J. Phys. Chem. 1987, 91, 261-216
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FEATURE ARTICLE Characterization of Surfaces by Excited States J. K. Thomas Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 (Received: June 16, 1986; In Final Form: September 4, 1986)
The techniques of photochemistry and photophysics, both steady-state and pulsed methods, have been successfully used to investigate interfaces. Spectral luminescence of an excited state reports back on the environment of the excited state, i.e. on the polarity of the surfaces and the nature of adsorption of the molecule to the surface; alterations of the surface by coadsorbed molecules are also monitored. Kinetic studies describe movement of molecules on the surface either of a luminescent probe molecule or of molecules that quench the luminescence. Access of nonadsorbed molecules to the surface is also monitored. Such parameters are of great utility in explaining photoinduced reactions at surfaces and in particular provide data to check special features of reactions at surfaces. Techniques of colloid chemistry enable one to prepare materials of small and selected dimensions which modify important properties of the material, e.g. semiconductors. Spectroscopic and excited-state techniques again provide invaluable information on the new materials. Many surfaces and interfaces are discussed, both inorganic (semiconductors, metal oxides) and organic (micelles, membranes, polymers), but these only provide a partial view of the rapidly developing field of excited-state chemistry at interfaces.
Introduction The role played by surfaces in chemical reactions is a source of intrigue for kineticists, colloid and polymer chemists, and biochemists. The exterior of a material is of prime importance to industrial chemistry, as it is here that corrosion chemistry begins or it is here that a selected feature of a chemical reaction is catalyzed. The surface contact can be of many kinds, air-solid, liquid-solid, liquid-liquid, etc., and a wide variety of physical methods have been developed to investigate the various “surface” systems of interest. This review indicates just four referen~esl-~ out of the many which are available. The crux of the matter with chemical reactions on surfaces is the enhancement of a selected chemical change or catalysis of a reaction; such processes are common in chemical industry, while membranes and membrane surface chemistry play a key role in many biosystems. The mimicry of the photosynthetic process has been of major interest over the past 15 years,5 due to its association with storage of light energy, leading the development of the field of photochemistry in nonhomogeneous media.6,7 This new area of research, which crosses the fields of photochemistry and colloid chemistry, is primarily concerned with the effects of colloidal systems on photochemical events. The colloidal systems are usually aqueous colloids of inorganic material, e.g. CdS, Ti02, clays, S O z , etc., or of organic materials formed via surfactants, e.g. micelles, microemulsions, polyelectrolytes,vesicles, etc. The active surface is the water-colloid interface. Thus, the chemistry of surface science can lie in the bulk colloid-gas, solid-liquid interface, as in heterogeneous catalyses, or in the (1) Somorjai, G. A . Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, N Y , 1981. (2) Thomas, J. M.; Thomas, W. .I. Introduction to the Principles of Heterogeneous Catalysis; Academic: New York, 1967. (3) Adamson, A . W. Physical Chemistry of Surfaces; Wiley: New York, 1982. (4) Rochester, G. H.; Smith, A. L. In Adsorption f o r Solution; Otteswill, R. H., Ed.; Academic: New York, 1983. (5) Fendler, J. H. J . Phys. Chem. 1985, 89, 2230. (6) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (7) Thomas, J. K. Chemistry ofExcitation at Interfaces; ACS Monograph 191; American Chemical Society: Washington, DC, 1983.
0022-3654/87/2091-0267$01.50/0
colloid-liquid interface. In all cases the surface plays a key role in the chemistry of interest. It has long been recognized that the sites of adsorption of the reactants are unique and give rise to the catalysis. In the case of solid catalysis, a selected bond of the reactant is influenced by the adsorption process in such a way as to promote the reaction of interest. Similar processes are rare in colloid systems; here the surface tends to directly affect the reaction, possibly the reaction transition state or the subsequent reaction of the product intermediates in the reaction cage. Quite often, as with micelles and similar systems, the surfaces may possess high charges (- lo6 V/cm), which directly affect the two processes stated above. In all cases the surfaces provide an unique environment for the reaction, and it is imperative that the nature of the surface and site of adsorption of reaction are described as well as possible. The earlier references describe many physical techniques for the study of surfaces. This article discusses a relatively recent approach, that of observing excited-state processes at interfaces. Rationale of Studies of Excitation at Interfaces. Excited states as probe ,molecules comment on their environment in two basic ways: spectroscopic measurements commenting on (a) the nature of the environment of the excited state and (b) the relative ease of movement at the site of solubilization, i.e. rotational movement at the site and gross lateral movement on the surface. Movement of other molecules (quenchers) to the excited state is also reflected in this type of kinetic measurement. If the probe molecule shows considerable mobility on the surface, then the spectroscopic measurements only comment on the average location of the probe molecule as it visits various surface sites. Kinetics at Interfaces. A molecule adsorbed at a surface may react with another molecule also adsorbed on the surface or with a molecule that visits the surface from the other phase of the system. The kinetics may be simple, as in homogeneous solution, i.e. Stern-Volmer in nature, or complex in that the rate constant may be time dependent, specific adsorption may give rise to a Poisson type kinetics, or the kinetics may be “static” in nature. Considerations outlining the various kinetics have been published.’ The most common form of kinetics with colloids is the Poisson type, as illustrated in Figure 1 for the reactions of excited 4-( 1pyreny1)butyltrimethylammonium ion (PN+)with dimethylaniline 0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
Thomas
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Figure 2. Time-resolved fluorescence decay of 5 X lo-' M PNf in 0.2 mM CTAB-laponite ( 5 g/L) system with various quencher concentrations. NM: (1) 0, (2) 1.0, (3) 2.5, (4) 5.0 mM. Broken lines are simulations based on eq 2. From ref 40.
