Spectroscopic Study
Oct., 1963
of
Methylene Blue
with
Montmorillonite
2169
A SPECTROSCOPIC STUDY OF METHYLENE BLUE MONOMER, DIMER, AND COMPLEXES WITH MONTMORILLONITE By K. Bergmann
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 h + fee1/”, the first term (NaM) in dilute aqueous suspension. The adsorption isotherm may be expressed u 80 meq. MB/100 g. NaM) arising from ion exchange, the second from physical adsorption. Large spectral (k, =
J. Phys. Chem. 1963.67:2169-2177. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/28/19. For personal use only.
=
changes (metachromasy) were found to accompany changes in the coverage, u, at values well below k¡. These are similar to 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 at moderate coverage. The spectra and dimer were determined quantitatively. The peak molar absorbancy index of the of free MB monomer The spectral variations up to concentrations of 2 X 10 “4 M were monomer was found to be 9.5 X 104 at 6640 A. interpreted quantitatively in terms of a monomer-dimer equilibrium, with a dimer dissociation constant of 1.7 X 10-4 at 25°. The dimer spectrum was found to 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. Trimethylthionme, 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 Metachroma tic 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 Á. to shorter wave lengths in the neighborhood of 6000 Á. as the dye concentration is increased. It was shown by Holst,1 Rabinowitch and Epstein,2 and others8-6 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 Curtis8 on pyridocyanine dyes confirmed and extended this picture. Sodium montmorillonite (NaM), the main mineral constituent of Wyoming bentonite, is known to consist of particles which possess a lamellar structure.9 Single crystals cannot be obtained, and powder X-ray diffraction data leave some uncertainties in the structure. It is generally accepted that each layer consists of a central octahedral alumina sheet bounded by two tetrahedral silica sheets. The nominal formula of this type of mineral is (OHjiSisALCbo ·% 20, where n is the number of interlayer water molecules. Montmorillonite always differs from this formula because of substitution of Al, or possibly P, for Si, and/or Mg, 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 atoms of the individual sheets, and which is neutralized by counterions. A typical formula is (1) G. Holst, Z. physih. Chem., A182, 321 (1938). (2) E. Rabinowitch and L. F. Epstein, J. Am. Chem. Soc., 63, 69 (1941). (3) G. N. Lewis, O, Goldschmid, T. T. Magel, and J. Bigeleisen, ibid., 65, 1150 (1943). (4) L. Michaelis and S. Granick, ibid., 67, 1212 (1945). (5) T. Vickerstaff and D. R. Lemin, Nature, 157, 373 (1946); D. R. Lemin and T. Vickerstaff, Trans. Faraday Soc., 43, 491 (1946). (6) M. Schubert and A. Levine, J. Am. Chem. Soc., 77, 4197 (1955). (7) T. Forster, Natunoissenschaften, 33, 166 (1946). (8) G. S. Levinson, W. T. Simpson, and W. Curtis, J. Am. Chem. Soc., 79, 4314 (1957). (9) R. E. Grim, “Clay Mineralogy,” McGraw-Hill Book Co., Ihc., New York, N. Y., 1953, Chapters 4 and 6.
(OH) 4Si8 (Al3 .eíMgo ,ßß) O20 1 Nao.ee
where the arrow indicates the exchangeable cations.9 A stack of about 10 sheets spaced about 15 Á. apart in aqueous environment forms a particle.10-12 Mering10 reports the formation of aggregates in dilute suspensions by the association of several particles. The Na+ ions are easily exchanged by methylene blue cations.13 In a particle consisting of many layers, most of the counterions will be found at interlamellar sites and only relatively few at the outer surface of the particles. Neutral organic molecules also appear to be adsorbed primarily at 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 coverages.18-17 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 adsorbent.16 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 surface.16·18·19 The main objects of this work were to study the binding of MB to montmorillonite and the spectra of the MB-M complexes, as the groundwork for a related study of electric dichroism on the same system, and a more complete discussion of the structure of the complexes.20 (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, Soil Sci., 47, 1 (1939). (14) S. B. Hendricks, J. Phys. Chem., 46, 65 (1941). (15) D. M. C. MacEwan, Trans. Faraday Soc., 44, 349 (1948). (16) R. Greene-Kelly, ibid., 51, 412 (1955). (17) A. Haxaire and J. M. Bloch, Bull. soc. franc, mineral, crist., 79, 464 (1956). (18) J. W. Galbraith, C. H. Giles, A. G. Halliday, A. S. A. Hassan, D. C. McAllister, N. Macaulay, and N. W. MacMillan, J. Appl. Chem., 8, '416 (1958). (19) D. M. C. MacEwan, Y. Canoruiz, and F. A. de la Cruz, Anales real, soc. españ.fis. quim, (Madrid), 55, 677 (1959).
K. Bergmann
2170
and
The first part of this paper deals with the quantitative spectrophotometry of aqueous MB solutions, including purification procedures. Many 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 MB solutions by the determination of the monomer-dimer equilibrium
constant and the spectra of pure monomer and dimer This leads us to the in the dimer species. discovery, spectrum, of the long wrave length peak theoretically predicted by Forster.7 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.
C. T.
O’Konski
Vol. 67 Table II Absorption Spectra Methylated Thionines
Peak Wave Lengths
of
the
.--No.
max
of
Various
of substituted methyl groups-------
Solvent
0
1
3
4
/h2o
6025 6053
6114 6147
6201 6178
6380 6301
6517 6424
6675 6574
561
-474
-295 -273
—158
0
-150
0
i,
C2H5OH
ÍH2°
1 C2H5OH
-650 -521
—
2(sym.) 2(asym.)
