1319
Langrnuir 1989,5, 1319-1324
A Method Using Total Internal Reflection Microscopy and Radiation Pressure To Study Weak Interaction Forces of Particles Near Surfaces M.A. Brown,* A. L. Smith, and E. J. Staples Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW, England Received December 6,1988. In Final Form: April 18, 1989 An apparatus has been built which allows, by radiation pressure forces, the manipulation of a single colloid particle, immersed in water, close to a flat surface. The particle movement is monitored by total internal reflection microscopy (TIRM). Preliminary results are presented which illustrate the usefulness of the technique for studying the weak interactions between a 10-pm polystyrene latex particle and a quartz glass surface. An interaction potential energy curve as a function of the relative separation between the particle and the surface has been obtained.
Introduction The DLVO theory of colloidal interactions is well established and provides a robust framework for the rationalization of the interactions present in lyophobic colloids. Recently, direct, though macroscopic, measurement of the forces involved have confirmed the essential correctness of the theories that describe both the attractive (van der Waals) and repulsive (electrostatic) contribution to interparticle forces.’ Some aspects of colloidal stability and deposition are, however, mediated by more subtle features, e.g., depletion forces that cannot be measured by (current) macroscopic methods. While scattering and particle-counting techniques can characterize aggregation states and rates resulting from any interaction of consequence, interpretation of the results in terms of the interparticle potentials involved is very indirect.2~~ Recent work by Prieve et a1.4 has shown that total internal reflection microscopy (TIRM) can be employed to monitor the position of a particle undergoing diffusion near an interface and thereby characterize weak potential energy profiles between a particle and a surface. In this paper, we describe an extension of this technique that involves the manipulation of colloid particles by the use of radiation pressure forces.
Experimental Rationale The ability of radiation pressure forcessi6to manipulate particles affords several modifications to the TIRM experiment described by Prieve et al.4 A particle located “off-axis” in a focused laser beam will be driven toward the beam center if the particle refractive index exceeds that of the solvent; the particle will also experience a net force in the direction of radiation propagation. With polystyrene particles of order 10 pm immersed in water and a focused laser beam of -300 mW, radiation pressure forces can be such as to both significantly modify the potential energy experienced by (1) Israelachvili, J. N.; Adams, G. E. J. Chem. SOC., Faraday Trans.
1 1978.74.975.
(2). ’ C&ill, J.; Cummins, P. G.; Staples, E. J.; Thompson, L. G. Collords Surf. 1986,18, 189. (3) Lips, A.; Duckworth, R. M. J . Chem. SOC.,Faraday Trans. 1
1988,84,1223. (4) Prieve, D. C.; Luo, F.; Lanni, F. Faraday Discuss. Chem. SOC. 1987, 83, 297. ( 5 ) Ashkin, A. Sei. Am. 1972,226,63. (6) Ashkin, A. Phys. Rev. Lett. 1970, 24, 156.
0743-7463/89/2405-1319$01.50/0
the particle and resist weak solvent flows orthogonal to the laser beam. This latter feature makes it possible to vary ionic strength, surfactant concentration, etc., via solvent exchange while obtaining all TIRM observations of the one particle a t the same location on the plane surface.
Radiation Pressure The theory of electromagnetic radiation developed by Maxwell correctly predicted that radiation must possess momentum but suggested that the magnitude of the force created by momentum transfer was too small to usefully employ. The development of the laser confounded this latter view, and the work of Ashkin et al. demonstrated the use of radiation pressure forces in the manipulation of microscopic particles and atom^.^^^ While the Debye analysis of radiation pressure forces provides an exact approach, the geometrical optics analysis described by Ashkin5l6provides a better insight into the direction of the forces involved. This is especially true when nonuniform incident radiation is involved, as is the case with a particle located close to focus of a laser beam. In the ray optics approach, the momentum transfer associated with the Fresnel reflection and refraction that occurs a t an interface can be characterized by scattering vectors. The resulting force on a particle is therefore obtained by summation of these vectors over the entire interaction region of the wavefront with the particle. For example, the tendency for particles of high relative refractive index to migrate to the center of a laser beam can be seen to arise from a simple refraction process. While the ray optics approach does neglect the effects of diffraction, for large particles this process involves negligible momentum transfer, and as a result this method can provide an adequate estimate of the forces involved. The scattering from a large particle in an intense focused laser beam is such that the “far field” intensity pattern can be projected onto a screen. For highly spherical particles, the various refraction and interference processes produce a symmetrical band pattern that ermits very accurate determination of the particle size? For a particle with surface asperities, the scattering pattern will contain local detail that permits particle rotational diffusion to be monitored. (7) Ashkin, A.; Dziedzic, J. M. Phys. Reu. Lett. 1977, 38, 1351.
