Langmuir 1988,4, 419-425
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Adsorption Characteristics of Polycyclic Aromatic Compounds on Clay: Pyrene as a Photophysical Probe on Laponite Pierre Labb6 and Gilbert Reverdy" Laboratoire de Photochimie, Universite' de Savoie, Faculte' des Sciences et des Techniques, B.P. 1104, 73011 Chambe'ry Cddex, France Received September 28, 1987 Pyrene is used as a fluorescence probe to study and to characterize the adsorption of polycyclic aromatic hydrocarbons on the surface of a smectite clay: laponite. The evolution of absorption and fluorescence spectra during air drying of a water-filled laponite paste containing a dispersion of pyrene microcrystals shows the destruction of these microcrystals in favor of adsorption of monomolecular pyrene. The maximum level of adsorption is in the region of 1%(w/w). The phenomenon is reversible: adding water to the dry film causes swelling of the clay and regeneration of microcrystals. The phenomenon of adsorption results from the considerable pressure forces involved when, in the final stage of drying, aggregates resulting from the assembling of elementary particles are ordered into a film. An identical phenomenon is obtained by dry crushing of a mixture of powdered laponite and pyrene crystals. On the other hand, evaporation of a methanol solution of pyrene in contact with laponite leads to low levels of adsorption (0.01%). The value 0.90 for the I373/1383 ratio of the relative intensities of the 373-nm and 383-nm vibronic baqds of pyrene emission spectra shows that the adsorbed molecules are situated in a weakly polar environment. This result concords with adsorption of pyrene on the hydrophobic siloxane planes of clay particles, by van der Walls interactions. The ratio value also shows that extensive dehydration of the clay places the pyrene in a highly polar medium (1373/13@ = 1.80),which certainly results from a partial desolvation of the exchangeable Na+ cation, which is present on the siloxane planes of the surface.
Introduction Much is known about the interlamellar adsorption of uncharged polar organic molecules such as alcohols, amines, etc., by clay minerals.l The intercalation of such molecules is essentially a process in which the organic species penetrate the interlamellar space and replace the water associated with the exchangeable interlayer cations. Little is known, on the other hand, about adsorption of nonpolar molecules, which are unable to replace the associated water. When montmorillonite is exposed to vapor or suspended in liquid n-hexane or n-dodecane, after extended contact, the aliphatic hydrocarbons insert themselves between the interlayer space.2 The adsorption is explained in terms of dipole-induced dipole interaction; that is, the electrostatic field a t the clay surface induces dipoles in the n-alkane molecule. Vapors of benzene, toluene, xylenes, and mesitylene are intercalated a t a very slow rate by montmorillonite saturated with copper(I1) At room temperature and 40% relative humidity, on the basis of infrared evidences, it is shown that benzene is physically adsorbed on the silicate s u r f a ~ e . ~When ,~ montmorillonite is partially dehydrated, benzene-copper(I1) coordination complexes are formed. In what follows, we shall show that the laponite, a smectite clay, is able to adsorb effectively on its surface pyrene in a monomolecular layer from microcrystals without it being necessary to make them soluble. We shall also show that with air-dried clay pyrene molecules will fix on the hydrophobic siloxanic oxygen planes which make up the surface of the clay particles. After extensive dehydration of clay film,adsorbed pyrene is exposed to a very polar medium. This probably results from a partial desolvation of the exchangeable Na+ cations, which are closed to adsorbed pyrene on the siloxanic planes of the surface.
The adsorption of polycyclic aromatic hydrocarbons on mineral solids is an important phenomenon in the trapping, dispersion, and chemical or photochemical transformations of these compounds in a natural medium.6 By its structure, clay should also play an important part in these areas. Laponite is a synthetic clay whose structure is similar to that of hectorite, a natural clay. I t was chosen as the model surface because it presents the advantage of having great structural regularity and of containing a low level of impurities. Its structure is derived from those of talc.'^^ This neutral mineral results from the packing of layers in which a central sheet of Q2- ions and OH- ions delimit octahedral sites occupied by Mg2+ions (Figure 1). This central sheet is sandwiched between two inverted sheets of tetrahedral silicates. The outer surface of these layers is thus made up of oxygen atoms which are involved in siloxane bonds. Hydroxyl groups are present on the edges of the particle, where the primary bonds are b r ~ k e n In .~ natural hectorite or synthetic laponite, substitution of Li+ ions for Mg2+ions is the source of the negative charge of layers. The excess negative charge is compensated by adsorption a t the surface of the layers of cations, Na+ in laponite, which are too large to penetrate inside the crystalline network where the negative charges are to be found. It is a property of these cations that they can very easily exchange with other mineral or organic cations.' The cation-exchange capacity of the laponite is 0.8 meq/g.g Elementary layers of laponite are parallelepipedal in shape, measuring 40 nm X 10 nm X 1 nm.lo Their flat surface can be estimated a t 750 m2/g (750 m2 is the ideal value for 1 g of an infinite layer 9.6 A t h i ~ k ) .The ~ experimental determination by the BET method of the total outer surface (flat outer surfaces and edges surfaces) of the
(1) Theng, B. K. G . The Chemistry of Clay Organic Reaction; Adam Hilger: London, 1974. (2) Eltantawy, I. M.; Arnold, P. W. Nature Phys. Sci. 1972,237, 123. (3) Doner, H. E.; Mortland, M. M. Science (Washington, D.C.) 1969, 166, 1406. (4) Mortland, M. M.; Pinnavaia, T. J. Nature Phys. Sci. 1971,229,75. (5) Pinnavaia, T. J.; Mortland, M. M. J.Phys. Chem. 1971, 75, 3957.
