Densities and volume properties of (water + tert-butanol) over the temperature range of (274.15 to 348.15) K at pressure of 0.1 MPa

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Abstract

The densities of {water (1) + tert-butanol (2)} binary mixture were measured over the temperature range (274.15 to 348.15) K at atmospheric pressure using “Anton Paar” digital vibrating-tube densimeter. Density measurements were carried out over the whole concentration range at (308.15 to 348.15) K. The following volume parameters were calculated: excess molar volumes and thermal isobaric expansivities of the mixture, partial molar volumes and partial molar thermal isobaric expansivities of the components. Concentration dependences of excess molar volumes were fitted with Redlich–Kister equation. The results of partial molar volume calculations using four equations were compared. It was established that for low alcohol concentrations at T  208 K the inflection points at x2  0.02 were observed at concentration dependences of specific volume. The concentration dependences of partial molar volumes of both water and tert-butanol had extremes at low alcohol content. The temperature dependence of partial molar volumes of water had some inversion at х2  0.65. The temperature dependence of partial molar volumes of tert-butanol at infinite dilution had minimum at ≈288 K. It was discovered that concentration dependences of thermal isobaric expansivities of the mixture at small alcohol content and low temperatures passed through minimum.

Introduction

The properties of {water + tert-butanol (tertiary butyl alcohol, TBA)} systems are of intent interest. Firstly, tertiary butyl alcohol is the only isomer of butyl alcohols which is soluble indefinitely in water at atmospheric pressure. Secondly, the dependences of many thermodynamic characteristics of (water + tert-butanol) mixtures on mixture composition have sharp extremes at very low alcohol concentrations. Thirdly, the tert-butanol molecule is, in some way, a model one, i.e. it can be considered as methanol derivative with hydrogen atoms replaced by methyl-groups. In the alcohol molecule three hydrophobic –CH3 groups are responsible for hydrophobic effects in water and one hydrophilic –OH group is able to form strong H-bond both with water and with other tert-butanol molecules. Fourthly, in tert-butanol nonspecific interactions are very strong, e.g. among other butyl alcohols it has the highest freezing point (298.65 K) and the lowest boiling temperature (355.35 K) [1], [2], [3], [4].

Volume properties of {water (1) + tert-butanol (2)} mixture were investigated earlier by many authors [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] but these data are rather scattered. So the most part of these studies dealt with solutions of low alcohol concentration [12], [13], [14], [15], [16], [17] or measurements were carried out at 298.15 K only [5], [6], [7], [8], [9], [10], [11]. In spite of the fact that TBA freezing temperature is higher of 298.15 K, in some works densities of liquid tert-butanol are presented at 298 K and even lower [23]. This demonstrates mainly the low quality of the alcohol used. In the works [11], [18] the results of density measurements are presented just graphically. The data on density determined at different temperatures and over the whole concentration range are given in papers [19], [20], [21], [22]. The density of (water + tert-butanol) system was measured using pycnometers at (288.15, 298.15, 313.15, and 323.15) K by Kenttamaa and co-workers [19]. Hvidt et al. [20] have measured the density using vibration densimeter with accuracy up to ±1 · 10−5 g · cm−3 at temperatures of (278.15, 283.15, 293.15, and 298.15) K and up to the alcohol content of 0.56 mole fraction. The densities at atmospheric pressure were determined by Kubota et al. [21] at (298.15, 323.15, and 348.15) K and at five mixture compositions (х2 = 0.05, 0,1, 0.25, 0.50, 0.75). Kim and Marsh [22] using pycnometers have measured the density at (303.15, 308.15, 313.15, 318.15, and 323.15) K over the entire concentrations range (36 compositions) with accuracy of ±1 · 10−5 g · cm−3.

For reasons given above it can be concluded that a majority of experiments on volume properties of (water + TBA) mixtures performed earlier were carried out either at one temperature, or over the short temperature or concentration range, or over the whole concentration range but with large intervals. These data cannot be used for careful calculation of thermal isobaric expansivities and its partial values for components, as well as partial molar volumes cannot be found from the data.

The present work is a continuation of our earlier researches of volume properties of binary systems with different nature of interactions depending on composition, temperature and pressure [24], [25], [26], [27], [28], [29], [30], [31], [32], [33].

Section snippets

Experimental

All mixtures were prepared gravimetrically from degasified solvents. All preparations and density measurements were carried out without a contact of solutions with atmospheric air. Only fresh-made solutions were used for all measurements. Tert-butanol (stated purity 0.995) was purified by double distillation according to [2], [3] and was kept under vacuum. Water content in the alcohol was determined with K. Fisher method and did not exceed of 0.015 wt.% (or 0.00004 mole fraction).

Densities (ρ)

Results

Excess molar volumes, VmE, were calculated directly from experimental data by equation (1):VmE=x1M1(1/ρ-1/ρ1)+x2M2(1/ρ-1/ρ2),where x1, M1, ρ1, and x2, M2, ρ2 are molar fractions, molar masses, and densities of water and TBA, accordingly, ρ is a mixture density. The uncertainty of excess molar volume determination did not exceed of ±0.02 cm3 · mol−1.

Concentration dependences of excess molar volumes, VmE, were fitted by Redlich–Kister equation (2) [35], [36]:VmE=x1x2i=0i=nAi(x1-x2)i,n=6.

The optimal

Discussion

Every individual solvent has its own structure and its own short-range order. The binary mixture formation is attended by destruction of individual structures as well as by formation of new bonds and structure of the mixture. At that water plays the dominating role in the volume properties changes of {water (1) + tert-butanol (2)} mixture. Water, as it is well known, is a solvent with extended hydrogen-bonds framework and with cavities in the structure [37], [38], [39], [40], [41], [42]. Tertiary

Conclusions

The volumetric properties of {water (1) + tert-butanol (2)} liquid system indicate that the system changes are mainly due to changes of water own structure. Extremes appearing at volume properties dependences at low tertiary butyl alcohol concentrations can be explained by the process of water self-packing destruction. This process consists in displacement of “free” H2O molecules, situated in cavities of water own structure and bound with hydrogen bonds network by means of weak H-bonds, by TBA

Acknowledgement

This work was financially supported by Russian Foundation for Basic Research (No. 09-03-97501-r_centr_a).

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