1350
Langmuir 1990, 6, 1350-1356
the hydroxy-A1 clay is expected to possess the greatest porosity, and this expectation is consistent with the result of the most efficient reaction with oxygen for this system. After the hydroxy-Al-pillared hectorite, the tetramethylmodified clay shows the next greatest efficiency of quenching by oxygen. Results for the tetraethylammonium clay are similar to that for the unmodified clay, while in the case of the tetrabutylammonium clay the extent of penetration by oxygen is extremely small, due to effective blocking of pathways within the clay structure by the spacefilling butyl substituents. Figure 7. Stern-Volmer plots for oxygen quenching of tris(bipyridine)rutheniumluminescence in various hectorite and modified hectorite powders all dried at 110 OC: (A) hydroxyA1 pillared; (B)tetramethylammonium pillared; (C)unmodified hectorite; (D)tetraethylammonium pillared; (E) tetra-n-
butylammonium pillared.
of the cation-exchange capacity. Figure 7 shows SternVolmer plots for the oxygen quenching of the probe luminescence for each of the modified clay samples. These plots are the ratio of luminescence intensity in the absence of quencher to that in the presence of various amounts of quencher versus the amount of quencher, indicated here as oxygen pressure. Not included in the figure are data for the unmodified hectorite which has been dehydrated by heating a t two different temperatures. As expected, for t h e unmodified clay sample, more extensive dehydration tends to collapse the clay layers, decreasing the effective pore size, and decreasing the accessibility of the interlayer region to oxygen. Among the pillared clays,
Conclusion Synthetically derived pillared clays possess unique properties which may be characterized through the use of photophysical and photochemical techniques. Proper choice of pillaring agent allows tailoring of the resultant properties of enhance adsorption characteristics from solution or modification of the effective microporosity. A unique chemical environment is created within the pillared clays. Photochemical quenching studies utilizinggasphase reagents can be used to monitor or characterize the microporosity. Acknowledgment. We thank the NSF and Miles Labs for support of this work. Registry No. Tetra-n-butylammoniumchloride, 1112-67-0; tetramethylammonium chloride, 75-57-0;tetraethylammonium chloride, 56-34-8;aluminum(II1) chloride, 7446-70-0; sodium hydroxide, 1310-73-2;tris(bipyridine)ruthenium, 74391-32-5; pyrene, 129-00-0; oxygen, 7782-44-7.
Photophysical Investigation of the Degree of Dispersion of Aqueous Colloidal Clay Victor G. Kuykendall and J. Kerry Thomas* Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 Received September 1 , 1989. In Final Form: December 22, 1989 The photophysical properties of water-soluble tetrakis(N-methylpyridy1)porphyrin(TMPyP)are altered upon adsorption to the surface of a smectite clay mineral in an aqueous dispersion. In particular, the Soret absorption band shifts from 421 nm in aqueous solution to 452 nm when the molecule is adsorbed on the external face of a clay sheet in contact with the bulk solution. Upon intercalation between clay sheets, the Soret band further shifts to 488 nm. The net Soret absorption band observed in a particular clay sample is a superposition of contributions from intercalated versus externally adsorbed porphyrin and provides a convenient method to monitor the degree of dispersion or extent of deflocculation of a clay dispersion to produce primary particles or single sheets. Further, the spectral changes can be utilized to assess the effect of chemical and physical modification to the clay dispersion on the extent of faceto-face particle aggregation. Shifts in the fluorescence spectrum parallel those of the Soret band. A mechanism which could account for the substantial spectral changes observed is proposed.
