ULTRASONIC INVESTIGATIONS OF MOLECULAR INTERACTIONS IN AQUEOUS ELECTROLYTIC SOLUTIONS AT VARYING TEMPERATURES

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1 Research Article ULTRASONIC INVESTIGATIONS OF MOLECULAR INTERACTIONS IN AQUEOUS ELECTROLYTIC SOLUTIONS AT VARYING TEMPERATURES 1 Shinde B. R, 2 Jadhav K. M. Address for Correspondence 1 Department of Engineering Sciences (Applied Physics), Sanjivani Rural Education Society s College of Engineering, Kopargaon. 2* Department of Physics, Dr. Babasaheb Ambedkar, Marathwada University, Aurangabad (MS), India. brscoe@gmail.com ABSTRACT Acoustical parameters such as adiabatic compressibility s, specific acoustic impedances, intermolecular free lengths and solvation numbers have been estimated using standard relations from measured values of Ultrasonic velocities, densities and viscosities in wide range of concentrations at , , and K temperatures for ZnSO 4 + water system to study the ion-solvent interactions. It is observed that ion-solvent interactions are increases in increasing temperature and the concentration of Zn ++ ions in water. However in very dilute solutions there is weak ion-solvent interactions and for moderate concentrations U increases steadily, which shows that ion-solvent interactions increases with concentrations suggesting structure promoting nature of Zn ++ ions in cluster of water molecules. KEYWORDS: ultrasonic velocity, ion-solvent, adiabatic compressibility, intermolecular free length. INTRODUCTION transition metal ions. In solution of ionic A survey of literature 1-13 indicates that acoustical parameters are useful in understanding the nature and strength of molecular interactions in the liquid mixtures. Ultrasonic energy is used in medicine, engineering, agriculture, defence and industry. In chemical industries ultrasonic energy is found useful in studying the chemical processes and in synthesis of chemical substances. Recently an ultrasonic wave has been extensively used to determine the ionsolvent interactions in aqueous solutions containing electrolytes. A number of researchers have studied acoustical properties of solutions containing solute the attraction between the solute and solvent is of ion-dipole type. When electrolyte is dissolved in solvent it causes volume contraction due to interactions between ions and solvent molecules and therefore other acoustical parameters may be affected. Literature survey 1-13 shows that attempt has not been made by any of researchers to study the ion-solvent interactions in aqueous solutions of some electrolyte at varying temperatures. In this paper an attempt has been made to calculate acoustical parameters such as acoustical impedance (Z), adiabatic compressibility (β), Intermolecular free length (Lf) and solvation number (Sn) with

2 the help of experimentally calculated parameters viz. ultrasonic velocity (U), density (ρ) and viscosity (η) for aqueous ZnSO 4 solutions in a wide range of concentrations and at varying temperatures to study the ion-solvent interactions of Zn ++ ions and water molecules. METHODS AND MATERIALS Ultrasonic velocity for the solutions was measured using a single crystal ultrasonic interferometer of 2 MHz frequency(model F-81) supplied by Mittal enterprises, New Delhi with an accuracy of ± 0.25 m/sec at 25 C. The temperature was kept constant by constant temperature water bath with an accuracy of ± 0.1K. The temperature of the circulating water near the cell was measured by using PT-100 sensor and was found to be accurate to C. The densities of various concentrations have been measured using 25ml capacity specific gravity bottle and digital balance (Shinko, model HTR-220E, made in Japan) with an accuracy of gm. The viscosities have been determined by using Ostwald s viscometer with an accuracy of 0.001Nsm -2. The Merck made hydrated salt ZnSO 4.7H 2 O of Anal-R grade having 99.5% assay was used without further purification for the present investigation. The aqueous solution of ZnSO 4.7H 2 O was prepared in double distilled water in the presence of perchloric acid to avoid the hydrolysis 3. THEORY AND CALCULATIONS Using measured values of ultrasonic velocity (U), density (ρ) and viscosity (η) some acoustical parameters have been calculated using following standard 4, 5, 6 expressions Specific acoustic impedance (Z) = ρu (Kgm -2 S -1 ) Adiabatic compressibility (β) (Kg -1 ms -2 ) Intermolecular free length (Lf) = (m) = 1/U 2 ρ Where k is temperature dependent constant called as Jacobson constant, it is , , and for , , and K respectively. Solvation number (S n ) = k n s β 1 ni β0 n s = number of moles of solvent (mol/kg) n i = number of moles of solute (mol/kg) β = Adiabatic compressibility of solution at a particular temperature (N -1 m 2 ) β 0 = Adiabatic compressibility of solvent at a particular temperature (N -1 m 2 ) β

