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Reversible Metachromasy of Crystal Violet on Titanium Dioxide: A New Surface Photophysical Phenomenon S. R. Coon,*,† T. Y. Zakharian,†,‡,§ N. L. Littlefield,† S. P. Loheide,†,| E. J. Puchkova,†,‡ R. M. Freeney,† and V. N. Pak†,‡ Department of Chemistry, University of Northern Iowa, Cedar Falls Iowa 50614-0423, and Department of Chemistry, Herzen State Pedagogical University, St. Petersburg, Russia Received August 10, 2000. In Final Form: October 2, 2000 Crystal violet adsorbed on Degussa P-25 titanium dioxide, under certain conditions of dye loading and after evaporation of liquid solvent, exhibits a reversible color change from pink-violet to blue-violet when exposed to water vapor. The spectral changes, measured by reflectance spectroscopy, correspond to metachromasy of crystal violet that occurs in solution at different dye concentrations, due to aggregation of dye molecules. We propose a mechanism, reversible metachromasy, in which the extent of aggregation of the dye molecules in the surface water monolayers changes as water molecules adsorb or desorb. Specifically, as water adsorbs, the aggregates separate into monomers, and then as water desorbs, the molecules reaggregate.
Introduction In this paper, we report on a new phenomenon: visible and reversible metachromasy of a dye on a non-polyelectrolyte solid in the absence of solvent. Metachromasy of dyes is a well-known phenomenon in solution, on chromotropic polyelectrolytes, and in the staining of biological tissues. Metachromasy is a change in the shape of the absorption spectrum and the color of the dye with concentration or adsorption but with no chemical change in the dye itself. This phenomenon can be seen in comparing concentrated and dilute aqueous solutions of crystal violet (CV+). For CV+ in dilute solution (10-3 M), λmax shifts even further to the blue. The change in the spectrum is due to aggregation of the dye molecules into dimers, trimers, and higher aggregates. The λmax of the dimer is 550 nm, and the λmax of the trimer is 520 nm.1 Metachromatic dyes are soluble in water because they are ionic compounds with either the cation or anion being the organic dye. The dye ions themselves, however, are made up of large hydrophobic groups, and the charge is delocalized over most of the molecule. Thus, even at relatively low dye concentrations, the ions have a tendency to aggregate to minimize the interaction with water molecules, and the resulting π-electron interactions cause the aggregates to have a different λmax. A similar effect is responsible for metachromasy of dyes on a chromotropic polyelectrolyte and for the metachromatic staining of biological tissues, in which the same dye produces different colors of stain depending on the type of tissue.2 In these * Corresponding author: phone 319-273-2059; telefax 319-2737127; email
[email protected]. † University of Northern Iowa. ‡ Herzen State Pedagogical University. § Present address: Department of Biochemistry, Rice University, Houston TX. | Present address: Department of Geology, Indiana University, Bloomington IN. (1) Lueck, H. B.; Rice, B. L.; McHale, J. L. Spectrochim. Acta 1992 48A, 819. (2) Kugel, R. W. Adv. Chem. Ser. 1993 No. 236, 507.
