Polyelectrolyte gels osmotic pressure

The nion term is simply an expression for the osmotic pressure generated across a semipermeable membrane effectively, the gel serves as a membrane which restricts the polyelectrolytes to one phase, while small ions can readily redistribute between phases. Assuming that the ions form an ideal solution, the expression for nion becomes simply... 

We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account Experimental results demonstrate a good correlation with theoretical analysis. 

On the other hand, for a macroscopic polyelectrolyte gel, which is a collection of an infinite number of macromolecules, the swelling is due to the osmotic pressure of counterions remaining within the sample of gel due to the macroscopic electroneutrality condition. The electrostatic interaction between charged units in this case is of secondary importance. 

We would like to stress that the physical reason of the polyelectrolyte gel expansion is the osmotic pressure of counterions, which originates from their translational entropy. Counterions possessing high translational entropy would like to leave the network sample. However, this is forbidden because of the condition of total electroneutrality. So the counterions create osmotic pressure on the sample of the polymer network. 

The placement of fixed charges in a swellable polymer network can endow the network with a variety of useful properties, perhaps the most important of which is a discontinuous swelling transition (262). For example, the volume of a polyelectrolyte gel may abruptly expand by several hundred times as the electrolyte concentration of a bathing solution decreases below a critical value (263). This property underlies the superabsorbency of many polyelectrol)d e gels. A discontinuous swelling transition can be imderstood in terms of concepts already discussed, with attention focused on coimterions and their impact on osmotic pressure. 

The important role of counterions in the swelling of polyelectrolyte gels was established long ago. At the beginning of 1950s, Katchalsky et showed that the polyelectrolyte gel swelling is determined by a balance between the elastic energy of polymer chains and the osmotic pressure of counterions. 

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