Oct., 1963
SPECTROSCOPIC STUDY OF METHYLENE BLUEWITH MONTMORILLONITE
2169
A SPECTROSCOPIC STUDY OF METHYLENE BLUE MONOMER, DIMER, AND COMPLEXES WITH MONTMORILLONITE BY K. BERGMANE AND
c. T. O’KONSKI
Department of Chemistry, University of California, Berkeley, California Received March 14, 1963 Spectrophotometry was employed to study binding of methylene blue (MB) dye to sodium montmorillonite (NaM) in dilute aqueous suspension. The adsorption isotherm may be expressed u = kl k d ’ n , the first term ( k , = 80 meq. XB/100 g. XaM) arising from ion exchange, the second from physical adsorption. Large spectral changes (metachromasy) were found to accompany changes in the coverage, u,a t values well below k l . These are similar t o the spectral shift accompanying dimerization of MB in aqueous solution and are attributed to dye-dye interactions. The spectral properties showed large aggregates were formed a t moderate coverage. The spectra of free MB monomer and dimer were detezmined quantitatively. The peak molar absorbancy index of the 1M were monomer was found to be 9.5 X lo4a t 0640 A. The spectral variations u p to concentrations of 2 X interpreted quantitatively in terms of a monomer-dimer equilibrium, with a dimer dissociation constant of 1.7 X 10-4 a t 25‘. ‘The dimer spectrum was found t o contain a long wave length peak which can be explained by a sandwich structure having the monomer transition moments at a mean angle of about 13’ to each other. Trimethylthionine, formed by base-catalyzed demethylation, was found chromatographically to be a common impurity in methylene blue; a spectral criterion for this impurity was introduced and an extraction procedure was developed for removing it.
+
Introduction Metachromatic effects in methylene blue (MB) have been reported previously, both in solutions of pure MB of varying concentration and in systems where the dye is adsorbed on crystal surfaces. They consist of shifts of the long wave length adsorption peak near 6600 A. to shorter wave lengths in t.he neighborhood of 6000 A. as the dye concentration is increased. It was shown by Holst,’ Rabinowitch and Epstein12 and that the metachromasy of aqueous MB solutions can be related to the formation of dimers. The blue shift of the absorption peak was first explained by Forster7 on the basis of a sandwich structure of the dimer. Later work by Levinson, Simpson, and Curtis* on pyridocyanine dyes confirmed and ext,ended this picture. Sodium montmorillonite (Nabf), the main mineral constituent of Wyoming bentonite, is known to consist of particles which possess a lamellar structure. Single crystals cannot be obtained, and powder X-ray diffraction data leave some uncertainties in the structure. It is generally accepted t,hat each layer consists of a central octahedral alumina sheet bounded by two tetrahedral silica sheets. The nominal formula of this type of mineral is (OH)4Si~1402a~nHz0, where n is the number of iiiterlayer waker molecules. bfontmorillonite always differs from this formula because of substitution of All or possibly P, for Si, and/or Xg, Fe, Zn, Ni, Li, etc., for Al. These substitutions give rise to a net negative charge, which can be regarded as being distributed over many atorns of thle individual sheets, and which is neutralized by oounte:rions. A typical formula is (1) G. Hoist, Z. p h y s i k . Chem,., A182,321 (1938). (2) E. Rabinowitch and L. 17.Epstein, J . Am. Chem. Sac., 63, 69 (1941). ( 3 ) G. N. Lewis, 0 . Goldsohmid, T. T. Magel, and J. Bigeleisen, iiiid., 66,
1150 (1943). (4) L. Michaelis and S. Granick, ibid., 67, 1212 (1945). ( 5 ) T. Viokerstaff and D. K. Lemin, Nature, 167, 373 (1946); D. 12. Lemin a n d T. Viokerstaff, Trans. Faraday Sac., 43,491 (1946). (6) M. Schubert artd A . Levine, J . Am. Chem. Sac., 77, 4197 (1956). (7) T. Forster, NaturmissenachaSten, 83, 166 (1946). (8) G. S. Levinson, W. T. Simpson, a n d W. Curtis, J . Am. Chem. Soc., 79, 4314 (1957). (9) R. E. Grim, “Clay Jfiaeralogy,” IlfcGraw-Hill Book Co., Inc., New York, N.Y., 1953, Chapters 4 and 6.’
