J Therm Anal Calorim
DOI 10.1007/s10973-012-2800-x
Solid–liquid equilibrium, thermal, and physicochemical studies
on salicylamide–4-nitrophenol and 2-cyanoacetamide–4aminoacetophenone organic eutectic systems
Manjeet Singh • Priyanka Pandey • R. N. Rai
U. S. Rai
•
Received: 1 August 2012 / Accepted: 25 October 2012
Ó Akadû`miai Kiadû˚, Budapest, Hungary 2012
Abstract The solid–liquid phase equilibrium data of two
binary organic systems, namely, salicylamide–4-nitrophenol
and 2-cyanoacetamide–4-aminoacetophenone show the
formation of a eutectic in each case. The values of enthalpy
of fusion of pure components and binary eutectics have been
determined using differential scanning calorimeter (Mettler
DSC-4000 system). The thermal properties of the materials,
such as, heat of mixing, entropy of fusion, roughness
parameter, interfacial energy, and excess thermodynamic
functions were computed using the enthalpy of fusion values.
The microstructures of eutectics were developed using unidirectional thermal gradient, and regions of interest for
microstructures were photographed.
Keywords Thermal analysis Phase diagram Microstructure Growth kinetics at different
undercooling Eutectic
Introduction
Organic systems are more suitable for detailed investigations than metallic systems [1] because of low transformation temperature, transparency, wider choice of
materials, and minimized convection effects. These are the
special features which have prompted a number of research
groups to study binary organic materials in detail [2–5]. As
a result, organic systems are being used as model systems
Manjeet Singh and Priyanka Pandey have made equal contribution in
this study.
M. Singh P. Pandey R. N. Rai U. S. Rai (&)
Department of Chemistry, Centre of Advanced Study,
Banaras Hindu University, Varanasi 221005, India
e-mail: [email protected]
for detailed investigation of several parameters which
control the mechanism of solidification and decide the
properties of materials. The fundamental understanding of
controlling parameters could be applied to metallic systems
in which experimentation is difficult. During the last two
decades, the potential use of organic compounds for nonlinear optical (NLO) materials and for other electronic
applications [6, 7] has prompted the researchers for
undertaking various physicochemical investigations in
detail. In addition, syntheses of binary organic materials
have shown the potential to produce, as well as to modify,
the NLO and white light-emitting diode materials [8, 9].
4-Nitrophenol (PNP) belongs to monoclinic system with
cell parameters: a = 6.09 Å, b = 8.79 Å, c = 11.61 Å,
and b = 103.15°. The SHG efficiency of PNP is reported
as 3.1 times greater than KDP (KH2PO4) [10]. Salicylamide (SAM) belongs to monoclinic system with space group
I2/a, lattice parameters: a = 12.90 Å, b = 4.98 Å, c = 20.
99 Å, and b = 91.50° [11], and it is a non-prescription
drug with analgesic and antipyretic properties. The
medicinal uses are similar to those of aspirin [12].
4-Aminoacetophenone (AAP) crystal belongs to monoclinic system with space group P21/a with lattice parameters: a = 17.63, b = 5.18, and c = 8.51, and b = 106.6°
[13], while 2-cyanoacetamide (CA) crystal belongs to
monoclinic unit cell with space group P21/c with lattice
parameters: a = 8.36, b = 13.56, c = 7.56 Å, and b = 11
1.2° [14]. The scientific contribution of phase diagram
study for crystal growth as well as in the synthesis of
binary materials is significant. With a view to synthesize
new materials, we have selected two binary organic systems, namely, SAM–PNP and CA–AAP for the study of
their phase diagram, growth kinetics, microstructure, and
thermal properties such as heat of fusion, Jackson’s
roughness parameter, excess thermodynamic functions, etc.
