Phase inversion (chemistry)

Not to be confused with Phase inversion.

Phase inversion (also called phase separation or demixing) refers to two related phenomena in chemistry and materials science: (1) a controlled phase-separation process widely used to fabricate porous polymer membranes, and (2) the inversion of continuous and dispersed phases in an emulsion.[1][2]

In membrane fabrication, phase inversion converts an initially homogeneous polymer solution into a solid, porous structure by inducing phase separation and solidification.[3]

Membrane fabrication

Phase inversion is one of the most common routes for producing polymeric membranes used in microfiltration, ultrafiltration, nanofiltration and related separations.[4] In practice, a polymer is dissolved in a solvent (sometimes with additives), shaped as a film or hollow fiber, and then phase-separated to form a polymer-rich phase (matrix) and polymer-lean phase (pores).[3][1]

Main methods

Diffusion-induced phase separation (DIPS)

DIPS is driven by mass transfer between the cast polymer solution and a contacting vapor or liquid, causing local composition changes that induce demixing.[1]

Common DIPS variants include:[1]

  • Immersion precipitation (often called non-solvent induced phase separation, NIPS): a cast film/fiber is immersed in a non-solvent coagulation bath, causing solvent ↔ non-solvent exchange and phase separation.
  • Vapor-induced phase separation (VIPS): the cast film is exposed to non-solvent vapor (e.g., humid air), allowing gradual uptake of non-solvent and demixing.
  • Evaporation/air casting: solvent evaporation changes the composition until phase separation occurs (often used with polymer–solvent–non-solvent systems).

Temperature-induced phase separation (TIPS)

In TIPS, phase separation is triggered primarily by changing temperature (e.g., cooling a polymer solution prepared at elevated temperature), causing demixing and solidification.[1]

Factors affecting membrane morphology

The pore structure produced by phase inversion depends on thermodynamics and mass transfer during demixing and solidification, which are influenced by polymer concentration, solvent/non-solvent choice, additives, and process conditions (e.g., bath composition, temperature, exposure time to vapor/air).[3][1]

In emulsions

In emulsions, phase inversion occurs when the dispersed and continuous phases exchange roles (e.g., water-in-oil to oil-in-water), which can be driven by formulation changes (such as surfactant affinity) or by changing phase volume fractions.[2]

See also

References

  1. ^ a b c d e f Kools, Willem F. C. (1998). Membrane formation by phase inversion in multicomponent polymer systems: mechanisms and morphologies (PDF) (PhD thesis). University of Twente. Retrieved 2026-02-25.
  2. ^ a b Perazzo, A.; Preziosi, V.; Guido, S. (2015). "Phase inversion emulsification: Current understanding and future perspectives". Advances in Colloid and Interface Science. 222: 581–599. doi:10.1016/j.cis.2014.10.010. hdl:10347/17068.
  3. ^ a b c Strathmann, H.; Kock, K. (1977). "The formation mechanism of phase inversion membranes". Desalination. 21 (3): 241–255. doi:10.1016/S0011-9164(00)88244-2.
  4. ^ Guillen, G. R.; Pan, Y.; Li, M.; Hoek, E. M. V. (2011). "Preparation and Characterization of Membranes Formed by Nonsolvent Induced Phase Separation: A Review". Industrial & Engineering Chemistry Research. 50 (7): 3798–3817. doi:10.1021/ie101928r. PMC 8146349.
  5. ^ "File:Polysulfone Hollow Fiber Membrane Cross-Section.jpg". Wikimedia Commons. Retrieved 2026-02-25.
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