Langmuir 2003, 19, 1143-1147
1143
Noncovalent Assembly of Nitroxide Spin Labels in Laponite Films: Formation of One-Dimensional Heisenberg Antiferromagnets L. Chavez,† E. Bain,† Michael Eastman,*,† T. L. Porter,*,‡ and R. Parnell§ Departments of Chemistry, Physics & Astronomy, and Geology, Northern Arizona University, Flagstaff, Arizona 86011 Received August 16, 2002. In Final Form: November 20, 2002 Laponite is a synthetic layered silicate with a particle size of 25 nm and a basic chemical composition similar to that of the naturally occurring smectite clay hectorite. Aqueous solutions of Laponite and nitroxide spin labels derived from 2,2,6,6-tetramethyl-1-piperidinyloxyl form gels which when air-dried form shiny orange films. Phase contrast scanning force microscopy shows that the freshly prepared films have surfaces with islands of spin label embedded in the Laponite matrix. X-ray diffraction shows a spacing between clay layers of about 1.54 nm and an interstratified phase with spacings of 0.9 nm. Angle-dependent electron spin resonance of the films shows that the spin probes produce a signal whose line width varies as |3 cos2 θ - 1|4/3 where θ is the angle between the perpendicular to the plane of the film and the applied magnetic field. These line width studies and supporting line shape studies provide evidence for noncovalent assembly of nitroxide radicals in the Laponite film into structures which act as one-dimensional Heisenberg antiferromagnets. The films are metastable, and the antiferromagnet properties degrade considerably over about 60 days.
Introduction Laponite (TM Laporte plc) is a synthetic layered silicate with a particle size of 25 nm and a basic chemical composition similar to that of the naturally occurring smectite clay hectorite.1 Hectorite, with particle sizes on the order of 1 micron, is well-known for its ability to exchange a variety of ions into the interlayer region of the clay. In particular, it is possible to make films of hectorite which have organic cations such as the protonated spin label 4-amino-TEMPO (TEMPO, 2,2,6,6-tetramethyl-1piperidinyloxyl) (Figure 1) exchanged into the interlayer region and onto the surface of the clay.2-4 Electron spin resonance (ESR) of films, exchanged with protonated 4-amino-TEMPO at 5% of the clay cation exchange capacity (CEC) and maintained at low humidity (20% CEC) 4-amino-TEMPO+ loading levels yield a broad isotropic single line ESR spectrum whose width narrows with increasing radical concentration.4 In this high radical concentration region, the ESR line width appears to be dominated by Heisenberg spin exchange interactions which arise as a result of collisions between diffusing free radicals.4-6 * Corresponding authors. E-mail:
[email protected];
[email protected]. † Department of Chemistry. ‡ Department of Physics and Astronomy. § Department of Geology. (1) Eastman, M. P.; Porter, T. L. Polymerization of Organic Monomers and Biomolecules on Hectorite. In Polymer-Clay Nanocomposites; Beall, G. W., Ed.; John Wiley & Sons, Ltd.: Chichester, 2000; p 65. (2) McBride, M. B. Clays Clay Miner. 1977, 25, 6. (3) McBride, M. B. Clays Clay Miner. 1977, 25, 205. (4) McBride, M. B. Clays Clay Miner. 1979, 27, 97. (5) Eastman, M. P.; Kooser, R. G.; Das, M. R.; Freed, J. H. J. Chem. Phys. 1969, 51, 2690.
Figure 1. The structure of the nitroxide radicals used in experiments leading to the Laponite films.
The properties of aqueous suspensions of Laponite show evidence for fluid-solid phase transitions; the structure of these suspensions has been elegantly probed using spherical maghemite particles.7 Laponite suspensions are used in a wide variety of industrial applications and can be used to form films. Such films are conductive/antistatic and have barrier and antiblocking properties. Modification of film properties is obtained by including organic binders such as polyurethane and vinyl acetate along with appropriate wetting agents. (6) Eastman, M. P.; Bruno, G. V.; Freed, J. H. J. Chem. Phys. 1970, 52, 2511. (7) Cousin, F.; Cabuil, V.; Levitz, P. Langmuir 2002, 18, 1466.
