Excess parameter studies on tetrahydropyran with 1-hexanol at T = 298.15 to 318.15 K using Anton Paar

Sound velocity, densities of binary mixture of Tetrahydropyran (THP) with 1-hexanol has been measured over the entire range of composition at T = 298.15 to 318.15 K. The excess parameters viz., excess sound velocity, deviations in isentropic compressibility, excess molar volume, excess free length and excess acoustic impedance are deduced from experimental values and discussed intermolecular interactions present in the mixture. At the end all the parameters have been fitted to Redlich-Kister equation and their coefficients are obtained.


INTRODUCTION
Ultrasonic velocity studies in binary liquid systems which are capable of supporting extensive hydrogen bonding networks have been carried out by many research groups [1][2][3][4][5][6]. Such studies can provide a lot of information on the molecular interactions. Ultrasonic wave propagation affects the physical properties of the medium and hence, can furnish information on the physics of the liquid and liquid mixtures. THP is used in polymerization processes [7], is a cyclic monoether, an excellent solvent very often used in the manufacture of special chemicals & 1-hexanol is a substance heavily used in the perfume industry. In this paper the sound velocity, densities of binary mixture THP with 1-hexnaol have been measured at 298.15 to 318. 15 K using Anton Paar. From the experimental values, excess sound velocity (u E ), deviations in isentropic compressibility (∆K s ), excess molar volumes (V m E ), excess free length (L f E ) and excess acoustic impedance (Z E ) for the binary system are estimated using standard equations that are reported by several authors [8][9][10]. Apart from that the deviations in isentropic compressibility are more related with structural effects and packing phenomena. The systematic investigations of these excess properties are therefore of great importance. The values of ∆K s and V m E which can be measured with great accuracy reflect the degree of deviation from ideality. The deviations form ideal behavior has been widely used for the study of structural variations and molecular interactions of mixtures [29][30][31][32][33][34].

EXPERIMENTAL DETAILS
THP, 1-pentanol were purchased from Aldrich chemical Company with purities >0.998, the purities of the purified liquids were checked by measuring their sound velocity, densities [recorded in Table 1] using Anton Paar DSA 5000M at the range of 298.15 to 318.15 K (± 0.01 K) and these agreed to within ±2 × 10 −3 kg m −3 with their literature values [11][12][13][14][15][16][17][18][19][20]. Sound velocity (u), densities (ρ) of the pure liquids and their binary mixture were measured using a vibrating U-tube digital density and sound analyzer as explained in the literature [21,22] which is the same as claimed by the manufacturer. The measurements are based on measuring the period of oscillation of a vibrating U-shaped hollow tube filled with the sample. The calibration of the apparatus was carried out with the double distilled, de ionized water before each series of measurements. The mole fraction of mixture was obtained with uncertainty of 1 × 10 −4 from the measured apparent masses of the components. All the mixtures were weighed on an electric balance Sartorius, model CP 225D, +/-0.01 mg. The uncertainties in the density and speeds of sound measurements are 2 × 10 −3 kg·m −3 and 0.1 m s −1 respectively.

THEORY
Using the measured values of data, calculate the various thermo acoustical parameters such as; Isentropic compressibility K s = Intermolecular free length
Further, the excess parameters were fitted to Redlich -Kister polynomial equation to estimate the adjustable parameters.
using least-squares regression method, the (a i ) coefficients are obtained by fitting above equation to the experimental values. The optimum number of coefficients is ascertained from an examination of the variation in standard deviation (σ) (7) where 'N' is the number of data points and 'n' is the degree of fitting

RESULTS AND DISCUSSION
The values of sound velocities and densities for pure liquids are experimentally measured and are compared with the literature values and they are good agreement with each other as given in the Table 1. The experimental data related to excess sound velocity, deviations in isentropic compressibility, excess molar volume, excess free length and excess acoustic impedance for the binary liquid mixture at different temperatures are given in Tables 2(a), 2(b). Further, parameters (a i ) and standard deviations σ (Y) for the Redlich-Kister equations are reported in table 3.
According to R J Fort et al. and A Ali et al. the sign and magnitude of ∆K s and V m E play a vital role in assessing the molecular interactions in the liquid mixtures. In general negative values of ∆K s and V m E indicates strong interaction in the mixture which include chargetransfer, dipole-dipole, dipole-induced dipole interactions and interstitial accommodation of the smaller molecules into the spaces created by bigger molecules, while positive signs of these parameters are indicative of weakening of interactions between the component molecules [23,24].
ILCPA Volume 36   1 shows the behavior of excess sound velocity (u E ). It exhibits less magnitude of deviations at the entire composition range of THP for the mole fraction 0.2445 to 0.7436 in the mixture at all five temperatures when compare to other mole fractions studied. According to A Ali et al [24] the negative (less magnitude) deviations of u E suggests the existence of dispersion forces in the system. Further, the deviations of u E are observed to be in opposite trends of ∆K s it suggests specific interactions are exists in the mixture.   fig. 3 for the whole mole fraction range of THP. This indicates specific interactions of the following [25]. At the temperatures 298.15, 303.15 & 308.15 K for the whole mole fraction of THP an increase of ∆K s denotes weakening of inter-molecular interactions.
The hetero-molecular i.e. hexanol-THP interactions not only disturb the homo molecular (hexanol-hexanol, THP-THP) interactions in components liquids, but also cause re-arrangement in the geometry of the clusters in such a way volume of the cluster increases. It will lead to increase in volume of the cluster i.e. excess molar volume (V m E ) will be positive.
Where as at temperatures 313.15 & 318.15 K as mole fraction of THP is < 0.2445 and > 0.7436, it is observed that ∆K s is negative & V m E is positive. The decrease in ∆K s indicates an attractive hetero molecular interactions leading to an association of molecules but still molar volume increases, the reason for increase in volume may be due to the larger size of molecular clusters (hexanol-hexanol, THP-THP). The variation of excess intermolecular free length (L f E ) is shown in Fig. 4. The deviation of L f E are well supports the deviation of isentropic compressibility. The sign of excess properties plays a vital role in assessing the compactness or extent of molecular interactions.
The various types of interactions that are operating between the molecules are dispersion forces, which should make a positive contribution to excess values and charge transfer, H-bonding, dipole-dipole interaction and dipole-induced dipole interactions expected to make negative contributions. In the present mixture as ∆K s and L f E are positive and negative suggesting dispersive and attractive forces are present in the mixture [26].     Further, the calculated values of excess acoustic impedance (Fig. 5) of binary mixture at all temperatures studied are negative suggests that the rupture of the hydrogen bonded chain of 1-hexanol dominates over that of the hydrogen bond formed between the unlike molecules. The similar observations are reported by the author in his earlier work on 1,4dioxane with 1-butanol [27,28], this shows weak molecular interactions between the components of the mixture exists.

CONCLUSION
Sound velocity and densities for binary mixture consist of THP with 1-hexanol system is measured at T = 298.15, 303.15, 308.15, 313.15 & 318.15 K using Anton-Paar. The calculated excess parameters are discussed and concluded the presence of weak dispersion forces in the mixture.

ACKNOWLEDGEMENT
The author (AK) is grateful to the Andhra Loyola College (Autonomous) NAAC re-accredited at 'A' grade with 3.65/4.00 points, Vijayawada-8. For allowing the author to avail laboratory facilities of the Department of Physics, thank s extended to Dr Ch Srinivasu for giving fruitful discussions and suggestions.