Electrical and optical properties of some azo dyes

PolymerPhotochemistr~2 (1982) 269-275

ELECTRICAL OF

AND OPTICAL PROPERTIES SOME AZO DYES

A. M. BISHAI,t S. SHAKRA:~and I. K. HAKIM'J" National Research Centre, Dokki, Cairo, Egypt (Received: 1 September, 1981)

ABSTRACT

The dipole moments of six monoazo dyes were obtained from measurements of their static dielectric constants and refractive indices of dilute solutions in benzene and carbon tetrachloride at temperatures between 20°C and 40°C. Also their lighrfastness and absorption spectra ()t,,~) were measured. From the results obtained, it was found that there is a linear relationship between the dipole moment, lighrfastness and maximum absorption (A,,ax) of the monoazo dyes under investigation.

INTRODUCTION

The relation of the dipole moment of a molecule to the arrangement of its positive and negative charges was first considered in the area of Lewis Langmuir theory of atomic structure. A molecule is described as being dipolar when its centres of positive and negative charges are not coincidental. Therefore such a molecule has the ability to orientate in an electrical field. A vertical characteristic of a dipolar molecule is that which has a value equal to the product of the magnitude of each charge and the distance between their centres. This is known as the dipole moment. In this paper we have measured the dielectric constants and optical properties of a series of azo dyes and related these values to the lightfastness properties of the dyes in nylon 6,6. ? A. M. Bishai and I. K. Hakim, Microwave physics laboratory. $ S. Shakra, Textile Research Division. Address for correspondence.

269 Polymer Photochemistry 0144-2880/82/0002-0269/$2-75OApplied England, 1982 Printed in Northern Ireland

Science Publishers Ltd,

270

A. M. BISHAI, S. SHAKRA, I. K. HAKIM EXPERIMENTAL

Preparation of the investigated dyes In the present work, six m o n o a z o dyes were prepared, recrystallised from suitable solvent and their melting points were measured and c o m p a r e d with those given in the literature (Table 1). TABLE 1

Dye No. Diazoniumcompound I

II III IV V VI

o-Methoxy aniline Aniline Aniline Aniline o-Nitroaniline o-Nitroaniline

Coupling compound Phenol Aniline Dimethyl aniline 2-Naphthol Dimethyl aniline 2-Naphthylamine

Literature H 109 H 370 E II 151 H 126 E II 152 H 370

mp (°C) 146 E 124 A 114 A 133 A 127 A 198 E

Where A = alcohol, E = acetic acid, H = Beilstein 16, E II = Beilstein 16 2nd edition.

Application of the dyestuff on nylon 6,6 fabric Dyeing in this work was m a d e on 1 g samples of nylon 6,6 fabric. Dispersion of the dyestuff was p r e p a r e d by grinding 3.3 mg on a metal plate with 2 ml of 0.2% aqueous dispersing agent (Irgasol D a G r a n C I B A - G E I G Y ) . Dyeing was carried out by washing the dispersion into the dyepot and making up the dye bath to 30 ml with distilled water. The previously wetted out fabric was placed in a cold dye bath, and the t e m p e r a t u r e raised to the boil in 20 min. Dyeing was continued for 1 h with the necessary agitation and the fabric was then rinsed, washed and air dried. No finish was u s e d )

Lightfastness measurements T h e dyed fabrics were exposed to daylight (in the A r a b Republic of Egypt) and the lighffastness was measured in accordance with 'The Standard Methods for Determination of Colour Fastness of Textiles'. 2

Measurement of the ultra-violet and visible spectrum A P M Q I I specrophotometer, Carl Zeiss, was used. T h e dyes were dissolved in absolute alcohol to give a concentration of 10 -4 mol/litre. A 1 cm cell was used for the measurements.

Recording of the IR spectrum A n U R 10 infra-red spectrophotometer, Carl Zeiss Jena, was used. The spectra

E L E C T R I C A L A N D O P T I C A L P R O P E R T I E S O F S O M E A Z O DYES

271

of the azo dyes were recorded in the solid state as alkali halide discs (each compound was ground with KBr). Dielectric constant measurements The dielectric constants eo of very dilute solutions of the investigated dyes in non-polar solvents were measured at a frequency 70 kHz using a dekameter of Schering bridge type W T W DK05 to an accuracy of 1%. The refractive index n D for each solution was measured by an Abbe refractometer Carl Zeiss, Jena, Model G to an accuracy of ---1 × 10 -4.