0 1989 American Chemical Society
1320 Langmuir, Vol. 5, No. 6, 1989
Brown et al.
In the work described here we have, however, relied on a direct measurement of the net radiation pressure force; i.e., we have determined the laser power required to support a particle of known diameter against the force of gravity. Under these circumstances, the particle was well separated from any surface yet within the laser focus defined by the confocal parameter. Total I n t e r n a l Reflection Microscopy Light traversing from a region of high to low refractive index will be totally internally reflected if the angle of incidence, 4, exceeds the critical angle, BC. BC = sin-’ (n2/n1) n, > n2
(1) where n, and n2 are the refractive indices. From Maxwell’s equations it can be shown that under these conditions there must exist a nonpropagating transmitted field in the low density medium:
E(h) = E(0) exp(-h/6) (2) The intensity thus varies exponentially with distance h away from the surface: I(h) = I(0)exp(-2h/6) (3) where 6, the “skin depth”, is dependent upon the angle of incidence and is given by8 6 = Xo/[27r(nl sin24 - n,)1/2]
As indicated earlier, the radiation pressure forces associated with a given laser flux ( P ) can be expressed as multiples of the flux necessary to just move a particle relative to the influence of gravity (Po). The energy associated with such radiation pressure forces can therefore be expressed as XRP = kXG(P/PO) (6) No exact analytical solutions exist for either the electrostatic or van der Waals contributions to the potential. For the case considered here, where the particle radius is much larger than the range of the potential, the electrostatic interactions can be approximated by following the method suggested by Derjaguin:” xEL
where R is defined as 1 6 e r ( Y 2 tanh
Characterization of the Interaction Energy In the experiment described here, we consider a particle close to a surface to be subject to the following influences: (1)gravity, (2) radiation pressure forces, (3) “double layer” electrostatic interactions, and (4) van der Waals attraction. The gravitational potential energy term for a particle a t a distance from a surface is XG = FGh with the gravitational force FG acting on the particle given by
FG= (4/3)ar3(&)g (5) where r is the particle radius, Ap is the relative density, and g is the acceleration due to gravity. (8) Klein, M. V. Optics; Wiley: New York, 1970. (9) Chew, H.; Wang,D.-S.; Kerker, M . Appl. Opt. 1979,18, 2679.
(7)
(3) (a) tanh
9, and q2are the Stern potentials of the particle and the plate, K = (S?rC,e2P/ekT)’f2, Z is the valence, C, is the ion concentration (ions/cm), e is the dielectric constant, k is the Boltzmann constant, and T is the absolute temperature. The van der Waals contribution can be approximated by
xv = -Ar/Gh
(4)
where A, is the wavelength of light in vacuo. The exponential decay of the “evanescent wave” can be detected if a second interface marking the boundary of a high refractive index medium is placed a t a distance just larger than the skin depth. Under these circumstances, the effects of multiple reflections are eliminated, and the transmitted intensity will “reflectnthe exponential decay. Chew et al.’ have obtained numerical solutions for the intensity of light scattered by a dielectric sphere placed in an evanescent wave. Their results indicate that the scattered intensity into any given solid angle also varies exponentially with separation from the surface (see eq 3). They assume that the boundary conditions a t the interfaces are identical with those found for the isolated surfaces, and therefore they suggest that their theory will apply only at large separations. The experimental work of Prieve et al.4 does, however, suggest that an exponential variation of scattered intensities with distance may well be preserved down to small separations.