Introduction p h o ~ p h y s i cinvestigations ~ are useful for the study of surface phenomena when the spectroscopic properties of the probe are altered upon adsorption to the surface, thus providing information about the environment 0743-7463/90/2406-1350$02.50/0
at the surface or interface. The particular surface system of interest here is the aqueous dispersion of a clay mineral. lp2
(1)Thomas, J. K. The Chemistry of Excitation at Interfaces; ACS Monograph 181; American Chemical Society: Washington, 19M. (2) Thomas, J. K. J. Phys. Chem. 1987, 91, 267.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 8, 1990 1351
Dispersion of Aqueous Colloidal Clays
However, while the viscosities and shear strength Interest in the clay minerals stems from their many measurementsprovide some informationabout interactions industrial applications and also the abundance of clays in between particles, it is difficult t o obtain precise nature, which makes their study of environmental information about the extent of layering, since clay particle importance. edge-edge or edgeface interactions can dominate changes The particular clays studied are part of the smectite in the viscosity.3J1J2 group of minerals. These clays are layer silicates with extremely flat morphology and are characterized by micron Recently, plate-plate particle interactions have been or submicron particle sue, cation-exchangecapability, and studied by the direct measurement of surface forces using unique adsorptive properties.3-6 Isomorphous substitution mica crystals a n d aqueous montmorillonite diswith cations of low valence leads to a net negative charge persions.gJ0J4 The results suggest that the small size within the clay structure, and this negative charge is fraction of lithium montmorillonite completely disperses compensated by adsorption of various cations at the surface as single sheets in water. From small-angle neutron of the clay sheets. In water, these clays tend to disperse diffraction studied and electron microscopy of lithium to form a suspension of hydrated particles. m~ntmorillonite,~J~ it was concluded that the major part An aqueous clay suspension contains hydrated clay of the montmorillonite sample consisted of single plates. particles, each consisting of one or more individual clay In the case of Laponite, rheological and turbidity sheets separated by several water layers. The absolute measurements suggest that below 2 % concentration Lapodegree of dispersion, or the number of colloidal particles nite platelets separate completely; i.e., they are fully compared to the number of individual clay sheets, is in dispersed.'8J9 general not known, although there have been numerous This paper reports the use of a water-soluble porphyinvestigations of aqueous dispersions of some specific clay rin as a photophysical probe which reports on the location samples.7-15 Experimentally, measurement of the degree of the adsorption environment. In particular, marked of dispersion in dilute aqueous colloidal clays has proven spectral changes occur depending on whether the probe to be difficult. is located within an interlayer region (an indication of low Dry smectite clays initially expand in the presence of dispersion) or whether the probe is located on an external water to take up one to three or four water layers in a stepclay face in contact with the bulk solution (an indication wise fashion, which increases the interlayer separation to of high dispersion). Thus, the porphyrin probe may be distances of 3-12 A.3 In contact with a bulk water phase, used to monitor the degree of dispersion of clays under the exchangeable cations a t the clay surface have a various conditions. The effects of chemical and physical tendency to diffuse into the bulk water, where their modification of the colloidal state on the face-to-face concentration is lower. As the diffusion of the cations aggregation of clay sheets may also be monitored. proceeds, significant particle-particle repulsions develop, The spectral properties of porphyrins and metalloporand this leads to dissolution or deflocculation of the clay phyrins have been studied in some detai1,20-30 and mineral (osmotic swelling) and to formation of a colloidal correlations have been developed which relate the suspension. Hydration forces may independently contribute to the swelling behavior of expanding c l a y ~ . ~ J ~ absorption band energies and spectral shapes with structure.m The Soret band generally shifts to lower energy The net behavior of a clay in water may be regarded as upon protonation of the porphyrin to the dication and is the sum of contributions from the Born repulsion, the van also