STRUCTURE OF WATER IN ASYMMETRIC CELLULOSE ESTER MEMBRANES - AN ATR-FTIR STUDY

The structure of water in the active layer of cellulose acetate and ce llulose acetate butyrate asymmetric membranes of nanofiltration/revers e osmosis is studied and correlated with the permeation properties. Me mbranes 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 s tructure in the active layer is investigated by attenuated total refle ction infrared spectroscopy (ATR-FTIR). ATR-spectra of cellulose aceta te and cellulose acetate butyrate membranes show bands due to OH stret ching centred in the 3370-3460 cm(-1) range, their position depending on the polymer, casting solution and casting conditions. The casting c onditions, namely the evaporation time, have a strong effect on the st ructure of the active layer: the spectra of cellulose acetate membrane s prepared using lower evaporation time show lower values of nu(OH) th is band shifting from 3372 to 3458 cm(-1) for samples prepared using e vaporation time of 1 and 10 min, respectively. ATR-FTIR spectra of the support show lower values of nu(OH) than those of the active layer wh ich is in agreement with the presence of larger pores and larger water clusters in the sublayer. The presence of a larger and more hydrophob ic substitute, such as butyryl, has a strong influence on the permeati on performance, leading to higher values off. The higher values of nu( OH) observed in spectra of cellulose acetate butyrate membranes have b een associated to the presence of weakly H-bonded water clusters. For both cellulose acetate and cellulose acetate butyrate membranes a rela tion between the permeation properties and the structure of the water in the active layer was found, lower values of apparent rejection bein g obtained for increasing size of the water clusters. (C) 1998 Elsevie r Science B.V.