Some physicochemical studies on organic eutectics and 1:l addition compound; p-phenylenediamine - benzoic acid system

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1 Some physicochemical studies on organic eutectics and 1:l addition compound; p-phenylenediamine - benzoic acid system U. S. RAI AND K. D. MANDAL Chemistry Departmetzr, Batzaras Hitzdu Universi~, Varanasi , U.P., lndia Received June 8, 1988 U. S. RAI and K. D. MANDAL. Can. J. Chem. 67, 239 (1989). The phase diagram of p-phenylenediamine - benzoic acid system, determined by the thaw-melt method, shows the formation of two eutectics and a 1: 1 addition compound. The linear velocity of crystallization of pure components, eutectics and addition compound, determined by measuring the movement of growth front in a capillary, suggests that crystallization data obey the Hillig-Tumbull equation. Using experimental values of heats of fusion, entropy of fusion and excess thermodynamic functions were calculated and the results are explained on the basis of cluster formation in the melts. X-ray diffraction data infer that these eutectics are not simply the mechanical mixture of the two components and there is preferential ordering of atomic planes during their formation. The infrared spectral studies suggest the formation of intermolecular hydrogen bonding between the components forming the molecular complex. Key words: organic eutectics, growth kinetics, phase diagram, thermochemistry, X-ray diffraction studies U. S. RAI et K. D. MANDAL. Can. J. Chem. 67,239 (1989) Le diagramme de phase du systkme p-phcnylknediarnelacide benzoique, dctermine par la mtthode de IiquCfaction, montre qu'il y a formation de deux eutectiques et d'un composc d'addition 1: 1. La vitesse lineaire de cristallisation des produits purs, des eutectiques ainsi que du produit d'addition, telle que dctermince par le mouvement du front de croissance dans un capillaire, suggkre que les donnces de cristallisation obcissent i l'kquation de Hillig-Tumbull. Utilisant les valeurs expcrimentales pour les chaleurs de fusion, on a calculc l'entropie de fusion ainsi que les fonctions thermodynamiques en exces et on a expliquc les rksultats expkrimentaux en se basant sur la formation d'agrcgats dans les produits de fusion. Les donntes de la diffraction des rayons-x suggkrent que les eutectiques ne sont pas simplement des mclanges mccaniques des deux produits et qu'il existe un ordre prcfcrentie1 des plans atomiques au cours de leur formation. Des Ctudes rcalisces par spectroscopic infrarouge suggkrent qu'il y a formation de ponts hydrogknes intramolcculaires entre les produits formant le complexe molcculaire. Mots elks: eutectiques organiques, cinktique de croissance, diagramme de phase, thermochimie, etudes par diffraction des rayons-x. [Traduit par la revue] 1. Introduction Since the early sixties, the chemistry (1-5) of eutectlcs and addition compounds has been a field of potential interest due to their unusual physical properties not normally shown by the parent components. The metallic (6,7) eutectics and intermetallic compounds constitute an active area of present investigation in metallurgy and materials science. However, the various studies carried out on these systems are inadequate and incomplete as high transformation temperature, opacity, and difficulties involved in purification present serious problems. Apart from these, wide difference in densities of the two components forming the metal eutectics causes density-driven convection effects, which in turn, affect their solidification. Due to low transformation temperature, ease in purification, transparency, low convection effects, and wider choice of materials, organic systems (8-12) are more suitable and are being used as model systems for detailed invest~gation of the parameters which control solidification. Further, the experimental techniques required for their investigation are simpler and more convenient as compared to those adopted in metallic systems. Most of the organic systems studied in the past are simple eutectic type. There is only a limited number of cases in which two components form a molecular complex (1 3-15) with congruent melting point. Such systems are of potential technical interest since these are quite common in metallurgical systems. Organic systems, giving addition compounds