cases, the dye molecules are adsorbed so close together, on a protein for instance, that they behave as aggregates spectrally. Metachromasy can also occur upon adsorption of a dye from solution onto inorganic solids such as clay minerals. Either aggregation of dye molecules3-7 or increased π-electron interactions with the substrate3,8-12 is thought to be responsible, depending on which clay is used and the dye and clay concentrations. Metachromasy was also observed in the adsorption of crystal violet from solution onto ordered zinc oxide, due to increased hydrostatic pressure on the solution.13 What all of the above examples of metachromasy have in common is the presence of liquid solvent (water). Metachromatic changes in the absence of solvent are rare. Metachromasy of methylene blue adsorbed on a polyelectrolyte acid (Nafion film) at various relative humidities has been studied.14 The adsorption of water by the film disrupts the acid-base interactions of the methylene blue and the sulfonic acid groups, causing the methylene blue molecules to aggregate into dimers. Desorption of water caused the acid-base interaction to proceed as far as monoand diprotonation of the methylene blue. As will be seen, a different mechanism is at work for reversible metachromasy on a non-polyelectrolyte solid. Experimental Section Certified reagent grade crystal violet chloride (hexamethylpararosaniline, Basic Violet 3, CI 42555) was obtained from Acros (3) Grauer, Z.; Grauer, G. L.; Avnir, D.; Yariv, S. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1685. (4) Cenens, J.; Schoonheydt, R. A. Clays Clay Miner. 1988, 36, 214. (5) Schoonheydt, R. A.; Cenens, J.; De Schrijver, F. C. J. Chem. Soc., Faraday Trans. 1 1986, 82, 281. (6) Yariv, S.; Nasser, A.; Bar-on, P. J. Chem. Soc., Faraday Trans. 1990, 86, 1593. (7) Yariv, S.; Ghosh, D. K.; Hepler, L. G. J. Chem. Soc., Faraday Trans. 1991, 87, 1201. (8) (a) Yariv, S.; Lurie, D. Isr. J. Chem. 1971, 9, 533. (b) Yariv, S.; Lurie, D. Isr. J. Chem. 1971, 9, 537. (9) Cohen, R.; Yariv, S. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1705. (10) Garfinkel-Shweky, D.; Yariv, S. J. Colloid Interface Sci. 1997, 188, 168. (11) Chernia, Z.; Gill, D.; Yariv, S. Langmuir 1994, 10, 3988. (12) Schramm, L. L.; Yariv, S.; Ghosh, D. K.; Hepler, L. G. Can. J. Chem. 1997, 75, 1868. (13) Clark, F. T.; Drickamer, H. G. J. Chem. Phys. 1984, 81, 1024. (14) Otsuki, S.; Adachi, K. Polym. J. 1993, 25, 1107.
10.1021/la001152a CCC: $19.00 © 2000 American Chemical Society Published on Web 11/11/2000
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Organics and was used without further purification. Degussa P-25 TiO2 (50 m2 per g, 21 nm average particle size, 70% anatase/ 30% rutile)15 was used as supplied by Degussa-Hu¨ls. Samples which exhibited reversible metachromasy were prepared by mixing an aqueous solution of crystal violet (4 × 10-4 to 2 × 10-3 M) with an appropriate amount of TiO2 to give a dye loading of about 1-5 µmol of CV per gram of TiO2. Typical proportions were 1 mL of solution to about 0.5 g of TiO2. The slurry was then spread thinly and as evenly as possible onto glass slides, and the solvent was allowed to evaporate off in the dark to minimize photodegradation of the dye. The consistency of the slurry was important for obtaining an easily spreadable, but not too runny, slurry. If the sample was too thick, “mud flats” formed as the sample dried and these were not as effective in displaying reversible metachromasy. Since all the liquid water evaporated away, all of the dye molecules were adsorbed onto the surface of the TiO2 particles, along with some few monolayers of water molecules in equilibrium with atmospheric water vapor.15 Therefore, the effective area per molecule can be calculated to be about 16-80 nm2 per molecule, depending on dye loading. The color changes that are visible in the sample were quantified by reflectance spectroscopy. To obtain enough sample for reflectance spectroscopy, several slides were prepared and the dyed powder was scraped off, collected, and pressed into the sample holder. Reflectance spectra were measured on a Shimadzu UV-2101PC UV-visible double beam spectrophotometer using the ISR-260 integrating sphere attachment. The spectra were referenced to undyed Degussa P-25 TiO2. For the purposes of these experiments, reflectance R is defined as the reflected light intensity from the sample divided by the reflected light intensity from the TiO2 reference, and absorbance is calculated by A ) -log R. Excess water vapor exposure was accomplished by placing an open container of hot tap water (∼90 °C) into the sample compartment. Absorption spectra of crystal violet solutions were measured on a Shimadzu UV-2401PC UV-visible double beam spectrophotometer, which is equivalent to the spectrophotometer used in reflectance measurements.