(6) Sims, R. C.; Overcash, M. R. Residue Reuiews 1983, 88, 1-68. (7) Van Oluhen, H. An Introduction to Clav Colloid Chemistry; Wiley-Interscience: New York, 1977. (8) Grim, R. E. Clay Mineralogy, 2nd ed.; MacGraw-Hill: San Francisco, 1968; p.86. (9) Van Olphen, H.; Fripiat, J. J. Data Handbook for Clay Materials and Other Non Metallic Minerals; Pergamon: 1979. (10) Neumann, B. S.; Sansom, K. G. Isr. J. Chem. 1970, 8, 315.
0743-7463/88/2404-0419$01.50/0
0 1988 American Chemical Society
420 Langmuir, Vol. 4, No. 2, 1988
Labbd and Reverdy
G7
TETRAHEDRAL
M"+x H,O
SHEET
OCTAHEDRAL SHEET
Mn?x H,O
--..
TETRAHEDRAL
.
SHEET
0 :0,-
o:
si4+
0 : OH-
o:
~ g ' + o r Li+
Figure 1. Schematic representation of the oxygen and hydroxyl network in a hectorite layer (adapted from ref 8). The nonexchangeable cations within the octahedralsites inside the layer have been omitted for clarity. M"+,xHZOis the hydrated exchangeable cation (Na+,rH,O in laponite).
particles which make up the grains of the powdered laponite leads to the value 350 m2/g.l0 From these calculated and experimental values it is possible to deduce that these particles are composed on average of three elementary layers. The edge surface represents 100 m2Jg,the flat outer surface 250 m2/g, and the interlayer surface 500 m2/g. The laponite particles can be expanded by adsorbing polar molecules, such as water, in the interlamellar space. When suspended in water, a t concentration 1 1 0 g/L, complete delamination process of the layers stacked together occurs. The colloidal dispersion contains elementary layers which do not interact strong1y.l' With concentration in the range 10-200 g/L gelation occurs because of strong interactions between the negative layers." At higher concentration, 200-400 g/L, the dispersion is incomplete and the system presents a pastelike appearance. As far as the adsorption properties of laponite are concerned, several surface sites seem at first able to constitute adsorption centers: oxygen of siloxane bonds, silanol groups, and Na+ counterions. We have chosen pyrene as our model aromatic compound because it presents some interesting fluorescence properties for carrying out the present study. Its high quantic yield of fluorescence offers a powerful technique for studying its adsorption. Ita crystals only present an excimer type of fluorescence;12monomeric emission thus allows the presence of its monomolecular form to be unambigously detected. Its dynamic excimer is strongly f l u ~ r e s c e n t . ' ~ JObservation ~ of this excimer can reveal mobility of the excited state of the adsorbed molecules. The vibrational structure of its monomolecular emission is strongly dependent on the polarity of the surrounding medium.15J6 This property will make it possible to (11) Avery, R.G.;Ramsay, J. D. F. J. Colloid Interface Sci. 1986,109, 448. (12) Birks, J. B.; Kazzaz, A. A.; Kina, T. A. Proc. R . Soc. London, A 1966, A 291, 556. (13) Forster, T.;Kasper, K. 2.Phys. Chem. (Munich) 1954, I , 275. (14) Birks, J. B. Photonhvsics of' Aromatic Molecules: Wilev-Interscience: New York, 1970-p -301. (15) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. Soc. 1977,99, 2039.
