Ternary Conjugates as a Result of Exchange Reactions Between Polyelectrolyte Complexes and Surfactants: Formation and Stability

Summary: The exchange of sodium polymethacrylate (Na‐PMA) for anionic surfactant sodium dodecylsulfate (SDS) in a polyelectrolyte complex of Na‐PMA and poly( L‐ lysine) (PLL) was studied by light scattering techniques. The exchange reaction proceeds inside the complex particles without any aggregation. It causes swelling of the particles and the formation of thick polymer shell around them. In order to prove the formation of the shell, the colloidal stability of the particles in salt solutions was studied. The addition of SDS to a complex solution dramatically increases colloid stability. The stability was found to be independent of the mixing ratio of the initial complex, depending only on the molecular weight of Na‐PMA and slowly decreasing with time. The mechanism of the exchange reaction was proposed. The addition of SDS causes fast substitution of charged units of Na‐PMA for surfactant molecules in the complex and formation of the PLL/SDS complex stabilized by hydrophobic interactions between aliphatic chains of SDS. The Na‐PMA chains cannot be released sufficiently fast from the particles due to entanglements of the chains inside the particles. They build a thick stabilizing shell around the particles and lead to their swelling. This shell protects the particles against coagulation at relatively high salt concentrations. However, such state of the particles does not correspond to thermodynamic equilibrium. Na‐PMA is very slowly released from the particles, decreasing the density of stabilizing shell and stability of the particles in salt solutions. The possible application of ternary conjugates in drug delivery is discussed.Schematic representation of changes in structure and stability of Na‐PMA/PLL complex particles upon the exchange of polyanion Na‐PMA for negatively charged surfactant SDS.