Apparent molal volumes of symmetrical and asymmetrical isomers of tetrabutylammonium bromide in water at several temperatures

doi:10.1016/j.jct.2013.08.031

The Journal of Chemical Thermodynamics

Volume 68, January 2014, Pages 117-121

https://doi.org/10.1016/j.jct.2013.08.031 Get rights and content

Highlights

•
Apparent molal volumes of five isomers of Bu₄NBr in water have been measured.
•
The structural effect of branched and linear chains is discussed.
•
The structural contributions to the ionic volume were calculated.

Abstract

Apparent molal volumes of a series of differently substituted quaternary ammonium bromides, namely tetra-iso-butyl-, tetra-sec-butyl-, tetra-n-butyl-, di-n-butyl-di-sec-butyl- and di-n-butyl-di-iso-butylammonium bromide have been determined as a function of molal concentration at (298.15, 303.15 and 308.15) K. Partial molar volumes at infinite dilution and ionic molar volumes of these quaternary ammonium cations were determined. Structural volume contributions to the ionic molar volume were also calculated. The symmetric and asymmetric quaternary ammonium cations are “structure making” ions. The contribution of the branched butyl chains predominates over the linear butyl chains in the asymmetric cations.

Introduction

Quaternary ammonium cations are widely used in several areas of chemistry [1]. Since their hydrophobicity is easily tuned by changing the length and/or the three-dimensional structure of the non-polar moieties these cations are often used as model systems for the investigation of ion–ion, ion–solvent and solvent–solvent interactions in electrolyte solutions [1], [2], [3].

Studies of the volumetric properties of symmetrical quaternary n-alkylammonium cations in aqueous solution are abundant in the literature and various reviews have summarized and commented the results [1], [2], [3]. However, corresponding investigations of symmetrical and asymmetrical quaternary ammonium cations having branched alkyl chains are still scarce [4], [5], [6]. These cations are of special interest because they provide a way of establishing the influence of the geometry of the non polar groups on the structural properties of water.

This contribution intends to widen the available data base on the effect of the geometry of the non-polar alkyl side chain on the solvent by a systematic study of the volumetric behavior of aqueous solutions of five isomers of tetrabutylammonium bromide as a function of concentration (0.01 to 0.1 mol · kg⁻¹) at three temperatures (298.15, 303.15 and 308.15 K). Three symmetrical cations, namely tetra-n-butylammonium (Bu₄N⁺), tetra-iso-butylammonium (isoBu₄N⁺) and tetra sec-butylammonium (secBu₄N⁺) were selected in addition to the asymmetric ions di-n-butyl-di-iso-butylammonium (Bu₂isoBu₂N⁺) and di-n-butyl-di-sec-butylammonium (Bu₂secBu₂N⁺), figure 1. Apparent molal volumes were determined by density measurements to an uncertainty of ±5 · 10⁻⁶ g · cm⁻³. Limiting partial molar ionic volumes of the cations were calculated from the limiting partial molar volumes at infinite dilution of the solutes. The concentration dependence of these apparent molal volumes was also examined.

Section snippets

Materials

The salt Bu₄NBr was obtained from Sigma and used without further purification, whereas isoBu₄NBr, secBu₄NBr, Bu₂isoBu₂NBr and Bu₂secBu₂NBr were synthesized and purified according to established methods in literature [7], [8]. The purity of all salts was better than 0.99 in mass. The water used was doubly distilled from an alkaline KMnO₄ solution and degassed before use. The conductivity of the water employed always was less than 2 μS · cm⁻¹. The detailed specifications of chemical samples are

Results and discussion

The densities, ρ, of the studied aqueous quaternary ammonium bromide solutions are given in tables S1 to S3 in Supporting Information. The apparent molal volumes, V_ϕ, of the solutes were calculated from ρ and the density of water, ρ_o, according to $V_{ϕ} = \frac{M_{2}}{ρ} - \frac{1000 (ρ - ρ_{o})}{m ρ ρ_{o}},$ where m is the molal concentration of the solute and M₂ its molar mass_. Water densities at the three studied temperatures were taken from reference [9]. Tables S1 to S3 also list the apparent molal volumes of the solutes with

Conclusions

The ionic molar volumes and the structural contribution to the ionic volume for the five isomers of tetrabutylammonium cations have been calculated. The results showed that the group contribution methods of chain contribution methods for the calculation of the molar volume are not suitable for quaternary ammonium cation with branched chains.

The structural effect of branched chains over linear chains is more pronounced as can be seen for the asymmetrical quaternary ammonium cations. In addition,

Acknowledgement

The authors wish to thank the Faculty of Science of the Universidad de los Andes for financial support.

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Apparent molal volumes of symmetrical and asymmetrical isomers of tetrabutylammonium bromide in water at several temperatures

Highlights

Abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgement

J. Mol. Liq.

J. Solution Chem.

Chem. Rev.

Trans. Faraday Soc.

Phys. Chem. Liq.

J. Solution Chem.

J. Org. Chem.

Monatsh. Chem.

J. Chem. Eng. Data

Chem. Rev.

J. Chem. Eng. Data