FASTNESS TO LIGHT OF MONOAZO DYES
363
REFERENCES
(1) ALEXANDER, J . : Colloid Chemistry, p. 305. The Chemical Catalog Company, Inc., New York (1931). (2) BLADE,E. : Ind. Eng. Chem., Anal. Ed. 11, 499 (1939). (3) CULBERTSON, J. L., AKD DUNBAR, A.: J. Am. Chem. SOC.69,306 (1937). J. L., AKD WEBER,M. K.: J. Am. Chem. SOC.60,2695 (1938). (4) CULBERTSON, (5) HEISIG,G. B., AND CAMERON, A. E.: Ind. Eng. Chem., Anal. Ed. 6,420 (1933). J., AND ADAMS,L. H.: J. Am. Chem. SOC.34,563, 554 (1912). (6) JOHNSTON, (7) LAMB,A. B., AKD LEE, R. E.: J. Am. Chem. SOC.36,1666 (1913). (8) REILLY,J., AND RAE,W. N . : Physico-Chemical Methods, 3rd edition, Val. I. D. Van Nostrand Company, New Y o l k (1939). (9) Reference 8, p. 59. (IO) SOSMAN, R. B.: The Properties of Silica, American Chemical Society Monograph, Chapter XVIII. The Chemical Catalog Company, Inc., New York (1927). (11) Reference 10, p. 291. (12) Reference 10, pp. 294-5. (13) Reference 10, p. 298. R. W . : J. Chem. SOC.101,2429 (1912). (14) WADE,J., AND MERRIMAN,
A CORRELATION OF STRUCTURE WITH FASTNESS TO LIGHT OF MOKOAZO DYES1 R. H. KIENLE, E. I. STEARNS,
AND
P. A. VAN DER MEULEIL’
Department of Chemistry, Rutgers University, New Brunswick, New Jersey Received March 6, 1.946 INTRODUCTION
It is impossible for the chemist to synthesize all possible azo dyes and test them for fastness to light, because it has been estimated that there are over 6,000,000 possible molecular structures with the intermediates in commercial use. Accordingly, this work was undertaken with the idea of determining some relationships between constitution and light fastness and thereby contributing to a method of selection of likely possibilities for improved azo dyes. According to the method of Hammett (1) for studying the effect of structure on reactivity, when the side-chain reactions of meta- and para-substituted benzene derivatives are studied, it is found that a simple and quantitative relationship appears when two series of rate constants are compared. The ionization constants of the substituted benzoic acids are recommended as a standard series of reference. It is thus possible to calculate a react,ion constant which is characteristic of the reaction of the unsubstituted compound and also substituent constants which are independent of the reaction. 1 This article is based on a thesis submitted by E. I. Stearns to the Graduate Faculty of Rutgers University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, July, 1945.
364
R. H. KIENLE, E. I. STEARNS AND P. A. VAN DER MEULEN
The fading of monoazo dyes by light may be analyzed by the method of Hammett. It is necessarily the destruction of the azo group by the reaction of light that results in the loss af color characteristic of the fading reaction. It is to be expected that this benzene side-chain reaction will be influenced by substituent groups in the same way as the many other reactions that have been studied. This means that once the reaction constant is found, the light fastness of any related dye differing in substituent only may be calculated provided the substituent constant is available in the literature. To evaluate fading three types of samples were studied. Water solutions were faded because it was believed that these would involve fewer complicating side reactions. Swatches of wool cloth dyed with the dyes were faded, because there is a great commercial interest in these dyes applied to wool. Films of gelatin were faded, because they combined the simplicity of analysis by light transmission and the chemical characteristics of wool. For all samples dye concentrations were determined spectrophotometrically and reaction rates were calculated. Unexpectedly, it was found that the reaction rate of the fading of water solutions was too random to permit calculation of reliable reaction constants. The fading of the dyed gelatin and dyed wool was reproducible and the reaction constant of each was calculated. I n addition to the meta and para derivatives, the ortho derivatives were studied. Ortho derivatives are not susceptible of analysis by the method of Hammett . EXPERIMENTAL PROCEDURE
Description of dyes The parent compound of the series of twenty-five dyes studied was aniline coupled t o R salt, which has the following structure:
Eight substituent groups were introduced into the benzene ring of this parent compound in all three positions. The dyes2 which were studied are listed in table 1.