which are organic analogues of intermetallic compounds, are important to material and metallurgical sciences since they permit visual observation of phase transformation and process during solidification. The formation of such molecular compounds can be established on the basis of phase diagrams which exhibit a characteristic maximum, surrounded by two eutectics, corresponding to the stoichiometry of the addition compounds formed. The growth morphology of eutectics and molecular compounds depend on the growth characteristics of individual constituent phases which can solidify either with faceted or with non-faceted interfaces. This behaviour is related to the nature of solid-liquid interface and can be predicted for pure materials from their entropy of fusion data. A unique crystallographic orientation relationship between the constituent phases and their mating planes was established by Hogan et 01. (16). Gruzleski and Winegard (17) observed perfect lamellar grains in Sn-Cd eutectics but in a number of other systems, the eutectic grains did not exhibit a fixed crystal orientation with respect toexternal lines of forces. Bassi and Sharma (18) studied the infrared spectra of naphthalene - benzoic acid eutectic system and inferred that there exists a specific orientation between the constituents with respect to each other. With a view to elucidate the chemistry of organic eutectics and the 1: 1 addition compound, p-phenylenediamine (PPD) - benzoic acid (BA) system was undertaken and its phase diagram, linear velocity of crystallization and heat of fusion were studied. From the heats of fusion data the enthalpy of mixing, entropy of fusion, and excess thermodynamic functions were calculated to throw light on the nature of interactions among the components forming the melts. The structure of eutectics and molecular compound was also studied by infrared and X-ray methods. 2. Experimental 2.1. Materials and plrrification p-phenylenediamine (High Purity Chemicals, India) was purified by repeated distillation under vacuum and was stored in coloured bottles to avoid exposure to light. Benzoic acid (BDH) was recrystallised Primed in Canada 1 lmprimc au Canada

2 240 CAN. J. CHEM. VOL. 67, 1989 from boiling water. The purity of the compounds was confirmed by determining their melting points which were in good agreement with literature values. 2.2 Phase diagram srudy Phase diagram of p-phenylenediamine - benzoic acid system was determined (19, 20) by the thaw-melt method. Mixtures of various proportion covering the entire range of composition were taken in long-necked glass test tubes and were sealed. The mixtures were subjected to number of alternate melting in liquid paraffin followed by chilling in ice to make it homogeneous. The mixtures were taken out by breaking the test tubes and were further homogenized by grinding in a mortar with due care to avoid moisture or any other contamination. The melting and thaw temperatures were determined by Toshniwal Melting Point Apparatus. 2.3 Linear velocity of crystallization The linear velocity of crystallization of pure components, eutectics and addition compound was determined (21, 22) by measuring the rate of movement of growth front in a capillary. The experimental details are reported earlier (22) Heats of fusiorl The heats of fusion of pure components, eutectics, and 1: 1 addition compound were determined (23, 24) by their DTA patterns obtained from Stanton Redcroft STA-780 Series Unit. All the runs were carried out with heating rate 2"C/min, chart speed 10 mm/min, and chart sensitivity, 100 pv/10 mv. The sample weight range was 5-10 mg for each estimation. Using phenanthrene as a standard substance, the heats of fusion of unknown compounds were determined by the following equation: ws A [I] AH = AH, x - x - W A, where AH is the heat of transition of unknown sample and AH, is the heat of fusion of standard substance. W and A are weight and peak area, respectively Infrared studies The infrared spectra of the pure components and adduct were recorded (25) in Nujol mull in the region cm-i, on a Perkin-Elmer 783 Infrared Spectrophotometer X-ray diffraction studies X-ray diffraction patterns of pure components, eutectics and 1:l addition compound were recorded (24) on a computerized X-ray diffraction unit, PW 1710 model, using Cu-K, radiation. 