The Nature of Free and Bound Water in Water and Solute Permeation of Cellulose Acetate Membranes in Reverse Osmosis

Abstract

Cellulose acetate membranes were prepared from several Manjikian type casting solutions by changing the ratio of weight percent of formamide (F) to cellulose acetate (CA), that is, F/CA-value from 0.74 to 2.94. For these membranes, their transport coefficients (L_p, σ, and P) in Spiegler-Kedem equation and coupling coefficient L_pπ were obtained by reverse osmosis experiment. The free and bound water content in these membranes were investigated by means of differential scanning calorimetry.
An attempt was made to obtain the correlation between both the free and bound water content and the phenomenological coefficients (L_p and L_pπ). The L_p and (-L_pπ) can be represented in terms of the external velocity of water (υ_w/ΔP) and the velocity of solute relative to that of water (υ_w-υ_s)/ΔP, per unit pressure, respectively, as reported by Katchalsky and Curran.
It was stated that the coupling coefficient (-L_pπ)=(υ_w-υ_s)/ΔP was concerned with the bound water content, and the hydraulic permeability L_p=(υ_w/ΔP), also with the bound water content so far as 2 of F/CA-value, but above 2, the L_p, with the free water content.
In the range (F/CA-value<2) that the role of bound water take spart in water permeation, the increase of the bound water content gave little effect on the solute permeation, but in the range (F/CA-value>2) that the role of free water does part in water permeation, the amount of solute permeation through the membranes increased steeply.