= R exp(-Kh)
(8)
where A is the appropriate Hamaker constant for a polystyrene particle, immersed in water and interacting with a quartz surface. This expression ignores retardation effects and is, therefore, suspect a t large separations; however, for present purposes a more exact expression is not justified. The probability of a particle being a t a separation h from the surface, p ( h ) , can now be related to the total interaction potential between the particle and the surface via the Boltzmann expression: p(h) = exp(-x(h)/kT)
(9)
where ~ ( h =) XEL + xV+ XRP Normalization with respect to a reference potential x(hREF)gives x(h) - X ( ~ R E F ) = k T l n [ P ( ~ R E F ) / P ( ~ ) (10) ] It is possible to obtain estimates of p(h) from the TIRM spectrum if the evanescent wave scattering intensity is a known (monotonic) function of particle position. The probability of P(1)dl that the evanescent scattering is within the intensity I to I + dl is equal to the probability p(h)dh that the particle will be found between h and h dh of the surface:
+
p(h) = P(I)W/dh)
(11)
therefore
where the evanescent wave scattering intensity varies exponentially with distance h - REF = 6/2 In [ I ( ~ R E F ) / ~ ( ~ ) ] (13) While kinetic processes, such as surface-related hin(10) Verwey, E. J. W.; Overbeek, J . Th. G. Theory of the Stability
of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
Langmuir, Vol. 5, No. 6, 1989 1321
Weak Interaction Forces of Particles Near Surfaces
styrene latex particle in a 1 X 10"' mol dm-3 NaCl solution, the effect of radiation pressure force is seen to significantly modify both the position and the width of the potential minimum. If we assume that the interaction between the particle and the surface is dominated by electrostatic repulsion, we can employ the following expression for the potential:
kT
x(h) = XRP + €2 exp(-Kh)
(14)
Hence
+
x(h) = Fh R eXp(-Kh) (15) From the stationary point of this function, the position of maximum probability is found to vary logarithmically with radiation pressure force F (laser power). h = - In (F/RK)
(16)
Combining this expression with eq 3, we obtain
l0
t 0
100
200
3 00
nm.
Figure 1. (a, Top) Theoretical potential energy profile of a 10-pmpolystyrene particle immersed in 1X lo-' mol dm-3 NaCl solution as a function of separation from a quartz glass surface: potential a, +10F ; potential b, FG; potential c, -1OF,. (F, is the gravitational ?or,, exerted on the particle, see eq 5). (b, Bottom) Theoretical potential energy profile of a 10-pm polystyrene particle immersed in 1 X lo4 mol dm-3 NaCl solution as a function of separation from a quartz glass surface: potential a, +lOFG; potential b, FG.
dered diffusion, do not affect the equilibrium probability distribution p ( h ) ,it should be appreciated that the distribution is not sampled randomly. For this reason, the particle position has to be monitored for a considerable length of time before the evanescent wave scattering intensities can be converted into an unbiased estimate of the interaction potential between the particle and the surface. Further, characterizing a potential that is (only) 7kT greater than the minimum of the total interaction potential, ~ ( h involves ), a position probability ratio of 1:lOOO with respect to the most probable position. It can be appreciated that the ability of radiation pressure forces to locate the particle a t a fixed site makes such long experiments feasible while eliminating any potential modification that may arise from surface heterogeneity. The (theoretical) variation of the total potential energy with distance is shown in Figure la,b. For the situation depicted in Fi ure la, a 10-Fm polystyrene latex particle in 1 X 10- mol dm-3 NaCl solution, the radiation pressure force (expressed as a multiple of the gravitational force, FG)is seen to only affect the width of the potential profile; however, the possibility of removing the particle from the surface is demonstrated. (Note a "radiation pressure" force of lOF, is generated by an unfocused laser power of -150 mW with our optical configuration.) In Figure l b , which represents a 10-wm poly-
Q
I(m) = I(0)(F/RK)2/6K (17) where I(m) is the evanescent wave scattering intensity associated with the most probable particle position. A graph of In I ( m )vs In F will therefore produce a gradient 2/6K from which the decay constant of the potential can be obtained. The absolute value of the potential energy cannot be established as the distance of the particle from the surface is unknown. However, the changes in the potential energy a t any separation (h)that results from a change in radiation pressure force is given by AXRP= (413)rr3ApgAph/ PO (18) where Ap is the change in laser power and the selection of the origin (h = 0) is arbitrary. By the addition of such terms, the potential energy profiles collected over a range of radiation pressure forces can be modified such that their