a physicochemical study on organic eutectics and addition compound; benzidine–pyrogallol system

A physicochemical study on organic eutectics and addition compound; benzidine-pyrogallol system

U. S . RAI AND SANTHI GEORGE Chemistry Department, Banaras Hindu University, Varanasi - 221 005, U.P. lndia

Received July 8, 1991'

U. S. RAI and SANTHI GEORGE. Can J. Chem. 70, 2869 (1992). The phase diagrams of the binary organic system of benzidine-pyrogallol was determined by the thaw-melt method.

The solidification behaviour of the pure components, their eutectics, and the addition compound were studied by measuring the movement of growth front in a capillary. From the data on X-ray diffraction, thermal and microscopic investigations it can be inferred that the eutectics are not simple mechanical mixtures of the components involved. The IR and NMR spectral investigations were canied out to throw light on the nature of bonding between the two components forming the addition compound.

U. S . RAI et SANTHI GEORGE. Can. J. Chem. 70, 2869 (1992). Faisant appel a la mCthode de fusion, on a determine les diagrammes de phase du systkme organique binaire benzi-

dine/pyrogallol. En mesurant le mouvement de croissance du front dans la capillaire, on a CtudiC le comportement lors de leur solidification des composants purs, de leurs eutectiques et de leur composC d'addition. En se basant sur des Ctudes thermiques, microscopiques et de diffraction des rayons X, on peut dCduire que les eutectiques ne sont pas de simples melanges des composants impliquCs. Des etudes par spectroscopies IR et RMN ont CtC entreprises afin de faire la lu- mikre sur la nature des liaisons entre les deux composants formant le composC d'addition.

[Traduit par la rCdaction]

1. Introduction The past decade has witnessed (1-6) an immense activity

both in the fundamental understanding of the solidification and properties of polyphase alloys and in the technological developments of in situ composites for particular applications. Owing to the low transformation temperature, ease in purification, transparency, minimised convection effects and wider choice of materials, the organic systems (7-12) are being used as model systems for a detailed investigation of the parameters which control solidification, which in turn, govern the properties of materials. As the organic eutectics and the molecular complexes are analogues of metal eutectics and intermetallic compounds, respectively, a system- atic physicochemical study of a model system involving organic compounds may be of potential importance in un- ravelling the mysteries of solidification, bonding, and microstructure, among others. In view of this, a binary organic system involving benzidine (BZ) and pyrogallol (PG) has been chosen to study its phase diagram, linear velocity of crystallization, thermochemistry, microstructure, X-ray diffraction, and spectral behaviour.

2. Experimental

2.1 Materials and purification AR grade benzidine (CDH, India) was directly used in the pres-

ent investigation. Pyrogallol (S.d. Fine-Chem Pvt Ltd., India) was purified by repeated distillation under reduced pressure and was stored in a coloured bottle to avoid exposure to light. The melting point of each compound (benzidine, 127°C and pyrogallol, 134OC) b a s compared wiih its literature value (benzidine, 1 2 8 " ~ and py- roeallol. 134°C) to assess its ~ u r i t v .

in ice. Their thaw and melting temperatures were determined using a Toshniwal melting point apparatus equipped with a precision thermometer which could read correctly up to "-0.5°C.

2.3 Linear velocity of crystallizatiorl The linear velocity of crystallization data for the pure compo-

nents, the eutectics, and the addition compound were determined (15, 16) at different undercooling temperatures by measuring the movement of the solid-liquid interface in the capillary tube of 15 cm length and 0.5 crn inner diameter.

2.4 Heat of fusion Heats of fusion of the pure components, the eutectics, and the

addition compound were determined (17) by using a Dupont-9900 thermal analysis DSC apparatus.

2.5 Microstrricture Microstructures of the eutectics and the addition compound were

recorded (18, 19) by placing the slide containing unidirectionally solidified sample on the platform of a Leitz Labourlux D optical microscope attached with a camera.

2.6 X-ray diffraction X-ray diffraction patterns of the pure components, the eutec-

tics, and the addition compound were recorded (20 ,2 1) on a com- puterized X-ray diffraction unit, PW 1710 model, using CuK, radiation.

2.7 Spectral studies Infrared spectra of the pure components, the eutectics, and the

addition compound were recorded (22) in the region 4000-625 cm-' in Nujol mull using a Perkin-Elmer 783 infrared spectrometer. CDCl, was used as a solvent for recording the PMR spectra on Jeol FX 90 Q Fourier Transform NMR spectrometer.