0.2
[OMAladsd./mM
Figure 1. Stern-Volmer plot (0) of PN' fluorescence quenching by DMA in the laponite-CTAB system and Poisson plot ( 0 )based on eq 1: [PN'] = 5 X 10" M, [laponite] -- 5 g/L, [CTAB] = 0.2 mM. From ref 40.
(DMA) on a clay surface coated with cetyltrimethylammonium bromide (CTAB). The figure shows a Stern-Volmer plot of fluorescence of PN+ in the absence of DMA, lo, and in the presence of DMA, Z,vs. [DMA], from the standard relationship
the site concentration is calculated as 8.1 X M. These results lead to a model in which at least some coadsorbed CTAB molecules are associated in the form of clusters on the clay surface. This is not unexpected as a single CTAB molecule does not solubilize DMA in solution, while micellar aggregates of CTAB are an effective host system. Each cluster is an independent system where the PN+ is located and where quenching by DMA takes place. Similar data are obtained if either C H 3 N 0 2(NM) or nitrobenzene is used in place of DMA. The time decay profiles of the PN+ fluorescence in this system are shown in Figure 2. The time-resolved fluorescence decay data can be satisfactorily explained by a Poisson type quenching kinetics, as shown in Figure 2.8H
where k is the quenching rate constant for DMA and excited PN+ and ko is the natural decay rate constant. The Poisson plot is also shown, i.e. In lo/l= [DMA]/[site]
(2)
Similar data are also obtained with nitromethane (NM) quenching in this system. The time-resolved fluorescence decay data can be explained by a Poisson type quenching, as used in micellar systems, i.e. In
- = -kot I(0)
- ~ ( -l exp(-k,t))
(3)
where I ( 0 ) and Z(t) denote the fluorescence intensities of PN' at t = 0 and t , respectively, and A denotes the average number of quenchers per micelle. Many examples of Poisson type kinetics have been reported in several colloidal systems. Analysis of the data provides information on k,, the quenching rate constant at the surface, and on iV,the aggregation number of the colloidal or reaction site on the surface. In many cases this is the only way in which IVcan be obtained. A good example is provided by the quenching of PN+ fluorescence by DMA in a CTAB-laponite clay system. The CTAB forms hemimicelles on the clay surface which are centers for the PN+-DMA quenching reaction. Quenching by DMA in CTAB-Luponite Systems. Coadsorption of CTAB with PN' onto colloidal laponite provides a simple system where the pyrene chromophore is attached to the surface of the clay under conditions where the PN' fluorescence exhibits a single-exponential decay and no quenching by laponite takes place. Addition of DMA to this system gives rise to efficient quenching of the PN' fluorescence. A small but significant fraction of the added DMA is also adsorbed onto the CTAB cosurfactant which surrounds the PN+, which is obtained from the adsorption isotherm of DMA on the CTAB-laponite system. Figure 1 shows a steady-state Stern-Volmer plot of PN+ fluorescence quenching by adsorbed DMA. The Stern-Volmer plot shows curvature, but a semilogarithmic plot is linear, indicating that the quenching kinetics are of a static type. The model suggested is a Poisson distribution of quencher molecules among reactive sites, i.e. a random distribution of quenching species about the location of the fluorescing molecule, a situation that is similar to that used in micelles.sa From the slope of this straight line
The model introduced is mathematically identical with that of micellar systems. Therefore, the time-resolved fluorescence decay of PN+ in the CTAB-laponite system should be expressed by eq 3. In the above model it is assumed that each cluster, which may contain a different number of CTAB molecules, offers the same environment regarding the quenching reactions. The time-resolved fluorescence decay data for N B and NM can be satisfactorily explained by eq 3, as shown in Figure 2 . The average number of quenchers contained in a cluster, A, is a function of the amount of quenchers adsorbed in the CTABlaponite system, [Qladsd, as follows: ri =
[Q]add/[cluster]
(4)
In eq 4 it is assumed that all the quencher molecules adsorbed exist in clusters and directly participate in quenching reactions. Using eq 4, one can calculate [QIadsd, since the cluster concenM. tration has already been determined as 8.1 X Spectroscopic Studies. Most spectroscopic studies of molecules, in particular aromatic molecules, at interfaces are obtained via emission spectroscopy of the excited state. This technique of fluorescence or phosphorescence probing of the system is sensitive both to probe environment and to low concentrations of probe, a necessity in order not to disturb the system under study. However, there are a few examples of significant changes in the absorption spectrum of molecules, as the absorption process disturbs the geometry of the ground ~ t a t e . ~ .Alternatively, '~ the absorption spectrum, arising from a fast physical process, can comment on the dielectric nature of the environment." Few direct studies on surfaces are available, but data' in homogeneous solvents imply correlation between the refractive index of a medium and the position of the absorption band. The spectral changes in the absorption spectrum are small, and great care is required with such work. Emission spectroscopy provides a more (8) (a) Turro, N. J.; Yekta, A. J . A m . Chem. SOC.1978, 100, 5951. (b) Infelta, P. P.; Gratzel, M.; Thomas, J. K. J . Phys. Chem. 1974, 78, 190. (c) Tachiya, C. M. Chem. Phys. Lett. 1975,33, 289. (d) Atik, S . S.; Singer, L. A. Chem. Phys. Lett. 1978, 59, 519. (9) Krenske, D.; AMo, S.; Van Damme, H.; Cruz, M.; Fripait, J. 5. 1. Phys. Chem. 1980, 84, 2447. (10) Dellaguardia, R.;Thomas, J. K. J . Phys. Chem. 1983, 87, 990. ( 1 1) Birks, J. B. Photophysics oJAromatic Molecules; Wiley: New York, 1969.