somewhat solvent dependent. Characteristic der Waals attraction, and the electric double-layer absorption spectral patterns are observed for the four repulsion. The specific countercations involved (Schulze visible bands in the free base spectrum, depending on the Hardy rule), the surface charge density, and particle size symmetry and nature of the substituents. all affect the degree and stability of a clay dispersion.3J3 The intense Soret absorption band has been identified The clay hydrates initially formed in the presence of bulk as an E, Al, transition, from the ground-state singlet water (three to four water layers) possess some stability, to the second excited singlet state, involving displacement and it is usual to apply some physical agitation of the of electron density toward the periphery of the porphysample to enhance the dispersion of the clay. rin ring. Thus, electron-attracting substituents tend to The rheological behavior of clay suspensions is related stabilize this excited state, and shifts to longer wavelength to the degree and nature of particle-particle aggregation are observed to correlate with the electron-attracting power and is of concern in many technological applications.3J6J7
-
(3) van Olphen, H. An. Introduction t o Clay Colloid Chemistry; Wiley: New York. 1977. .(4) Theng, B. K. G. The Chemistry Reactions; Wiley: . of. Clay-Organic . New York, 1974. (5) Brindley,G. W., Brown, G.,Eds. Crystal Structures of Clay Minerals and their X-Rav Identification: Minerolonical Societv: London. 1980. (6) Barrer, R.M. Zeolites and Clay Miner& os Sorbents and Moiecular Sieoes; Academic Press: London, 1978. (7) Cebula, D. J.; Thomas, R. K.; Middleton, S. R. Clays Clay Miner. 1979, 27, 39. (8) Cebula, D. J.; Thomas, R. K.; White, J. W. J . Chem. Soc., Faraday Trans. I 1980, 76,314. (9) Viani, B. E.; Roth, C. B.; Low, P. F. Clays Clay Miner. 1985,33, 244. (10) Viani, B. E.; Low, P. F.; Roth, C. B. J . Colloid Interface Sci. 1983, 96,229. (11) Foster, W. R.; Savins, J. G.; Waite, J. M. Clays Clay Miner. 1954, 3, 246. (12) Wood,W. H.; Granquist, W. T.; Krieger, I. M. Clays Clay Miner. 1955, 4, 240. (13) van Olphen, H. Clays Clay Miner. 1954,2,418. (14) Lubetkin, S. D.; Middleton, S.R.; Ottewill, R. H. Philos. Trans. R. SOC.London 1984, A31 1 , 353. (15) Cebula, D. J.; Ottewill, R. H. Clays Clay Miner. 1981,29, 73. . _-
----I
(16) Neumann, B. S.;Sansom, K. G. Isr. J. Chem. 1970,8,315. (17) Perkins, R.; Brace, R.; Matijevic, E. J. Colloid Interface Sci. 1974, 48,417. (18) Moyea, J. R. Minerals Erg.SOC.Tech.Mag. (U.Birmingham) 19751976,41-53,19. (19) Lawrte Industries. Technical Brochure no. L64. (20) St&, A.; Wenderlein, H. Z. Physik Chem. l936,176A, 81. (21) Gouterman, M. J. Mol.Spectrosc.. 1961, 6, 138. (22) Fleischer, E. B.; Webb, L. E. J. Phys. Chem. 1963, 67, 1131. (23) Fleiecher, E. B. Inorg. Chem. 1962; 1,493. (24) Kim, B. F.; Bohandy, J. John Hopkins APL Tech. Dig. 1981,2, 153. (25) Darwent, J. R.; Douglas, P.; Harriman, A,; Porter, G.; Richoux, M. Coord. Chem. Reo. 1982,44,83. (26) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. Inorg. Chem. 1980, 19, 386. (27)Kalyanasundaram, K.; Neuman-Spallart, M. J. Phys. Chem. 1982, 86, 5163. (28) Falk, J. E. Porphyrins and Metalloporphyn'ns; Elsevier: New York, 1964. (29) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 111. (30) Falk, J. E., Lemberg,R., Morton, R. K., Eds. Haematin Enzymes; Pergamon Press: New York, 1961.
1352 Langmuir, Vol. 6, No. 8, 1990
of the substituents. Similarly, lower electronegativity of the metal in metalloporphyrins leads to lower energy transitions. The four visible bands (I-IV) observed in the free base have been identified as a E, Az,, transition, populating the first excited singlet excited state. These bands are believed to correspond to a symmetry-forbidden (0-0) transition (bands I and 111) and a symmetry-allowed (01)transition (bands I1 and IV), which are split into four bands due to lower than overall square symmetry. Protonation to the dication, or metallation, which brings the porphyrin nucleus closer to square symmetry results in the observation of only two prominent visible bands, a lowintensity (0-0)band of lower energy and a more intense band (0-1) at higher energy. It has been observed29that bands I1 and IV are less affected by changing substituents than bands I and 111, as expected from the above discussion. The initial impetus for choosing the water-soluble porphyrin TMPyP as a potential photophysical probe of faceto-face clay particle aggregation was based on simple empirical considerations. The high positive charge (+4) was expected to result in complete probe adsorption to the clay surface