cosolvent

A cosolvent, or co-solvent, is a solvent added to a primary solvent to improve the solubility of solutes, alter phase behavior, or influence chemical reactivity. By changing the overall solvent environment, a cosolvent can modify properties such as polarity, dielectric constant, hydrogen-bonding capability, and solvent–solute interactions, thereby enabling dissolution or stabilization of species that are poorly soluble in the main solvent alone.

Cosolvents work through several mechanisms. They can disrupt organized solvent networks, adjust solvation parameters to better match those of the solute (as described by Hansen solubility parameters), and change solvent viscosity or microstructure. The choice of cosolvent depends on the target solute, the main solvent, temperature, and practical considerations such as toxicity and regulatory constraints.

Common cosolvents include alcohols like ethanol and methanol, ketones such as acetone, nitriles like acetonitrile, and polar aprotic solvents such as dimethyl sulfoxide and dimethylformamide. Glycerol and ethylene glycol are used for high-boiling applications, while water remains the intrinsic solvent in many aqueous systems.

Applications span pharmaceuticals (solubilizing poorly soluble drug candidates), polymer science (controlling dissolution and precipitation), extraction and crystallization processes, and various chemical reactions where solvation conditions influence yield or selectivity.

Safety and environmental concerns are important: cosolvents may be toxic, flammable, or environmentally persistent. Selection often aims to balance solubility improvement with availability, cost, and safety considerations.