mehring, M. Z. Phys. Chem. 1987,161, 1. (2) StBbner,B.;Kndzinger, H.;Conard,J.;Fripiat,J. J. J.Phys. Chem.
1978,82, 1811.
(3) Gottlieb, H. E.;Lw,2. J . Magn. Reson. 1983,54,257. (4) Majors, P. D.; Ellis, P. D.J . Am. Chem. Soc. 1987,109,1648. (5) G r o w , R.; Boddenberg, B. Z. Phys. Chem. 1987,152,259. (6) Horstmann, W.; Auer, G.; Boddenberg, B. Z. Phys. Chem. 1987, 152, 281.
sample preparation and measurements are given in section 111and our findings and their interpretations are discussed in section IV. 11. Background
Boehmite Structure and Morphology. The unit cell dimensions and atomic structure of boehmite are shown in Figure la. Two layers of A1 atoms octahedrally coordinated to oxygen are connected by H-bonding bridges? The unit cell is uniquely oriented in the platelike, ultrafine boehmite primary particles obtained in the solgel synthesis of metal oxides from metal alkoxides. The average size of these particles along three mutually perpendicular axes (loo), (OlO), and (001) are 6.0,2.5, and 4.0 nm, respectively (Figure lb). The b axis of the unit cell is along the (010) direction; hence the platelike particle's thickness corresponds to twice the b-dimension of the unit The morphology of macroscopicboehmite samples-the relative orientations of primary particles-is dependent on the drying method. During the process of synthesizing boehmite, macroscopic, glassy, monodomain sampleswith uniform, long-range structural order may be prepared. This macroscopic order is achieved by two distinct routes: (1) air-drying or (2) freeze-dryingof the solsand gels obtained from the corresponding aluminum alkoxide. When the boehmite sol is dried at room temperature and atmospheric pressure (air-drying),a transparent, porous glass is formed.0 Fukasawa and Tsujiilo showed that in air-dried glasses a uniplanar arrangement of the primary particles results wherein the platelike boehmite particles close-pack, forming thin layers with the (010) axis normal to the layer. The layers in turn stack (like the pages of a book) forming a macroscopic, glassy, transparent film having a thickness on the order of 0.5 mm (Figure 2 (left),where the normal to the film F is shown). Freeze-drying the sol, on the other hand, produces a glassy plug with a uniaxial (7) Fripiat, J. J.; Boemane, H.; Rouxhet, P. G. J . Phys. Chem. 1967, 71, 1097. (8)Fukasawa, J.; Tsujii, K. J. Colloid Interface Sci. 1988, 126, 166. (9) Yoldae, B. E. Am. Ceram. SOC.Bull. 1976,54, 286. (10) Fukasawa, J.; Teujii, K. Unpublished data.
0743-7463/91/2407-1727$02.50/0 0 1991 American Chemical Society
Fukasawa et al.
1728 Langmuir, Vot. 7, No. 8, 1991 b (12.24A) A
J-
I
c (2.86A)
10 pm
(010) 25A
I .b I I
(100) 60A (001) 4O.A
bl
Figure! 1. (a) Unit cell dimensions and atomic structure of boehmite (after ref 7); the dashed lines indicate the H-bonding bridges. (b)Ultrafine boehmiteprimary particle dimensionswith the three mutually perpendicular axes (100), (010)’ and (001) indicated. F
Rp
10 pm
Figure 3. SEM photomicrographs of the channel morphology in glassy boehmite plugs.
Fj ,..........’_ ......
Figure!2. A schematicillustrationof the hierarchicalmorphology of a glassy film and a glassy plug of boehmite; the macroscopic symmetry axes (the film normal F and plug axis P)are indicated and the arrangement of primary particles is shown;n is the local surface normal and the (010) axis (parallel to the b axis of the boehmite unit cell-see Figure 1) are shown in successive magnifications.