at levels below the cation-exchange capacity, and indeed this is observed. The overall approximately flat porphyrin shape and peripheral location of the N-methylpyridyl positive charges were expected to result in a more or less parallel orientation of the molecule to the flat clay surface, and X-ray powder diffraction measurements confirm this mode of adsorption. The established environmental sensitivity of the photophysical properties of porphyrins to solvent, acidity, axial ligation, etc., held promise for distinguishing external particle face adsorption versus intercalation between clay sheets. As is discussed below, the magnitude of the spectral effects observed under conditions corresponding to the two situations was substantial. The spectral effects are interpreted in terms of a specific interaction of the porphyrin molecule with the clay surface, the origin of which is a combination of the surface acidity, electrostatic, and steric factors. +
Experimental Section Chemicals. Laponite XLG and RDS (Laporte Industries), tetra-n-butylammonium chloride (Aldrich), tetramethylammonium chloride (Eastman), calcium chloride (Fisher), spraydried natural hectorite (National Lead), and hydrochloric acid (Fisher) were used as received. TMPyP (Midway) was generously provided by Marie Ivanca and was used as received. Butylammonium chloride was prepared by reacting butylamine with HCl. Distilled-deionized water was used for all aqueous solutions. Natural hectorite (SHCa-1, San Bernadino, CA), was reacted for 24 h with 1 M sodium acetate/acetic acid buffered at pH 5 for removal of carbonate impurities and exchanged with 1 M sodium chloride for several days for conversion to the sodium clay. The hectorite was centrifuged, resuspended, washed with distilled-deionized water, and finally dialyzed, until a negative chloride test was obtained with 0.1 M silver nitrate. Montmorillonite (Georgia Kaolin) Gelwhite L was prepared similarly. Equipment. Absorption spectra were obtained on a PerkinElmer 552 spectrophotometer with the option of utilizing an integrating sphere type reflectance mode accessory for the study of solids. Steady-state luminescence measurements were performed on an SLM SPF5OOC. Laser excitation was provided by a Photochemical Research Associates LNlOO nitrogen laser with pulse width 120 ps, energy 50 gJ/pulse, and wavelength 3371A. Time-resolved emission was observed via the signal from an R928 photomultiplier tube connected to a Tektronix 7912AD programmable digitizer interfaced to a Zenith 80286 based microcomputer. X-ray diffraction measurements were performed by using a Philips diffractometer and a PW1710 control unit.
Kuykendall and Thomas
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Figure 1. (a, Top) Steady-state absorption spectra for TMPyP
in Soret band region under three conditions: (1)aqueous solution; (2) adsorbed state on fresh 1.0 g/L Laponite XLG colloid; (3) Laponite film. (b, Bottom) Same as for a, but absorption was monitored in visible region.
Results and Discussion Absorption Spectra. Figure 1 presents the steadystate absorption spectra for TMPyP in both the Soret and visible regions under various conditions. In aqueous solution, one observes the Soret band with characteristic absorption maximum at 421 nm. When adsorbed on a freshly prepared aqueous Laponite colloid, the Soret maximum is shifted to 452 nm, with a shoulder at about 490 nm. When TMPyP is adsorbed in the interlayer region of a Laponite film, the Soret maximum is red-shifted to 488 nm. These Soret band shifts are interpreted to correspond to largely external clay particle face adsorption (452-nm maximum with 490-nm shoulder) and t o intercalation (488-nm absorption maximum). The shoulder a t about 490 nm in the case of the aqueous colloidal spectrum suggests that some layering occurs, even in a dilute freshly prepared Laponite colloid. Figure 2 shows the diacid absorption spectrum compared to that of the free base. As is generally found, the Soret band is red-shifted, and the four Q bands collapse into only two prominent bands. At pH 0.1, the Soret maximum is at 449 nm. Thus, a part of the explanation of the Soret absorption band shifts on the clay surface is that the Brmsted acidity at the clay surface is capable of protonating the very weakly basic porphyrin (p&, p K a 1). The protonation of organic species adsorbed on clays beyond that which might be predicted from simple bulk solution pH measurements is a common phen~menon.~l.~* Addition
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(31) Karickhoff, S. W.; Bailey, C. W. Clays C h y Miner. 1976,24,170. (32) Theng, B. K.G. Clays Clay Miner.1971,19,383.
