Density and Comparative Refractive Index Study on Mixing Properties of Binary Liquid Mixtures of Eucalyptol with Hydrocarbons at 303 .

Density and refractive index have been experimentally determined for binary liquid mixtures of eucalyptol with hydrocarbons (o-xylene, m-xylene and toluene) at 303.15K, 308.15K and 313.15K. A comparative study of Lorentz-Lorenz (L-L), Weiner (W), Heller (H), Gladstone-Dale (GD), Arago-Biot (A-B), Eykman (Eyk), Newton (Nw), Eyring-John (E-J) and Oster (Os) relations for determining the refractive index of a liquid has been carried out to test their validity for the three binaries over the entire mole fraction range of eucalyptol at 303.15K, 308.15K and 313.15K. Comparison of various mixing rules has been expressed in terms of average deviation. From the experimentally measured values, refractive index deviations at different temperatures have been computed and fitted to the Redlich-Kister polynomial equation to derive the binary coefficients and standard deviations.


Introduction
Accurate knowledge of thermodynamic mixing properties of binary mixtures has great relevance in theoretical and applied areas of research.These data are needed for design process in chemical, petrochemical and pharmaceutical industries.
Refractive index and density measurements of binary liquid mixtures are essential for determination of composition of binary 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 intermolecular interactions.The most widely used rules for predictivity of refractivity in case of binary liquid mixtures are Arago-Biot 1 , Gladstone-Dale 2 , Lorentz-Lorenz [3][4] , Eykman 5 , Weiner 6 , Heller 7 , Newton 8 , Oster 9 and Eyring-John 10 .Many authors [11][12][13][14][15][16][17][18][19] have applied these properties to study the structures, solvent-solute interactions and the solvation behaviour in binary liquid mixtures.

Experimental
The chemicals used were of A. R. grade.The purity of compounds was checked as per the method given earlier 22 and results agreed reasonably with corresponding literature values.Mixtures were prepared by mixing the appropriate volumes of liquids in specially designed ground glass air tight ampules and weighed in single pan balance (Mettler Toledo AB 204 electronic balance) to an accuracy of + 0.0001 gm.Preferential evaporation losses of solvent from the mixture were kept to a minimum as evidenced by repeated measurements of physical properties over an interval of 2-3 days during which time no change in physical properties were observed.The possible error in mole fraction is estimated to be around 0.0001.Density of pure liquids and their binary mixtures in the composition range of 0.0660 to 0.8689 mole fraction increments were measured by using pycnometer having bulb volume of 10 cm 3 and capillary with the internal diameter 1 mm for each measurement.Sufficient time was allowed to attain thermal equilibrium in High Precision Water Bath, Cat No. MSW-274 thermostat, the bath temperature was monitored to + 0.01 o C with a calibrated thermometer.The reported density at 303.15K, 308.15K and 313.15K are significant at four figures.An average of triplicate measurements was taken into account and their reproducibility was with in range of + 0.00001.
Refractive index for the sodium D line was measured with a thermostatic Abbe's refractometer, SER.No. 95033 with is error less than 0.0001 units.These data were approximated to four digits of decimal.Water was circulated into the instrument through the thermostatically controlled bath.The refractometer was calibrated using glass test piece of known refractive index supplied with the instrument.

Results and Discussion
The experimental refractive indexes of eucalyptol with hydrocarbons are presented in Table 1 and the data have been used to evaluate refractive index deviations, via following equation n E = n m -∑x i n i (1) Where x i and n i represent the mole fraction and the refractive index of the i th component respectively and n m is the refractive index of the binary liquid mixture.
For all the three binary mixtures, Weiner relation has exhibited deviation higher in value than Heller, Eyring-John and Eykman relations at all temperatures.It should be noted that the calculated average deviation values between the Arago-Biot and Gladstone-Dale relations are exactly same for all systems at all temperatures.This is expected because of similarities in the functional forms of these equations; as per literature 21 .Gladstone-Dale relation is more frequently used than the Arago-Biot equation.However, most generally, the Heller relation gives the least deviation in value as compared to other mixing rules.

Conclusions
All nine mixing rules could be successfully applied at lower concentration of eucalyptol omitting other factors such as volume reduction, volume addition and temperature.Weiner equation could not give better results.All nine theoretical mixing rules performed well within the limits of experimental error.The deviation between theoretical and observed values of refractive indexes for all system taken under consideration can be reduced if the concept of excess molar volumes is taken into consideration.Results from Redlich-Kister polynomial equation reveal that strong intermolecular interactions related to decrease in molar volume and negative enthalpy change takes place on mixing.

Table 1 .
Experimental Densities(ρ), Refractive Indexes(n exp ) and Excess Refractive Indexes (n E ) of binary liquid mixtures of Eucalyptol with Hydrocarbons at 303.15K, 308.15K and 313.15K

Table 2 .
Parameters, a i and standard deviation, σ for Eucalyptol + Hydrocarbon

Table 3 .
Average deviations* in the Refractive Index from different mixing relations for Eucalyptol + Hydrocarbon binary mixtures at 303.15 K, 308.15K and 313.15K