Nonsolvent

Nonsolvent refers to a liquid in which a given polymer or solute has little or no solubility. It is described in relation to a good solvent for the same polymer. When a polymer solution is contacted with a nonsolvent, solvent molecules tend to diffuse out while nonsolvent molecules diffuse in. This exchange lowers the polymer’s solvency and can drive phase separation or precipitation, enabling solid formation or structural changes without adding new polymer.

In polymer processing, nonsolvents are widely used to precipitate polymers from solution and to form membranes through phase inversion. A polymer solution immersed in a nonsolvent bath undergoes solvent–nonsolvent exchange, which controls the timing and nature of phase separation and influences the final morphology, such as pore size and porosity. The properties of the nonsolvent, including miscibility with the solvent and its affinity for the polymer, as well as temperature and concentration, strongly affect outcomes. Nonsolvent exchange can also be used to remove residual solvent or to recover solid polymer by precipitation.

Outside membrane fabrication, nonsolvents play roles in protein precipitation, purification, and various washing steps in chemical processing. In each case, a nonsolvent is chosen for its poor ability to dissolve the target polymer or solute, while still being compatible enough to mix with the solvent system to enable controlled phase changes. The concept of a nonsolvent highlights how solvent quality and solubility relationships govern precipitation, extraction, and the formation of solid materials from solutions.