3. Results and discussion u . ,

2.2 Phase diagram 3.1 Phase diagram

The phase diagram of the benzidine-pyrogallol system was de- The sO1id-liquid data on the BZ-PG 'ystem termined by the thaw-melt method (13, 14). In this method, mix- are given in Fig. 1 in the form of a temperature-composi- tures of two components covering the entire range of composition tion curve. 'The plot shows the formation of a 1 : 1 addition were prepared in different long-necked test tubes. These mixtures compound with congruent melting point surrounded by two were homogenized by melting in silicone oil followed by chilling eutectics E, and E,, containing 0.902 and 0.139 mole frac-

tions of benzidine, respectively. The melting point of pure 'Revision received July 28, 1992. benzidine is 127.O"C and it decreases continuously with the

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2870 CAN. J . CHEM. VOL. 70. 1992

I I I I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Mole fraction of pyrogallol

FIG. 1. Phase diagram of the benzidine-pyrogallol system. 0, Melting temperature; a, thaw temperature.

addition of pyrogallol and attains a minimum at El (1 18.0°C). With continued addition of pyrogallol the melting point rises to a maximum of 145.0°C at C after which it decreases to a minimum of 120.5"C at EZ. At point C the solid and the corresponding liquid have identical composition. This phase diagram may be viewed as consisting of two simple eutectic type phase diagrams placed side-by-side. The flatness in the maximum at C suggests that the addition compound is dis- sociated (23, 24) in the molten state.

3.2 Linear velociv of crystallization According to Hillig and Turnbull (25), the linear velocity

of crystallization (v) of each of the pure components, the eutectics, and the addition compound is related to the undercooling ( T ) by the equation

where u and n are constants. The constants u and n are calculated from the linear plots (Fig. 2) of log v versus log AT and the values are reported in Table 1. It is evident from the table that most of the n values are close to 2, thereby sug- gesting a square relationship between v and AT. However, the deviations in the values of n from 2, observed in some cases, may be due to the difference in the bath temperature and the temperature. of,the growing interface. In the present investigation, both components have a high enthalpy of fusion, consequently each crystallization step results in a re- lease of heat causing the interface to attain a higher temperature than the bath.

From the values of u (Table 1) for the pure components and the addition compound, it can be inferred that the crystallization rate of the addition compound of the BZ-PG system is less than that of the parent components. Studies on the crystal morphology of the addition compounds indicate that they crystallize as a definite chemical entity. However, during crystallization, the two components of the melt have to enter the crystal lattice simultaneously in such a way that the composition of the melt conforms to the respective molar ratios of the components. Thus the linear velocity of crystallization of the addition compound may be expected to be of the order of the growth velocity of the species crystalliz-

log At ('C) - 1.0 1.2 l.L 1.6 1.8

/ I / I I I / I I

/ m FyrcgalWnzidine Addillon compound

FIG. 2. Linear velocity of crystallization of the benzidine-pyrogallol system.

TABLE 1 . Values of u and 11 of benzidine-pyrogallol system

U

Material (mm s-' deg-I) n

Benzidine 0.000105 4.00 Pyrogallol 0.007244 2.50 Eutectic- 1 0.000724 1.86 Eutectic-2 0.006918 1.50 1 : 1 Addition compound 0.000005 3.33

ing with the lower rate. The value of u for E, (formed between benzidine and the addition compound) is higher than that of either component while for E, (formed between the addition compound and pyrogallol) it lies between the corresponding components. These results may be explained on the basis of the mechanism proposed by Winegard et al. (26) and substantiated by others. Accordingly, the solidification of both the eutectics of BZ-PG system takes place by the side-by-side growth of the two components. In the eutectics under investigation the addition compound behaves as one of the components, and its melting point is higher than those of the components it nucleates first.

3.3 Thermochemical studies One may get an idea on the mode of crystallization,

structure of eutectic melt, and the nature of interaction between two components forming the eutectics and the addition compound from their heats of fusion. For the purpose of comparison, the experimental and calculated values of heat of fusion for the eutectics are reported in Table 2. It is evident from the data in the table that the calculated (27) values of the heat of fusion are higher than the experimental

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RAI AND GEORGE

TABLE 2. Heat of fusion and heat of mixing of benzidine-pyrogallol system

Heat of Entropy of Roughness Heat of fusion fusion parameter mixing

Material (kJ mol-') (kJ mol-' K-I) Asp /R (kJ mol-I)

Benzidine Pyrogallol Eutectic- 1

(exptl.) (calcd.)

Eutectic-2 (exptl.) (calcd.)

