2808 Langmuir 1991, 7, 2808-2816

Study of Surface Properties of Clay Laponite Using Pyrene as a Photophysical Probe Molecule

Xinsheng Liu and J. Kerry Thomas'

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556

Received May 6,1991. In Final Form: June 24, 1991

Surface properties of the clay laponite, following thermal activation at different temperatures, were investigated by using photophysical and electron spin resonance techniques taking pyrene as a probe molecule. The results show that the surface properties of the laponite such as polarity, adsorption, and electron accepting ability change with change of preactivation temperature. The surface polarity increases in the temperature range of 100-350 OC, while above 440 OC, the polarity decreases. The adsorption capability of the clay for pyrene molecules increases dramatically at preactivation temperatures higher than 100 OC. Pyrene cation radicals start to form on the lateral surface of laponite at 115 O C . The effect increases linearly with temperature up to 350 "C, and then decreases dramatically as the temperature increases beyond 440 OC. The effects of various factors such as pyrene concentration and time of pre- activation on the formation of pyrene cation radicals, as well as the effects of solvent, 02, and water on the pyrene species present on the laponite surface, were also examined. At a constant temperature (200 OC), the amount of pyrene monomer cation radical formed increases with increasing pyrene concentration, and at high pyrene concentrations, pyrene dimer cation radicals are obtained.

Introduction

Clays have long been used as catalysts for organic and photochemical reactions,lV2 as adsorbents for purification and clarification of petroleum fraction^,^ as hosts for intercalation of various organic molecules,4 and as pre- cursors for pillaring of inorganic oligocations and clus- ters.3~5~8 Studies of the surface properties of clays are essential for an understanding of the functions the clay surface plays in these processes. Even though some research has been reported on the properties of active surfaces of ~ l a y s , ~ ~ ~ - 9 nevertheless the exact nature of the active sites on the clay surface and the modification of the active sites on physical and chemical treatments are still not clearly understood. Among many techniques to explore surface properties, photophysical and photochem- ical techniques using aromatic molecules as probes play important roles.2 Using photophysical and photochemical probes, surface properties such as polarity and the nature of active sites for electron charge transfer can be clarified. In the present study, attention is concentrated on changes of the surface properties of laponite, a synthetic clay with a similar structure to that of hectorite, a natural clay, on changing the preactivation temperature; pyrene is used as a probe molecule in these studies. Laponite is chosen as it has great structural regularity and a low level of impurities. Pyrene is used as a probe molecule due to its high quantum yield of fluorescence,lO its excimer-forming

(1) Theng, B. K. G . The Chemistry of Clay-Organic Reactions; Adam Higler: London, 1978.

(2) Thomas, J. K. Acc. Chem. Res. 1988,21,275-280and the references therein.

(3) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Mo- lecular Sieues; Academic Press: London, 1978.

(4) Thomas, J. M. Intercalation Chemistry; Academic Press: London- New York, 1982; p 55.

(5) Rozengart, M. I.; V'yunova, G. M.; Isagulyanta, G. V. Russ. Chem. Rev. (Engl. Transl.) 1988,57 (2), 115 and the references therein.

(6) Brindley, G. W.; Sempels, R. E. Clay Miner. 1977, 12,229. (7) Mortland, M. M.; Raman, K. W. Clays Clay Miner. 1968,16,393. (8) Solomon, D. H.; Swiff, J. D.; Murphy, A. J. J . Macromol. Sci.,

Chem. 1971,5,587. Solomon, D. H.; Loft, B. C.; Swiff, J. D. Clay Miner. 1968, 7, 387. Ibid. 389.

(9) Labbe, P.; Reverdy, G. Langmuir 1988, 4, 419-425. (IO) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic

Molecules, 2nd ed.; Academic Press: New York and London, 1971.

0743-7463/91/2407-2808$02.50/0

capacity,l1 and the great sensitivity of its photophysical properties to its su r r~und ings .~~J~ The study showed the following: (1) The surface of laponite changed its polarity on changing the preactivation temperature. (2) The surface of laponite activated at the temperature greater than 100 "C could act as an electron acceptor with pyrene. The phenomenon of electron transfer from pyrene mol- ecules to the surface of laponite disappeared when the preactivation temperature was over 440 "C. (3) The sites which accept electrons from pyrene molecules upon activation were mainly located on the lateral surface of the laponite particles. (4) At high pyrene concentration, pyrene dimer cation radicals formed on the surface. (5 ) Adsorbed pyrene molecules migrated and formed dimers on exposure of the samples to water.

Experimental Section Chemicals. Laponite RDS (Laporte Industries) was used as

received. The composition of the laponite is Si02,55.6%; MgO, 25.1%;Li~0,0.7%,Na~O,3.6%;K~O,0.2%;andFe~0~,0.04%.14 n-Pentane (Aldrich Product HPLC grade) was dried and purified by using molecular sieve 5A pallets. Pyrene crystals (Aldrich Product, 99%) were purified by column chromatography (silica gel and cyclohexane were used as adsorbent and eluant, respec- tively), followed by recrystallization from cyclohexane solution.

