The dynamic development of exclusion zones on cellulosic surfaces

The dynamics of the development of solute exclusion zones produced by water in contact with various cellulosic surfaces are reported. The term “exclusion zone” (EZ) refers to the ordered water volume present immediately contiguous to hydrophilic surfaces. As such, three examples of cellulose-based films, i.e., cellophane, cellulose acetate with 0.94 degree of substitution (CA0.94) and cellulose acetate with 2.51 degree of substitution (CA2.51) were compared by exposure to sulfated-polystyrene microspheres (2 μm diameter) suspended in water. Zones that were not penetrated by the microspheres were observed in each case and measured by means of an optical microscope. The thickness of these exclusion zones adjacent to the cellulosic surfaces increased progressively with time and reached a maximum value of 190 μm after 1 h of exposure. Zone formation was influenced by roughness, crystallinity, and the degree of acetylation of the surfaces. For the cellulosic surfaces, the rougher the surface, the thicker was the EZ formed. Both the accessibility and availability of the hydroxyl groups on the cellulosic surfaces also influenced the thickness of the resultant EZ in the sequence, cellophane > CA0.94 > CA2.51. The surface of cellulose acetate with the greater substitution degree (CA2.51) produced the thinnest EZ. The diminution of the capacity of these cellulose derivatives to instigate water-molecule ordering is considered to be due to the hydrophobicity imparted by the acetyl group content.