Preparation of samples for fading The dyed wool samples corresponded to what the textile trade would call a 1 per cent dyeing. The dyeing method was as follows: A solution was prepared which consisted of 0.050 g. of dye and 0.10 g. of sulfuric acid in 200 cc. of distilled water. A 5-g. piece of wool serge cloth of medivm weight mas wet out by im2 The authors are indebted t o the Calco Chemical Division, American Cyanamid Company, for furnishing the dyes together with chemical analyses for their purity.
365
FASTNESS TO LIGHT OF MONOAZO DYES
mersing in boiling water and then cooled down but not dried. The solution of dye and the wet wool wese placed together in a porcelain beaker a t a temperature of 25"C., and the beaker placed in a bath of calcium chloride solution a t 105°C. During a 5-10 min. interval the dye solution was raised to the boiling temperature TABLE 1 Purity of dyes CODE
CROUP
POSITION
PURITY'
MOLAR EXTINCTIONt
2 3
OH OH OH
Ortho Meta Para
88.5 28.2 82.9
19,100 7 ,160 15,200
4 5 6
OCHI OCHI OCH,
Ortho Meta Para
97.2 95 99.5
13,500 19,200 17,500
7 8 9
CGH5
CBH~ CeHs
Ortho Meta Para
99.4 97.2 97.3
18,600 20,600 26 ,500
12
C Ha C H3 CHI
Ortho Meta Para
100.6 97.7 77
19,600 20,300 14,700
13 14 15
c1 c1 c1
Ortho Meta Para
99.1 98.0 100. 1
18,200 18,800 18,800
16 17 18
COOH COOH
Ortho Meta Para
100
97.4 99.6
19,800 17,320 21,000
19 20 21
SO,H SOsH
Ortho Meta Para
99.2 86 95
16,100 16,500 20,800
22 23 24
K0 2
Ortho Meta Para
78.5 95.0 100.3
18,700 18,700 27 ,000
25
H
99.2
19,600
1
10 11
COOH
NO2 pi0
2
* Purity was determined by the titanium trichloride reduction of the azo group. t Molar extinction coefficients in square centimeters per mole a t the absorption maximum for water solutions at p H 5.0.
and kept boiling for a period of 1 hr., with occasional additions of distilled water to maintain the volume between 180 and 200 cc. approximately. During the dyeing process the cloth was agitated frequently by brief removal with a glass rod and by stirring. This agitation was sufficient to produce a uniform distribu-
366
R. H. KIENLE, E. I. STEARNS AND P. A. V.4N DER MEULEN
tion of dye throughout the cloth but insufficient to cause felting of the wool. The pieces of moo1 cloth were then removed, washed in cold water, tacked on a wooden board under slight tension, and air dried at 105°C. Measurement of the dye solution before and after the dyeing showed that in every case a t least 97 per cent of the dye was exhausted from the bath into the wool. The dyed gelatin film samples were acid dyeings at a concentration' such that the transmission of the dyed film to light of the wave length of the absorption maximum was between 20 and 30 per cent. By developing and fixing unexposed photographic negatives a layer of clear gelatin 23 microns thick on a cellulose acetate backing sheet was obtained. Solutions of the dyes in 0.3 per cent acetic acid were prepared and the gelatin films dipped into the solutions at 25°C. The films were removed and the transmission determined. If the measured transmission was between 20 and 30 per cent, the filmwas rinsed in mater at 25°C. and air dried at 25°C. In no case was the concentration of the dye bath substantially changed by the removal of the dye in the gelatin. The mater solutions were at a pH of 5.0 and of such a concentration that the transmission of a 1-cm. thickness to light of the wave length corresponding to the absorption maximum was between 14 and 22 per cent. The dyes were dissolved in distilled water by boiling the solution for 1 min., buffered t o a pH of 5.0 with a potassium phthalate buffer, and always measured a t 25°C. for transmission. None of the dyes were phototropic. Reproducibility of 1 per cent (twice the standard deviation) in the preparation of duplicate solutions, and ageing stability such that less than 4 per cent apparent concentration change was observed over a period of a month indicated that satisfactory solution conditions had been found and that therefore observed changes were caused by fading and not by unintentional solution variables.