arrangement of primary particles.a This morphology is a result of unidirectional growth of ice crystals in the sol phase which form on freezing a cylindrical sample of the sol solutions in a vertical temperature gradient (- looo/ cm). This results in a honeycomb of channels whose lengths span the macroscopic, vertical dimensions of the glassy plug. The channel morphology, which remains after the water is removed, consists of delicate, corrugated thin
walls (thickness = several tens of nanometers) which are comprised of primary boehmite particles (Figure 3). Approximately cylindrical channels form at high sol concentrations.8 The platelike boehmite particles are segregated and compressed between unidirectionallygrowing ice crystals which propagate along the temperature gradient used to prepare the plug. This unusually regular morphology, stable at room temperature after freezedrying, is schematically illustrated in Figure 2 (right), where the plug’s symmetry axis, P, is indicated. Electron microscopy, optical microscopy, and X-ray diffraction confiim in detail the hierarchical morphologies schematically illustrated in Figure 2, showing how the primary boehmite particles are arranged in the thin layers and channel walls of the glassy films10and plugs: respectively. We briefly review these findings. In both the films and plugs, optical microscopy and X-ray diffraction experimental results were essentiallythe same. Optical anisotropy is not observed in the plane of the films and in the plane of the channel walls of the plug; it is observed in their respective cross sections. The intensity of the diffraction from the (020) plane is much stronger than that of other scattering when the samples were irradiated with an X-ray beam in the plane of the glassy film (perpendicular to the surface normal F) or along the axisof the glassy plug (parallelto P);the opposite intensity distribution is observed when the X-ray beam is normal to the film (along the layer surface normal F) or normal to the plug axis P. These results indicate that
Deuterium NMR Studies of Boehmite in the glassy films and plugs, the (010) direction of the boehmite primary particle is normal to the thin layers comprisingthe film and normal to the channel wall surface; the particles are stacked in the layers and the walls with the unit cell's b axis essentially parallel from particle to particle. Furthermore, in the case of air-dried glassy films, the resulting large specimen thickness enables one to perform small angle X-ray diffraction experiments (in the plane of the glassy film) that yield a Bragg distance of =2.8 nm, which is close to the primary particle's thickness (2.5 nm), Le., twice the unit cell b dimension. In summary, the uniplanar structure of the glassy films (and locally uniplanar channel wall in the glassy plugs) is composed of densely stacked, primary platelike particles of boehmite. The average direction of the b axis of the boehmite particles is uniquely defined for these two morphologies. Since the air-dried film is a single, homogeneous, macroscopic film,the b axis is, on average, aligned along the film normal F. In the freeze-dried morphology, the b axis is perpendicular to the channel walls. Hence in the macroscopic plugs the b axis is, on average, perpendicular to P. Note, however, that because the wall of the channel forms a closed surface (with variable curvature-see Figure 3), the b axes are distributed in a uniplanar orientational pattern in a cross section normal to P. Both glasses are very porous with high surface area. And, their macroscopic anisotropy suggests that adsorbate guests may be uniquely suited for study with 2H NMR. Herein we will show that the 2H NMR measurements of deuterated benzene delineate the orientationaldistribution of the b axes in these two different boehmite morphologies. Deuterium NMR. The utility of the 2H NMR technique derives from the fact that the relevant NMR interactions are entirely intramolecular, i.e., the dominant interaction is between the nuclear quadrupole moment of the deuteron and the local electric field gradient (efg) at the deuterium nucleus. The static efg tensor is generally defined in terms of the quadrupolar interactions tensor q. This is a second rank tensor that is usually axially symmetric for deuterium covalently bonded to carbon; its principal component q is along the C-D bond. In mobile phases with long range molecular orientational constraints, deuterium-labeled molecules exercise rapid anisotropic reorientation which incompletely averages the static quadrupolar interaction. A partially averaged tensor results with its principal component ( q ) along the local symmetry axis of the anisotropic molecular motion (the local symmetry axis is denoted by the unit, apolar director n). Anticipating uniaxial adsorbate motion relative to a director identified with the local substrate surface normal, the average ( q ) is simply related to the static component q (defined in a molecular fixed frame) by the factor (Pz(cosa ( t ) )), where&) is the time-dependent angle between n and the C-D bond vector
(4)E q (p2(cos a ( t ) ) ) (1) ( Pz(cos & ) ) ) represents a time average of the second Legendre polynomial, PZ(C0S &)) = (3C0S2&) - 1)/2, over the rapid motion of the bond vector's orientation relative to the local director. In an idealized microsystem-mobile adsorbates at the interface of a single crystallite-an averaged quadrupolar interaction may be observed with essentially high-resolution NMR techniques. The adsorbate's deuterium NMR spectrum consists of a resolved pair of resonances at frequencies um ( m = i l ) centered
