Refractometric Studies on Molecular Interactions in Six Binary Liquid Mixtures

Refractive indices of the six binary mixtures between N-Butyl bromide with Aniline, Benzene, Xylene, Carbon tetrachloride, n-Heptane and Toluene for the entire concentration range have been measured at 303.15 K. Furthermore, deviation in refractive indices from ideal mixture, molar refraction, deviation in molar refraction from ideal values for these mixtures has also been evaluated. The results are discussed in term of molecular interactions between the components of the binary mixtures.


Introduction
The studies of refractive indices are being increasingly used as tools [1][2][3][4][5] for investigation of the physical properties of pure components and the nature of intermolecular interaction between the liquid mixture constituents.Refractive index measurements of binary liquid mixtures are essential for determination of composition of binary mixtures usually for non-ideal mixtures where experimental measurements are performed directly over the entire range of composition.The review of literature [6] on acoustical studies of solution reveals that refractive indices measurements are also used to estimate the different elastic properties of the molecule from which the type of molecular interactions can be very well understood.Pandey et al. [7] have made refractive indices measurements in liquid mixtures and have suggested that such studies are very much helpful for understanding of the molecular interactions in the components of the mixture.In addition to binary liquid mixtures, a few ternary, [8,9] mixtures have also been studied employing refractive index measurements.However, literature survey shows that study of molecular interactions between N-Butyl bromide and hydrocarbon molecules either by refractive index measurements or other methods is rare.Taking these into considerations, in the present study we have reported refractive index data for binary mixture of N-Butyl bromide with Aniline, Benzene, Xylene, Carbon tetrachloride, n-Heptane and Toluene for the entire concentration range.

Materials and Methods
Six binary systems, viz N-Butyl bromide + Aniline, N-Butyl bromide + Benzene, N-Butyl bromide + Xylene, N-Butyl bromide + Carbon tetrachloride.N-Butyl bromide + n-heptane and N-Butyl bromide + Toluene, were studied.All chemicals were of Rankem Ltd. (AR grade, 98% -99% pure) chemicals were purified by fractional distillation method [10].Binary mixtures of N-Butyl bromide + Aniline, N-Butyl bromide + Benzene, N-Butyl bromide + Xylene, N-Butyl bromide + Carbon tetrachloride.N-Butyl bromide + n-heptane and N-Butyl bromide + Toluene, were prepared by weight covering the entire mole fraction range.The components of binary mixtures were injected by means of syringe in to the glass vials of sealed with rubber stopper in order to check evaporation losses during sample preparation.Refractive indices of pure liquids and liquid mixtures were measured using white light by an Abbe refract meter (Model R-8 M/S Mittal Enterprised, New Delhi) Refract meter was calibrated with kept constant at 303.15 K ± 0.03 K by circulating water of the thermostat with the help of pump through both the prism boxes of the refract meter.Refractive indices of liquid were measured after attainment of constant temperature.An average of five measurements was made for each sample.Density of pure liquids and liquid mixtures at experimental temperature were determined using double steam capillary type pyknometer.Pyknometer was also calibrated with liquids of known density values.The weight of the sample was measured using electrical single pan analytical balance (K-Roy).

Results and Discussion
The experimental values of refractive indices (n mix ) and density (ρ mix ) at 303.15 K with mole fraction of second components for all binary mixtures of N-Butyl bromide and second components are reported in Table 1.Values of deviation in refractive indices (Δn) for all the systems studied are evaluated using experimental values of refractive indices for pure liquids and liquid mixtures employing following equation [11] (Equation ( 1) where n mix and n ideal are refractive indices of actual binary mixture and ideal binary mixtures.The refractive index deviation, Δn, has been calculated on a molefraction basis and n ideal has been evaluated by Equation (2).
where X 1 , X 2 and n 1 , n 2 are the mole fraction and refractive indices of the components 1 and 2 respectively.Values of deviation in refractive indices, Δn, with compositions of the binary mixtures studied at experimental temperature are also recorded in Table 1.
The experimental values of refractive indices and densities of the mixtures have been used to evaluated molar refraction, R, employing Lorentz-Lorenz equation,

(
) ( ) where ( )  is the molar volume of the mixtures.Deviation in molar refraction, ΔR is evaluated by Equation (4), where The value of molar refraction and ΔR are recorded in Table 1.Perusal of Table 1 shows that experimental refractive indices decrease with mole fraction of N-Butyl bromide increases, for the binary mixtures of all the N-Butyl bromide with Aniline, Benzene, Xylene, Carbon tetrachloride, n-Heptane and Toluene, except that refractive indices increases with mole fraction of N-Butyl bromide increases in type system N-Butyl bromide + n-heptane.Values of Δn for the entire composition range of N-Butyl bromide in binary mixture of the Aniline, Benzene, Xylene, Carbon tetrachloride, n-Heptane and Toluene, are negative.
On the basis of the above, it seems that molecular interactions play an important role in governing the deviation in refractive indices for the binary systems studied in the present investigation.It may also be concluded that there is a gradual change in molecular interactions between the components of binary mixtures.Values of molar refraction, R, evaluated by Equation (3) and deviations in molar refractions, ΔR at experimental temperature for all binary mixtures are reported in Table 1.perusal of Table 1 shows that for binary mixtures of different hydrocarbon and CCl 4 with a common N-Butyl bromide, R, values have the following trend in the entire composition range, Benzene < carbon tetrachloride < Aniline < Toluene < Xylene < n-Heptane.
The above order shows the graded interactions between components of the mixtures.Since R can be interpreted as a hard-core volume of one mole of molecules, its values have been associated with atoms, groups of atoms and even types of chemical bonds [12].Fermeglia and Torriano [13] define deviation in molar refraction on mole fraction basis while it is defined on the basis of volume fraction [14] by others.Herein, we have reported ΔR values on the mole fraction basis.Table 1 shows that ΔR values for all the systems studied are negative [15,16].It may be concluded that the interactions resulting in the interstitial accommodation of benzene in to Toluene are the predominant factor over dipole-dipole and dipole induced-dipole interaction.

Conclusions
The present investigations show that the interactions resulting in the interstitial accommodation of benzene into Toluene are the predominant factor over dipole-dipole and dipole induced-dipole interaction between the unlike molecules.The existence of molecular interaction in the mixture is in the order: Benzene < carbon tetrachloride < Aniline < Toluene < Xylene < n-Heptane.It may also be concluded that there is a gradual change in molecular interactions between the components of binary mixtures.The values of deviation in molar refraction for all the systems studied are negative.