Preparation of Pyrene/Laponite Samples. Laponite pow- der (1 g) was heated at temperatures ranging from 20 to 650 O C

for 24 h, transferred into a desiccator, and allowed to cool. The pyrene-n-pentane solution with a concentration of 2 X 1V mol/L was prepared by dissolving purified pyrene crystals in n-pen- tane. A 5-mL porition of the solution was mixed with the activated laponite present in a glass vial immediately after cooling and capped. The suspension was shaken for several minutes and left in the dark for about 24 h. The solid was then separated from its supernatant by filtration and washed with n-pentane. The supernatant was collected for UV-visible spectrophotometric

(11) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-

(12) Kalyansundaram, K.; Thomas, J. K. J. Am. Chem. SOC. 1977,99, science: New York, 1970; p 301.

2039. (13) Kalyansundaram, K. Photochemistry in Microheterogeneous

Systems; Academic: New York, 1987. Thomas, J. K. Chemistry of Excitation oflnterfaces; ACS monograph Series 181; American Chemical Society: Washington DC, 1984.

(14) Larpote Industries Technical Brochure L64.

0 1991 American Chemical Society

Surface Properties of Clay Laponite Langmuir, Vol. 7, No. 11, 1991 2809

Tetrahedron

Tclrahrcfral shwt

OcbRedmn

Figure 1. Schematical structure of laponite.

measurement to estimate the amount of pyrene adsorbed on the clay. The final solid pyrene samples were used for the photo- physical measurements.

Instruments. The electron spin resonance (ESR) spectra were recorded on a Varian E-lines Century Series electron spin resonance spectrometer operated in the X-band frequency region. Quartz tubes were used for samples under vacuum. The tubes were connected to a vacuum line. X-ray diffraction patterns were collected on a Philips PW1710 X-ray diffractometer with Cu KO radiation operated at 40 kV and 30 mA. Infrared spectra were recorded on Perkin-Elmer Model 1600 Fourier transform infrared spectrophotometer, with the KBr wafer technique being used. Diffuse reflectance absorption measurements were carried out on a Perkin-Elmer 552 UV-visible double beam spectro- photometer with tungsten bromide and deuterium sources and a wavelength range of 190-750 nm with automatic source crossover at 315 nm. The machine is equipped with an integrating sphere reflectance unit for measurement of solid opaque samples. Steady-state emission and excitation measurements were mea- sured on an SLM SPE 5ooc fluorimeter with a 250-W xenon arc lamp, over a wavelength range of 200-900 nm with 0.2-nm accuracy, and a 0.1-nm minimum band-pass and a Hamamatsu R928P PMT.

Results and Discussion At this stage it is pertinent to summarize what is known

of the structure of laponite. Laponite is a synthetic clay with a layered structure similar to that of hectorite, a natural clay. The layer is composed of two Si04 tetrahedral sheets and one MgO6 octahedral sheet arranged in a TOT sandwich (T = Si04 tetrahedral sheet, and 0 = MgO6 octahedral sheet). The two Si04 tetrahedral sheets are inverted in the layer where each Si04 tetrahedron shares three oxygens with three other Si04 tetrahedra in the same sheet and with one oxygen of the MgO6 octahedron in the central octahedral sheet. Each MgO6 octahedron in the MgO6 sheet shares edges with other Mg06 octahedra in the same sheet in a manner reminiscent of a trioctahedral

\

/

0 0'- 0 . S I ' + M: Na+

on- 0 Mg'+or LI+

network.1s The Mg2+ ions in the octahedra may be partially substituted by Li+ ions, which provides the source of negative charge of the sheet. The negative charge produced by the substitution is compensated by Na+ cations located in the interlayer space. The interlayer space can also accept water molecules and as a consequence is expanded to an extent, depending on the actual conditions of hydration. In an aqueous solution, laponite layers may separate completely and are present in the €orm of individual layers. On drying, the layers aggregate together and form particles with three or four layers in a ~ t a c k . ' ~ J ~ The external surface of a laponite particle is built up of the outer surface of the stack which involves siloxane bonds, and the lateral surface of the layers which include broken and terminated Si-0 and Mg-0 and/or Li-0 bonds. Because of these differences, the lateral surface of the laponite ismore polar than the outer surface. The hydrated form of laponite exhibits an outer surface which is covered by water, while the lateral surface is covered by -OH and/or -0Na groups as well as water molecules. The interlayer space is filled by a monolayer of water molecules. Figure 1 shows schematically the structure of laponite with emphasis on the outer and lateral surfaces.

