Cosolvents fraction

More recent work with cosolvency in dilute systems seems to indicate that the magnitude of the solubility enhancement is linear up to some 10-20% cosolvent fraction [55,172,184,250-262]. At very low concentrations of cosolvent, the assumption of non-interaction between the cosolvent and water cannot hold. In dilute solutions the individual cosolvent molecules will be fully hydrated and, as a result, will disrupt the water network structure. If the total volume disrupted is regarded as the extended hydration shell, and if Sc is the average solubility within this shell, then the overall solubility Sm in the water-cosolvent mixture will be approximated by... 

Most solubilization curves, as shown in Figure 14.21.2.1, exhibit significant curvatures which are not accounted for by the log-linear model. A closer look at the solubilization curves in Figure 14.21.2.1 reveals that the deviation can be concave, sigmoidal, or convex. In many cases, especially with amphiprotic cosolvents, a negative deviation from the end-to-end log-linear line is often observed at low cosolvent concentrations, followed by a more significant positive deviation as cosolvent fraction increases. 

Additional deviations related to the solute s behavior may occur. For organic electrolytes, the acid dissociation constant may decrease as cosolvent fraction f increases. ... 

Additional deviations related to the solute s behavior may occur. For strong electrolytes, the acid dissociation constant may decrease as cosolvent fraction f increases. This, in turn, will affect the patterns of solubilization by cosolvents. Furthermore, a high concentration of solutes may invalidate the log-linear model, which presumes negligible volume fraction of solute and no solute-solute interactions. For solid solutes, solvent induced polymorphism may also bring additional changes in their solubilization profile. 

Then, a plot of AH versus AG is made, and the position of the peaks is used to indicate the shift of dominance from entropy at low cosolvent fractions due to the lessening of the water structure, to enthalpy at high cosolvent content. Thus, this approach is considered to be able to detect a true enthalpy-entropy relationship independent of a statistieal compensation. 

The cosolvent will lower the dielectric constant of the mixed solvent, independent of the properties of the solute molecule. The ionization constant of acids will increase and that of bases will decrease (see Sections 3.3.3 and 3.3.4), the result of which is to increase the fraction of uncharged substance in... 

Walters and Guiseppe-Elle [108] studied the sorption of 2,3,7,8-tetrachlorodibenzo-p-dioxin to soils from aqueous methanol mixtures and evaluated the applicability of the cosolvent theory to such sorption. Sorption kinetics were influenced by the fraction of methanol in the liquid phase and the soil type. Linear equilibrium sorption isotherms were... 

Using values of ac determined from solubility measurements, Fu and Luthy (50) found an average value of a equal to 0.51, which shows that the decrease in K c was half of that expected from the increase in solubility. This was interpreted as evidence that the cosolvent was swelling the organic fraction of the soil and consequently increasing the accessibility to the organic matter (50) in accordance with the gel-partition model (31). 

It has been generally considered that there is an exponential increase in the solubility of a solute as the fraction of the cosolvent increases linearly. The only requirement for the log linear relationship seems to be that the solute must be less polar than the mixed solvent [19]. The validity of the log-linear nature of the cosolvent process has been well validated in the literature [110,188, 247-249, 262,263]. The effect of a cosolvent on solubility can be calculated according to... 

Cosolvent and temperature effects on various types of noncovalent forces involved in protein-protein interactions are now well documented. These effects have been intensively studied in Douzou s laboratory through their impact on protein fractionation (Douzou and Balny, 1978). Mixed solvents at carefully controlled concentration and temperature variations in the range of normal and subzero temperatures ap-... 

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. 

Crystalline salts of many organic acids and bases often have a maximum solubility in a mixture of water and water-miscible solvents. The ionic part of snch a molecule requires a strongly polar solvent, snch as water, to initiate dissociation. A mixture of water-miscible solvents hydrates and dissociates the ionic fraction of pollutants at a higher concentration than wonld either solvent alone. Therefore, from a practical point of view, the deliberate nse of a water-soluble solvent as a cosolvent in the formnlation of toxic organic chemicals can lead to an increased solnbility of hydrophobic organic contaminants in the aqueous phase and, conse-qnently, to a potential increase in their transport from land surface to groundwater. 

Table 6.2 presents data showing the effect of various CMOS on the activity coefficient or mole fraction solubility of naphthalene, for two different solvent/water ratios. To examine the cosolvent effect, Schwarzenbach et al. (2003) compare the Hildebrand solubility parameter (defined as the square root of the ratio of the enthalpy of vaporization and the molar volume of the liquid), which is a measure of the cohesive forces of the molecule in pure solvent. 

The quasi-linear relationship between the logarithm of the retention factor and volume fraction organic cosolvent in the mobile lase seems to be the general rule in RPC. However, special effects can ur to cause this rule to be violated. Marked deviation from linearity was observed by Melander et al. (158) with retention data o poiy(ethylene glycol) derive-... 

Figure 3.6-5 Change in the conductivity of [EMIM]CI/AICl3 ionic liquids with the mole fraction of cosolvent ( ) benzene or (O) CH2CI2 added to a 55.56-44.44 mol % [EMIMJCI/AICIj ionic liquid, and (O) benzene added to a 40.00-60.00 mol % [EMIMJCI/AICIj ionic liquid.

Solution-based systems are common to both nebulizers and nasal formulations. In general, water will form the greatest fraction of the formulation, but, in some cases, cosolvents such as ethanol and propylene glycol may be added for increased stability. Acidifying and alkalizing excipients may also be added to optimize pH from the perspective of the drug stability as well as the physiological effect on the airways. Similarly, iso-osmotic and iso-tonic solutions are preferred. 

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