Thermodynamic studies of some non-electrolytes in aqueous solutions at low temperatures

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Abstract

The experimental data of density (ρ) and speed of sound (u) in the temperature range (275.15 to 283.15) K have been obtained for aqueous solutions of di-methyl sulphoxide (DMSO), ethylene di-amine (EDA), polyethylene glycol-200 (PEG-200) and polyethylene glycol-400 (PEG-400). The specific heat (Cp) data for aqueous solutions of DMSO, EDA, and PEG-200 have also been obtained at T = 279.15 K. The data obtained are used to calculate the derived parameters like isentropic compressibility (βS), isothermal compressibility (βT), apparent molar volume (ϕV), coefficient of thermal expansion (α), apparent molar isentropic compressibility (ϕKS), and internal pressure (Pi). The limiting values of apparent molar volume (ϕV) and apparent molar isentropic compressibility (ϕKS) were also obtained. From these data, the temperature coefficients of limiting properties were computed. The results obtained were interpreted in terms of solute–solvent and solute–solute interactions and structure making and structure braking abilities of the solute. The results of Pi are used to discuss the extent of hydrogen bonding between solute and solvent (water).

Introduction

It is interesting to study the effect of dissolved solutes on thermodynamic properties of aqueous solutions of non-electrolytes at and around temperature of maximum density (TMD), i.e. T = 277 K, as changes in extent of hydrogen bonding between water molecules on dissolution of solute become more prominent at this temperature. At this temperature, the internal pressure of water is also zero. The thermodynamic properties of aliphatic alcohols and some non-electrolytes have been reported from T = (275.15 to 283.15) K [1], [2], [11], [12]. In the present communication, we report the measurement of density (ρ) and speed of sound (u) at T = (275.15, 277.15, 279.15, 281.15, and 283.15) K for aqueous solutions of dimethyl sulphoxide (DMSO) and ethylene diamine (EDA) in the concentration range (0.01 to 0.8) x2 and polyethylene glycol-200 (PEG-200) and for polyethylene glycol-400 (PEG-400) in the concentration range (0.002 to 0.1) x2, where x2 is the mole fraction of the solute. The specific heat measurements (Cp) for aqueous solutions of DMSO, EDA, and PEG-200 were carried out at T = 279.15 K. The derived properties like isentropic compressibility (βS), isothermal compressibility (βT), expansivity (α), and internal pressure (Pi) of aqueous solutions have been obtained. The solute properties, apparent molar volume (ϕV), apparent molar isentropic compressibility (ϕKS) and apparent molar expansivity (ϕE) of solutes in aqueous solutions have been calculated and from these limiting quantities ϕV, ϕKS, and ϕE were obtained. The results are interpreted in terms of solute–solvent and solute–solute interactions. Thermodynamic properties for above systems have been reported by several workers, but at higher temperatures [3], [4], [5], [6], [7], [8], [9]. In the present work, it was thought worthwhile to investigate effect of solute on thermodynamic properties of water in the neighbourhood of TMD, where water is in a highly ordered state.

Section snippets

Materials and methods

The solutes dimethyl sulphoxide (DMSO) and ethylene diamine (EDA) supplied by SRL India were of A.R. grade. They were purified by following standard methods [10]. The PEG-200 was furnished by E. Merck and PEG-400 by Qualigens. They were dried under reduced pressure and used without further purification. All the solutions were prepared in doubly distilled water on a molality basis. A Mettler balance having precision of ±0.1 mg was used for weighing.

The densities were measured at T = (275.15,

Calculations of derived parameters

The isentropic compressibility (βS) at different concentrations has been computed by using the relationβS=1u2ρ,where u is the speed of sound and ρ is the density of solution. The isothermal compressibility (βT) is calculated using the relationβT-βS=α2TCPρ,where α is 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 the thermal

Results and discussion

It has been observed that density of aqueous solutions of DMSO increases with increase in concentration of solute and reaches a maximum around x2  0.5 at all the temperatures, whereas for aqueous solutions of EDA, the density initially increases goes through a flat maxima (x2  0.3) and then decreases continuously. The variation of density (ρ) against mole fraction of solute (x2) for (DMSO + H2O) and (EDA + H2O) is shown in the figure 1 at T = 279.15 K. The same type of graphs is obtained at other

Acknowledgements

The authors are thankful to Professor M.V. Kaulgud and Professor K.J. Patil for their valuable discussion. The authors thank the reviewers for important suggestions.

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