3. Results and discussion 3.l. Phase diagram Phase diagram of p-phenylenediamine - benzoic acid system, given in Fig. 1, shows the formation of one 1:l addition compound with congruent melting point surrounded by the two eutectics El and E2 having and mole fraction of benzoic acid, respectively. The melting temperatures of molecular compound and eutectics El and E2 are 145.O, 124.5, and C, respectively. For each eutectic, the addition compound serves as one of the components. The flat maximum, observed in this system, suggests (13) that the addition compound dissociated in the molten state. From the phase diagram it can also be inferred that the addition compound is capable of existing as a solid compound in equilibrium with a liquid of the same composition. From the first eutectic point El onwards, on addition of the second component, the melting point again rises, attains a maximum at C, where the composition of liquid and solid phases are identical. The existence of an eutectic point on either side of the maximum provides an I 8 I Moletraction ot Benzoic ocid - FIG. 1. Phase diagram of p-phenylendiamine - benzoic acid system. 0, Melting temperature; 0, thaw temperature. 1' / I / Addition ill-ppd- Benroic acid Compound IV-PPD- Benzaic acid d V- PPD-6enzoic acid 1; FIG. 2. Linear velocity of crystallization of pure components, eutectics and 1: 1 addition compound at different undercoolings. information about the large stability of the molecular complex formed Growth kinetics The linear velocity of crystallization, u, of pure components, eutectics and 1: 1 addition compound, determined by measuring the rate of movement of growth front in a capillary at different undercoolings, AT, is given in Fig. 2 in the form of log u versus log AT. The linear dependence of growth velocity and undercooling suggests that the crystallization data obey Hillig- Turnbull (26) relation: where u and n are constants depending on the nature of solidification of the sample under investigation. The experimental values of these constants are given in Table 1. The values of n being close to 2 suggest a square relationship between the linear growth velocity and undercooling. The slight deviations (20) in the values of n from 2, observed in some cases, may

3 TABLE 1. Values of u and n U Compound (mmlsdeg) n p-phenylenediamine Benzoic acid Eutectic Eutectic : 1 Addition compound RAI AND l be due to the difference between the bath temperature and the temperature of growing interface. It is evident from the values of u, given in Table 1, that the crystallization velocity of eutectic-1 (E,) and eutectic-2 (E2) are lower than those of pure components and higher than the addition compound. The first eutectic (El), having a low molar concentration of benzoic acid, solidifies more slowly than the second eutectic (E,) with a high molar concentration of benzoic acid. These results may be explained on the basis of the mechanism proposed by Winegard et al. (27). According to them, in binary system the eutectic solidification begins with the formation of the nucleus of one of the phases. This would grow until the surrounding liquid becomes rich in the other component and a stage is reached when the second component also starts nucleating. Now there are two possibilities. First, the two initial crystals may grow side by side. This explains the case in which the rates of solidification of eutectics are not lower than those of the parent components. The second possibility is that there may be alternate nucleation of the two components. This explains the solidification phenomena, in cases, where the crystallization velocity of the eutectics is lower than that of the parent components. Thus, the solidification of El and E2 takes place by side by side growth of the two components. For both eutectics, addition compound acts as one of the components and nucleates first. Thus, p-phenylenediamine and addition compound act as the parent components for El, while the molecular complex and benzoic acid do so for eutectic E2. For El and E2 the nucleation of addition compound is followed by the nucleation of p-phenylenediamine and benzoic acid, respectively. From the values of u, given in Table 1, it can be inferred that the 1 : 1 addition compound of p-phenylenediamine with benzoic acid crystallizes at a rate slower than that of pure