Densities, excess molar volumes, speeds of sound and isothermal compressibilities for {2-(2-hexyloxyethoxy)ethanol + n-alkanol} systems at temperatures between (288.15 and 308.15) K

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Abstract

The densities, ρ and the speeds of sound, u, for {2-(2-hexyloxyethoxy)ethanol (C6E2) + methanol, +1-propanol, +1-pentanol, and +1-heptanol} have been measured as a function of composition using an Anton-Paar DSA 5000 densimeter at temperatures (288.15, 293.15, 298.15, 303.15, and 308.15) K and atmospheric pressure over the whole concentration range. The ρ and u values were used to calculate excess molar volumes, VE, and excess molar isentropic compressibility, KS,mE, respectively. Also, thermal expansivity, α, partial molar volume, V¯i, and partial molar volume of the components at infinite dilution, V¯i0, have been calculated. The variation of these properties with composition and temperature of the mixtures are discussed in terms of molecular interactions.

Introduction

The mixing of different compounds gives rise to solutions that generally do not behave ideally. The deviation from ideality is expressed by many thermodynamic properties, particularly activity coefficients and excess or residual properties. Excess thermodynamic properties are useful in the study of molecular interactions and arrangements. In particular, they reflect the interactions that take place between solute–solute, solute–solvent, and solvent–solvent species. In general, positive excess molar volumes may be due to compensation between strong like interactions (such as those present in alcohols) and equally unlike H-bond interactions (such as those present between alcohol and ether). Negative excess molar volume will occur when the unlike interactions prevail over self-association. Binary mixtures are an important class of solvents and solutions, and the behavior of their physical properties is still unclear. The effect of temperature on the molar volume and excess molar volume can be expressed by the coefficient of isobaric thermal expansion α = −(1/V)(δVT)P.

Non-ionic surfactants poly(oxyethylene glycol) monoethers CH3(CH2)i−1O(CH2CH2O)jH (symbolized by CiEj) are widely used in the industrial processes as liquid–liquid extraction emulsifying agents [1], tertiary oil recovery detergents [2], and herbicides.

Non-ionic surfactant CiEj has been successfully applied to separate or concentrate proteins and other biological materials using two-phase aqueous micellar systems [3], [4]. Recently, (liquid + liquid) equilibrium data of systems containing CiEj have been measured for binary and ternary mixtures [5], [6], [7], [8], [9], as thess data were necessary for the design of liquid–liquid extractors and of decanter in distillation systems.

Densities and ultrasonic speeds of (C6E2 + water) have been measured at 5 °C by Douheret et al. [10]. Specific heat capacities for the same system have been measured from (2 to 55) °C by Piekarshi et al. [11]. There has been no temperature dependent study on these systems from the point of view of their volumetric and ultrasonic speed behavior.

As a part of our ongoing program of research on thermodynamic and acoustic properties of binary liquid mixtures containing alkoxyethanols, we report herein experimental data for density and speed of sound of {C6E2 + methanol, +1-propanol, +1-pentanol, and +1-heptanol} and those of pure liquids at atmospheric pressure and temperatures (288.15, 293.15, 298.15, 303.15, and 308.15) K, covering the entire composition range. The experimental values of ρ and u were used to calculate the excess molar volume, VE, excess molar isentropic compressibility, partial molar volume, V¯i, and partial molar volumes, V¯10 and V¯20, at infinite dilution. The aim of this work is to provide a set of data for the characterization of molecular interactions of the oxygen (–O–) and hydroxyl (–OH) functional groups of alkoxyethanols with alkanols and to study the composition–temperature dependent behavior of these mixtures. An attempt is also made to compare the results of the excess molar volumes and speeds of sound with those of our precious results for (C2E2 + alkanols).

Section snippets

Materials

Methanol (CH3OH), 1-propanol (C3H7OH), and 1-pentanol (C5H11OH) (all SD Fine Chemicals, India, HPLC, spectroscopic and analytical grade) were stored over sodium hydroxide pellets for several days and fractionally distilled twice [12]. The middle fraction of the distillate was used. The 2-(2-hexyloxyethoxy) ethanol (C6E2) (Merck, Schuchardt purity >0.98) and 1-heptanol (Acros, USA, purity >0.98) were used without further purifications. Prior to experimental measurements, all liquids were stored

Equations

The isentropic compressibility, κS, can be calculated using the Newton–Laplace’s equation [29],κS=1/u2ρ=V(Mu2)-1.The molar isentropic compressibilities, KS,m, can be obtained from the equation,KS,m=-(δV/δP)s=VκS=xiMi/(ρu)2,where ρ is the density, V is the molar volume, and xi and Mi are the mole fraction and molar mass of component i in the mixture, respectively. These equations (1), (2) appear to be valid provided there is no absorption of the energy associated with the ultrasonic wave. The

Discussion

FIGURE 1, FIGURE 2 show the values of the speed of sound and density at equimolar composition and 298.15 K. For the investigated systems the ρ values decrease at the same temperature with an increase of temperature in the sequence: methanol < 1-propanol < 1-pentanol < 1-heptanol, where the reverse is the trend in the values of u at any particular temperature.

For all the mixtures, VE is negative over the whole mole fraction range with the exception of C6E2 with 1-heptanol at 308.15 K. Figure 3 shows VE

Acknowledgement

Financial support for this project (Grant No. SR/SI/PC-33/2003) by the Government of India through the Department of Science and Technology (DST), New Delhi, is gratefully acknowledged.

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