Water desorption isotherms of cellulose-acetate membranes

Abstract

The water desorption isotherms are determined in three cellulose acetate membranes with different acetyl content as a function of p/p 0 at 10–40 °C. The partition coefficients (adsorbed water over water pressure) show a minimum at p/p 0=0.5–0.6. This indicates a two energy mechanism. The agreement of our results with the BET adsorption isotherms only till p/p 0<0.3 shows that a two energy adsorption mechanism is valid only for small water contents, probably one hydrate layer and a second more liquid-like water layer. At large p/p 0, the adsorbed water becomes more and more liquid like by polarization of the hydrogen bonds. The heat of desorption is larger than the vaporization heat of water ΔH vap(H2O). It decreases with increasing water content asymptotically to ΔH vap(H2O). The cause may be a larger van der Waals interaction of the hydrate layer due to coordination numbers larger than 4.4 as in liquid water. Additionally, we found a hole adsorption process by sorbing unpolar solvents. The water and methonal adsorption are 100 times larger due to a swelling mechanism depending on the number of acetyl groups in the membranes. The amounts of n-alcohols sorbed decrease with their chain length.