Volume properties of liquid mixture of water + glycerol over the temperature range from 278.15 to 348.15 K at atmospheric pressure
Introduction
The solvents forming spatial H-bond network and their mixtures possess some particular properties such as relatively large free volume, small isobaric expansion and isothermal compressions, high viscosity and its strong dependence on temperature, unlimited miscibility with other H-bonded solvents, and so on. Triols, along with water and diols, belong to the solvents with developed spatial network of hydrogen bonds. The simplest representative of triols is glycerol, being studied extensively [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Earlier we have studied the volume properties of water–ethylene glycol mixture [12] and in this connection the volume properties of water–glycerol mixture arouse interest.
In practice water + glycerol mixture is widely used as cryoprotectors for preservation of biomaterials used for damping of intracellular ice formation and damage of biological tissues [13], [14], [15]. In nature marine algae, salt-tolerant plants, insects and fish living at low temperatures contain glycerol as a common cellular component. The stability of biological molecules is explained precisely by glycerol presence [16], [17]. Thus, it is applied as general cryoprotector which increases liquids ability to osmosis and preserves them from ice formation [18].
Glycerol (Gly, 1,2,3-propanetriol) has the following physical properties: ɛ = 41.14, μ = 0.28 D, η = 1.45 Pa s at 293 K; Tmelting ≈ 291.3 K; Tboiling ≈ 563 K (with decomposition) [19], [20], [21]. In Gly molecule there are three hydrophilic alcoholic groups forming intra- and intermolecular hydrogen bonds. These three alcohol groups are considered to be responsible for Gly unlimited solubility in water. Liquid glycerol demonstrates anomalous temperature behavior of viscosity coefficient [22], [23] and dielectric relaxation times [24], [25]. Both these phenomena are determined, according to literature, by H-bond network formation [26]. Physical properties of glycerol depend on external parameters and H-bonds number varies with temperature and pressure [27], [28]. Glycerol can exist as supercooled liquid that makes its crystallization possible just with special technique [5], [28]. Mere temperature decreasing (down to 185 K) brings usually to glassy state formation [29]. Glycerol molecule is very flexible and can theoretically form, according to results of computer simulation, 126 conformers [4], [30], [31]. If in crystalline state Gly molecule is as αα conformer [5], then the data on quality composition of conformers in other aggregate states are rather inconsistent. In [6] it is asserted that glassy glycerol forms also one but another conformation – βγ. In liquid state the presence of two dominating conformers – αα and αγ – has been confirmed experimentally. However the possibility of another conformers existing is not ruled out [1], [2], [7], [31].
Last years water–glycerol system was studied intensively by various methods such as molecular dynamic [4], [8], [26], [31], [32], [33], [34], [35], [36], [37], [38], [39], diverse thermodynamic measurements [40], [41], [42], NMR [43], infrared [44] and Raman spectroscopy [43], [45], [46]. The system volume properties have been studied also. Before, the density of water–glycerol system, due to its technological importance, has been measured for the first time as far back as 1884 [47]. In the reference-book [48] published in 1928 all results obtained were summarized and the mixture densities were performed at 15, 15.5, 20 and 25 °C (±0.1 °C). But surprisingly till now the majority of density precise measurements of water–glycerol mixture has been carried out either only at one temperature (293.15 or 298.15) or within narrow temperature interval. So these data do not allow calculating the mixture expansion with high accuracy. Besides the works dealing with supercooled glycerol are rather scarce.
The present work continues our earlier investigations of volume properties of binary mixtures with different nature of intermolecular interaction depending on composition, temperature, and pressure [49], [50], [51], [52], [53].
Section snippets
Experimental part
In the work the solvents of the highest available purity were used. The initial glycerol (stated purity 99.5%, “Khimreactive”) was purified by double distillation according to reference [21] and was kept under vacuum. The water content was determined by K. Fischer's method and did not exceed 0.02 wt.% (or 4 × 10−5 molar fraction). For solutions preparation the deionized freshly prepared bidistillate was used.
The mixtures were prepared by gravimetric method with accuracy 1 × 10−3 g from degasified
Calculations
Excess molar volumes of the mixture :where Vm is the mixture molar volume, , x1, and , x2 are molar volumes of pure components and their molar fractions (indexes 1 and 2 refer to water and glycerol, accordingly). values were found directly from experimental data by equation:where M1, ρ1, and M2, ρ2 are molar weights and densities of water and glycerol, accordingly; ρ is the mixture density. Uncertainty of excess molar volumes determination
Discussion
Intermolecular interactions in the mixed solvent of {water (1) + glycerol (2)} are mainly determined by hydrogen bonds between hydroxyl groups. Variation of the mixture volume, first of all, depends on alteration of H-bonds energy and their quantity. Moreover the mixture volume reflects conformational equilibrium of glycerol molecules. It is known that in liquid phase several conformations of glycerol molecules can exist simultaneously [1], [2], [3], [4], [30], [31], but depending on external
Conclusions
Volume characteristics of liquid {water(1) + glycerol (2)}mixture reveal that in the mixture complex processes run upon temperature and composition variation. They are caused by rearrangement of H-bond components participating into intermolecular interaction that also leads to conformational variations.
The mixture formation is attended by more compact packing formation. On rising the temperature the absolute value of excess molar volume at a certain composition decreases.
Partial molar volumes of
Acknowledgment
This work was supported by the Russian Foundation for Basic Research (project 12-03-97525-r_centre_a) and grant of the President of the Russian Federation (No. MK-1288.2013.3).
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