Table I gives the weight losses of laponite activated a t different temperatures and their corresponding percent- ages of uptake for pyrene molecules from a pyrene-pen- tane solution containing 1.0 X 10-6 mol of pyrene. I t is seen from Table I that, for the original air-dried laponite sample (20 "C), only 18% of the total amount of pyrene is adsorbed. Filtration of the sample shows that moat of the pyrene molecules are left in the supernatant phase. However, preactivation a t 115 "C dramatically increases

(16)Brindley, G. W., Brown, G., Eds. Crystal Structure of C h y Minerals and Their X-ray Diffraction; Mineralogical Society: London, 1980.

(16) Determined by using X-ray diffraction techniques.

2810 Langmuir, Vol. 7, No. 11, 1991

Table I. Weight LOSS of the Laponite at Different Preactivation Temperatures and Uptake for Pyrene

Molecules at a Concentration of 1.0 X lod mol/a

Liu and Thomas

~ ~ ~~~

uptake for preactivation weight pyrene, 5% of

temp, O C loss, YO the total in solution 20 18.0 115 8.8 99.0 205 10.8 99.4 350 11.2 99.5 440 12.5 99.3 480 12.5 99.2 650 14.3 96.4

the pyrene uptake of the laponite which reaches up to 98.9% of the total. At this temperature, the laponite loses 8.8% of ita weight compared to the original sample (see Table I). From X-ray diffraction studies, it is known that the original air-dried laponite has a basal spacing of 12.6 A, corresponding to a monolayer water coverage, and that the 115 "C activated sample does not show any significant change of basal spacing on weight loss. Infrared spec- troscopic studies of the framework vibrations also reveal that the structure of the laponite is not distorted on activation. Therefore, the amount of weight loss at this temperature mainly corresponds to removal of the water on the outer and lateral surfaces (physically adsorbed), and the water present in the outer coordination sphere of cations in the interlayer spacel'as well as the water created through dehydroxylation of OH groups on the lateral surface. The occurrence of dehydroxylation is proved by the presence of the electron accepting sites (see below).

The dramatic change of adsorption properties of the laponite on preactivation at 115 "0 indicates that a blocking of the interlayer space and a covering of the outer and lateral surfaces by water molecules prevent interac- tions of the pyrene molecules with the laponite surface. The low uptake of the original air-dried laponite for pyrene is a consequence of weak interactions of pyrene with the surface of laponite and with the adsorbed water.'*

Increasing the preactivation temperature (200-440 "C) leads to a further increase in uptake for pyrene molecules. Within experimental error, all the pyrene molecules present in the solution were taken up by the laponite, >99% (see Table I). In this range of temperature, the weight loss of the sample is increased from 10.8% at 205 O C to 12.5% at 440 "C. The further weight loss of 4% (for the sample activated at 440 "C compared to the sample activated at 115 "C) is considered to be due to water located in the interlayer space (the water coordinated to the Na+ cations) and that created through further dehydroxyla- tion. A t this stage, it is not possible to compare these samples according to their uptakes for the pyrene mol- ecules owing to the limited amount of pyrene used, but it is possible to distinguish them according to the change in yield of excimers (dimers) (see below). On further increase of preactivation temperature (from 480 to 650 "C), the uptake of pyrene by the laponite decreases (96.4% of the total amount of pyrene was taken up by the 650 "C activated laponite). This decrease is not due to further dehydroxylation of the surface (2% more of weight loss), but to the change of the interlayer space of the laponite. A t 650 "C, the basal spacing of laponite decreased to 9.6 A, which excludes adsorption of pyrene molecules in the interlayer space.

(17) Walker, G. F. In The X-ray Isentification and Crystal Structure of Clay Minerals; Brown, G., Ed.; Mineral Society: London, 1961; pp

(18) The Merck Index, 10th ed.; Merck and Co., Inc.: Rahway, NJ. 297-324.

111 I

250 350 450 550 650 750 Wavelength (nm)

Figure 2. Diffuse reflectance absorption spectra of pyrene on the laponites activated at different temperatures.

Table 11. Data Obtained from Diffuse Reflectance Absorption Spectra

preactivation temp, O C bands observed,' nm ~ a s o / ~ s 1 0

20 300 (sh), 315,330,350 (sh), 360 (sh), 370 5.5 115 300,315,330,350 (sh), 360 (sh), 370,450 2.3 205 300 (ah), 315,330,350 (sh), 360 (sh), 370, 2.5

350 300 (sh), 315,330,350 (ah), 360 (sh), 370, 2.7

440 300 (ah), 315,330,350 (sh), 360 (sh), 370, 3.1

480 300,315,330,350 (sh), 360 (ah), 370 4.8 650 300,315,330,350 (sh), 360 (ah), 370 4.4

450,470 (sh)

420,440 (sh), 450,460 (ah)

450

sh means shoulder.

