Fabrication of Free-Standing Gelatin Thin Films via the Gelation and Drying of Liquid Foam Films

Ultrathin foam films can be prepared from numerous compounds under a wide range of conditions but have a narrow application scope because of their small size, susceptibility to rupture, and other drawbacks arising from low mechanical stability of the bilayer structure. To address this gap, we herein prepared ultrathin foam films of gelatin using a facile method via drying thin liquid films of aqueous gelatin solutions and the gelation. Their thickness ranges between nanometers to micrometers in response to changes in process parameters. The effects of various process parameters, including the solution concentration, frame size, drying temperature, and humidity, were systematically investigated. In addition to surface tension affecting the initial formation of the liquid film, the high viscosity of the solutions stabilized the film and caused gelation. The key formation factor of the dried foam films was explained by the sol-gel behavior of aqueous gelatin solutions. The prepared films are mechanically more stable than surfactant films and therefore have a wide range of potential applications as separation membranes. Glutaraldehyde cross-linking makes these films water-insoluble and therefore suitable for use as a nanoseparation membrane. Fully cross-linked gelatin thin films showed a rejection performance of 100% for 5 and 2 nm gold colloids, Direct Yellow 1, and potassium ferricyanide.