1 : 1 Addition compound (exptl.) (calcd.)

values. If an eutectic is a simple mechanical mixture of two TABLE 3. Excess thermodynamic functions for benzidine-pyro- components involving no heat of mixing or any type of as- gall01 system sociation in the melt, the heat of fusion will be simply given by the mixture law (19). However, when a solid eutectic gE hE sE melts there is considerable likelihood of association and Material (J mol-I) (J mol-I) (J mol-I K-I)

mixing, both causing violation of the mixture law. The dif- ~ ~ ~ ~ ~ ~ i ~ - ~ 580.02 1201.28 1.62 ference between the experimental and calculated values can Eutectic-2 712.32 4383.36 9.34 be attributed to the formation of clusters in the eutectic melt. In the present eutectic system where one component has an -NH, group and the other contains an -OH group, both the end members have a tendency for hydrogen bond formation, resulting in a favourable condition for cluster (28) formation in the melt. The values of heat of mixing (29) (AH,), which is the difference between the experimental and calculated values of the heat of fusion, were calculated for both eutectics and the data are reported in Table 2. It is clear that the values are highly negative. According to thermochemical studies (13) the structure of the eutectic melt depends on the sign and magnitude of the enthalpy of mixing. Three types of structures are suggested; quasieutectic AH, > 0, clustering of molecules for AH,, < 0, and molecular solution for AH, = 0. The negative values of AH, for the eutectics of the BZ-PG system suggest clustering of molecules in the eutectic melt. To see a small value of heat of mixing in the case of EZ of the system, one might be tempted to consider the formation (30) of a molecular solution in- stead of a weak interaction. It may be noted that this system is quite different from simple eutectic systems where merely ordering of the parent phases has been suggested in the melts. It seems that there is considerable enhancement in the inter- actions due to the presence of molecular complex in the melt.

The experimental values of the heat of fusion of the addition compound determined by the DSC method are reported in Table 2. Its theoretical value was calculated by a method reported earlier (28). It is evident from Table 2 that the heat of mixing is highly negative. This suggests (30) that the presence of an addition compound enhances the attrac- tion among the components. The association is also fa- voured by the presence of hydroxyl and amino groups in the components.

The deviation from ideal behaviour can best be expressed in terms of excess thermodynamic functions which give more quantitative ideas about the nature of the molecular inter- actions. It is defined as the difference between the thermodynamic function of mixing for a real system and the

corresponding value for an ideal system at the same temperature and pressure. To understand the nature of interaction between two components forming the eutectics, some thermodynamic functions such as excess free energy (gE), excess enthalpy (hE) and excess entropy (sE) were calculated by a method reported earlier (27). The positive gE values (Table 3) suggest (31) that the interaction between like molecules is stronger than that between unlike molecules. The values of hE and sE correspond to the excess free energy and are a measure of the excess enthalpy of mixing and excess entropy of mixing, respectively.

3.4 Microstructure It is well known that in a polyphase material the micro-

structure provides data on the size, shape and orientation of the grains in addition to the distribution of the phases involved. The arrangement of grains and phases in a material controls its mechanical properties and decides its various applications. The shape (32) that a crystal adopts in the melt subsequent to nucleation is controlled by the way in which atoms molecules are added onto the solid, which in turn is determined by the atomic/molecular structure of the solid- liquid interface. However, the interface structure depends upon the chemical character of the material in question and also upon the thermal environments in which the crystal is growing. For a material with entropy of fusion less than 2R (where R is the gas constant), the interface which is about 50% populated is atomically rough and results in non-faceted growth. On the other hand, for a material with entropy of fusion greater than 2R, the interface is perfect with a few atoms missing from it, and results in faceted growth. In the present investigation, the pure components have their entropy of fusion greater than 2R, thereby resulting in faceted growth. The morphology of eutectics developed in each case is influenced by the entropy of fusion of the phases involved. The microstructures of the eutectics under investi-

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FIG. 3. Microstructure of the benzidine-pyrogallol eutectic (El) x 100.

FIG. 4. Microstructure of the benzidine-pyrogallol eutectic (E?) x 100.

gation are given in Figs. 3 and 4. While the eutectics E, of the BZ-PG system given in Fig. 3 shows lamellar growth, the second eutectic E, (Fig. 4) of this system indicates that one of the phases nucleates first and the other phase radiates outward perpendicularly. The microstructure of the addition compound (Fig. 5) shows faceted growth like pure components.