-427
-406
The color change of TMT, also reported by7 other workers,23>24 a change of pH. In aqueous solutions which are acidicoor neutral, the absorption peak of very dilute TMT is at 6500 A., and in basic solutions, it is near 5500 A. By means of spectrophotometric measurements at various pH values in buffered solutions, the equivalence point was found at pH 12.2. The acid-base equilibrium may be expressed as has its origin in
Experimental Methylene Blue.—To check the purity of our commercial methylene blue (MB) sample (Merck “reagent methylene blue”) we submitted it to a chromatographic test. A small column contained Merck “reagent aluminum oxide, suitable for chromatographic adsorption.”21 A few milliliters of a dark blue aqueous MB solution was applied to the column, and the chromatogram was developed with 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 With water inzone emerged from the column it turned blue. stead of ethanol as a developer, the chromatogram developed Two to three day7s were required for a clear sepamore slowly7. ration 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 C02 atmosphere. The pink color could be restored by adding some drops of dilute NH4OH to the liquid, and the blue was quickly7 restored with dilute acid; thus it became clear the pink impurity was an acidbase indicator, evidently7 in its pink form on the aluminum oxide, and converted to a blue form visually indistinguishable from methylene blue on contact with C02 from the air. It was learned that the aluminum oxide is adjusted by- the manufacturer to a basic pH value. For a dilute suspension (~10%) in water we found a pH of 10.8. The composition of the two eluted chromatographic zones was determined from their spectra, which were very similar in shape and had, at very low concentrations (^lO-5 M), peaks at the The material in the pink zone wave lengths listed in Table I. was observed in its acidic (blue) form in this comparison. Table I Peak Wave Lengths of Methylene Blue and Chromatographically Separated Impurity
the
f--Developed and measured in-s Zone
H20
C2H¡OH
Blue
6640 6500
6520 6390
Pink
-140
-130
Formanek22 reported the absorption peaks of a number of spectra of substituted thionines. Because these old observations were visual we cannot trust the absolute values of peak wave lengths, but can have more confidence in the wave length shifts, , with respect to MB. Formanek’s data22 are presented in
Table II. It seems evident that the blue zone reported in Table I corresponds to the pure MB, column 4 in Table II, and that the pink zone on the chromatographic column corresponds to column 3, trimethydthionine (TMT). called methylene azure B.23'24 (20) C. T. O’Konski and K. Bergmann, J. Chem. Phys., 37, 1573 (1962)! ibid., to be published. (21) P. Ruggli and P. Jensen, Helv. Chim. Acta, 18, 624 (1935). (22) J. Formanek, “Untersuchung und Nachweis organischer Farbstoffe auf spektroskopischem Wege,” 2. Áuflage, 1. Teil, Berlin, 1908, pp. 142-164.
At low pH the TMT molecule is charged, whereas at the high pH of the chromatographic column it apparently7 is neutral. In contrast, the methylene blue remains charged. The fact that the pink zone elutes faster with ethanol than the blue zone is consistent with the greater solubility7 of the neutral molecule in the
ethanol. From the different partition coefficients of the ionized MB and the neutral TMT between aqueous and organic phases, we derived an extraction procedure for the purification of our commercial MB sample. Approximately 0.1 g. of the Merck sample was dissolved in 100 ml. of 0.15 iff NH4OH and extracted ten times with 100 ml. of thiophene-free, redistilled benzene. Other solvents were tried, including aniline, dimethylaniline, xylene, and mesity'lene. None of these was appreciably more efficient than benzene. It was essential to do the extraction as fast as possible since in basic solution MB was converted to TMT. 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 rapidly7 in the course of extractions carried out under more basic conditions, KOH. Through experiments in which oxy7gen was e.g., 0.01 excluded, it was found that oxy7gen was not needed for this conversion. The demethylation reaction evidently is base catalyzed; therefore, the pH of the aqueous ammomacal phase was lowered after the extraction from 10.9 to 8.0 in order to prevent further reaction of the MB. This was achieved conveniently7 during evaporation in a vacuum-drying apparatus at room temperature to concentrate the MB solution. The purity7 of the MB was estimated in the following way. The spectrum of MB was taken at neutral pH and such low concentrations that there was very little dimer present. From the heights d and e of the peak and the inflection point of the adsorption curve, respectively7, R = d/e was calculated. Since all of the demethydation products have absorption peaks at lower wave lengths than MB, demethylation tends to decrease R. Also, the position of the peak, Xmax, is shifted toward lower wave lengths in the presence of demethydation products, but this shift is not as sensitive as the change in R. During the purification of our MB sample, the R value in5.7 X 10"6 M; at the same creased from 1.78 to 2.01 for c time kmax shifted from 6620 to 6640 k. Our ymox is in good agreement with the value given by Schubert and Levine,6 6650 A., and differs appreciably from Rabinowitch and Epstein’s value,2 —
(23) G. Schultz, “Farbstoff-Tabellen,” 7th Ed., Akademische Verlagsgesellschaft, Leipzig, 1931, No. 1039. (24) W. J. Me Neal and J. A. Killian, J. Am. Chem. Soc., 48, 740 (1926).
Oct.,
19G3
6560 A. 6600 A.
Spectroscopic Study
of
Methylene
Blue
with
Montmorillonite
2171
Michaelis and Granick1 and Auskáps2'’ found „ :
Attempts to purify the MB by recrystnllizatiori from hot water and from hot ethanol failed. It was found that such recrystallizations decreased the li value instead of increasing it. Chromatographic observations confirmed that the impurity TMT concentration was increased by recrystallization. To compare our purified MB sample with the data published by other workers, we plot in Fig. 1, R vs. the concentration of MB, c, in moles/1. Because of dimerization, R depends upon c, and the R values of different authors should be compared at constant c. We see from Fig. 1 that our R values are close to the best literature data by Schubert and Levine.6 The R values of Rabinowitch and Epstein2 and of Auskáps26 are far off, thus indicating an impure sample. The R value of an alcoholic MB solution is almost equal to the one of a low concentration aqueous solution, as is indicated by one point of Michaelis and Granick4 in Fig.
1.