The varied adsorption behavior of laponite for pyrene molecules on preactivation at different temperatures cor- responds to surface and structural changes of the lapo- nite. The surface and structural changes of the laponite on activation involve dehydration and dehydroxylation of the surface, and collape of the interlayer space.

Diffuse reflectance absorption spectra of pyrene on lapo- nite samples preactivated at different temperatures show the following features (see Figure 2): (1) overlapping of the bands at 330,315, and 300 nm; (2) appearance of three new bands at 370,360, and 350 nm (shoulder); (3) change in relative intensities of the bands at 370,360, and 350 nm with preactivation temperature; (4) appearrence of the absorption bands (450 nm) of pyrene cation radicallg for the samples activated at 115,205,350, and 440 "C; and (5) observation of the maximum intensity of the pyrene cation radical bands at 350 "C. Table I1 summaries the data obtained from the diffuse reflectance absorption spectra.

The broadening and overlapping of the spectral ab- sorption bands of pyrene on the laponite are due to the surface of laponite, and to the interactions between ad-

(19) Shida,T. Electronic AbsorptionSpectraofRadical Ione;Elsevier: Amsterdam, 1988; pp 85-86.

Surface Properties of Clay Laponite

5.0 1 0 . ~

Langmuir, Vol. 7, No. 11, 1991 2811

pentane solution are due to enhancement of the transition- forbidden bands of pyrene by the surface, and are not due to the microcrystalline pyrene. Pyrene microcrystals exhibit absorption at 360 nm,9 but in the present case, the fluorescence study of the samples excludes the assignment of this band to pyrene microcrystals. The change in the relative intensities of the absorption bands does not follow the change of the relative intensity of the excimer fluorescence (see below). The behavior of the fluoreecence intensity of the excimer (dimer) would parallel the absorption intensity of pyrene microcrystals if present.= However, the change of the relative intensities of the absorption bands follows the change of the surface polarities33 with preactivation temperature. The relative intensity ratios of the absorption bands, 1930/1370, for the samples are given in Table 11. The significant change of the 1330/1370 ratio occurred at 115 and 480 "C, indicating that significant changes of the surface properties of lapo- nite (polarity and active sites) have taken place at these temperatures. The behavior of pyrene cation radicals (the band at 450 nm) at these temperatures supporta this interpretation.

The formation of pyrene cation radicals on preactivated laponite (see Figure 2) suggests that, a t temperature as low as 115 "C, the surface of laponite already contains electron acceptor sites (Lewis acid sites). Because of the low content of impurities in the synthetic laponite, the electron acceptor sites (Lewis acid sites) are considered to be due to dehydration. A consideration of the structure of laponite helps to understand the location of the electron accepting sites. As indicated earlier, the external surface of laponite is composed of two components: the outer surface involved in the siloxane bonds and the lateral surface built up of the unsaturated Si-0 and Mg-0 bonds. When hydrated, the laponite surfaces are covered by water and/or OH groups. On activation, the outer surface is unaffected while the lateral surface changes due to de- hydroxylation. Studies of dehydroxylation of yA1203 surfaces by Knozinger and RatnasamyZ3 provide a model for events on the surface of laponite upon activation at different temperatures. Processes on the r-Al203 surface may resemble those on laponite, particularly on the lateral surface. For both outer and lateral surfaces, the Lewis acid sites are only created on the lateral surface on de- hydroxylation, as shown below:

l a

Wavelenglh (nm)

Figure 3. (a) Diffuse reflectance absorption spectrum of pyrene with different loadings on the laponites activated at 190 O C for 24 h. (b) Emission spectra of the samples in (a), excited at 370 nm.

sorbed pyrene molecules. The resolution of the absorption bands is improved when the amount of adsorbed pyrene is decreased. Figure 3a shows absorption spectra of lapo- nite samples with different amounts of adsorbed pyrene, of which the laponite had been preactivated at 190 "C for 24 h. The improved resolution of the bands in the spectrum of the sample with lower pyrene loading clearly demonstrates the marked effect of interactions between the pyrene molecules. A calculation of the average distance between two pyrene molecules on the laponite surface using the surface area of laponite (370 m2/g)I4 gives an average separation of two pyrene molecules of about 80 A. The broadening and overlapping of the adsorption bands in the spectra, therefore, suggest that the pyrene molecules are not uniformly distributed on the surface of laponite, but are clustered. The inhomogeneous distribution of pyrene molecules on the laponite surface is illustrated further by results obtained from dimer (excimer) formation as a function of pyrene concentration. A trace amount of pyrene dimer (excimer) can be observed even at concen- tration as low as mol/g (see Figure 3b). The inho- mogeneous distribution phenomenon of the aromatic molecules on the surface of the clays has been previously observed.mVz1

The absorption bands observed at 370,360, and 350 nm which are not observed in the spectrum of the pyrene-

(20) Nakamura, T.; Thomas, J. K. J. Phys. Chem. 1986,90,641. (21) Viaene, K.; Crutzen, M.; Kuniyama, B.; Schoonheydt R. A.; De

Schryver, F. C. h o g . Colloid Polym. Sci 1988,266, 242-246.

