ISSN : 0975-3699
EISSN : 0975-9131
THIRUMARAN S.1*, KARTHIKEYAN N.2
1Department of Physics (DDE), Annamalai University, Annamalainagar-608 002.
2Department of Physics (DDE), Annamalai University, Annamalainagar-608 002.
* Corresponding Author : thirumaran64@gmail.com
Received : 15-04-2011 Accepted : 06-05-2011 Published : 15-12-2011
Volume : 3 Issue : 3 Pages : 83 - 98
Int J Chem Res 3.3 (2011):83-98
DOI : http://dx.doi.org/10.9735/0975-3699.3.3.83-98
Conflict of Interest : None declared
The observed experimental parameters such as density (Ï), viscosity (η) and ultrasonic velocity (U) of ternary liquid mixtures of substituted benzenes such as chlorobenzene, bromobenzene, nitrobenzene, ethylbenzene and toluene added in the fixed binary solvent mixtures of (ratio 4:2) acetophenone and 1-heptanol at 303.15, 308.15 and 313.15K were determined. The observed experimental data have been utilized to evaluate some of the excess thermo acoustical parameters such as adiabatic compressibility (βE), intermolecular free length (LfE), free volume (VfE), internal pressure (Ï€iE), Gibb’s energy (ΔGE), viscosity (ηE) molar volume (VmE)and Grunberg’s Nissan interaction parameter (d). It is eventually observed that weak molecular association between acetophenone and 1-heptanol is through hydrogen bonding and substituted benzenes through weak dipole-dipole interactions. The existence of formation of donor – acceptor complexes in the present study has been identified and promptly discussed. Further, the impact due to the elevation of temperature on these liquid systems has also been interpreted.
adiabatic compressibility, dipole-dipole interaction, hydrogen bonding, internal pressure, intermolecular free length.
In recent years, the measurement of ultrasonic velocity has been adequately employed in understanding the nature of molecular interactions in pure liquids and liquid mixtures. The ultrasonic velocity measurements are highly sensitive to molecular interactions and can be used to provide qualitative information about the physical nature and strength of molecular interaction in the liquid mixtures [1-3] . Ultrasonic velocity of a liquid is fundamentally related to the binding forces between the atoms or the molecules and has been adequately employed in understanding the nature of molecular interaction in pure liquids [4-6] . The variation of ultrasonic velocity and related parameters throw much light upon the structural changes associated with the liquid mixtures having weakly interacting components [7-8] as well as strongly interacting components.
The study of molecular association in organic ternary mixtures having alcohol as one of the components is of particular interest, since alcohols are strongly self-associated liquid having a three dimensional network of hydrogen bond and can be associated with any other group having some degree of polar attractions. Accurate knowledge of thermodyanamic mixing properties such as adiabatic compressibility, intermolecular free length, free volume, internal pressure and molar volume and their excess values for mixtures of protic, non-protic and associated liquids has a great importance in theoretical and applied areas of research. The deviations from ideality and specific or non-specific interactions have been revealed. Alcohols [1-3] and aromatic ketones such as acetophenone [9-10] exist as associated structures in liquid state. Thus, upon mixing of ketones with alcohols provide interesting properties due to specific interactions arising from charge-transfer, dipole-dipole, donor–acceptor and hydrogen bond formation forces may be observed.
The thermodynamic properties of systems containing acetophenone are helpful to better understanding the molecular interaction and to design and simulate the different process of separation [11] . Acetophenone is typical aromatic ketone has been used in perfumery and fragrances. 1-heptanol is one of the primary alkanols with a seven carbon chain and it is a clear colourless liquid, slightly soluble in water, but miscible with ether and ethanol. It is commonly used in cardiac electrophysiology experiments. Nitrobenzene have higher dipole moment and dielectric constant values than those of chlorobenzene and bromobenzene. Nitrobenzene is supposed to be a relatively complex molecule and its non-ideality in all probability may be due to the polarity arisen out of nitro-group is concerned, it rotates freely along the C-N axis where it likely to give more flexibility to the interactions arising due to the two highly polar N®O bonds. Chlorine atom in chlorobenzene is an electron-withdrawing atom, tends to attract to p-electrons of the benzene ring, thereby, decreases the electron density of ring. As a result, the benzene ring in chlorobenzene becomes relatively poor electron-donor towards the electron-seeking proton of any groups. Chorobenzene has low dielectric constant e’= 5.649 and dipole moment m=1.69D. Its boiling point and melting points are 132ºC and -45ºC respectively. Chlorobenzene is neither acidic nor basic colourless liquid with a pleasant smell. Chlorobenzene is insoluble in water and soluble in alcohol, benzene and ether. Chlorobenzene is more reactive because the chlorine atom is bonded with SP3 hybridized carbon atom and consequently can be removed easily. So, the rate of reaction with chlorobenzene is faster. Bromobenzene is less reactive because of its double bond character between carbon and bromine atom. The slow reactions in case of bromobenzene may also be attributed to its being heavier. Thus, the molecular interaction is likely to be more affected resulting in a greater degree of variation with respect to the ultrasonic related parameters.