Feature Article sensitive and versatile tool for the investigation of environments. Emission Spectroscopy. Two major events can affect the luminescence of an excited molecule at an interface, the general polarizability and dielectric relaxation of the surroundings which can lead to marked red shifts in the luminescence, a prime example being N-phenylnaphthylamine, and a change in the symmetry properties of the molecule due to its adsorption, an example here being pyrene, benzene, and other molecules of high symmetry. The adsorption process may significantly alter the state of the molecule as with aminopyrene which may exist in the free base or protonated forms, or it may restrict molecules in the excited state as exhibited by tris(bipyridine)ruthenium(II) or binaphthyl adsorbed on silica. Examples of the above will be given in the body of the text. However, before proceeding with a detailed spectroscopy of molecules adsorbed at surfaces, it is pertinent to first consider surfaces that are inherently luminescent. Luminescent Surfaces. Oxide Surfaces. A wide variety of oxide catalysts exist, but it is only recently that information has been available on the surface states of oxygen in these catalysts.'* Early work monitored the 0- of the oxides by EPR and its reactivity with various adsorbed gases. Optical evidence of surface states is quite marked and highly dispersed (Le. high surface area); alkaline earth metal oxides contain optical absorption bands that are not present in bulk pure crystals. The excitation process leads to a charge transfer of an electron of the oxide ion, creating an excitation which migrates on the surface. Ions on the surface experience a reduced Madelung potential due to their low coordination, and the spectral absorption edge is markedly red-shifted compared to ions in the bulk phase. Typically, absorption bands are in the 4-5-eV region with emission in the blue at about 3 eV. Emission is only seen if the surfaces are thoroughly outgassed in order to remove adsorbed gases which quench the excited states; the lifetimes of the emissions may be as long as microseconds. Analysis of the optical spectroscopic data in conjunction with EPR and work suggests that oxide ions of low coordination, e.g. 02-4c 02-3c, are present on the oxide surfaces and that they can act as electron-transfer sites with 02,CO,, and nitrobenzene. Normal are not reactive in this way. The surface oxide sites, e.g. 02-5c, optical data are of great use as monitors to determine the nature of the catalytic action of oxide surfaces. Similar low-energy absorption and emission bands have been reported for TiO, and for porous Vycor glass. Emission has been reported for TiO, both as a colloid13and as a guest molecule in porous Vycor glass.14 TiO, in the glass exhibited an activation spectrum with a maximum of 210 nm and an emission spectrum with a onset at 420 nm, the lifetime being 10 ns. AP on Si02 surfaces with geminal OH groups produces a structured emission on the 370-400-nm region with a lifetime of > 100 ns, which is reminiscent of pyrene. Studies in homogeneous solvents show that the pyrene-like emission arises from protonated AP where no conjugation exists between the NH3' group and pyrene. The fluorescence data on Si02geminal O H groups indicate that these entities donate a proton to the base A P and that the excited protonated species is stable over the lifetime of the excited state. Earlier work with aniline adsorbed on SiOzsurface suggests33that geminal O H groups adsorb water and act as proton donors. The aminopyrene acts as a sensitive monitor of the surface O H groupings. Interfaces between Condensed Phases. The preceding section has dealt mainly with photophysics on surfaces with solid-gas interfaces. This section deals with solid-liquid interfaces as with Si02/cyclohexane, colloidal clays, etc., and liquid-liquid interfaces as with micelles, microemulsions, or vesicle systems. The concept behind all the work is the adsorption or location of a fluorescent probe molecule at the interface, via observations of the excitedstate probe to ascertain features of the interface. Simple spectroscopy comments of the nature of the surface, i.e. polarity, rigidity, etc., while kinetic spectroscopy or quenching of the excited probe describes movement at the interface. Solid-Liquid Interfaces. SiO,-Hydrocarbon. Early indicated that, due to close refractive index matching, the system SO,-cyclohexane was optically transparent. Arenes dissolved in the hexane phase were strongly adsorbed onto S O 2 , and spectroscopic studies were used to indicate the different nature of the SiOZ-C6H,, interface compared to that of bulk C6H,z. This convenient method of refractive index matching can be used to study several arenes in Sio2-C6Hl2 mixtures. With aminopyrene the various vicinal and geminal OH groups can be assessed, as with the dry SiOz powder. Kinetic studies can be made of
Colloidal Clay Systems Aqueous colloidal clay systems which occur quite readily in nature provide a particularly interesting challenge for fluorescence
(32) Hite, P.; Krasnansky, R.; Thomas, J. K. J. Phys. Chem. 1986, 90, 5195. (33) Hair, M. L.; Herl, W. J. Phys. Chem. 1969, 73, 4269. (34) Leermakers, P. A.; Thomas, H. J.; Weis, L. D.; James, F. C. J. Am. Chem. SOC.1966,88, 5075.
(35) Wheeler, J.; Thomas, J. K. ACS Symp. Ser. 1982, No. 177, 99; J . Phys. Chem. 1982,86,4540; J . Phys. Chem. 1984,88, 750; J . Photochem. 1985, 28, 285. (36) Willner, J.; Yang, J.-M.; Laone, C.:Otvas, J. W.; Calvin, M. J. Phys. Chem. 1981, 85, 3277.
