990
J. Phys. Chem. 1983, 87,990-998
as many as four adjacent methanol molecules as nearest neighbors. However, if the radical were formed in a channel then one would expect a similar geometry as found for H-mordenite with an end-to-end configuration of the methanol molecules in the channel. The difference in the structural parameters and the differences with adsorbate pressure clearly suggest that the structural model deduced for H-mordenite does not apply, at least in a dominant sense, for methanol adsorbed in H-ZSM-5 zeolite. In Figure 13 the hydroxymethyl radical is oriented with its CO bond perpendicular to the plane of the paper so that the radical carbon is equidistant from each of the four channels ending at the intersection. Then at the lowest adsorbate pressure studied of 10 torr it appears that there is only one adjacent methanol molecule in one of the channel entrances to the intersection adjacent to the radical at the intersection. I t is proposed that the methanol molecule is oriented as shown in Figure 13 in an approximate end-bend configuration across the channel as suggested by the results in H-mordenite. It is also deduced that the hydroxyl proton is oriented away from the intersection since no modulation from a deuteron in this position is observed. With this orientation the distance to the hydroxyl deuteron is about 0.6 nm for which distance no modulation should be expected to be detectable. Increase of the adsorbate pressure to 40 torr gives results consistent with two methanol molecules in two channels adjacent to the radical at the channel intersection with both hydroxyl groups oriented away from the intersection. At still higher adsorbate pressure of 100 torr the
structural parameters indicate four methanol molecules adjacent to the radical occupying all four channels opening to an intersection. Furthermore, the structural results on CH30D adsorbate indicate that two of the four molecules have their hydroxyl proton oriented toward the intersection at a distance of 0.47 nm from the radical carbon at the intersection. It is not clear why radical formation should be favored for those molecules at channel intersections; however, this postulate does seem consistent with the observed structural data. Perhaps the additional space available at the channel intersections promotes radical formation for those methanol molecules at that location. It has also been suggested that the most acidic sites inthe H-ZSM-5 lattice are located at or near the channel interse~tions;'"'~this seems consistent with the methanol adsorbate geometry suggested for H-ZSM-5 zeolite. Acknowledgment. This research was supported by the National Science Foundation and the Robert A. Welch Foundation. We thank Dr. J. Rabo of Union Carbide Corp. and Dr. C.D. Chang of Mobil Research and Development Corp. for providing samples of mordenite and ZSM-5 zeolite, respectively. Registry No. Methanol, 67-56-1; hydroxymethyl, 2597-43-5. (14)N.Y. Topsoe, K. Pedersen, and E. G. Derouane,J. Catal., 70,41 (1981). (15)J. R.Anderson, T. Mole, and V. Chritov, J.Catal., 61,477(1980). (16)P. A. Jacobs and R. Von Ballmoos, J. Phys. Chem., 86, 3050 (1982).
Photoprocesses on Colloidal Clay Systems. Tris( 2,2'-blpyrldlne)ruthenlum(I I ) Bound to Colloidal Kaolin and Montmorillonite R. A. DellaGuardla and J. K. Thomas' Chemistry Department, University of Notre Dame, Notre "e.Indiana 46556 (Received: August 19, 1982; In Final Form: October 27, 1982)
Photochemical reactions have been studied in aqueous colloidal montmorillonite and kaolin clays. The photochemical probe used was tris(2,2'-bipyridine)ruthenium(II), Ru(bpy)32+, which is quenched by Cu2+,Eu3+, nitrobenzene, and dimethylaniline (DMA). The processes occur in the domain of the colloidal particles and are dynamic with kql equal to 2 X 10') 2 X lo8, 2 X lo8 M-' s-' for Cu2+,nitrobenzene, and dimethylaniline, respectively. The absorption and emission spectroscopyat room temperature and 77 K of Ru(bpy),2+comments on the environment of the probe on the kaolin surface and in the internal layers of montmorillonite. Quenching occurs when the quenchers are adsorbed into the layers (Langmuir type) or react from the bulk solution with the probe located on the surface. The rate data show that both the cationic and uncharged molecules move quite freely on the clay surfaces and also in the clay layers. Inert electrolytes such as KC1 markedly affect the kinetic data in montmorillonite colloids by decreasing the particle association.
Introduction Catalytic studies have shown the importance of interfacial effects on chemical reactions.'-3 Photochemists have extended these studies to colloid systems, mostly organic (1)J. H.Fendler and E. J. Fendler, "Catalysis in Micellar and Micromolecular System", Academic Press, New York, 1975. (2)N.J. Turro, A. M. Braun, and M. Gratzel, Angew. Chem., 19,675 (1980). (3) J. K. Thomas, Chem. Reu., 80,283 (1980).
