Chapter I

General Introduction and Literature Review

GENERAL INTRODUCTION AND LITERATURE REVIEW

Liquid mixtures are indispensable in almost all industries and all biological

sciences. There exists an imperative need to understand these systems and to be

able to predict their behavior from the molecular point of view. The use of

measurable macroscopic or global quantities to probe into the microscopic, or the

local properties of the system has become an essential area of research. The types

of properties probed by these tools are local composition, local change of order, or

structure etc. The traditional characterization and study of the properties of liquid

mixtures by means of the global excess thermodynamic functions has become

handy as it provides richer and more detailed information on the immediate

environment of molecules in the mixture.

1.1 Molecular Interaction Studies

Molecular studies lie in the ability to assess the information stored in the

structure of molecule as a function of their physical and chemical properties. There

had been many developments in Chemistry and Physics during the 19th and 20th

centuries particularly in the fields of statistical mechanics, thermodynamics and the

nature of chemical bond. The ensuing atomistic view shall be presented and

discussed in the context of molecular interactions. Molecular interactions are

generally electrostatic in nature. The strength of these interactions and the forces

among atoms, can be analyzed according to their thermodynamic and kinetic

behaviour1.

1.1.1 Types of Molecular Interactions

There exist a wide variety of physical interactions relevant to the structure

and function viz., attractive or repulsive electrostatic interactions like ion-ion, ion-

dipole, dipole-dipole, dipole-induced dipole, induced dipole-induced dipole,

hydrogen bonded and hydrophobic interactions. The strength of intermolecular

interaction determines the physical properties of a substance2. These weak forces

are responsible for the secondary structure of biomolecules like carbohydrates,

proteins, nucleic acids and lipids and these forces are also responsible for the

specific biological functions3. Physical interactions can be either long range non-

specific or short range specific interactions. The long range interactions are the

2

first to control the repulsion or attraction between the molecules thereby

determining whether short range biophysical interactions come into play4.

Ion-ion interaction exists between point charges whereas ion-dipole

interaction is a force of attraction between a point charge and dipolar molecule.

The strength of these interactions determines the solubility of ionic and polar

compounds. Van der Waals forces between atoms and molecules are responsible

for the condensation or freezing even when there is no covalent or ionic bond

present between the atoms. Van der Waals interactions are categorized as weak

forces and they are relatively short range forces. The strength of interaction varies

inversely as the sixth power of distance between the molecules. Van der Waals

interaction represents three types of interactions namely orientation interaction

between two permanent dipoles, induction interaction between permanent and

induced dipoles and London dispersion interactions. The relative contribution of

each of these interactions to the total forces depends on the type of molecule. The

energy associated with van der Waals interaction is less than 1 kcal mol-1.

Hydrogen bond is a bond formed between two electronegative atoms with

hydrogen atom acting as a bridge. A hydrogen atom has only one ‘s’ orbital, which

becomes saturated after the formation of one covalent bond thus making it

incapable of forming a second covalent bond. However, the positive hydrogen can

polarize the lone pair orbital of another atom, delocalize the lone pair of electrons

and there by form a weak long covalent bond. Thus hydrogen bond is a simple

case of dipole-dipole (or ion-dipole) attraction. Hydrogen bonds have also been

found to exist between a carbon atom and an electronegative atom provided there

are some electronegative atoms attached to the carbon atom to activate the

hydrogen atom.

H - bonds have the following special features:

1. These are weak in nature, for example, they have energy of only ~ 2-

10 kcal mol-1 where as an ordinary covalent bond has energy of

about 80-100 kcal mol-1.

2. Increase in electronegativity of an atom increases its power of

forming hydrogen bonds.

3

3. Hydrogen ion in a hydrogen bond is not centrally situated but is

nearer to one of the two adjacent electro negative atoms5.

Although the hydrogen bond is not a particularly strong bond it has great

significance in determining the properties of substances. Because of its small bond

energy and the small activation energy involved in its formation and dissociation,

the hydrogen bond is expected to play an important part in reactions occurring at

normal temperatures. For example, the hydrogen bond is believed to be responsible

for the retention of the native configuration of protein molecules. Since it plays

such an important role in a number of different phenomena much work has been

carried out in recent years with the hope of the eventual elucidation of the nature of

the hydrogen bond6.

A variety of methods have been employed in the investigation of the

formation, strength and structure of hydrogen bonds. Among various methods,

physico-chemical and spectroscopic methods are widely used to study the

hydrogen bonds in organic liquids. Since the hydrogen bond is largely responsible

for the interaction between molecules of associated liquids, a comparison of the

properties displayed by substances capable of forming hydrogen bonds with those

of closely related compounds in which hydrogen bonding takes place unlikely

requires a qualitative understanding of the bonding. Hydrogen bonds and van der

Waals interactions are categorized as weak forces. Substances in which

intermolecular hydrogen bonds exist are usually associated and it influences heat

of vaporization, melting and boiling points and to some extent the dielectric

constants7, acidic strength basic strength reactivity of organic compounds8. The

hydrogen bonding is weaker than covalent bond but stronger than van der Waals

interactions9.

1.1.2 Molecular Interactions in Liquid Mixtures

The phenomenon of preferential interaction between molecules, between

unlike molecules, solute-solvent and similar ones, solute-solute is observed

depending on the degree of affinity between components. When solute dissolves in

the solvent, the structure of both the components may change due to reorganization

of the component molecules and the phenomenon of solvation and association are

4

observed depending on the nature of solvent. The molecular interaction studies in

solution provide information regarding the internal structure, molecular

association, complex formation and internal pressure.

