Cosolvents osmotic effects

Ophthalmic formulations sometimes contain cosolvents, such as propylene glycol or PEG 300, as part of the vehicle. The greatest limitation to the use of cosolvents in ophthalmic preparations is their irritation potential. For example, ethanol is too irritating to be used in the eye (Table 3). Osmotic effects of cosolvents are also important, and the strong osmotic effect of glycerin combined with its poor solubilizing power limit its usefulness in ophthalmic preparations. 

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. 

The most commonly used generic term for a dissolved substance is solute, and this is the term that we will employ in most contexts, for both large and small compounds, that is, for macromolecules and micromolecules. A closely related term, cosolvent, is often used by physical chemists when the issue in question involves a dissolved substance that either stabilizes or destabilizes the structures of macromolecules. For instance, cosolvent is often used in literature on the effects of solutes on protein stability. A more restrictive and specific term that will be employed when we discuss the osmotic relationships of organisms is osmolyte. 

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