in nature, and have found that interfacial photochemistry can efficiently promote chemical reactions that are quite inefficient, or even do not occur, in simple homogeneous solutions. A wide choice of colloidal systems is available to the photochemist, and at this stage it is pertinent to inquire into inorganic colloids, in particular clay colloids, as this important group of materials is of general interest. For many years clay minerals have been the focus of intensive research due to their ability to intercalate various molecules and their catalytic properties. The two minerals
0022-365416312087-0990$0 1.5010 0 1983 Amerlcan Chemical Society
Photoprocesses on Colloidal Clay Systems
studied in this work were montmorillonite and kaolin. Montmorillonite possesses a layered structure and strong sorptive properties due to the expandability of the internal layers. It is commonly referred to as a 2:l layer mineral, indicating that an aluminum in octahedral configuration shares oxygen atoms with two silica sheets in tetrahedral confiiation, one of each side. Kaolin, on the other hand, is a nonexpandable 1:l layer mineral, i.e., there is a sharing of oxygen atoms between one silica sheet and one aluminum sheet in a continuous network that cannot be easily disrupted for the intercalation of ions or organic molecules. Hence, only the surface of kaolin particles participates in chemical reactions whereas both the surface and the internal layers are available in montmorillonite particles. These mineral surfaces are not inert. Montmorillonite has been shown to induce the thermal transformation of alkylammonium ions adsorbed on its surface at temperatures well below the thermal decomposition temperature of the amine? The mechanism is attributed to that of acid catalysis due to the highly acidic nature of the montmorillonite surface5 (K,x 10-9. The extreme acidity of the “dry” mineral surface is postulated to be due to the polarization of residual water molecules by the exchangeable cations located at the mineral’s surface. Both montmorillonite and kaolin have been shown to catalyze the polymerization of some unsaturated organic compounds such as styrene and hydroxyethyl methacrylate and yet to inhibit polymer formation from other structurally related monomers such as methyl metha~rylate.~~’ This behavior is believed to be due to the electron-accepting or electron-donating sites on the clay mineral. The electron acceptor sites are throught to be aluminum at the crystal edges and transition metals, such as iron, in an oxidized valence state in the silicate layer. The aluminum sites a t the crystal edges arise from defects or fissures in the crystal structure of the clay.8 I t has been suggested that the catalytic activity at this site is due to aluminum in octahedral coordination with the mineral acting as a Lewis acid. Correspondingly, the electron donor sites are transition metals in the reduced state. The presence of transition metals is due to isomorphous substitution within the lattice structure of the clay mineral. This involves the replacement of quadrivalent silicon in the tetrahedral sheet with trivalent species such as aluminum. In the octahedral sheet, aluminum may be replaced by divalent iron or magnesium. The small size of these atoms permita them to take the place of the Si and Al atoms. The replacement of an atom of higher positive valence for one of lower valence results in a net negative charge. This excess of negative charge is balanced by the adsorption of cations on the layer surfaces. In the presence of water, these charge-balancingcations may be exchanged with other cations available in solution. The cation exchange capacity (cec) is greater for montmorillonite due to the possibility of exchange in the internal layers and the resulting increased surface area of this mineral. The property of cation replaceability of clays has been extensively studied and is responsible for many of the unique properties of montmorillonite and kaolin. The present work makes use of the cation exchange capacity (4) B. Durand, J. J. Fripiat, and R. Pelet, Clays Clay Miner., 20, 21 (1972). (5)R.Touillaux, R. Salvador, C. van der Meersche, and J. J. Fripiat, Isr. J. Chem., 6,337 (1968). (6)A. Blumstein, R. Blumstein, and T. H. Vanderspurt, J . Colloid Interface Sci., 31, 236 (1969). (7)D. H.Solomon and J. D. Swift, J. Appl. Polym. Sei., 2,2567(1967). (8) B. K. G. Theng, “The Chemistry of Clay-Organic Reactions”,Adam Hilger, London, 1978.
The Journal of Physical Chemistry, Vol. 87, No. 6, 7983 991
by exchanging tris(2,2’-bipyridine)ruthenium(II),Ru(bpy)32+,for the clay Na+ ions. This process locates a luminescent probe molecule on the clay and enables us to gain information about (1)the mobility of the probe on the clay particle, (2) the accessibility of various quenchers to the probe, and (3) the local environment of the probe and the nature of the colloidal clay particle.
Experimental Section Chemicals. Tris(2,2’-bipyridine)ruthenium(II)chloride (G. Fredrick Smith), nitrobenzene (Eastman), copper sulfate (Baker), europium chloride (Alfa Inorganics), sodium chloride (Fisher), potassium chloride (Fisher), potassium ferricyanide (Baker), and sodium hexametaphosphate (Sargent Welch) were used as received. The NJV-dimethylaniline (Matheson Coleman and Bell) was vacuum distilled before use. Equipment. A nitrogen laser from Photochemical Research Associates (pulse width, 120 ps; 50 pJ/pulse; X = 337.1 nm) was used to observe the phosphorescence decay. The signals from a RCA lp28 photomultiplier tube were fed into a Tetronix 7912 AD programmable digitizer and displayed on a Tetronix 546B storage oscilloscope. AU data were analyzed with a Tetronix 4051 computer which was interfaced to the digitizer for rapid data reduction. The decay curves were fitted to a high degree of precision, i.e., u < 0.0006 where u = (l/N) x
over the 512 points of the decay curve. The absorption spectra were obtained by using a Perkin-Elmer 552 spectrophotometer. Phosphorescence spectra, quenching, and polarization studies were obtained with a Perkin-Elmer MPF-44B fluorescence spectrophotometer. Particle size measurements were made on a Nicomp particle analyzer. The centrifugation of clay samples was performed by using a Sorvall high-speed centrifuge. Clay Minerals. The montmorillonite and kaolin primarily used in this work were obtained from the Georgia Kaolin Co. as the pure mineral. The sodium-exchanged form was prepared by mixing the clay sample for several days with 1N NaC1. This was followed by several washings with distilled water and centrifugation cycles to remove the excess sodium chloride from solution. The clay mineral was then resuspended and dialized until a negative chloride test was obtained with 0.1 M AgN03. Kaolin (Mesa Alta, NM) was obtained from Ward’s Natural Science Establishment, Inc., as an API Standard clay. The crude mixture was dispersed in solution and treated with 1M NaCl as described above; however, after each washing with NaCl, the supernatant liquid containing the clay gel was decanted into another beaker before centrifugation in order to separate the clay gel from the mineral sediment. The clay colloids were made by dispersing the mineral with a high-shear Waring blender for approximately 5 min. The montmorillonite particles are tactoids, or platelike particles approximately three layers thickg (Na+-exchanged form) with a measured hydrodynamic radius of 2000 A. The surface area and the cation exchange capacity of the montmorillonite used in this work were 750 m2/g and 1 X equiv/g. The kaolin particles are similar in that they also consist of stacked plates. However, the kaolin (9)H.van Olphen, “An Introduction to Clay Colloid Chemistry”, Wiley, New York, 1977.