\ Ai 'OH

\ Ai / 'OH

In order to confirm the above proposal, the laponite was treated with an aqueous solution of sodium metapoly- phosphate. The metapolyphosphate molecules interact preferentially with the lateral surface of clays and prevent interactions between the lateral surfaces of individual clay particles.24 In comparison with untreated laponite sam- ples, these treated samples show a significant decrease in the intensities of the pyrene cation ESRsignals (see Figure

(22) Birks, J. B.; Kazzaz, A. A.; King, T. A. Proc. R. Soc. London, A

(23) Knozinger, H.; Ratnasamy, P. Catal. Rev.-Sci. E M . 1978, I7 (11,

(24) van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd

1966, A291,556.

31-70.

ed.; John Wiley and Sons: New York, 1977.

2812 Langmuir, Vol. 7, No. 11, 1991 Liu and Thomas

Table 111. Data Obtained from Emission Smctra

Figure 4. ESR spectra of pyrene cation radicals on laponite (a) and metapolyphoephate-treated laponite (b). Both laponitm were activated at 205 "C for 15 h before loadingpyrene molecules. For clarity, the spectra were recorded at the same instrumental conditions using a rather large value of modulation, 1 X 10 G.

.. -- -__

I I I I I 400 500 600

Wavelength (nm)

Figure 5. Emission spectra of pyrene on the laponites activated at different temperatures.

4). This confirms that the electron acceptor sites created on the surface of laponite are mainly located on lateral surfaces. The residual electron acceptor sites may be created by the grinding of the treated laponite samples prior to loading with pyrene.

at the same temperature, 115 "C, forms Lewis acid sites which are much weaker than those formed on laponite. This result is approved by the yield of pyrene cation radicals on the two surfaces, a higher activation temperature being required for ?-A1203 compared to laponite.

Fluorescence emission studies of the samples show emissions from monomeric pyrene and pyrene excimers (dimers) (Figure 5). The vibronic structure of the mon- omeric pyrene is sensitive to its environment and therefore provides information on surface polarity.2J1-1s The data obtained from the emission spectra are given in Table 111. The III/I ratio of pyrene (the intensity ratio of the vi-

However, dehydroxylation of

~ ~~~~~

(25) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991,95,6990.

preactivation temp, "C 11111 LlL" 20 0.95 0.42 115 0.88 0.14 205 0.81 0.12 350 0.74 0.21 440 1.15 0.23 480 1.31 0.31 650 1.00 0.53

bronic bands at 382 nm (111) and 372 nm (I)) on the surface of laponite decreases from 0.95 to 0.74 on changing the preactivation temperature from 20 to 350 "C. It then increases to a value greater than unity with a further increase of temperature. The change of the III/I ratio of pyrene fluorescence with preactivation temperature in- dicates a change in the nature of the surface on activation. Compared to that of the original sample where pyrene experiences a hydrated surface, the surface of laponite becomes more polar as the temperature increases. On dehydration and dehydroxylation of the laponite surface, adsorbed pyrene molecules experience increased contact with the uncovered laponite surface. The uncovered surface is more polar than that covered by water, and exhibits a lower III/I ratio for pyrene fluorescence. At preactivation temperatures over 440 "C where dehydration and dehydroxylation 88 well as changes of the surface are complete, the adsorbed pyrene molecules experience an environment which is significantly different from that at lower preactivation temperatures (see Table 111). The increase of the III/I ratio of pyrene fluorescence to about 1.1 indicates that the environment of pyrene molecules becomes less polar. The change of the III/I ratio of pyrene is significant and cannot be attributed to interactions between pyrene molecules.

The change in the laponite surface from polar to less polar can be understood on the basis of the difference between outer and lateral surfaces, and the changes in these surfaces on activation. At lower temperatures (C350 "C), changes in the surfaces are mainly dehydration and dehydroxylation. The exposure of the outer surface and the lateral surface on activation is reflected by changes of environment experienced by pyrene molecules and by the formation of pyrene cation radicals. When the preacti- vation temperature is higher than 440 "C, significant change occurs on the lateral surface, as exemplified by the disappearance of the absorption band of the pyrene cation radicals at 450 nm (see Figure 2). The lateral surface no longer acts as an electron acceptor with pyrene molecules, and behaves similarly to the outer surface. At this tem- perature, Na+ cations present on the outer surface (if any) should penetrate into the aperture of the six-membered ring of Si04 tetrahedra in the T sheet, away from interaction with pyrene molecules adsorbed on the surface. The framework vibrations of the laponite studied by infrared spectroscopy show corresponding band broad- ening and shifts toward higher frequencies, indicating changes in the Si-0 and Mg-0 (or Li-0) bond strengths in the layer of laponite. On the other hand, following the change of the structure, the collapse of the basal spacing expels the pyrene molecules occupying the interlayer space. As a consequence, the pyrene molecules are onIy located on the external surface. The observation of increased yield of excimers (dimers) on the sample activated at 650 "C confirms this conclusion (see Table 111). Because of the hydrophobicity of the outer ~ u r f a c e ~ ~ t ~ ' (with screening of