1.2 Different Investigating Techniques & Literature Survey

The molecular interactions are investigated by employing different

techniques by many researchers. The nuclear magnetic resonance10,11,

microwave12,13, ultraviolet14,15, infrared spectroscopy16,17, X-ray18, viscometric19,20,

refractive index measurement21 and neutron scattering studies22 are frequently

employed techniques to identify the molecular interactions; the NMR technique

reflects the effect on the proton bearing molecule, the Microwave absorption and

refractive index measurement provide information through the dielectric constant,

the X-ray and neutron scattering help in the study of molecular motion, the

spectroscopic techniques provide information on molecular interactions. Weak

molecular interactions cannot be resolved from the observed spectra. Thus, the

molecular interactions cannot be identified thoroughly by a single technique.

However, the ultrasonic technique is a simple, non-destructive and low cost

technique that reveals successfully the inter-molecular interactions.

1.2.1 Ultrasonic Investigation of Molecular Interactions

Among various experimental techniques developed to obtain information on

the nature and strength of intermolecular interactions in dilute solutions, ultrasonic

technique has established itself as a promising tool. The propagation, dispersion

and attenuation of ultrasonic wave in a medium are intimately connected with the

structural aspect of the medium. The successful applications of acoustical methods

to physico-chemical investigation of the solutions become possible after the

development of adequate theoretical approaches and methods for precise ultrasonic

velocity measurements in small volume of liquids. In the year 1945 Lagemann23

successfully used the sound velocity approach for qualitative estimation of the

interactions in liquid mixtures. The measurement of the ultrasonic speed enables

the accurate determination of some useful acoustic and thermodynamic parameters

which are highly sensitive to molecular interactions thus provide physical nature

and strength of intermolecular interactions in liquid mixtures.

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1.2.1.1 Literature Review on Ultrasonic Investigation of Molecular Interactions

in Liquids and Liquid Mixtures

Ultrasonic technology finds many applications in the fields of Chemistry,

Physics, Biology and Medicine. Ultrasonic measurements are very useful in

chemical and food processing, material testing, under water ranging and cleaning.

Ultrasonic vibrations are commonly employed in mechanical machinery of

materials24, preparation of colloids or emulsions, the pre-germination of seeds,

imaging of biological tissues and non-destructive testing (NDT). Ultrasonic AFM

(Atomic force microscope) can improve fabrication technologies on nanometer

scale25. Ultrasound eliminates friction at a nanometer scale26.

A study of ultrasonic propagation provides an important means of probing

the liquid state, as different molecular processes taking place in liquids can

influence directly or indirectly, the propagation constant of sound wave in the

medium. A wealth of data accumulated on this subject has appeared in excellent

reviews by Hertzfeld and Litovitz27, Nozdrev28, and also in the volumes edited by

Mason29, Flugge30, Bergmann31, Beyer and Letcher32. Ultrasonic investigations

find extensive application in characterizing physicochemical behaviour of liquid

mixtures. Tumikoski and Nurmi33 Fort and Moore34,35, Flory and co-worker36,37

have studied the non-ideal behaviour of binary liquid mixtures.

Ultrasonic methods are important to study the molecular association,

dissociation and complex formations. The intermolecular forces of liquids in a

liquid mixture show considerable effect on the physical and chemical effects. From

the knowledge of ultrasonic velocity, density and viscosity of a liquid, various

acoustic parameters such as intermolecular free length, isentropic compressibility,

free volume and internal pressure etc., can be obtained. In an ideal mixture in

which the components are non-interacting, the variation of density, viscosity and

velocity with the concentration is expected to be linear. Addition of another liquid

changes the values depending upon the nature of components. The ultrasonic

velocity and isentropic compressibility reflect the degrees of deviation from

ideality. Deviations from ideal behaviour have been widely used for the study of

structural variations and molecular interactions of the mixtures. These studies also

6

yield information about changes with composition in the packing efficiencies that

take place in solution during the mixing process38-41.

Schaaff16 introduced sound velocity in van der Waal’s equation of state and

obtained a formula for molecular radius from sound velocity and density values.

He compared the molecular radius calculated from sound velocity and density with

the value evaluated from molecular refraction. It was found that the molecular

radius obtained from sound velocity and density represents a very useful measure

for size of molecules. In another paper Schaaff17 derived an improved formula for

molecular radius by replacing the isothermal sound velocity with adiabatic sound

velocity.

Ultrasonic studies have been carried out to find out ion-solvent interactions

in electrolytic solutions containing different electrolytes and found that ultrasound

speed in these solutions depend on the size of the ion, polarity of the solvent and to

some extent on the electronic configuration of the transition metal ions. Ultrasound

has been extensively used to determine the ion-solvent interactions in aqueous and

non-aqueous solutions containing electrolytes42-48.

Number of studies has been focused on the ultrasonic properties of polymer

solutions49-51. The degree of rubber blends52 and other polymer blends53,54 by

ultrasonic technique is reported. The glass transition temperature, an important

parameter of polymers, is reported by Shanthi for some synthetic polymers55.

Soap–solvent interactions in soap solution are investigated in view to

determine critical micelle concentration (CMC) and action of mixed surfactants by

several authors using acoustical investigation56,57. In recent years, the ultrasound

technique is used to detect the charge transfer complexes in binary and ternary

liquid mixtures by nonlinear variation in the ultrasonic speed and other acoustical

parameters as a function of composition58,59.

