Indian Journal of ChemistryVol. 27 A, December 1988, pp. 1021-1024

Mechanism of Solidification of Binary Naphthalene-Benzoic Acid EutecticSystem

P S BASSI-t, B L SHARMA, N K SHARMA & SHIV KUMARDepartment of Chemistry, University of Jammu, Jammu 180004

Received 27 July 1987; revised 14 March 1988; accepted 30 March 1988

The nonideal character of binary naphthalene-benzoic acid eutectic system is ascertained from its ideal solid-li-quid equilibrium data and the excess thermodynamic functions G~ and SE The behaviour of this system has beeninvestigated by spontaneous crystallization, unidirectional velocity of solidification, viscosity measurements and mic-roscopic studies.

The eutectic composites have considerable indus-trial importance 1. While nonfaceted-nonfaceted(nf/nf) eutectic composites have been extensivelyinvestigated, the faceted-nonfacted (flnf) class ofeutectic composites have received comparativelylesser attention. But very little attention has beenpaid to the facted-faceted (flf) eutectic composites.Presently, we have explored the nature, mechan-ism of solidification and structures both in t,he sol-id and liquid phases of flf naphthalene-benzoic ac-id eutectic system.

Materials and MethodsNaphthalene (BDH) was purified by repeated

sublimation (m.p. 353.5K). Benzoic acid (BDH,AR, m.p. 394.40K) was used as such. The solid-li-quid equilibrium diagram for naphthalene-benzoicacid system and its spontaneous crystallization-,linear velocity of crystallization- and the viscositiesof the eutectic, pre-, post- and non-eutectic meltswere measured by the methods reported earlier".The microscopic studies were carried out using apolarising microscope.

Results and DiscussionThe ideal liquidus temperatures (T's) at differ-

ent compositions of naphthalene (xl) were calculat-ed from Eq. (1)5, assuming that the naphthalene-

-In ~Y:=~~[~- ~lbenzoic acid system behaved ideally, i.e. activitycoefficient, y! = 1. The value of heat of fusion,~ fH P of component x] (i = 1,2) used in the afore-mentioned calculation depends upon whether the

. . . (1)

tPresent address: Department of Chemistry, Guru Nanak DevUniversity, Amritsar

mixture is rich in component x \ or x ~ with thetemperature of melt T? or Tg.ICurve II in Fig. 1represents the ideal phase diagram of this systemand curves III and IV represent the temperaturesof spontaneous crystallization and thaw points re-spectively. The eutectic composition is found at0.6710 mol fraction of naphthalene with a temper-ature of melt 342.30 K. The excess thermodynam-ic functions, free energy and entropy of mixing atthe temperature T and constant pressure were de-termined=? using Eqs. (2) and (3) and the data areprovided in Table L

E DTf 1 1 1 In I]G =,UlXtlnYt+xz Y2E [I 1 1 I I]S = -Rx1lnYl+X2 nY2

... (2)

... (3)

The ideal phase diagram (curve II, Fig. 1) andthe excess thermodynamic functions ct- and SEshow that naphthalene-benzoic acid binary systemdeviates from the ideal behaviour. The excess freeenergy (ct-) is minimum for the mixture at the eu-tectic composition (Table 1) which is due to thereason that the eutectic composite has the lowestfreezing point; whereas as expected, SE is maxi-mum at the eutectic composition, since at the eu-tectic composition there are three phases, viz. twosolid phases and one liquid phase in equilibrium.Hence the molecular distribution at the eutecticcomposition is more probable than that in anyother mixture of naphthalene-benzoic acid system .

As is evident from Table 2 the ~Sm values of.naphthalene and benzoic acid differ appreciablybut for the mixtures these values should be higherby an amount contributed by the entropy of mix-ing (Table 1) in the melt, yet the ratio ~ TITm forthe mixtures is nearly constant and also the crys-tallographic factor 1; approaches unity (Table 2). Itfollows that a definite relationship exists between

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INDIAN J. CHEM., VOL. 27A, DECEMBER 1988

Table 1- Excess Thermodynamic FunctionsTemp Mol fr of GEx 10- 2 SEX 10-4

Tm(K) naphthalene (kJ mol : ') (kJ mol-I K -I)390.50 0.1 28.73 - 07.34383.40 0.2 31.47 - 08.20376.40 0.3 34.25 - 09.08368.80 0.4 36.09 - 09.80358.20 0..5 31.64 - 08.83345.80 0.6 18.70 - 05.44342.30 0.671O(E) 12.67 -03.77343.20 0.7 18.59 - 05.44347.30 0.8 38.35 -10.88351.00 0.9 37.09 - 10.47

E = Eutectic composition

Table 2-Heterogeneous Nucleation DataMol frof ~Sm x 10-3 of !;=T/Tm ~T/Tm

naphthalene nucleating phase(kJmol-1 K-')

0.0000 43.74 0.95 0.0460.0963 43.74 0.95 0.0460.1994 43.74 0.95 0.0470.3003 43.74 0.95 0.0530.3985 43.74 0.94 0.0540.4887 43.74 0.94 0.0570.5908 43.74 0.94 0.0570.6968 54.04 0.96 0.0360.7898 54.04 0.98 0.0240.9029 54.04 0.98 0.0221.0000 54.04 0.98 0.016

Table 3- Linear Velocity of Crystallization(V) at~T=9.lOK

Mol fr of naphthalene VXlO-2

(ern s-')

87.5005.90

192.3026.1015.70

0.00000.56990.67100.76961.0000

nucleation (determined from the limit of under-cooling) and the structure of the melt (determinedfrom the entropy of fusion) and hence this obser-vation verifies the nucleation theory.