Feature Article probing, These aluminosilicate systems are similar to colloidal Si02, being negatively charged with metal c o u n t e r i o n ~ . ~Clays ~,~~ also contain aluminum in an octahedral configuration sharing oxygen atoms with silicon, which is in a tetrahedral configuration. Montmorillonite is referred to as a 2: 1 layered mineral because its aluminmum shares oxygen atoms with silicon on either side of it. There then occurs an expandable layer into which water, organic molecules, or cations may be intercalated. The mineral possesses a periodic negative charge along its structure due to the isomorphous replacement of aluminum for ferrous or magnesium ions. The small size of these atoms permits them to take the place of the Si and A1 atoms. The replacement of an atom of higher positive valence for one of lower valence results in a negative charge. This excess of negative charge is balanced by the adsorption of cations on the mineral's surface. In the presence of water, these charge-balancing cations may be exchanged with other cations available in ~ o l u t i o n . ~ ~ ~ ~ ~ The cation-exchange capacity of the clay can be utilized to located cationic probe molecules,(3941)e.g. R ~ ( b p y ) ~ ~at' ,the surfaces of the following clays: laponite, which is a synthetic clay, and natural hectorite and montmorillonite. R ~ ( b p y ) , ~is+adsorbed completely by the clay by ion exchange and on excitation gives rise to a luminescence spectrum in the red part of the spectrum with a lifetime of about half a microsecond. The lifetime, quantum yield, and nature of the absorption spectrum are dependent on whether the R ~ ( b p y ) , ~is+ adsorbed in layers as in the natural clays or whether it is adsorbed on the surface as with laponite. At low concentrations of laponite, Ru(bpy)t' is adsorbed on the outer layers and is in contact with the aqueous phase. However, at higher clay concentrations or in the presence of calcium chloride, layering of the clay occurs and the probe molecule is placed progressively between the layers where its photophysics are altered. The casting of a film from the laponite-R~(bpy),~+exhibits maximum spectral changes associated with maximum colloid layer formation. Such changes are not as readily observed with hectorite or montmorillonite, and this indicates that, for the most part, these systems exist as layered colloids and that R ~ ( b p y ) , ~is+already adsorbed between the layers. Other molecules such as Cu2+, dimethylaniline, and nitrobenzene react with excited R ~ ( b p y ) , ~ + through electron-transfer reactions and are also adsorbed to varying extents on the clay surface. The rate constants are 2 X lo7, 2 X lo8, and 2 X IO8 M-' s-', respectively; concentrations are estimated by assigning a lO-A-thick layer around the clay surfaces where reactions occur. Cu2+is adsorbed strongly, and the kinetics are simplified due to the strong adsorption. Here, Stern-Volmer type kinetics are observed and a quenching rate constant is obtained which is lower than in the aqueous solution, which gives an estimate of the degree of movement of cupric ions on the clay surface. Dimethylaniline and nitrobenzene are adsorbed weakly on the clay. However, the clay catalyzes the reaction of the R ~ ( b p y ) , ~with + these quenchers as both are adsorbed in a small value, i+e.the clay surface. The kinetics that describe these latter reactions are of the Poisson form, and the kinetics indicate that the reactive quencher molecules are adsorbed around the R ~ ( b p y ) , ~ +in, a zonelike effect, rather than being adsorbed randomly throughout the system. This tends to indicate that the sites of adsorption are not uniform on the clay surface but occur in regions. Inert electrolytes such as KCl markedly affect the kinetic cluster in montmorillonite colloids by decreasing particle association. ~
(37) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Wiley: New York, 1974. (38) van Obhen, H. An Introduction to Clay Colloid Chemistry; Wiley: New York, 1963. (39) Dellaguardia, R.; Thomas, J. K. J . Phys. Chem. 1983.87.990, 3550 1984, 88, 964. Dellaguardia, R.; Kovar, L.; Thomas, J. K. J . Phys. Chem.
1984, 88, 964. (40) Nakamura, T.; Thomas, J. K. Langmuir 1985, I , 568; J . Phys. Chem. 1986, 90, 641. (41) (a) Abdo, S.; Canesson, P.; Cruz, M.; Fripiat, M. J.; Van Damme, H. J . Phys. Chem. 1981, 85, 792. (b) Bard, A. J.; Ghosh, P. K. J . Phys. Chem. 1984, 88, 5519. Schoonheydt, R. A,; Pauw, P. D.; Vliers, D.; Deschrijver, F. C . J . Phys. Chem. 1984, 88, 51 13.
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 273 Organic molecules tend to cluster on being adsorbed to clay surfaces. Adsorption of the cationic fluorescent probe PN" onto either colloidal montmorillonite or kaolin causes a clustering of probe molecules on the surface. This leads to pyrene excimer formation, as illustrated by its characteristic emission spectrum. Fluorescence polarization measurements indicate that the molecule is rigidly bound to the surface. The PN+ can be dispersed over the surface of the mineral by the addition of quaternary ammonium surfactants which cause the emission from the pyrene monomer to increase and that from the excimer to decrease. The monomer is quenched by the clay mineral surface, but its interaction with the surface can be significantly decreased by coadsorbing a quaternary ammonium surfactant with a hydrocarbon chain length of 10 carbons or more. Excited PN+ exhibits a double-exponential decay of its transient fluorescence, suggesting that it is located in two different regions of the particle. Quenching studies demonstrate that the surfactant cetyltrimethylammonium bromide (CTAB) reduces the accessibility of quenchers not adsorbed by the clay to the PN+ on montmorillonite, but it has a negligible effect when PN+ is located on kaolin. For molecules that are adsorbed by the montmorillonite particles, the quenching rate constants are reduced by a least 1 order of magnitude compared to those for homogeneous solution due to the limited accessibility of PN' and the reduced diffusion of molecules on the particle surface. All of the quencher molecules used exhibit a mixture of static and dynamic quenching. The addition of CTAB to a montmorillonite colloid in an amount that is twice the cation-exchange capacity of the colloid reverses the charge on the particles due to the formation of a CTAB bilayer around their surface. The emission spectrum and steady-state quenching studies comment on the location of pyrene in this environment as well as on the nature of these colloidal particles. Organization of the adsorbed molecules on the clay surface also controls the observed