123
M. Singh et al.
Experimental procedure
Growth kinetics
Materials and purification
The influence of undercooling temperature on growth
kinetics of the pure components and their eutectics was
studied [17, 19] by measuring the rate of movement of the
solid–liquid interface at different undercooling in a capillary
tube (U-shape) with 150-mm horizontal portion and 5-mm
internal diameter. Molten samples of the pure components
and the eutectics were separately taken in a U tube and placed
in a silicone oil bath. The temperature of the oil bath was
maintained using microprocessor temperature controller of
accuracy ±0.1 °C. At a particular undercooling, below the
melting point of the sample, a seed crystal of the same
composition was added to start the nucleation, and the rate of
movement of the solid–liquid interface was measured using a
traveling microscope and a stop watch. The same procedure
was repeated at different undercooling for different material.
The starting materials SAM (99 %), PNP (99 %), CA
(99 %), and AAP (99 %) were obtained from SigmaAldrich, Germany. The melting points as received of SAM,
PNP, CA, and AAP are found to be 140.0, 115.0, 121.0,
and 106.0 °C, respectively. SAM used as such as purchased, while remaining starting materials were purified by
recrystallization from ethanol. The melting points of the
purified PNP, CA, and AAP were found to be 115.0, 121.5,
and 107.0 °C, respectively. The assessed purity of each
compound was found to be more than 99 % when compared with the literature data of melting temperatures [15]
and NMR.
Phase diagram
Microstructure
The phase diagrams of SAM–PNP and CA–AAP systems
were studied using the thaw–melt method [16, 17]. In order
to establish the phase diagram, mixtures of different
compositions covering the entire composition were prepared. They were taken in different glass test tubes, and the
mouth of each test tube was sealed. The mixtures were
homogenized by melting and mixing in oil bath followed
by chilling in ice-cooled water, and the process was repeated four times. During homogenization, the temperature of
the oil bath was maintained slightly above the melting
temperature of the parent components. The melting temperatures of mixtures of different compositions were
determined with the help of a melting point apparatus
(Toshniwal melting point) attached with thermometer
which could read correctly up to ±0.5 °C, and the rate of
rise of temperature, during melting point determination,
was 0.5 °C min-1 around the melting point. The phase
equilibrium graphs were plotted between melting temperatures on Y-axis and their respective compositions on the
X-axis.
Enthalpy of fusion
The values of heat of fusion of the pure components and
the eutectics were determined by differential scanning
calorimeter (Mettler DSC-4000 system) [17, 18]. Indium
and Zinc samples were used to calibrate the DSC unit. The
DSC experiments were performed under nitrogen gas
environment, and the gas flow rate was maintained to be
35 mL min-1. The amount of test sample and the heating
rate were about 5–7 mg and 10 °C min-1, respectively.
The values of enthalpy of fusion are reproducible within
±0.01 kJ mol-1.
123
Microstructures of the pure components and the eutectics
were recorded [18] by placing a drop of molten compound
on a hot glass slide. To avoid the inclusion of the impurities
from the atmosphere, a cover slip was glided over the melt,
and it was allowed to cool to get a supercooled liquid. The
melt was nucleated with a seed crystal of the same composition at one end, and the care was taken to have unidirectional freezing. The directionally solidified crystal
system on the glass slide was then placed on the platform
of an optical microscope (Leitz Laborlux D). The different
regions of the slide were viewed, and the photographs of
regions of interest were recorded at a suitable magnification using a camera attached with the microscope.
Results and discussions
Phase diagram
The phase diagrams of SAM–PNP (Fig. 1) and CA–AAP
(Fig. 2) systems, reported in terms of melting temperature–
composition curves, show the formation of simple eutectics.
The melting point of SAM is 140.0 °C, which decreases with
the addition of PNP and attains the minimum melting temperature, i.e., the eutectic temperature of SAM–PNA system.
Further addition of PNA increases the melting point and
attains 115.0 °C, which is the melting point of PNA. The
eutectic temperature and composition of SAM–PNP systems
are 89.0 °C and 0.625 mol fraction of PNP, respectively.
Similarly, in case of CA–AAP, the melting temperature of
CA 121.5 °C decreases with the addition of AAP and attains
the minimum melting temperature, and further addition of
Solid–liquid equilibrium, thermal, and physicochemical studies on SAM–PNP–CA
fraction of AAP, respectively. It should be noted that composition other than eutectic composition, in both cases, does not
melt completely at a particular temperature, rather they melt
in a range of temperature. The reported temperatures in figures are the temperature where melting process completes.