Fading of samples The fading was carried out in the Fade-Ometer, an instrument manufactured by the Atlas Electric Devices Co., Chicago, Illinois. It has been described in the literature (3) and the energy distribution of its radiation, which resembles that of sunlight, has been measured (7). The dyed wool samples mere clamped in metal frames provided with positioning pins., The metal frames facilitated the handling involved in the alternate exposing in the Fade-Ometer and measuring in the spectrophotometer and insured that the cloth was held flat at all times. The positioning pins were fitted to corresponding holes in the spectrophotometer and insured that exactly the same area of cloth was measured before and after each fading exposure. This procedure eliminat,eserrors of measurement which might arise from slight irregularities in distribution of dye throughout the cloth. The dyed gelatin film samples were stapled to cardboard frames in which a hole had been cut. The cardboard frames facilitated mounting the samples in the Fade-Ometer for exposure and in the spectrophotometer for measurement. The holes permitted the films to be measured as transmission samples. This method
FASTNESS TO LIGHT OF MONOAZO DYES
367
of mounting led to exposure primarily by radiation passing through the film in one direction only, although a small amount of back reflection from the metal sides of the Fade-Ometer would contribute some exposure by light passing in the reverse direction. The water solutions of dye were exposed and Zeasured in glass absorption cells 1 cm. thick. The water solutions were covered with a layer of toluene in which some mineral oil mas dissolved, and the water boiled for a minute to expel oxygen. The solutions were then transferred to the exposure cells with a pipet in such a manner that there was always a layer of toluene solution preventing contact with air. A toluene solution layer was maintained over the mater solutions at all times during the exposure and measurement. While this technique was designed to eliminate oxygen from the water solution, it is probable that it did not accomplish its purpose.
Measurement of samples The samples were measured in a General Electric automatically recording spectrophotometer (2). The gelatin films and water solutions were measured as transmitting samples and the wool cloth measured as reflecting samples in the conventional manner. The transmission of the gelatin films was measured relative to an unexposed and undyed gelatin film as the reference standard, the water solution transmissions were measured relative to a similar cell filled with water, and the wool cloth reflectances were measured relative to magnesium oxide as a reference standard. I n this way data were obtained for per cent of incident light transmitted or per cent of incident light reflected after the various times of exposure. CALCULATIONS
Calculation of dge concentrations from optical data In all cases the Concentration of the dye in the sample before exposure in the Fade-Ometer was taken as 100. In the cases of the gelatin films and the water solutions the relative concentrations after various times of exposure were calculated on the basis of Beer’s law, using the wave length of the absorption maximum. In the case of the wool samples the relative concentration was calculated on the basis of the Kubelka and Munk theory (4),assuming a constant scattering coefficient, as is suggested by Pineo ( 6 ) ,and using the wave length of the absorption maximum. I n the case of solutions these calculations lead to a straightforward dye concentration. I n the case of gelatin films these calculations lead to an over-all dye concentration but the dye may not be uniformly distributed throughout the film, since the front surface layer may be faded more than the back surface layer and the unfaded dye may not distribute itself uniformly in the film. In the case of the wool cloth the interpretation of the calculation is more difficult. If the exposed side of the wool cloth after fading appears to be as darkly colored as a wool cloth dyed a t one-half the concentration would appear, then the concentration of
368
E. H. RIENLE, E. I. STEARNS AND P. A. VAN DER MEULEN
