Structure of water in asymmetric cellulose ester membranes — and ATR-FTIR study

Abstract

The structure of water in the active layer of cellulose acetate and cellulose acetate butyrate asymmetric membranes of nanofiltration/reverse osmosis is studied and correlated with the permeation properties. Membranes are prepared by phase inversion, varying the casting solution and the casting conditions. Their preferential permeation performance is tested with pure water and a model solution of NaCl and the water structure in the active layer is investigated by attenuated total reflection infrared spectroscopy (ATR-FTIR). ATR-spectra of cellulose acetate and cellulose acetate butyrate membranes show bands due to OH stretching centred in the 3370–3460 cm −1 range, their position depending on the polymer, casting solution and casting conditions. The casting conditions, namely the evaporation time, have a strong effect on the structure of the active layer: the spectra of cellulose acetate membranes prepared using lower evaporation time show lower values of ν oh this band shifting from 3372 to 3458 cm −1 for samples prepared using evaporation time of 1 and 10 min, respectively. ATR-FTIR spectra of the support show lower values of ν oh than those of the active layer which is in agreement with the presence of larger pores and larger water clusters in the sublayer. The presence of a larger and more hydrophobic substitute, such as butyryl, has a strong influence on the permeation performance, leading to higher values of f. The higher values of ν oh observed in spectra of cellulose acetate butyrate membranes have been associated to the presence of weakly H-bonded water clusters. For both cellulose acetate and cellulose acetate butyrate membranes a relation between the permeation properties and the structure of the water in the active layer was found, lower values of apparent rejection being obtained for increasing size of the water clusters.