T H E EFFECT OF SOLVENTS ON T H E ABSORPTION SPECTRUM OF A SIMPLE AZO DYE* BY WALLACE R. BRODE
Before starting an extended study of the absorption spectra of a number of mon-azo dyes, a series of experiments was made to determine the effect of various solvents on the absorption spectrum of benzeneazophenol and precautions necessary in the determination of the absorption spectra of dyes of this type. The literature contains considerable theorizing and some practical data on the question of the effect of the solvent on the absorption band of dyes.l Kundt2 said that the absorption band of dye is shifted towards the red end of the spectrum, Le., to lower frequencies, with an increase in the refractive index of the solvent. This rather broad statement has since been known as Kundt’s law. However, there have been nearly as many data reported in contradiction of it as in support of it. It is certain that the refractive index of a compound is a function of its molecular constitution, and that in homologous series, the molecular refraction always increases with an increase in molecular weight, But it is not true that two isomeric substances such as methyl formate and acetic acid, or ethyl alcohol and di-methyl ether, have t’he same refractive index. Conversely, substances with the same refractive index do not necessarily have the same molecular constitution, and it follows that a dye dissolved in such solvents should not necessarily give the same molecular vibration. For example, benzyl alcohol and tetralin (tetrahydronaphthalene) have about the same refractive index, (1,540) which is rather high for organic solvents, Benzeneaeophenol gives in benzyl alcohol a frequency of 845 f , I 5 lower than in ethyl alcohol, while in tetralin the band is a t a frequency of 875 f , some 15 higher than in ethyl alcohol, giving in all a difference of 30 in the center of the absorption bands in the two solvents used for the dye. I n general, little work has been done on the effect of the solvent on the absorption bands of azo dyes, and most of what has been done has been confined to the two solvents, water and alcohol. Only a few of the simple azo dyes which were to be studied in the course of this work, are soluble in water, hence it was necessary to use some organic solvent. The dye selected for the determination of the effect of the solvent on the absorption band, was benzeneazophenol. This dye was carefully prepared from purified intermediates, recrystalized repeatedly from alcohol, and dried in a vacuum oven at 60’
* This communication is an abstract of a portion of a thesis presented for the degree of Doctor of Philosophy in Chemistry at the University of Illinois. Published by permission of the Director of the Bureau of Standards of the U. S. Deparhment of Commerce. Presented before the Physical and Inorganic divisions of the American Chemical Society a t Baltimore Md. April 8, 1924. 1 Kavser: “Handbuch der Smktroscooie”., 3., 80-89 , (IQOCI). . 2Pogg. Ann. Jubelband, 615 (1874); Sitzungsber. Bayr. Akad. 7, 234 (1877); Ann. Physik, (3) 4, 34 (1878). ~
- I
EFFECT O F SOLVENTS ON ABSORPTION SPECTRUM
57
under reduced pressure. The dye gave a melting point of 152' corr. and analyzed 1007~ pure by titanous chloride titration. I n the neutral solution of benzeneazophenol in absolute ethyl alcohol there appears a very weak band near frequency 700, Fig. I., so weak that it can scarcely be called a regular band, although the effect has been observed by a number of observers. This is followed by the principal band at a frequency 860 f , following which comes a region of fairly high transmission and then a third band at a frequency of 1 2 8 0 f . The last or third band is at about the limit of the spectrophotometric measurements obtainable in the
FIG.I Absorption Spectra of Benzeneazophenol in Alcohol.
ultra-violet. It will be noticed that the third band is an exact multiple of the second band, the third band being 3 / 2 of the second band, cr the bands correspond to the second and third members of a series of whici the first would be 430 f or the fundamental frequency and the difference beiween each band being 430 f on the frequency scale. I n an aqueous alkaline solution there appear bands at 710 and 770 f and a third weaker one at I 15of, which is again 3/2 of the second band; this time, however, the first band is slightly greater in intensity than the second band. In alkaline alcoholic solution there appear bands a t 660 and 73 5 f and a weaker band at I I 2 5 f which is : / 2 of the second band. This time the first band is somewhat weaker than the second band. In concentrated HC1 a third set of bands occurs with frequencies of 630, 940 and 1 2 0 0 f.