Ethyl benzene, which is one aromatic hydrocarbon, is important in the petrochemical industry as an intermediate in the production of styrene, which in turn is used for making polystyrene, a common plastic material. Toluene, which is also one of the aromatic hydro carbons – also known as mono-substituted benzene, i.e. one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group. It is widely used as a solvent and also used as an inhalant drug for its intoxicating properties. Toluene is an important organic solvent, but is also capable of dissolving a number of notable inorganic chemicals such as sulphur.
Owing to these physical characteristics of substituted benzenes, ketones and primary alcohols which motivated the authors to carry out the present study of molecular interactions of ternary liquids mixtures of substituted benzenes with binary solvent mixtures of acetophenone and 1-heptanol (in the ratio of 4:2) at different temperatures. Though, a number of workers have studied the behaviours of substituted benzenes with alcohols [12-13] , almost no systematic study was carried out the combined effect of ketones with alcohols as binary solvents in which substituted benzenes were added as solutes on varying temperatures.
The present ternary liquid systems taken up by the authors at different temperatures, say at 303.15, 308.15 and 313.15 K are
System I acetophenone + 1-heptanol + chlorobenzene
System II acetophenone + 1-heptanol + bromobenzene
System III acetophenone + 1-heptanol + nitrobenzene
System IV acetophenone + 1-heptanol + ethylbenzene, and
System V acetophenone + 1-heptanol + toluene
In the present work, we have used chemicals, which are analytical reagent (AR) and spectroscopic reagent (SR) grades of minimum assay of 99.9% obtained from E-Merk, Germany and Sd fine chemicals, India. The purities of the above chemicals were checked by the density measurements at 303.15, 308.15 and 313.15K and were compared with available literature values [14-25] . The ternary liquid mixtures of different known composition were prepared by mole fraction basis. The density, viscosity and ultrasonic velocity were measured as a function of the ternary liquid mixture of substituted benzenes which were added to the binary mixtures of acetopheneone and 1-heptanol at 303.15, 308.15 and 313.15K. For this purpose, binaries with fixed ratio X1/X2 = 4:2 respectively were prepared by volume. The density was determined using a specific gravity bottle by relative measurement method. The weight of the sample was measured using a electronic digital balance with an accuracy of ± 0.1 mg (Model: Shimadzu, Japan Make, AX-200). An Ostwald’s viscometer (10ml) was used for the viscosity measurement. Efflux time was determined by using a digital chronometer to within ± 0.001s. An Ultrasonic Interferometer having a frequency of 2 MHz (Mittal Enterprises, New Delhi. Model: F-81) has been used for velocity measurement. The overall accuracy in the velocity measurement is ±2ms-1. An electronically digital operated constant temperature water bath (RAAGA Industries, Chennai) has been used to circulate water through the double walled measuring cell make up of steel containing the experimental solution at the desired temperature. The accuracy in the temperature measurement is ± 0.1K.
Various acoustical and thermodynamical parameters are calculated from the measured data such as Adiabatic Compressibility
(1)
Intermolecular free length
(2)
where K is a temperature dependent constant. Its values are 631×10–6, 636×10–6 and 642×10–6 respectively at 303.15, 308.15 and 313.15K.
Free volume
(3)
where Meff is the effective molecular weight (Meff = mixi, in which mi and xi are the molecular weight and the molefraction of the individual constituents respectively). K is a temperature independent constant which is equal to 4.28 × 109 for all liquids.
Internal Pressure
(4)
where b is the cubic packing which is assumed to be ‘2’ for all liquids and solutions, is the viscosity, R is a gas constant and T absolute temperature.
The Gibb’s free energy can be estimated from the following relation.
(5)
Where K is the Boltzmaan’s constant (1.23 × 10–23 JK–1), T the absolute temperature, ‘h’ the Planck’s constant and is the relaxation time
.