3.5 X-ray diffraction A critical scanning of the current literature (19) reveals that

there are two conflicting ideas about the nature and bonding in eutectics. While one group of workers believes that a binary eutectic is a mixture of two kinds of crystals, favour- ably oriented with respect to each other, the other group is of the opinion that the eutectic grains do not exhibit a fixed orientational relationship. To clarify this, the X-ray diffraction patterns of the pure components, the eutectics, and the addition compounds were recorded and the results are reported in Tables 4 and 5. It is evident from the reported data that the number of reflections of pure components and the addition compounds is comparable with the number of reflections of their corresponding eutectics. It can be inferred from this observation that they belong to the same crystal system and have similar lattices. It is evident from the re-

FIG. 5 . Microstructure of the benzidine-pyrogallol addition compound (A) x 100.

sults reported in Table 4 that for the BZ-PG system strong reflections of benzidine and the addition compound are either absent in the eutectic E l or show a variation of intensity. Accordingly, reflections of benzidine and d values 4.82, 4.45, 3.12 A show an increase in intensity and those at 4.23, 3.83, and 3.35 A show a decrease in intensity in the eutectic E l . Similarly, reflections of the addition compoupd at d values 5.25, 4.47, 4.35, 3.94, 3.84, 3.30, and 3.14 A show a decrease in intensity and those at d values 4.58, 3.53, and 3.10 A show an increase in intensity in the eutectic El .

The X-ray patterns of the two components should be ex- actly superimposed on the eutectic composite if a eutectic is a simple mechanical mixture of two components. From the diffraction data on the pure components, the eutectics, and the addition compound, it can be inferred that there is a marked difference in the interplanar distance and the relative intensity. The variation in relative intensity of the reflections of pure components in the eutectics and the absence of reflections of pure components in eutectics and those of eutectics in pure components suggest that the eutectics are not simply a mechanical mixture of two components. There is orientation of some atomic planes during the formation of the eutectics.

3.6 Spectral studies The IR spectrum of benzidine in Nujol shows three bands

at (i) 3190, (ii) 3320, and (iii) 3400 cm-I due to NH stretching vibrations. The IR spectrum of the pyrogallol in Nujol exhibits four bands at (i) 3280, (ii) 3250, (iii) 3380, and (iv) 3480 cm-' due to OH stretching vibration in the compound. The addition compound gives four peaks: (i) 3300, (ii) 3355, (iii) 3380, and (iv) 3485 cm-I. This may be attributed to the merging of a peak of benzidine at 3190 cm-' and that of pyrogallol at 3280 cm-I resulting a new peak at 3300 cm-I in the addition compound because of intermolecular interaction between the two components. However, no definite conclusion can be drawn from the IR data. The proton NMR spectrum of benzidine shows a peak at 6 3.52 due to NH proton, and the ring protons are obtained as mul- tiplets in the range 6 6.48 - 6 7.37. Pyrogallol gives a proton signal at 6 1.6. The molecular complex of the BZ-PG system gives a broad band in the range 6 1.58 - 6 1.14 and shows the disappearance of the OH proton signal which may

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TABLE 4. d values and relative intensity (Rl) of benzidine, eutectic-1 and the addition compound of the benzidine-parogallol sys-

tem

I : 1 Addition Benzidine Eutectic- 1 compound

be due to a slow exchange (33) between the OH and NH protons. This upfield shift of NH proton signal may be due to intermolecular hydrogen bonding between N of NH, and H of OH of pyrogallol.

Acknowledgement

We thank Professor K. N. Mehrotra, Head, Chemistry Department, Banaras Hindu University, for providing re- search facilities.

TABLE 5. d values and relative intensity (Rl) of benzidine, eutectic-2, and the addition compound of the benzidine-pyrogallol sys-

tem

I : 1 Addition Pyrogal lo1 Eutectic-2 compound

d (A) R1 d (A) R 1 d (A) R1

- - 17.20 5 - -

12.30 100 - - - A

- - 10.70 100 - - - - - - 8.99 8 8.70 30 - - - -

- - 8.42 9 - - - - - 8.00 8 6.80 10 - - 6.85 10 - - - - 6.57 12 6.10 60 - - - -

- - 5.99 19 - - - - - - 5.85 15 - - - - 5.70 9 - - 5.57 11 - - - - 5.57 1 1 A

- 5.50 50 5.50 29 - - - - 5.34 44 5.34 18 - - - - 5.25 24 - - - - 5.01 23 - - - - 4.90 43 - - - - 4.84 100 - - 4.60 7 4.58 45 - - 4.5 1 9 4.53 5 1 - - - - 4.47 54 - - 4.40 14 4.35 57 4.08 20 4.10 7 4.11 73 - - - - 3.98 24 3.91 20 - - 3.94 37 - - - - 3.84 3 1 - - - - 3.76 22 3.67 30 3.65 26 - - - - - - 3.60 14 3.52 100 3.56 7 3.53 17 - - - - 3.39 37 3.35 20 3.36 14 3.35 39 3.23 75 3.27 27 3.30 20 - - 3.21 9 3.20 2 1 - - 3.17 12 3.14 3 2 - - - - 3.10 24 - - - - 3.06 3 1 3.01 50 - 3.03 25 -