dye in the faded wool cloth is found to be 50 per cent by the method of calculation used. Since the wool cloth is opaque, the unexposed side will appear unchanged. Hence the calculated concentration is definitely not the average concentration throughout the wool. However, the average concentration throughout the wool is not the desired concentratiok anyway, because the back side of the cloth was not exposed to the Fade-Ometer radiation. The method of calculation of the wool data is justified by the following l i e of reasoning: For the spectrophotometric measurement of initial color, only a certain portion of the dyed wool participates in the partial absorption of light and hence is involved in an estimate of dye concentration. It is assumed that this same portion of dyed wool partially absorbs the Fade-Ometer radiation. Then the same portion of dyed wool participates in the partial absorption of light by which the estimate of the faded concentration is made. On the basis of this assumption it may be said that the dye content of a certain volume of wool is measured, faded, and remeasured. The extent of this certain portion of wool is unknown, but it is believed that since irradiation by light was used both to evaluate the concentration and to produce the change in concentration, the calculated concentrations have a usable significance in studying the fading reaction. The calculations can be thrown off by two factors: namely, failure of Beer’s law (or the Pineo theory), and the formation of decomposition products having absorption at the wave length of calculation. For reasons which are discussed immediately below, it is believed that neither of these two factors introduces an error except in the case of the fading of the wool samples of dyes No. 23 and No. 24. Beer’s law was checked in water solution and found to hold over the concentration range of 2 to 1 within 1 per cent for all the dyes. This check involved two calculations: first, that the relative calculated concentration a t one wave length (the absorption maximum) was correct; second, that the relative calculated concentration at all wave lengths was the same. The concentration law was not checked directly for gelatin or wool. For the faded water solutions identical values of percentage of dye destroyed were obtained at all wave lengths, indicating that the products of decomposition of the fading process did not absorb appreciably within the range of wave lengths included in the spectrophotometric measurements. Incidentally, this identity of results a t all wave lengths is an indirect check of Beer’s law, for usually a Beer’s law failure of dyes exhibits different calculated results a t different wave lengths (5). For the faded gelatin films essentially identical results were obtained a t all wave lengths; slight differences apparent in regions of high transmissions where the calculations are not accurate anyhow were presumably due to the fact that unfaded gelatin rather than faded gelatin was used as the reference standard in measurements. The agreement at all wave lengths showed that no decomposition products with interfering absorption were formed and was also an indirect indication of the validity of Beer’s law. For the faded wool samples essentially identical results were obtained a t all wave lengths for all samples except No. 23
FASTNESS TO LIGHT OF MONOAZO DYES
369
and No. 24. For all samples but these twcj exceptions, this showed that no interfering decomposition products were formed and was an indirect indication of the validity of the Pineo theory. Some slight discrepancies a t wave lengths of high reflectance were attributed to the fact that the unfaded and undyed wool was considered to be contributing constant absorption to all the samples in accordance with the conventional interpretation of Pineo’s theory. In the case of dyes No. 23 and No. 24 the samples tended to change from red to violet visually as they faded, suggesting the presence of absorbing decomposition products. Calculations of concentration were made a t the absorption maximum of the unexposed dye, a practice which tended to reduce the error due to the decomposition products, but these data are put in parentheses in the tables to indicate their inferior precision. 0.2
-
0.1
-
0
0.5
FIG.1. Plot of the data for wool. Ordinate, logarithm of the ratio of the reaction rate of the substituted dye t o that of the unsubstituted dye; abscissa, logarithm of the ratio of the ionization constant of the substituted benzoic acid t o that of the unsubstituted benzoic acid.
Calculation of reaction rates from dye concentrations Because the solution data showed such a deviation from both a first- and a second-order reaction, no attempt was made to calculate reaction rates. The reproducibility of the solution fading was so poor that even if rates had been calculated, a reaction constant based on them would be grossly inaccurate. In order to reduce the fading data of gelatin and wool dyeings to a quantitative basis, it was assumed that the fading was a first-order reaction. Plots of logarithm of concentration versus fading time were not quite straight lines. The
370
R. H. KIENLE, E. I. STEARNS AND P. A. VAN DER MEULEN
rates were calculated as the average rate over the time interval zero to 80 hr. exposure. The specific reaction rates were calculated in the usual manner and expressed in units of reciprocal seconds.