characterize the adsorption sites. Pyrene has already been used to study the adsorption sites of various solid surfaces: silica," zeolites,le and p01ymers.l~
Experimental Section Products and Conditions of Use. Pyrene, from Aldrich, was
purified by chromatography on alumina and then recrystallized twice in ethanol. Laponite was obtained from Laporte Industries and was used as received. Water WBS purified and deionized, just before use, by being passed through four Bioblock columnsplaced in series: first a column of active carbon, then two columns of ion-exchangingresin, and finally a column of macroreticulate resin. Filtering through a 0.45-pm porosity membrane completed preparation. Methanol Lichrosolv from Merck was used without further purification. Preparation of Samples of Pyrene Adsorbed on Laponite. The colloidal suspensions of laponite used in this study are of 1g of air-dried clay/L of water. The level of pyrene is expressed as a weight percentage in relation to the air-dried clay. 1. Clay Pastes and Air-Dried Clay Films. The various samplesprepared contain 0.5%, 1%,and 2% pyrene. Suspensions of microcrystalline pyrene are prepared by adding respectively 125 pL of 2 mg/mL, 125 pL of 4 mg/mL, and 250 pL of 4 mg/mL pyrene methanolic solutionto 50 mL of a 1g/L colloidalaqueous suspension of laponite under high-speed stirring. After filtering on a cellulose acetate membrane with a porosity of 0.05 pm under a pressure of 106 Pa, a paste is obtained,in which the microcrystals of pyrene are trapped. This paste is spread on a quartz window; as it dries in the surrounding air, a film is formed which adheres to the quartz window. In the absence of pyrene, paste and airdried film display 100% transmission of light above 300 nm (Figure 4, spectrum c). 2. Evaporating a Methanol Solution of Pyrene in Contact with the Laponite. Samples containing0.01%, 0.1%, and 0.5% of pyrene were prepared. Laponite, 500 mg, is placed in a suspension in 50 mL of methanol containing the necessary quantity of pyrene. The solvent is evaporated in a vacuum by means of a rotary evaporator. Traces of solvent are eliminated by placing the powder in a vacuum of 1.3 Pa for 6 h. The powder is studied as soon as it is placed in the open air. (16) Dong, D. C.;Winnik, M. A. Photochem. Photobiol. 1982,35, 17. (17) Bauer, R. K.; De Mayo, P.; Ware, W. R.; Wu, K. C. J . Phys. Chem. 1982,86, 3781. (18) Suib, S.L.;Kostapapas, A. J. Am. Chem. SOC.1984, 106, 7705. (19) Rice, M. R.; Gold, H. S. Anal. Chim. Acta 1984, 164, 111.
Langmuir, Vol. 4 , No. 2, 1988 421
Pyrene as a Photophysical Probe on Laponite
W
0
z 4
m a
0
m
m
4
200
300
400
A nm Figure 2. Absorption spectra of pyrene in (a) laponite paste loaded with 0.5% microcrystalline pyrene, (b) aqueous suspension of colloidal laponite (1g/L), and microcrystalline pyrene (5 mg/L), and (c) aqueous solution of pyrene, 4 X lo-' M. (The signal has been expanded by a factor 10 for clarity). EXCITATION
EMISSION
hem = 480 nm
Figure 4. Evolution of the absorptionspectrum of a clay paste loaded with 0.5% microcrystalline pyrene during the last 15 min of 2 h of air drying: (a) first spectrum (b) last spectrum, (c) absorption spectrum of clay paste without microcrystallinepyrene. to study the fluorescence of samples in a vacuum down to 1.3 X Pa.
hem= 373nm
C
I
300
0
400
500
h nm Figure 3. Fluorescence and excitation spectra of pyrene in (a) laponite paste loaded with 0.5% microcrystalline pyrene, (b) aqueous suspension of colloidal laponite (1g/L) and microcrystalline pyrene (5 mg/L), and (c) aqueous solution of pyrene, 4 x 10-7 M. 3. Dry Crushing of a Laponite and Crystalline Pyrene Mixture. Samples containing0.5%,1%,and 2% pyrene were prepared by manually crushing 2.5, 5, and 10 mg of crystalline pyrene, respectively, and 500 mg of air-dried laponite mixture in an agate mortar. Apparatus and Measurements. Absorption spectra were recorded on a UV 240 Shimadzu spectrophotometer. The steady-statefluorescence data were recorded on a Perkin-Elmer LS-5 spectrofluorimeterequipped with a cell holder specially designed to hold the cell at a 45' angle to the incident excitation light beam. The nominal bandwidth is 2.5 nm. The recording speed is 30 nm/min. Studyingthe evolution of fiis during drying requires a high recording speed (240 nm/min). Experiments under vacuum degassing were carried out with a home-made air-tight cell, equipped with a QZS quartz window which makes it possible