potential energy profiles are relevant to any selected radiation pressure force. However, as the minima in the original potential energy curves a t each radiation pressure force are set to zero, by definition the absolute values of the corrected potentials obtained a t separate radiation pressure forces remain unknown. Where the potential energy curves overlap, however, the addition of a position invariant term results in coincidence of the potential energies in the overlap region such that a composite curve with extended dynamic range is obtained. This convolution process was performed by a least-squares fitting procedure, and the parameters obtained result in an independent measure of the radiation pressure force. Data of very high quality must be obtained to reduce the cumulative errors, and while the positions of the potential minimum can be determined quickly and accurately, very good mechanical and laser stability are required if other parts of the potential profile are to be characterized. Experimental Section Figure 2 shows schematically the experimental setup. The apparatus essentially allowed for the manipulation of a single particle close to a quartz glass surface by application of radiation pressure forces from a tightly focused Ar+ laser, 514.5 nm (Lexel Model 85). The laser was capable of delivering -600 mW at maximum power. The particle movement induced could be followed by monitoring the scattering of the evanescent wave which was established at the glass/water interface by total internal reflection of s-polarized radiation from a He/Ne laser, 632.8
1322 Langmuir, Val. 5, No. 6,1989
Figure 2. Experimental apparatus: A, Ar laser (514.5 nm); B, He/Ne laser (632.8 nm); E, eyepiece (ocular); F, interference filter; G, Glan-Thompson prism; H, half-wave retardation plate, I, iris diaphragm; L, lens; M, mirror; 0,objective lens; SB, SoleilBabinet compensator; , vertically polarized laser radiation; 0,horizontally polarized laser radiation. nm (Spectra-Physics Model 120 Stabalite, 5 mW). The angle of incidence at the quartz glass/water interface was typically 62.40°, implying a “skin depth” 6 of 3.25 X nm. On the basis of polarization direction, the Ar+ laser radiation could be constrained to impinge on the particle from above or below the glass surface. Thus, radiation pressure forces could be used to drive the particle toward and away from the surface. A polarization rotator (Soleil-Babinet compensator) mounted on the Ar+ laser allowed the polarization of the light to be smoothly rotated between the vertical and horizontal planes. The polarized laser light incident on the Glan-Thompson prism, (a) in Figure 2, was split into two mutually orthogonal components such that the horizontally polarized component was reflected upward and the vertically polarized component passed through the prism. With the optical arrangement as shown in Figure 2, vertically polarized light leaving the Glan-Thompson prism could be aligned and focused on the particle from beneath, whereas light which was horizontally polarized focused on the particle from above, consequently minimizing the Ar+ laser radiation transmitted to the ocular. The inclusion of a X/2 mica retardation plate ensured that light directed from below the particle had the same p-polarization as that from above, consequently minimizing the Ar+ laser radiation transmitted to the ocular. In both cases, the beam waists at the focii were of comparable size to the 10-Hm polystyrene particles used. The confocal parameter of the objectives was not only longer than the evanescent skin depth but also much larger than distances over which colloid forces act. A 632.8-nm interference filter interposed between the Glan-Thompson prism, (b) in Figure 2, and the microscope ocular ensured that primarily light from particles scattering the evanescent wave was detected. A pinhole mounted in the ocular allowed for viewing of only one particle. This pinhole lay on the principal axis along which the Ar+ laser radiation could counterpropagate. A background due to plasma radiation from the Ar+ laser was removed by placing a 514.5nm interference filter in front of the laser. Laser radiation which was directed onto the particle from above was brought to a focus at a point X (see Figure 2) which was the same distance away from the particle as the focal plane of the objective. An iris centered at this point also provided an alignment criterion for the collinear nature of the counterpropagating beams. Light from the ocular was conducted via a fiber optic to a photomultiplier tube (EM1 9865B, 5-20 response). The subsequent signal was transferred with A/D conversion to a Hewlett Packard 9826 computer for data acquisition and analysis. The distribution of separations of the particle from the surface (TIRM spectrum) was displayed as a real-time histogram. The Ar+ laser power and peristaltic pump motion (see procedure) were also under computer control.
