Thermodynamic properties of (LiCl + N,N-dimethylacetamide) and (LiBr + N,N-dimethylacetamide) at temperatures from (323.15 to 423.15) K
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.
References (26)
- et al.
Carbohydr. Res.
(1995) - et al.
Polymer
(2003) - et al.
J. Chem. Thermodyn.
(2004) - et al.
Comprehensive Cellulose Chemistry
(1998) - et al.
J. Polym. Sci., Polym. Lett.
(1979) Carbohyd. Polym.
(1997)- et al.
J. Macrom. Sci. Rev. Macrom. Chem. Phys.
(1990) - et al.
Sen’i Gakkaishi
(1992) Chem. Rev.
(1969)- et al.
Trends polym. Sci.
(1996)