photochemistry. In this respect the fluorescence quenching of 4-(1-pyrenyl)butyltrimethylammonium ion (PN') adsorbed on colloidal laponite clay by coadsorbed alkylpyridinium ions showed unusual behavior.40 Increasing the quencher concentration at first leads to an efficient quenching of PN+ fluorescence, but on increasing the quencher concentration further, a reverse effect is observed, whereby the fluorescence started to recover, only to be followed by a smaller degree of quenching. The degree of recovery was strongly dependent on the chain length of alkylpyridinium ions; the longer the chain length, the larger the degree of recovery. PN+ fluorescence was also quenched by laponite clay itself or cupric ion in copper-clay, where cupric ion was constructed in the clay lattice. However, coadsorbed hexadecyltrimethylammonium ions (CTAB cations) dramatically reduced the quenching. These results can be explained in terms of the change of geometrical arrangement of PN' adsorbed on clay. Quenching of PN+ fluorescence by dimethylaniline, nitrobenzene, and nitromethane in the CTAB-laponite system obeyed Poisson type kinetics, indicating that the adsorbents exist in the forms of clusters or zones on the clay surface. Clustering of adsorbed molecules on a clay surface is a quite general phenomenon, an important example being the interaction of the cation of tetramethylbenzidine (TMB+) with colloidal montmorillonite. TMB' reacts rapidly at a diffusion-controlled rate with the clay particles with a rate constant k = 3 X lo'* M-' s-', A further rapid reaction on the clay surface of TMB' produces the dimer cation of the TMB. All events were identified by rapid spectrophotometry, which shows significant differences in the absorption spectra of the cations when absorbed onto the clay particles. In homogeneous solution the TMB' shows a spectrum with a pronounced maximum at X = 470 nm. This maximum disappears when TMB' is absorbed on clays, and the spectrum contains a maximum in the 400-nm region. The presence of other anionic assemblies, e.g. micelles in the system, bind TMB' and decrease the rate of cation exchange on the clay. Neutral molecules also adsorb to clay surfaces creating hydrophobic host sites. The fluorescent probe pyrene and 1-dodecanol have been incor-
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The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
porated in the interlamellar spaces of the clay mineral montmorillonite. This powder has been suspended in aqueous solution to form colloidal clay particles containing these molecules. Upon suspension, a fraction of the pyrene and dodecanol molecules form micelles that incorporate pyrene. The critical micelle concentration (cmc) of these micelles is approximately 3 X lo4 M. They render a nonpolar environment, and fluorescence quenching studies with hydrophobic and hydrophilic molecules show the expected trends of enhanced quenching rates in the former case and reduced rates in the latter. Quenching studies with the suspension containing pyrene in the interlamellar spaces of the clay and in the dodecanol micelles indicate that the pyrene excimer exists only in the micelles but not in the clay particles. The results show that the diffusion of molecules within the domain of the montmorillonite particles is significantly reduced compared to that of homogeneous aqueous solution. The system models natural conditions when clay colloids coexist with organic micelles. Fluorescence techniques enable kinetics in both particles to be observed independently in addition to ions interacting between the two colloids. Organic Colloids
There are many different types of organic colloids, as indicated in ref 5-7. For the most part these are constructed from surfactant molecules. The final structures which in an aqueous sample consist of an interior portion which is hydrophobic, consisting of hydrocarbon chains, and some surface region which is hydrophilic, usually consisting of ionic groups such as sulfates or quaternary ammonium compounds. Organic probe molecules such as pyrene will associate with these structures, and in the smaller colloids such as micelles they associate with the surface. Micelles Micelles are constructed from a variety of different surfactant molecules, and for the sake of simplicity sodium lauryl sulfate will be considered as an anionic micelle, cetyltrimethylammonium bromide as the cationic micelle, and Triton XI00 as a neutral micelle. Pyrene and other hydrophobic molecules which have extremely low solubilities in water are readily solubilized by the micellar forms of these surfactants. Typically, the micelles consist of anywhere from 60 to 100 molecules aggregated together. NMR studies4*and fluorescence s t ~ d i e sindicate ~ ' ~ ~ ~that probe molecules are solubilized close to the surface of the micelle at the head group water interface. Pyrene has been used to illustrate that the polarity of this region approaches that of butanol. Other probes such as pyrenecarboxaldehyde, which are sensitive to the solvent region, can also be used to measure the local dielectric constant, although the precise location of the probe in this measurement is not known; nevertheless, it is in the general region of the head region of the micelle.44 The surface dielectric constant measured by means of carboxaldehyde is in the vicinity of 30 for several different micelles and much less than that measured for water (80) but much larger than that for hydrocarbons ( 2 ) . Probes such as pyrene and pyrene carboxaldehyde give the polarity in the interface region. Studies using the polarization of the fluorescence of probes such as methylanthra~ene~~ can be used to give the rigidity of the probe site at the micellar surface, and rather high viscosities approaching the region of 10-100 cP, Le. higher than that of water or simple hydrocarbons, are found. A description of chemical reactions requires some idea of the lateral motion of the reactant or probes on the micellar surfaces. This has been achieved quite readily by the incorporation of quencher molecules into the system, either in the aqueous or in the micellar phase. Quenching of the probe fluorescence by a quencher then comments on the access of the micelle surface to quencher molecules from the solution phase or on movement of probe and quencher around the micelle. In particular, the probe pyrene produces pyrene excimers on the (42) Kalyanasundaram, K.; Gratzel, M.: Thomas, J. K. J . Am. Chem. SOC. 1975, 97, 3915. (43) Thomas, J. K. Acc. Chem. Res. 1977, IO, 133. (44) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1977,81,2176. (45) Weber, G. Annu. Rev. Biophys. Bioeng. 1972, I . 553.