3 RESULT AND DISCUSSION Table1:Ultrasonic velocities (U), densities (ρ), specific acoustic impedances (Z), viscosities (η), adiabatic compressibility s(β), intermolecular free lengths(l f ), solvation numbers(s n )for ZnSO 4 + water system at K and K,313.15K and K Temp K K Conc./M (U) ms -1 (ρ) Kgm -3 ( η ) /10-4 Nsm -2 (Z) /10 6 Kgm -2 S -1 ( β) /10-10 Kg -1 ms -2 ( L) /10-11 m S n Continued on next page)

4 Table 1. (Continued) Temp K K Conc./M (U) ms -1 (ρ) Kgm -3 ( η ) /10-4 Nsm -2 (Z) /10 6 Kgm -2 S -1 ( β) /10-10 Kg -1 ms -2 ( L) /10-11 m S n

5 The experimental values of ultrasonic velocity, density, and viscosity and the calculated values of acoustic impedances, adiabatic compressibility s, Intermolecular free lengths and solvation numbers for ZnSO 4 + water system for the wide range of concentrations for four different temperatures are given in table 1. At all temperatures the ultrasonic velocity, density and viscosity of aqueous solutions of Zn ++ ions increases with increasing concentrations and corresponding decrease in these experimental parameters with increase in temperature except ultrasonic velocity. The increase in the density with increasing concentration of solutes suggest a moderate strong electrolyte nature in which the Zn ++ ions tends to attract the water molecules. The gradual increase in velocity, density and viscosity with ZnSO 4 concentration at all temperatures may be due to interactions between Zn ++ ions and water molecules 5. From fig.1 it is observed that the ultrasonic velocity (U) is found to increase with increase in Zn ++ ions concentrations. There is slight increase in velocity for very dilute solutions, suggesting weak ion-solvent interactions. However for moderate concentrations U increases steadily, which suggest that ionsolvent interactions increases with concentrations. This is because increase in U in any solution indicates the maximum ion-solvent interactions due to the hydrogen bonding between the solute and solvent molecules. At M and M dilute solutions for all temperatures there is a slight decrease in U which suggests that Zn ++ ions behave as a structure breaker for the cluster of water molecules suggesting weak ion-solvent interactions. The ultrasonic velocity of aqueous solutions of ZnSO 4 was found to increase with increasing temperature 6. In aqueous Zn ++ ions solution, Zn ++ ions form a core compact structure with the solvent molecules through hydrogen bonding. Due to this effect the compressibility decreases. When temperature increases the mean distance between the Zn ++ ions and water molecules tends to increase with corresponding increase in the compressibility and hence increase in the ultrasonic velocity showing strong ion-solvent interactions 6, 7. Table1 shows that adiabatic compressibility (β) decreases with increase in concentrations and temperature of Zn ++ ions in water; this confirms the presence of ion-solvent interactions through ion-dipole interactions between Zn ++ ions and surrounding water molecules. Because when an ion is added to water, it attracts certain water molecules towards itself by moving with a violent twist in the bulk of the water molecules due to the force of electrostriction.

6 Because of this, the available water molecules for the next incoming ion get decreased. This closeness is called the compression. Every solvent has a limiting compressibility. The compressibility of a solvent is higher than that of solution and it decreases with the increase in concentration of the solution. With increase in ionic concentration, their electrostrictive forces cause the structure to break and the solute surrounded water molecules are more compactly packed. This effect in turn reduces the compressibility with increase in the Zn ++ ions concentration 6, 7, 8. When acoustic wave travel through a medium there is a variation of pressure from particle to particle. The ratio of instantaneous pressure excess at any particle of the medium to the instantaneous velocity of that particle is known as specific acoustic impedance (Z) of the medium. This factor is governed by the inertial and elastic properties of the medium 7. From table 1 and figure 2, it is observed that the values of acoustic impedance vary linearly with the increase in the Zn ++ ion concentrations in water. The variation in Z for very dilute solution is very less which shows weak ion-solvent interactions similar to U. However increase in Z is appreciable with increase in temperature for very weak concentration shows the increase in ion-solvent interactions. For moderate concentration of Zn ++ ions in water, Z linearly increases with concentration as well as for all temperatures showing strong ion-solvent interactions 6, 9. With increase in temperature specific acoustic impedance (Z) values increases showing structure promoting nature of Zn ++ ions in water suggesting increase in ion-solvent interactions 7.

7 Journal of Engineering Research and Studies

8 Intermolecular free length shows a similar behavior as reflected by the compressibility values. The decreased compressibility brings the molecules to a closer packing resulting in a decrease of intermolecular free length. The variation of ultrasonic velocity in a solution depends upon the increase or decrease of intermolecular free lengths after mixing the component 10, 11, 12. The Ultrasonic velocity should increase if the intermolecular free length decreases as a result of mixing the components. If the free length decreases on increasing the concentration of solute then it indicates significant interactions between the ions - solvent molecules suggesting a structure promoting tendency of the added solute 3, 5. The solvation number of solute molecules is explained on the basis of Frank & Wen model of solute-solvent interactions1 3, 14, which picture three different solvent structure regions in the neighborhood of solute. Just outside the molecule, there is a layer of immobilized and compressed solvent molecules as a result of electrostrictive and other attractive forces exerted by the solute. This is known as primary sheath of solvation. This layer is surrounded by slightly less compressed or structure - broken region of solvent molecules distantly affected by these forces. This layer is called as secondary sheath of solvation.