Langmuir, Vol. 7, No. 8, 1991 1729 about the Larmor frequency 4 1 1 um = uL
3 1 + m-(q)-(3 cos2e - I), m = &I 4 2
(2)
8 is the angle between the magnetic field Bo and the local symmetry axis n. The Larmor frequency (center of the spectrum) in its most general form is given by
where y is the magnetogyric ratio of the deuteron and ( Au) is the partially averaged chemical shift anisotropy (uniaxial symmetry is assumed). For hydrogen (deuterium) Au is rather small, and in motionally averaged spectra this source of angular dependence for 4is negligible (( Au) = 0). In single crystals and inhomogeneous solids with long range order, however, a difference in the diamagnetic susceptibility along n,X I I , and normal to n,XI, A x = (xi - xl) could, according to eq 3, result in an angular-dependent centroid of the two resonances um. Apart from these subtleties in the dependence of the location of the center of the spectrum, the dominant feature of the deuterium NMR spectrum associated with a monodomain (crystallite) exhibiting an incompletely averaged quadrupolar interaction is a quadrupolar doublet Au (the frequency span between u+1 and u-1) whose magnitude is a direct measure of the efficacy of the motional averaging 3 1 A~ = -(q)-(3 cos2e - 1) 2 2
(4)
Hence, in the case of a mobile, partially aligned adsorbate, the Au values may be readily interpreted in terms of the average orientation of adsorbed molecule's C-D bond vector relative to the local symmetry axis. In the absence of a peculiar adsorbate-substrate interaction, this local symmetry axis will coincide with the substrate's local surface normal. For symmetric adsorbates such as benzene, the experimentallydetermined value of ( q ) , Le., (P2(cosa ( t ) ) ,may be further decomposed to give the average of the time-dependent orientation of the CSaxis of benzene, (Pz(c0sacJ) (= -'/~(Pz(cos &)I)), relative to the surface normal.5 We emphasize that the NMR spectra are particularly simple to interpret if labeled adsorbates are examined in monodomain hosts (single crystals): A resolved quadrupolar doublet is observed in the NMR spectrum of the adsorbate for each motionally inequivalent deuteron in the guest molecule. The boehmite samples we will study fall into this category. However, randomly oriented powders of inorganic crystalline substrates (the most readily prepared specimens) complicate this otherwise straightforward measurement. Deuterated guest compounds adsorbed on powdered inorganic crystals yield broad, often featureless, NMR spectra as a consequence of the random distribution of local symmetry axes of the crystallites in such powders. Nevertheless, considerably dynamical information about the adsorbate can be obtained from 2H NMR studies of powder substrates.24 111. Experimental Section Sample Preparation. Boehmitesole were prepared according to literature procedure^.^ A dry glassy film of boehmite was made by slowly drying a 6 w t % boehmite sol (64 mM HCl). The sol (-5 mL)was placed in a flat glass vessel which was covered with a porous (1mm diameter pores) plastic film and allowed to standat room temperature and atmospheric pressure for a month. (11) Abragam, A. The Principles of Nuclear Magnetkm; Oxford University Press: London, 1961; Chapter 7.
Fukasawa et al.
1730 Langmuir, Vol. 7, No. 8, 1991
The sol transforms into a transparent thin glassy film after going throughthe gel state.0 The thickneas of suchglassy films is about 0.6 mm. The glassy plug is prepared from a 16wt 9% sol (160mM HCl) in a 8 mm (LdJ X 6 mm glass tube fitted with a 8 mm (o.d.1 X 4 mm brass bottom; the solution is unidirectionally frozen by cooling the braes bottom in a -70 O C bath and then lyophilizing the frozen plug (freeze-drying).The boehmite particles aggregate into sheeta forming parallel channels that are continuous from the top to the bottom of the cylindrical plug (-7 mm in diameter and -5 mm long).' The morphology of the glassy plug was observed with a JOEL JSM-840 scanning electron microscope (SEMI. Both types of glasses were dehydrated at 110 O C under vacuum for 2 days before being placed into benzene-& vapor; after 24 h at room temperature the benzene was adsorbed as a liquid on the surface of the boehmite glasses. NMR Experimentm. NMR spectra were recorded with a Bruker MSL-360wide-bore spectrometer using a high power probe with a transversesolenoid coil (theB1 field is perpendicular to the Bo field). For the glassy film experiments,a 5-mmsolenoid was used and the 90Dpulse width was 2 ps; in some cases for the glassy plug, we used a transverse 10-mm solenoid with a 90° pulse width of 6 ps. All of the NMR spectra were obtained by using a single pulse aequence. In order to conduct angular dependent experiments (i.e. to vary 0 in eqs 2-41, a small piece of air-dried glassy film adsorbed with benzene-& was centered in a 5 mm tube closed by a Teflon cap having radial markings at loo intervals. This geometry in the transverse solenoid enabled us to vary the angle between the magnetic field BOand the plate normal F. Similarlya plug adsorbedwith benzene-dein the same configuration enabled us to record spectra with symmetry axis of the plug, P,set to either Oo or 90° with respect to the magnetic field.