10.1021/la020728o CCC: $25.00 © 2003 American Chemical Society Published on Web 01/17/2003
1144
Langmuir, Vol. 19, No. 4, 2003
Chavez et al.
In the course of investigating the properties of the films formed from Laponite and nitroxide spin labels, we found that certain Laponite/spin label systems are capable of forming unique films. The purpose of this paper is to report the preparation of these films and their properties as determined by ESR, scanning force microscopy (SFM), and X-ray diffraction. The results provide evidence for noncovalent assembly of nitroxide radicals in the Laponite film into structures capable of acting as one-dimensional Heisenberg antiferromagnets. Such magnetic behavior involving organic free radicals in a composite is unusual and complements previous studies of magnetic behavior in organic radical systems.8-10 Experimental Methods Laponite RD was supplied by Southern Clay Products, Inc. (Gonzales, TX), and the nitroxide free radicals 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), 4-amino-TEMPO, and 4-hydroxy-TEMPO were obtained from Aldrich Chemical (Milwaukee, WI). The structure of the nitroxide radicals is shown in Figure 1. The Laponite, 4-amino-TEMPO, 4-hydroxy-TEMPO, and TEMPO were used as received. To prepare a Laponite/nitroxide film, 1 g of Laponite was dispersed in approximately 50 mL of Nanopure water for approximately 20 min and then the nitroxide was added at a ratio of 10% by mass and the mixture was stirred for a period of 24 h. At the Laponite concentrations employed in these experiments, aqueous dissolution of Laponite is negligible.11 The resulting gel was poured into level polyethylene weighing boats in 5 mL portions that were allowed to dry overnight. The resulting films were relatively easy to remove from the weighing boats. Small changes ((1-2%) in the weight percent ratios for Laponite and spin label did not seem to affect the properties of the films that were formed. X-band ESR experiments were carried out on a Bruker ESP 300E (Bruker Instruments, Billerica, MA). Film samples were placed in a Wilmad glass ESR tissue cell for angle-dependent studies. The bronze clips ordinarily used to hold on the cover plates of the tissue cell were replaced by a small amount of rubber cement and Teflon tape. The purpose of this modification was to make it possible to tune the ESR at all orientations of the tissue cell. SFM imaging of Laponite films was carried out on a Park Scientific Instruments “Auto Probe” using standard noncontact and phase contrast techniques. X-ray diffraction (XRD) measurements were performed on the Laponite/nitroxide films as oriented mounts using a Siemens D-500 diffractometer. Sigma Scan (SPSS Science, Chicago, IL) image processing software was used to analyze the SFM phase contrast images described in the results section.
Results and Discussion In the case of the nitroxides 4-amino-TEMPO and 4-hydroxy-TEMPO, the Laponite/organic gel dried to transparent, shiny orange films of approximately 40 µm thickness. Films with diameters on the order of 5-8 cm were easy to prepare. Several attempts were made to prepare a film using the spin label TEMPO, but all were unsuccessful. When hectorite was substituted for Laponite in the synthetic procedure described above, warped and highly fractured films were formed. The size of the individual units in the fractured film was too small for experimental work. Thus, the small platelet size and (8) Nogami, T.; Ishida, T.; Yasui, M.; Iwasaki, F.; Takeda, N.; Ishikawa, M.; Kawakami, T.; Yamaguchi, K. Bull. Chem. Soc. Jpn. 1996, 69, 1841. (9) Togashi, K.; Imachi, R.; Katsuyuki, T.; Tsuboi, H.; Ishida, T.; Nogami, T.; Takeda, N.; Ishikawa, M. Bull. Chem. Soc. Jpn. 1996, 69, 2821. (10) Mito, M.; Takeda, K.; Mukai, K.; Azuma, N.; Gleiter, M. R.; Krieger, C.; Neugebaurer, F. A. J. Phys. Chem. B 1997, 101, 9517. (11) Thompson, D. W.; Butterworth, J. T. J. Colloid Interface Sci. 1992, 151, 236.