The data for linear velocity of crystallization foreutectic, pre-, post-, and non-eutectic materialsobey Eq. (4).

V=K(~T)n ... (4)

In Eq. (4), V is the velocity of crystallization andK and n are constants. The velocity of crystalliza-tion for the mixture at the eutectic compositionshows anomalous behaviour (Fig. 2) which can be

1022

400

380

" 360

1&1

i 3401&1Go::lIE~

320

300 0·0 o·~0·2 06 0·8 I{)

MOL FRACTION OF NAPHTHALENE

Fig. 1- Diagrams of state and undercooling for naphthalene-benzoic acid system [(I) Melting points; (II) ideal temperatures;(III) temperatures of spontaneous crystallisation; and (IV)

thaw points]

+120

•. eo

+«1

-40

~ -80

>

Fig. 2-Linear velocity of crystallisation for naphthalene-ben-zoic acid system at various degrees of undercooling [(I) Purenaphthalene; (II to IV) pre-eutectic, eutectic and post-eutecticsamples containing 0.5699, 0.671 and 0.7696 mol fraction of

naphthalene, respectively; and (V) pure benzoic acid)

expected because of dimer character of benzoicacid. For the mixtures other than the eutectic, thevelocity of crystallization decreases when one purecomponent is gradually added to the other. Thedata in Table 3 indicate that the velocity of crys-tallization of the eutectic composite is much higherthan those of the pre-, post-, and non-eutectic ma-terials for fixed undercooling (~T= 9.10 K) indi-cating that the growth of the component is go-verned by a diffusion-controlled process".

BASSI et al.: MECHANISM OF SOUDIFICATION OF EUTECTIC SYSTEM

+40

+20

-20

-40

-80

"12 -80••F'(!)or::

-140

-160

-180

D

2·86 2·88

~.

Fig. 3-Plots of log T] versus liT for naphthalene-benzoic acid system [(I) Pure naphthalene; and (Il to IV) pre-eutectic, eutecticand post-eutectic samples containing 0.5701, 0.6671 and 0.7678 mol fraction of naphthalene, respectively]

Though the velocity of crystallization for theeutectic melt is much higher, the components can-not escape the influence of each other while grow-ing from the melt as has been observed in micros-tructure of solid eutectic.

Microscopic studies reveal that the solid sepa-rating out from the eutectic melt has entirely dif-ferent structure from those of its constituentphases. The solid structures of the mixtures werefound to be opaque. Growth of the two phases isquite interdependent and cross-diffusion, ahead ofthe interface, is required to sustain the growthprocess. Neither phase can escape the influence ofthe other and the normal dendrite growth is pre-vented. The microstructure of the eutectic compo-site of naphthalene-benzoic acid is faceted-faceted(flf) which is in accordance with the Jackson and

Table 4-Activation Energy and Viscosity Data

Mol frof Temp.(K) Viscosity, T] e.;« 10-6

naphthalene (cp) (kJ mol-I)1.0000 350.20 0.7850

353.20 0.7582 16.90357.20 0.7009361.20 0.6568

0.5701 348'.20 1.0791350.20 1.0509

16.76352.20 1.0148354.20 0.9844

0.6670(E) 344.20 1.0070346.20 0.9388

25.54348.20 0.9036350.20 0.8635

0.7678 349.20 0.9152350.20 0.8839 17.42352.20 0.8507353.20 0.8392

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INDIAN J. CHEM., VOL. 27A, DECEMBER 1988

Hunt hypothesis"; since a values for both thecomponents are greater than two.

At 350.20K, the viscosity is minimum (0.8635cp) for the mixture at the eutectic composition(Table 4). The viscosities of pure components andvarious other mixtures were determined as a func-tion of temperature using Eq. (5).

Y] = Y] oeE,;,jRT ••• (5)

Log Y] values for any composmon were plottedagainst liT (Fig. 3) to find the activation energy(Evi.) for that composition. The activation energiesare given in Table 4. It is found that the activationenergy is maximum for the mixture at the eutecticcomposition (25.5436 kJ mol-I).

The lower value of viscosity at the eutecticcomposition indicates that the specific interactionsare insignificant whereas the higher value of acti-vation energy predicts the structural changes oc-curring in the eutectic melt, which may compriseof microregions or clusters rich in one component

1024

or the other. Hence two factors are contributing tothe activation energy for the eutectic liquid: (i) acti-vation energy for viscous flow, and (ii) energy re-quired for breaking the clusters. The clusteringnumber decreases with rise in temperature, and ata temperature when all the clusters have broken,only first factor will contribute to E vis'

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