Brawn et al. The monodisperse 10-pm polystyrene latex particles (Dynospheres Inc.) immersed in water were contained in a 1-mmpathlength quartz spectrophotometer cell. The monodisperse latex particles, as supplied, were stabilized in a 5 X mol dm-3 sodium dodecyl sulfate (SDS) solution. The cell was optically coupled to a dove prism with glycerol, and the assembly was securely mounted on an X-Y translation stage. Particles could be introduced and aqueous solutions exchanged via PTFE tubing attached to a peristaltic pump. Narrow-bore stainless steel tubing connected the cell to the PTFE. Procedure. All solutions used were prepared from distilled water and filtered (0.22-pm pore size) to remove any residual particulates. Initially, the cell and PTFE tubing were carefully filled with water so as to avoid the inclusion of air bubbles. Polystyrene latex particles were introduced to the reservoir and pumped into the cell. When sufficient particles arrived in the field of view, the flow was stopped and the reservoir thoroughly cleaned and refilled with new distilled water. A particle was then translated into the center of the observation region defined by the pinhole in the ocular and “trapped” close to the lower glass surface by radiation pressure forces. Thus, at a sufficiently high laser power the particle could be held in the same position above the surface while aqueous solutions were exchanged. For a volume flow rate of 2 cm3 min-’, the duration of solution exchange was typically 300 s. This time was more than sufficient to allow mixing and complete flushing of the cell with new solution. During exchange, the position of the particle above the surface was monitored by measuring continuously the evanescent wave-scattering intensity, thus recording a TIRM spectrum. Once the particle was immersed in the solution required, ita variation in position above the surface at various laser intensities was determined. For a fixed laser intensity, a distribution of evanescent wave-scattering intensities (TIRM spectrum) was obtained over a 60-5 sampling time. These distributions were measured at a range of laser intensities up to a maximum unfocused power of 300 mW and cycled through the selected powers to check the reproducibility of the mean particle position at a given laser intensity.
-
Results and Discussion The flux densities involved in the radiation pressure aspects of this experiment are very high, and any absorption process may lead t o thermophoretic or even multiphoton processes. Thermophoretic motion may arise from absorption and consequent local heating of the particle surface, which results in momentum transfer to the solvent. Alternatively, the general direct heating of the solvent may establish a convective flow which again perturbs the mean particle position. It is obviously a prerequisite for the simple interpretation of TIRM spectra that thermophoresis must be negligible. We have established that this is the case with the 10-pm polystyrene latex particle and water system by showing that the TIRM spectra are invariant with laser power when equal and opposite radiation pressure forces are incident on the particle. Prolonged exposure of polystyrene latex to very high flux appears to result in multiphoton absorption. This results in a detectable fluorescence rapidly followed by an irreversible distortion of the particle as detected by the modification of the projected “far field” scattering pattern. In the absence of fluorescence, thermophoretic motion was never detected. Figure 3 shows a set of TIRM spectra obtained from a 10-pm polystyrene latex particle in water. Each “distribution” corresponds to a different intensity of the Ar+ laser with the optics configured such that radiation pressure forces push the particle toward the surface. In Figure 4, the log of the scattering intensity associated with the separation from the surface of maximum probability is plotted as a function of the log of the laser power. The associated gradient, which may be approxi-
Langmuir, Vol. 5, No. 6, 1989 1323
Weak Interaction Forces of Particles Near Surfaces
-
200 180 160
i40
m
120
2
100
t-
0
3 00
-
50
-
40
-
0
0 m
E
2 0 H
+
A
m
E
2 00
0
3 00
(arb.
1 0-4
units)
VI
+ C 3 Q i
m Y
t
+
H
ffl
z w t-
z
H
I
I
I
I
1
10 2
L A S E R POWER
(mW)
Figure 4. log of the evanescent wave scattering intensity associated with the separation from the surface of maximum probability as a function of the log of the laser power.
