OSMOTIC PROPERTIES OF POLYELECTROLYTE MEMBRANES - POSITIVE AND NEGATIVE OSMOSIS

The osmotic properties of two types of hydrated polyelectrolyte membra nes with negatively charged fixed ionic groups in aqueous solutions of NaCl and SrCl2 are studied. The membranes are condensation products o f p-hydroxybenzenesulfonic acid and formaldehyde (abbreviated -SO3-gel ) and (hydroxyphenyl)methanesulfonic acid and formaldehyde (abbreviate d-CH2SO3-gel),respectively. The osmotic properties are characterized b y measurements of the reflection coefficient sigma(s) and its concentr ation dependence. The system-CH2SO3-/SrCl2 shows negative osmosis in c oncentrated solutions (negative osmosis is the transport of water acro ss the membrane under isobaric conditions against its gradient of the chemical potential). For the other systems (-CH2SO3-/NaCl, -SO3-/SrCl2 , -SO3-/NaCl) positive osmosis is observed at all concentrations. The experimentally determined concentration dependence of sigma(s) is comp ared with that predicted by the model of the membrane with narrow pore s. The values of the parameters of the model are determined by indepen dent experiments. A satisfactory agreement between the experimental si gma(s)(c(s)) data and the prediction of the model is found when an eff ective fixed ion concentration of the gels is used for the calculation instead of the analytically determined value. The experiments indicat e that the gels have an inhomogeneous structure.