992
The Journal of Physical Chemlstty, Vol. 87, No. 6, 1983
particles are nonswelling and have no internal surface area for cation exchange. The surface area and the cation exchange capacity of kaolin were much less at 10 m2/g and 3x equiv/g. Adsorption of Molecules from Solution onto the Clay Particles. It is well established that clay minerals have a strong affinity for organometallic cations such as Ru(bpy),2+.1° Complete adsorption of Ru(bpy),2+onto the clay particles used in this study was determined by adding the R ~ ( b p y ) ~ to~a+ typical montmorillonite or kaolin colloid, sonicating for 5 min, and then centrifuging the colloid at 20 000 rpm for 1h. Spectroscopic examination of the supernatant showed that virtually all of the Ru(bpy);+ was adsorbed on the clay particles at the concentration of clay colloid and Ru(bpy)t+used in this work. X-ray powder diffraction measurements were made on a montmorillonite powder which contained R ~ ( b p y ) ~ The ~+. Ru(bpy);+ was added to an aqueous colloid and allowed to mix for several minutes. The mineral was then centrifuged from solution, washed, and allowed to dry in air. The measured doolspacing of this sample was 18.3 A, in agreement with values obtained for Ru(bpy),2+intercalated in clay fi1ms.l' It was also determined that, at the concentrations of Cu2+ or Eu3+ used in the quenching experiments, the cations did not displace the Ru(bpy),2+into solution. This preferential adsorption of R ~ ( b p y ) , ~was + also observed at concentrations of Cu2+and Eu3+ well beyond the cec of the clay where the Cu2+or Eu3+ions had saturated the clay and were also in the solution. The R ~ ( b p y ) is ~~+ strongly bound to the montmorillonite particle. When a montmorillonite colloid containing Ru(bpy)t+was mixed with a 0.1 M sodium lauryl sulfate solution containing anionic micelles with approximately 100 times the exchange capacity of the clay, the Ru(bpy),2+ could not be removed from the clay. Calculation of the Effective Concentration of Molecules Adsorbed on the Montmorillonite Particles. Due to the adsorption of molecules on the colloidal montmorillonite particle, the effective concentrations of R ~ ( b p y ) , ~and + quencher are much greater than the bulk solution concentration. Unrealistically high quenching rate constants of 10" M-l s-' or greater were obtained and are a result of the use of a bulk concentration of quencher instead of an effective local concentration in the calculations. The montmorillonite particles are known to exist as platelike sheets in solution. Calculations based on the surface area, concentration, and charge of the particles yield a surface are per charge of approximately 92 A2, or about 9.6 A between each negative charge. Based on the cec and surface area of this mineral, this value is in reasonable agreement with other estimated2of the surface area per unit charge for montmorillonite. The surface and internal volume of the montmorillonite colloid were calculated from an assumed width of the Stern layer on the outside of the particle surface (10 A), the interlayer spacing of the silicate sheets in an aqueous solution (20 A), the length of the particles (4000 A), the thickness of an aluminosilicate sheet in montmorillonite (10 A), and the number of montmorillonite particles in the solution, e.g., 5.7 x 1014particles/g for a colloid concentration of 1g/L. In this instance, the total volume rendered by the colloid is calculated to be 0.55 cm3. This value is in good agree(10) M. F. Traynor, M. M. Mortland, and T. J. Pinnavaia, Clays Clay Miner., 26, 318 (1978). (11) D. Krenske, S.Abdo, H. Van Damme, M. Cruz, and J. J. Fripiat, J . Phys. Chem., 84, 2447 (1980). (12) D. T. B. Tennakoon, J. M. Thomas, M. J. Tricker, and J. 0. Williams, J. Chem. Soc., Dalton Trans. 2207 (1974).
DellaGuardia and Thomas a
T
I
c
+ \
ICC
1
'
I
35 3
400
500
9oc
W A Y E__ LCMW
Flgure 1. Absorptlon spectrum of 8.5 X lo-' M R~(bpy),~+ in water and adsorbed on 1 g of montmor#lonite/Lwith a montmorillonitecolloid (1 g/L) in reference cell. (b) Absorption spectrum of 8.5 X lo-' M R~(bpy),~+ adsorbed on 1 g of kaolln/L with a kaolin colloid (1 g/L) in reference cell.
ment with other values obtained for bentonite colloids which contain layered particles of similar size and structure.13
Results and Discussion Ru(bpy),2+ Adsorbed on Montmorillonite or Kaolin. Absorption Spectra. The absorption spectra of Ru(bpy)gS+ adsorbed on montmorillonite and kaolin are shown in Figure 1. The absorption spectrum of R ~ ( b p y ) in ~ ~water + consists of four bands in the 200-600-nm region. The one at 285 nm has been assigned to an intraligand a-*a transition and the one at 452 nm with a shoulder at 425 nm to a metal-to-ligand (d-*?r) charge-transfer transition. There are significant shifts in the absorption maxima, appearance of a new peak at 272 nm in montmorillonite colloids, and a significant decrease in the extinction coefficient for the transition in the 280-nm region upon adsorption of R ~ ( b p y ) , ~on+ a clay colloid. Other work with Ru(bpy),2+adsorbed on hectorite films has shown that the probable causes for some of these changes is distortion of the bipyridine ligands due to steric constraints which arise from adsorption on the mineral surface. X-ray photoelectron spectroscopy (XPS) of these (13) 0. J. Greenland, R. H. Laby, and J. P. Quirk, Trans. Faraday Soc., 61, 2024 (1965).
