Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.21.2 Predicting cosolvency 997
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14.21.2 PREDICTING COSOLVENCY FOR PHARMACEUTICAL AND
ENVIRONMENTAL APPLICATIONS
An Li
School of Public Health, University of Illinois at Chicago, Chicago, IL, USA
14.21.2.1 Introduction
Cosolvency refers to the effects of adding one or more solvents (cosolvents), which are different
from the existing solvent in a solution, on the properties of the solution or behavior of
the solute. Cosolvency has found its applications in numerous engineering and scientific
disciplines. The discussion in this section will be limited to aqueous phase cosolvency (the
primary solvent is water), and cosolvents will include only pure organic solvents which are
miscible with water either completely (in any proportion) or partially (in only certain proportions).
The extent of cosolvency will be quantitatively described by the difference in solute
solubilities in pure water and in a mixture of water and cosolvent(s).
Cosolvency has been studied for decades. However, it remains a poorly understood
phenomenon due in large measure to our limited awareness of the liquid structure and the
intermolecular forces. At present, practical approaches to predicting cosolvency are to develop
models based on established theories and to make use of correlation between experimental
observations and properties of the substances involved. As with all modeling efforts,
it is essential to make judicious simplifications at various levels. The efforts to date have
given rise to several models, including the extended regular solution theory 1-3 and its modification,
4 excess free energy model, 5-7 the phenomenological model, 8,9 modified Wilson
model, 10,11 the combined nearly ideal binary solvent (NIBS) model, 12 the mixture response
surface model, 13 and others. Many of these models, however, are considered to be more descriptive
than predictive, because they inevitably involve one or more model parameters
which are usually specific to a particular solute/solvent/cosolvent(s) system, and must be
estimated from experimental data of solubility obtained for that system. On the other hand,
purely empirical models, e.g., the double-log exponential equation, 14 aim at satisfying
mathematical descriptions of measured data, and often offer little insight to the process.
Comparisons among cosolvency models have been made in several published papers. 15-19