Excess values of the above parameters can be determined using
AE = Aexp - Aid (6)
where Aid = Ai Xi, Ai is any acoustical parameters and Xi the molefraction of the liquid component. Grunberg [26] and Nissan formulated the following relation between the viscosity of a binary liquid mixture and pure components.
ln ηmix = X1 ln η1 + X2 ln η2 + X1 X2 d (7)
On applying to a ternary liquid mixture, this equation takes up the form
ln ηmix = X1 ln η1 + X2 ln η2 + X3 ln η3 + X1 X2 X3 d (8)
where d is an interaction parameter regarded as a measure of the strength of molecular interactions between the mixing components.
The [Table-1] and [Table-2] provide the experimentally determined values of density (Ï), viscosity (η) and ultrasonic velocity (U) of pure liquids and for the ternary systems at 303.15, 308.15, and 313.15 K. The excess values of adiabatic compressibility (βE), free length (LfE), free volume (VfE), and internal pressure (iE) are tabulated in [Table-3] . The excess viscosity (ηE), excess Gibbs Energy (∆GE), excess molar volume (VmE) and Grunberg and Nissan’s interaction parameter (d) are listed in [Table-4] .
It is seen that in all the five ternary liquid systems, the density (Ï) and the ultrasonic velocity (U) increases with increasing concentrations of substituted benzenes, such as chlorobenzene, bromobenzene, nitrobenzene, ethyl benzene and toluene, whereas the viscosity (η) shows the decreasing trend. These are tabulated in [Table-2] . The variation of ultrasonic velocity in a mixture depends upon the increase (or) decrease of intermolecular free length after mixing the components. On the basis of a model, for sound propagation proposed by Eyring and Kincaid [27] , ultrasonic velocity should decrease, if the intermolecular free length increase and vice-versa. This is in fact observed in the present investigation for all the five liquid systems.
The thermodynamic excess properties are found to be more sensitive towards intermolecular interaction between the component molecules of liquid mixtures. The sign and magnitude of deviation of excess properties depend on strength of interaction between unlike molecules. That is in order to understand the nature of molecular interactions between the components of the liquid mixtures, it is of interest to discuss the same in term of excess parameter rather than actual values. Non-ideal liquid mixtures show considerable deviation from linearity in their physical behaviour with respect to concentration and these have been interpreted as arising from the presence of strong or weak interactions [28] .
The perusal of Table 3 exhibits the values of excess adiabatic compressibility (βE) for all the five liquid systems. The negative values of βE is associated with a structure-forming tendency, while positive values are an indication of structure-breaking tendency due to hetero-molecular interaction between the component molecules of the mixtures. The positive values of excess adiabatic compressibility which indicates the loosely packed molecules in the mixtures resulting due to shape and size. From [Table-3] , it is evident that the excess adiabatic compressibility are all almost negative in all the five ternary liquid system, except in systems I and II, where a slight positive deviations are seen in lower mole fraction range (See [Fig-1] (a,b)) and becomes negative on going to the higher mole fraction range. The negative values of the βE decrease with increasing concentrations of substituted benzenes in all the five liquid systems as well as increase of temperature. This may be ascribed as that in acetophenone, the carbonyl group is highly polar and hence has a high percentage of ionic character [29] and there exists is a negative charge on the carbonyl oxygen atom of acetophenone. Hence, the decreasing trend of negative values will make one would expect a weak interactions with substituted benzenes such as chlorobenzene, bromobenzene, nitrobenzene, ethyl benzene and toluene due to weak dipole-dipole interactive forces. Also, in the case of substituted benzenes, the bromine, chlorine and nitrogen all having a tendency to act as an electron acceptors. The decrease in adiabatic compressibility may further be attributed to the internal interactions between -electrons of C = O bond and
-electrons of the benzene ring [30] . A plausible explanation may be that the oxygen atom of carbonyl group of acetophenone being strongly electronegative would act as a good electron acceptor towards the
-electrons of the aromatic ring, forming donor-acceptor complexes [31] . It is observed that decreasing negative deviations in βE increasing with molar concentrations seems to be an indication of weak molecular interaction between unlike molecules increases. Such decreasing negative values of βE may also due to the dominance of dispersive interaction forces resulting from the breaking up of the hydrogen bonds of substituted benzenes makes the compressibility to decrease. The magnitude of the βE values are of the order: Systems III > I > II > IV > V.
