Isentropic compressibilities of (amide + water) mixtures: A comparative study
Introduction
Among the different solvents, amides have a particular position in detecting the effect of solvents in the solutions of different substances. The interest in these solvents arises from a theoretical view as well as from the biological, pharmaceutical and other applications of pure solvents and their aqueous solutions. The volumetric properties of amide aqueous solutions have been extensively studied: formamide [1], [2], [3], [4], [5], N-methylformamide [5], [6], [7], N,N-dimethylformamide [3], [4], [5], [7], [8], [9], [10], N,N-dimethylacetamide [1], [2], [3], [4], [11], [12], [13], [14], pyrrolidin-2-one [5], [7], [15], [16], N-methyl-2-pyrrolidinone [5], [7], [16], [17], [18]. The ultrasonic behavior of amides in water [4], [9], [16], [19], [20] or other solvent mixtures [21], [22], [23], [24], [25] has been also reported as well as their dielectric constants [15], [26] and viscosity behavior [4], [5], [13], [14], [16], [17], [18], [23], [25]. Some of these amides have been involved in our studies [27] and invoked our interest to the detailed evaluation of their ultrasonic properties (ultrasonic velocity, isentropic compressibility, intermolecular free length, apparent molar compressibility) across the entire range of mole fractions. As far as we know, a systematic study of ultrasonic properties in (amide + water) mixtures is not available. The present work aims to provide a comparative study of the ultrasonic properties in aqueous solutions of the most used amides at ambient temperature which besides the well-studied volumetric properties would be a useful tool in elucidating the intermolecular and structural interactions among the water and amides molecules. Moreover, their knowledge would be useful in elucidating the ultrasonic behavior in various substances in the aqueous mixtures of these solvents and the role of interactions between solute–solvent molecules in these solutions.
Evidence for the interpretation of the experimental ultrasonic properties in the liquids mixtures provides the deviation from their ideal values. Various approaches [28], [29], [30], [31], [32], [33] have been used for the calculation of ideal ultrasonic velocity or ideal isentropic compressibility and other thermodynamics properties. Therefore, the evaluation of their excess values and the investigation in the light of new approaches [28], [29], based on a detailed thermodynamical formulation, are very significant for the interpretation of molecular interactions and critical for the comparison of the behavior of various solvents.
Section snippets
Experimental
The solvents were supplied: formamide (stated purity >99%-microselect for molecular biology), N-methylformamide (stated purity >99%), N,N-dimethylformamide (stated purity >99.8%), N,N-dimethylacetamide (stated purity >99.5%), N-methyl-2-pyrrolidinone (stated purity >99%) by Fluca, pyrrolidin-2-one (stated purity >99%) by Acros, and were used without other purification. The solvents were kept over molecular sieves. The water was deionized and distilled prior to be used for making up solutions or
Results and discussion
Densities ρ and ultrasonic velocity u have been measured for all pure amides and their binary aqueous mixtures at 298.15 K across the entire range of mole fractions. From these data isentropic compressibility κs, of pure amides and (amide + water) mixtures were calculated [28], [40] from the Newton–Laplace equation:
In addition various thermodynamics parameters as apparent molar compressibility κΦ [41], [42], intermolecular free length Lf [24], [25], [46], relative association RA [43], [45],
Conclusions
The ultrasonic properties in (amide + water) mixtures exhibit a maximum in ultrasonic velocity and relative association and a minimum in isentropic compressibility and intermolecular free length vs. x2 except in the case of FA/W where they change monotonically. The sequence of compressibility minima is increased in the order PYR < NMP < DMA < DMF < NMF revealing that compressibility in the cyclic (amide + water) mixtures is less than in dialkylated and these than the monoalkylated amides. The behavior of
Acknowledgement
We thank ELKE (The Special Research Account of National and Kapodistrian University of Athens) for financial support.
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