1.2.2 Excess Thermodynamic and Viscometric Investigation of Molecular

Interactions in Binary Liquid Mixtures

Fundamental thermodynamic and viscometric properties are essential

sources of information necessary for a better understanding of the non-ideal

behaviour of complex systems because of physical, chemical and geometrical

7

effects, which are caused by molecular interactions, intermolecular forces, etc., of

unlike molecules. From a practical point of view, these properties are necessary for

the development of thermodynamic models required in adequate and optimized

processes of the chemical, petrochemical, pharmaceutical, and other industries. In

addition, extensive information about structural phenomena of mixtures is essential

for the development of theories of the liquid state and predictive methods.

The excess thermodynamic functions introduced by Scatchard in the year

1931, provided a way to represent directly the deviation of solution from ideal

behaviour. The excess functions are very useful in understanding molecular

interactions between components of liquid mixtures. By definition, the excess

function E idY Y Y represents the excess of a given quantity Y of a real

mixture over its value for an ideal mixture idY at the same conditions of pressure,

temperature, and composition60.

Various theories of solutions were developed to predict the properties of

liquid mixtures composition and independently observable properties of pure

components. These theories were formulated to account for the departure of a real

solution from the ideal behaviour. Theories concerning excess volumes of binary

liquid mixtures were thoroughly reviewed and discussed by Rowlingson61-63,

Flory64, Scott and Fenby65, Hijmens and Holleman66, Baattino67, Kehiaian68, Handa

and Benson69.

Van der Waals70 and van Laar71 proposed the initial theories of binary liquid

mixtures. These theories successfully explained certain excess properties in critical

region of liquid mixtures. In an attempt to improve van Laar’s theory, Hildebrand

and Scott72 and Scatchard73-76 used Hildebrand’s77 concept of regular solutions to

formulate a relation for excess volume. Priogogine et al78 have used the cell model

to extend the theory of corresponding states to chain molecules. In this approach

the chain molecules were considered as a series of quasi-spherical segments.

Eyring79 developed the cell model, which was extensively used by Lennard-Jones

and Devonshire80,81 and Prigogine et al82. This model relates the thermodynamic

properties of liquid mixtures to the intermolecular energy parameters.

8

Redlich and Kister83 proposed an empirical equation to predict EY values

for binary mixtures.

1

12121 )(

j

j

E

cal xxAxxY

where Aj-1 are adjustable parameters.

In general, the excess thermodynamic functions such as excess molar

volume, excess compressibility, excess internal pressure, excess free volume and

excess Gibb’s free energy are much useful in understanding the nature of

molecular interaction in binary and ternary liquid systems. Extensive studies of the

ultrasonic velocity in liquids and liquid mixtures and their interpretation in the

light of molecular structure were also made by several investigators84-89. The study

of the ultrasonic velocity and their interpretation in the molecular structure was

made by several workers90-95. From the study of ultrasonic velocity measurements

in a large number of organic liquids, the following general conclusions were drawn

about the dependence on molecular structure:

1. Liquids having higher density give lower ultrasonic velocity but not

necessarily in proportion.

2. Long molecules generally give rise to higher velocity even though their

density is higher.

3. Aromatic compounds have usually higher velocities than the aliphatic

compounds even though the density of aromatic compounds is higher.

4. A double bond of unsaturation is found to be resulting in low velocity.

5. Polar molecules have higher velocities in alcohols, ketones, acetic

anhydride, nitrobenzene, aniline, acetophenone, cyclo hexanol and

water.

6. A decrease in velocity is observed by substitution of a heavier atom in

place of lighter atom.

7. In non-polar groups, the viscosity of amines and alcohols will not differ

much and increase with increase in chain length.

8. In isomeric amines, the branched amines exhibit greater viscosity than

those of the straight chains.

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1.2.2.1 Previous Work on Excess Thermodynamic, Thermo-Acoustic and

Viscometric Investigation of Molecular Interactions in Binary Liquid Mixtures

Positive excess molar volumes and negative deviations in isentropic

compressibilities are reported by Syamala et al96 in the binary mixtures of dimethyl

sulphoxide with chloro and nitro substituted aromatic hydrocarbons. The viscosity

data is analyzed on the basis of corresponding states approach and the positive

deviations in viscosity explain the intermolecular interactions between the unlike

molecules in these mixtures.

Gadzuric et al97 calculated the isobaric thermal expansion coefficients,

partial molar volumes, apparent molar volumes, partial molar excess volumes and

excess thermal expansions for the binary liquid mixtures of N-ethylformamide

with tetrahydrofuran, 2-butanone and ethyl acetate over the entire composition in

the temperature range 293.15 to 313.15 K.

Sastry et al98,99 analyzed the effect of hydrophilic additives on volumetric

and viscosity properties of amino acids viz., glycine, l-valine, l-phenylalanine,

l-leucine, and l-aspargine in aqueous solutions of sucrose, urea and 1,3-butanediol

from 283.15 to 333.15 K and suggested that the solute–co-solute interactions are

more favored at elevated temperatures and in presence of high concentration of

sucrose. Otherwise the hydrophobic side chains facilitate the solute–solute

interactions and also time induced hydrophobic hydration in the bulk water.

To discover the nature and type of bulk state interactions present in the

binary mixtures of Alkyl (Methyl, Ethyl, Butyl, and Isoamyl) Acetates + Glycols

Satry et al99 determined densities, excess molar volumes, viscosities, speeds of

sound, excess isentropic compressibilities, and relative permitivities at different

temperatures. Further, they have correlated the experimental viscosities and

ultrasonic speeds with several theoretical models viz., Grunberg-Nissan,

McAllister, Auslander and collision factor theory etc.