The perusal of [Table-3] shows the values of excess free length (LfE) for all the five ternary liquid systems. It was noticed that the LfE values are negative in all the five liquid systems and found to be decreased with increasing molar concentration of substituted benzenes and elevation of temperature, except in systems I, II, and III, where a slight positive deviations are seen in lower mole fraction range, and however such positive deviation are found to be decreasing and becomes negative on going to the higher mole fraction range (See [Fig-2] (a,b)). According to Kannappan et al. [32] , the negative values of LfE indicate that sound waves cover long distances due to decrease in intermolecular free length describing the dominant nature of hydrogen bond interaction between unlike molecules. Fort and Moore [33] indicated that the positive excess values of free length should be attributed to the dispersive forces and negative excess values of should be due to charge transfer and hydrogen bond formation. In the present investigation, one can notice that the decreasing trend of negative excess values of intermolecular free length (LfE) with the increasing concentration of substituted benzenes as well as increase of temperature may be interpreted as aromatic ketones such as acetophenone which is being a highly polar molecule (μ = 2.96) and it may enhance the polarities of the substituted benzenes. The
- electron density in derivatives of benzene ring depends upon that is attached to it. The heteromolecular interaction between component molecules necessarily depends upon the net electron density in the ring. Further, these interactions seemed to depend on relative orientation of the two groups in the ring. As the separation between the two groups increases, the intermolecular interaction is expected to decrease. Hence, a decrease in excess free length (Lf) is noticed. The magnitude of the Lf E values are in the order: Systems III > I > II > IV > V.
The perusal of [Table-3] provides a qualitative picture of excess free volume (VfE) values for all the five ternary liquid systems. The present parameter indicates the extent of deviation from ideal with the mole fraction of the mixtures. The excess values for all the five ternary liquid systems found to be negative. These values found to be decreased on increasing the molar concentration of substituted benzenes as well as elevation of temperature. The results can be explained in terms of molecular interaction, structural effect and interstitial accommodation along with the changes in free volume. The sign of the VfE depends on the relative strength between the contractive forces and expansive forces. The factors responsible for volume contraction are (i) specific interactions between the component molecules and (ii) weak physical forces, such as dipole-dipole or dipole-induced dipole interactions or Vanderwaal’s forces. The factors that cause expansion in volume are dispersive forces, steric hindrance of component molecules, unfavourable geometric fitting and electrostatic repulsion. The negative values of excess free volume in all the systems assert that the combined effect of the factors responsible for volume contraction and vice-versa [34] . Adgaonkar et al. [35] showed positive values of VfE indicating the existence of weak molecular interactions in the liquid mixtures and the negative values of excess free volume suggesting the specific interactions among unlike molecules.
In our binary mixture solvent, 1-heptanol, a primary alcohol also a polar one which is in association with acetophenone forms hydrogen bonding. Grace selvarani et al. [36] had made it clear that the molecular association is due to the hydroxyl group of 1-alkanol and acetophenone. According to them, aromatic ketones such as acetophenone are more associated with alcohols than the aliphatic one. Since alkanols are liquids which are associated through hydrogen bonding and in the pure state, they exhibit an equilibrium between multimer and monomer species. Further, when the substituted benzenes such as Nitrobenzene is mixed with 1-heptanol, the NO2 group can interact with OH group [37,38] . The aromatic derivatives set up on interaction between the p-electrons cloud and the hydroxyl group. Though, this interaction is of minor intensity compared with hydrogen bonding, but they may lead to formation of intermolecular complexes [39] .
In the study of liquid mixtures, the variation of internal pressure may give some suitable information regarding the nature and strength of the forces existing between the molecules. In fact, the internal pressure is a broader concept and it is a measure of the totality of forces of the dispersion, ionic and dipolar interaction that contribute to be overall cohesion of the liquid systems. The present study shows that the excess internal pressure values (πEi) are negative in all the five liquid systems. Such negative values of πEi are increasing on increasing the molar concentration of substituted benzenes indicate that only dipolar forces are operating between the unlike molecules. Further, the observed increasing behavior of excess internal pressure values reveals that the weakening of cohesive forces resulting perhaps due to making up of the structure of the solvent.
One should observe from the [Table-4] , the variation of excess Gibbs energy ∆GE for the five ternary liquid systems. The values of ∆GE are all negative in all the systems concerned and increase with molar concentration of substituted benzenes (See [Fig-3] (a,b)). According to Read et al. [40] , the positive values of excess Gibbs energy values may be attributed to specific interactions like hydrogen bonding and charge transfer, while negative ∆GE values may be ascribed to the dominance of dispersion forces [41] . In the present investigation, our close observation suggest that the increasing negative values of ∆GE shows that the strength of interaction gets weakened with increasing of substituted benzenes. Further, the negative values of ∆GE indicate the easier flow of ternary mixture compared with the behaviour of pure components [42] .