Photoprocesses on Colloldal Clay Systems
thin films showed that the effective charge on the nitrogen nucleus was greater (increased electron density) than that observed for pure samples of R ~ ( b p y ) ~ C l ~ Corre.~~ spondingly, the Ru 3dSl2 binding energy was strongly shifted toward higher energies, i.e., toward those observed for the R ~ ~ ( b p y )salt, ~ C in l ~which ruthenium is formally in a 3+ valence state. The XPS data indicate that adsorption on the clay surface promotes a quasi metal-toligand charge-transfer state. The absorption band normally occurring at 452 nm is red shifted to 472 nm for montmorillonite and 465 nm for kaolin samples and the extinction coefficient is increased. The shift to the red for the metal-to-ligand charge-transfer band could possible be due to a destabilization of the Ru(bpy),2+ground state. This is explained in conjunction with the XPS data which show that the adsorbed ruthenium has a spatial 3+ valence. In this partially oxidized state, the energy of the d orbitals would be raised, which would decrease the energy required for the d-*r transition. It is also apparent that in both samples there is a significant decrease in the extinction coefficient for the transition occurring in the 280-nm region. For montmorillonite samples, there is a primary peak at 272 nm with a secondary peak at 285 nm. In kaolin systems, a single peak occurs at 280 nm. The appearance of two peaks for R ~ ( b p y ) , ~adsorbed + on the montmorillonite colloids indicates that it is in a different environment or adsorbed state than that on the kaolin samples. Since the X-ray diffraction data and emission spectra at 77 K indicate that the R ~ ( b p y ) , ~is+adsorbed in the layers, it would appear that its intercalation can also be detected by the appearance of a second absorption peak at 272 nm. A blue shift and a decrease in the extinction coefficient are observed for the r-*r transition occurring in the bipyridine ring, which suggests that one of the pyridine rings is twisted out of the plane of the other pyridine ring upon intercalation. A distortion of the bipyridine ligand in this manner would inhibit delocalization of the excited electron through both rings, thus decreasing the transition probability and requiring more energy for the transition to occur. Emission Spectra. The phosphorescence spectra for Ru(bpy),2+adsorbed on kaolin or montmorillonite colloids had an emission maximum at 612 nm at 298 K which is identical with that observed in aqueous solution. The emission spectra of Ru(bppy),2+at 77 K in 50% ethylene glycol/water glasses (v:v) are shown in Figure 2. In the ethylene glycol/water glass there is a distinct spectral separation of two of the triplet states yielding emission maxima at 578 and 620 nm. These results are understood in terms of the thermal redistribution of the relative populations of the triplet states and a matrix-induced distortion of the complex which shift the luminescence to lower energy.15 An increase in the intensity of the emission maximum in the 630-nm region is also observed with increasing distortion of the complex. These effects are the most pronounced in the ethylene glycol glass. In the kaolin system the R ~ ( b p y ) , ~is+adsorbed on the surface and is apparently not as immobilized and distorted by the glass matrix. This is evidenced by the appearance of only a broad shoulder in the 630-nm region. In the montmorillonite samples, intercalation in the layers of the particles separates the complex from the glass matrix to a greater extent, thereby decreasing the matrix-induced distortion such that the emission maximum is not as blue (14)S. Abdo, P. Canesson, M. Cruz, J. J. Fripiat, and H. Van Damme, J . Phys. Chem., 85,797 (1981). (15)F. Felix, J. Ferguson, H. U. Gudel, and A. Ludi, J . 102, 4096 (1980).
The Journal of Physical Chemistry, Vol. 87, No. 6, 1983 993
550
6 00
650
Wovelengfh ( n m )
Flgure 2. Phosphorescence emission spectra of Ru(bpy),'+ at 77 K In an ethylene glycoVwater glass (-), adsorbed on montmorillonite in an ethylene glycoVwater glass (- - -), and adsorbed on kaolin in an ethylene glycoVwater glass (. -).
-
shifted and the maximum in the 630-nm region appears as only a slight shoulder. Studies by the standard methods1'jJ7of the polarization of phosphorescence from these samples at 298 and 77 K also indicate that Ru(bpy),2+is located in an environment different from that on kaolin. For ethylene glycol, kaolin, and montmorillonite the degree of polarization, p , of the phosphorescence at 298 and 77 K was 0,0.20; 0.10, 0.15; and 0.11, 0.11, respectively. The larger values of p at 77 K in the ethylene glycol glass indicate greater restriction to movement placed on the probe molecule. These results support the conclusions reached from the emission spectra at 77 K. For the montmorillonite samples, the lack of change in the degree of polarization upon cooling the solution to 77 K (0.11, 0.11) indicates that the local environment of the Ru(bpy)? must be such that it is still able to rotate. With kaolin, since only surface sites are available, the Ru(bpy),2+is at least partially exposed to the rigid matrix that forms in the solution at 77 K and an increase in the degree of polarization is therefore observed (0.10, 0.15). As expected, the maximum effect is observed in ethylene glycol-water mixtures in which the R ~ ( b p y ) , ~ + becomes trapped in the rigid matrix formed at 77 K and essentially all rotation stops. Figure 3 shows the time-resolved phosphorescence decay of Ru(bpy),2+ adsorbed on montmorillonite and kaolin in a degassed solution. The decay curves can be fitted by a double exponential function I = Io[ae-klt + (1 - c,)e-kzt] which is simply the sum of two first-order decay functions. Each first-order decay has a characteristic rate constant, k l or k 2 , and each is weighted by the factor a or 1 - a, respectively. The figure shows the excellent agreement (16)J. M.Price, M. Kaihara, and H. K. Howerton, Appl. Opt., 14,521 (1962). (17)S. Cheng and J. K. Thomas, Radiat. Res., 54, 49 (1973).
The Journal of phvsical Chemistry, Vol. 87, No. 6, 1983
DellaGuardia and Thomas
ro
20 jCoocen+rotion]
I 100
3w
200
4w
500
Time ins)
I
100
200
300
4!m
500
T l m e ins)
Figure 3. (a) Time-resolved phosphorescence decay of 1 X 10-5 M R~(bpy),~+ adsorbed on a kaolin colloid (1 g/L); a = 0.50, k, = 1.0 X lo7 s-I, k, = 1.4 X 10es-l. Smooth line is calculated from the values of k,,k,, and a. (b) Tlme-resolved phosphorescence decay of 3 X lod M Ru(bpyh2+adsabed on a montmor#bnitecoilold (1 g/L); a = 0.62, k , = 3.2 X lo7,k, = 2.4 X 10'. Smooth line calculated from the values of k,,k,, and a.