Thomas micellar surface, and the kinetics give the mobility of pyrene on the ~ u r f a c e . If ~ both probe and quencher (Le. as with pyrene excimers) are incorporated into the micellar structure, then Poisson kinetics prevail, as described earlier, and the number of micelles as well as the reactivity of the molecules in the micellar phase can be ascertained. Fromfhese studies the rate constant for the reaction of two pyrene molecules at the surface of a micelle is given as 2 X lo7 s-l, indicating that a molecule such as pyrene traverses micelles such as SDS or CTAB at greater than 10 million times per second. Cosurfactants such as long-chain alcohols, i.e. hexanol, interact with the micellar surface, an event which is readily monitored by the probe m o l e c ~ l e s .The ~ ~ ~cosurfactants ~ lead to a more hydrophobic environment for the probe and also to a loosening of the structure of the micellar surface. Incorporation of an oil into the core of the micelle cosurfactants system leads to microemulsions, which are larger than micelles being several hundred angstroms in radius compared to tens of angstroms for a micelle. The large oil phase acts as an ideal host for the probe which is reflected in its photophysics, which reflect spectra indicative of a completely hydrophobic type of environment identical with the oil phase in which the probe resides. However, if polar groups are placed on the probe, N H 2 in aminopyrene and S 0 3 H as in pyrenesulfonic acid, then because of the polar nature of these molecules they naturally tend to reside at the surface of the surfactant structure, and the photophysical properties are indicative again of the surfactant-water surface. Reversed Micelles Surfactants in aqueous solution form micelles where a small hydrocarbon-like core of the micelle is screened from the bulk aqueous phase by polar surfactant head groups. The reversed situation where small pockets or pools of water are solubilized in bulk hydrocarbon by a simple surfactant is not as common, although many microemulsion systems utilizing an additional cosurfactant are k n ~ w n . One ~ , ~ system, Aerosol OT, an anionic surfactant, is widely used to form discrete water clusters in heptane and other hydrocarbons. In these systems hydrophobic probes tend to reside in the hydrophobic region or oil rather than at the micelle. Hence, most probes used are charged or very hydrophilic; much used probes are anilinonaphthalenesulfonate (ANS), pyrenesulfonic acid, or other very polar arene derivatives. These probes tend to reside, either in the water phase or close to the interface of the water micellar surfactant region. The nature of the water bubbles in these systems excites some interest as their properties differ from bulk water and often catalyze chemical reactions. Small pockets of aqueous media also exist in living systems in the midst of lipid regions, and reversed micelles can provide simple models for such species in nature. In earlier work on AOT, NMR and fluorescence spectroscopic studies clearly showed a variation in physical properties of the water structure with increasing water content, physical properties reminiscent of bulk water being approached only at high water contents when (H,O)/(Na+) was greater than 6.46 The kinetic properties, electron transfer from bulk hydrocarbon, influence on photoionization, and photochemical reactions all showed a strong dependence on water s t r ~ c t u r e . ~ The ~ , ~ interchange ~ of ions between micellar pools is readily achieved by fluorescence quenching methods$* the fluorescence probe and quencher having to migrate from pool to pool for quenching to occur. The exchange is quite dependent on the constituency of the head group region, being relatively inefficient in the unadulterated micellar system, but increasing on addition of cosurfactants such as benzyl alcohol which tends to disrupt the head group structure and to promote solute penetration through the surfactant layer. Other molecules such as toluene and benzene also interact with the head group (46) (a) Wong, M.; Grazel, M.; Thomas, J. K. J . Am. Chem. SOC.1976, 98,2391. (b) Wong, M.; Nowak, T.; Thomas, J. K. J . Am. Chem. SOC.1977, 99, 4730. (47) Reverse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum: New York, 1984. (48) Atik, S . S.; Thomas, J. K. Chem. Phys. Lett. 1981, 79, 351; J . Am. Chem. SOC.1981, 103, 3543; J . Phys. Chem. 1981,85, 3921.
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 275
Feature Article
TABLE I: Fluorescent and Excited-State Probes for Monitoring Surfaces and Microassemblies probe pyrene pyrenecarboxaldeh yde N-phenylnaphthylamine (NPN) anilinonaphthalenesulfonate(ANS) alkylpyridinium salts indoles coumarins tris(bipyridine)ruthenium(II) (Ru(bpy)32+) NPN, ANS, phenylene, rhodamine dyes diphenyl- and dipyrenylpropane various dyes pyranine quinopyrene pyrenecarboxylic acid pyrene 4 4 1-pyrenyl)butyltrimethylammoniumbromide (PN’) pyrenesulfonic acid (PSA) pyrenebutyric acid probes for biology
utility environment polarity
measurement fluorescence fine structure; y change in fluorescence A,
polarity
change in A,, change in A,
kinetic
T
various
various
67
polarity polarity polarity polarity polarity, rigidity rigidity rigidity PKa PKa nature of surface OH surface acidity pressure measurements kinetic environment kinetic
structure but tend to block passage of solute from one micelle to another. Vesicles To round off the discussion of simple organic colloids, it is worthwhile to consider vesicle structures which are constructed from natural biological materials such as lecithins or from double-chained surfactant molecule^.^-^ These structures are double layered and enclose an inner volume of water which is separated by the surfactant double layer from the exterior water phase. The systems are quite analogous to biological membranes, hence their intrinsic interest. They behave in many ways like simple micellar systems whereby the probe molecules are located in the surfactant structure. However, these systems are more organized than simple micelles or microemulsions and exhibit additional properties, such as phase changes. Temperature-induced phase changes are well established in these systems by calorimetry and other physical studies; luminescent probes such as pyrene have also been used to highlight the t r a n ~ i t i o n . ~ ~ -Excitation ~l of pyrene leads to extremely rapid formation of excimer at temperatures below the phase tran~ition.4~3”However, as the temperature is raised, there is an abrupt change in the yield of the pyrene excimer compared to that of the monomer. A drop in yield is first observed followed by an increase at higher temperatures. This is explained as follows:s1 pyrene is mainly located in the head region of the vesicle, and raising the temperature produces the phase transition or a melting of the hydrocarbon chains which gives a ready access of pyrene to the internal phase. The pyrene fluorescence fine structure III/I ratio also shows an increase at the phase transition temperature, i.e. an increase in hydrophobicity. At the phase transition a greater volume of the vesicle interior becomes available to the pyrene, causing a decrease in its local concentration, Le. a decrease in the excimer yield, and a more hydrophobic environment. Further heating above the phase change leads to a further increase in the rate of excimer formation, giving an increase in excimer yield. Interactions of other surfactant molecules of biological interest with the vesicle structures can also be monitored by pyrene. Polyelectrolytes Polyelectrolytes such as polymethacrylic acid (PMA) in aqueous solution exist as tightly coiled structures at low pH and as open chains at high pH. This is entirely due to the ionization of COOH groups of PMA which give rise to the open structure. A number of fluorescence studies have been reported on such system^.^^^^^ (49) Morris, D. A.; Thomas, J. K. Micellations, Solubilisation, and Microemulsions; Plenum: New York, 1971; Vol. 2, p 913. (50) Morris, D. A,; Castellino, F. J.; McNeil, R.; Thomas, J. K. Biochim. Biophys. Acta 1980, 599, 380. (51) Galla, J.-J.; Sackmann, E. Ber. Bunsenges. Phys. Chem. 1974, 78, 949.
