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Abstract An account has been given of sorption and diffusion of water by ethyl cellulose. In contrast with diffusion coefficients of some other penetrants, those of water decrease with increase of water sorption in the manner previously observed by Rouse for water–polythene systems. The sorption isotherms have the opposite curvature to Langmuir or Freundlich isotherms, and hence also to the isotherms of benzene, C 4 and C 5 paraffins, or acetone sorbed at low penetrant concentration by the same polymer. The results have been interpreted in terms of cluster formation of water molecules either in preexisting cavities in the polymer or, less probably, in the polymer matrix itself. Visual evidence of cluster formation is provided by the onset of light scattering by the membrane as the relative humidity reaches a critical range. The opacity disappears again as the humidity is decreased.
Barrer et al. (1958) studied this question.