INTRODUCTION:

Refractive index is one of the physical properties of a solvent which affects the solution of a number of problems in Pharmaceutical and chemical areas. Refractive index values of most of the common solvents are available in literature. Binary and higher order solvent mixtures were used to overcome a number of practical limitations of mono-solvent systems including poor solubility of a solute or insufficient analytical performance of mono solvent systems in chemical analysis. Experimental of physical properties such as refractive index are required for a full understanding of the thermodynamic properties of liquid mixtures as well as for practical chemical engineering work..

Refraction arises from the presence of extra nuclear electron of atom, tend to follow the oscillation of the electro-magnetic field associated with light. As per Clausius-Mosotti equation, at a particular frequency of light,

[R] = 4π . N α /3

Where, N is Avogadro’s number and α is electronic polarizability of the molecules of medium, polarizability of medium is sum of polarizabilities of constituent atoms. Refractive index depends on the wave length of light.

Some of the important investigations, which contributed to the development of the treatment of refractive index of liquid mixtures have been reported^1,2. Some of them, equations suggested for binary solvents are not suitable when there is a large change of volume on mixing, resulting from physical and chemical interactions. The behavior of binary system in a particular mixture is specific and depends on its composition.

Refractive index and density measurements of binary liquid mixtures are essential for determination of composition of binary liquid mixtures, usually for non-ideal mixtures, where direct experimental measurements are performed over the entire composition range. Most empirical approach for calculating the excess properties is an attempt to explain non-ideality in terms of specific and non-specific molecular interaction. The most widely used rules for predicting refractivity in case of binary liquid mixtures are Arago-Biot⁴, Gladstone-Dale⁵, Lorentz-Lorenz^6,7, Weiner⁸, Heller⁹, Newton¹⁰, Oster¹¹ and Eyring-John¹². Many authors¹³ have applied their properties to study the structure, solvent- solute interactions and the solvation behavior in binary liquid mixtures. Alaksandar Z. et.al³. reported the refractive indices for binary system benzene-cyclohexane, acetone-benzene and acetone-cyclohexane at 25° C over the composition range.

Sangita Sharma et.al.¹⁴determined density and refractive index for binary liquid mixtures of methyl acetate, ethyl acetate, propyl acetate and butyl acetate with n-butanol and iso-butanol at 303.15, 308.15 and 313.15K. S. Dhillon and Chugh H.S.¹⁵ measured the refractive indices of mixtures of 1,2-dibromomethane with cyclohexane, benzene, toluene , o-xylene, m-xylene and p-xylene at 303.15K as a function of composition. Molar refractions of the mixture were calculated from refractive index measurement. Jagan Nath and B. Narain¹⁶ reported the refractive indices for binary liquid mixtures of tetrachloroethylene with benzene, toluene, p-xylene, carbon tetrachloride and cyclohexane. Juan Ortega¹⁷ measured the refractive indices of normal alcohols form methanol to 1-decanol in the temperature range from 20 to 80° C.

Since the refractive index can be determined for a small amount of samples sealed in a cell in wide range of temperature and pressure with a high precision, the refractive index method have been used to obtain density of liquids and liquid mixtures. Among many theoretical and empirical equations, the Lorentz-Lorenz formula and the Gladstone-Dale law have been widely employed to evaluate the density of liquids from observed refractive indices. The density derivative of the refractive index for the L-L formula has been found to be Ca 5-10% higher than the observed one¹⁸. The estimation of the density of binary liquid mixtures from the observed refractive index is affected by another uncertainly caused by the assumption of the additivity rule. It has been known that the experimental values of the molar polarization for various binary mixtures usually do not differ by more than a few parts in a thousand from the values calculated with L-L formula on the basis of additivity rule¹⁹. From a fractional viewpoint, this small deviation is not serious in the determination of density. However, this uncertainty yields a large error for the excess volume, which reflects the properties specific to binary liquid mixtures . Therefore, the determination of the excess volume with refractive index data requires a careful examination of the additivity rule and equations of refractive index. Heller²⁰ scrutinized various equations of refractive index by calculating refractive indices from old data for a liquid mixture, although attention was not paid to the excess volume.

Milsuo Nakato and Masao Sakurai²¹ reported the refractive indices of binary mixtures of 1-chlorobutane-2-ol,1-chlorobutane-2-methoxyethanol, and isopropyl acetate-methylethyl ketone. The excess volume obtained from densimeteric measurements were compared with the ones calculated from the refractive indices.

Jimenez et.al.²² compared these models using refractive index data of 1,2-ethane diol + 1- propanol and 1,2-ethanediol + 1-butanol and found that the accuracy order of the models was as Wiener, Heller, Newton, Gladstone-Dale, Lorentz-Lorenz, Eykman, Arago-Biot and Oster equations . The main limitation of these models is that they correlate refractive index values to the solvent composition as independent variable at a constant temperature and the models could not be used to predict refractive index values at other temperatures. However, refractive index of the solvent depends strongly on temperature and mixed solvent systems at various temperatures have been used in many analytical methods where refractive index detectors were used as detection system. It is obvious that by changes in the refractive index of the system, fluctuations appear on the detection output and make noises. To provide a single model to correlate refractive index of mixed solvents at various temperatures Jimenez and co-workers proposed a two independent variables model as –

RIm,T = J₀ + J₁f₁+ J₂f²₁+ J₃( T-298.15 ) + J₄( T-298.15 )²+ J₅f₁( T-298.15) + J₆f₁²( T-298.15 ) + J₇f₁( T-298.15 )²+ J₈f₁²( T-298.15 )² ……………………………….…….(1.44)

Where RIm_,T is refractive index of the mixture at temperature T, J₀-J₈ are the model constants, f₁ is the mole fraction of the solvent 1 in the mixture. Lee and co-workers²³ have used equation (2) for computing RIm_, T and shown its accuracy using R.I. data of water + 1,3-propanediol at 298- 323K.

RIm_,T=M₀+M₁f₁+M₂f²₁+M₃T+M₄f₁T+M₅f²₁T+M₆T²+M₇f₁T²+M₈f²₁T²………………………………………..…..(1.45)

Where, M₀ – M₈ are the model constants.

The Jouyban-Acree model has been presented for calculating different physico-chemical properties of solvent mixtures at various temperatures including dielectric constants²⁴, surface tensions²⁵, absolute viscosities²⁶ and density²⁷. The adopted model for calculating refractive index of binary solvent mixtures at various temperatures is –

…………..(1.46)

Where,

f₂ is the volume (mole/weight) fraction of solvent 2 in the mixture, RI₁, T and RI₂T are the values of solvents. 1 and 2 at T, and Aj is the model constant.

The performance of the equations for calculating the refractive index of the systems at various temperatures has been tested with 29 systems including aqueous and non-aqueous binary liquid mixtures collected from literature^22,23,28,29. The agreements between experimental (RI exp) and calculated (RI cal) refractive indices for the systems were shown in the form of average percentage deviation (APD);

……………………(1.47)

Where N is the number of data points in each set. The overall APD (OAPD) was defined as –

………..(1.48)

The refractive index, excess refractive index, etc. are used to explain the nature of solvent-solvent interactions. The refractive index along with density values of mixtures are used to test the accuracy of available refractive index mixing relationships in predicting binary mixture refractive index data.

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Received on 19.09.2012 Modified on 03.10.2012

Asian J. Research Chem. 5(10): October, 2012; Page 1300-1302