\
1.1
kT
X I 0
2
DISTANCE ( n m )
Figure 5. Convolved potential energy profile.
mated by 2 6 / ~is found to be 0.557, i.e., 1 / ~ = 171 nm, consistent with an ionic strength of 2.6 X lo* mol dm-3. mol dm-3 A similar analysis for the particle in 1 X NaCl solution resulted in a measured 1 K of 11 nm (theoretical value 9.2 nm) while in 1 X 10- mol dm-3 NaCl ~ 2.3 nm is obtained (theoretisolution a value for 1 / of cal value 2.9 nm). While the results are in reasonable agreement with the assumed potential (see eq 15), they do not constitute a very stringent test. Figure 5 represents the composite potential obtained from the overlapping data of Figure 3 when normalized
1
1 0-3
ELECTROLYTE
Figure 3. Distribution of evanescent wave-scattering intensities (TIRMspectra) obtained at different Ar+ laser intensities.
I
100 0
100
1
0
H
INTENSITY
60
200
Z
80 -
0
I
1
10-2
CONC.
10-1
(M)
Figure 6. Relative particle position as a function of electrolyte concentration (mol dm-3).
to a radiation pressure force equivalent to SF,. The solid line is a least-squares fit to the data obtained by using eq 15 from which F = 0.062 (lOF,) is obtained, and the effective 1 / is ~ found to be 140 nm. This decay constant is consistent with an ionic strength of 4.4 X lo* mol dm-3. Although the expected constants are not obtained, the assumptions implicit in eq 7 and 15 would appear justified in that the convolution procedure has resulted in a smooth potential energy profile. The variation of particle position with electrolyte addition is shown in Figure 6. The results obtained a t the various electrolyte compositions were collected by using only one particle, and it was found that the mean evanescent wave-scattering intensity associated with any given ionic strength could be reproduced accurately. This confirmed that the particle potential was not dependent on the adsorbed sodium dodecyl sulfate (SDS)present in the initial particle stock. All the particles examined were subject to thorough solvent exchange prior to observation; however, considerable variation in the effects of electrolyte was observed from particle to particle. In some cases, the addition of electrolyte in excess of 1 X lo-* mol dm-3 NaCl resulted in a very small change in the evanescent wave-scattering intensity. Subsequent solvent exchange to low electrolyte established that the particle could "escape" from the surface, confirming that the particle had been in a restricted primary minimum and not in direct contact with the surface. By applying radiation pressure forces to the particle such as to push it away from the surface, it was found that, while it is not possible to remove particles from the surface for ionic strengths exceeding 7 X mol dm-3 NaC1, the maximum separation of the particle from the surface varied significantly with electrolyte. We therefore conclude that the abrupt potential barrier that some particles experience on approach to the surface is steric in nature, possibly resulting from a diffuse polymeric layer that itself contributes negligibly to the evanescent wave scattering intensity. In contrast, some particles demonstrated a much "softer" interaction with the surface than predicted by eq 15. For example, in one case, the change in position with radiation pressure force was consistent with a value for 1 / of ~ 40 nm while the electrolyte concentration was 1 X mol dm-3, implying a 1 / value ~ of 9.2 nm. This is consistent with a low surface potential and a corresponding greater contribution of the attractive Hamaker term to the total interaction potential.
Langmuir 1989,5, 1324-1325
1324
Conclusions The preliminary observations described above serve to illustrate the sensitivity of TIRM technique for the study of the interactions between a particle and a surface. The manipulation of a particle close to a surface by the application of radiation pressure forces has been demonstrated to be a valuable extension to the TIRM technique. However, determination of the absolute separation of the particle from the surface still has to be established if a more complete analysis of the potentials obtained is to be performed. One way to establish the absolute separation of the particle from the surface is to subject the particle to a force that varies with distance
in a defined manner. Lubrication theory as developed by Brenner" indicates that the frictional force of a spherical particle moving orthogonal to a surface increases monotonically as the separation is decreased, The transient response of a particle subject to an instantaneous change in radiation pressure force is dependent on the measured gradient and this position-dependent frittional term, The results of such an analysis are to be the subject of a future publication. Registry No. Vitreous silica, 60676-86-0; polystyrene, 900353-6. _ _ -. (11) Brenner, H. Chem. Eng. Sei. 1961, 16, 242.