ref 31, 56 44, 57 68 45, 46, 58, 59 61 66 62 IO, 40 45 63 64 6 32, 33 60 65 35 46 46
change absorption A,, change in A,, change in A, change in emission A,
fluorescence polarization monomer to excimer ratio change in spectral maxima change in spectral maxima spectral change spectral change fluorescence five structure fluorescence spectrum; T 7
At low pH the hydrophobic coiled PMA acts as a “micelle” and solubilizes fluorescent hydrophobic molecules, the fluorescence commeting on the nature of the solubilization. Stop flow exp e r i m e n t ~ whereby ,~~ alkali metal is rapidly mixed with PMA+ probe, show complex kinetics as the probe leaves the expanding PMA for the water phase. Cationic probes are electrostatically bound to the high pH ionized form but not to the low pH form. However, with copolymers of PMA and vinyl or acrylate derivatives of the fluorescent probe, the probe always remains with the PMA and comments directly on the molecular events of the pH-induced phase transition. Various stages of the transition have also been investigated by other probes, Le. R ~ ( b p y ) , ~ +Here . the partially open and ionized PMA binds Ru(bpy),’+ electrostatically under conditions where the ligands are held rigidly by the environment. The excited state (a CT metal ligand state) cannot completely relax, and the photophysics of R ~ ( b p y ) , ~in+ partially ionized PMA resemble that observed in frozen media at low temperatures or in porous silica. Fluorescent probes may be used to investigate the interactions of surfactants and colloids53awith polymer.
-
Cellophane
Photochemistry in polymer films enjoys wide appeal, and a comprehensive statement of the field is beyond the scope of this article. However, work in cellophane gives the general flavor of the area. It was identified early that arenes in cellulose materials often exhibit phosphorescence in the presence of air. This is attributed to the protective environment of the cellulose which prevents diffusion of solutes within its structure, and in particular 02.The capability of observing both fluorescence and phosphorescence in a room-temperature sample in air is of great benefit to the photochemist. The most useful feature is that of low solute mobility. Most photoinduced energy transfer and electron transfer can be observed in the absence of d i f f ~ s i o n . ~ ~ Varying amounts of water added to the system markedly affect (52) Barone, G.; Crescenzi, V.; Qumdrifoglio, F. J . Phys. Chem. 1967, 71, 2341. (53) Chen, T. A.; Thomas, J. K. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1103; Radiat. Phys. Chem. 1980, 15, 429; J . Chem. Educ. 1981,58,140. Chu, D.; Thomas, J. K. Macromolecules 1984,17,2142; J . Phys. Chem. 1985, 89, 4065; J . A m . Chem. SOC.,in press. (a) Harrop, R. A.; Williams, P. A.; Thomas, J. K. J . Chem. SOC.,Chem. Commun. 1985, 280, 1366. (54) Schulman, E. M.; Walling, C. J . Phys. Chem. 1973, 77, 902. (55) Milosavljevic, B.; Thomas, J. K. J . Phys. Chem. 1983, 87, 616; Int. J . Radiat. Chem. 1984, 23, 231; Macromolecules 1984, 17, 2244; J . Chem. Soc.,Faraday Trans. 1 1985,81,135; J. Phys. Chem. 1985,89, 1983; J . A m .
Chem. SOC.,in press. (56) Lianos, P.; Georghiou, S . Photochem. Photobiol. 1979, 30, 355. (57) DeDeren, J. C.; Loosemans, L.; DeSchryver, F. C.; Dormaeh, A. V. Photochem. Photobiol. 1979, 30, 443. (58) Dodiuk, H.; Kanety, H.; Kosower, E. M. J . Phys. Chem. 1979, 83, 515.