(26) Grauer, 2.; Avnir, D,; Yariv, S. Can. J. Chem. 1984,62, 1889. (27) Yariv, S.; C r a , H. Geochemistry of Colloid System; Springer-

Verlag: Berlin, 1979.

Surface Properties of Clay Laponite Langmuir, Vol. 7, No. 11, 1991 2813

I I " " I " " I " " I " ' ~ f

400 450 500 550 600 650

wwmlength (nm)

Figure 6. Emission spectra of the pyrene excimer (dimer) on the laponites activated at different temperatures.

cations) and the change in nature of the lateral surface, the III/I ratio of pyrene exhibits a value characteristic of a nonpolar surface.

Figure 5 and Table I11 also show the varying yield of pyrene excimers (dimers) formed on samples preactivated at different temperatures. At low temperature (20 "C), a larger amount of pyrene excimers (dimers) is formed owing to blocking of the interlayer space and covering of the external surface by water molecules. When the tem- perature increases (115-350 "C), the yield of pyrene ex- cimers decreases even though the total amount of pyrene adsorbed dramatically increases. This is due to dehy- dration of the interlayer space which enhances penetration of pyrene. As the temperature increases further (440-650 "C), the yield of pyrene excimers again increases. This corresponds to exclusion of pyrene molecules from the interlayer space. This type of behavior on preactivation is consistent with the conclusion drawn from absorption and pyrene monomer emission studies.

I t is pertinent to note that the broad excimer (dimer) emission band is composed of two overlapping bands centered at 480 and 500nm, and that the relative intensities of the two bands change with preactivation temperature (see Figure 6). For samples preactivated at 20 "C, the band maximum is at 500 nm, while those preactivated at higher temperatures exhibit a band maximum at 480 nm, and the 500-nm band appears as a shoulder. This phenomenon implies that two types of excimer are formed and that the environments experienced by these pyrene excimers (dimers) are not all the same. The maximum emission position of the pyrene excimer depends on the kinds of materials where the excimer is formed, i.e., on the environments the excimer experiences. Compared to the excimer emission of microcrystalline pyrene powder, the red shift of the maximum position of the excimer on several materials such as Si02 and A1203 is observed.34 A significant red shift of the pyrene excimer emission is also observed for the pyrene-containing Nay. At dry condition the maximum position is at 460 nm, whereas after adsorption of water, the position is shifted to about 510 nm.36 The changes of the maximum position and the relative intensities of the pyrene excimer present on the laponite in response to preactivation temperature reflect the changes of the surface properties of laponite.

Excitation spectra (Figure 7) of the samples (EM = 480 nm) show features of ground-state association of pyrene molecules. The red shift of the bands compared to the bands observed at EM = 397 nm and the characteristics

20% n

300 350 i wwilanglh

Figure 7. Excitation spectra of pyrene on the laponites activated at different temperatures.

of bands (no features of monomeric pyrene) suggest that excimers observed are present as ground-state dimers. The spectra also clearly show that different dimers are formed on surfaces preactivated at different temperatures. On the 20 "C preactivated sample, the spectrum exhibits bands at 313,327,350,356,374, and 397 nm. On preactivation at 115 and 205 "C, the spectra show better resolution with a shift of the band at 374 nm to 382 nm, and also a change of relative intensities of the bands. The band centered at 382 nm is now the most intense band at these preacti- vation temperatures. The 350 "C activated sample, however, gives a spectrum exhibiting the most significant red shift. The relative intensities of the bands at wave- lengths below 390 nm are decreased, and new bands at 414 and 420 nm appear (see Figure 7). This significant change of the spectrum reflects the dramatic change in the lapo- nite surface where the highest yields of pyrene cation radicals are reached (see Figure 2). When the preacti- vation temperature is over 440 "C, the excitation spectra of the samples (activated at 440,480, and 650 OC ) revert back to spectra similar to those of samples activated at 115 and 205 "C. The changes observed in the excitation spectra of these samples show that the surface change of the laponite on activation is also reflected in the pyrene dimers formed on the surfaces.

