Structure in very permeable collodion gel films and its significance in filtration problems

The problem of gel structure is one of general scientific interest and importance, but it is in regard more particularly to its significance in ultra-filtration that the present work has to deal. The observations concern collodion gel films, and have been made during an investigation into the problem of filtration and filterable viruses, which has involved a close study of the behaviour of various types of gel membranes employed for ultra-filtration purposes. The most widely used membranes are made from collodion, a solution of nitro-cellulose in some suitable solvent, like acetic acid or ether/alcohol mixture. Two methods are available for preparing such membranes, according as to whether the solvent is volatile, like ether/alcohol, or non-volatile like acetic acid. In the former case the collodion is spread in a uniform layer over a glass or mercury surface and the solvents allowed to evaporate under standard conditions until the film just “sets,”i. e. , incipient gelation occurs. The remaining solvent is then washed out by immersing the film in water, which completes the gelling process. The alternative method, used when employing acetic acid collodion, consists in impregnating filter-paper (which serves to support the delicate film) with the collodion and then washing in water to replace the solvent acetic acid, and so gel the collodion.