- - - - 2.98 17 - - - - 2.92 10 2.85 40 - - 2.84 9

1. R. Elliott. Eutectic solidification processing. Buttenvorths, London. 1983.

2. M. M. Schwartz. Composite materials hand book. McGraw Hill Book Company, New York. 1984.

3. K. A. Jackson. Mater. Sci. Eng. 65, 7 (1984). 4. C. Schafer, M. H. Johnson, and R. A. Parr. Acta Metall. 31,

1221 (1983). 5. R. Elliott. Cast iron. Butterworths, London. 1988. 6. W. Kurz and R. Trivedi. Proc. Third Int. Conf. on Solidifi-

cation Processing, Sheffield. 1987. p. 1. 7. N. B. Singh and K. D. Dwivedi. J. Sci. Ind. Res. 41, 98

(1982). 8. R. P. Rastogi, D. P. Singh, N. Singh, and N. B. Singh. Mol.

Cryst. Liq. Cryst. 73, 7 (1981).

Can

. J. C

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11/1

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onal

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2874 CAN. J . CHEM. VOL. 70. 1992

K. Pigon and A. Krajewska. Thermochim. Acta, 58, 299 (1982). M. E. Glicksman, N. B. Singh, and M. Chopra. Manuf. Space, 11, 207 (1983). N. B. Singh, N. N. Singh, and R. K. Laidlaw. J. Solid State Chem. 71, 530 (1987). H . Song and A. Hellawell. Metall. Trans. 20A, 171 (1989). U . S. Rai, K. D. Mandal, and N. P. Singh. J . Thermal Anal. 35, 1687 (1989). B. M. Shukla, N. P. Singh, and N. B. Singh. Mol. Cryst. Liq. Cryst. 104, 265 (1984). N. B. Singh and N. B. Singh. Krist. Tech. 13, 1175 (1978). U. S. Rai and K. D. Mandal. Bull. Chem. Soc. Jpn. 63, 1496 (1990). U . S. Rai, 0. P. Singh, and N. B. Singh. Can. J. Chem. 65, 2639 (1987). R. P. Rastogi and V. K. Rastogi. J . Cryst. Growth, 5 , 345 (1969). U. S. Rai and K. D. Mandal. Mol. Cryst. Liq. Cryst. Liq. Cryst. 182, 387 (1990). U. S. Rai and K. D. Mandal. Thermochim. Acta, 138, 219 (1989).

N. P. Singh and B. M. Shukla. Cryst. Res. Technol. 20, 345 (1985). U . S. Rai and K. D. Mandal. Curr. Sci. 58, 784 (1989). R. P. Rastogi. J. Chem. Educ. 41, 443 (1964). M. Radomska and R. Radomslu. 'Thermochim. Acta, 40, 415 (1980). W. B. Hillig and D. Turnbull. J . Chem. Phys. 24, 914 (1954). W. C. Winegard, S. Mojka, B. M. Thall, and B. Chalmers. Can. J . Chem. 29, 320 (1957). U . S. Rai, 0. P. Singh, N. P. Singh, and N. B. Singh. Ther- mochim. Acta, 71, 373 (1983). R. P. Rastogi, N. B. Singh, and K. D. Dwivedi. Ber. Bunsen- ges. Phys. Chem. 85, 85 (1981). U . S. Rai and K. D. Mandal. Mat. Sci. Forum, 50, 117 (1989). N. P. Singh, B. M. Shukla, N. Singh, and N. B. Singh. J. Chem. Eng. Data, 30, 49 (1985). J. Wisniak and A. Tamir. Mixing and excess thermodynamic properties. Elsevier, New York. 1978. G. A. Chadwick. Metallography of phase transformations. Butterworths and Co. Publishers Ltd., London. 1972. R . M. Silverstein, G. C. Basslev, and T. C. Morill. Spectro- metric identification of organic compounds. 4th ed. John Wiley &Sons, Inc., New York. 1981. pp. 95-105.

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a physicochemical study on organic eutectics and addition compound; benzidine–pyrogallol system

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