Isentropic compressibilities of (amide + water) mixtures: A comparative study

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Abstract

The density and ultrasonic velocity of aqueous solutions of formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), pyrrolidin-2-one (PYR), N-methyl-2-pyrrolidinone (NMP), and their pure phases have been measured at 298.15 K and atmospheric pressure. Densities and ultrasonic velocities in pure amides have been also measured at the temperature range 288.15 K to 308.15 K for the computation of their thermal expansivities. Isentropic compressibility, intermolecular free length, relative association, apparent molar compressibility, as well as the excess quantities, ultrasonic velocity, isentropic compressibility, intermolecular free length, have been evaluated and fitted to the Redlich–Kister type equation. The deviation from ideal mixing law in ultrasonic velocity is positive while the deviations in isentropic compressibility and intermolecular free length are negative for all (amide + water) mixtures. This behavior reveals the nature and the magnitude of intermolecular interactions between the amide–water molecules. The sequence of superimposed curves of various ultrasonic parameters vs. the amide mole fraction is related to the strength of interactions between the unlike molecules and the role of –CH3 substitution in amides. The comparison of ultrasonic to volumetric properties reveals differences on the position of the extrema and their relation with the degree of substitution while the interpretation of these differences is discussed. Two different approaches on the computation of excess functions, applied in this work, brought out a difference in the magnitude of deviations and a partial reversion to the sequence of amides curves suggesting a different estimation in terms of deviations from ideal mixing law and therefore of the relative molecular interactions.

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:κs=1u2ρ

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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