Langmuir, Vol. 6, No. 8, 1990 1353
Dispersion of Aqueous Colloidal Clays
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Figure 2. TMPyP diacid absorption spectrum in comparison to TMPyP free base. Free base absorbs at higher energy.
of HC1 to the 1 M level has essentially no effect on the absorption spectrum in the presence of the Laponite colloid, confirming that the initial adsorption to the clay particle involves protonation to the diacid level. In the visible region of the absorption spectrum, the characteristic free base four-band phyllo type spectrum is observed for the aqueous colloidal solution. For the aqueous colloidal Laponite, a four-band Q absorption region pattern is also observed, in spite of the greater symmetry normally associated with the diacid (D4h), analogous to the metalloporphyrin, compared to the free base (&h). I t is reasonable to suggest that a structural distortion of the porphyrin electronic core occurs due to the electrostatic attraction of the N-methylpyridyl groups to the negatively charged clay surface, resulting in less than D4h symmetry. In the case of the Laponite film, a two-band visible spectrum is again observed. I t seems likely that the approximate D4h symmetry is restored when the porphyrin molecule is flanked on both sides by the clay sheets (both sides become equivalent again) in the case of a clay film. The extreme red shift to about 490 nm of the Soret band, however, cannot be explained on the basis of symmetry considerations. It has been observed that the position of the Soret maximum is sensitive to the electrondonating or -withdrawing character of external substituents.28.B In studies of the interaction of TMPyP with various DNA samples, Pasternack et a1.34*35 have found that rotation of the N-methylpyridyl groups away from a perpendicular orientation with respect to the porphyrin ring is accompanied by a significant red shift of the Soret maximum (up to 21 nm), under conditions which do not involve protonation. In that work, the exact extent of rotation of the pyridyl groups was unknown, but significant rotation was presumed to occur based on steric considerations for intercalation, and the observation that the o-N-methylpyridyl-substitutedporphyrin, which has a greater rotational barrier, did not intercalate at all. I t is proposed that between the clay sheets (clay film) the combination of electrostatic attraction and the rigidity of the clay surface results in a significant rotation of the N-methylpyridyl groups from their nominal perpendicular orientation. A more closely parallel orientation may decrease the extent of electronic isolation of the N-methylpyridyl groups and increase their effective electron(33) Moet-Ner, M.; Adler, A. D. J. Am. Chem. SOC. 1976,97,5107. (34) Pasternack, R. F.; Gibbs, E. J.; Villafranca, J. J. Biochemistry 1983, 22,2406. (35) Pasternack, R. F.; Gibba, E. J.; Villafranca, J. J. Biochemistry 1983, 22, 5409.
WAVELENGTH (nm)
Figure 3. TMPyP absorption spectrum in water and on a fresh natural hectorite colloid for comparisonwith the Laponite results
in Figure 1.
withdrawing character, resulting in the large red-shifts observed. At most, it is expected that a 60' or possibly 40' angle with respect to the porphyrin electronic core might be achieved36 in the case of extreme distortion. In view of Meot-Ner and Adler's o b s e r ~ a t i o nt h~a~t , apparently, significant electronic resonance interactions exist between perpendicular substituted phenyl substituents and the porphyrin electronic core, it seems reasonable that the electronic interaction might be considerably enhanced by a less perpendicular orientation of the substituents, resulting in greater orbital overlap between the ring core and the N-methylpyridyl substituents. X-ray diffraction results indicate that TMPyP occupies a maximal effective thickness of about 4.4 A (at most 4.5 A if maximum Ykeying"4is presumed) in a clay film. These X-ray diffraction signals were by necessity recorded with a high loading of TMPyP on the clay between 20% and 90% of the cation-exchange capacity. This would correspond to an upper limit angle of 69-73' between the porphyrin core and the substituent N-methylpyridyl rings. It is possible that under conditions of low loading as used in the spectral measurements (0.01-1 % cation-exchange capacity) the porphyrin is further compressed by stronger clay layer compression forces, resulting in a smaller angle. There was only a small 3-nm shift of the Soret band position on going from water to micellar sodium laurel sulfate, micellar Triton X-100, or ethanol. When adsorbed to a highly charged liquid crystal assembly, there were also only small changes in the Soret absorption, indicating that not only the high charge density on the clay surface but the rigidity of the clay surface as well is involved in the observed effects. Figure 3 shows the Soret absorption region for a natural hectorite (under conditions analogous to those described above) for comparison to the Laponite results. The natural hectorite possesses an overall larger particle size and more extensive face-to-face aggregation in the colloidal state (and lower stability). The spectra support this expectation. In the colloidal solution case, bands of almost equal intensity are observed, corresponding to about half of the TMPyP being adsorbed in the interlayer region and about half on external clay particle surfaces. Fluorescence Spectra. Figure 4 shows the steadystate emission from the lowest excited singlet state observed for TMPyP in water, on a Laponite colloid, and within a Laponite film. In water, a broad emission maximum around 680 nm is observed. There has been a suggestion3' (36) Scheidt, W. R. Personal communication.