There is also little agreement among the molar absorbancy index values of methylene blue in the literature. This can be seen from Table III, in which arc compared the effective molar absorbancy indices, a (see eq. 2), at the monomer peak (m peak) The MB concentrations c at which a values were near 6600 Á. measured arc also given, since a depends on concentration (see Table III). In this research, the concentration of purified MB stock solution was determined by Ferrey’s method.26 It consists of precipitation of MB with potassium dichromate. Drying of the commercial MB sample at temperatures from 105 to 130° did not. produce constant weight. All solutions were buffered with 10""5 M IIAc and 10~3 M NaAc (pH 4.75) and were kept in tightly stoppered polyethylene vessels. It was shown that adsorption effects and possible photochemical reactions were not important. Schubert and Levine6 observed that a lowering of pH decreased the adsorption by the spectrophotometric cell and also increased the reproducibility of the measurements considerably. These observations were confirmed. The precision was well within Ferrey’s claim of 2% accuracy.
Table III Peak Molar Absorbancy Index Values Reported Methylene Blue Author
AuSkaps”
Yen
i·
10-4«
1930
7.5
1941
3.9
'-
i
Lewis, Goldschmid, 1943 Magel, and Bigel1943 eisen' 1945 Michaelis and Granick’1 1945 Michaelis and Granick'1 1955 Schubert and Levine' Present authors “ Reference 25. 6 Reference 2. 4. Reference 6.
8.4 9.2 8.0 8.8 6.2 9.5 c
II20
2.0
h2o
C
0.03 h2o 95% EtOH 0.35 h2o 2.54 Alcohol 1.05 0.1.VHC1 0 HgO, buffer
Reference 3
d .
Referee
e
Rabinowitch and Epstein’s value is much lower than the others. From the low R value, it seems clear that impurity of their sample is the explanation. Since Schubert and Levine’s sample has a high R value (Eig. 1), their low value of a can only be interpreted as due to an erroneous concentration determination. It is not clear from their paper how the concentration was deter8 X 104, mined. The rest of the values scatter around a probably because of adsorption of the methylene blue by glass vessels and the spectrophotometric cell at low concentrations. This statement, is supported by Fig. 2, where the effective molar absorbancy index a, defined below, is plotted against the MB concentration. The drawn curve is a plot of eq. 2 (sec Results and Discussion), which gives an excellent fit of the experimental points fore > 20 X 10 M. For c < 20 X 10“° Af, the points lie This suggests that some MB was being rebelow the curve. moved from the solution by the vessels. A comparison of Fig. 2 with the above cited literature values shows that all molar absorbancy indices of MB in the literature are too small. Na Montmorillonite.—Sodium montmorillonite (NaM) was obtained from Dr. W. T. Higdon and H. van Olphen of the Shell Development Company. Wyoming bentonite clay had been =
(25) .7. AuSkaps, Acía Unit'. Late. (Riga), 279 (1930). (26) G. J. W. Ferrety, Anatysf, 69, 54 (1944).
I06,
for
Rabinowitch and Epstein6
X
Solvent
0
.
C
Fig. 1.—Peak to shoulder absorbancy ratio, R, vs. concentration of aqueous methylene blue solutions. Experimental points by: O, Schubert and Levine6: V, Michaelis and Granick4; *, Rabinowitch and Epstein2; , Auskáps; ·, this research. The curve represents the best available values for pure methylene blue.
x
I06.
Fig. 2.—Effective molar absorbancy index of methylene blue = at the monomer 6640 A., as a function of methylene peak, blue concentration. The points are experimental; the curve is drawn according to eq. 2, with am 9.50 X 104, «i¡ 2.52 X 104, and K = 1.7 X 10~4 mole/1. =
=
suspended in water and the coarse particles were allowed to settle out. The suspended material had been passed through a Na+ Dowex-50 ion-exchange column until Na+ was the only exchangeable cation in the clay. After the clay had been washed free of soluble electrolyte by repeated centrifugations from water, it had been fractionated with the Spinco preparative centrifuge. The fraction we used contained particles with the equivalent spherical radius 70 to 230 A. The concentration of stock gel used in this research was determined by drying the gel in air at 180° for 16 hr.; it was 5.25 weight %. Dilutions were prepared by shaking vigorously a weighed amount of the Na montmorillonite gel with a known volume of distilled water. Shortly before the adsorption measurement the suspension was buffered with (10~3 N IIAc, 10~3 N NaAc) buffer. The pH of the buffered suspension was 4.75. A relatively highly concentrated NaM sol, w 0.36 g./l., showed only very little light absorption, A < 0.015, between 5000 and 7000 Á.; at X 2420 there was an absorption peak, AmnI 1.03. This absorption in the ultraviolet was proportional to the NaM concentration and was used from time to time to check the concentration of the sol. Binding Studies and Spectrophotometry.—Since the spectral absorption regions of free and adsorbed dye overlap and both are dependent upon the concentration, it was necessary in the general case to measure the absorption spectrum of tire MB-NaM suspension, then centrifuge out the MB-NaM complex and measure tire spectrum of the supernatant. Tire spectrum of the MBNaM complex was then obtained by difference. At low MB: NaM ratios, the MB was quantitatively removed from the aqueous =