58
WALLACE R. BRODE
We are primarily interested in the neutral series of bands, however, and will limit this discussion to that set of bands. The first band, if it may be called such, Fig. I , is very much weaker than any of the others, so much so that its height and center of vibration can not be accurately determined in solutions where the other bands are a,t a height that can be accurately measured. We will therefore disregard the first band and confine our attention to the second or principal absorption band of the dye (860 f.), One of the principal reasons for choosing this particular dye was the fact that its main absorption band, although so far out in the ultra-violet as to prohibit the use of some solvents, owing to their absorption, is very sharp and apparently consists of only one band, in contrast to the apparent double band formation present in the more complex azo dyes. No conclusions can be drawn as to the effect of solvents on the third band owing to the position it occupies in the extreme ultra-violet and the fact that nearly all the solvents used absorbed in that region so as to make observation impossible.
Method The Hilger sector photometer method with quartz spectograph was used in the determination of the ultra-violet spectral transmission of these solutions. The mdiant energy source was an arc between tungsten electrodes under distilled water, which gave a continuous spectrum from the visible throughout the ultra-violet as far as the plates recorded, A high voltage high frequenc: current was used for this arc, the source of it being a large Tesla coil. T h apparatus and the method of manipulation are described in detail in Scientific Paper 440 of the Bureau of Standards. The extinctbn coefficient1which is plotted as the logarithm of the transmittancy, is shcwn on the accompanying graphs as a negative value and as such is plotted bwnwards on the ordinate. As a result the “peak” or point of maximum absorption of the band is really the lowest point rather than the highest. I n this paper this point of maximum absorption will be referred to as Dhe peak. Tie transmittancy is defined as the ratio of the energy passing through the cell containing the solvent and the dye, to that which passes through the cell containing only the solvent, (I/I’). The transmirtancy is thus given as
T = iobCi‘ or
- log T I/bc
=
k,
where b is the thickness in cm., c the concentration in cg. per liter, and k the extinction 2oefficient. I n this paper the cell thickness was always I cm., while the conceriration was maintained a t 1.44 cg. per liter, and the extinction coefficient Efers to the absorption of a solution containing 1.44 cg. per liter rather than I. cg. per liter. Bureau of Staidards Scientific Paper, KO. 440; J. Opt. SOC.America, 10, 169 (1925).
EFFECT O F SOLVENTS ON ABSORPTION SPECTRUM
59
The frequencies are plotted as true frequencies which is equivalent to 105 times the reciprocal of the wave length, rather than 105 times the reciprocal of the wave length as has often been done. The unit of frequency is defined as vibrations divided by (seconds X 1 0 ~ ~=) f , the fresnel. The exposure ranged in general from about three minutes at. the maximum to I O seconds a t 100% transmittancy. Each exposure resulted in two spectra, parallel and in contact with each other, the lower one representing the energy passing through the solvent containing the dye, this beam of energy being unimpeded by any absorbing agent other than the dye in the solvent. The upper band represents the energy passing through the clear solvent. In the path of this latter beam there is a rotating sector which may be set at various 3 X
2.001-
lo F n f l h
C Y
1000
V/ORAT/O,YS i[SECONDS
I
1200
/0’2)
1400
FIG.2 Benzeneazophenol in ,4bsolute Alcohol. Cone. = 1.44 cg per liter, thickness 5 I cm.