Isabel et al100 reported the ultrasound speeds and molar isentropic

compressions of aqueous 1-propoxypropan-2-ol mixtures from 283.15 to 303.15 K

probing the role of branching and chain length on mixing and especially on

aggregation patterns. Kondaiah et al101 determined the ultrasonic velocities,

10

densities, and excess molar volumes of binary mixtures of N,N-dimethyl

formamide with methyl acrylate, or ethyl acrylate, or butyl acrylate, or 2-ethyl

hexyl acrylate at 308.15 K and published the positive values of VmE indicating the

presence of dispersion forces between the DMF and acrylic ester molecules.

Narendra et al102 investigated the excess parameters of binary mixtures of

anisaldehyde with o-cresol, m-cresol and p-cresol at 303.15, 308.15, 313.15 and

318.15 K. The negative and positive values of deviation or excess thermo-acoustic

parameters observed have been explained on the basis of the intermolecular

interactions present in these mixtures. Sangeeta et al103 measured experimental

densities, speeds of sound, and refractive indices of the binary mixtures {1-butyl-

3-methylimidazolium methylsulphate and methanol, or 1-propanol, or 2-propanol,

or 1-butanol} over the whole range of composition at T = 298.15, 303.15, 308.15,

and 313.15 K. From the experimental data, excess molar volumes, excess

isentropic compressibilities, deviation in refractive indices and molar refractions

were calculated. The Lorentz–Lorenz equation was applied to correlate the

volumetric properties and predict the density or the refractive index of the binary

mixtures. For all the systems studied, the excess molar volume and excess

isentropic compressibility are negative, while the change in refractive index on

mixing is always positive over the entire composition range and at all

temperatures.

The short-range dipolar interactions which lead to structural changes in

water - methyl, ethyl, n-propyl alcohol binary systems at 298.15 K are reported by

Hulya et al104. A comparative study of excess thermodynamic properties and

deviations in the thermodynamic properties in the binary mixtures using different

approximations is carried out by several researchers105-106. Anjali Awasthi et al107

studied the acoustic, volumetric, and spectroscopic properties of formamide with

2-alkoxyethanols at different temperature and reported the presence of extensive

hydrogen bonding between oxygen atom of CO group of formamide and hydrogen

atoms of the H–O group of 2-alkoxyethanol molecules in these binary liquid

mixtures.

11

The density and speed of sound of the binary mixtures of ethylenediamine

(EDA) with alcohols (1-hexanol, 1-octanol, and 1-decanol) were measured by

Dubey et al108 from 293.15 to 313.15 K and viscosity from 298.15 to 308.15 K

over the entire composition range and at atmospheric pressure. Using the

experimental values of density, viscosity and speed of sound, the excess molar

volume, viscosity deviation, deviation in speed of sound, deviation in isentropic

compressibility were calculated and fitted to the Redlich–Kister type polynomial

equation. Further, the viscosity data was analyzed in terms of Heric−Brewer and

McAllister models. Gonzalez et al109 enlightened the thermodynamics of (ketone +

amine) mixtures considering volumetric and speed of sound data at 293.15, 298.15,

and 303.15 K for (2-heptanone + dipropylamine or dibutylamine or triethylamine)

systems and indicated that structural effects increase with the ketone size in

mixtures with a fixed amine. They concluded that (i) interactions between unlike

molecules are more easily created in solutions containing shorter amines and (ii)

this effect predominates over the disruption of the amine–amine interactions.

These general trends were confirmed by the treatment of the mixtures using the

PFP theory, and the internal pressure concept.

Specific interactions between unlike molecules through hydrogen bonding

and dipole-dipole interactions between unlike molecules in the mixtures of 2-

chloroaniline or 3-chloroaniline with diisopropyl ether or oxolane are reported by

Pandiyan et al110 at different temperatures. Further, they have estimated the speed

of sound in these mixtures using several empirical and theoretical models to

determine their relative predicting ability in terms of pure component properties.

The values of ultrasonic velocity, density and viscosity measured by

Parveen et al111 in binary mixtures of tetrahydrofuran with methanol and o-cresol

at different temperatures along with estimated values of various thermo-acoustic

parameters suggest the existence of hydrogen bonding between unlike molecules

in all the mixtures studied and dispersive forces in therhydrofuran and methanol

mixture. An estimation of physical properties made by different theoretical

procedures, due to the strong dependence of the adequate industrial design on

12

computation and simulation, are found to be in good agreement with experimental

values.

Density, speed of sound, viscosity, and surface tension of the binary

mixtures N-ethyl-2-pyrrolidone + ethanolamine or diethanolamine or

triethanolamine were measured at different temperatures from 293.15 to 323.15 K

over the entire range of concentrations by Antonio et al112. The excess molar

volumes and isentropic compressibility deviations were calculated. Moreover, the

excess molar volumes were fitted using a Redlich−Kister equation, and the surface

tension data were fitted by the Connors−Wright model. They have concluded that

an increase in the substitution degree in the nitrogen atom causes an increase in the

magnitude of each property.

Slobodan Gadzuric et al113 calculated the isobaric thermal expansion

coefficients, partial molar volumes, apparent molar volumes, partial molar excess

volumes and excess thermal expansions for the binary liquid mixtures of

N-ethylformamide with tetrahydrofuran, 2-butanone, and ethyl acetate over the

entire composition in the temperature range 293.15 to 313.15 K.