9 The outermost layer is bulk solvent which is not affected by any of the above forces. The solvation number in the primary sheath corresponds to coordination number and it is concentration independent, while the solvation number in the secondary sheath is concentration dependent 3, 6, 14. The computed values of solvation number for Zn ++ ions in water for different temperatures are given in table1. Fig.4 shows the graph of S n values against concentration for four different temperatures for given system. The solvation number in aqueous ZnSO 4 solution decreases with increase in concentration and it may attain the primary solvation in pure crystalline state. The positive values of solvation number indicate an appreciable solvation of solute 6. This suggests the structure promoting nature of solution as well as the presence of ion-dipole interaction between Zn ++ ion and water molecules. Solvation number decreases with increase in temperature shows the weakening of ionsolvent interactions. CONCLUSION Ultrasonic investigations of molecular interactions in aqueous ZnSO 4 solutions have been carried out at wide range of concentrations at different temperatures. The experimental parameters such as

10 ultrasonic velocity, density, viscosity and the other acoustical properties gives valuable information regarding ion-solvent interactions in aqueous solutions. There is uniform decrease in density and increase in intermolecular free length with increase in temperature indicating the loosening of intermolecular forces due to thermal agitation of the molecules in aqueous solutions of ZnSO 4 at high temperatures. The ultrasonic studies provide comprehensive investigations between Zn ++ ion + water molecules arising due to hydrogen bonding. ACKNOWLEDGEMENT One of the authors (B.R.Shinde) is very much thankful to BCUD, University of Pune for providing financial assistance. REFERENCES 1. Jacobson B, velocity in liquids and liquid mixtures. J. Chem. Phys.1952; 20: Lageman R. J., Dunbar W S., Sound velocity approach for qualitative determination of the degree of association in liquids. J. Phys. Chem, 1945; 49: Kannappan V. & Chidambara V. S., Ultrasonic investigation of ion-solvent interactions in aqueous and non-aqueous solutions of transition and inner transition metal ions. Indian J of Pure & App. Phy, 2007; 45: Palani R and Jayachitra K, Ultrasonic study of ternary electrolytic mixtures at 303,308 and 313K. Ind. J. Pure & App. Phys 2008; 46: Frank S & Wen W Y, Disc. Faraday Society 1957; 24: Kannappan A.N. & Palani R., Acoustical behavior of glycerine, dextrose and sucrose in Na 2 CO 3 and NaHCO 3 buffer solution, Ind J of Pure & App. Phy 2007; 45: Sethu Raman M & Amirthaganesan G, Ultrasonic study of intermolecular association through hydrogen bonding in aqueous solutions of D mannitol. Ind. J. Physics 2004; 78(12): Awasthi A, Shukla J P, Ultrasonic and IR study of intermolecular association through hydrogen bonding in ternary liquid mixtures. Ultrasonics 2003; 41: Seetharaman V, Kalyansundaram S and Gopalan A, Ultrasonic studies of aqueous solutions for ploy diallyl dimethylammonium chloride. Ind. J. Pure and Appl. Phy. 2004; 42: Yadava S.S, Yadav A., Ultrasonic study on binary liquid mixtures between some bromoalkanes and hydrocarbons. Ultrasonics, 2005; 43: A, Nain A K and Ahmad S., ultrasonic velocity and other acoustical parameters of ion-solvent in DMSO + water + KBr system at K. Acoustic Letters 1997; Dash U N, Roy G S, Mohanty S & Ray M, Thermo-acoustic study of cobalt (III) complexes in aqueous and binary aqueous media. Indian J. of Pure & App. Phy.2007; : Kannapan V & Jaya Santhi R, Ultrasonic study of induced dipole-dipole interactions in binary liquid mixtures. Indian J of Pure & App. Physics 2005; 43: Jayakumar S, Karunanidhi N and Kannappan V, Ultrasonic velocity and other acoustical parameters in binary mixtures of O-cresol, P-cresol, O- chlorophenol and P-chlorophenol with CCl 4 at K. Ind. J. Pure and Appl. Phy. 1996; 34: Frank S & Wen W Y, Disc. Faraday Society 1957; 24: Atkins P W, Physical Chemistry, 6 th edn. Oxford University press: 1998.

Ultrasonic investigation of ion-solvent interactions in aqueous and non-aqueous solutions of transition and inner transition metal ions

Indian Journal of Pure & Applied Physics Vol. 45, February 2007, pp. 143-150 Ultrasonic investigation of ion-solvent interactions in aqueous and non-aqueous solutions of transition and inner transition