Acoustical studies of some synthesized Schiff base derivatives in dimethylsulphoxide at 303.15 K, by ultrasonic velocity measurement

The density, viscosity and sound velocity of five Schiff bases (1-5) derivatives in DMSO solutions have been studied at 303.15 K over a wide range of concentration. From these experimental data, some acoustical parameters such as Molar volume (Vm), Specific Acoustic Impedance (Z), Adiabatic compressibility (β ad ), Intermolecular Free Length (L f ), Rao’s Constant (R), Molar compressibility (W), Relaxation time(τ), van der Waals constant (b), Relaxation strength (r), Relative association (R A ), Isothermal compressibility ( ) , Isothermal expansion co-efficient (α) ,Free volume (V f ) and Internal pressure (π i ) and Ultrasonic attenuation (α/f²) have been evaluated. A fairly good correlation between a given parameter and concentration is observed. The results are interpreted in terms of molecular interactions like solvent-solvent, solvent-solute and solute-solute interactions.


INTRODUCTION
In the recent years, measurements of the Ultrasonic velocity are helpful to interpreted solute-solvent, ion-solvent interaction in aqueous and non aqueous medium [1]. Ultrasonic investigations find extensive applications in characterizing aspects of thermodynamics and physico chemical behavior of solutions of organic compounds. The measurement of ultrasonic speed enables to accurate determination of some useful parameters, which are highly sensitive to molecular interactions [2].
Literature survey on ultrasound velocity measurement shows that very little work has been done for solid organic compounds. The ultrasonic velocity along with other experimental data such as density and viscosity, furnish wealth of information about the interaction between ions, dipoles, hydrogen bonding, multi-polar and dispersive forces. This molecular interactions between like and unlike molecules are influenced by structural arrangement along with shape and size of the molecules [3].
We have synthesized some new Schiff base derivatives. In continuation of our work we intended to investigate the solute-solute, solute-solvent molecular interactions of these newly synthesized Schiff base derivatives in DMSO. By simply measuring density of liquid and its corresponding ultrasonic velocity, many parameters like Molar volume (Vm), Specific Acoustic Impedance (Z), Adiabatic compressibility (β ad ), Intermolecular Free Length (L f ), Rao's Constant (R), Molar compressibility (W), Relaxation time(τ), vander Waals constant (b), Relaxation strength (r), Relative association (R A ) Isothermal compressibility (β ), Isothermal expansion co-efficient (α), Free volume (V f ) and Internal pressure (π i ) and Ultrasonic attenuation (α/f²) etc. can be determined. From the observations of these properties the molecular interactions are predicted.

EXPERIMENTAL
The solvent used in the present work of AR grade and were purified according to the standard procedure described in the literature [4]. The Compounds were recrystalized before use. The structure of the synthesiszed compounds are given in Scheme 1.
Solutions of different molarity were prepared for each binary system. The ultrasonic velocity in the mixture was measured using a variable path fixed frequency ultrasonic interferometer working at 2 MHz frequency (Mittal enterprises, New Delhi). The accuracy of sound velocity was ±0.1 ms -1 . The density was determined at the experimental temperature using 10 ml capacity specific gravity bottle immersed in a thermostatic bath (accuracy +0.01 °C). The volume of the bottle at the experimental temperatures, viz. 303.15 K was ascertained using doubly distilled water. The densities of water at these temperatures were obtained from literature. The viscosity of pure liquids and liquid mixtures at 303.15 K were determined using an Ostwald viscometer.

RESULTS AND DISCUSSION
Ultrasonic velocity (u), density (ρ) and viscosity (η) of the solutions were obtained using the relations; From the experimental data of ultrasonic velocity (u), density (ρ) and viscosity (η), the various thermodynamic parameters such as adiabatic compressibility (β ad ), intermolecular free length (L f ), specific acoustical impendence (Z), Rao's molar sound function (R), Vander Waals constant (b), etc., were evaluated using the following standard equations [5][6][7][8][9][10][11][12][13][14][15][16][17] Some of these calculated parameters are given in Table 1 Figure 1 show the variation of ultrasound velocity (U) with concentration in DMSO solutions. It is observed that overall ultrasonic velocity (U) increases with concentration for all the compounds in DMSO solvent. The velocity depends on intermolecular free length (Lf). The velocity increases with decrease in Lf or vice versa. It is evident from Table 2 that Lf decreases continuously, which suggests that there is strong interaction between solvent and compound molecules.
Specific acoustic impedance is defined as the impedance offered to the sound wave by the components of the mixture. Acoustic impedance increases with increase in concentration. Increasing trend of acoustic impedance further support the possibility of molecular interaction due to hydrogen bonding between Schiff base and DMSO. Specific acoustic impedance is directly proportional to ultrasonic velocity and inversely proportional to adiabatic compressibility and shows similar behaviour to that of ultrasonic velocity and opposite to that of adiabatic compressibility.    This is further supported by adiabatic compressibility and relaxation strength (r) values. The variation of adiabatic compressibility with concentration of these compounds is also shown (Fig. 2). Both adiabatic compressibility and relaxation strength. Table 2 and 3 are  observed to decrease with concentration for all the compounds. The decrease in adiabatic compressibility is attributed to the fact that the Schiff base molecules in dilute solutions are considerably ionized and these ions are firmly bound to surrounding solvent molecules. The orientation of solvent molecules around the ions is attributed to the influence of the electrostatic field of the ions, which lowers the compressibility of the Schiff base solution.   (Table 3). show the variation of Vander Waals constant (b) with concentration. All the three parameters vary linearly with concentration. This linear change of these parameters suggests the absence of complex formation in these systems.
The internal pressure (π i ) is the results of forces of attraction and repulsion between the molecules in a solution. Figures 5 show the variation of internal pressure with concentration for schiff base derivatives in DMSO. Although decrease in compressibility, intermolecular free length, relaxation strength and increase of velocity, viscosity suggest predominance of solute-solvent interaction, the decrease in internal pressure indicates the existence of solutesolute interactions also in these systems. It is clear from these figures that π decreases with concentration, which indicates the decrease in cohesive forces.
Relaxation time decreases with increase in concentration (Fig. 3). The relaxation time which is order of 10 -10 sec is due to structural relaxation [18][19] process in such a case it is suggested that molecules get rearranged due to co-operative process [20][21][22][23][24][25][26].     The free volume of solute molecule at particular temperature and pressure depends on the internal pressure of liquid, in which it was dissolved (Fig. 5) shows the variation of free volume with concentration for all the compounds in DMSO. The decrease in molecular association causes an increase in free volume. Thus, free volume is an inverse function of internal pressure. It is evident from (Fig. 5) that free volume increases with concentration for all the compounds, indicating the presence of solute-solute interactions also. This suggests that both solute-solute and solute-solvent interactions exist in these systems

CONCLUSION
Further, as these systems are characterized by hydrogen bonding, the solute-solvent interactions can be interpreted in terms of structural change that arises due hydrogen bond interactions between various components of the solvent solution systems. DMSO belong to the solvent having functional group of S=O, While Schiff base derivatives belong to solute having functional group CH=N and aromatic ring containing substituent. Thus, the overall finding suggests that the studied compounds exhibit solute-solvent interactions i.e. structure forming tendency of these compounds in DMSO solvent.