Different physical properties (density, speed of sound, viscosity, and

refractive index) were measured for the system NMP + water + ethanol over the

entire composition range by Blanco et al114. They have reported that density was

found to increase with NMP concentration and excess molar volumes shows

negative values with large deviations that indicate the important interaction

Satyanarayana et al115 computed the excess isentropic compressibilities by

measuring the density and speed of sound of binary mixtures of N-methyl

acetamide with ethyl acetate, ethyl chloroacetate, and ethyl cyanoacetate over the

entire range of volume fraction in the temperature range of 303.15 to 318.15 K.

The results showed that the volume reduction factors are dominating in all the

systems over the entire range of temperatures studied.

Fan-Li et al116 measured densities as a function of composition for a

ternary–pseudo binary mixture of {(styrene + ethyl acetate or benzene) + (N,N-

dimethylformamide + ethyl acetate or benzene)} at atmospheric pressure and the

temperature 298.15 K and showed that the third component, ethyl acetate or

13

benzene, have a significant influence on the interaction between styrene and

N,N-dimethyl formamide.

Excess volumes, speeds of sound and viscosities were measured for binary

mixtures of methyl acetate, ethyl acetate, butyl acetate, isoamyl acetate with n-

butyl amine and tert-butyl amine at 303.15 K over the entire range of composition

by Sankara Reddy et al117,118. Isentropic compressibilities were computed from

speed of sound and density data. Speeds of sound were evaluated using Jacobson

free length theory and Schaaff’s collision factor theory. The viscosity data were

analyzed on the basis of corresponding states approach and Grunberg and Nissan

treatment. The experimental results on excess volume, deviation in isentropic

compressibility and in viscosity were discussed in terms of molecular interactions

between unlike molecules.

Nikam et al119,120 measured the densities and viscosities at 298.15, 303.15,

and 308.15 K for binary mixtures of ethyl acetate with pentan-1-ol, hexan-1-ol,

3,5,5-trimethylhexan-1-ol, heptan-1-ol, octan-1-ol, decan-1-ol, linear and branched

alkanols (C1-C4). Molar excess volume and deviation in viscosity were computed

from the measured data and fitted to the Redlich-Kister polynomial. From the

results, it was observed that VE values of all binary systems were found to increase

with an increase of temperature due to decreased ester-ester and alkanol-alkanol

contacts and to the structural contributions arising from the geometrical fitting of

one component (methanol) into the other (ethyl acetate) owing to differences in the

molar volumes between components. Further, they reported that even though the

geometrical contribution seems to be negligible with alkanols (C2-C4) but it was

found to be increasingly significant in the case of higher alkanols and deviations in

viscosity were negative in all the systems and become more negative with an

increase in chain length and branching of alkanols.

Aminabhavi et al121,122 investigated the thermo-physical properties of the

binary mixtures of Methyl Acetate, Ethyl Acetate, n-Propyl Acetate, and n-Butyl

Acetate with 2-Chloroethanol and acetonitrile. They observed that the relative

Lorenta-Lorentz molar refraction values decrease systematically with increasing

14

size of the ester molecules i.e., form methyl acetate to butyl acetate in these binary

mixtures.

Palaiologou et al123,124 and Lee et al125 established specific interactions

between component molecules in the binary mixtures of Dimethyl Sulfoxide +

butyl acetate , 4-chloro toluenes + alkyl ester and nitro methane + alkyl ester by

investigating densities, viscosities, refractive indices, and surface tensions of these

binary mixtures at 293.15, 298.15, 303.15, and 313.15 K. Negative excess molar

volumes are reported by several researchers126-129 in the binary mixtures of NMP

indicating strong interactions between unlike molecules owing to dipole-dipole,

charge-transfer or hydrogen bonded interactions.

Awwada et al130 reported the negative excess molar volumes and difference

in the partial molar volumes in the binary mixtures of NMP with some aromatic

hydrocarbons, which follow the order: toluene > p-xylene > benzene > o-xylene >

m-xylene. Further, they suggested that the contraction in the volume of the mixture

on mixing, is due to the charge transfer complex formation and the partially

interstitial accommodation of aromatic molecules in the empty spaces in 2-

pyrrolidone structure. Partial molar volumes and isentropic compressibilities of

polycyclic aromatic hydrocarbons in 1-methyl-2-pyrrolidone at 298.15 K are

reported by Takuya et al131. The results are attributed to the formation of

intermolecular interactions in mixture, like dipole–induced dipole intermolecular

interactions between the polycyclic aromatic hydrocarbon and NMP. A shift to the

long wavelength was observed in the peak of UV spectra of benzene and

naphthalene in the polar solvents suggesting the existence of an induced effect in

the intermolecular interactions.

The excess molar Gibb’s energies for the binary mixtures of NMP with

several chloroalkanes are reported by Gimenez and co-workers132 through the VLE

studies in these mixtures. The theoretical DISQUAC group model used by them to

examine the results, has reproduced the experimental data quite well.

1.2.3 Spectroscopic Investigation of Molecular Interactions

Spectroscopic techniques like infrared (IR) and nuclear magnetic resonance

(NMR) have been extensively used to investigate solution structure and provide

15

physical information about intermolecular interaction133. The infrared spectrum

which gives significant information about the functional groups can be

substantially influenced by the surrounding condensed medium. The resultant

effects in IR spectra of liquid mixtures, due to intermolecular association (between

unlike molecules) are mainly peak position shifting (solvatochromic shift), change

in the intensity of the spectral line and change in the shape and width of the

spectral band etc.

