WATER DESORPTION ISOTHERMS OF CELLULOSE-ACETATE MEMBRANES

The water desorption isotherms are determined in three cellulose aceta te membranes with different acetyl content as a function of p/p(0) at 10-40 degrees C. The partition coefficients (adsorbed water over water pressure) show a minimum at p/p(0) = 0.5-0.6. This Indicates a two en ergy mechanism. The agreement of our results with the BET adsorption i sotherms only till p/p(o) < 0.3 shows that a two energy adsorption mec hanism is valid only for small water contents: probably one hydrate la yer and a second more liquid-like water layer. At large p/p(0), the ad sorbed 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 Delta H-vap(H2O). It decreases with increasing water co ntent asymptotically to Delta H-vap(H2O). The cause may be a larger va n der Waals interaction of the hydrate layer due to coordination numbe rs larger than 4.4 as in liquid water. Additionally; we found a hole a dsorption 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-alco hols sorbed decrease with their chain length.