1
1
1
1
1
-2
-1
0
1
2
-
kHZ
Figure 4. The deuterium NMR spectrumof benzene-de ( 109%
by wt) adsorbed on a boehmite film; the plate normal is parallel to the magnetic field of the spectrometer (0 = Oo in eq 4).
0 Ax
0"
IV. Results and Discussion At the rather high fraction of benzene in the boehmite glasses used in this study (SlO%benzene by weight), we can assume that rapid molecular reorientation and chemical exchange among adsorbate molecules quite effectively average the quadrupolar interaction. In this mobile adsorbate phase, the residual interaction is assumed to be defined relative to the local substrate surface normal, I), Le., the residual quadrupolar interaction tensor associated with the mobile absorbate is axial with its principal component ( q ) along n. This is confirmed by the general finding that the observed Au values (-5 kHz) are on the order of a few percent of the static (maximum) value of the splitting (-250 kHz). Thus the high (slightly anisotropic) mobility in conjunction with monodomain samples gives resolved spectra wherein the magnitude of Au is dependent on the orientation of the director relative to Bo. In solvated films with the film normal F (coincident with the director n) parallel to the magnetic field Bo (8 = OO), the deuterium NMR spectrum of benzene-de exhibits a quadrupolar doublet with Au 4 kHz (Figure 4). When the angle 8 is changed, the quadrupolar splitting Au is scaled by the factor of 11/2(3 cos2 8 - 1)1 (Figure 5); the magnitude of Au is plotted versus the angle 8 in Figure 6. The quadrupolar splitting Au diminishes as F approaches the magic angle ( 5 4 O 4 4 3 , and AU (8 = 90') = l/2Au (8 = ) ' 0 as predicted by effective uniaxial averaging of q with respect to F. (Small deviations from the 1'/2(3 cos28 - 1)1 dependence in Figure 6 are due to changes in the amount of benzene-de adsorbed on the film by evaporation during the course of the experiments; the magnitude of Au increases as the adsorbate concentration decreases.) It is also interesting to note in Figure 5 that the center of the spectrum shifts with 8 according to P&os 8). According to eq 3 and the anticipated negligible ( Au), this observation implies that the sample has a nonnegligible magnetic susceptibility contribution-an apparent chemical shift anisotropy. There could be several con-
-
b ',I h
40" 50"
-+n 1 kHz
70"
A !a
-I
A
80"
'IAx12
Figure 5. The 0 dependenceof the deuterium NMR spectra of benzene-deadsorbed on a boehmite film, the angulardependence of Av and the shift of the center of the doublet (vertical bar) are
apparent.
tributions to the observed Ax (d.2xAuin frequency units): the shape anisotropy of the glassy film itself or intrinsic contributions stemming from the lamellar arrangement of the inherently anisotropic primary boehmite particles themselves. Independent of ita origins, however, Ax clearly influences the location of the centroid of Au in the 2H NMR spectra (Figure 5). In Figure 7 we show 2H NMR spectra of benzene-& adsorbed on the glassy plug for 6 = Oo (part a) and /3 = 90° (part b), where 6 is the angle of the P axis with respect to the magnetic field Bo. A resolved doublet is observed in Figure 7a since the direct&), Le., the local symmetry axes corresponding to the set of normals to the channel
Deuterium NMR Studies of Boehmite
Langmuir, Vol. 7, No. 8, 1991 1731
I loOo
1
01
3
1
I
I
"
v
I
I
I
l
l
0
1
I
Figure6. Quadrupolar splittingsfrom Figure5 are plotted versus the angle 8; the expected IP2(cos 8)l dependence (solid line) is indicated.