1.2.3.1 Introduction to Infrared Spectra

An invaluable tool in organic structure determination and verification

involves the class of electromagnetic radiation with frequencies between 4000 and

400 cm-1. Chemical bonds in different environments will absorb varying intensities

and at varying frequencies. Thus IR spectroscopy involves collecting absorption

information and analyzing it in the form of a spectrum. The frequencies at which

there are absorptions of infrared radiation ("peaks" or "signals") can be correlated

directly to bonds within the compound under study134.

A molecule absorbs only selected frequency (frequencies) of infrared

radiation and is excited to a higher energy state which occurs when frequency of

infrared radiation exactly matches a natural vibrational frequency of the molecule.

A net transfer of energy takes place during energy absorption. The energy

absorbed serves to increase the amplitude of the vibrational motions of the bonds

in the molecule and changes the bond angles. Not all bonds in a molecule are

capable of absorbing infrared energy even if frequency of the radiation exactly

matches that of the bond motion. Only those bonds having a dipole moment absorb

infrared energy135.

Hydrogen bonding is formed between a function group and an atom or

group of atoms in the same or different molecules. It can exist in the solid and

liquid phases or in solutions. Hydrogen bonding can be detected in many ways, but

the most important way is by the infrared (IR) spectroscopy.

Because each inter atomic bond vibrates in a different way (stretching or

bending) individual bonds may absorb more than one infrared frequency.

Stretching absorptions usually produce stronger peaks than bending, however the

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weaker bending absorptions can be useful in differentiating similar types of bonds

(e.g. aromatic substitution). It is also important to note that symmetrical vibrations

do not cause absorption of infrared radiation. A molecule consisting of n atoms has

a total of 3n degrees of freedom, corresponding to the Cartesian coordinates of

each atom in the molecule. In a nonlinear molecule, 3 of these degrees are

rotational and 3 are translational and the remaining corresponds to fundamental

vibrations; in a linear molecule, 2 degrees are rotational and 3 are translational.

The net number of fundamental vibrations for nonlinear and linear molecules is

therefore136 3n-6 and 3n-5 respectively.

Symmetric stretching

vibrations

In-plane bending

vibrations (Scissoring)

Out-of-plane bending

vibrations (Wagging)

Asymmetric Stretching

vibrations

In-plane bending

vibrations (Rocking)

Out-of-plane bending

vibrations (Twisting)

Fig. 1.1 Different types of stretching and bending vibrational modes.

The stretching frequency of a bond can be approximated by Hooke’s Law.

In this approximation, two atoms and the connecting bond are treated as a simple

harmonic oscillator composed of 2 masses (atoms) joined by a spring.

17

Figure 1.2 Energy curve for a simple harmonic oscillator (left) and energy

constrained to quantum mechanical model (right).

The energy curve for a simple harmonic oscillator is illustrated in Figure

1.2. According to Hooke’s law, the frequency of the vibration of the spring () is

related to the mass and the force constant of the spring by the following formula:

1

2

k

m

where k is the force constant, m is the mass.

A molecule is not just two atoms joined on a spring, of course. A bond can

come apart, and it cannot be compressed beyond a certain point. A molecule is

actually an anharmonic oscillator. As inter atomic distance increases, the energy

reaches a maximum, as seen in Figure 1.3. Note how the energy levels become

more closely spaced with increasing inter atomic distance in the anharmonic

oscillator. The allowed transitions, hν, become smaller in energy. Therefore,

overtones can be lower in energy than predicted by the harmonic oscillator theory.

Figure 1.3 Energy curve for an anharmonic oscillator.

18

The following formula has been derived from Hooke’s law. For the case of

a diatomic molecule, ( has been substituted for ν, recall that ν = c ):

1 2

1 2

( )1

2

f m m

c m m

where is the vibrational frequency, m1 and m2 are the mass

of atoms 1 and 2, respectively, ‘c’ is the velocity of light, ‘f’ is the force constant

of the bond. This equation shows the relationship of bond strength and atomic

mass to the wave number at which a molecule will absorb IR radiation. As the

force constant increases, the vibrational frequency (wave number) also increases.

The force constants for bonds are: single bond 5 x 105 dyne cm-1; double bond 10 x

105 dyne cm-1 and triple bond 15 x 105 dyne cm-1

As the mass of the atoms increases, the vibration frequency decreases. The

regions of an IR spectrum where bond stretching vibrations are seen depends

primarily on whether the bonds are single, double, or triple or bonds to hydrogen.

Although a useful approximation, the motion of two atoms in a large molecule

cannot be isolated from the motion of the rest of the atoms in the molecule. In a

molecule, two oscillating bonds can share a common atom. When this happens, the

vibrations of the two bonds are coupled. As one bond contracts, the other bond can

either contract or expand, as in asymmetrical and symmetrical stretching. In

general, when coupling occurs, bands at different frequencies are observed, instead

of superimposed (or degenerate) bands as you might expect from two identical

atoms in a bond vibrating with an identical force constant.

If hydrogen bonding is possible between solute and solvent, this greatly

increases solubility and often results in large or even infinite solubility. There is

evidence that double and triple bonds, aromatic rings can form hydrogen bonds

with polar functional groups, but these bonds are very weak. In many cases, there

is partial hydrogen bonding in dilute solution, that is, some functional groups are

free and some are hydrogen bonded. In such cases two IR bands appear. IR

spectroscopy can also distinguish between inter- and intra-molecular hydrogen

bonding, since intermolecular bands are intensified by an increase in concentration

while intra-molecular bands are unaffected.