Figure 1. Photographs of a typical CV+/TiO2 sample under various conditions: left, wet slurry; center, freshly dried sample; right, sample from center immediately after water vapor exposure (from breath). Dye loading was 1 µmol of CV+/g of TiO2.
Results and Discussion The reversible metachromasy effect was first observed when an attempt was made to speed the drying of a slurry by blowing on it. It was found that as the slurry dried, its color changed gradually from lavender (created by the whitening of the CV+ solution by the TiO2 powder) to pink-violet, with or without blowing on it. Hand warming of the underside of the glass was found to be useful to speed the color change. When the slurry was freshly dry, an immediate color change from pink-violet to blue-violet was noticed when the sample was blown on gently. The color then reverted to pink-violet over a few seconds’ time. This was repeatable, even after the slurry had been dry for several hours. The color of the sample under various circumstances is shown in Figure 1. By use of steam, compressed air, and dry ice, it was determined that it was the water vapor in the breath that caused the color change. Reflectance spectra of a freshly dried sample were measured with increasing duration of excess water vapor exposure. The absorbance spectra, calculated from the reflectance, are shown in Figure 2a. The initial spectrum has weak absorbance (strong reflectance) in the violet and red region, resulting in the pink-violet color. The water vapor exposed spectra have increasingly less absorbance (stronger reflectance) in the blue and green region, resulting in the increasingly blue-violet color of the sample. After the container of water was removed from the compartment and the compartment was left open to the air for 15 min, the spectrum was virtually identical to the initial spectrum, and the sample was again pink-violet. (15) Technical Bulletin Pigments No. 56, 6th ed., Degussa AG, March, 1995; pp 13-17.
Figure 2. (a) Spectra of a 5 µmol of CV+/g of TiO2 dry sample exposed to excess water vapor for the amounts of time shown in the legend. Fifteen minutes after water vapor exposure ceased, the spectrum labeled “initial” was reproduced (not shown). When the sample was exposed to the air for 18 h after water vapor exposure ceased, the spectrum labeled “18 hours air” was obtained, showing that very little change in the spectrum occurred after the sample had equilibrated with atmospheric humidity. (b) Spectra of crystal violet in aqueous solution: solid line, 5 × 10-5 M CV+ “dilute”; broken line, 5 × 10-3 M CV+ “concentrated”.
Longer exposure to air does not change the spectrum dramatically. The spectrum labeled “18 hours air” in Figure 2a was measured after the same sample had sat in the open compartment overnight. For comparison, the spectra of dilute and concentrated crystal violet solutions are shown in Figure 2b. As can be seen, the spectrum of the freshy dried sample resembles that of the concentrated solution, although its peak is at a lower wavelength. This observation is well-known in adsorption of dyes from solution onto clay minerals.3,5,7 Increasing exposure to water vapor increased the resemblance of the absorption
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Figure 3. A proposed structure of the dimer formed by crystal violet ions in aqueous solution. The molecule drawn with a dottedline region lies below and parallel to the molecule drawn with only solid lines. Indicators of aromaticity have been omitted in the overlapping regions of the molecules for graphical clarity. Adapted from ref 1.
spectrum to the dilute solution, and decreasing exposure to water vapor restored the resemblance to the concentrated solution spectrum. When a freshly dried (pink-violet) sample was resuspended in water and then centrifuged, the spectrum of the supernatant solution was identical to a dilute CV+ solution. Therefore, no permanent chemical changes had occurred to the CV+ on the titanium dioxide surface that would explain the color change from the lavender of the wet slurry to the pink-violet of the freshly dried sample. It should be noted that the response time of the sample is much faster than is indicated by the exposure times in Figure 2a. The increase or decrease in partial pressure of water vapor above the sample in these experiments is limited by diffusion of water molecules through the air of the sample compartment. As shown by the instantaneous color change when the sample is breathed upon, the response of the surface layers to an increase in water vapor pressure is very rapid (