Results and Interpretation Pyrene-Loaded Clay Pastes and Clay Films. The suspensions of microcrystals are produced by dilution of an alcoholic pyrene solution into water. By this technique, in the presence of surfactant, pyrene colloids with a diameter of 0.12 pm have been stabilized.20 In absence of surfactant the small particles agglomerate and precipitate. By immediately filtering the particles on membranes with different porosities we observed that particles have diameters larger than 0.05 vm and smaller than 0.45 vm. Ten minutes after the preparation, the particles have diameters smaller than 0.65 vm. In our experimental conditions, pyrene suspensions containing the clay colloid are filtered in 10 min. Consequently, microcrystals with diameters 0.05-0.65 wm must be trapped in the clay paste. 1. Clay Paste with 0.5% Pyrene. Figure 2 shows the absorption spectra of a clay paste loaded with 0.5% pyrene (spectrum a), of an aqueous suspension of colloidal laponite (1g/L) and microcrystalline pyrene (5 mg/L) (spectrum b), and of an aqueous solution of pyrene (4 X M) (spectrum c). The presence in the paste's spectrum of a broad red-shifted band a t A,, = 360 nm indicates that pyrene is still present in microcrystalline form.12p21-22The fluorescence emission and excitation spectra of the same preparations confirm this (Figure 3). The emission spectra of the paste (spectrum a) and of the aqueous suspension (spectrum b) show a weak monomeric emission due to the low fraction of pyrene dissolved and a broad excimeric band emission at A,- = 460 nm characteristic of crystalline pyrene.12 The excitation spectra monitored at A,, = 480 nm (spectra a and b) exhibit the 360-nm band followed by a shoulder a t 375 nm, which are characteristic of crystalline p y r e n e . " ~ ~ ~It will be noticed that the (20) Thomas, J. K; Hashimoto, S. New J. Chem. 1987, 11, 145. (21) Ferguson, J. J. Chem. Phys. 1958, 28, 765. (22) Calvert, J. G.; Pitta, J. N. Photochemistry; Wiley: New York, 1966, p 282.
422 Langmuir, Vol. 4, No. 2, 1988
LabbP and Reuerdy
-h e m = 480 nm
b
300
400
500
h nm Figure 6. Fluorescence and excitation spectra of an air-dried clay film loaded with 0.5% pyrene, under ambient atmospheric pressure. 360
440
520
h nm Figure 5. Evolution of the emission spectrum with excitation monitored at A,, = 334 nm of a clay paste loaded with 0.5% microcrystallinepyrene during the last 15 min of 2 h of air drymg: (a) first spectrum, (b) last spectrum. fluorescence excitation spectra of the paste with emission monitored at 480 and 373 nm (spectra a) allow a clear distinction between the microcrystalline and the monomolecular pyrene. 2. Evolution of Clay Paste with 0.5% Crystalline Pyrene under Air-Drying Conditions: Adsorption of Pyrene. Air drying of the laponite paste loaded with 0.5% pyrene causes profound modification to the absorption and emission spectra, revealing the transition of pyrene from its microcrystalline state to its monomolecular state. The transformation does not occur until the final moment of drying. A record of the evolution of these spectra during the last 15 min of 2 h of air drying is shown in Figures 4 = 360 nm, and 5. The broad absorption band a t ,A, characteristic of microcrystals (Figure 4, spectrum a), gradually disappears and is replaced by the spectrum b of monomolecular pyrene (A= 307, 320,335 nm). In the same way, monomolecular emission increases a t the cost of excimer emission (Figure 5 ) . The transition of pyrene from its microcrystalline state to its monomolecular state involves its adsorption in the air-dried film. This phenomenon is reversible and can be repeated several times. Adding water to the film causes it to swell, and microcrystals reappear. Absorption (Figure 4a) and emission (Figure 5a) spectra are observed again. After the film redries, only the monomolecular pyrene is present. 3. Air-Dried Clay Film with 0.5% Pyrene. The emission spectrum (Aex = 334 nm) of dry film (Figure 6) is principally monomeric with bands quite sharp, narrow, and structured at A, = 373.5, 379 (81, 384, 390 (s), and 393.5 nm. These wavelengths are weakly red-shifted compared to those obtained for the fluorescence spectrum (Figure 3 spectrum c) of pyrene in aqueous solution, where A,, = 372, 378.5 (s), 383, and 393 nm. A weak eximerictype emission remains. The excitation spectrum with emission monitored a t A,, = 480 nm (Figure 6) looks like the excitation spectrum (Aem = 373 nm) of the monomer but with its bands broadened and red-shifted by 4 nm. The nature of this excitation spectrum does not fit with
v)
-
e
IT
? +i .
