Polyelectrolytes conformation when adsorbed

Hesselink23) attempted to calculate adsorption isotherms for flexible polyelectrolytes. He assumed that, when adsorbed on a surface, a flexible polyelectrolyte takes a conformation consisting of one train and one tail. The theoretical treatment of Hoeve et al.4I) (cf. B.3.1) for non-ionic polymers was extended by taking into account the change in electrical free energy that occurs when the polyelectrolyte is brought from the solution onto the interface. The partition function Q for a system of N polyelectrolytes each consisting of n units, in which Na polyions are adsorbed on the surface of area S and Nf(Nf = N - N ) polyions remain in the bulk solution of volume V, is then represented by... 

Dissolved polymer molecules can be adsorbed by polymer particles via electrostatic attractive force or hydrophobic interaction. When polyelectrolyte is adsorbed on an opposite-charge particle, the polymer molecules usually have a loop-and-tail conformation and, as a result, inversion of charge occurs. For example, sulfatecarrying particles behave as cationic ones after they adsorb poly(lysine). Then poly(-styrene sulfonate) can be adsorbed on such cationic particles and reinvert the charge of particles to anionic (14). Okubo et al. pointed out that the alternate adsorption of cationic and anionic polymers formed a piled layer of polyelectrolytes on the particle, but the increment of adsorbed layer thickness was much less than expected. This was attributed to synchronized piling of two oppositely charged polyelectrolytes (15). 

The second adsorbed layer was formed due to the pumping through the capillary with preadsorbed polyelectrolyte layer of anionic surfactant solution of different concentrations below erne. To clear out the influence of the first adsorbed PE layer on the formation of the second anionic surfactant layer, we studied the adsorption of SDS on PE layers of different structures (see Deformation of Adsorbed Layers) when the PE molecules adsorbed in flat conformation (CSDAPM at C = 10—4 g/1) and when the extended layer with loops and tails was formed (CSDAPM at C = KT2 g/1 and PDADMAC at C = KT2, KT3 and 10-4g/l). 

Smaller surface charge densities will be necessary to trigger polyelectrolyte adsorption for other boundary conditions, when, e.g., the maximum of the polymer peak is positioned right on the adsorbing interface, instead of being displaced from it by entropic repulsion of polymer chains from the surface, as assumed above. Also note that for weak adsorption, we work in the limit of vanishing polymer concentration in the bulk, to avoid a nontrivial computation of the conformational entropy of free polyelectrolytes in an electrolyte solution. 

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