1354 Langmuir, Vol. 6, No. 8, 1990
Kuykendall and Thomas 1
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that this featureless emission is due to the existence of aggregates of TMPyP, which might be the dominant species down to 0.1 pM or less, making an assessment of aggregation by routine Beer's law plots difficult. With adsorption to the aqueous Laponite colloid, a structured emission red-shifted with maxima at 700 and 757 nm is observed, with similar appearance to the spectrum observed in methanol where TMPyP aggregates are believed to be de~troyed.~'Within the clay film, the emission is further red-shifted. For the Laponite colloid and the natural hectorite colloid, excitation spectra taken a t different observation wavelengths are different and support the existence of two distinct species under these conditions. Modificationsof the Colloidal State. One feature of the colloidal state of aqueous dispersed clays which may be monitored by using TMPyP is the stability of the dispersion over time. Thus, upon aging of the clay colloid, gradual flocculation occurs due to spontaneous faceto-face aggregation and is reflected by changes in the absorption spectra. Figure 5a shows the spectral changes observed in the Soret absorption region on aging a freshly prepared natural hectorite colloid for up to 34 h. Initially, a significant degree of layering is observed, and this layering increases markedly over the observation period. The presence of an isobestic point in this case confirms that the spectral changes are due to creation of the layered species at the expense of the externally adsorbed species. In the case of the Laponite colloid, which possesses much greater stability against flocculation, the changes upon aging are less pronounced. The spectra in the bottom of Figure 5b show the normalized Soret absorption bands as they evolve over a 3-month period. For both the Laponite and natural hectorite, no changes in the colloidal state over the time periods studied were apparent to the eye. These data illustrate a source of variability which can influence the chemical and physical properties of clay dispersions over relatively short periods of time. The Soret absorption spectra for TMPyP on a 15 g/L aqueous Laponite colloid, a 25 g/L viscous colloid, and a 40 g/L gel were compared. In spite of the marked changes in the physical properties of the system on going from the colloid to the gel, the spectra were remarkably similar. These data illustrate the well-established fact that gel formation is primarily due to linking of the clay particles through edge-face interactions rather than face-face interactions. To check the response of the colloidal clays to addition of flocculating electrolytes, calcium dication was added to (37)Kano, K.; Miyake, T.; Uomoto, K.; Sato, T.; Ogawa, T.; Hashimoto, S. Chem. Lett. 1983, 1867.
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Figure 5. (a, Top) Spectral changes in Soret band for adsorbed TMPyP as fresh natural hectorite colloid is aged over a 34-h period illustrating isobesticpoint: (A) freshly prepared; (B) 2 h; (C) 5 h; (D) 34 h. (b, Bottom) Normalized absorption spectra for TMPyP on Laponite colloid as it is aged over a 3-month period: (A) freshly prepared; (B) 30 min; (C)6 h; (D)30 h; (E) 38 h; (F) 115 days.