ESR studies of the above samples give direct proof for electron transfer from pyrene molecules to the laponite surface. The ESR spectra of samples preactivated at tem- peratures ranging from 115 to 650 "C show hyperfine structure, indicating coupling of the unpaired electron with the protons of the pyrene molecule. The g value of the pyrene cation radical is 2.0024. A typical spectrum is given in Figure 8. Figure 9 shows the change of the intensity of the ESR signal of pyrene cation radicals as a function of the preactivation temperature. On activation at tem- peratures below 350 "C, the intensityof the signal increases linearly. The rapid decrease of the signal at temperatures above 350 "C indicates a corresponding decrease in the electron accepting sites on the laponite surface. The observation of small ESR signals for the samples activated at higher temperatures (>480 "C) is due to the sensitivity of the technique;B diffuse reflectance absorption spec- troscopy gives no signal for these samples.

The formation of pyrene cation radicals as a function of time of preactivation at a temperature of 215 "c was

(28) Gordy, W. Theory and Applications of Electron Spin Resonance; John Wiley and Sone: New York, 1980.

2814 Langmuir, Vol. 7, No. 11, 1991

f 20 - L (D

. I

5

Liu and Thomas

EID

0-

Figure 8. Electron spin resonance spectrum ofthe pyrene cation radical on the surface of laponite activated at 350 O C .

Temperature ("C)

Figure 9. Change in intensity of ESR signal of pyrene cation radicals as a function of activation temperature.

= o 1 0 20 30 4 0 50 Pre-activation tlme ( h )

Figure 10. Change in intensity of the ESR signal of pyrene cation radicals on the surface of laponite preactivated at 215 OC as a function of time of activation.

also examined by using ESR spectroscopy in order to understand the process of the formation of electron accepting sites on the surface of laponite, and the data are shown in Figure 10. It is seen from Figure 10 that pyrene cation radicals are created a t the onset of activation. The yield reaches a maximum in about 8 h and then remains unchanged with further heating. These results indicate that formation of the electron accepting sites (Lewis acid

Pyrene adaorbed on iaponite (mol x E-6/g)

-

Figure 11. (a) Change in intensity of the ESR signal of pyrene cation radicals on the surface of laponite preactivated at 200 O C

for 24 h as a function of pyrene concentration. (b) ESR spectra of the samples in (a) with the order of increasing concentration of pyrene.

sites) is rapid and that an equilibrium is reached quickly. The rapid change of the surface on activation is also reflected by the rapid change of the surface polarities exemplified by the III/I ratio of the pyrene fluorescence. The ratio changes to 0.7 in 1 h and remains constant for the rest of the process.

Figure l la,b illustrates the formation of pyrene cation radicals on the laponite surface preactivated a t 200 OC for 24 h as a function of pyrene concentration using the ESR technique. Over the range of pyrene concentration used (from 10-7 to 10-4 mol/g adsorbed on the surface), the yield of pyrene cation radicals increases with increasing pyrene concentration and an equilibrium is not reached. Concomitantly with the increase of the ESR signal of pyrene cation radicals, the hyperfine structure of the spectrum becomes blurred (see Figure l lb). The change of the ESR spectrum with increasing pyrene concentration is understood using reflectance absorption spectroscopy

Surface Properties of Clay Laponite Langmuir, Vol. 7, No. 11, 1991 2815

Ail II

(29) Badger, B.; Brocklehurst, B. Trans. Faraday SOC. 1969,65,2588. (30) Kira, A.; Arai, S.; Imamura, M. J. Chem. Phys. 1971,54, 4890. (31) Rooney, J. J.; Pink, R. C. Trans. Faraday SOC. 1962,1632-1641.

Figure 13. Change of the ESR spectrum of pyrene cation radicals upon exposure of the sample to air.

350 450 550 650 750

Wavelength (nm)

Figure 12. Diffuse reflectance absorption spectra of pyrenel laponite samples in Figure Ila. The concentration of pyrene is increased in the order from bottom to top. The dashed line is for the background.

as shown in Figure 12. Here it is seen that the relative intensity of the pyrene monomer cation radicals (bands around 450 nm) increases with pyrene concentration and passes through a maximum at a concentration of about 1.1 X 10+ mol/g. This is accompanied by an increasing yield of pyrene dimer cation radicals having absorption bands at 510, 570, and 720 nm. By pulse radiolysis in benzonitrile, acetone, and dichloroethane solution, Badger et a1.B and Kira et al.30 have reported observations of pyrene dimer cations with absorption bands at 500,580, and 750 nm. The difference in the band positions of the laponite samples compared to those in the solution is due to the laponite surface. The obscurity of the ESR hy- perfine structure of the samples is the consequence of the formation of the pyrene dimer cation radicals. The absence of the equilibrium of pyrene cation radicals with increasing pyrene concentration, as shown by the ESR studies therefore reveals that preference of pyrene molecules for the electron accepting sites on the laponite surface does not occur because if the preferential occupation toward these sites were present, it would be expected that an equi- librium should be observed.

