Isothermal compressibility and internal pressure studies of some non-electrolytes in aqueous solutions at low temperatures

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Abstract

The experimental data of density (ρ) and sound velocity (u) in the temperature range (275.15 to 293.15) K have been obtained for the systems (dioxane + water), (dimethylformamide + water), (tetrahydrofuran + water), and (acetonitrile + water). The specific heat (Cp) data for the above systems have been obtained at T = 279.15 K. The data obtained are used to calculate the derived parameters of adiabatic compressibility (βS), at T = 275.15 K to T = 283.15 K, isothermal compressibility (βT), and internal pressure (Pi) at T = 279.15 K for different concentrations. The solute properties: apparent molar volume (ϕV), apparent molar expansivity (ϕE), and apparent molar compressibility (ϕKS) have been studied and the limiting values for these properties are reported. The variation in apparent molar properties with concentration and the corresponding temperature and pressure effects are discussed in terms of hydrophobic hydration (–H bonding interaction) and hydrophobic interaction (non-polar group solute–solute association in water). It is noted that the internal pressure of solutions is quite insensitive in the region of solute–solute association, while its variation with concentration in the dilute region is sensitive in contrast to the aqueous alcohol solutions. The molecular interactions also exhibit individualistic behaviour and are much dependent on structural alterations in water structure.

Introduction

Over many years, aqueous solutions of non-electrolytes have been of interest to scientists of major disciplines. The thermodynamic and spectral properties of aqueous alcohol, aqueous urea, and solutions of aqueous carbohydrates and aqueous amines [1], [2], [3], [4], [5], [24] at T = 298.15 K have been studied in detail. The concentration variations of thermodynamic properties show extrema, e.g. minimum in partial molar volume of the solute, minimum in excess entropy of mixing, maximum in viscosity, etc. These are being attributed to water-structure strengthening by H bond formation or formation of clathrate-like equilibria in the dilute region, while the extrema at moderate concentration region can be viewed as a manifestation of solute–solute association equilibria [6]. Similarly, the solute properties in the infinite dilute solution such as partial molal volume, partial molal entropy, etc. have been used to understand solute–water (solvent) interaction. The internal pressure (∂u/∂v)T of such solutions also yields useful information since this is related to the radial distribution function of the solute and solvent [7]. However, such studies are limited to a few systems [8], [9]. Internal pressure and cohesive energy density of binary liquid mixtures are closely related to the extent and strength of intermolecular interactions. However, internal pressure is more convenient to interpret the solute–solvent interactions than cohesive energy density. It is known that ultrasonic absorption is quite insensitive in the dilute concentration range for alcohols in water, but increases steeply after certain concentration and goes through a maximum [10]. Similar observations were noted for aqueous tert-BuOH solution at T = 298.15 K for the variation of the internal pressure parameter [11]. Thus, it seems that there is some correlation between internal pressure (Pi) and ultrasonic absorption in solutions. At the same time, Pi has a zero value at T = 277 K, since α, the coefficient of expansion, is zero at the TMD, the temperature of maximum density for water. However, not much information about Pi is available in the temperature region of (273 to 283) K. The calculations of Pi are difficult as they involve measurement of isothermal compressibility, or adiabatic compressibility, expansivity and specific heat at constant pressure.

We report in this communication, the measurements of density, and speed of sound measurements at T = (275.15, 277.15, 279.15, 281.15, and 283.15) K for aqueous solutions of THF, ACN, DO, and DMF in the concentration range of (0.02 to 0.9) x2, where x2 is the mole fraction of the solute. The Cp measurements were carried-out at T = 279.15 K only. The adiabatic compressibility (βS), isothermal compressibility (βT), expansivity (α), and internal pressure (Pi) of solution have been obtained. The solute properties of apparent molar volume (ϕV), apparent molar adiabatic compressibility (ϕKS), and apparent molar expansivity (ϕE) have also been calculated and used to obtain the limiting quantities (ϕV, ϕKS, and ϕE). These are discussed below in terms of solute–solvent and solute–solute association in solutions. These properties are also compared with the data at higher temperature (298.15 K), which are well known.

Section snippets

Material and methods

The solutes N,N-dimethyl formamide (DMF), 1,4-dioxane (DO), tetrahydrofuran (THF), and acetonitrile (ACN) were of A.R. grade. They were purified by following standard methods [12]. The DMF was allowed to stand over KOH pellets for 24+ h before use. The DO was boiled over NaOH and then distilled and the middle fraction was used. The THF was distilled under partial vacuum and stored over CaO, the ACN was used as obtained. All the solutions were prepared in doubly distilled water on a molality

Calculations of derived parameters

The adiabatic compressibility (βS) at different concentrations has been computed by using the relationβS=1u2ρ,where u is the sound velocity and ρ is the density of solution.

The isothermal compressibility (βT) is calculated using the relationβT-βS=α2TCpρ,where α is the thermal coefficient of expansion, T is absolute temperature, Cp is the specific heat, and ρ is density of solution.

The internal pressure (Pi) of solutions was calculated by using the equationPi=αTβT-P,where α is thermal

Results and discussion

The variations of u, βS, and βT with concentration of the apolar solutes studied in this work at T = 279.15 K are depicted in FIGURE 1, FIGURE 2, FIGURE 3, respectively. It is observed that for (DMF + H2O) and (DO + H2O) systems density and sound velocity both increase with concentration initially, go through a maximum at a particular concentration, then further decrease with increase in concentration at all temperatures. For (ACN + H2O) and (THF + H2O) systems, density decreases continuously with

Acknowledgements

The authors are thankful to Prof. M.V. Kaulgud and Prof. K.J. Patil for valuable discussion.

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