a dilute Laponite/TMPyP colloid. As Figure 6a shows, as expected, the spectral response to addition of small amounts of calcium ion is due to an increase the extent of clay layering in solution. Figure 6b also illustrates the effect of sonication on the Soret absorption of TMPyP on a natural hectorite colloid. After sonication for 5 min, a noticeable decrease in the amount of layered structures is observed. Relation of the Soret Energy to Interlayer Distance. In order to estimate the dependence of the Soret band spectral shifts to the interlayer distance between the clay sheets, the X-ray powder diffraction patterns for clay samples under various conditions were monitored and correlated with the corresponding absorption spectra under the same conditions. Figure 7a shows (top) the X-ray diffraction pattern observed for a natural hectorite film under ambient conditions and after brief saturation by immersion under water. Initially, a monolayer of water is present between the clay layers. Upon immersion in water, the clay sheets expand to an 18.6-A repeat distance corresponding to three layers of water in the interlayer region. The TMPyP spectra corresponding to these conditions are also shown in Figure 7b. A 5-nm blue shift is observed on increasing the between-layerspace available t o the TMPyP from 4.4 to 9 A. This shift is small compared t o the 36-nm shift observed between the externally adsorbed and intercalated TMPyP. Thus, if N-methylpyridyl ring rotation is important in producing the observed spectral changes, then a significant ring rotation is maintained even when the clay layers are
Langmuir, Vol. 6, No. 8, 1990 1355
Dispersion of Aqueous Colloidal Clays 5
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Figure 6. (a, Top) Effect of calcium ion addition on Soret absorption of TMPyP/Laponite colloid: (1) before calcium addition; (2) after calcium addition to 1mM level. (b, Bottom) Effect on brief sonication of Soret band of TMPyP on a natural hectorite colloid (1) before sonication; (2) after 5 min of sonication. separated by 9 A. Attempts to achieve a uniform interlayer spacing at greater distances for correlation with spectra were uncertain due to low angle measurement limitation of the X-ray diffractometer and the likelihood that a distribution of interlayer distances was obtained, leading to broadened and weak diffraction peaks. It is established that beyond the stepwise hydration which takes place to the three or four water layer level, there is a "jump" to an interlayer spacing of about 30 The results obtained suggest that a t a free interlayer distance of 30 the TMPyP behaves entirely as when adsorbed to the external clay particle face. Butylammonium-exchanged clays are established to readily expand in the presence of water, although they also exhibit a "jump" to relatively long spacing following the three water layer hydration The Soret absorption maximum for TMPyP adsorbed within a butylammoniumexchanged air-dried Laponite film was broadened and slightly blue-shifted by 7 nm compared to the sodium Laponite film. Figure 8 shows the effect of wetting the butylammonium clay film on the Soret band. A marked blue shift is observed, indicating that the clay sheets have expanded to a distance at which the TMPyP can interact effectively with only one of the two clay sheets between which it is initially adsorbed. Conditions were not established under which an X-ray diffraction pattern corresponding to the externally adsorbed spectral behavior could be obtained. (38) Fukushima, Y.Clays Clay Miner. 1984, 32,320. (39)Garret, W. G.; Walker, G. F. Clays Clay Miner. 1960, 9,557.
Figure 7. (a, Top) X-ray powder diffraction pattern obtained for natural hectorite film: (1)air dry; (2) after 1min of immersion under water. (b, Bottom) Soret absorption spectra corresponding to the two conditions in a. Od 7
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, 560
WAVELENGTH (nm)
Figure 8. Soret absorption band of TMPyP within butylammonium-saturated Laponite film: (1)before wetting; (2) after brief exposure to water. Conclusion .The water-soluble porphyrin TMPyP is useful as a photophysical probe of face-to-face clay particle aggregation. It is believed that electronic interaction of the external substituents with the porphyrin core may be enhanced by rotation of the substituents from a 90" to a 70" orientation with respect to the porphyrin core, although the exact mechanism responsible is unknown. The protonation of TMPyP combined with steric constraints imposed by the flat, rigid clay surface and the electrostatic attraction of TMPyP to the clay lead to large shifts in the Soret transition energy on going from aqueous solution to adsorption at an external particle face to intercalation between clay sheets.
Langmuir 1990,6, 1356-1359
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Acknowledgment. We thank the National Science Foundation (CHE 8911906) and Miles Inc. for support of this work.
Registry No. TMPyP, 50391-14-5;calcium chloride, 1004352-4; tetra-n-butylammonium, 10549-76-5;tetramethylammonium, 51-92-3.