transmittancies. These transmittancy values are so chosen as to be rquivalent to units of .05 on the log of the transmittancy scale, Le., actual trarsmittances such as 100, 89.4, 79.5, 71.0, 63.1, 56.4, etc. which are equivdent to log transmittancy values of .oo, .05, .IO, .15, .zo, . 2 5 , etc. This rrquired from 35 to 40 sets of exposures for each solvent. These values were wecked again a t the maximum and a t 100% transmission by some IO moreexposures on a newly made solvent solution so as t o verify the previous neasurements. In Fig. 2 are given curves of benzeneazophenol by various oservers which have been recalculated on the basis of percentage of thicknes of a solution of constant transmission and to true frequencies of vibration Solvents and Preparation of Solutions The solvents which are listed below were used for thee observations. All but carbon disulfide permitted the determination of the &sorption maxi-
60
WALLACE R. BRODE
2 a
9
r
E F F E C T O F SOLVENTS ON ABSORPTION SPECTRUM
61
mum of the dye. The solvents used were thought to be of high purity, with the possible exceptions of butyl alcohol and butyl acetate, which showed indications of impurities in their ultra-violet spectrum. A number of the solvents used were redistilled in the laboratory. It is entirely possible that the carbon disulfide used although of c.p. grade contained a small amount of free sulfur, which would have increased the absorption of the solution. The black portion in Fig. 3 indicates the region where total absorption takes place as determined through a I centimeter cell on exposure of not over
ETHYL ALCOHOL METHYL ALCOHOL ETH€R GLYC€ROL
CHLOROFORM /soPROPYL ALCOHOL
AC&T/C AC/O
AM/L A C f TAT€
FORMlG /IC/# f THYL A C€TA Tf
ETHYL FORMAT€ cAR/30N 7€TRACHf OR/D€ fTHYL PROP/ONATS BUTYL ACfTATf 5€/VZ€N€ TULU f Nf KYL EN€ L/GRO/N PfTROLATffM f TH YZ BENZOATE ?? BUTYL ALCOHOL ETHYL OXALATE GASOL/NE A M Y L ALCOHOL ETHYL M€THrZ KETONE T€TRAL/N ACETONf PYR/D/NE 5ENZYL AL COMU CAR/3nn/ /3/s//L/ 7 ” ~
400
WAF€ L f N G T H
300
225
my- MEiiEi$?!x 10-9
FIG.3 Absorption of solvents used for I em. thickness. Slope of start of band indicates whether the absorption is sharp or gradual. 3 minutes nor less than 2 minutes. The slope of the start of the absorption strip indicates whether the absorption commences abruptly or gradually, as determined by a series of exposures varying from I O seconds to more than two minutes. The accuracy of the determination of the limit of transmission of these solvents can better be determined by direct examination of the plates (not reproduced in this paper). The esters were the only solvents which showed satellite bands. In all probability the other solvents would have shown bands if the cell thickness had been reduced, as it requires a relatively weak band to exhibit anything but total absorption in a thickness of I cm. The graph of the absorption limits of these solvents gives a series of solvents with gradual absorptions from 13 5 0 to 800 f and in practically all cases, except where indicated, the absorption is very sharp so that they may be used conveniently as radiant energy filters. The limit of the spectrograph used was about frequency 1360 t o 1380 so that values in the neighborhood of this limit do not have as high an accuracy as those of lower frequencies.
62
.WALLACE R. BRODE
By experiment it was found that a concentration of 1.44centigrams per liter would give a curve with a maximum extinction coefficient of about 1.80, which was a convenient value to work with. The solutions used were made up in two ways, first, by dissolving goo g. of the dye in 500 cc, pipetting off 40 cc of this and making it up to zoo, and pipetting off 40 cc of this solution and making it up to zoo cc. This procedure was followed when the dye dissolved with difficulty or the solvent was easily obtainable in large quantities. The second method was to dissolve .1440g. in IOO cc of solvent, pipette off
FIG.4 Absorption Spectra of Benxeneasophend in Pyridine latum (3),Ethy! Acetate (4), Benzene (5).
(I),
Methyl Alcohol (z),Petro-
I O cc of this and make up to IOO cc and pipette off I O cc of this solution and make it up to I O O cc. The solutions were made up at 20' and samples of 6he clear solvent removed a t the same time. I n the case of glycerol, considerable error was possible, owing to the viscosity of the solvent, so that it was necessary to wash the pipette out several times with the diluting solvent so as to insure as complete a draining of the pipette as possible. In the case of the other solutions the pipettes were simply allowed to drain, The dye dissolved with difficulty in the petroleum solvents and glycerol, it dissolved quite readily in the other solvents. The temperature of the cells when the observations were made was between 21' and 23'.
Results The results obtained are best illustrated graphically. I n Fig. 4 are given curves for the dye in methyl alcohol, pyridine, benzene, petrolatum and ether. In the following table are given values for the extinction coefficient and frequency at the peak, for all the solvents used.