.,
K. Bergmann
2172
and
C. T.
O’Konski
Yol. 67
and it follows that
K
=
2ca2/(1
a)
-
(1)
If A is the absorbancy of the solution at a total MB concentration c and l is the path length, we may define an effective molar absorbancy index a
=
A/lc
Assuming that Beer’s law holds for each component, it follows that a
aam
=
+
(1
—
a)a¿/2
(2)
Experimental absorbancy indices of MB solutions, after extraction, are listed in Appendix A. A check of the validity of eq. 2 was obtained by plotting the molar absorbancy indices for constant wave length vs. a, assuming certain values for K. In the neighborhood of 2 X 10~4, straight lines were obtained in the range K 5000 A. < < 7000 A., as shown in Fig. 3. By extrapolating the straight lines to a 0, the absorption the of dimer was obtained. spectrum pure Extrapola1 at 6640 A. gave the value czmax tion to a 9.5 X 104. Trial and error fitting involving the straight line plot was not sensitive enough to distinguish between several equilibrium constants in the neighborhood of K 2 X 10-4 mole/1. It was possible to find an accurate value for the dimer dissociation constant, K (1.7 ± 0.2) X 10-4 mole/1., from the condition that the dimer spectrum should show neither a shoulder nor a hollow at 6640 A. The standard free energy of dissociation is AF° 5.1 kcal./mole. Kwas RTlnK found not to depend on the buffer concentration. This excludes the possibility that the acetate anions are involved in the aqueous MB dimer, in contrast to the situation with a pyridocyanine dye in a rigid glass at 77°K, where anion participation was indicated.8 Previous values for K were 1 X 10-4 < K < 2 X 10-4 2.8 X 10-4 mole/1. (Rabinowitch (Holst1) and K =
Fig. 3.—Straight line plot of the effective.molar absorbancy index of aqueous methylene blue solutions at various wave 1.7 X 10-4 fraction a for K lengths (in A.) vs. monomer and dimer mole/1. The molar spectra of the pure monomer 1 and 0, respec(Fig. 4) were obtained by extrapolation to a tively. The points are experimental and the lines represent calculated values. =
=
phase, so the centrifugation step and spectrophotometry of the supernatant were not necessary. Centrifugation at 1.1 X 106 the montmorillonite quantitag for 2 hr. was found to remove tively. Measurements showed there was no rise in temperature during the centrifugation. All spectrophotometry was done at room temperature (25 ± 2°). Glass vessels were avoided as far as possible for the adsorption studies, because MB is adsorbed on their surfaces. The sols were prepared in polyethylene containers and centrifugated in polypropylene tubes. At high speed centrifugation the polypropylene tubes adsorbed a little MB, apparently in fine cracks, but this adsorption was reproducible, so corrections could be applied. Apparatus.—The spectroscopic studies were carried out with the Cary recording spectrophotometer, Model 14 M, of the Applied Physics Corporation in Pasadena, California. The wave length calibration was checked periodically with a hydrogen lamp and found stable to ± 1 A. For the centrifugation of the clay suspensions, the Spinco ultracentrifuge, Model L, of the Beckman Instruments, Inc., Spinco Division, Palo Alto, California, was used.
Results and Discussion Spectra of Methylene Blue Monomer and Dimer and the Monomer-Dimer Equilibrium Constant.—-It was confirmed that the absorption spectra of aqueous methylene blue solutions could be interpreted quantitatively on the assumption of a monomer-dimer equilibrium2 for concentrations up to 2.15 X 10~4M. For the
equilibrium
(MB)
2
=
2MB
may define the dimer dissociation constant
we
K
=
CmVCd
and cd are the molar concentrations of the and dimer, respectively. Neglecting higher polymers, the total molar concentration of methylene blue computed as monomer is
where
cm
monomer
c
If
a
~
Cm
2Cd
is the fraction of MB present a
cn/c
as
monomer, then
=
=
=
=
=
=
—
=
=
and Epstein2). Figure 4 shows the spectra of the pure MB monomer and dimer. The spectrum of the monomer, of course, differs but little from the spectrum of a very dilute MB solution. The value of the absorbancy index at the peak of 6640 A., which we shall call the m peak, is 9.5 X This exceeds all direct experimental determina104. tions at low concentrations (see Fig. 2). There is a shoulder at approximately 6100 A. which has been identified,3 with the aid of fluorescence observations, as a vibrational component of a single electronic band. The spectrum of the dimer is strikingly different from that of the monomer, which indicates a strong interaction of the electrons of the two MB molecules in a dimer. There are two peaks, one at 6050 A., which we will call the di peak with molar absorbancy index 13.2 X 104, and the other at approximately 6970 A. (d2 peak) with molar absorbancy index 2.2 X 104. The long wave length region was examined carefully in this study, as the d2 peak might be expected from theoretical considerations7·8 and has been found for the dimers of similar dyes,8 27 Apparently this is the first time it was observed in methylene blue dimer. The transition moment lengths q of the MB monomer and dimer Were determined from areas under the absorption curve, using8 eq. 3. (27) T. Forster and E. Konig, Z. Elektrochem., 61, 344 (1957).
Spectroscopic Study
Oct., 1963 g2
-1.09 X KT19
=
fa
Methylene Blue
of
d log
with
Montmorillonite
(3)
2173 rm
A
2.2 A., and For the aqueous monomer we found gm 0.24 A. 2.0 Á. and qd, for each MB in the dimer, qdl Oscillator strengths were obtained by integrating over the absorption bands, employing the relation 28 29
PE AK
664 3
A
=
=
/ where
P
is the
4.32 X 10 SR
=
=
fa
dP
number, and R is
wave
function given by
an
i
d, PEAK 6 050 A
/ /
i
(4)
internal field
Qd 2
/
~~7
(Lorentz internal field)
„
(n-o2
+
no (
——)
2)2
\
3 n02 \ I-—^n°
/
Q3
(Onsager internal field)
Results n0 is the refractive index of the solvent. given in Table IV. From the oscillator strengths, also from the transition moments, hypochromic shifts Included in this are seen to accompany dimerization. table are oscillator strengths of the bound polymer spectra, discussed below. The hypochromic effect is even greater there. The oscillator strength of the is higher than the value 0.45 reported earlier.2 monomer and are
Oscillator Band m
di di
pd p¿
Strengths
of
Table IV Free and Bound Methylene ----Caled, Lorentz
0.68 .60 .032 .49 .003
d2PEAKJ
0T--1---i-^- -
6000
5500
5000
0.59 .52 .026 .42 .003
The dimer probably is held together by London dispersion forces and “hydrophobic bonding”30 which overcome the electrostatic repulsion between two MB cations. The dispersion forces should be greatest when the monomer units are in a sandwich with principal molecular axes parallel. The coulombic repulsion
(am) and Fig. 4.—Molar absorbancy indices of pure monomer dimer (eta) of aqueous methylene blue solutions. The points are extrapolated from plots like Fig. 3.