NMR Analysis of Chemisorbed Ligands for Isomeric Products: The p-SulfonylbenzylGroup on Silica Gel D. Slotfeldt-Ellingsen,f*tH. A. Resing,**+K. Unger,§ and J. F r y e l Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000, Fachbereich Chemie, Johannes Gutenberg Universitaet, 0-6500 Mainz, FRG, and Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received July 22, 1988. In Final Form: May 24, 1989 13C NMR spectra (cross-polarization magic angle spinning) have been recorded for the benzylsilyl derivative of silica gel and for its chlorosulfonated derivative. Resolution is sufficient to provide for the latter a four-line spectrum in the aromatic region with intensities and chemical shifts as expected for the sulfonyl group substituted para to the methylene group. The product of a surface reaction has thus been analyzed for its isomeric content.
The CPMAS (cross-polarization magic angle spinning) technique of carbon-13 NMR (nuclear magnetic resThe onance) has proven its utility in surface surface of silica gel can react to provide a surface-modified, chemically significant derivative;' the new surface functional group may itself be modified.' Such a preparative sequence may be used to provide specific reagents or even catalysts useful in the cleanup of air streams through contact of a gas stream with the porous modified adsorbent.6 In this note, it is shown by the CPMAS technique that when silica gel with attached benzyl groups
* Deceased. Address correspondence to: A. N. Garroway, Naval Research Laboratory, Chemistry Division, Code 6122, Washington, D.C. 20375-5000. Naval Research Laboratory. Current address: Center for Industrial Research, P.O. Box 124 Blindern, 0314 Oslo 3, Norway. Johannes Gutenberg Universitaet. Colorado State University. (1) Chang, J. J.; Pines, A.; Fripiat, J. J.; Resing, H. A. Surf. Sci.
*
1975, 47, 661.
(2) Resing, H. A.; Sloffeldt-Ellingsen, D.; Garroway, A. N.; Weber, D. C.; Pinnavaia, T. J.; Unger, K. Magnetic Resonance in Colloid and Interface Science; Fraissard, J., Resing, H. A., Eds.; Reidel: Dordrecht, 1980; p 239. (3) Shoemaker, R. K.; Apple, T. M. J.Phys. Chem. 1985,89,3185. 1986, 108, (4) Bronnimann, C. E.; Maciel, G. E. J.Am. Chem. SOC. 7154. (5) Unger, K.; Berg, K.; Nyamah, D.; Lothe, Th. Colloid Polym. Sci. 1974, 252, 317. (6) Carhart, H., personal communication. (7) Stewart, J. J. Chem. SOC.1922, 121, 2556.
is sulfonated,' sulfonation only occurs para to the methylene surface link. Sulfonation of toluene itself with chlorosulfonic acid yields a mixture of isomers;' thus the surface reaction might be expected to do the same. To show by other means that only the para isomer is formed would not be as straightforward. The preparation of the surface derivative has been described.' After reaction, the final concentration of the benzyl moiety was 4.1 pmol/m2; the BET area was 211 m2/g. Sulfonation with chlorosulfonic acid proceeds with little loss of organic ligand.' Spectra were recorded at NRL at 1.4 T and/or a t 3.5 T a t CSU. Experimental parameters are given in the figure captions. Spectra for the benzyl derivative with and without magic angle spinning are shown in Figure 1. A spectrum for the sulfonated benzyl derivative is given in Figure 2. Spectra similar to spectra c and d of Figure 1 have been analyzed earlier in terms of motion and orientation of the phenyl ring of the benzyl group.s For the purposes of this paper, they confirm the presence of both aromatic and aliphatic carbon in the benzyl derivative. Spectrum b of Figure 1, the high-resolution CPMAS spectrum, again shows these two contributions: aromatic (around 130 ppm) and aliphatic (at 21.1 ppm). Spectrum b of Figure 1could be simulated rather well by the 1:1:4:1 spectrum of shown as a, where the line a t 138 ppm is assigned to the carbon atom attached to the methyl~~
~~~
~~~
(8) Slotfeldt-Ellingsen, D.; Resing, H. A. J. Phys. Chem. 1980, 84,
2204.
0 1989 American Chemical Societv
