Volumetric and compressibility properties of liquid water as a solute in glycolic, propylene carbonate, and tetramethylurea solutions at T = 298.15 K
Introduction
Water is singular as a liquid because of its ability to form three-dimensional network of molecules, mutually hydrogen bonded [1], [2]. From the point of view of H-bonding interactions, hydrophobic interactions (association of non-polar groups in presence of water) and ion–water (dipole) interactions, binary aqueous solutions of variety of electrolytes, non-electrolytes, etc. have been studied in detail [3], [4]. However, such studies for water as a solute in organic polar and organic non-polar solvents are very much limited [5], [6], [7].
Recently, we undertook a comprehensive programme of making volumetric, compressibility and dielectric constant property measurements for binary solutions involving solvents such as alcohols, apolar THF, dioxane, acetonitrile, benzene, etc. and water as a solute in the concentration range of 0.1–1 mol · kg−1 of water at T = 298.15 K [8], [9] for solutions having enough solubility or up to the saturation if solubility is limited. Our dielectric studies revealed that water dissolves in benzene may be in cyclic trimer form exhibiting trimer–monomer type of dissociative equilibria [9]. We also examined possibilities of the presence of water-centered complexes or participation of water in the chain like solvent structure (alcohols) and in apolar solvents like THF, dioxane, acetonitrile and DMF on the basis of volumetric and compressibility studies [8]. We observed that in aqueous methanolic and aqueous ethanolic solutions water exhibits differences in excess volumes and compressibilities as compared to other apolar solvents because of the presence of H-bonded linear structures of the solvents themselves. Thus, while the behaviour of alcohols is rather different from that of simple, non-associating liquids [10] no unusual properties ascribable to three-dimensional structures are to be expected. The situation is different with slightly more complex molecule such as ethylene glycol. In this substance, due to the presence of two –OH groups per molecule, one might conceive the formation of three-dimensional networks [11]. Thus, a priori, physical and solvent properties intermediate between those of water and the monohydric alcohols are to be expected. Similarities between water and ethylene glycol are also suggested by studies of protein denaturation, ethylene glycol is a much weaker denaturant than other monohydric alcohols [12]. The conformational analysis of ethylene glycol molecule has shown that sufficient number of molecules exist in non-hydrogen bonded conformations forming intramolecular hydrogen bonds, allowing the possibility of three-dimensional structure formation [11].
Therefore in continuation of our work related to studies of interactions of water in organic solvents, we have now extended it to the solutions of water in diols (glycols), propylene carbonate (PC) and tetramethylurea (TMU), propylene carbonate has been used extensively as a reference solvent (relative permittivity = 64.7) to understand the solvation enthalpy effects and other thermodynamic transfer functions (from water to other non-aqueous solvents) in terms of the important interactions like ion–solvent, ion–ion, polarization–dipole and dipole–dipole for electrolyte and non-electrolytes like alcohols [13], [14], [15]. Tetramethylurea is a liquid at room temperature, having four methyl groups and a relative permittivity of 23.4 at T = 298.15 K. It is widely used as a reaction medium for organic synthesis [16]. We report in this communication, the sound speed and density measurements carried out for solutions (up to 1 mol · kg−1) involving water as a solute at T = 298.15 K. The data are used to obtain isentropic compressibility (βS) of solution and apparent molar volume (ϕV) and apparent molar isentropic compressibility of water as a function of concentration of water. The appropriate determination of partial/apparent molar volume and compressibility of water at infinite dilution and , respectively) and its variation with concentration yield information about water–solvent and water–water interactions. These aspects are presented and discussed below.
Section snippets
Experimental
EG and TMU were procured from MERCK (purity of about 0.99 mass fraction each), PG was procured from Thomas Baker (purity greater than 0.995 mass fraction) while PC was procured from s.d.fine (purity of about 0.99 mass fraction). These solvents were used without any further purification.
In pure solvents the water concentrations were determined using a microprocessor based automatic Karl–Fischer Titrator, TKF-55, Chemito from the M/s Toshniwal company. Pyridine free Aquanil-5 Karl–Fischer reagent
Results
The variations of the speed of sound parameter (Δu = usolution − usolvent) as a function of the molality of water in EG, PG, PC, and TMU are shown in Fig. 1. Isentropic compressibility (βS) of solvents and solutions were obtained using the Laplace equation βS = 1/(u2ρ). The variations of βS values with the molality of water are shown in Fig. 2. The uncertainty in βS(δβS) values was obtained using method of propagation of errors and standard deviations obtained for the experimental parameters
Discussion
It is well known that for associated liquids, the sound velocity and isentropic compressibility values are higher and lower as compared to non-associated liquids such as carbon-tetrachloride and benzene. From table 2, it reveals that EG is much more associated due to intramolecular and intermolecular H-bonding even more than water (u for water is 1497.5 m · s−1 at T = 298.15 K; βS = 44.74 · 1011 Pa−1) [20] while, PG and PC also appear to be associated but comparatively less. TMU is more compressible than
Acknowledgment
One of the authors Sopan K. Kushare is thankful to UGC, India for the award a teacher fellowship to carryout the research work at Shivaji University, Kolhapur.
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