between the experimental and calculated data.18 Although the decay constants, kland kz, for Ru(bpy)gP+ adsorbed on kaolin samples are smaller than for montmorillonite samples, the phosphorescence decay is much faster than that observed in aqueous solution, i.e., k = (1.66 f 0.14) X lo6 s-l.19 The reason for this enhanced decay upon adsorption on either kaolin or montmorillonite has not yet been conclusively determined but one significant factor appears to be the amount of iron present in the aluminosilicate lattice of the mineral. Ferric and ferrous ions are known to quench Ru(bpy),2+ by an electron-transfer mechanism in aqueous solution with rate constants of 2.7 X lo9 and 2.8 X lo6 M-' s-l, respectively.z0 The iron usually present in the most abundant amount in the clay minerals used is in the ferric form. For the montmorillonite colloids studied in this work, there was significantly more quenching of Ru(bpy),2+ on a montmorillonite containing 4 % iron than when 1% was present. The kaolin had an iron content of 0.3% and, as shown in Figure 2b, k1 and k z are about 30% of those for the montmorillonite sample containing 1% iron. Hydrazine has been used to reduce the lattice iron from ferric to ferrous and a total conversion of the iron present to the ferrous form would be expected to cause a decrease in the quenching of R ~ ( b p y ) , ~adsorbed + on the montmorillonite since the quenching rate constant for ferrous is less than that for ferric. However, pretreatment of the montmorillonite with a 60% hydrazine solution for 1 day (followed by several washings with water to remove the (18) We thank James Wheeler for writing the computer programs used to fit the experimental data. (19) N. Sutin and C. Creutz, Adu. Chem. Ser., No. 168, 91 (1978). (20)C. Creutz and N. Sutin, Inorg. Chem., 15, 496 (1976).
30 x
40
sc
/a4
Figure 4. Steady-state Stern-Volmer plot of *R~(bpy),~+phosphorescence quenching. (A) Cu2+ quenching of 'Ru(bpy),* adsorbed on a montmorillonite colloid (1 /L) wlth a cec of 1 X lo-, equivlg. (B) Eu3+ quenching of 'Ru(bpy))+ adsorbed on a kaolin colloid (1 g/L) with a cec of 3 X lo-' equlv/g. (C) Eu3+ quenching of *Ru(bpy),2+ adsorbed on a montmorillonite colloid (1 g/L) with a cec of 1 X lo-, equivlg. Concentration of quencher Is that added to solution.
adsorbed hydrazine) had no effect on the phosphorescence quantum yield or the decay rate. The lack of any reduction in the decay rate could, however, be due to oxidation of the iron back to the ferric form when the clay is suspended in water.21 Further studies are in progress on the effects of iron and clay structure on the phosphorescence decay rate of Ru(bpy)3z+adsorbed on the mineral surface. Quenching Studies. Cu2+and Eu3+. Both Cu2+and Eu3+quench the luminescence of Ru(bpy)gP+adsorbed on colloidal clays. Typical steady-state data are shown in Figure 4, where a plot of Io/Z for the Ru(bpy),2+ luminescence is plotted vs. the added concentration of Cu2+ and Eu3+. The data were analyzed by the Stern-Volmer equation l o / I = 1 + KsJQI where Io and I are the emission intensity in the absence and the presence of quencher, Q, respectively. K,, is the Stern-Volmer constant. The plots tend to plateau, indicating a decreased efficiency of quenching at higher concentration of quencher. This is due to the limited adsorption of the quenching cation on the clay, which cannot exceed the cec. Attempts to increase the adsorption of the cation lead to flocculation of the clay. Cu2+is much more efficient than Eu3+ in quenching *Ru(bpy),2+in accord with its greater efficiency in water. The rate constants for Cu2+and Eu3+quenching in aqueous solution are 3 X lo7 and 1 X lo6 M-' s-l , r espectively.22~23 The cationic quenchers increase the rate of decay of *Ru(bpy),2+adsorbed on the clay. Analysis of this decay in the presence of quencher gave k l and k2 as a function of Cu2+concentration. Figure 5 shows the variation in k l and kz with the effective concentration of Cu2+adsorbed on the montmorillonite surface. The linear plots permit a calculation of the second-order rate constants for quenching by Cu2+,kql = 2 X lo7 M-l s-l and kq2 = 3 X lo6 M-' s-l. The decay constants obtained from the transient phosphorescence decay curves allow calculation of the ratio Zo/I from the equation a (1 - 4
-+-
Io _ -- kl0 I
122'
-+- (1-a) (Y
kl
kz
(21) I. Rozenson and L. Heller-Kallai, Clays Clay Miner., 26,&3 (1976). (22) D. Meisel and M. S. Matheson, J. A m . Chem. Sac., 99,6577 (1977). ~~
(23) C. T.Lin, W. Bottcher, M. Chou, C. Creutz, and N. Sutin, J. Am. Chem. SOC.98,6536 (1976).