When a solution of the eutectic composition is cooled below
eutectic temperature, it dissociates into two solid phases as
150
Melting temperature
140
Temperature/°C
130
L ! S1 + S2
120
L
These three phases, namely, a binary liquid phase, L, and
two solid phases, S1 and S2, are in equilibrium at the
eutectic point which is an invariant point of a system.
110
L + S1
100
Growth kinetics
L + S2
90
E
S1+ S2
80
0.0
0.2
0.4
0.6
0.8
1.0
Mole fraction of PNP
Fig. 1 Phase diagram of SAM and PNP system
130
Melting temperature
120
Temperature/°C
110
L
100
90
L + S1
L + S2
80
E
S1 + S 2
70
0.0
0.2
ð1Þ
0.4
0.6
0.8
1.0
Mole fraction of AAP
In order to study the crystallization behavior of the pure
components and the eutectic the crystallization rates (v) are
determined at different undercooling (DT) by measuring
the rate of movement of solid–liquid interface in a capillary. The plots between log DT and log v are depicted in
(Fig. 3). The linear dependence of these plots is in accordance with the Hillig and Turnbull [20] equation:
v ¼ uðDT Þn
ð2Þ
where u and n are constants and depend on the solidification behaviors of the materials involved. The values of
u and n are given in Table 1. These findings may be
explained by the mechanism given by Winegard et al. [21].
According to them, the crystallization of eutectic begins with
the formation of the nucleus of one of the components. This
phase grows until the surrounding liquid becomes rich in
phase of the other component, and a stage is reached when
the second component also starts nucleating. Now, there are
two possibilities: either the two initial crystals grow side-byside, or there may be alternate nucleations of the two phases.
The values of n for the eutectics being close to 1, in some
cases, suggest that there is direct relationship between
growth velocity and undercooling. The values of n being
close to 2, in other cases, suggest the square relationship
between growth velocity and undercooling (DT). The deviation of n values from 2, observed in a few cases, is due to the
difference between the temperature of bath and the temperature of growing interface. From the values of u (Table 1), it
can be concluded that the growth velocities of eutectics lie
between the corresponding values for parent components in
both the systems (SAM–PNP and CA–AAP). These findings
suggest that the eutectics in both the systems solidify via the
side-by-side growth mechanism.
Fig. 2 Phase diagram of CA and AAP system
Thermochemistry
AAP increases the melting point, until it attains the melting
point of AAP (107.0 °C). The eutectic temperature and
composition of CA–AAP system are 82.0 °C and 0.55 mol
It is well known that the values of heat of fusion of the pure
components and the eutectics are important in understanding
123
M. Singh et al.
0.4
Table 2 Heat of fusion, entropy of fusion, and roughness parameter
of PNP, SAM, CA, AAP, and their eutectics
I - PNP
II - SAM
III - Eutectic (SAM – PNP)
IV - CA
V - AAP
VI - Eutectic (CA – AAP)
II
0.2
0.0
–0.2
IV
–0.4
log v/mm s–1
–0.6
–0.8
VI
–1.0
Roughness
parameter/
a
25.59
0.062
7.5
18.97
0.049
5.9
0.051
6.1
Heat of
fusion/
kJ mol-1
SAM
PNP
Heat of
mixing/
kJ mol-1
Eutectic (SAM–PNP)
V
I
Entropy of
fusion/
J mol-1 K-1
Materials
III
Exp.
18.33
Cal.
21.45
-3.12
CA
18.81
0.048
5.7
AAP
18.53
0.049
5.9
0.031
3.7
Eutectic (CA–AAP)
–1.2
Exp.
10.87
Cal.