components and both eutectics. Studies (14) on crystal morphology of the addition compounds indicate that they crystallize as a definite chemical entity. However, during crystallization, the two components from the melt have to enter the crystal lattice, simultaneously, in such a way that the composition of the melt corresponds to a 1 : 1 molar ratio of the two components. Due to this, the linear velocity of crystallization of the addition compounds may be expected to be of the order of the velocity of the species crystallizing at a slower rate Thermochemistry 'The experimentally determined values of heats of fusion of parent components, eutectics, and addition compound are given in Table 2. If the eutectics were a simple mechanical mixture of the two components involving no heat of mixing or any type of association in the melt, the heats of fusion, (Afh),, would simply be given by the mixture law (28): where x, and x2 are the mole fraction and Afh I and Afh2 are the experimental values of heats of fusion of parent components 1 and 2, respectively. The heats of fusion, calculated from the mixture law, are also given in the same table. It is evident from the table that the values of heat of fusion calculated from eq. [3] are higher than the experimental values. This difference can be attributed to the formation of clusters in the eutectic melt. It can be imagined that during cluster formation, heat liberated may lower the actual value of heat of fusion. The cluster formation will be favoured if the molecules can associate themselves by certain weak intermolecular forces. In eutectic systems where one or both components contain hydroxyl groups, there would be a tendency to form hydrogen bonds and as a result, the cluster formation will be favoured. This is one of the reasons why the experimental values of heat of fusion are lower than those calculated from eq. [3]. The experimental value of heat of fusion of addition compound and its theoretical value, calculated by the mixture law, is also given in the Table 2. It is evident that the calculated value is higher than the experimental value. A similar type of cluster formation and explanation can also be suggested to justify this observation. Heat of mixing (AH,) which is the difference between the experimental and calculated values of heats of fusion is given by [4] AH,=(Afh)exp-C~iAfh; where (Afh),,, is the heat of fusion of the eutectic, determined experimentally, xi and Afhi are the mole fraction and heat of fusion of the end components, respectively. It is evident from Table 2 that in all cases the heats of mixing are negative. 'Thermochemical studies suggest that the structure of eutectic melt depends on the sign and magnitude of the enthalpy of mixing. Three types of structures (29) are suggested; quasieutectic for AH, > 0, clustering of molecules for AH, < 0, and molecular solutions for AH, = 0. The negative values of AH, of eutectics of system under investigation, suggest clustering of molecules in the eutectic melt and substantiate our earlier conclusion drawn on the basis of the mixture law. It is also very interesting to note that the molecular complex and one of the eutectics have low values of enthalpy of mixing, and one might argue for the possibility of simple molecular solutions instead of weak interactions. These results (30) are quite different from those of 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 interactions due to the presence of molecular complex in the eutectic melts. In order to know the nature of interaction between the components forming the eutectics in the present systems, some thermodynamic functions, such as excess free energy, ge, excess enthalpy of mixing, he, and excess entropy of mixing, SE, were calculated using the following equations:

4 CAN. J. CHEM. VOL. 67, 1989 TABLE 2. Heat of fusion and entropy of fusion Roughness Enthalpy Entropy parameter Heat of fusion of mixing of fusion Material (kj/mol) (kj/mol) (kj mol-' K-I) p-phenylenediamine Benzoic acid Eutectic-1 (Expt.) (Calc.) Eutectic-2 (Expt.) (Calc.) 1: 1 Addition compound (Expt.) (Calc.) TABLE 3. Excess thermodynamic functions for eutectic-l and eutectic-2 g h s Material (J/mol) (J/rnol) (J mol-' K-I) where yi and T P are activity coefficient and melting temperature of the component indicated by the suffix and T and R are the eutectic temperature and gas constant, respectively. Equation [5] is obtained by considering the general condition of phase equilibrium for the two phases and assuming that heat of fusion is independent of temperature and the two components are miscible in the liquid phase only. This equation was used to calculate the activity coefficient of the end members at the eutectic point. The values of a In y,!/at were calculated by differentiating eq. [5] and taking slope of the liquidus line near the eutectic point. The details of calculation of excess thermodynamic functions are reported earlier (28)... Values of excess fgnctions are given in able 3. The positive values of excess free energy give measure of the departure of the system from ideal behaviour and suggest interaction among the components forming the eutectic melt. The values of he and se of Table 3 correspond to the excess free energy and are measures of excess enthalpy of mixing and excess entropy of mixing, respectively. The entropies of fusion, AS, of pure components, eutectics and addition compounds were calculated using the following relation: where Afh is the heat of fusion and T is the fusion temperature, and are given in Table 2. In all the cases AS values are positive, indicating an increase of randomness during melting. According to Hunt and Jackson (I), the type of growth from a eutectic melt depends upon a factor cx as defined by equation: where 5 is a crystallographic factor depending upon the geometry of the molecules and has the values less than or equal to one. ASIR, also known as Jackson's roughness parameter, FIG. 3. Microstructure of eutectic El ( X 200). is the entropy of fusion in dimensionless unit and R is the gas constant. When cx < 2, non-faceted growth occurs whereas a faceted growth appears if cx > 2. In all cases, ASIR values are greater than 2, which indicate that they exhibit faceted growth. The microstructures of eutectic, recorded on a microscope (Leitz Laborlux D, West Germany) are given in Figs. 3 and 4, confirm this conclusion Infrared studies The infrared spectrum of p-phenylenediamine shows a strong band at 3380 cm-'. In the case of pure benzoic acid, there is no peak in the cm-' region. But the 1:l addition compound of p-phenylenediamine with benzoic acid has three bands at 3230,3340, and 3460 cm- ' in this range. The strongest band being at 3340 cm-' shows a negative shift of 40 cm- ' as compared to the strong band observed in PPD at 3380 cm-'. The other two bands in the 1 : 1 addition compound are attributed to free -NH2 group present therein which is not involved in hydrogen bonding with benzoic acid. A band observed at 1690 cm-' due to C=O stretching of benzoic acid was found to shift to 1640 cm-' in the addition compound. These observations clearly indicate the formation of intermolecular

5 RAI AND MANDAL 243 TABLE 4. Preliminary X-ray data (i) p-phenylenediamine Eutectic- 1 Addition compound Relative Relative Relative d (A) intensity d (A) intensity d (A) intensity FIG. 4. Microstructure of eutectic E2 ( X 200). hydrogen bonding between the two components, which is represented below: 3.5. X-ray diffraction studies Experimental results on some preliminary investigations of X-ray diffraction of pure components, eutectics, and addition compound are reported in Tables 4 and 5. It can be observed from the X-ray diffraction data that for a particular d value there is marked difference in the relative intensity of the components. This may be explained on the basis of the structure considerations of the compounds under investigation. It is evident from Tables 4 and 5 that the number of reflections of El, E2, and addition compound are comparable. This suggests that they belong to the same crystal system and have similar lattices. In general, common reflections of benzoic acid and eutectic E2 have smaller intensities in the case of eutectic. Common reflections of addition compound and eutectic E2 also show decrease in intensities in the eutectic. Strong reflections of benzoic acid show a drastic decrease in intensity in the eutectic phase showing that the structure of the two differ very significantly. Whereas this is not true for the eutectic and addition compound. The structure of eutectic seems to be governed more by the addition compound as compared to benzoic acid because the high angle