Thermodynamic properties of (LiCl + N,N-dimethylacetamide) and (LiBr + N,N-dimethylacetamide) at temperatures from (323.15 to 423.15) K

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Abstract

Precise vapor pressure data for LiCl and LiBr solutions in N,N-dimethylacetamide are given for T = (323.15 to 423.15) K. The molality ranges covered in this study are about m = (0.073 to 1.89) mol · kg−1 for lithium chloride and m = (0.06 to 1.75) mol · kg−1 for lithium bromide. Osmotic coefficients are calculated by taking into account the second virial coefficient of N,N-dimethylacetamide. The parameters of the extended Pitzer-ion interaction model of Archer, of the MSA-NRTL model and of the chemical model of Barthel are evaluated. These models accurately reproduce the experimental osmotic coefficients within different concentration ranges. The parameters of the Pitzer-ion interaction model of Archer are used to calculate the mean molal activity coefficients and excess Gibbs free energies. The non-ideal behaviors of these systems are discussed in terms of the model parameters.

Introduction

Lithium chloride and lithium bromide solutions in N,N-dimethylacetamide (DMAc) belong to a small group of non-degrading solvents of cellulose [1]. The (LiCl + DMAc) was first applied for the solubilization of cellulose in 1979 by McCormick et al. [2] and since then established as a powerful solvent system for polysaccharides in general. Especially its use for analytical purposes [3] and its application in the preparation of cellulose derivatives seem to be promising [4].

The (LiBr + DMAc) is a suitable solvent system for bromination of carbohydrates [5]. Bromine is a better leaving group than chlorine and the C–Br bond is higher in stability than the C–I bond, as was observed in the nucleophilic substitutions of halodeoxycelluloses [6]. DMAc is known to interact strongly with metal cations to leave non-solvated anions. The order of nucleophilicity of non-solvated halide ions decreases in the order, C1 > Br > I [7]. However, there is still some disagreement about the conditions under which this solvent must be applied [8], [9]. Literature survey revealed that the information about the (LiCl + DMAc) and (LiBr + DMAc) is rather scare [10].

Therefore, a systematic study has been undertaken to determine the vapor pressures and osmotic coefficients of (LiCl + DMAc) and (LiBr + DMAc) with high precision in the temperature range T = (323.15 to 423.15) K and for concentrations ranging from m = (0.0738 to 1.8889) mol · kg−1 for LiCl and m = (0.0649 to 1.7530) mol · kg−1 for LiBr solutions.

The extended Pitzer-ion interaction model of Archer [11], [12], the recently developed MSA-NRTL model [13] and the chemical model of Barthel [14] were applied to reproduce the experimental osmotic coefficients. The parameters of the extended Pitzer-ion interaction model of Archer are used to calculate mean molal activity coefficients and excess Gibbs free energies.

Section snippets

Chemicals

The lithium chloride and lithium bromide were obtained from Merck. They were all suprapur reagents and were dried in an evacuated electrical oven at about 393 K for 24 h prior to use.

DMAc, obtained from Sigma–Aldrich as HPLC grade (⩾0.999 mass fraction purity), was dried over 0.4 nm molecular sieves, and was used without further purification. The water content in dry DMAc as determined by Karl-Fischer titration was <20 ppm. The organic impurities were lower than 30 ppm detected by gas

Experimental results

The vapor pressures of (LiCl + DMAc) and (LiBr + DMAc) were measured from T = (323.15 to 423.15) K in 10 K intervals for m = (0.0738 to 1.8889) mol · kg−1 for LiCl and m = (0.0649 to 1.7530) mol · kg−1 for LiBr solutions. The salts are not volatile and therefore the total measured pressure at equilibrium p is the vapor pressure of DMAc. The experimental data are given in TABLE 1, TABLE 2. Activities of solvent and osmotic coefficients for the (LiCl + DMAc) and (LiBr + DMAc) as a function of molality m were

The osmotic coefficients – a qualitative discussion

There are some qualitative and simple rules to interpret the osmotic coefficients as a function of concentration:

  • The larger the ion radius, the larger the repulsion between the ions and consequently the higher are the values of the osmotic coefficients.

  • The stronger the ion solvation, the higher the osmotic coefficient.

  • The higher the ion association, the smaller are the values of the osmotic coefficients.

According to the first rule, it seems natural that the osmotic coefficients of the bromide

Acknowledgments

We thank the Arbeitsgemeinschaft industrieller Forschung AiF “Otto von Guericke e. V.” (AiF) for financial support. K.N. is grateful to the Iranian Ministry of Science, Research and Technology for a grant. We thank also Prof. R. Buchner for providing pure DMAc.

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