18.66
-7.79
–1.4
–1.6
–1.8
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
o
log ΔT / C
Fig. 3 Linear velocity of crystallization at various degrees of
undercooling for SAM, PNP, CA, AAP, and their eutectics
Table 1 Values of n and u for pure components and their eutectics
S. N.
Systems
n
u/mm s-1 deg-1
1
SAM
2.25
1.30 9 10-2
2
PNP
2.24
1.19 9 10-3
3
Eutectic (SAM–PNA)
1.44
7.40 9 10-3
4
CA
1.17
3.60 9 10-3
5
AAP
4.61
7.53 9 10-7
6
Eutectic (CA–AAP)
1.25
4.17 9 10-3
the mechanism of solidification, structure of eutectic melt,
and the nature of interaction between two components
forming the eutectics. The values of enthalpy of fusion of the
pure components and the eutectics, determined by the DSC
method, are reported in Table 2. For comparison, the values
of enthalpy of fusion of eutectics, calculated by the mixture
law [22] are also included in the same table. The values
of enthalpy of mixing, which is the difference between
the calculated and experimentally determined values of
the enthalpy of fusion, were found to be -3.116 and
-7.785 kJ mol-1 for the eutectics of SAM–PNP and
CA–AAP systems, respectively. Based on enthalpy of fusion
three types of structures are suggested [23]: quasi-eutectic
for DmixH [ 0, clustering of molecules for DmixH \ 0, and
molecular solution for DmixH = 0. The highly negative
values of enthalpy of mixing in both cases suggests that there
123
is associative interaction in the molecules of eutectic melt
[24]. The entropy of fusion (DfusS) values, for different
materials have been calculated by dividing the enthalpy
values of fusion by their corresponding absolute melting
temperatures (Table 2).
The deviation from the ideal behavior can best be
expressed in terms of excess thermodynamic functions,
namely, excess free energy (gE), excess enthalpy (hE), and
excess entropy (sE), which give more quantitative idea
about the nature of molecular interactions. The excess
thermodynamic functions could be calculated by means of
the following equations [18, 25]:
gE ¼ RT x1 ln cl1 þ x2 ln cl2
ð3Þ
o ln cl1
o ln cl2
þ x2
hE ¼ RT 2 x1
ð4Þ
oT
oT
o ln cl1
o ln cl2
þ x2 T
sE ¼ R x1 ln cl1 þ x2 ln cl2 þ x1 T
oT
oT
ð5Þ
ln cl
where ln cli , xi and oT i are activity coefficient in the liquid
state, the mole fraction, and the variation of log of activity
coefficient in liquid state as a function of temperature of
the component i, respectively. It is evident from Eqs. 3–5,
that activity coefficient and its variation with temperature
are required to calculate the excess functions. Activity
coefficient ðcli Þ could be evaluated [18] using the equation,
Dfus Hi 1
1
ln xi cli ¼
ð6Þ
Tfus Ti
R
where xi, DfusHi, Ti, and Tfus are the mole fraction, the
enthalpy of fusion, the melting temperature of component i
and the melting temperature of eutectic, respectively. The
variation of activity coefficient with temperature could be
Solid–liquid equilibrium, thermal, and physicochemical studies on SAM–PNP–CA
Table 3 Excess thermodynamic functions for the eutectic
E
-1
E
h /k J mol
-1
E
Material
g /kJ mol
s /J mol
Eutectic (SAM–PNP)
0.03
36.72
0.10
Eutectic (CA–AAP)
0.52
-15.93
-0.05
Table 4 Critical radii of PNP and SAM, and their eutectics
-1
K
-1
ð7Þ
where qxi/qT in this expression can be evaluated by considering two points around the eutectic. The positive value
of excess free energy (Table 3) in both cases indicates that
there is an association between like molecules [26].