02, water, and solvent affect the pyrene species formed on the laponite surface. With pyrene cation radicals, introducing air or 02 causes a broadening and decrease of the ESR signals, suggesting interactions between pyrene cation radicals and 02 molecules (see Figure 13). Removal of oxygen reinstates the hyperfine structure of the pyrene cation radicals, showing that the process is reversible. A similar phenomenon has been observed by Rooney and Pink31 for perylene on silica-alumina.

Figure 14 shows the effects of pentane on the ESR signal of pyrene cation radicals. There is a difference in response of the ESR signals for the “wet” (with residual solvent) and “dried” (under vacuum) samples on changing the

0 20 40 60 80 100 120 140 Microwave power (mW)

Figure 14. Change in intensity of the ESR signal of pyrene cation radicals as a function of microwave power: (a) a sample containing residual pentane; (b) under vacuum.

microwave power. For the wet sample, the intensity of the ESR signals increases with increasing microwave power without saturation. However, for the same sample under vacuum, the spectrum starts to show saturation of the signals a t about 15 mW. The behavior of the pyrene cation radicals with respect to increasing microwave power cor- responds to changes of spin-lattice relaxation time28 (2’1) of the cation radicals. The saturation of the ESR signals indicates that the spin-lattice relaxation time of the sample under vacuum is longer compared to that of the sample with residual solvent. Interactions of the solvent molecule with pyrene cation radicals shorten ita spin-lattice re- laxation time significantly, and the saturation phenomenon is no longer observable.

Exposure of the samples to water causes migration of the adsorbed pyrene molecules on the surface (Figure 15), and the yield of pyrene excimers (dimers) increases. This is due to occupation of the surface by the polar water molecules, which tends to force the pyrene molecules together, forming dimers, a phenomenon which has been observed for the pyrene molecules on silica.32

Studies of the surface properties of laponite on thermal activation reveal that the surface of laponite changes

(32) Kraananeky, R. Ph.D. Thesis, University of Notre Dame, IN, 1990. (33) The concept of surface polarity here is defiied as the ability of

a solid surface to polarize the organic probe molecules such as pyrene adsorbed on it (surrounding effects on the probe molecules). Originally the concept is from the fact that, in solutions, the polarities of solventa have effecta on the photophysical properties of the organic probe moleculea di~solved.1~

(34) Bauer, R. K.;de Mayo, P.; Ware, W. R.; Wu, K. C.J.Phye. Chem. 1982,87, 3781.

(35) Iu, K. K.; Liu, X.; Thomas, J. K. Submitted to MRS Symposium, ACS Spring Meeting, California, 1991.

2816 Langmuir, Vol. 7, No. 11, 1991 Liu and Thomas

perature range, 115-440 "C. On adsorption of pyrene molecules on the laponite surface, electron-transfer re- actions occur, and as a consequence, pyrene monomer cations and pyrene dimer cations are formed depending on the pyrene concentration.

Conclusion From the present study, we conclude the following. (1) Due to dehydration and dehydroxylation, the

adsorption properties of the surface of laponite increases its capability for adsorbing pyrene molecules.

(2) The polarity of the surface changes on preactivation from polar to less polar, corresponding to a change in the nature of the lateral surface.

(3) The electron accepting ability is temperature-de- pendent and is observed in the range of 115-440 "C, indicating unstable nature and the specific location of these electron acceptor sites.

(4) At high pyrene concentration, pyrene dimer cation radicals are formed on the surface.

(5) The electron acceptor sites are mainly located on the lateral surface, in agreement with the location for these sites proposed in the (6) At 200 "C preactivation, the amount of the pyrene

cation radicals (monomeric and dimeric) formed on the surface increases with increasing pyrene concentration without an equilibrium being reached, indicating that the preferential occupation of the electron accepting sites by the adsorbed pyrene molecules does not occur.

(7) Solvent, oxygen, and water affect the pyrene species present on the surface due to various interactions of the species and to modification of the laponite surface.

Acknowledgment. We thank the Environmental Pro- tection Agency for support of this work.

W*v*!+nplh (nm)

Figure IS. Emission spectra of the pyrene/laponite samples before and after exposure to water vapor. The solid line is for the sample before exposure to water vapor, and the dashed line is for the sample after exposure to water vapor.

significantly with this process. Dehydration (mainly occurred at temperatures below 350 "C39 and dehydrox- ylation (mainly occurred at temperatures above 350 0C36) of the surface at different temperatures not only change the adsorption property and surface polarity of laponite, but also alter the number of the electron accepting sites on its surface. The electron accepting sites are mainly located on the lateral surface, as proved by the blocking experiment shown above. The decrease of the electron accepting sites at higher temperature as examplified by the dramatic decrease of the ESR signal of pyrene cation radical and by the disappearance of the absorption band at 450 nm indicates their inherent unstability. This kind of electron accepting sites is only created in a certain tem-

(38) Grim, R. E. Clay Minerology; McGraw-Hill Book Co., Inc: New York, 1953.