-
f ?r
c
v)
z
W
t-
z
300
400
A
500
nm
Figure 7. (a) Fluorescence spectrum of an air-dried clay film loaded with 0.5% pyrene. (b) Fluorescence and excitation spectra of the same film placed in a vacuum of 1.3 X Pa. the hypothesis of a dynamic excimer of pyrene. A phenomenon of this type has already been observed with regard to pyrene adsorbed on silica" or zeolitela or in a frozen solution.23 The shifts, which vary from 5 to 20 nm, are explained by the existence of associations in the ground state between two or more pyrene molecules. 4. Clay Film with 0.5% Pyrene under Vacuum. Exhaustive degassing and dehydration of film placed in a vacuum of 1.3 X Pa modifies its fluorescence spectrum (Figure 7 ,spectrum b). There is, on the one hand, a fivefold increase in the fluorescence signal, due to the suppression of the quenching by oxygen.24 On the other hand, comparing this spectrum with that observed under atmospheric pressure (Figure 7, spectrum a) one sees that (i) there is an increased bandwith such that only the three vibrationnal peaks a t A = 373,383 (s), and 393 nm can be seen and (ii) there is an increase in the 1373/13s3 ratio of the relative intensities of the vibronic bands at A = 373 and 383 nm. The ratio of initially 0.90 (Figure 6 A,, = 334 (23) Loewenthal, Y.; Tomkiewicz, Y.; Weinreb, A. Spectrochim.Acta, Part A 1969,25A,1501.
Pyrene as a Photophysical Probe on Laponite
a io,o1 400
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Langmuir, Vol. 4 , No. 2, 1988 423
%)
A nm Figure 8. Emission spectra of laponite powders loaded with 0.01% (a),0.1% (b), and 0.5%pyrene (c)by methanol evaporation. nm) for air-dried film has value of 1.80 after it has been placed in the vacuum. This change reflects a modification in the pyrene molecule's environment and will be used in the discussion to characterize the absorption sites on laponite. The bandwith increase may be attributed to an increasing inhomogeneity of the clay surface.l' This anisotropic interaction of pyrene with the mineral surface can also be confirmed by comparing the excitation bands (Figure 7, spectrum b, A,, = 373 nm) with the narrower excitation bands of adsorbed pyrene in air-dried film (Figure 6, A, = 373 nm). 5. Air-Dried Clay Films with 1 % and 2% Pyrene. For a pyrene level of 1%the dry film principally shows monomer-type emission, identical with that of a film a t 0.5% (Figure 6 ) . For a level of 2%, the excimer emission increases. Analysis of the excitation spectrum (spectrum identical with spectrum a, Figure 3, A, = 480 nm) enables us to attribute excimer emission to the presence of pyrene microcrystals; 2% pyrene is therefore already not entirely adsorbed in monomolecular form by this method. Loaded Clay Powders. 1. With 0.01%, 0.1%, and 0.5% Pyrene Prepared by Evaporation of a Methanol Solution of Pyrene. The emission spectra of a series of samples of clay powders loaded with 0.01%, 0.1%, and 0.5% pyrene are shown in Figure 8. The samples were prepared by evaporation of a methanol solution of pyrene brought into contact with laponite. In methanol, laponite does not lead to a colloidal suspension but remains in the form of flocks. For this reason, the clay does not arrange itself into a film during methanol evaporation but remains in powdered state. Where the pyrene load is very low (0.01%) the same results are obtained as for a 0.5% dry film. As the fluorescent emission shows (Figure 8, spectrum a), pyrene is present principally in monomolecular form. The excitation spectrum with emission monitored at A, = 480 nm (spectrum identical with Figure 6 , A,, = 480 nm) shows that the excimeric emission band is due to the association of a few pyrene molecules in the fundamental state. On the other hand, when the level of pyrene increases to 0.1% and 0.5%,the quantity of monomolecular pyrene adsorbed does not increase, and the excimer band in the fluorescence spectrum increases (spectra b and c, Figure 8). This excimer band is principally due to the presence of crystals, as is shown by the analysis of excitation spectra with emission monitored a t 480 nm (spectra identical with spectrum a, Figure 3, A,, = 480 nm). A phenomenon of emission and reabsorption" by the crystals (24) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: New York, 1978; p 354.
300
400
500
A nm Figure 9. Fluorescence and excitationspectra of laponite powders loaded with pyrene by dry crushing (a) 0.5%, (b) 1%and (c) 2% crystalline pyrene and laponite.
limits the intensity of the vibronic bands (principally the band a t A, = 373 nm) of the fluorescence spectrum of the monomer. Comparison of the fluorescence spectra of a film containing 0.5% pyrene (Figure 6) and of a powder containing the same level of pyrene by evaporation of methanol (Figure 7, spectrum c) shows the low efficiency of adsorption by this method compared with that of drying a paste loaded with microcrystalline pyrene. 2. With 0.5%, 1 %, and 2% Pyrene Prepared by Dry Crushing a Mixture of Laponite and Crystalline Pyrene. The emission spectrum of a powder containing 0.5% pyrene (Figure 9, spectrum a) is principally of monomeric type. It shows the adsorption of monomolecular pyrene. The excitation spectrum (Figure 9, spectrum a, A, = 373 nm) which is characteristic of monomer pyrene confirms this. The weak excimeric emission intensifies with pyrene level of 1%and 2% (Figure 9, spectra b and c). This emission is due to the presence of pyrene crystals, as is demonstrated by the recording of the excitation spectrum of the 2% sample with emission monitored a t A, = 480 nm (Figure 9, spectrum c). It is therefore difficult by this method to adsorb more than 0.5% pyrene without residual crystals being presents.
