Ultrasonic study on binary mixture containing dimethylformamide and methanol over the entire miscibility range (0 < x < 1) at temperatures 303–323 K

Abstract

The experimental density and speed of ultrasound measurements in connection with literature data have been measured for pure N,N-dimethylformamide (DMF), methanol and their binary mixtures over the whole miscibility range at different temperatures 303, 308, 313, 318 and 323 K. These parameters were used to determine the adiabatic compressibility, intermolecular free length, molar compressibility, molar sound velocity, acoustic impedance, relaxation strength and their excess values. The variation of these parameters with composition of mixture indicates the nature and extent of interaction between unlike molecules. The non-ideal behavior of the system studied was explained on the basis of the dipole-induced dipole interactions and hydrogen bonding. The complex formation through intermolecular hydrogen bonding was confirmed from the recorded FTIR spectra. Available thermal energy breaks the bonds between the associated molecules into their respective monomers on increasing the temperature.

Introduction

The concentration and temperature dependence of acoustic properties has proved to be a significant observation of intermolecular interactions in liquids, liquid mixtures and solutions [1], [2], [3], [4]. Since liquid mixtures have found applications in medicine, engineering, agriculture and other industrial applications, the study and understanding of thermodynamic and transport properties are more essential [5], [6]. Intermolecular interactions in liquid mixtures modify the structural arrangements and hence change the shape of the molecule. The study of molecular interactions in the liquid mixtures is therefore important in elucidation of the structural properties of the molecules. Measurements of ultrasonic velocity and density have been used to calculate various parameters related to different types of molecular interactions in liquid mixtures. The compressibility behavior of solutes, which is the second derivative of the Gibbs energy, is a very sensitive indicator of molecular interactions and can provide useful information about these phenomena [7], [8], [9], [10]. The non-rectilinear behavior of ultrasonic velocity, compressibility and other thermodynamical parameters of liquid mixtures also reveal the strength of interactions. To get additional information about the nature and strength of molecular interactions, other related acoustical parameters such as free length, adiabatic compressibility, free volume, acoustic impedance and their excess parameters have been calculated in the liquid mixtures [11], [12]. The excess thermodynamic functions are sensitive to the intermolecular forces as well as to the size of the molecules. In order to study all these molecular-kinetic properties of liquids and liquid mixtures, low amplitude ultrasonic wave is very valuable. Ultrasonic methods have established a permanent place in science and new applications and found for the solution of many theoretical and practical problems. Most important features of ultrasonic systems are robustness, non-invasiveness, precision, low cost, rapidity and easy automation.

Since acoustic parameters provide a better insight into molecular environments in liquid mixtures, it seems important to study molecular interactions in binary liquid mixtures. The present investigation is concerned with the study of N,N-dimethylformamide (DMF)–methanol mixtures covering the whole miscibility range. DMF is highly polar (μ = 3.86 D and ɛ = 36.71 at 298.15 K), yet is practically unassociated and behaves as a good solvent in chemical and technological process [13], [14], [15]. Moreover, DMF is reported to be a powerful breaker of polymerized structures of hydroxy compounds and the dipoles of DMF are randomly oriented [16]. Methanol is an interesting non-aqueous solvent, in particular because it is strongly self-associated through hydrogen bonding despite its low dipole moment and dielectric constant (μ = 1.70 D and ɛ = 33 at 298.15 K). Many researchers have studied the hydrogen bonding and volumetric behavior of amide–water [17], [18], amide–alcohol [19], [20] and ternary mixture containing amide and alcohol mixed with other organic liquids [21]. Most of the authors have studied the acoustic parameters only at a single temperature and therefore a clear understanding of the solution structure of DMF–methanol system is lacking.

In this report, we estimate densities, viscosities and ultrasonic velocities of mixtures of DMF with methanol at temperatures 303, 308, 313, 318 and 323 K covering the whole miscibility range expressed by the mole fraction ‘x’ of DMF (0 < x <1). These properties have been used to determine adiabatic compressibility, intermolecular free length, molar sound velocity, molar compressibility, acoustic impedance, etc. The significance of these parameters has been emphasized in understanding the intermolecular interaction between DMF and methanol molecules. The variation of these parameters with composition of mixture and temperature is useful in understanding the nature and extend of interaction between unlike molecules in the mixture. The excess parameters such as excess velocity (U^E), excess adiabatic compressibility (β^E), excess free length (L_f), excess acoustic impedance (Z^E) and excess free volume (V_f) have significant role in determining the molecular interaction amongst the atoms of the liquid mixture. The presence of dispersion forces, dipole–dipole, dipole-induced dipole, charge transfer and hydrogen bonding interaction has been understood from the positive and negative values of the mixture. Based on the above facts, excess values are used to study the strength and nature of interaction namely strong, weak and complex formation.