calculated from and Curtis8
a
relation given by Levinson, Simpson, =
9d±
2
,2(gr
+
9b)
where qd+ and qd_ are the dimer transition moments and qt, 9b are the transition moments of the top and bottom molecules. This equation leads to the relations =
9d+
9d-
=
+
9m (1 9m (1
-
cos cos
)1/2
(a)
)1/2
(b)
transition where gm is the magnitude of the monomer moment. These equations are not rigorous, no allowThat is, ance being made for possible hypochromism. be from should and be different What qm. gT gB may employed in eq. a and b is the transition moment of unit in the dimer structure, in place of each monomer moment. To cancel any hypo9m, the free monomer chromic effect we divide (a) by (b) and solve for cos with the result cos
=
(9d+2/9d-2
-
l)/(9d+2/9d-2 + 1)
2.0 From the transition moment lengths, qd+ qdl 0.24 A., gd+2/9d-2 72, and 9d2 Á., and gd13 ± 2°. Considerations based on the well known free electron model28 suggest to us that the transition moment for the visible absorption lies along the long axis of the molecule. Thus, the mean angle of 13° units in between transition moments of the monomer the dimer, calculated with the aid of the (modified) relation of Levinson, Simpson, and Curtis, may be interpreted as the mean angle between the long axes of the monomer units of the relatively loose dimer struc=
Probable structure of methylene blue dimer. (Only one of four possible resonance configurations is shown.)
will be minimized when the charged amino groups lie
along opposite edges of the sandwich; hence we expect the structure depicted above, and originally proposed by Forster,7 to be the most stable. Since there are no hydrogen bonds or attractive coulombic interactions to lock the two pieces of the sandwich, we expect a rather low restoring force constant for the torsional deformation about an axis through the central rings and perpendicular to the planar ring systems. The values of the transition moments can be related to the angle characterizing this deformation. An averbetween the transition moments can be age angle (28) W. J. Kauzmann, “Quantum Chemistry,” Academic Press, Inc., New York, N. Y., 1957, p. 581. (29) R. S. Mulliken and C. A. Rieke, Rept. Progr. Phys., 8, 231 (1941); H. Kuhn, Helv. Chim. Acta, 34, 1308 (1951). (30) W. J. Kauzmann, Adran. Protein Chem., 14, 1 (1959).
7500
X(A).
Blue
with R of---· Onsager
7000
6 500
=
=
=
=
=
ture. The Binding of Methylene Blue by Na Montmorillonite.—The binding of methylene blue to Na montmorillonite was measured in 0.001 M acetic acid plus 0.001 M sodium acetate buffer. The isotherm is plotted in Fig. 5. The amount of bound (or adsorbed) 105 X c'/w, is MB in meq. per 100 g. of NaM, u plotted as a function of c, the conilibrium concentra=
K. Bergmann
2174
and
C. T.
O’Koxski
Yol. 67
tive charges on the NaM should be canceled by MB + cations if u is equal to 88. Beyond the neutralization point, the NaM particles become positively charged and the suspension should become stable again. This is apparently occurring near 125 meq./100 g. The curve of Fig. 5 fits the equation u
and
=
fci
+
constants) which
=
(6)
proposed long recently discussed by Plesch and Robertson.32 The latter investigators interpreted their adsorption measurements of methylene blue on different clays by eq. 6, but they did not report the constants ki and k2. The curve in Fig. 5 is drawn with ki 80 meq./100 g., k2 0.168. 48, and l/n According to Robertson and Plesch, eq. 6 describes two different adsorption processes: (a) irreversible ion k2c1/n exchange, iti ki, (b) physical adsorption, u2 80 meq./ (Freundlich isotherm). Since our value, ki 100 g., is essentially the exchange capacity of the clay (generally about 88 meq./100 g.), Robertson and Plesch’s assumption is wrell supported by our data. The rather low7 exponent l/n is in good agreement w7ith literature data33 and seems to be typical for basic dyes. At equilibrium MB concentrations greater than 10~4 M, the adsorption isotherm was not reproducible for reasons wrhich w7ere not clear; these data are not reported here. Spectra of the MB-M Complexes.—The spectra of MB at the NaM surface for various coverages u are showm in Fig. 6; this is a reproduction of the Cary chart showing the spectra of the buffered solutions containing MB at constant concentration (9.0 X 10-6 M) with NaM of various concentrations. Large spectral changes occur between coverages of 3 and 75 meq./100 g., w7hich corresponds to the part of the adsorption isotherm that coincides with the ordinate axis (see 75, the spectrum did not change Fig. 5). Above u except for a further decrease in absorbancy at 6700 A., w7hich is the main peak of the bound monomer (to/ 80 was found to be the limit between peak). Since u ion exchange and physical adsorption, this means that the w7hole metachromatic shift occurs in the ion-exchange region of the adsorption isotherm. The spectra of Fig. 6 w7ere measured directly, all MB having been adsorbed by the clay, but for c > 0 the spectra of MB at the NaM surface w7ere obtained from the difference in the spectra before and after the clay w7as separated from the aqueous MB solution by centrifugation. The accuracy of the difference spectra was generally less than that of the directly obtained ones. Kinetics of the MB Binding.—The rate of equilibration of methylene blue on the montmorillonite can be measured by following the spectral changes which occur wdien NaM is added to a dilute suspension of the MB-M complex. In Fig. 7 are showm the spectral changes at the peaks seen in Fig. 6 when additional NaM is added to dilute suspensions, buffered and unbuffered, of the MB-M complex. From the experimental decay of the to' peak, the time constants w7ere found to be 2.4 min. for the buffered system and 54 min. for the unbuffered one. These experiments w7ere con(/ci, k2,
n are
ago by Gurwitsch31 and was
Fig. 5.—Adsorption isotherm of methylene blue on Na montmorillonite at 25°. The points are experimental, the curve is drawn according to the equation u 80 + 48c5·158.
k2cl/n
=
was
more
=
=
=
=
=
6000
5000
7000
(A.).