Discussion Nature of the Adsorption Sites of Pyrene. The tests carried out show that it is possible to adsorb pyrene effectively without the laponite being previously activated. There are three kinds of possible binding functions on the particles of laponite: silanol groups on the edge, siloxane groups, and sodium counterions on the surface. In airdried film which still contains at least 10% humiditg6 and air-dried powder, the polar silanol groups and sodium ions are certainly strongly solvated. The most important binding agents appear to be the h y d r o p h o b i ~siloxane ~~,~~ (25) Parker, C. A. Photoluminescence of Solutions; Elsevier: New York, 1968, p 225. (26) Prost, R. Etude de l'hydratation des argiles; Thesis, University of Paris VI, 1975; p 468. (27) Grauer, Z.; Avnir, D.; Yariv, S. Can. J . Chem. 1984, 62, 1889.
424 Langmuir, Vol. 4, No. 2, 1988 groups, according to the value 0.90 for the 13,,/13,,peak ratio for pyrene emission. It is well-known that the 1373/1383 ratio is very sensitive to the local environment of pyrene,15J6and this molecule is often used as a fluorescent probe for microenvironmental polarity. A value close to or below 1characterizes a weakly polar environment and a value much above 1a highly polar environment. In M solution in cyclohexane, benzene, and methanol, and 4 X lo-, M in water, we observe, in our experimentalconditions, a ratio of 0.58, 1.13, 1.35, and 1.74, respectively (0.58, 1.05, 1.35, and 1.87 in the published literature9. When pyrene is adsorbed on polar dehydrated silanol groups on dry silica gel, or on alumina, the ratio is respectively 1.81 and 1.89.17 On hydrated silica gel the ratio is in the 1.60-1.35 range;29in air-dried ndion membranes, where interaction with the hydration water molecules occurs, the ratio is 1.28.,O When pyrene is adsorbed on a nonpolar polypropylene surface the ratio is 0.59.19 The value 0.90 for pyrene adsorbed on air-dried film and airdried powder of laponite implies a local environment much like that found in nonpolar aromatic solvent.16 In air-dried conditions, only the hydrophobic siloxanic oxygen planes can justify the adsorption of the pyrene. Everything seems to indicate that this adsorption takes place through van der Walls type dispersion interaction with the surface oxygen atoms. Since pyrene is believed to reside on the siloxanic surface of the particle, and therefore close to the hydrated sodium, the demonstration of an interaction between them could only reinforce this hypothesis. The local environment of the adsorbed pyrene becomes highly polar when a film is strongly dehydrated by being placed in a secondary vacuum (Figure 7, spectrum b). The value of the ratio 13,,/Im is 1.80. This striking increase, corresponding to the departure of the highly polar water, must be rationalized by the creation of a pyrene-cation interaction. The departure of a part of the solvation water of the cation favors interaction with the vicinal pyrene, which is then in a highly polar environment. The properties of benzene inserted from vapor between the layers of air-dried montmorillonite are characteristic of the same type of behavior?" Mortland et al. suggest an interaction with the siloxane surface rather than with the counterion. When the cation is a Cu2+, extensive drying of the clay favors a benzene-Cu2+ complex, which goes as far as electron transfer, that is, the formation of a radical cation of benzene. Modes of Pyrene Adsorption. The adsorption faculties of laponite are such that pyrene initially in the form of microcrystals can be adsorbed in a principally monomolecular layer, without needing to be solubilized. This unexpected result is obtained in the final stage of drying of a water-filled paste resulting from the filtration of a clay colloid containing a suspension of microcrystals or by crushing a mixture of clay powder and pyrene crystals. The structure of the clay paste and its evolution during drying make it possible to interpret the phenomenon that leads, in the final stage of drying, to adsorption of pyrene. A laponite paste is the last state in which particles remain in equilibrium under the effect of electrostatic repulsion" (in our study, in the paste resulting from filtering of the laponite colloid containing a suspension of pyrene microcrystals, particles of clay and microcrystals coexist). (28) Yariv, S.; Cross, H. Geochemistry of Colloid System; SpringerVerlag: Berlin, 1979. (29) Bauer, R. K.; De Mayo, P.; Natarajan, L. V.; Ware, W. R. Can. J. Chem. 1984,62, 1279. (30) Lee, P. C.; Meisel, D. Photochem. Photobiol. 1985, 4 2 , 21.