Figure 2. (a) A 2 × 2 µ SFM topographical image of a 4-aminoTEMPO Laponite film. (b) Phase contrast image of the same region.
discotic structure characteristic of Laponite appear to be important or necessary for film preparation. Films with protonated 4-amino-TEMPO located exchangeable cation sites in the interlayer region of the clay were easy to prepare using hectorite and published techniques but could not be prepared when Laponite was substituted for hectorite. Figure 2a shows a SFM topographical image of a 4-amino-TEMPO Laponite film, and in Figure 2b the corresponding phase contrast image is shown. The topographical image shows a relatively smooth surface made up of the 25 nm Laponite platelets or aggregates of platelets. In Figure 3, a 500 nm × 500 nm phase contrast image of the same film is shown. In a two-component system such as the Laponite/ nitroxide films, the interpretation of phase contrast images is relatively straightforward. In general, for systems such as this, light contrast in an image will represent areas of high material hardness or stiffness, while darker contrast areas will correspond to regions in which the material is softer. Thus, Figure 3 can be interpreted as a clay matrix surrounding tiny “islands” of organic material. The lateral dimensions of these irregular islands range from under
Assembly of Nitroxide Spin Labels in Laponite
Langmuir, Vol. 19, No. 4, 2003 1145
Figure 5. A comparison of the experimental line shape for a Laponite/4-amino-TEMPO film oriented at θ at 30° ((), 45° (2), and 105° (9) to a Lorentzian line shape.
Figure 3. A 500 × 500 nm phase contrast SFM image of the Laponite/4-amino-TEMPO film. Here, individual 25 nm Laponite platelets (arrows) are interspersed with small regions of organic material.
Figure 6. A comparison of the experimental line shape for a Laponite/4-amino-TEMPO film oriented at θ at 30° ((), 45° (2), and 105° (9) to a Gaussian line shape.
Figure 4. ESR line width as a function of the angle of the plane of a 4-amino-TEMPO/Laponite film with respect to the applied magnetic field (9). The least-squares fit to the experimental data is shown by the solid line.
5 nm to over 70 nm, with the average size being approximately 5 nm. For reference, in Figure 3 small arrows point to isolated Laponite platelets, whose dimensions are approximately 25 nm. The many larger light contrast regions in Figure 3 correspond to aggregates of Laponite in which the individual platelets cannot be resolved. Analysis of the phase contrast image shows that the organic islands cover approximately 30% of the exposed surface of the film. ESR provides a convenient method for analyzing the behavior of the paramagnetic organic molecules in the islands exhibited in the phase contrast images. The ESR spectrum of the film consisted of a single line; the ESR spectrum from a freshly prepared film of Laponite/4-aminoTEMPO oriented with its plane perpendicular to the applied magnetic field had an approximate width of 37 G, while the width of the line observed for the parallel orientation of the same film was approximately 24 G. The single line ESR spectrum indicates strong coupling between free radical sites in the film, and the anisotropy in the line width indicates that the radicals are strongly oriented in the film. To explore these aspects of the films more closely, we determined the ESR line width as a function of the angle (θ) formed by the plane of the film and the applied magnetic field. Figure 4 shows the line width data for a 4-amino-TEMPO/Laponite film as the angle was varied from 0° to 360° in 15° increments (9).
This dependence of the ESR line width on angle is characteristic of a one-dimensional Heisenberg antiferromagnet.12-14 Such antiferromagnets, for spin 1/2 systems, are sometimes found in single crystals of Cu2+ complexes where metal ions are arranged in linear chains with spacings in the range of 3-6 Å.13,14 Both theory and experiment show that the ESR line width can be described by equations of the form A|3 cos2(ω) - 1|4/3 where the angle ω is the angle between the field and the linear chain of spin 1/2 sites.12,13 Figure 4 shows that ω differs from θ by 90°; this indicates that the ferromagnetic ordering in the Laponite films arises from chains of spin 1/2 sites arranged perpendicular to the surface of the film. Figure 4 also shows the least-squares fit to the experimental data (solid line); the equation used to generate the least-squares curve is ∆H ) 8.5 Gauss|3 cos2(θ) - 1|4/3 + 11.8 Gauss where ∆H is the first derivative line width for the single line ESR spectrum. The nonzero intercept in the line width equation probably arises from incomplete averaging of hyperfine splittings for the 4-amino-TEMPO radical. An ESR line shape analysis of the single line spectrum as a function of angle has been carried out using techniques previously described.15 Figure 5 shows a comparison of the experimental line shape for a Laponite/4-aminoTEMPO film oriented at θ at 30°, 45°, and 105° to a Lorentzian line shape. Figure 6 shows the same type of comparison for a Gaussian line shape. In Figures 5 and (12) Dietz, R. E.; Merritt, F. R.; Dingle, R.; Hone, D.; Silbernagel, B. G.; Richards, P. M. Phys. Rev. Lett. 1971, 26, 1186. (13) Bartkowski, R. R.; Morosin, B. Phys. Rev. B 1972, 6, 4209. (14) Eastman, M. P.; Horng, M.-L.; Freiha, B.; Sheu, K. W. Liq. Cryst. 1987, 2, 223. (15) Eastman, M. P.; Hagerman, M. E.; Attuso, J. L.; Bain, E. D.; Porter, T. L. Clays Clay Miner. 1996, 446, 769.