=
Fig. 6.—Spectra of 8.9 X 10-6 M methylene blue bound to Na montmorillonite at various concentrations. The spectra a, b, c, d, e, f correspond to a coverage u of 9.4, 12.5, 17.4, 26.1, 46.9, and 75.1 meq./100 g. (Reproduction of the chart of the Cary recording spectrophotometer.)
tion of MB in solution. Here w is the weight concentration of the NaM, in g./l., and c' is the molar concentration of MB on the montmorillonite, or the molar concentration of MB removed from solution by the NaM, that is c'
=
c0
—
c
(5)
where co is the initial molar concentration of methylene blue and c is the equilibrium concentration in the solvent. Since u is proportional to the coverage of the NaM particles with MB, we will call it simply the “coverage.” The first part of the isotherm coincides completely with the ordinate axis, thus indicating that all the MB is bound by the clay particles; after the centrifugation, the solution was colorless. In the 120 meq./100 g., some of the neighborhood of u suspensions precipitated. These are marked by triangles in Fig. 5. Whether or not a suspension pre100 depended cipitated in the critical range around u mainly on the MB concentration of the suspension. The higher the concentration, the easier the suspension precipitated. Since a formula weight of NaM, 754 g., ordinarily has a charge of —0.66 mole unit,9 all nega=
=
=
(31) L. Gurwitsch, Z. physik. Chem., 87, 323 (1914). (32) P. H. Plesch and R. H. S. Robertson, Nature, 161, 1020 (1948). (33) X. Freundlich, “Kapillarchemie,” Akademische Verlagsgesellschaft, Leipzig, 1922, p. 273.
Spectroscopic Study
Oct., 1963
of
Methylene
ducted at low u values (compare Fig. 6) where there is no detectable amount of free MB in the solution. Since the equilibrium is achieved so rapidly in the buffered sol, and probably would be even faster at higher u values, a time span of 0.5 to 1 hr. was regarded sufficient to achieve equilibrium before centrifugation or spectrophotometry. Interpretation of the Spectra.—Figure 6 shows an isosbestic point near 6000 A. for curves a to d. This 9 to 26 we have on suggests that within the range u the surface an equilibrium between two distinct forms of the adsorbed MB. At higher values of u the curves do not run exactly through the isosbestic point, but nearly do so. Assuming that the adsorbed MB may be treated, in a first approximation, as a mixture of bound monomer and. n-mer, we may write an expression for the effective molar adsorbancy index, a', of the MB at the surface, similar to eq. 2
Blue
with
Montmorillonite
2175
=
a'
=
a'a'm + (1
—
a')a'n/n
(7)
Here a' is the fraction of MB at the surface present as and a'm and a'n are the molar absorbancy indices of the monomer and -mer, respectively. The molar absorbancy index of the bound -mer was found by subtraction of curve a of Fig. 6 from curve e, according to eq. 7, and is shown in Fig. 8 as a'n/n, the absorbancy index per formula weight of MB. Curve a of Fig. 6 at first appeared to correspond spectrum as the approximately to the bound monomer m\ peak absorbancy was not increased by further decrease of u. It was observed subsequently that the m'i peak decreased at lower u values, and that this decrease had a time lag associated with it. Also, there was a concomitant increase of the peak around 6000 Á. This suggests that there are a few sites at which MB dimers are very strongly bound, and these become important at low coverages. A comparison of Fig. 8 with Fig. 4 shows that the MB monomer peak wave length is changed by the binding, shifted being up by 60 Á. Similar changes have been observed upon binding dyes to nucleic acids34·36 and have been attributed to an electronic interaction between dye and adsorbent. Recent investigations by Bradley and Wolf36 indicate that at very low coverages of nucleic acids by acridine orange, a dye similar to MB, the absorbancy index at the maximum of the bound monomer peak is approximately the same as that of free monomer, though it is less at coverages corresponding to curve a of Fig. 6. Thus, unless the tendency of methylene blue to aggregate on the montmorillonite is considerably less than the tendency for acridine orange to aggregate on nucleic acids, spectrum a of Fig. 6 contains a significant contribution from dimers or other low polymers of bound MB. Therefore the a'm curve of Fig. 8 should not be regarded as an accurate reproduction of the bound monomer spectrum. Determination of a more accurate curve would require elimination of the anomalous fading of the m\ peak at low u values, mentioned above. This uncertainty will not affect appreciably the curve for the bound polymer spectrum also shown in Fig. 8. monomer
(34) F. W. Northland, etal., Exptl. Cell. Res., 7, 201 (1954). (35) L. Michaelis, Cold Spring Harbor Sympos. Quant. Biol., 12, 131 (1947). (36) D. F. Bradley and . K. Wolf, Proc. Natl. Acad. Sci. U.S., 45, 944 (1959).
Fig. 7.—Change of absorbancy with time of a 10-3 A (HAc, NaAc) buffered and an unbuffered methylene blue-Na mont0 the concentration of Na morillonite suspension. At t montmorillonite was suddenly changed so as to decrease the coverage u from 43.1 to 8.0 meq./100 g. in both suspensions. =
3
s
3 —
p1
PEAK
'5790Á a'n a
5000
/
X
5500
6000
X
6500
VV
7000
(A.).
Fig. 8.—Molar absorbancy indices of very dilute (a'm) and aggregated (a'n/n) methylene blue species at the Na montmorillonite surface. The a'm curve probably contains a significant contribution, at lower wave lengths, from bound dimers.
As in the case of the aqueous MB dimer the bound MB polymer exhibits a second absorption peak (p'2 peak) whose wave length is larger than that of the m\ peak. Although the intensity of this peak is very small (amax 0.1 X 104) it was located in separate experiments (not shown) at a tvave length (7620 ± 20) A. A corresponding long wave length peak was first found by Scheibe for the “reversible polymers,” as he calls the aggregates of pseudoisocyanine in aqueous solutions37 and in the adsorbed state.38 With this dye the peak is distinguished by its height and very small band width, and is referred to as a “/-band.” The p\ and p\ peaks of MB polymer correspond to the d, and d2 peak of the =
=
The spectra of dye dimers and higher aggrea qualitative way by Forster.7·39 The positions of the respective absorption bands are compared in Table V. In the second column of Table
dimer.
gates were discussed in
(37) G. Scheibe, Kolloid Z„ 82, 1 (1938). (38) W. Appel and G. Scheibe, Z. Naturforsch., 13b, 359 (1958). (39) T. Forster, “Fluorescenz organischer Verbindungen,” Vandenhoeck und Ruprecht, Gottingen, 1951, pp. 245-260.