J . Phys. Chem. 1987, 91, 276-282
276
+
the rates of e- transfer. The system Ru(bpy)32+ MV2+,which gives rise to reduced methylviologen, MV+’, has been studied in cellulose at temperatures varying from 77 K to 25 “C. The marked acceleration with temperature of the rate of photoinduced electron tunneling for Ru(bpy)?+* to MVZ+is explained by phonon-assisted transfer. The data is cellophane are of vital importance to other colloidal systems, e.g. micelles. The cellophane eliminates diffusion effects, and only reactions “over a distance” are monitored, whereas (59) Matthews, M. B.; Hirschhorn, E. J. J. Colloid Sci. 1953, 8, 86. (60) Nakamura, T.; Stramel, R.; Thomas, J. K. J. Colloid Interface Sci., in press. (61) Mukerjee, P.; Ray, A. J. Phys. Chem. 1966, 70, 2144. (62) Fernandez, M. S.; Fromberg, P. J. Phys. Chem. 1977, 81, 1755. (63) Zachariasse, K. A. Chem. Phys. Lett. 1978, 57, 429. (64) Fendler, E. J.; Fendler, J. H. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. Montal, M.; Gitler, C. Bixnergetics 1973,4, 363. Beckman, L. S.; Brown, D. G. Biochim. Biophys. Acta 1976, 428, 720. (65) Offen, H. W.; Turley, W. D. J. Phys. Chem. 1982, 86, 3501. (66) Schon, N. E.; Turro, N . J. J. Am. Chem. SOC.1975, 97, 2488. (67) Brand, L.; Gohlke, J. R. Annu. Rev. Biochem. 1982, 41, 8437. Waggoner, A. J. J. Membr. Biol. 1976, 27, 317. (68) Overath, P. Trauble, H. Biochemistry 1973, 12, 2625; Biochim. Biophys. Acta 1973, 302, 492.
the other systems the former effects are often mixed in with those due to diffusion.
List of Probes Table I contains a list of fluorescent probes that have been used to monitor surfaces and colloidal structures. The list is by no means complete as new probes are reported frequently. Summary The techniques of photochemistry may be used to investigate the nature of interfaces between various materials. In particular, fluorescence and phosphorescence spectroscopy, which are sensitive to environment, provide valuable information on the rigidity, polarity, and modification of surfaces. Photokinetic studies provide details of factors that affect chemical reactions at interfaces. The wealth of data provided by the above techniques comments directly both on factors affecting catalysis on surfaces and on the nature of the surfaces themselves. Acknowledgment. The author thanks the National Science Foundation, the Army Research Office, and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work.
ARTICLES Ground-State Vibrational Assignments of cis- and trans-l,3,5-Hexatriene R. McDiarmid and A. SabljiC* Laboratory of Chemical Physics, National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892 (Received: February 1 1 , 1986)
The infrared and Raman spectra of cis- and trans-hexatriene have been remeasured and reinterpreted. Several new low-frequency fundamentals were observed. The new set of fundamental frequencies presented here for each molecule differs somewhat from that previously identified. Vibrational frequencies observed in other electronic states-both totally symmetric sequence bands and nonsymmetric enabling vibrations-were used to aid in the assignments of the ground-state vibrations. The new sets of fundamental frequencies were used to evaluate sets calculated by a semiempirical valence force-field method.
The importance of both the fundamental frequencies of hexatriene and their assignments is manifest in the number of published papers addressed to this topic. These investigations include both ab initio] and semiempirica12 theoretical calculations of the ground electronic state vibrational spectrum, resonance Raman investigations of the ground electronic state vibrational m a n i f ~ l d , ~ , ~ the use of “known” ground-state vibrational frequencies to identify less stable geometric isomers in matrix isolation ~ t u d i e s both ,~ theoretical and experimental investigations of the vibrational manifolds of excited electronic states,6-8 and transient resonance ~ Raman investigations of the excited states of other t r i e n e ~ . The ground-state vibrational fundamental frequencies employed in all these investigations are either those originally p r o p e d l o ~ lorl their subsequent revisions” although the accuracy of these sets of fundamentals can be questioned because of (1) the limited resolution originally used, (2) the original recognition that the given assignments are “essentially reasonable” but not proven,” and (3) the subsequent observations that the calculated spectra do not concur with the experimentally observed vibrational ~ p e c t r a . ~ . ~
* Perrnanent address: Institute Rudjer BoSkovii., 41001 Zagreb, Croatia, Yugoslavia
More recent investigations attempting to correlate excited-state with ground-state fundamental frequenciesc8 also found internal inconsistencies in the experimentally derived vibrational frequencies and assignments. Because we too needed the ground-state fundamental frequencies of cis- and trans-hexatriene for our analysis of the excited (1) Fogarasi, G.; Pulay, P. Annu. Reu. Phys. Chem. 1984,35, 191-213. (2) Lasaga, A. C.; Aerni, R. J.; Karplus, M. J. Chem. Phys. 1980, 73, 5230-5243. (3) Warshel, A.; Dauber, P. J. Chem. Phys. 1977, 56, 5477-5488. (4) Myers, A. B.; Mathies, R. A,; Tannor, D. J.; Heller, E. J. J . Chem. Phys. 1982, 77, 3857-3866. (5) Furukawa, Y.; Takeuchi, H.; Harada, I.; Tasumi, M. J. Molec. Struct. 1983, 100, 341-3.50. (6) Leopold, D. G.; Pendley, R. D.; Roebber, J. L.; Hemley, R. J.; Vaida, V. J . Chem. Phys. 1984,81, 4218-4229. (7) Sabljie, A.; McDiarmid, R. J. Chem. Phys. 1985, 82, 2559-2565. (8) SabljiE, A.; McDiarmid, R. J. Chem. Phys. 1986, 84, 2062-2067. (9) Langkilde, F. W.; Wilbrandt, R.; Jensen, N.-H. Chem. Phys. Lett. 1984, 111, 372-378. (IO) Lippincott, E. R.; White, C. E.; Sibilia, J. P. J . Am. Chem. SOC.1958, 80, 2926-2930. ( 1 1 ) Lippincott, E. R.; Kenney, T. E J. Am. Chem. SOC. 1962, 84, 364 1-3648.
This article not subject to U S . Copyright. Published 1987 by the American Chemical Society
