Reverse osmosis separations for some alcohols and phenols in aqueous solutions using aromatic polyamide membranes

The performance of aromatic polyamide membranes for reverse osmosis separations of eight alcohol and four phenol solutes in dilute aqueous solutions has been studied. The Taft polar parameter σ* for the solutes studied were in the range of −0.3 to 1.388. Positive solute separations were obtained for each one of the solutes. In the σ* value range of −0.3 to 0, data on PR/PWP ratio scattered close to 1, and solute separation decreased with increase in σ*. For the phenol solutes, PR/PWP ratio decreased and solute separation increased with increase in σ*. The results are interpreted on the following basis. The aromatic polyamides are more nonpolar than cellulose acetates. In the σ* range of −0.115 to −0.3, solute separation is governed primarily by polar interactions; in this range, solute transport parameter DAM/Kδ is well correlated by the expression DAM/Kδ = C* exp (ρ*σ*). The solute separation for ethyl and methyl alcohol solutes (σ* = −0.1 and 0, respectively) is reduced by the nonpolar character of the membrane material. Positive solute separation for each of the phenolic solutes is due to preferential sorption of solute at the membrane‐solution interface caused by both the nonpolar character of the membrane material and acidity of the solutes.