No abstract

The interaction of charged surfactants with oppositely charged polyelectrolyte networks has been investigated. The effective absorption of the surfactant ions by charged gel is observed. The surfactantnetwork complex formation results in gel collapse. The influence of charge density and mobility, ionic strength, and network topology on complex formation has been studied. It has been shown that the surfactant ions in charged gel form micelle-like aggregates. Due to this fact the surfactant-network complexes are effective absorbers for hydrophobic organic substances.

Several experimental facts indicate that the collapse transition in weakly charged polyelectrolyte gels or single macromolecules can take place with an increase in the degree of ionization (e.g., during titration of a weak polyacid). This unusual behavior can be explained by the energy gain from the formation of ion pairs in the collapsed state of low polarity, which competes with the swollen state where most of the counterions are dissociated. An increasing degree of ionization increases the thermodynamic advantages of the collapsed state with an ionomeric multiplet structure over the swollen polyelectrolyte state. In some regimes, this effect can lead to reentrant phase transition behavior, i.e., to an initial decollapse of the gel upon charging with subsequent jumplike collapse at a higher fraction of charged monomer units.

SUMMARY:The theory of collapse of weakly charged polyelectrolyte gels is generalized by taking into account the possibility of counter ion trapping with the formation of ion pairs, which becomes progressively important as the gel shrinks and the dielectric constant of the gel medium decreases. A phenomenon well known in the theory of ionomers, namely the aggregation of ion pairs due to dipole-dipole interactions with the formation of multiplets, is also taken into account. These multiplets act as additional physical cross-links. It is shown that accounting for the two effects mentioned above generally leads to an increase of the region of stability of the collapsed phase and to an increase of the jump in volume at the transition point. The most important, qualitatively new effect is the possibility of existence of a new supercollapsed state of a polymer gel which is very close to the densely packed dry gel. The reason for the thermodynamic stability of the supercollapsed state is a loop of positive feedback: the decrease of the volume of the gel leads to a decrease of the dielectric constant and hence to progressive formation of ion pairs, thus the concentration of mobile counter ions and the corresponding osmotic pressure decrease, the gel shrinks further etc. It is possible to realize the phase transitions between all three states of a polymer gel: swollen, ordinary collapsed and supercollapsed.