Cosolvents linear function

Addition of a cosolvent is an alternative mechanism to increase contaminant solubility in an aqueous solution. When a contaminant with low solubility enters an aqueous solution containing a cosolvent (e.g., acetone), the logarithm of its solubility is nearly a linear function of the mole fraction composition of the cosolvent (Hartley and Graham-Bryce 1980). The amount of contaminant that can dissolve in a mixture of two equal amounts of different solvents, within an aqueous phase, is much smaller than the amount that can dissolve solely by the more powerful solvent. In the case of a powerful organic solvent miscible with water, a more nearly linear slope for the log solubility versus solvent composition relationship is obtained if the composition is plotted as volume fraction rather than mole fraction. 

Alternatively, Rubino and Yalkowsky found that a was a linear function of cosolvent polarity for a given solute. This is illustrated in Fig. 3 for the three lipophilic compounds phenytoin, diazepam, and benzocaine. Thus, knowledge of the solubility of a given drug in water and at least two cosolvents would permit cr to be estimated for other cosolvents by interpolation using an index of the desired cosolvent polarity. These studies permit the use of Eq. (4) as a means to rationally choose or eliminate solvents for formulation studies based on limited experimental solubility data and commonly obtained indexes of solute and solvent polarity. 

As noted in the Introduction, the Cohn equation, (Eq. (1)), considers that log of protein solubility is a linear function of the cosolvent molarity. In reality [5,14], the above dependence is not linear. Fig. 1 presents some accurate experimental data regarding the aqueous solubility of lysozyme and shows that linearity occurs only in the dilute region (C3<0.5). Our Eq. (12) allows one to explain this behavior. Only the dilute region (c3<0.5) is considered because in this composition range the preferential binding parameter is proportional to the concentration of the cosolvent [58,64,69] and Eq. (12) involves this approximation. Eq. (12) reveals that the linearity or nonlinearity of In y2 versus cosolvent concentration depends on the water activity in the protein-free aqueous mixed solvent. Eq. (12) was used to examine the log protein solubility versus eosolvent molarity in water/protein/ polyethylene glycol (PEG) mixtures [42]. It was shown that there were almost linear behaviors for PEG 1000 and PEG... 

Equation 14 shows that J21 can be calculated from experimental data regarding In the dilute cosolvent region, the preferential binding parameter F is a linear function of cosolvent concentration,3939-4i nd J21 can be considered to be a constant provided by the slope of F versus C3. 

The physical properties of liquids, such as viscosity (Figure 7.1.23) and surface tension (Figiue 7.1.24), also change during the evaporation process. The viscosity change for this system was a linear function of the amount of solvent evaporated. This study on waterborne coatings showed that the use of a cosolvent (e.g. i-butanol) caused a reduction in... 

In dilute solutions, the solute will, on average, contact only one hydrated cosolvent molecule at a time, and the degree of solubilization should be a linear rather than a logarithmic function of cosolvent content. Thus, it is expected that the log-linear relationship between Sm andf. that applies at high cosolvent concentrations will become linear at low cosolvent levels due to a change in the mechanism of solubilization. If S is defined as solubility enhancement... 

A generally accepted viewpoint is that the deviation from the log-linear solubilization is mainly caused by the non-ideality of the solvent mixture. This is supported by the similarities in the patterns of observed log and activities of the cosolvent in solvent mixture, when they are graphically presented as functions of f Based on the supposition that solvent non-ideality is the primary cause for the deviation, Rubino and Yalkowsky examined the correlations between the extent of deviation and various physical properties of solvent mix-... 

All the investigations cited so far have been in aqueous solution medium dependence of dissociation rate constants in binary aqueous solvents (cosolvents methanol, r-butyl alcohol, 1,2-ethanediol, 1,2,3-propanetriol, and sucrose) has been reported for the 4-cyanopyridine and 4,4 -bipyridyl penta-cyanoferrates(II). For water-rich media (IQi lD < 16.5), plots of logarithms of rate constants against mole fraction organic component are linear, against 1/D almost linear. Slopes differ greatly between cosolvents, but results can be correlated quite satisfactorily with a three-parameter equation incorporating acidity and basicity parameters and the excess function... 

---