The sol used to prepare top layer of the membrane was spread over a glass slide (instead of graphite support)
and subjected to treatment identical to that used for synthesis of membrane. The top-layer so-formed was peeled-off and powdered. The powder was dried in an air oven at 80 C for 2 h and then was mixed with KBr and made into a pellet. Functional groups of this powder were determined using FTIR