Calculation of reaction constants In accordance with the recommended procedure of Hammett, the wool data are plotted in figure 1and the gelatin data in figure 2. Least-square straight lines
0.2
-
0.1
-
0 -
I
0
0.5
FIG.2. Plot of the data for gelatin. Ordinate, logarithm of the ratio of the reaction rate of the substituted dye to that of the unsubstituted dye; abscissa, logarithm of the ratio of the ionization constant of the substituted benzoic acid to that of the unsubstituted benzoic acid.
were calculated by minimizing the sum of the squares of the ordinate deviations. The slopes of these straight lines are the reaction constants. In calculating the reaction constants the data for dyes No. 23 and No. 24 on wool were omitted, because the reaction Tates mere questionable. The data for dye No. 18 on wool and on gelatin were omitted; Hammett did not believe that his ionization constants applied to this type of compound. Datum 6 on wool was omitted, because it fell so far from the others that t.here is probably some different type of reaction mechanism occurring in this case.
37 1
FASTNESS TO LIGHT OF MONOAZO DYES
TABLE 2 Fading results REACTION R A T E / ~ O ~ (RECIPBOCAL SECONDS)
PEB CENT CONCENTRATION LOSS IN 80 EX.
Gelatin
Wool
Gelatin
55 58 53 50 45
63 57 52 56 61
2.78 3.02 2.63 2.41 2.08
3.45 2.94 2.54 2.85 3.25
22 23 53 25 100
55 54 57 48 49
65 65 62 69 62
2.78 2.71 2.94 2.28 2.35
3.45 3.35 3.35 4.05 3.35
34 88
45 46 87 54 52
62 62 93 72 70
2.09 2.15 7.10 2.70 2.55
3.35 3.35 9.50 4.42 4.19
1.M)
Water
Wool
100 64 94 20 44 6 7 8 9 10 11 12 13 14 15
loo* 100 loo*
16 17 18 19 20
loo* 30 57 100 37
25 47 50 36 45
43 62 70 62 66
2.21 2.41 1.56 2.09
1.92 3.36 4.25 3.36 3.75
21 22 23 24 25
loo* 26 34 29 100
37 74 (50) (58) 45
73 90 78 84 65
1.61 4.70 (2.41) (3.02) 2.09
4.54 8.00 5.25 5.96 3.62
0.152
0.376
Standard deviation..
..
40
2.1
3.5
* These samples faded t o 100 per cent destruction in less than 80 br. TABLE 3 Statistical data of figures 1 and B WOOL
Least-square line: Zero intercept., , . , . , , . . . . . . . . . . . . . . , Slope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , Standard deviations: Points from line.. . . . . . . . . . . . . . . . . . . ,
I
$0.024 +O. 182 0.042
GELATIN
-0.010 $0.217
I
0.061
372
R. H. KIENLE, E. I. STEARNS AND P. A. VAN DER MEULEN
RESULTS
Fading results Table 2 presents the per cent concentration loss during an 80-hr. exposure and the specific reaction rates in reciprocal seconds. The reproducibility of each set of fading data as determined by replicate fadings is expressed as standard deviation. Reaction constants Table 3 presents the statistical data of figures 1 and 2. DISCUSSION OF RESULTS
The fact that both slopes are positive means that the reaction rate is increased, and hence the fastness to light decreased, by substituent groups which are metadirecting inpecondary benzene substitution, and that the reaction rate is lowered by ortho-para-directing substituents. In addition, the fact that both slopes are less than unity means that fading is less affected by the nature of the substituent group than the ionization of benzoic acid is affected by the substituent group. .The position ortho to the azo group is the most sensitive to the effect of a substituent. The smallest reaction rate (fast to light) was found for the o-carboxy dye. SUMMARY