According to Fort et al. [33] the variation of excess viscosity (ηE) gives the strength of molecular interaction between the molecules. For systems, where dispersion, induction, and dipolar forces which are operated by the values of excess viscosity are found to be negative, whereas the existence of specific interactions leading to the formation of complexes in liquid mixtures tends to make excess viscosity positive. In the present study, from [Table-4] the excess viscosity are negative in all the five ternary liquid systems and increase with the further addition of substituted benzenes as well as elevation of temperature. Hence, our present close observation of excess viscosity reveals that presence of weak dipolar forces are existing in the liquid mixtures. The increasing negative values of ηE further suggests that the dominance of dispersive interaction resulting from the breaking up of the hydrogen bonds of substituted benzenes make the mixture to flow more easily. Further, the strength of interaction get weakened in view of rise in temperature due to the presence of thermal dispersion forces in the liquid mixtures. The strength of the molecular interactions among the liquid systems by making use of the magnitude of the ηE values are of the order: Systems III > I > II > IV > V.
The perusal of [Table-4] exhibits the excess molar volume (VmE) are all negative over the entire range of concentration range for the mixtures of acetophenone and 1-heptanol and substituted benzenes. According to VmE data for ternary mixtures of acetophenone and 1-heptanol with substituted benzenes depend upon the balance between two opposing contributions [43,44] .
(a) A positive term from the rupture of alkanol-alkanol hydrogen bonds and physical dipole-dipole interactions between alkanol monomers and multimers, and
(b) A negative term from the formation of the OH π-electron hydrogen bonded complexes, changes of free volume and interstitial accommodation.
The experimental data in the present investigation suggest that factor (b) which is responsible for the negative excess volume is dominant over the entire composition range in the mixtures of acetophenone and 1-heptanol with substituted benzenes. On the other hand, both the factors, (a) and (b) compete with each other to varying degrees in the mixtures which contain benzene and toluene as non- common components. According to Awwad et al. [45] measured excess molar volume obtained in the present study can be qualitatively explained as arising to due to differences in the sizes of the component molecules and in the interactions between them. Differences are mainly due to negative contributions arising from changes of free volume in the real mixtures, comprising alkanol monomers and multimers and acetophenone, benzene and substituted benzene molecules. The experimental data in the present study suggest that the factors which account for the existence of dipole-dipole interactions present in the liquid mixtures. The decreasing trend of negative values of VmE for the further addition of substituted benzenes in all the five liquid systems clearly predicting this.
The interaction parameter d in the Grunberg and Nissam equation is a measure of strength of interaction between the mixing components. d-values were said to indicate various type of interactions [46] . Large and positive d-values indicate strong specific interaction, small positive values indicate weak specific interaction and large negative values indicate no specific interaction. It is evident from [Table-4] . The d-values are negative (except in system – III) and decrease with increasing of molefraction X3, and the same varies non-linearly with the rise of temperature. Further, the decreasing behavior of d-values exhibits weaker interaction between the unlike molecules, and also their non-linear behaviour towards the rise of temperature which may suggest the weakening of intermolecular interaction due to thermal dispersion forces.
Ultrasonic method is a powerful probe for characterising the physico-chemical properties and existence of molecular interactions in the liquid mixtures. The evaluated ultrasonic excess values of ternary mixtures of substituted benzenes in the binary solvent mixture of acetophenone and 1-heptanol (in the fixed ratio of 4:2) at 303.15, 308.15 and 313.15K which may eventually concluded that there exist a molecular interaction between the acetophenone and 1-heptanol is through hydrogen bonding and with substituted benzenes through dipole-dipole interactions. The strength of the molecular interactions gets weakened on further addition of substituted benzenes as well as rise in temperature. Weak dipolar and cohesive forces are existing in the present systems of liquid mixtures. Our present study also predicts the formation of donor-acceptor complexes in the component mixtures. Further, the elevation of temperature in the liquid mixtures lead to the weakening of molecular interactions due to weak intermolecular forces and thermal dispersion forces. From the magnitude of some of our excess [] LfE and ηE] which confirms the present molecular interaction is of the order: Systems III > I > II > IV > V.
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 | Fig. 1- a & b. Excess adiabatic compressibility Vs mole fraction |
 | Fig. 2- a & b. Excess free length Vs mole fraction |
 | Fig. 3- a & b. Excess Gibb's free energy Vs mole fraction |