I
l
l
-1
0
1
Figure 8. Comparison of simulated static lineshapee for (a) standard 2-D powder pattern and (b) 2-D powder pattern with the influence of AX (=0.2).The quadrupolarsplittingwas taken as unity, and the line width is 0.05.
a
b
I
-1
\
-
Figure 7. Deuterium NMR spectra of benzene-& adsorbed on
a glassy plug: (a) @ = Oo, channel axes are parallel to Bo and the set of channel wall normals {no]are (ideally)normal to Bo (8 90'). (b) fl = 90', a motionally averaged 2-D powder pattern with Ax # 0 is apparent. (c) Simulated spectrum of @ = 90' sample after ref 10; DR = 0.02, Ax = 0.18, see text.
wall {n], ideally have a single orientation with respect to the field. That is, for @ = Oo, 8 = 90°, where 8 is the angle between the (n]and Bo;some deviations from 8 = 90° (undulations in the channel wall or misaligned channels) contribute to the width of the resonances in Figure 7a. In Figure 7b a distorted, partially averaged,two-dimensional powder pattern is seen as a result of Ax effects in conjunctionwith "effective reorientation of n" (see below). In the 6 = 90° orientation of the plug, there is ideally a uniplanar distribution of 8 which would correspond to a uniform 2-Ddistribution of the local directors (n)and an associated classical 2-Dpowder spectrum (Figure 8a). We consider the origin of deviations from this ideal case in more detail now. We approximate an ideal morphology of the freeze-dried boehmite plug as regular matrix having cylindrical channels of radius r spanning the length of the plug (Figure 9a). When the spectrometer magnetic field Bo makes an angle @ with the plug axis P (refer to Figure 9b), a distribution in director orientations (multiple 8 valuesdue to the azimuthal distribution of local normals to the channel wall) will be encountered. In the absence of exchange of adsorbate between wall surface segments
X
Figure 9. Schematicillustrationof the idealized morphology of
a glassy plug: (a)orientationof the cylindrical axis P with respect to the magnetic field Bo; (b) different orientations of the set of local directors ire with respect to BO.
having the local directors nd and ny at different orientations e(&) and e(#') with respect to Bo, the NMR spectrum I ( v ) will consist of a superposition of quadrupolar doublets, i.e., I ( v ) = ZdAv(p,d), where Av (@d) = (4)'/2(3 COS2 8 - 1)
= (q)1/2(3 sin26 sin24 - 1) (5) For 6 = 90°, equally weighted azimuthal orientations (a uniform distribution of 6 values) result in the classical 2-Dpowder pattern lineshape (Figure 8a);this spectrum is distorted when we introduce the influence of a finite A x on the location of the centroids of the subspectra Av(@,4)
Fukasawa et al.
1732 Langmuir, Vol. 7, No. 8, 1991
will effect reorientation of n. Since diffusion parallel to both z (the channel axis) and c are equally probable, a mean sqaure distance x 2 = z2 + c2 = 2r2must be traversed before n reorients by 1 radian. Using the Einstein-Smoluchowski relation for surface diffusion,we find for the time required for n to reorient by 1 rad
b
a
7R = r2/DS
(6)
and thereby obtain for the planar diffusion constant n
n
M
rsi
l.o
I
l
l
I
-I
0
I
-I
AX = O
l
l
0
I
Ax = 0.2
Figure 10. Simulated lineshapes for planar reorientation with different planar diffusion coefficienta DR: (a) standard 2-D powder pattern; (b)2-Dpowder pattern with the influence of Ax (-0.2). The quadrupolar splitting was taken as unity, and the
line width is 0.05.
according to eq 3 (Figure 8b). We now consider how translational diffusion of the adsorbate along the circumference of a cylinder (tantamount to the reorientation of n itaelf)will influence these 2-D powder spectra by further averaging ( q ) . In order to understand the influence of adsorbate translational diffusion on the NMR spectra, we follow closely the method Luz, Poupko, and Samulski12 used to simulate partially averaged NMR lineshapes in cholesteric liquid crystals, fluids having a helicoidal distribution of locally uniaxial directors (seeAppendix of ref 12).What we seek is the "planar diffusion coefficient" DR that describes the effectiveangular reorientation of n brought about by translational diffusion of the adsorbate between "surface segments" characterized by different orientations n4 (see Figure 9b). The result of such surface diffusion might be equivalently thought of as reorientation of the local director n in the xy plane normal to a cylinder axis (z 11 P).As shown by Luz et al., such motion will strongly influence the lineshape when Bo is not coincident with z (6 # Oo) and when the reorientation rate DR of n (the circumferential translational diffusion of the adsorbate) is sufficiently rapid. Examples of the influence of DR on the 2-D powders (@=90') for ( q ) = 1 and Ax = 0 and Ax = 0.2 are shown in parts a and b of Figure 10,respectively. In the latter case, the formulation of the motional average presented in ref 12 was modified to include Ax # 0.lS As DRincreases, for both cases the powder patterns collapse to resolved doublets with Au = l/* (q). In order to make contact with the work of Luz et al. and to extract values of adsorbate self-diffusion from spectral simulations, we first address surface diffusion on a cylindrical surface having a radius of curvature r. Only diffusive steps along the circumference c of the cylinder (12) Luz, Z.; Poupko, R.; Samuleki, E. T. J. Chem. Phyr. 1981, 74, 5825.