Twenty-five monoazo dyes were used to study the relation of fastness to light to structure. These comprise aniline coupled to R salt and the derivatives of aniline containing the substituents OH, OCH3,CaH6,CHs, C1, COOH, S03H, and NOz in the ortho-, meta-, and para-positions. Dyes of a high degree of purity were used and were faded in water solution, on wool, and in gelatin. The measurements of the relative dye concentrations were made with a spectrophotometer. Solution and gelatin concentrations were calculated on the basis of Beer’s law, and apparent concentrations for wool on the basis of the Pineo relation. The conditions of the measurement of light absorption were chosen so that the standard deviations in per cent concentration were 1 per cent for the solution preparation, 2.1 per cent for wool fading of 80 hr., and 3.5 per cent for gelatin fading of 80 hr. This indicates that ample consideration was given to the factors that affect the measurements. The fading was carried out in a Fade-Ometer and the fading results expressed in terms of destruction of dye on a concentration basis. It was found that the solution fading was random, but the wool and gelatin fading quite reproducible. The fading of dyes in solution was very slow for a period of time, after which the fading was very rapid. This suggested an induction period followed by a different reaction mechanism and made i t ,impossible to calculate reaction rates. Graphical representations of results of fading wool and gelatin dyeings give curves which fall between the curves for a first- and a second-order reaction. On the
VISCOSITY AND TEMPERATURE RELATIONSHIP OF LIQUIDS
373
assumption that the fading was a first-order reaction, the reaction rates were calculated for wool and gelatin. The position ortho to the azo group is the most sensitive to the effect of a substituent. The smallest reaction rate (fast to light) was found for the o-carboxy dye. Large reaction rates (fugitive) were found for the o-chloro and o-nitro dyes. The reaction rates of the meta- and para-substituted dyes were analyzed in accordance with the method of H a m i e t t and were found to follow the usual linear relationship with the substituent constants. The reaction constant (Hammett’s) was found to be 0.18 for dyes on wool and 0.22 for dyes in gelatin. Values less than unity, such as these, indicate that the substituent is a less important factor in fading than is the substituent in the ionization of derivatives of benzoic acid. REFERENCES
(1) HAMMETT, L. P . : Physical Organic Chemistry, pp. 184-207. McGraw-Hill Book Company, Inc., New York (1940). (2) HARDY,A. C.: J. Optical SOC.Am. 28,360 (1938). (3) HARVEY, E. H.: Am.Dyestuff Reptr. 31, 13 (1942). P.,AND MUNK,F.: Z. tech. Physik 12,593 (1931). (4) KUBELKA, R. H . : Ind. Eng. Chem., Anal. Ed.13, 692 (1941). (5) MULLER, (6)PINEO,0. W.: U. S.patent 2,218,357(1940). (7) WALKER, P.H . , AND HICKSON, E. F.: Bur. Standards J. Research 1, 1 (1928).
ON T H E APPLICATION AND DERIVATION OF T H E NEW VISCOSITYTEMPERATURE RELATIONSHIP OF LIQUIDS M. S. TELANG Laxminarayan Institute of Technology, Nagpur University, Nagpur, India Received February 7, 1946
A new correlation of viscosities of liquids with temperatures has been recently proposed by the author (71). It has been shown that the proposed equation is closely obeyed by seven normal organic liquids over wide ranges of temperatures and is applicable to two associated liquids over a limited range of temperatures, not far below the boiling points. It is felt necessary to investigate whether this remarkably good application of the equation to a few normal organic liquids is merely accidental or otherwise, and whether the equation has any theoretical basis. Further, if the equation is not applicable to associated liquids, what might the reasons be? In trying to discover the causes of the failure of the equation, we may be able to get an insight into the true picture of the mechanism of the viscosity of pure liquids. The fact that the equation breaks down when applied to a certain class of liquids is regrettable from the experimental point of view, as we are unable to calculate viscosity values a t any desired temperatures JIiithout recourse to experimental determination. But from the theoretical
