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Cellulose acetate membranes are film layers of an ester, composed of the plant fiber cellulose and a varying number of acetyl groups. Such membranes allow water to pass through while keeping salts from permeating them, so they are used in cold sterilization and ultrafiltration procedures. Cellulose is a chain of linked glucose molecules, and acetyl groups are small structures of carbon, hydrogen and oxygen molecules. These membranes are described as being asymmetric, because they have a dense skin on their surface with a more porous layer underneath them. Both the skin and support are composed of chemically identical cellulose acetate, even though their outward structure and appearance are different.
Generally, cellulose acetate membranes are composed of a polymer of fibrous cellulose, with each cellulose molecule being bound to either two or three acetyl groups from a source such as acetic acid. These membranes might have only diacetyl or triacetyl groups bound to cellulose, or they might have a mixture of the two. After cellulose and acetic acid are combined, along with a catalyst such as sulfuric acid, the resulting film is then placed in a water bath, which washes away excess acetic acid and creates the asymmetric layers in the membrane. Although cellulose acetate membranes are inexpensive to produce and can easily filter chlorinated water, unlike other types of membranes, they are limited in their ability to withstand extreme potenz hydrogen (pH) levels.
In addition to effectively filtering salts, cellulose acetate membranes do not bind organic molecules easily and are quite strong, so they are an excellent choice for filtering organic compounds such as proteins and enzymes in a laboratory setting. The strength of these membranes makes them useful for holding proteins for recovery during experiments or processes in which these substrates must be retrieved, which is why they are used as food analysis products and environmental monitoring products. The properties of cellulose acetate membranes can be limiting in lab techniques requiring imaging, though, because the porous underside of the cellulose acetate tends to absorb liquids and becomes transparent. This means that it can be difficult to determine how well these membranes adhere to other surfaces or to substrates such as proteins.
The transparency and strength of cellulose acetate membranes are two qualities that led to their use as a film in the 1930s. These membranes, however, tend to degrade when exposed to acids or extreme heat, leading to unusable film. For this reason, cellulose acetate is no longer a common film component, but it has continued to be used in ultrafiltration.