Labb&and Reverdy During air drying, the removal of water liberates surface tension forces which favor face to face aggregation between elementary particles., Isolated aggregates form first. They are ordered into a film in the final stage of drying. This process involves pressures in a range from less than 0.1 atm to more than several times 10 atm.7 In the final stage of drying, during the movement of aggregates which are ordered and united into a film, the captured pyrene microcrystals are therefore submitted to considerable crushing and shearing forces and are brought into contact with the hydrophobic siloxane surface made available by the removal of water. This results in destruction of the microcrystals by the pyrene molecules being dragged onto the adsorption sites situated on the surface of the aggregates. Only a small fraction remains in the form of an association of several molecules (Figure 6). Introduction of water in the air-dried film causes its swelling by an osmotic phenomenon., In the reconstituted paste, the released pyrene reorganizes into microcrystals. Contact between a montmorillonite and n-hexane and n-dodecane liquids2 or benzene should give access to all the interlayer spaces of the clay. Adsorption rates of the order of 17.5% (w/w) for n-hexane are achieved. It is obvious that this mechanism through interlayer diffusion is not suitable for solid pyrene. In the mechanism of adsorption by formation of a laponite film, it is only the spaces resulting from the uniting of external surfaces of the aggregates that are available to the solid pyrene. The adsorption surface is thus considerably reduced. This seems to be indicated by the experimental results, since it is observed that 1% of pyrene is adsorbed in a film but that 2% can no longer be completely adsorbed. Taking as the actual surface occupied by a molecule of pyrene adsorbed flat on a surface a value of 150 A2?l 2% of pyrene corresponds to an occupation of 90 m2/g of the flat surface of clay, as compared to the 750 m2/g of elementary particles. Crushing (Figure 9, spectrum a) a mixture of an air-dried powder of laponite and pyrene crystals at 0.5% also causes removal of the pyrene molecules on the hydrophobic siloxane surface of the grains of powdered laponite (1373/1383 = 0.67). Because of the association of the clay particles (made up of three layers on average) in larger sized grains, the adsorption surface available by this method is therefore reduced. For this reason, a load of 1% cannot be completely adsorbed by manual crushing. The evaporation of a methanol solution in contact with the mineral limits adsorption of pyrene to a level below 0.1% in the case of laponite (Figure 8 spectrum a). In methanol, which has a distinctly lower dielectric constant than water, the range of clay particle repulsion is reduced. The colloid does not form, and the clay is found in the form of flocks. Evaporation of the solvent does not lead to an ordered system, but to a powder, and does not permit effective adsorption of the pyrene. Comparison of the fluorescence spectra of a dry film (Figure 6) and a 0.5% pyrene powder (Figure 8, spectrum c) shows up relative ineffectiveness of preparation by evaporation. This lack of effectiveness is due only to the mode of preparation. Crushing the dry 0.5% powder leads once again to adsorption of the 0.5% of pyrene (fluorescence spectrum identical with spectrum a, Figure 9). The absence of dynamic excimer formation in the samples studied indicates that the mobility of pyrene is restricted whatever the mode of preparation. (31) Snyder, L. D. Principles of Adsorption Chromatography. T h e Separation of Non Ionic Organic Compounds; Marcel Dekker: New York, 1968; p 200.
Langmuir 1988,4,425-429 Conclusion This study demonstrates that, by air drying, a waterfilled smectite, laponite, is able to adsorb pyrene principally in a monomolecular form from microcrystals. In our experimental conditions, pyrene monomolecular adsorption is limited to below a level which corresponds to a coverage of 12% of the whole clay surface. The adsorption results from the crushing of the microcrystals during the clay drying. Dispersion and delamination in aqueous medium followed by the restacking of clay particles during the air-drying process cause the phenomenon which substitutes for the diffusion of liquid or gaseous hydrocarbons inside the interlayer space. Adsorption by dispersion forces on the air-dried clay lattice, already observed with gaseous benzene and liquid n-hexane and n-dodecane, seems to be a property which
425
concerns nonpolar molecules. This adsorption must be favored, from an energetic point of view, since it is possible despite the need to destroy a crystalline structure. In the particular case of aromatic molecules, the adsorption phenomenon is influenced by the solvation state of the exchangeable cation. Thus, with benzene adsorbed on Cu2+montmorillonite, dehydration results in the formation of a Cu2+-benzene complex or even in the benzene radical cation formation if extensive dehydration occurs. In sodium laponite, adsorbed pyrene can only be exposed to polar influences when the clay is dehydrated. Exchanging sodium ions for transition-metal ions in laponite could be a pathway for chemical transformations of adsorbed molecules. The behavior of pyrene adsorbed on such modified laponite is a t present under study. Registry No. Laponite, 53320-86-8;pyrene, 129-00-0.