1146
Langmuir, Vol. 19, No. 4, 2003
Chavez et al.
Figure 7. A schematic illustration showing nitroxide radicals with hydrogen bonding groups on the γ position incorporated into a Laponite matrix. Blue in this illustration represents hydrophobic methyl groups on the spin label, while yellow represents the hydrogen bonding -OH or NH2. The region of high unpaired spin density is represented in red. The gray disks represent the Laponite platelets.
6, deviations from the theoretical line shape are represented by the distance of the data points from the solid line. The units on the axes represent multiples of the half width of the first derivative line. Figures 5 and 6 show that at 30° and 45° the line shape is Lorentzian while at 105° the line shape is Gaussian. The theory of the ESR of one-dimensional Heisenberg antiferromagnets predicts that line shapes in the region of ω ) 55° (θ ) 35°) are Lorentzian and at ω ) 0° (θ ) 90°) the line shape is expected to be Gaussian or deviate slightly from Gaussian with a line shape determined by the Fourier transform of exp(-t3/2).12 Our ESR line shape data appear to be Gaussian for the Laponite/4-amino-TEMPO system at angles around ω ) 0° (θ ) 90°). Both the untreated and nitroxide-treated Laponite were analyzed by X-ray powder diffraction following techniques of Moore and Reynolds.16 The untreated Laponite sample produces an X-ray diffraction profile consistent with previous studies of this material.11,16 That is, the X-ray diffraction profile is consistent with a Mg-rich trioctahedral smectite with a single interlayer water layer consisting of hydrated sodium ions. The sample’s X-ray diffraction pattern presents a rational sequence of reflections based upon a 001 repeat spacing of 1.30 nm ((0.01 nm). The contribution of the interlayer of exchangeable hydrated sodium ions to the total spacing is approximately 0.3 nm. Powder X-ray diffraction analysis of the nitroxidesaturated Laponite reveals a significant change in the nature of the Laponite. The treated sample shows a nearrational sequence of reflections based upon a 001 repeat spacing of 1.54 nm ((0.06 nm). The X-ray diffraction pattern demonstrates that the treatment has expanded the Laponite interlayers by 0.24 nm per unit cell. Thus, an interlayer complex of 0.54-0.6 nm thickness is present in the Laponite/nitroxide composite. This repeat spacing, approximately 0.6 nm, is consistent with the thickness of the nitroxide molecule and suggests that intercalation of the Laponite by nitroxide molecules has occurred. The small deviation from an ideal rational sequence of reflections (i.e., the 002 reflection is not exactly half of the 001 reflection) indicates a small degree of interstratification with a second phase.16 On the basis of onedimensional computer modeling of the X-ray diffraction pattern using NEWMOD2.5, the second phase repeat spacing is between 0.9 and 1.0 nm. Between 10 and 20% of this second phase present in the Laponite sample adequately accounts for the near-rational sequence of (16) Moore, D. M.; Reynolds, R. C., Jr. X-ray Diffraction and the Identification and Analysis of Clay Minerals; Oxford University Press: New York, 1997.