Na0.68 where the arrow indicates the exchangeable cations. A stack of about 10 sheets spaced about 15 A. apart in aqueous environment forms a particle. Mering lo reports the formation of aggregates in dilute suspensions by the association of several particles. The Na+ ions are easily exchanged by methylene blue cations.l 3 I n a particle coiisisting of many layers, most of the counterions will be found a t interlamellar sites and only relatively few at the outer surface of the particles. Neutral organic molecules also appear to be adsorbed primarily a t interlamellar sites. The orientation of neutral organic molecules has been found from X-ray measurements of the (001) spacing to be flat to the montmorillonite surface at low c o v e r a g e ~ . l ~ -At ~~ higher coverages, an increase in the spacing of the montmorillonite sheets was observed; this was attributed to the formation of double layers of the ad~ 0 r b e n t . l ~It has been suggested that, as the MB content increases, there might be a change in orientation of the flat methylene blue molecule from a parallel position to an orientation normal to the s ~ r f a c e . ~ ~ J * ~ ~ ~ The main objects of this work were to study the binding of MB to montmorillonite and the spectra of the ?L/IB-R;I complexes, as the groundwork for a related study of electric dichroism on the same system, and a more complete discussion of t,he structure of the complexes. 2o (10) J. Mering, Trans. Faraday Soc., 42B, 205 (1946). (11) R. K. Iler, “The Colloid Chemistry of Silica and Silicates,” Cornell University Press, New York, N. Y., 1955, p. 193. (12) T. W. McBain, “Colloid Science,” D. C. Heath and Co., Boston, Mass., 1950, Chapter 26. (13) J. E. Gieseking, SoilSci., 47, 1 (1939). (14) 8 . B. Hendricks, J . Phys. Chem., 46, 65 (1941). (15) D. M. C. MacEwan, Trans. Faraday Sac., 44, 349 (1948). (16) R. Greene-Kelly, ibid., 51, 412 (1955). (17) A . Haxaire and J. >I. Bloch, Bull. sac. franc. mineral. mist., 79, 464 (1956).
(18) J. W. Galbraith, C. H. Giles, A. G. Hallidey, A. S. A. Hassan, D. C . McAllister, N. Macaulay, a n d N. W. Maclllillan, J . Appl. Chem., 8, ,416 (1958). (19) D. A t , C. XacEwan, Y. Canoruiz, and F. A . de la Cruz, Anales real. s’oc. espah. 5s. quim. (Madrid), 55,677 (1959).
2170
Irol. 67
The first part of this paper deals with the quantitative spectrophotometry of aqueous MB solutions, including purification procedures. Vany previous authors apparently used impure samples for their studies, and the literature contains a wide range of absorbancy values. There follows an interpretation of the spectra of the aqueous LIB solutions by the determination of the monomer-dimer equilibrium constant and the spectra of pure monomer and dimer species. This leads us to the discovery, in the dimer spectrum, of the long wave length peak theoretically predicted by Forster.‘ Finally, we deal with the adsorption of methylene blue on a montmorillonite suspension, report the adsorption isotherm, and give interpretations of the spectra as a function of coverage of the montmorillonite surface in terms of interactions between the dye molecules. Experimental Methylene Blue.-To check the purity of our commercial methylene blue (MB) sample (Rlerck “reagent methylene blue”) we submitted it to a chromatographic test. A small column contained RIerck “reagent aluminum oxide, suitable for chromatographic adsorption.”21 A few milliliters of a dark blue aqueous RlB solution was applied to the column, and the chromatogram was developed a ith 95% ethanol. Right after the application of the ethanol, a pink ring started descending down the column. It was followed by a blue ring. As the pink zone emerged from the column it turned blue. With water instead of ethanol as a developer, the chromatogram developed more slowly. Two to three days were required for a clear separation into two zones. Also, the sequence of the zones was reversed; the blue zone led the pink zone. Again the pink zone turned blue as it emerged from the column. When the pink zone emerged from the column under a nitrogen atmosphere, it stayed pink, but the color turned blue under a COS atmosphere. The pink color could be restored by adding some drops of dilute ”,OH to the liquid, and the blue was quickly restored with dilute acid; thus it became clear the pink impurity was an acidbase indicator, evidently in its pink form on the aluminum oxide, and converted to a blue form visually indistinguishable from methylene blue on contact with COZ from the air. It was learned that the aluminum oxide is adjusted by the manufacturer to a basic p H value. For a dilute suspension (-10%) in water we found a p H of 10.8. The composition of the two eluted chromatographic zones was determined from their spectra, which were very similar in shape and had, a t very low concentrations (-lop5 AT), peaks at the wave lengths listed in Table I. The material in the pink zone mas observed in its acidic (blue) form in this comparison.
