The Study of Propagation of Ultrasonic Waves and Molecular Interactions in the Solutions of Magnesium Salicylate Tetrahydrate

: Present study deals about the experimental results of ultrasonic velocity, density and viscosity along with estimated parameters for the liquid mixture of magnesium salicylate tetrahydrate (MST) as solute and 50% ethanol as a solvent at 298.15K, 303.15K and 308.15K temperatures. The frequency selected for measurements of velocity is 4MHz. The measured values of density (ρ), ultrasonic velocity (v) and viscosity (η) are used to calculated various thermoacoustic parameters lik e Adiabatic compressibility (β s ), free length (L f ), Specific acoustic impedance (Z), Apparent molar compressibility ( ϕ K ), free volume(V f ), Relaxation time (ι), Relative association (R A ) Rao’s constant (R) and Wada’s constant (W). The calculated values are utilized to study the intermolecular interactions existed between magnesium salicylate tetrahydrate in 50% ethanol. The variation in the values of these parameters with composition of the mixture at the temperatures 298.15K, 303.15K and 308.15K has been discussed in terms of molecular interaction between the components of liquid mixtures.


INTRODUCTION
Ultrasonic velocity measurement and calculation of associated parameters is one of the key factor to study the nature of liquid mixture. Ultrasonic wave propagation in liquid has been the subjects of exhaustive research which has been carried out theoretically and practically. The number of techniques are used to study the molecular structures in mixtures. Ultrasonic methods have the added advantage of being less cost with efficiency comparable to other methods. Many researchers have used ultrasound to study the molecular interactions in aqueous solutions containing electrolytes [1]. Ultrasonic wave propagation affects the physical properties of the medium and hence can furnish information on the liquid and liquid mixtures [2]. Studies on speed of sound, viscosity, acoustic thermodynamic, excess thermodynamic parameters and their deviations in the binary systems have been the subject of many investigations in the recent years. These investigations on different systems reveal specific interactions between the molecules of the component liquids, it has been reported by several workers [3]. Theoretical evaluation of speed of sound in binary liquid mixtures and its correlation to study the molecular interactions has been successfully done in recent years [4] using different theoretical relations. The deviations in the values of velocities and adiabatic compressibilities of liquid mixtures with composition are useful to describe the difference in molecular size and strength of interaction between the molecules [5]- [6]. The present work is focused on the study of acoustic, viscometric properties, by measuring the ultrasonic velocities, densities and viscosities of the mixtures of Magnesium salicylate tetrahydrate + 50% ethanol as a solvent. The data obtained can be used to understand intermolecular interactions between the unlike molecules and to test the theories of solutions. By knowing the interaction between the drug molecules and solvent molecules, this work may be helpful to understand the action of drug and this may be useful to create new drugs.

A. Materials
Magnesium salicylate tetrahydrate used in the present work was of analytical reagent (AR) grade. The solution was prepared by using 50% mixture of distilled water and 99.99% pure ethanol as a solvent. Weights have been taken on digital electronic balance.(Model-CB/CA/AT-Series).

B. Methods
The ultrasonic velocities have been measured by using ultrasonic Interferometer(Model-M-83). Mittal Enterprises, New Delhi) operating at 4MHz frequency with an accuracy of ±2m/s. The viscosities (ƞ) of solution and solvent were determined using Ostwald's viscometer by calibrating with distilled water. The densities (ρ) of the solution were measured accurately using digital densitometer (Model -DMA-35, Anton Paar). The ultrasonic velocity was measured at 4MHz frequency at 298.15K, 303.15K. and 308.15K. The temperature of cell was maintained with continuous circulation of water at constant temperature by using thermostat.

III.
RESULTS AND DISCUSSION The acoustical parameters were calculated from v, η and ρ S values using standard formulae.         . The viscosity of the system decreases with increase in the concentration and afterwards viscosity increases with increase in concentration of magnesium salicylate tetrahydrate. It is well known that ethanol and water molecules are highly polar and in addition, they can accept as well as donate protons. Hence the dipole-dipole and acceptor-proton type interactions are also possible in addition to the already explained hydrogen bonding interactions.
In system magnesium salicylate tetrahydrate + 50% ethanol, the increase in the adiabatic compressibility (Fig. 4)  The decrease in acoustic impedance with increase in concentration (Fig. 5) of the solution can be explained on the basis of molecular interactions between solutes and solvent molecules which decreases the intermolecular distance making relatively shorter gap between the molecules and become responsible for the propagation of ultrasonic waves. From fig. 6, it is observed that the intermolecular free length increases with increase in concentration of solute in the liquid system. It shows that there are weaker molecular interactions [10]. The values of relaxation time for system are found to increase with increase in concentration (Fig. 7). These values are in the order of 10 -14 seconds due to the structural relaxation process showing the presence of molecular interactions. Variation in the values of relative association shows that salvation process is predominant over the breaking of solvent molecules and results in the dipole-dipole interactions because both the solvent molecules (water and ethanol) are polar in nature. The higher molar compressibility of magnesium salicylate tetrahydrate at lower concentration may be due to breakdown of associated alcohol cluster in the bulk phase creating concentrated less efficient packing and hence relatively higher compressibility. The linear decrease in W with concentration shows the weaker molecular interactions due to competition between water and ethanol in the process of salvation of solute. The variation in the values of free volume is graphically shown in fig. 12 respecively. Here it is found that free volume decreases with increase in concentration [11]- [12] of the solution in both the solvents. It indicates that the space between the solute molecules and solvent molecules decreases. It means they are closely packed.

IV. CONCLUSION
Linear variation in ultrasonic velocity and adiabatic compressibility, acoustic impedance, free length, relaxation time relative association apparent molar compressibility, free volume, Rao's constant and Wada's constant is observed in solvent 50% ethanol, It shows that significant molecular interactions are observed in solvent 50% ethanol. As both the functional groups -COOH and -OH of salicylic acid molecule are blocked by Mg 2+ in magnesium salicylate tetrahydrate, the salvation process is significant and more effective in 50% ethanol. Negative values of apparent molar compressibility are due to solute-solvent molecular interactions.Due to highly polar nature of water and ethanol, the dipole-dipole and acceptor-proton type interactions are also possible in addition to the hydrogen bonding interactions. REFERRENCES