K. Bergmann
2176
and
C. T.
O’Konski
then the
mass
Vol. 67
action law may be expressed gn/h
Kn
=
where g and h are the surface concentrations of monomer and ra-mer species, respectively, in milliequivalents of MB per 100 g. of NaM. Taking logarithms, we get n
log
are
monomers
Assuming
log h
-
g
log Kn
=
(8)
in equilibrium with
n-mers
only u
and, since g becomes Fig. 9.—Plot after Scheibe37 to find the size of the methylene blue aggregates at the Na montmorillonite surface.
V the wave lengths of the six observed absorption peaks are given. All primed values refer to bound dye. The weak p\ peak wave length appeared determinable to about 20 Á., though aa/n is very low there. The wave lengths are converted into wave numbers in the third column. In the fourth column the shifts of the d peaks relative to the m peak and of the p' peaks relaTable V Position of Absorption Peaks of Free Adsorbed Methylene Blue Species
Relative Band
max.
A.
Pmax
X 10
di
6050
1.652
\
m
6640
1.507
{
di P' i
6960 5790 6700 7620
1.437 1.728 1.492 1.312
m'i
p'i
X 10-4
4
+ 0.145
|
X 10-
j Í
/
|
and
0.107
-0.070 j
1
{
j
+0.236
)
-0.180
J
0.208
J
tive to the m\ peak are given. It can be seen that the positive shift in both cases exceeds the negative shift. The average shift of the bound n-mer is about equal to twice the average shift of the free dimer (column 5). A result similar to this was found by Simpson40 in a treatment of the electronic spectra of carotenoid pigments. In a first-order perturbation calculation of weakly interacting oscillators, he found the excited electronic state to be split into n levels, n being the number of interacting molecules. Within his approximations, the splitting is symmetric, with a separation =° of extreme levels for which would be double the splitting for a dimer. If the p\peak corresponds to the highest excited electronic state of bound polymer and the p'i peak to the lowest, the extreme mean splittings are in the ratio 0.208/0.107, in good agreement with the theory. The fact that the split spectrum is not centered about the monomer absorption peak may be the result of a shift in either the ground or excited electronic state, or shifts in both, produced by the replacement of some of the surrounding solvent by dye molecules, an effect not included in Simpson’s calculation. =
The size of the adsorbed MB
by Scheibe’s method.87·41 If surface can interact to form a
nMB
n
n-mers
can be
estimated on the
MB molecules
n-mer
—(MB),
(40) W. T. Simpson, J. Am. Chem. Soc., 77, 6164 (1955). (41) V. Zanker, Z. physik. Chem., 199, 225 (1952).
n
log a'u
—
=
=
g
a'u and nh log (1
—
a')u
+ nh u
=
—
(9) g
=
log Kn
=
(1
—
—
log
a')u, eq. 8 n
(10)
It follows that plotting log (1 a’)u vs. log a'u gives a if n is line constant. straight This is done in Fig. 9, using the spectra of Fig. 8. The slope of the curve is approximately 30 at the begin—
ning, increasing to infinity at the end of the curve. This indicates that even at the lowest coverages aggregates tend to form. The analysis is subject to some uncertainty due to the anomalous fading of the m\ peak at low u values. Bradley and Wolf’s study of acridine orange36 showed that the spectrum of the bound to DNA is very similar to free monomer monomer except for a shift to longer wave lengths. If we assume this to be the case for our system, in place of using the curve in Fig. 8, another plot like Fig. 9 is obtained. It shows n starting at 3 for the lowest coverage, curve a of Fig. 6, increasing to about 9 at curve b, and rapidly rising beyond. Thus, the conclusion of high aggregates at low coverages is not sensitive to details of the monomer spectrum. Aggregation of dyes at crystal surfaces has been observed by several authors. Giles and co-workers found evidence for the aggregates of MB at silica42 and graphite48 surfaces. From a comparison of surface determinations of the adsorbent with the area of MB molecules these authors concluded that the MB molecules cannot be oriented in a flat monolayer with molecular planes parallel at high coverages and therefore assumed the presence of MB aggregates at the surface. Giles and co-workers’ proposal42 that the micelles are present on the surface because they are more rapidly adsorbed from the solution appears to be invalidated by our measurements, which show that from essentially monomeric aqueous MB solutions, polymers of MB are formed at the adsorbent surface. From the kinetic studies above, it is clear that equilibrium between various species is achieved within the times ordinarily involved in binding studies. Bradley and Wolf86 have made spectrophotometric studies of acridine orange bound to various polymers. They interpreted their data in terms of a “stacking coefficient,” a measure of the tendency for association of the bound dye. They conclude, “that the dye molecules are not randomly distributed among the available sites but prefer to occupy sites adjacent to each other.” This is analogous to our conclusion for the MB-M system. (42) . M. Allingham, J. M. Cullen, C. H. Giles, S. K. Jain, and J. S. Woods, J. Appl, Chem. (London), 8, 108 (1958). (43) J. W. Galbraith, C. H. Giles, A. G. Halliday, A. S. A. Hassan, D. C. McAllister, N. Macaulay, and N. W. MacMillan, ibid., 8, 416 (1958).
Polarography
Oct., 1963
Studies have been made of the dichroism of the MBM complexes produced by orienting dilute suspensions of the lamellar particles in pulsed electric fields. These have given the information that monomeric and associated bound dye molecules
Ditelluride
of the
theoretical findings on electric dichroism are of general interest in macromolecular structure studies, and a separate article is being prepared.20
Acknowledgments.—Financial support of this
re-
search by a Grant-in-Aid of the Petroleum Research
Fund, administered by the American Chemical Society, We wish to thank Dr. D. F. Bradley and Mrs. N. Stellwagen for helpful comments in reading the manuscript before publication. is gratefully acknowledged.