Crystal Growth of Polycrystalline a-CdS on Conducting Polymers Evangelos Dalas,+ Sotirios Sakkopoulos,i Jannis Kallitsis,' Evangelos Vitoratos,t and Petros G. Koutsoukos*l§ Department of Chemistry, University of Patras, Greece, Department of Physics, University of Patras, Greece, and Department of Chemical Engineering and the Research Institute of Chemical Engineering and Processes at High Temperatures, P.O. Box 1239, Patras, Greece Received December 8, 1989. In Final Form: February 23, 1990 Conducting polypyrrole polymers were doped during polymerization with a-CdS crystals. Doping made the polymers effective nucleators of a-CdS. Once the doped polypyrrole specimens were introduced into supersaturated cadmium sulfide solutions, a-CdS precipitated immediately without any precursor phase or induction period. The rates of precipitation were found to depend on the solution supersaturation and were maximum at an optimum dopant concentration of 6.4 % w/w. Increasing dopant concentrations decreased the polymer conductivity and increased its p-semiconductingcharacter, possibly through structural changes induced by doping. Introduction Organic polymers are increasingly replacing metals and other inorganic materials, in numerous applications ranging from photoconductors and piezoelectric devices to car parts. This trend is primarily based on advantages shown by the organic polymers, such as performance, durability, processability, and relatively low cost. The field of applications has recently been widened tremendously with the discovery of the first conducting polymers.'-5 Polymers such as polyacetylene and poly(pheny1ene chalcogenide^)^.^ have acquired semiconducting or metallic conductivities through doping with electron donors or electron acceptors. It is believed that the polymer chain structure, both in the presence and in t h e absence of dopants, is of key importance for the electronic properties of the polymer t
Department of Chemistry, University of Patras.
* Department of Physics, University of Patras.
JI Department of Chemical Engineering and the Research Institute of Chemical Engineering and Processes at High Temperatures. (1) Mac Diarmid, A. G.; Heeger, A. J. J. Synth. Met. 1980,1, 101. (2)Kanazawa, K. K.; D i u , A. F.; Geiss, R. H.; Gill, W. D.; Kwak, J. F.; Logan, J. A.; Rabolt, J. F.; Street, G. B. J. Chem. Soc., Chem. Commun. 1979.854. (3)Yam'amoto, T.;Sanechika,K.; Yamamoto,A. J.Polym. Sci., Polym. Lett. Ed. - 1980. ~... , 18. - - ,9. Sibson, H. W.; Bailey, F. C.; Pochan, J. M.; Epstein, A. J.; Rom-
cawa, H.; Ikeda, S. J. Polym (6)Shacklette, L. W.; Elsenbeumer, R. L.; Chance, R. R.; Eckhardt, H.; Frommer, J. E.; Baughman, R. H. J. Chem. Phys. 1981,75,1919. (7)Rabolt, J.F.; Clarke, I. C.; Kanazawa, K. K.; Reynolds, J. R.; Street, G.B. J . Chem. SOC.,Chem. Commun. 1980,348.
0743-7463/90/2406-1356$02.50/0
dopant systems.8 Besides polyacetylene, other polymers with interesting electronic properties include polythiophene, polypyrrole, polydiacetylene, and polyparaphenylene. Doped polypyrrole has advantages over other polymers, the most important of which are that it may be obtained as filmsgJ0 and it is the most environmentally stable conducting polymer. An interesting prospective application of polypyrrole would be the construction of photovoltaic cells by using cadmium sulfide deposits as windows. Polypyrrole, doped with perchlorate, has a band gap of 3.0 eV," and cadmium sulfide with a band gap of 2.43 eV would make a good window material for a CdS/polypyrrole heterojunction solar cell. It has recently been shown that modification of polymers by introducing the appropriate active groups enables them to serve as substrates which nucleate sparingly soluble salts selectively. The nature of the active groups determines the salt which may nucleate.I2-l4 In the present work, we have doped polypyrrole with polycrystalline cadmium sulfide, instead of modifying the polymer. This resulted (8)Baughman, R. H.; Bredas, J. L.; Chance, R. R.; Elsenbauma, R. L.; Shacklette, L. W. Chem. Reu. 1982,82, 209. (9)Dall'Olio, A.; Dascola, Y.; Varacco, V.; Bocchi, V. C. R. Seances Acad. Sci., Ser. C 1968,267, 433. (10)Soga, K.; Kobayashi, Y.; Ikeda, S.; Kawakami, S. J.Chem. Soc., Chem. Commun. 1980,931. (11)Kaufman, J. H.; Colaneri, M.; Scott, J. C.; Street, G. B. Phys. Rev. Lett. 1984, 53,1005. (12)Addadi, L.; Moradian, J.; Shay, E.; Maroudas, N. G.;Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1987,84, 2732. (13)Dalas, E.;Kallitsis, J.; Koutsoukos, P. G . J. Cryst. Growth 1988, 89,287. (14) Dalas, E.; Koutsoukos, P. G. J . Colloid Interface Sci. 1989,127, 273.
0 1990 American Chemical Society