The Journal of Physical Chemhtty, Vol. 87, No. 6, 1983 995
Photoprocesses on Colloidal Clay Systems
TABLE I: Comparison of the Experimental and Calculated Values of Quenching of Ru(bpy),2' Adsorbed on on Montmorillonite cul+ nitrobenzene dimethylaniline 105(concn), M
)obsd
3 6 10 15 20
1.09 1.12 1.21 1.35 1.36
(Io/I)calcd
(zo/z)obsd
(I@/z)calcd
(Io/I)obsd
(Io/z)calcd
1.07 1.17 1.28 1.38 1.45
1.17 1.32 1.44
1.21 1.31 1.46
1.66
1.71
1.17 1.29 1.42 1.51 1.59
1.19 1.36 1.44 1.64 1.80
800 0
1
2 [COnCentrohon Added]
50
150 [CY"]
250
350
x 10 3
Figure 5. Variation of the decay constants k , and k 2 with Cu2+ concentration. The abclssa represents the local or effective Cu2+ concentration on the montmorlllonite particle calculated as ,described in the Experimental Section. An effective concentration of 0.150 M M. corresponds to a bulk concentration of 8.25 X
where kl and k2 are the decay constants obtained in the presence of quencher and k t and k,O in the absence. Table I shows the good agreement between the calculated &/I and that obtained from the steady-state experiments. These data indicate that little, if any, static quenching of the *Ru(bpy)?+ occurs on montmorillonite colloids. The quenching rate constants, kql and k,,, are to be compared to the rate constant for quenching of *Ru(bpy)?+ by Cu2+in water, 3 X lo7 M-' s-'. The value of kql agrees well with that observed in aqueous solution of ionic strength similar to that of the clay colloid solution, indicating that the reactants move readily on the clay surface, and diffusion in the layers of the clay is comparable to that in water. The kq2value is lower and may be due to quenching in a more restricted region of the clay or may be due to a lower effective concentration of Cu2+ in this region than that calculated. Quenching of *Ru(bpy)? by Cu2+in localized regions, such as on poly(viny1 sulfate), also gives rate constants which are comparable to those in homogeneous aqueous solution.24 The quenching on sodium lauryl sulfate micelles is increased, however, due to restriction of the reactants on the micelle surface.25 Quenching by Organic Molecules. Montmorillonite. Two nonionic molecules, nitrobenzene and dimethylaniline, were used to observe the quenching behavior of neutral species. For the montmorillonite colloids used in this work, the X-ray powder diffraction data showed that the d, (24)S. Kelder and J. Rabini, J.Phys. Chem., 85, 1637 (1981). (25)D.Meisel, M. Matheson, and J. Rabini, J. Am. Chem. SOC.,100, 117 (1978).
3
4
5
x io3
Flgure 6. Adsorption Isotherms obtained for dimethylaniline (A) and nitrobenzene (E) with a montmorillonite colloid concentration of 1 g/L containing 3 X M Ru(bpy),2+. C represents the decreased adsorption of nitrobenzene In the presence of 0.01 M KCI.
spacing had increased to 18.3 A, which demonstrated that the Ru(bpy),,+ was intercalated in the layers of the particles. The adsorption isotherms and Stern-Volmer plots indicate that nitrobenzene and dimethylaniline are also adsorbed in the layers of montmorillonite. Therefore, in order to accurately interpret the quenching of Ru(bpy)? by nitrobenzene and dimethylaniline in this environment, the adsorption isotherms were prepared to determine their extent of partitioning between the layers of the particle and the aqueous solution and to calculate their local or effective concentration from these data. Unlike the adsorption isotherms depicted in Figure 6 for dimethylaniline and nitrobenzene adsorption on montmorillonite, no significant adsorption of these molecules on kaolin colloids occurs as the molecules cannot penetrate into the internal structure of this mineral. The quenching results obtained, however, indicate that although these molecules do not spin down with the clay upon centrifugation small concentrations may be associated or weakly bound to the kaolin colloid under experimental conditions. The adsorption of DMA by montmorillonite is not surprising in view of the extensive number of amines, especially aniline derivatives, which have been shown to be adsorbed by this mineral.26 Aliphatic amines are more strongly adsorbed by sodium montmorillonite than aromatic amines; the difference is more pronounced when studied as their aqueous salt solutions at pH 7 where they are usually taken up to, and sometimes beyond, the exchange capacity of the clay.27 The mechanism of adsorption of these amines varies from ion exchange to physical adsorption depending on the basicity of the amine. (26)A. Haxaire and J. M. Bloch, Bull. SOC.Fr. Mineral. Cristallogr., 79,464 (1966). (27)G. W.Brindley and A. Tsunashima, Clays Clay Miner., 20, 233 (1972).
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The Journal of Physical Chemistry, Vol. 87, No. 6, 1983
Hendricks has shown that bases such as benzidene, paminodimethylaniline, p-phenylenediamine, and a-naphthylamine are capable of neutrallizing most of the exchangeable hydrogen ions in montmorillonite while 0- and m-nitroanilines, being relatively weaker bases, do not form salts with the m i n e d B Karikhoff and Bailey have shown that the protonation of amines upon adsorption in claywater systems is dependent upon the adsorptive proerties of the organoclay species as well as the structure and degree of hydration of the clay system.29 They also found, in agreement with the results of Hendricks, that molecules with the ability to stabilize the cationic charge over two or more aromatic rings give rise to greater surface-induced protonation than single ring molecules with similar pK,'s. The bonding of aniline, and probably therefore the bonding of dimethylaniline, is due to the cation present in the exchange position of the clay.30 With sodium-exchanged clays, the aniline molecule is believed to be coordinated to the sodium ion through a water bridge since its uptake requires the presence of interlayer water and the removal of this water is difficult, even when the sample is boiled in aniline. The interposition of a water molecule between the interlayer cation and the adsorbed species has also been shown to occur in montmorillonite-nitrobenzene comp l e x e ~ . ~From l X-ray studies, Yariv has shown that nitrobenzene is intercalated in clay films yielding a basal spacing of 15.35 A with sodium montmorillonite. Infrared data on these films indicate that nitrobenzene is involved in hydrogen bonding through its nitro group with water that is coordinated to the polar interlayer cation. Figure 7 shows the steady-state quenching data obtained with nitrobenzene and dimethylaniline. By a comparison of the adsorption isotherms in Figure 6 with the quenching results in Figure 7, it can be seen that the quenching behavior of these neutral molecules follows their adsorption isotherms in that DMA quenches a higher fraction of the *Ru(bpy)?