2177
Appendix A Molar Absorbancy Indices (a X 10-4) of Aqueous Methylene Blue Solutions at 23° and Various (Total) Concentrations
oriented
are
with transition moments approximately parallel to the particle surfaces. The new experimental and
Ion
----------Concentration X
(A.)
5000 5500 5875 6000 6125 6250 6375 6500 6640 6750 6875 7000
9.80
16.5
19.6
31.8
0.35 0.75 2.30 3.45 4.24 4.40 5.18 6.95 8.45 6.50 2.31 0.70
0.31 0.79 2.45 3.72 4.48 4.49 5.22 7.37 8.30 6.31 2.31 0.76
0.34 0.81 2.47 3.72 4.54 4.43 5.21 6.90 8.21 6.06 2.33 0.74
0.31 0.88 2.64 4.09 4.88 4.63 5.09 6.73 7.93 6.01 2.24 0.82
POLAROGRAPHY OF THE DITELLURIDE
of MB 59.3
0.34 0.96 2.87 4.22 4.89 4.38 4.65 5.90 6.91 5.19 1.97 0.77
(c
X 106)· 107.5 143.2
0.40 1.07 3.16 4.49 4.96 4.23 4.27 5.18 6.05 4.79 2.01 0.91
215
0.38 0.36 1.12 1.17 3.27 3.35 4.67 4.93 5.17 5.18 4.27 4.15 4.10 3.77 4.94 4.65 5.67 5.07 4.39 3.91 1.86 1.72 0.84 0.84
ION
By Armand J. Panson Westinghouse Research Laboratories, Pittsburgh 85, Pennsylvania Received March 23, 1963
A polarographic investigation of the ditelluride ion, Tea-2, has been made for concentrations up to 0.25 mJf. Single well defined waves were obtained with anodic and cathodic portions of equal height. The waves were interpreted in terms of a disproportionation of Te2-2 at the mercury electrode as Te2-2 —*Te + Te-2. Analyses of half-wave potentials vs. pH indicate that the potential-determining step is the reaction H2Te -*· Te + 211+ + 2e_. This analysis gives a new value for the second acid dissociation constant of H2Te, 5 X 10_13. The Te + 2H+ + 2e~ was found to be 0.51 v. vs. the standard hydrogen standard potential for the reaction H2Te electrode. From these data F° for the disproportionation reaction Te2~2 3^ Te + Te-2 was calculated to be 5.1 kcal. These values differ markedly from the previous accepted values of K% 10—11, E° 0.72, and AF° 14.0 kcal. =
=
—
=
=
Introduction Recent interest in tellurides arises from their importance as thermoelectric materials. This paper reports results obtained in the course of a study of telluride ion solutions. The purpose of the study was to gain understanding and control of the chemistry involved in preparing telluride compounds from solution by electrolytic methods. In the study, polarography was used to analyze solutions for Te-2 ions. The polarographic waves of the Te2-2 ion were interpreted for the first time and these results are reported here. Apart from the very interesting polarographic information obtained, a key result of this paper is the determination of equilibrium constants which are essential in defining the pH limits for the preparation reactions. Polarographic investigation of the ditelluride ion, Te2~2, has not been reported previously in the literature. Lingane and Niedrach1 found that Te-2 ions give well defined anodic waves stemming from the oxidation of H2Te to Te at the dropping mercury electrode Te + 2H + (d.m.e.) according to the reaction H2Te + 2ew These authors concluded that the Te formed in the reaction is insoluble in the mercury electrode. The current investigation reveals that the Te2~2 ion disproportionates at the microelectrode according to the reaction Te2~2 w- Te + Te-2. Interestingly, the zero-valent tellurium is reduced at the dropping mercury electrode and the Te-2 is oxidized. The polarographic =
(1) J. J. Lingane and L. W. Niedrach, J. Am. Chem. Soc., 70, 4115 (1948).
thus consists of an anodic and a cathodic part of equal heights. The anodic portion of the polarographic wave of Te2-2 thus is identical with the anodic waves obtained by Lingane and Niedrach from Te-2 solutions. wave
Experimental Te2-2 ion solutions were prepared by cathodic dissolution of Te directly in the cell used for polarograph measurements. A Pt anode used for the electrolysis was isolated by means of a KCl-saturated agar salt bridge. Tellurium electrodes, 2.5 cm. long and 5 mm. in diameter, were cast in Vycor tubes under high vacuum. The tellurium used was American Smelting and Refining Co. semiconductor grade (99.999 + %). Solutions were degassed with purified N2 prior to electrolysis. The cathodic dissolution of tellurium was performed at constant currents of 2.5 or 5 ma. (0.6 or 1.2 ma./cm.2) and the concentrations of the solutions were determined from the number of Faradays passed on electrolysis. Gelatin (0.003%) was added to the solutions to suppress polarographic maxima. Polarograms were obtained with a Sargent Model XV polarograph using a dropping mercury microelectrode and saturated calomel reference electrode. In the stud)' of halfwave potential vs. pH, NaOH solutions of 1,0.1, and 0.01 M were used for the pH 14, 13, and 12 experiments. Solutions of 1 M NHjCl with sufficient XH4OH added to adjust the pH were used for the lower pH experiments. The reaction was tested and found to be diffusion controlled by measurements of current vs. height of the mercury column. The current in diffusion controlled reactions is proportional to the square root of the head of mercury while kinetic currents are independent of the mercury head.2 Table I presents the data. (2) P. Delahay, “New Instrumental Methods in Electrochemistry,” Interscience Publishers, Inc., New York, N. Y., 1954, p. 90. (3) I. M. Kolthoff and J. J. Lingane, “Polarography,” Vol. 1, 2nd Ed., Interscience Publishers, Inc., New York, N. Y., 1952, p. 86.