RESULTS A N D DISCUSSION

The measured values of the dielectric constants e0 and squares of the refractive indices An 2 for the six monoazo dyes in benzene or carbon tetrachloride solutions are given in Table 2 at different temperatures together with the dispersion step S/X, dipole m o m e n t / x , lightfastness, maximum absorption Am~, and the wave number VN=N. The dispersion step S I X = A e 0 - n E / x (where X is the concentration in mol % and A means the difference between solution and solvent) is obtained and used to calculate the dipole moment tz using the equation: 2 27 K T M S / X I~ = 4 TNd (es + 2) 2

(1)

where K is Boltzman's constant, T is the absolute temperature, N is Avogadro's number, M, d and es are molecular weight, density and dielectric constant of solvent. The dekameter was calibrated using known chlorobenzene solution in benzene at 20°C and the temperature was kept constant by an ultrathermostat. The dipole m o m e n t for chlorobenzene is given in Table 2 and it agrees well with that found in the literature. 3'4 The dielectric constant eo and refractive index n D were measured for four concentrations ranging from 0.046 to 0.4 mol % and at temperatures 20°C, 30°C and 40°C for each molecule. Figure 1 shows the relation between the dispersion step S I X versus concentration for the six dyes at different temperatures. By extrapolation, the value of S I X at infinite dilution is obtained and used to calculate the dipole m o m e n t of a single molecule. For the molecules 4-aminoazobenzene and 4-dimethyl aminoazobenzene, there is no concentration dependance of S I X as shown in Fig. 1 (b, c). But for the other four molecules, there is a concentration dependence of S I X at different temperatures as shown in Fig. 1 (a, d, e and f). Making use of eqn. (1) the dipole moment /x was calculated and is given in Table 2.

272

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A. M. BISHAI, S. SHAKRA, I. K. HAKIM

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The dipole moment of 4-aminoazobenzene was found experimentally to be 2.59 D, while the literature value for this molecule is reported to be 2.71 D. 5 The literature value was obtained by an optical method. For 1-benzene-azo-2naphthol, the value of the dipole moment in benzene solution was found to be 1.63 D. This value is lower than that in the literature 5 for the same molecule in dioxane solution. It was found that the dipole moment in d i o x a n e 6'7 solution is higher than that in benzene solution and this was supposed to be due to the formation of hydrogen bonds. Figure 2 gives an interesting linear relationship between the measured value of the dipole moment for the monoazo dyes under investigation and their lightfastness. It is clear that as the dipole moment increases, the lightfastness decreases. The increase in the dipole moment may be due to an increase in the charges on the ends of the molecule or the distance between the two charges. It is clear from the molecular structures of the dyes under test, that they are of a nearly equal molecular length. This implies that the increase in dipole moment is due to an increase in the molecular charge. Then a molecule with increased charges is active for photooxidation or reduction and this in some way induces the fading of the molecule to a greater extent than those dyes having lower molecular charges, i.e. an increase in the molecular charges increases the dipole moment on one hand and decreases the lightfastness on the other. Debye -18

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ELECTRICAL AND OPTICAL PROPERTIES OF SOME AZO DYES

275

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F o r discussing t h e results in Figs. 3 a n d 4, it has b e e n f o u n d s'9 t h a t t h e lighffastness of azo d y e s d e c r e a s e s as t h e i r m a x i m u m a b s o r p t i o n Xmax increases. A s f o u n d above, t h e lightfastness d e c r e a s e s with an i n c r e a s e in the d i p o l e m o m e n t . F r o m t h e s e two r e l a t i o n s , it can b e said t h a t t h e d i p o l e m o m e n t i n c r e a s e s as t h e m a x i m u m a b s o r p t i o n )'m~ of the a z o g r o u p i n c r e a s e s which is c l e a r f r o m Fig. 3. T h e s a m e can b e said to a c c o u n t f o r t h e results in Fig. 4. It has b e e n m e n t i o n e d in e a r l i e r w o r k 9 t h a t t h e lighffastness i n c r e a s e s as the w a v e n u m b e r v~=N increases. F r o m Fig. 2 it is c l e a r t h a t t h e lighffastness d e c r e a s e s as t h e d i p o l e m o m e n t increases. F r o m t h e s e two r e l a t i o n s , it w o u l d a p p e a r t h a t t h e d i p o l e m o m e n t d e c r e a s e s as the w a v e n u m b e r VN~N increases, This is c l e a r l y d e m o n s t r a t e d b y t h e results s h o w n in Fig. 4.

REFERENCES 1. SOOETYof DYERS and COLOUmSTS,Recent advances in the colouring of man made fibres, 19 Piccadilly, Bradford, Yorkshire, 1957. 2. SOCIETYof DYERS and COLOUVaSTS,The standard methods for determination of colour fastness of textiles, 19 Piccadilly, Bradford, Yorkshire, 1962. 3. HANNA,F. F. and AJ3DELNOUR, K. N., Proc. Math. Phys. Soc., UAR, 32 (1968) 1. 4. HANNA,F. F. and BISnAL A. M., Z Phys. Chemie, Leipzig, 257 (1976) 6, 1241. 5. LANDOLTBORNSTEIN, Tabellonwerk 1,3, Berlin, 1951. 6. Paw, A. V. and SMITH, J. W., J. Chem. Soc. (1949) 753, 2663. 7. CUmO,AN, C. and ESTAK, G. K., 3". Am. Chem. Soc., "/2 (1950) 4575. 8. SHAKaA,S., M.Sc. Thesis, Cairo University, 1969. 9. SHAKY, A, S., Thesis for Candidate, Moscow, USSR, 1975.