63
EFFECT O F SOLVENTS ON ABSORPTION SPECTRUM
TABLE I1 Frequency and height of the absorption band of benzeneazophenol in the solvents used. Solvent I 2
3
4 5 6
7 8 9 IO
I1 I2
I3
I4 15
16 I7
18 I9 20 21
22
23
24 25 26 27
28 29 30
Methyl Alcohol Ethyl Alcohol is0 Propyl Alcohol n Butyl Alcohol act. Amyl Alcohol Benzyl Alcohol Ethyl Acetate Butyl Acetate Amyl Acetate Ethyl Formate Ethyl Propionate Ethyl Oxalate Ethyl Benzoate Ethyl Ether Acetane Ethyl Methyl Ketone Formic Acid Acetic Acid Chloroform Carbon Tetrachloride Carbon Disulfide Glycerol Ligroin Gasoline Petrolatum Benzene Toluene Xylene Tetralin Pyridine
*See Fig. 5 .
Extinction Coefficient I
.90
.90 1.88 I
1.91 I
.92
I
.90
1.98 1.90 1.95 I .98
1.86 1.89 I .84 1.95 2.00
1.90 I . 15"
1.85 1.85 1.80
1.60 I .92 1.92
1.84 1.80 I . 70
1.74 1.74 1.64
64
WALLACE R. BRODE
From the above data it can be seen that as far as the height of the band is concerned, with a few exceptions, the band is lower in non-polar solvents. The measurements on the height of the bAnd, however, are not as accurate as the frequency measurements and allowance must be made for instrumental errors involved in the photographing and in the reading of the plates after photographing. This is in part due to the fact that the band is flat for a very short distance at the maximum, or rather gives the appearance of such, and in such cases the greater accuracy is obtained by changing the transmission
FIG.j Absorption Spectra of Benxeneazophenol in 85% Formic Acid Note acid band which is produced by formic acid.
(I)
and Glacial Acetic
(2).
valves gradually, a process which is impossible photographically. The error involved in the determination of the frequency does not amount to more than f 2 in most of the cases observed except where the absorption of the solvent was near enough t o the peak of the band to decrease the intensity. The general shape of the band is the same for all of the solvents, indicating a general shift of the entire band rather than the increasing of certain frequency amplitudes within the band structure with the decreasing of others, as it is highly improbable that if the latter were the case the change would be exactly analogous in all cases so as to give the same shaped band but with a different vibrational center. From Fig. 6 it is evident that there is very little if any relation between the refractive indices of the various solvents used and the frequency of the absorption maximum, although in the case of certain homologous series such as the alcohols there seems to be a general shift with an increasing molecular weight. It is also to be noticed that similar solvents in general group together
E F F E C T O F SOLVENTS ON ABSORPTION SPECTRUM
65
although the order in which they arrange themselves is not consistent with the increase of their refractive indices. Although there are a large number of exceptions, in fact too many to permit such a conclusion, it appears that the average shift is towards higher frequencies with increasing refractive indices, an effect exactly opposite to Kundt's iaw. In Fig. 6 are compared the refractive indices and frequencies of the peak of the band in different solvents which are numbered as in Table I and 11. In like manner the effect) of the dielectric constant which is also considered a
40
1.70
7
1
Relation between the refractive index at 20' and the absorption frequencies. The numbers refer to the solvent numbers in Table 11.
at
Relation between dielectric constants and absorption frequencies.
20'
molecular property of the solvent, appears to have very little effect, Fig. 7, but here again, it may be pointed out that in homologous series there may be some relation. This relation in regard to homologous series may and probably is due to some other effect than a change in the dielectric constant or refractive index, although this effect and the force which changes the dielectric constant and refractive index are probably fundamentally related. As would be expected, similar substances such as xylene and tetrahydronaphthalene (tetralin) give similar frequencies of vibration and also closely related dielectric constants and refractive indices. By plotting the frequency values of the absorption band of the dye in some of the various solvents against their refractive indices or dielectric constants
66
WALLACE R. BRODE
figured over to the theoretical value at their critical temperatures a somewhat better agreement is obtained and some of the discrepancies which existed before, as in the case of the saturated hydrocarbons and the aromatic hydrocarbons are corrected. There are still $00 many exceptions and deviations to draw any conclusions, Figs. 8-9. The values have also been plotted for the dielectric constants a t their boiling points with about the same agreement, or disagreement, depending upon the angle from which it is viewed, Fig. IO. The objection may be raised, and it is quite legitimate, that the observations
-’-I 2.2
1.8
j
1.6 -%
1.4-$ 3
5 & 5
1.2 - 2
? 1.1 -
1
840
I
I
014
fR€qu€/vcY,
860
j
380
FIG.8 Relation between refractive indices at the critical temperature and absorption frequencies of the solvent.
