Indian Journal of Pure & Applied Physics Vol. 46, March 2008, pp. 169-175 Study of molecular interactions in binary liquid mixtures R Kumar # , S Jayakumar ∗ & V Kannappan + # Department of Physics, The New College, Chennai 600 014 ∗ Department of Physics, R K M Vivekananda College, Chennai 600 004 + Department of Chemistry, Presidency College, Chennai 600 005 E-mail: jk_5454@yahoo.co.in Received 20 March 2006; revised 11 April 2007, accepted14 December 2007 The acoustical parameters for two binary liquid mixtures namely, acetone – carbon tetrachloride (CCl 4 ) and acetone – benzene have been determined at three different temperatures. The acoustical parameters such as the adiabatic compressibility κ, the free length of interaction L f , interaction parameter χ, Lennard Jones Potential (LJP) and also excess parameters such as excess velocity u E , excess compressibility κ E , excess volume V f E and excess free length L f E are computed for the two systems at 303K, 308K and 313K from the measured ultrasonic velocity and density values. The extent of interactions existing between component molecules has been found out. In acetone – CCl 4 system, the interaction parameter values have been out to be negative suggesting the presence of weak dipole –induced dipole interactions in this system. However, in acetone – benzene system, there are strong dipole - induced dipole interactions. With increase in temperature, the extent of interaction becomes weak in both the systems due to thermal agitation of the component molecules. Keywords: Binary mixtures, Free length of interaction, Dipole–induced dipole interaction 1 Introduction Ultrasonic velocity measurements have been employed extensively to detect and assess weak and strong molecular interactions in binary 1,2 and ternary 3,4 mixtures, because mixed solvents find practical applications in many chemical and industrial processes. To meet the needs of applications, ultrasonic velocity measurements are generally carried out at different temperatures. The parameters such as ultrasonic velocity (u), density (ρ) and viscosity (η) and computed parameters such as adiabatic compressibility (κ), free length (L f ), interaction parameter (χ), internal pressure (πi), acoustic impedance (Z) and Lennard Jones Potential (LJP) provide better insight into intermolecular interactions. The present investigation is carried out to study molecular interactions in the binary liquid mixtures of acetone – CCl 4 and acetone – benzene in the temperature range 303 – 313K. 2 Materials and Methods High purity acetone, CCl 4 and benzene (S.D. Fine Chem. Ltd.) are redistilled and purified by the standard methods 5 . Binary mixtures are prepared by mixing appropriate volumes of the liquid components in the standard flasks with airtight caps and the mass measurements are performed on high precision digital balance with an accuracy of ±1 mg. The uncertainty in mole fraction is ±0.0001. The accuracy in the measurements of density and viscosity is of the order of ±0.01 and ±0.001, respectively. The required properties are measured for freshly prepared solutions. The ultrasonic velocity is measured with an uncertainty of ±0.3% using a single crystal ultrasonic interferometer (Mittal Enterprises, New Delhi) operating at 2 MHz (Model F81), which is calibrated with water and benzene. The temperature stability is maintained within 0.1 K by circulating thermostated water around the interferometer cell that contains the liquid, with a circulating pump. In order to minimize the error of measurements, several maxima of ultrasonic velocity are allowed to pass and their number n is counted. All maxima are recorded with highest swing of the needle on the micrometer scale of the interferometer. The total distance, d moved by the reflector of the interferometer cell is given by