(13)Poupko, R.Private communication.
As shown in Figure 10, D g is the primary parameter influencing the simulations of partially averaged 2-D powder lineshapes (see eq A7 in ref 12). According to eq 7, D g will depend on the radius of curvature r of the channels in the boehmite plug and the surface diffusion coefficientDs of the adsorbate. If r can be estimated from electron microscopy, then by matching the experimental lineshape with simulated lineshapes, i.e., selecting the appropriate D g , one may infer the diffusion constant of the adsorbate on the boehmite surface, Ds. In Figure 7c we show an optimized (with respect to the experimental lineshape in Figure 7b), partially averaged 2-D powder spectrum simulated with a single parameter DR (=0.02 in units of Au, Ax = 0.18 Au; in the simulation we use the Au value of Figure 7a). If we approximate an effective channel radius r ( 1 5 pm) from electron micrographs (Figure 3), we infer with eq 7 that the surface diffusion constant for the benzene adsorbate on boehmite is Ds = 2X cm2/s. This estimate is close to that of liquid benzene itself (D = 2.14 X cm2/s)14and estimates of the self-diffusionconstant of benzene solubilized in a liquid crystal (D = 6 X cm2/s).I2 The difference between the experimental lineshape and the simulation (Figure 7b,c) stems in part from the use of a single (average) value for r and the neglect of angular deviations of {ndwith respect to z-a distribution of (6 + 8) values (see Figure 9)-apparent from the broadening in the spectrum with 6 = Oo (Figure 7a). Both affects would tend to redistribute spectral intensity more uniformly across the simulated lineshape yielding closer resemblance to experiment (Figure 7b) but at the expense of introducing additional parameters into the simulation. That we find Ds = D for pure benzene is not too surprising at the high adsorbate concentrations studied. The adsorbate is undoubtedly exchanging between surface-bound sites (with preferred surface orientations) and bulk, isotropic benzene. The rather low values of the observed absorbate quadrupolar splittings (i.e., about 3% of the Au value expected for having the c6 axis of benzene fixed normal to the surface) are also consistent with a chemical exchange process averagingq. Hence, there may be rather small influences by the surface on the adsorbate's average mobility in the present samples. In more regular etructures8 (havinga narrow distribution of r values), this novel method for estimating Ds may yield more precise insights into the surface diffusional properties of the adsorbates.
V. Concluding Remarks We have shown that the quadrupolar splittings and lineshapes exhibited by a deuterated organic adsorbate benzene on boehmite glasses provide unique information about the morphology of ultrafine boehmite particles in the glasses and reflect motionally averaged interactions (14) McCall, D. W.; Dough,D.C.; Anderson, E. W. Ber. BumenCes. Phy8. Chem. 1963,67, 336.
Deuterium NMR Studies of Boehmite between the boehmite surface and the adsorbate. We see that the anisotropic diamagnetic susceptibility of the glasses qualitatively influence the spectra. In glassy plugs the uniplanar distribution of surface normals (cylindrical channel walls) together with rapid adsorbate diffwion gives a unique, partially averaged 2-Dpowder pattern. When the channel morphology is idealized,the adsorbate surface diffusion constant may be inferred from simulated spectra. In summary, 2H NMR studies of labeled adsorbates on macroscopically oriented boehmite glasses give novel
Langmuir, Vol. 7, No. 8, 1991 1733 insights into substrateadsorbate interactions and the substrate morphology and local geometry.
Acknowledgment. We are indebted to 2.Luz and R. Poupko for modifying the simulation program for computing a motionally averaged 2-D powders to include effects of chemical shift anisotropy. This work was supported in part by Kao Corp. (Postdoctoral support for J.F.)and NSF (Instrumentation Grant CHE8821173).