TABLE I PEAKKAVE LEXGTKSOF METHYLENE BLUEA S D THE CHROMATOGRAPHICALLY SEPARATED IMPURITY --Developed Zone
320
Blue Pink
6640 6500 - 140
AX
and measured inC2HsOH
6520 6390 - 130
Formanek22 reported the absorption peaks of a number of Epectra of substituted thionines. Because these old observations were visual me cannot trust the absolute values of peak wave lengths, but can have more confidence in the wave length shifts, A x , with respect to MB. Formanek’s data22 are presented in Table 11. I t seems evident that the blue zone reported in Table I corresponds to the pure MB, column 4 in Table 11, and that the pink zone on the chromatographic column corresponds to column 3, trimethylthionine (TMT). called methylene azure B.23,24 (20) C. T. O’Konski and K . Rergmann, J . Chem. P h g s . , 87, 1673 (1962): i b i d . , t o be published. (,21) P. Ruggli and P. Jensen, He/?. Chim. Acta, 18, 624 (1935). (22) J. Formanek, “Untersuchung und Nachaeis organischer Farbstoffe auf spektroskopischem Wege,” 2. Auflage, 1. Teil, Berlin, 1108, pp. 142-164.
TABLE I1 PEAKWATELENGTHSOF
ABSORPTIOK SPECTRA OF VARIOUS METHYLATED THIONIKES
----No.
Amax
AX
Solvent
0
) HzO
6025 6053
\
CzHiOH
4 H20 CLH,OH i
-650 -521
THE
of substituted methsl y~ou~is------1 2(ssm.) 2(asLin.) 3 4
6114 6147
-561 -427
6201 6178 -474 -406
6380 6301 -295 -2i3
6517 6675 6424 6574
-158 -150
0 0
The color change of TLIT, also reported b y other norkers,23124 has its origin in a change of pH. I n aqueous solutions which are acidicoor neutral, the absorption peak of very +lute TRIT is a t 6500 A., and in basic solutions, it is near 5500 A. By means of spectrophotometric measurements a t various p H values in buffered solutions, the equivalence point was found a t p H 12.2. The acid-base equilibrium may be expressed as
At low p H the T M T molecule is charged, whereas at the high p H of the chromatographic column it apparently is neutral. In contrast, the methylene blue remains cliarged. The fact that, the pink zone elutes faster Tvith ethanol than the blue zone is consistent’ with the greater solubility of the neiitral molecule in the ethanol. From the different partition coefficients of the ionized MB and the neutral T M T bet’ween aqueous and organic phases, we derived an extraction procedure for t,he purification of our commercial l I B sample. -4pproximately 0.1 g. of the llerck sample was dissolved in 100 ml. of 0.15 M S-H40H and extracted ten times with 100 ml. of thiophene-free, redistilled benzene. Other solvents were tried, including aniline, dimethylaniline, xylene, and mesitylene. Sone of these was appreciably more efficient than benzene. I t was essential to do the extraction as fast as possible since in basic solution RIB was converted to TWT. During the extraction the red color of the benzene phase decreased quite rapidly and the tenth benzene phase was almost colorless Separate experiments showed that the pink component was produced from methylene blue more rapidly in the course of extractions carried out under more basic conditions, e.g., 0.01 M KOH. Through experimenh in which oxygen was excluded, it was found that oxygen was not needed for this consTersion. The demethylation reaction evidently is base catalyzed; therefore, the pH of the aqueous ammoniacal phase was lowered after the extraction from 10.9 to 8.0 in order to prevent further reaction of the XIB. This n-as achieved conreniently during evaporation in a vacuum-drying apparatus a t room temperature to concentrate the MB solution. The purity of the MB was estimated in the follon-ing way. The spectrum of MB was taken a t neutral p H and such low concentrations t,hat there was very little dimer present. From t,he heights d and e of the peak and the inflection point of the adsorption curve, respectively, R E d / e was calculated. Since all of the demethylation products have absorption peaks a t lower wave lengths than MB, demethylation t’ends to decrease R . Also, the position of the peak, A,, is shifted toward lower wave lengths in the presence of demethylation products, but this shift is not as sensitive as the change in R . During the purification of our RIB sample, the R value increased from 1.78 to 2.01 for c = 6 . 7 x 10-6 X ; a t the same time A,, shifted from 6620 to 6640 A . Our A,, is in good agreement wit,h the value given by Schubert and Levine,6 6650 and differs appreciably from Rabinowitch and Epetein’s value,2 I
i.,
(23) G. Schultz, “Farbstoff-Tabellen,” 7th E d . , .4kademisohe Verlagsgesellschaft, Leiprig, 1931, No. 1039. (24) W.J. MciYealand J. 4 . Killian, J . Am. Cliem. Soc., 48, 740 (1926).
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