reflections observed.16 This second phase is assigned to the islands of spin label observed in the SFM images. The SFM and ESR results can be interpreted in terms of a model where spin labels with two hydrogen bonding sites on the molecule self-assemble to form columns of radicals within individual islands; radicals within columns are antiferromagnetically coupled. The ordering of nitroxide radicals within individual columns must arise from noncovalent interactions, such as hydrogen bonding, dipole-dipole forces, and London dispersion forces, involving spin labels, water molecules, and Laponite platelets. Figure 7 presents a schematic illustration of our ideas concerning the structure of the Laponite/nitroxide films. In this figure, nitroxide radicals are included between clay particles giving an expanded interlayer spacing of 1.5 nm. Within the islands of spin label, hydrogen bonding between -OH or -NH2 groups γ to the N-O group on one nitroxide (yellow) and the paramagnetic N-O group on a neighboring nitroxide (red) aligns the nitroxides in one direction while dispersion forces between hydrophobic methyl groups on the R position of the nitroxide rings (blue) provide alignment in the perpendicular direction. These dispersion forces would also provide alignment down a column. The nitroxides are able to interact with the clay platelets through hydrogen bonding involving -OH, -NH2, or N-O. The TEMPO molecule, having no hydrogen bonding group γ to the N-O group, could not align in the way described. Thus, this compound would not be expected to form an antiferromagnetic composite. In solutions containing ions, nitroxide radicals can form micelles; such micelles represent a preassembly structure capable of contributing to the formation of islands of spin labels in the film.6 The density of Laponite (2.5 g/cm3) is substantially greater than that of nitroxides (0.8-0.9 g/mL); thus, clumps of nitroxide radicals might be expected to rise in a Laponite gel.17 Such a process might be capable of generating a column of radicals perpendicular to the surface of the film. As the Laponite/spin label systems age, the spectrum continues to consist of a single Lorentzian line but the anisotropy observed in the ESR of freshly prepared films slowly degrades, in a process that is nonlinear with time, over a period of approximately 60 days. This change is also reflected in the SFM phase contrast scans which show an eventual loss in the previous island/platelet structure and a transition to a more homogeneous mixture of organic/ inorganic components (Figure 8). (17) Aubrey, T.; Bossard, F.; Moan, M. Langmuir 2002, 18, 155.
Assembly of Nitroxide Spin Labels in Laponite
Figure 8. Α 2 × 2 µ SFM phase contrast image of a 4-aminoTEMPO/Laponite film approximately 3 months old. Owing to degradation effects, the microstructure of the clay film is lost on the topmost surface.
Conclusions The spin labels 4-amino-TEMPO and 4-hydroxyTEMPO form free-standing films with the synthetic clay Laponite at concentrations of 10 wt %, but the spin label TEMPO with no hydrogen bonding site in the 4 position does not. SFM phase contrast images show that freshly formed films have islands of spin label in the Laponite matrix; with time these islands begin to disperse in the
Langmuir, Vol. 19, No. 4, 2003 1147
matrix. ESR indicates that within these islands the spin labels are antiferromagnetically coupled and apparently arranged in stacks with the principal axis of the stack perpendicular to the plane of the film. Several workers have used susceptibility measurements to demonstrate antiferromagnetic coupling in solid samples of TEMPObased radicals and verdazyl radicals.8-10 In particular, single crystals of 3-nitro-1,5-diphenylverdazyl have the properties of a 1D Heisenberg antiferromagnet.10 Here neighboring molecules are arranged in columns with the verdazyl rings close to one another and the unpaired spins strongly coupled. Our data are consistent with columns of exchanged coupled nitroxide radicals arranged perpendicular to the surface of the composite film. The apparent spacing of clay layers in the composite is 1.5 nm with a second phase, accounting for approximately 1020% of the diffraction intensity, having a spacing of approximately 0.9-1.0 nm. It appears reasonable to assign this spacing to stacks of nitroxide radicals in the Laponite matrix. Present experiments are exploring the role of radical concentration, radical structure, and temperature on the antiferromagnetic behavior in these systems. The work is also being extended to organizing monomers in polymerizable organic systems. Acknowledgment. This work was supported by the National Science Foundation (DMR-9703840). We thank Southern Clay Products of Gonzales, TX, for a gift of Laponite. L.C. was funded by the Arnold and Mabel Beckman Foundation through a Beckman Scholars Grant to Northern Arizona University. LA020728O