When liquid is cooled below its melting temperature, it
does not solidify spontaneously because, under equilibrium
condition, the melt contains number of clusters of molecules
of different sizes. As long as the clusters are well below the
critical size [27], they cannot grow to form crystals and,
therefore, no solid would result. During growth, the radius of
critical nucleus is influenced by undercooling as well as the
interfacial energy of the surface involved. The interfacial
energy (r) is related to the critical size (r*) of the nucleus and
enthalpy of fusion by the following equation:
r ¼
2 r Tfus
Dfus H DT
ð8Þ
where Tfus, DfusHi, and DT are the melting temperature, the
heat of fusion, and the degree of undercooling,
respectively. The computed values of the size of critical
nucleus at different undercoolings using Eqs. 8 and 9 of the
two systems are given in Tables 4 and 5. However, the
interfacial energy (r) is given by
r¼
C Dfus H
ðNA Þ1=3 ðVm Þ2=3
Critical radius 9 10-8/cm
SAM
calculated by differentiating Eq. 6 with respect to
temperature,
o ln cli Dfus Hi
oxi
¼
oT
RT 2
xi oT
Undercooling DT/°C
ð9Þ
where NA is the Avogadro Number, Vm is the molar volume,
and the parameter C lies between 0.30 and 0.35. The density
values used for the calculation of interfacial energies of SAM,
PNP, CA, and AAP are 1.33, 1.23, 1.16, and 1.219 g cm-3,
respectively. However, to compute the interfacial energy of
the eutectic, the mixture law was used. The calculated values
of interfacial energy are reported in Table 6.
Microstructure
It is well known that, in polyphase materials, the microstructure gives information about shape and size of the
crystallites, which plays a very significant role in deciding
mechanical, electrical, magnetic, and optical properties of
materials. According to Hunt and Jackson [28], the type of
4
4.432
5
3.546
6
2.955
7
2.533
8
9
PNP
Eutectic
2.252
1.971
1.970
1.752
10
11
1.577
1.433
12
1.314
20
0.883
22
0.803
25
0.707
27
0.654
29
0.609
31
0.570
Table 5 Critical radii of CA and AAP, and their eutectics
Undercooling DT/°C
Critical radius 9 10-8/cm
CA
4.5
4.757
5.5
3.892
6.5
3.293
7.5
2.854
AAP
Eutectic
5
5.749
7
4.107
12
2.396
14
2.053
11
1.418
13
1.200
15
1.040
17
0.917
growth from melts depends upon the interface roughness
(a) defined by
a ¼ nDfus H=RT
ð10Þ
where n is a crystallographic factor which is generally equal
to or less than one (we have used one while calculating
roughness). The values of a are reported in Table 2. If
a [ 2, then the interface is quite smooth, and the crystal
develops with a faceted morphology. On the other hand, if
a \ 2, then the interface is rough, many sites are continuously available, and the crystal develops with a non-faceted
morphology. In the present system, the values of a being
123
M. Singh et al.
Table 6 Interfacial energy of PNP, SAM, CA, and AAP and their
eutectics
S. N.
Systems
Interfacial Energy/erg cm-2
1
rSL1 (SAM)
55.06
2
rSL2 (PNP)
38.54
3
Eutectic (SAM–PNP)
44.73
4
rSL1 (CA)
51.33
5
rSL2 (AAP)
38.03
6
Eutectic (CA–AAP)
44.02
Conclusions
The phase diagrams of two binary organic systems, SAM–
PNP and CA–AAP, show the formation of simple eutectics
with 0.63 mol fraction of PNP in the first case and
0.55 mol fraction of AAP in the second case. The growth
kinetics of both the systems suggests that eutectics solidify
via the side-by-side growth mechanism. The highly negative value of enthalpy of mixing in both cases suggests that
there is associative interaction in the molecules in the
eutectic melt. The positive value of excess free energy
(0.0322 and 0.5227 kJ mol-1), for SAM–PNP and CA–
AAP, respectively, indicates the associative interaction
between like molecules. The Jackson’s roughness parameters for binary eutectics suggest that phases grow with
faceted morphology. Microstructural studies of eutectics
have shown the cellular and feather morphologies.
Acknowledgements The authors would like to thank the Head,
Department of Chemistry, B.H.U., Varanasi, for providing the necessary infrastructure facilities.
References
Fig. 4 Directionally solidified optical microphotographs of SAM–
PNP, CA–AAP eutectics (a) and (b), respectively
greater than 2 in all the cases suggests that phases grow
showing facets.
The unidirectionally solidified microstructures of
eutectic of SAM–PNP and CA–AAP systems are shown in
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eutectic melts. The microstructure of eutectic CA–AAP
system shows the feather morphology (Fig. 4b) where the
two phases of eutectic has grown almost together.
123
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