+ because more of it is adsorbed. The leveling-off of the quenching is a result of the fact that the amount of nitrobenzene (Langmuir adsorption) or dimethylaniline reaches a maximum, beyond which only minimal adsorption occurs. Analysis of the phosphorescence decay rate can be used to obtain a pseudo-first-order rate constant for the quenching of clay-bound *Ru(bpy):+ by nitrobenzene and dimethylaniline. Plotting kl vs. the added quencher concentration yields quenching rate constants of 5 x 10'O and 1 X 10'l M-l s-l for nitrobenzene and dimethylaniline, respectively. These values are much higher than those obtained in water and do not take into account the localization of dimethylaniline or nitrobenzene on the montmorillonite particle due to adsorption. A calculation of the local concentration of these quenchers yields a realistic measure of their mobility on the particle surface and in the layers as reflected in their respective quenching rate constants. From a treatment of the data in this way a quenching rate constant, kql, of 2 X lo8 M-' s-l for nitrobenzene is obtained. This is somewhat less than the value of 3 X lo9 M-' s-l in homogeneous aqueous solution. However, it indicates that there is movement of nitrobenzene on the surface and in the layers of the montmorillonite particles. The mobility may be less than the (28) S. B. Hendricks, J. Phys. Chem., 46,65 (1941). (29)S.W.Karickhoff and G . W. Bailey, Clays Clay Miner., 28, 170 11976). (30) S. Yariv, L. Heller, and N . Kaufherr, Clays Clay Miner., 17,301 (1969). ( 3 1 ) S. Yariv, J. D. Russell, and V. C. Farmer, Isr. J. Chen., 4,201 (1966). '
DellaGuardia and Thomas
, nn
4
co
fl [Dmelhylom/,m] x IO4
Figure 7. (a) Steady-state Stern-Volmer plot of 'Ru(bpy);+ phosphorescence quenching by nitrobenzene: (A) 1 X M Ru(bpy),*+ M R~(bpy),~+ adsorbed on adsorbed on kaolin (1 g/L); (B) 3 X montmorillonite (1 g/L), with 0.01 M KCI added; (C) 3 X lo-' M Ru(bpy),'+ adsorbed on montmorillonite (1 g/L). (b) Steady-state phosfhotescence quenching by dk Stern-Volmer plot of *R~(bpy),~+ methylanlline: (A) 3 X lo-' M Ru(b y), adsorbed on montmorillonite M R~(bpy),~'adsorbed on kaolin (1 g/L). (1 g/L), (B) 1 X +
Ru(bpy),2+/Cu2+system where the quenching was approximately equal to that in homogeneous solution, indicating that the mobilities of the *Ru(bpy)32+and Cu2+ ions are close to that in water. The quenching rate constant, kql, for dimethylaniline, after correcting to the effective concentration on the montmorillonite particle, was 2 X lo8 M-' s-l and is about twice the value in homogeneous solution. This suggests that the dimethylaniline may have a much higher mobility on the clay than nitrobenzene. Table I again shows good agreement of calculated and measured values for the ratio Io/I. This is taken to indicate that no static quenching of * R ~ ( b p y ) by ~ ~dimethylaniline + occurs. Dynamic Quenching in Kaolin. The steady-state results for Ru(bpy),2+adsorbed on kaolin are distinctly different from those for montmorillonite (Figures 7, a and b). They are exemplary of the quenching behavior expected when the fluorescent probe is located only on the surface of a particle and the quenching molecule resides in the bulk solution. That is, a plateau is not reached in the SternVolmer plot as it is with montmorillonite because all of the Ru(bpy)32+is accessible to the quencher. As discussed above, it was found that neither nitrobenzene nor dimethylaniline was adsorbed by the kaolin. However, on the basis of the quenching rate constants determined from the analysis of the transient decay curves,
Photoprocesses on Colloidal Clay Systems
it appears that an association or affinity exists between nitrobenzene or dimethylaniline and the kaolin particles in solution. For nitrobenzene, the quenching rate constant, k,,, was 3 X 1O'O M-' s-l, which is about 1 order of magnitude larger than in homogeneous solution. Similarly, analysis of the decay curves upon the addition of dimethylaniline yielded 3 X log M-l s-l, about a 30-fold increase. With montmorillonite systems, the initial rate constants obtained by using bulk concentrations were 1 X 10,' and 5 X 1O1O M-' s-l, for dimethylaniline and nitrobenzene, respectively. These were only a result of using the added concentration of quencher, however, and when the adsorbed concentration was converted to the effective concentration more realistic rate constants were obtained. The conditions with kaolin may be similar and, although centrifugation and subsequent analysis of the supernatant revealed that neither of these molecules remained in the clay, a high local concentration on the surface of the kaolin particle through weak association with the surface would give similar results. Also, comparison of the steady-state quenching and that calculated from the transient decay curves indicates the existence of a static component (10-15%). The static component would arise from the association of quencher molecules with the particle surface in a region close to * R ~ ( b p y ) ~ ~ + . Quenching by Fe(CN)63-. Figure 8a shows the results obtained from the K,Fe(CN), quenching of R ~ ( b p y ) , ~ + adsorbed on a kaolin colloid. For the Na+-washed kaolin particles, K,, or the slope of the curve in the initial part of the Stern-Volmer plot was 8820 M-l compared to 180oO M-' in homogeneous solution. This result is not unexpected since a dramatic reduction in the quenching ability of Fe(CN)6* would result from its repulsion from the negatively charged kaolin surface. Because montmorillonite and kaolin have similar surface charge densities,32 the quenching behavior of Fe(CN)63-should be approximately the same. However, the site of Ru(bpy)?+, whether on the kaolin surface or in the montmorillonite layer, significantly affects the quenching behavior. This is quite apparent from Figure 8, a and b; the extent of quenching is much larger on kaolin. Pretreatment of the clay with hexametapolyphosphate dramatically reduces the quenching efficiency of Fe(CN)c3- on kaolin and montmorillonite, Figure 8, a and b. Various kaolinites are known to have an anion exchange capacity up to 20% of their c ~ c . This ~ ~ is due to the presence of positively charged sites at the edge surfaces of the particle. The atomic structure of the edge surfaces is entirely different from that of the flat-layer surfaces where a diffuse negative charge exists. Therefore, a clay particle may have two different double-layer structures due to the fact that two crystallographically different surfaces are exposed by the clay particle. Hexametapolyphosphate is commonly used as a peptizing agent for deflocculating clay colloids. The basic mechanism of the stabilization phenomenon is due to adsorption of the peptizer at the positively charged sites on the clay particle causing a charge reversal at this site and then inhibiting the edge-to-face association of the particles. Also, it is possible that these highly negatively charged anions significantly increase the negative charge on the particle upon adsorption. Based on the known properties of colloidal clay particles, the quenching behavior of Fe(CN)63-on untreated kaolin can be viewed as an association of the Fe(CN)6* with the (32)A. Weies and L. Kanter, 2.Nuturforsch. B, 15,804 (1960). (33)V. Hofmann, A. Weiss, G . Koch, A. Mehler and A. Scholz, N. AS'.-N.R.C., Publ., 456,273 (1956).