Bragg reflections are absent, both in addition compound and eutectic while they are present in benzoic acid. A drastic decrease of intensity of strong common reflections and the absence of high angle Bragg reflections are also observed in eutectic El containing p-phenylenediamine as one of the components. There is no regular change of intensity of reflections of El and addition compound but the tendency of decrease of intensity in the addition compound is dominant. Moreover the intensity of strong reflections of addition compound show drastic decrease in the eutectic El. TABLE 5. Preliminary X-ray data (ii) Benzoic acid Eutectic- 1 Addition compound Relative Relative Relative d (A) intensity d (A) intensity d (A) intensity It is clear from Tables 4 and 5 that the intensities of the reflections diminish with decreasing d values. It is also noted from both the tables that some reflections of pure components and addition compound are absent in the corresponding eutectic. The first eutectic (El) formed between p-phenylenediamine and the addition compound has some strong reflection! having d values 11.79,6.58,4.85,4.41,4.01,3.93,and3.85Awhereas some reflections of PPD and molecular complex are missing in

6 244 CAN. 1. CHEM. VOL it. In the case of the second eutectic (Ez), formed between benzoic acid and the molecular complex, a similar trend is observed aild strong regections correspond to d values 11.79, 10.91, 5.15, and 4.00 A. These experimental results infer that the eutectics are not simply the mechanical mixture of the two components. In them, there is orientation (30) of some atomic planes and preferential ordering also takes place during their formation. These findings are in accordance with our earlier conclusions drawn in the case of p-phenylenediamine-catechol system reported earlier (23). AcknowIedgements Thanks are due to Professor I. S. Ahuja, Head, Chemistry Department, Banaras Hindu University, Varanasi, for providing laboratory facilities. The authors are also thankful to CSIR, New Delhi, for financial assistance. 1. K. A. JACKSON and J. D. HUNT. Trans. Met. Soc. AIME, 236, 1129 (1966). 2. N. B. SINGH and K. D. DWIVEDI. J. Sci. Ind. Res. 41,96 (1982). 3. K. PIGON and A. KRAJEWSKA. Thermochim. Acta, 58, 299 (1982). 4. R. P. RASTOGI, D. P. SINGH, NAMWAR SINGH, and NARSINGH B. SINGH. Mo1. Cryst. Liq. Cryst. 73, 7 (1981). 5. N. B. SINGH and NARSINGH B. SINGH. J. Cryst. Growth, 28, 267 (1975). 6. R. M. JORDAN and J. D. HUNT. Met. Trans. 2, 3401 (1971). 7. R. ELLIOTT. Int. Met. Rev. 22, 161 (1977). 8. B. DERBY and J. J. FAVIER. Acta Metall. 31, 1123 (1983). 9. R. N. GRUGEL and A. HELLAWELL. Met. Trans. 15A, 1626 (1984). 10. J. E. SMITH, D. 0. FRAZIER, and W. F. KAUKLER. Scr. Metall. 18, 677 (1984). 11. M. E. GLICKSMAN, N. B. SINGH, and M. CHOPRA. Manuf. Space, 11, 207 (1983). 12. P. E. ARNDT, J. G. DUNN, and R. L. S. WILLIX. Thermochim. Acta, 79, 55 (1984). 13. B. M. SHUKLA, N. P. SINGH, and NARSINGH B. SINGH. MoI. Cryst. Liq. Cryst. 104, 265 (1984). 14. R. P. RASTOGI, N. B. SINGH, and K. D. DWIVEDI. Ber. Bunsenges Phys. Chem. 85, 85 (1981). 15. A. KRAJEWSKA and K. PIGON. Thermochim. Acta, 41, 187 (1980). 16. L. M. HOGAN, R. W. KRAFT, and F. D. LAMKEY. Advances in materialsresearch. Vol. 1. H. Herman Edition. New York J. E. GRUZLESKI and W. C. WINEGARD. J. Inst. Metals, 96, 301 (1968). 18. P. S. BASSI and N. K. SHARMA. Indian J. Chem. 14A, 692 (1976). 19. N. P. SINGH and B. M. SHUKLA. Cryst. Res. Technol. 20, 345 (1985). 20. N. B. SINGH, U. S. RAI, and 0. P. SINGH. J. Cryst. Growth, 71, 353 (1985). 21. U. S. RAI, 0. P. SINGH, and N. B. SINGH. J. Chim. Phys. 84, 483 (1987). 22. N. B. SINGH and NARSINGH B. SINGH. Krist. Techn. 13, 1175 (1978). 23. U. S. RAI and K. D. MANDAL. Thermochim. Acta. In press. 24. U. S. RAI and K. D. MANDAI.. Crystal Res. Technol. 23, 871 (1988). 25. U. S. RAI and K. D. MANDAL. Z. Phys. Chem. In press. 26. W. B. HILLIG and D. TURNBULL. J. Chem. Phys. 24,914 (1956). 27. W. C. WINEGARD, S. MOJKA, B. M. THALL, and B. CHALMERS. Can. J. Chem. 29, 320 (1951). 28. U. S. RAI, 0. P. SINGH, N. P. SINGH, and NARSINGH B. SINGH. Thermochim. Acta, 71, 373 (1983). 29. NAMWAR SINGH, NARSINGH B. SINGH, U. S. RAI, and 0. P. SINGH. Thermochim. Acta, 95, 291 (1985). 30. N. P. SINGH, B. M. SHUKLA, NAMWARSINGH, and NARSINGH B. SINGH. J. Chem. Eng. Data, 30, 49 (1985).