19

Table 1.1 Characteristic IR Absorptions

Frequency (cm-1) Bond Functional Group

20

1.2.3.2 Introduction to Nuclear Magnetic Resonance

NMR spectroscopy is one of the principal techniques used to obtain

physical, chemical, electronic and structural information about molecules due to

either the chemical shift, Zeeman effect, or the Knight shift effect, or a

combination of both, on the resonant frequencies of the nuclei present in the

sample. It is a powerful technique that can provide detailed information on the

topology, dynamics and three-dimensional structure of molecules in solution and

the solid state. Thus, the structural and dynamic information is obtainable from

NMR studies of quadrupolar nuclei even in the presence of magnetic "dipole-

dipole" interaction broadening (or simply, dipolar broadening) which is always

much smaller than the quadrupolar interaction strength because it is a magnetic

versus an electric interaction effect.

Table 1.2 Characteristic 1H1 NMR chemical shifts for different classes of

compounds

1.2.3.3 Literature Review on spectral analysis of molecular interactions in

binary liquid mixtures

Study on intermolecular interactions in liquid acetonitrile - propan-1-ol

mixtures is carried out by Kinart et al137 measuring the 1H NMR spectra and

analyzing their physicochemical properties. It is concluded that active electron

donor sites of acetonitrile lead to the formation of H-bonds with OH proton of

alcohols in these mixtures.

21

Mehta et al138 reported the positioning of maxima/minima at about the same

mole fraction of DMF in excess chemical shifts of 1H and 13C NMR values and

excess molar volume for the binary mixtures of DMF with butane diols. It is

concluded that carbonyl group of DMF and hydroxyl groups of butanediol are the

two active sites of interactions in these mixtures whereas in butanediol and

pyrrolidone systems139, hydroxyl groups of butanediol is the only active site of

interactions

Bricknell et al140 found some relationships between the infrared

spectroscopic properties of water molecules hydrogen bonded to a number of bases

viz., amines, ethers, nitriles etc., in binary liquid mixtures, and the partial molar

excess enthalpies at infinite. The results suggest new approach to the use of the

well-known Badger-Bauer relationship. The general adoption of the use of the

partial molar excess enthalpy at infinite dilution of a solute in a solvent as the

property of choice for correlating infrared spectroscopic and thermodynamic data

for such binary liquid systems is proposed.

Solvation properties of aliphatic alcohol–water and fluorinated alcohol–

water solutions are probed by Takamuku et al141 taking amide molecules as solutes

using infrared (IR) and 1H and 13C NMR techniques. They suggested that

hydrophilic moieties of both carbonyl and amino groups of amide molecules are

hydrogen-bonded with water molecules in aqueous amide solutions and gradual

weakening of hydrogen bonds with the increase of alcohol content is observed in

both aliphatic alcohol–water and fluorinated alcohol–water solutions.

Aashees Aswathi et al reported the presence of hydrogen bonding between

unlike molecules in the mixtures of pyridine and quinoline with phenol and in the

mixtures of DMSO with phenol and o-Cresol142,143 based on the FT-IR spectral

analysis of their mixtures. The C H--O hydrogen bond is of increasing interest

because of the significant role in protein conformation, recognized lately144-146.

Dependence of 1:1 intermolecular complex existing in the whole range of the

mixture composition on the environment is established by Stangret et al147 using

IR spectral analysis of DMF + methanol binary mixtures.

22

1H-NMR spectra in binary mixtures of sulfolane with ethylene glycol,

diethylene glycol, triethylene glycol and tetraethylene glycol have been recorded

over the whole composition range at 303 K under atmospheric pressure by Kinnart

et al148. They have suggested that the specific interaction between sulfolane and

glycols increases as the glycol carbon chain length increases and it is also as a

result of molecular size differences.

Based on the concept of local composition, the 1H NMR chemical shift data

of the OH proton over the whole concentration for alcohol + hydrocarbon systems

are correlated by Yingjie et al149. They have concluded that using 1H NMR

spectroscopy coupled with only one activity coefficient at infinite dilution, the

viscosity for alcohol + hydrocarbon systems can be accurately predicted.

The application of IR spectroscopy in the development of thermodynamic

models for the solutions in which association and solvation occur simultaneously

is presented by Asprion et al150. Extensive IR spectroscopic studies of binary and

ternary solutions where solvation either occurs between an alcohol and a solvent or

between two alcohols were performed at around 283–313 K. The results showed

that a quantitative interpretation of the monomer-band is possible in the systems

studied. They reported that the IR spectrum of the monomer band splits into two

bands, one resulting from free, the other from solvated monomers.

The existence of hydrogen bond between of –C=O group of

N,N-dimethylacetamide with –OH group of propan-1-ol and propan-2-ol is

confirmed through FT-IR spectra by Lakshminadh et al151. They have concluded

that the theoretical FT-IR values determined using Hartree-Fock and Density

Functional Theory methods (with 6-31+G* and 6-311+G** basis sets) are in

reasonable agreement with the experimental values.

1.3 Nature and Scope of the Present Work

Ultrasonic velocity in a medium is fundamentally related to the binding

forces between the molecules. Ultrasonic velocities of the liquid mixtures are of

considerable importance in understanding intermolecular interaction between

component molecules and find applications in several industrial and technological

processes152-154. Ultrasonic velocity measurements have been employed

23

extensively to detect and assess weak and strong molecular interactions in binary

mixtures, because mixed solvents find practical applications in many chemical and

industrial processes. Increasing use of N-methyl-2-pyrrolidone (NMP) in many

industrial processes have greatly stimulated the need for extensive information on

the acoustic and transport properties of it and its mixtures.

