Ultrasonic velocity measurements have been used to study the nature of molecular interactions in various pure liquids ^1-3, liquid mixtures ^4-10 and in solutions ^11-17. However, little work has been done for some organic compound solutions ^18-21 especially Schiff bases ^22-25.

Some of these bases are known to possess a wide spectrum of biological activities and are used in pharmaceutical science ^26-29. The presence of different functional groups cause different type of interactions with different solvents which is an important parameter for the selection of these compounds as starting material, intermediate or product ^30-32. Further, binding or interaction of a compound or drug affects their pharmacokinetic and pharmacodynamic properties ^33-35. For the prediction of biological activities and transport phenomena also, physiochemical parameters are required ^36,37 and one of important parameter is interaction between solute and solvent molecules. The ultrasonic study is one of the non-destructive tools to study different types of interactions occurring in solutions ^38,39.

Thus, in present paper, acoustical properties of some vanillin Schiff bases are studied in methanol and THF over entire concentration range at 308.15 K. The results are interpreted in terms of molecular interactions occurring in the solution.

The methanol and THF used in the present work were of AR grade and were purchased from Spectrochem Pvt. Ltd (Mumbai) and were purified according to the standard procedure ⁴⁰. The Schiff bases were synthesized in the laboratory and were recrystallized before use. The common structure of synthesized Schiff bases and their substitution groups (R) are given in Figure 1.

Figure 1

The densities, ultrasonic velocity and viscosity of pure solvents and their solutions were measured by single capillary pycnometer, single crystal variable path ultrasonic interferometer operating at 2 MHz (Mittal Enterprises) and Ubbelohde viscometer respectively. The accuracy of density, velocity and viscosity are ± 0.0001 g/cm³, ± 0.1% cm/sec and 0.05% respectively. All the measurements were carried out at 308.15 K. The uncertainty of temperature is ± 0.1 K and that of concentration is 0.0001 mol/dm³.

The experimental data of ultrasonic velocity, density and viscosity are given in Table 1.

Table 1

From the experimental data, various acoustical parameters were calculated using equations reported earlier ⁴¹.

Figure 2 shows the variation of ultrasonic velocity (U) increases with concentration for all the compounds in methanol and THF. It is observed that ultrasonic velocity increases with concentration for all the compounds in both the solvents.

Figure 2

The velocity depends on intermolecular free length (L _f ). Figure 3 show that L _f decreases continuously with concentration. Thus, intermolecular free length is reverse of velocity. In a solution, when the distance between molecules of solvent and compound decreases, L_f decreases which causes velocity to increase. The decrease in distance suggests interaction between solvent and compound molecules.

Figure 3

This is further supported by isentropic compressibility (Xs) and relaxation strength (r). The variation of these two parameters with concentration of these compounds is also shown in Figures 4 and 5.

Figure 4

Figure 5

It is observed that both isentropic compressibility and relaxation strength decrease with concentration for all the compounds in both the solvents. The decrease of isentropic compressibility and relaxation strength with increasing concentration might be due to aggregation of solvent molecules around compound molecules which causes interaction between molecules of compound and solvent.

For all the compounds in both the solvents, Rao's molar sound function (Rm), molar compressibility (W) and Van der Waals' constant (b) vary linearly with concentration. Table 2 shows the correlation equation and correlation coefficient values for all these solutions. The linear change indicates that there is no complex formation in solution.

Table 2

The type and magnitude of interactions in solution is further confirmed by apparent molar properties. The apparent molar compressibility's ((Фk) of the solutions is fitted to Gucker's relation (42).

From the plot of Фk verses and S _k values are evaluated from the intercept and slope. The isentropic compressibility of all the solutions was also fitted to the following Bachem's relation ⁴³:

The values of A and B were evaluated from the intercept and slope respectively. k⁰ _s is the isentropic compressibility of pure solvent. All these values of intercept and slopes are given in Table 3.

Table 3

Table 3 shows that in both solvents A and ( Ф ⁰ _k values are negative or very low whereas B and S _k values are positive. The low or negative A and Ф ⁰ _k values and positive B and S _k values suggest predominance of solute-solvent interactions.

It is concluded that in studied solutions of vanillin Schiff bases, compound-solvent interactions exist. This suggests that these compounds are more bonded to solvent and some modification is required so that it can easily be absorbed by the target.