The Journal of Physlcal Chemistry, Vol. 87, No. 6, 1983 997
1.120
eate d
1.080
Io -
I 1.040
Peared w i t h poiyphasphofe
1.000
20
40
[Fe ( C N ) 9 3 ] x
60
80
100
IO
Figure 8. Steady-state Stern-Volmer plot of *Ru(bpy),2+ phosphorescence quenchlng by potassium ferricyanide: (A) 1 X lo-' M Ru(bpy);+ adsorbed on kaolln (1 g/L), (B) 1 X M Ru (bpy),2+ adsorbed on a kaolin colloid (1 g/100 mL) which had been previously treated with sodium hexametaphosphate. (b) Steady-state SternVolmer plot of *Ru(bpy);+ phosphorescence quenching by potassium ferricyanide in montmorillonite colloids.
positive edge sites and a repulsion from the negatively charged layer surface of the particle. The * R ~ ( b p y ) ~ ~ + bound to the kaolin particle in the region of the edge is able to be quenched by the Fe(CN)6* residing at the edge sites. On the basis of the steady-state Stern-Volmer data, ~+ due to more than 70% of the * R ~ ( b p y )is~ quenchable these effects. However, after pretreatment of the kaolin with polyphosphate, the K,, is reduced further to 500 M-' and only 30% of the * R ~ ( b p y ) is~ ~ quenchable + at a Fe(CN)63-concentration of 1 X M. Figure 8b shows the results for montmorillonite colloids before and after polyphosphate treatment. Before polyphosphate treatment, there is a small amount (- 10%)of quenching observed (K,, = 600 M-l) followed by an inability of Fe(CN)6* to further quench the *Ru(bpy)2+. As with the kaolin colloids, it is possible that the small amount of quenching initially observed is due to the association of Fe(CN)63-with a positive double layer at the edge of the tactoid particle. However, since the number of positive sites on montmorillonite is known to be much less than on kaolin,= less quenching is expected, as observed. Upon pretreatment of the montmorillonite with polyphosphate to reverse the charge of these sites, the quenching ability (34)L.A. Dean and E. J. Rubins, Soil Sci., 63,377 (1947).
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The Journal of Physical Chemistry, Voi. 87, No. 6, 1983
of Fe(CN)63-was essentially eliminated up to 2 X lod3M. If the montmorillonite colloid containing the F C ? ( C N ) ~ ~ was allowed to stand for approximately 30 min, an increase in the quenching of *Ru(bpy)z+was observed (dotted line). Upon the addition of more Fe(CN)63-to a concentration of 3 X M, no immediate change in the phosphorescence intensity resulted, but upon standing for 30 min, the intensity decreased. It is well-known that the addition of a 1:l electrolyte to the montmorillonite colloid causes the particle association, due to edge-to-edge and face-to-face association, to decrease.35 The scaffoldlike structures break apart, exposing more surface and decreasing the amount of interlayer volume. Hence, more of the Ru( b ~ y ) is ~ ~now + located on the more accessible outside surface and the situation approaches that of a kaolin system. Light scattering measurements made in this laboratory show that the average particle size decreases upon the addition of electrolyte due to the above processes and a concomitant increase in the number of particles takes place. These processes take a finite amount of time, however, as this effect is demonstrated in Figure 8b. Effect of KC1 on Nitrobenzene Quenching. The effect of KC1 on a montmorillonite colloid can also be seen from Figures 6 and 7b, which show the effects of KC1 on the adsorption of nitrobenzene by a montmorillonite colloid and its quenching efficiency on Ru(bpy)p that is bound to a montmorillonite particle. Nitrobenzene becomes a more efficient quencher upon the addition of 0.01 M KCl. These results would appear to contradict those obtained from adsorption isotherms of this system which indicate that at least 50% less nitrobenzene is adsorbed by montmorillonite when 0.01 M KCI is present. However, (35)I. Shainberg and H.Otoh, Isr. J. Chem., 6, 251 (1968).
DellaGuardia and Thomas
as already discussed, the scaffoldlike structures break up, exposing more surface and decreasing the amount of interlayer volume available for adsorption of nitrobenzene. Thus, the adsorption of nitrobenzene decreases because of a decrease in the total internal volume available but the quenching increases due to the exposure of more Ru(bpy)?+ to the nitrobenzene in solution.
Conclusion The localization of R ~ ( b p y ) ~on~ + either kaolin or montmorillonite particles alters the spectroscopic properties of this probe as evidenced by changes in the absorption spectrum. The emission spectra and polarization studies at 77 K indicate that adsorption on the surface of kaolin or in the layers of montmorilloniteprovides a unique environment. The data show that, when a luminescent probe such as R ~ ( b p y )is~ bound ~ + to either kaolin or montmorillonite particles in aqueous colloids, the observed rates for quenching reactions are enhanced due to the localization of probe and quencher in the domain of the particle. Reevaluation of the data using the effective concentration of quencher molecules on the montmorillonite particles indicates that bound cations such as Cu2+,as well as the neutral molecules nitrobenzene and dimethylaniline, diffuse on the surface and in the internal layers of the particle at rates close to those observed in homogeneous aqueous solution.
Acknowledgment. We thank the Army Research Office via grant No. OAAG29-80-KO007 for support of this research. Registry No. Tris(2,2’-bipyridine)ruthenium(II),15158-62-0; nitrobenzene,98953; dimethylaniline,121-69-7; Cu2+,15158-11-9; Eu3+,22541-18-0.
