Liquid–liquid equilibrium in binary systems of isomeric C8 aliphatic monoethers with nitromethane
Introduction
Among fluid-phase equilibria, that of liquid–liquid seems to be one among most important and simultaneously probably the most difficult to describe. Practical importance is due to the extraction processes which take an advantage from different solubilities of a component in a liquid biphasic system. Numerous papers and monographs were published dealing with practical and theoretical problems but a satisfactory model description for the liquid–liquid equilibrium has not been achieved yet [1], [2], [3].
This difficulty is usually explained by a temperature dependence of activity coefficients which plays a crucial role for the equations binding parameters in the equilibrium state and generally is not well described by any thermodynamic model. This is common to relate some, usually energetic parameters, to temperature using strictly empirical equations, i.e. power expansion.
Another problem which we underline is an ambiguity in the group definition when a group-contribution approach is applied [4]. In the most general approach, group contribution models equate isomers having the same groups but differing as to their localization in a molecule. This is a serious shortcoming which, at least partly, can be overcome by a definition of numerous subgroups taking into account a character of neighboring ones. Such an approach makes the model more flexible but considerably increases the number of model parameters what simultaneously deprives the model its predictive abilities and may be applied with some restrictions only. Majority of group contribution models, as that of modified UNIFAC [5], combine polar groups with a short aliphatic adjacent neighbor thus forming a few complex groups as CH3X, CH2X, CHX and CX, where X denotes a polar part.
This paper begins our systematic study on thermodynamic properties of binary systems in a series of isomeric compounds dissolved in a common solvent. Its aim is to determine how a position of a polar group influences thermodynamic properties. The liquid–liquid-equilibrium was selected for this purpose as even small changes of thermodynamic parameters has a considerable impact on this property.
Aliphatic monoethers is a group of compounds for which such a study can be easily performed. If an ethereal group is being shifted from the middle position into periphery of a molecule, a family of positional isomers can be formed. On the other hand, changes in the aliphatic chain length of an ether makes possible to induce a miscibility gap for a well-defined polar solvent.
According to Sazonov et al. compilation [6], the miscibility gap in systems of aliphatic monoethers with nitromethane is observed for the C12 ethers, with the UCST located between 328 and 351 K. From the observed tendency it is clear that the liquid–liquid equilibrium appears for lower ethers, too. It must be underlined that the thermodynamic data of the nitromethane with lower aliphatic monoethers are extremely scarce and apart from a separate data point of the activity coefficient at infinite dilution for the dibutyl ether + nitromethane system, they concern mixtures with diethyl ether only.
The preliminary examination of the nitromethane + a symmetric ether mixture at approximately equimolar composition showed that a limited miscibility appears for dibutyl ether at the room temperature.
Section snippets
Apparatus and method
The liquid–liquid-equilibrium was determined using a synthetic method or a cloud-point technique. A biphasic sample of known composition stirred rigorously was slowly heated and disappearance of turbidity was taken as the solubility temperature. The extent of turbidity was measured by intensity of light beam crossing the measuring cell. The breakdown on the intensity versus temperature dependence corresponded to the solubility temperature. Similar apparatuses were frequently described in the
Results and discussion
The binary liquid–liquid equilibrium were measured for the binary systems of nitromethane and one among the following C8 aliphatic unbranched monoethers: dibutyl (C4OC4), heptyl methyl (C1OC7), ethyl hexyl (C2OC6) and pentyl propyl (C3OC5). Over 20 experimental points were determined per each systems including the critical solubility point. The data were correlated by the following form of the NRTL equation [17]with
Conclusions
It was shown that the new apparatus measuring liquid–liquid- and solid–liquid-equilibria has been able to provide accurate data. Its advantages include small amount of a sample, precise and objective determination of the equilibrium point and controlling of the heating rate. The equipment gives a possibility of considerable automation of the experimental procedure.
The measured data of the liquid–liquid equilibria for the systems of nitromethane + {C1OC7 or C2OC6 or C3OC5 or C4OC4 normal aliphatic
Acknowledgments
This work has been supported by Warsaw University of Technology.
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