Detailed literature survey shows that work has been carried out by different

researchers on excess molar volumes of binary mixtures containing NMP + an

alkanols or hydrocarbon156-158, + an ether159, + ketone160,161, + aromatic

hydrocarbons162,163, + water164, + methanol165 etc. However, no effort appears to

have been made to investigate the physico-chemical properties, for the binary

mixtures of NMP with substituted benzenes or aliphatic esters or alkyl amines.

The literature survey on the ultrasonic studies indicates that enormous work

has been carried out in binary and ternary liquid mixtures of weak and strong

interacting systems and very few studies are reported with NMP as main

component. Moreover, thermodynamic properties of these liquid mixtures are of

interest for different branches of science and engineering and also play significant

role in technological processes, biological process of living organisms and in

nature. This fact has encouraged the author to carry out a series of systematic

investigations on the solvent properties of these liquid mixtures.

N-methyl-2-pyrrolidone (NMP) is an important solvent as it is water-

miscible, hygroscopic, colorless, and strongly polar liquid. NMP has the potential

for use in, solvent extraction process as strong solubilizing agent166, purification

and crystallization of drugs167. It is also used in the manufacture of various

compounds, including pigments, cosmetics, insecticides, herbicides, and

fungicides. Its non-hazardous and ecological properties account for the reality that

it is increasingly being used as an alternative for chlorinated hydrocarbons.

In the chemical industry, there exists a continuing need of reliable

thermodynamic data of binary liquid systems. This is particularly true for systems

involved in industrial process. With this objective in mind and the lack of

availability of the correlation studies between spectroscopic and thermo acoustic

parameters in the NMP and substituted benzenes or aliphatic esters or alkyl amines

24

systems in literature has encouraged the author to carry out a series of systematic

investigations on the properties of the binary mixtures of NMP with substituted

benzenes, aliphatic esters and alkyl amines.

The present work is focused on the study of acoustic, volumetric and

viscometric properties, by measuring the ultrasonic velocities, densities and

viscosities of the binary mixtures of NMP viz.,

I N-methyl-2-pyrrolidone (NMP) + Aminobenzene (AB)

N-methyl-2-pyrrolidone (NMP) + Chlorobenzene (CB)

N-methyl-2-pyrrolidone (NMP) + Bromobenzene (BB)

II N-methyl-2-pyrrolidone (NMP) + Methylacetate (MA)

N-methyl-2-pyrrolidone (NMP) + Ethylacetate (EA)

N-methyl-2-pyrrolidone (NMP) + Butylacetate (BA)

III N-methyl-2-pyrrolidone (NMP) + Propylamine (PA)

N-methyl-2-pyrrolidone (NMP) + Butylamine (BAM)

N-methyl-2-pyrrolidone (NMP) + Dipropylamine (DPA)

over the complete composition range at temperatures of 303.15, 308.15, 313.15

and 318.15 K and atmospheric pressure. It is also aimed to study the molecular

interactions between the unlike molecules of the binary liquid systems under

investigation, with special reference to dipole–dipole and hydrogen bonding

interactions with the help of spectral (Fourier Transform Infrared and 1H NMR

Spectra) data of these mixtures. Further, the experimental data of these binary

mixtures is used to test the applicability of empirical relations of, Nomoto, Van

Dael and Vangeel, Schaaff’s collision factor theory, Rao’s specific velocity,

impedance relation and Junjie’s equation for ultrasonic velocity and Grunberg–

Nissan, Katti–Chaudhri, Heric–Brewer, McAllister (4-body model) and Hind et al

for viscosity. Also, the effect of temperature on the shapes of the interacting

molecules is analyzed using Scaled Particle Theory (SPT).

25

The following thermodynamic and thermo-acoustic parameters evaluated

from the measured values are used for understanding the nature of the interactions

in the binary mixtures under study:

molar volumes ( mV ) in m3 mol-1,

partial molar volumes ( imV , ) in m3 mol-1,

apparent molar volumes ( iV , ) in m3 mol-1,

excess molar volumes ( E

mV ) in m3 mol-1,

excess partial molar volumes ( E

imV , ) in m3 mol-1,

isentropic compressibility (s) in Pa-1,

excess isentropic compressibility ( E

s ) in m2 N-1,

isobaric thermal expansion coefficient (p) in K-1

excess isobaric thermal expansion coefficient ( E

P ) in K-1

inter molecular free length (Lf) in m,

excess intermolecular free length ( E

fL ) in m,

deviation in viscosity () in m Pa.s,

acoustic impedance (Z) kg m-2 s-1,

excess acoustic impedance (ZE) in kg m-2 s-1,

excess velocity (UE) m s-1,

free volume ( fV ) in m3 mol-1,

excess free volume ( E

fV ) in m3 mol-1,

internal pressure ( i ) in N m-2,

excess internal pressure ( E

i ) in N m-2,

viscous synergy (Is)

viscous relaxation () in s,

classical absorption coefficient (/f2)

cohesive energy (H) in J mol-1,

Gibb’s free energy of activation of viscous flow ( EG* ) in J mol-1.

26

The data obtained can be used to understand intermolecular interactions

between the unlike molecules and to test the theories of solutions. The departure of

these real mixtures from ideal behaviour can be explained in terms of effect of

hydrogen bond breaking, loss of dipolar association, differences in size and shapes,

dipole-dipole interactions between different component molecules etc.

As far as possible, S.I. units are used but there are some circumstances

where this system has no particular advantage. Duplication of some mathematical

symbols has been unavoidable but confusion should not arise because their

meanings are clearly explained in the appropriate parts of the text.

27

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