1 .o-
840
860
880
9 Relation between the dielectric constants at the critical temperature and the absorption frequencies. FIG.
were not made at these temperatures and that comparison is being made between two unrelated sets of figures. It may be pointed out, however, that the shift of the absorption band with temperature is slight, especially with azo dyes1whereas the changes in the refractive indices and dielectric constants with change in temperature are large and the values widely varied, but that a t the critical temperature or boiling point they fall into a more regular series with a more uniform change.
Mixed Solvents I n determining the effect of a solvent on the absorption maximum of a dye it was quite essential to determine the effect of mixed solvents, or the effect of an impurity on the shift in the absorption band. I n these experiments, solutions of benzene and alcohol, carbon tetrachloride and alcohol, ligroin and alcohol, water and alcoho1,andligroin and benzenewere used. The first two 1
Bureau of Standards Sei. Papers, No. 440; Kayser; “Handbuch der Spektroscopie”,
3,92.
EFFECT O F SOLVENTS ON ABSORPTION SPECTRUM
04
/f
0-0-
'6 Q
-P
12
-c
' '4-0860 I,
/O'
P /
67
$
O
870
3 880
' $
I cc24
fa0 % I
890
AL C0HO.L
of these were of special interest as data were available on the dielectric constants and refractive indices of these mixtures. From these data a careful check could be obtained on the relation between the absorption shift and the refractive indices and dielectric constants. The results are indicated in Figs. 11-13. The effect, although one which might be predicted, is far from being a function of either of these two physical properties. As small an amount of alcohol as 1% was sufficient to shift the band to the alcohol band in both benzene and carbon tetrachloride. The shift is quite apparent and would
840
8 90 8 60 870
880
8 90
FIG.12 Effect oqmixtures of alcohol and benzene on the dielectric constant and absorption frequency.2 Dotted line =frequency. King and Patrick: J. Am. Chem. SOC.41, 1143(1921).
68
WALLACE R. BRODE
undoubtedly have taken place for much smaller concentrations of alcohol, the 1% solution being the lowest percentage tried. It is apparent that the more polar the solvent the greater tendency it has to shift the band to its own vibratory frequency giving the effect of the dye being entirely dissolved in the solvent, and this solution being suspended in the other solvent. In the case of ligroin and benzene the effect appeared to be a gradual change as compared with the sharp change due to alcohol. In both the benzene and the carbon tetrachloride mixtures the band seemed to shift to a point somewhat beyond the ordinary frequency of alcohol (855) and then on increasing the percentage of alcohol it gradually returned to 860. The same effect was noticed on di-
FIG.13 Effect of mixtures of alcohol and benzene on the refractive index and absorption frequency. Dotted line =frequency.
lution of the alcohol solution with water. The band was gradually reduced in frequency with an increase in the water percentage to about the same values, indicating possibly that the effect is merely one of dilution. The dye being insoluble in water, there is no shifted band in this case, the main band being simply reduced in intensity. This reduction does not take place until the percentage of alcohol becomes very small. This effect accounts to some extent for some of the wide variations in the shifting of the band, as in the case of the chloroform which was known to contain a small amount of alcohol and in which the band was not shifted t o anywhere near the extent to which it was shifted in carbon tetrachloride.
Summary I. The absorption limits of some 30 organic solvents have been determined and their region of total absorption has been observed throughout the ultra-violet to a frequency of 1360 for a thickness of I cm. 2. The absorption spect,rum of benzeneazophenol has been observed in these solvents.
EFFECT O F SOLVENTS ON ABSORPTION SPECTRUM
69
a. From these data it appears that for this simple azo dye, and in all probability for other simple azo dyes, Kundt’s law does not hold. b. There appears to be no definite relation between the refractive indices or the dielectric constants of these solutions and the frequency of the absorption band of the dye dissolved in them. 3. I n mixtures the dye appears to give the absorption frequency for the most polar solvent, even if this solvent is present in a very low percentage and there is a slight dilution effect in such cases. 4. The height of the band appears to be greater for polar solvents than for non-polar solvents.