Interaction of Water with the Surface of SrF2. 1. Strongly Adsorbed Water on SrF2 Yasushige Kuroda and Tetsuo Morimoto* Department of Chemistry, Faculty of Science, Okayama University, Okayama 700, Japan Received July 10, 1987. In Final Form: September 25, 1987 Infrared absorption spectra were measured to make clear the adsorbed state of strongly adsorbed HzO on SrF,. The IR spectra of the strongly adsorbed HzO involve three absorption peaks at 2561,1947, and 10oO cm-', which did not appear on metal oxides, besides the peaks characteristic of HzO adsorbed on the latter substances. These three peaks can be elucidated to originate from molecularly adsorbed H20 on the (100) plane of SrFz,the molecule being located at the F- ion-vacant site and hydrogen-bonded to the neighboring F- ion. A model for the H20 molecule adsorbed at the special site on SrF, is postulated. Introduction Recently, it was found that two-dimensional (2D) condensation of H20 occurs on the surface of CaF:-4 and SrF2? especially on the H20-chemisorbed surface. The chemisorbed H20 on these solids was classified into three kinds on the basis of the surface H20content m e , which exhibits three peaks at temperature ranges of 25-100 (A), 100-200 (B), and 200-500 "C (C), respectively. Among them, the chemisorbed H20 corresponding to peaks A and B is reproducible when the sample degassed a t temperature lower than 500 OC is exposed to saturated HzO vapor, being associated with the occurrence of a 2D condensation of H20. Thus, it is interesting to depict the adsorption model of the chemisorbed HzO on these solids. The present work was undertaken to clarify the adsorbed state of H20 on SrF2,by measuring the IR spectra together with the surface H20 content. Experimental Section Materials. The SrFzsample used was prepared by the precipitation method as described previously: Le., by mixing a 0.5 M Sr(N03)2solution with a 1.0 M NH4Fsolution, the latter being used in 10% excess. The precipitate formed was then washed sufficiently with deionized HzO. The DzO- and methanol-exchanged samples were also prepared from the H20-preadsorbed (1) Morimoto, T.; Kadota, T.; Kuroda, Y. J . Colloid Interface Sci. 1985. 106. 154. _.._ ~ - (2) Kuroda, Y.; Sato, H.; Morimoto, T. J. Colloid Interface Sci. 1985, I
- - - 7
-108. - -, 241. - - -.
(3) Kwoda, Y.; Takenaka, T.; Umemura, J.; Kittaka, S.; Morishige, K.; Morimoto, T. Langmuir 1985, I , 679. (4) Kuroda, Y. J. Chem. SOC.,Faraday Trans. 1 1985,81, 757. (5) Kuroda, Y.; Kittaka, S.; Miura, K.; Morimoto, T. Langmuir, in press.
0743-7463/88/2404-0425$01.50/0
sample. Four kinds of adsorbates were used: HzO,DzO,CH30H, and CD30H. Among them DzO, CH30H, and CD30H were supplied from Nakarai Chemical Co. Each liquid was purified by removing dissolved gases through a "freeze-evaporate-thaw" cycle. Measurement of Surface Water Content. Prior to the measurement of the surface H,O content, the sample was evacuated at 500 "C for 4 h, rehydrated by exposing it to saturated HzOvapor at 25 "C, and outgassed at 25 O C for 4 h. The surface HzO content was measured by means of the successive ignition loss method.6 Measurement of IR and NIR Spectra. For the measurement of IR spectra, a self-supporting disk 20 mm in diameter was prepared by compressing a 0.2-g sample under a pressure of 400 kg cm-z. The disk wm set in an in situ cell, equipped with windows Torr for 2 h made of KRS-5, and outgassed in a vacuum of at increasingly elevated temperatures, 25, 100, 150,200, and 500 "C, respectively; the IR spectrum was measured at room temperature before each heat treatment, by using a spectrometer, IR-810,made by the Nippon Bunko Co. The NIR (near-infrared) spectra were measured by means of the diffusional reflection method, in a vacuum cell made of quartz, using a spectrometer, Hitachi U.V.-320; Teflon powder was employed as the reference. Results a n d Discussion
IR Spectra of Adsorbed H,O on SrF,. The IR spectra measured on adsorbed H,O on SrF2 in the wavenumber range 4000-400 cm-' are shown in Figure 1. When the original sample is treated at 500 "C in vacuo, only the lattice vibration of SrF, appears near 500 cm-' (Figure la).7 After the 500 "C treated sample is equilibrated with sat(6) Morimoto, T.; Naono, H. Bull. Chem. SOC.Jpn. 1973, 46, 2000. (7) Calder, G. V.; Mann, D. E.; Seshadri, K. S.; Allavena, M.; White, D. J. Chem. Phys. 1969,51,2093.
0 1988 American Chemical Society
