Complexation of two oppositely charged Polyeletrolyte chains: Role of entropy and enthalpy

We report a study of the complexation process of two oppositely charged polyelectrolyte (PE) chains in dilute solution interacting through screened Coulomb potential. The two PE chains, as well as the neutral complex, are considered to be hypothetical spheres whose radii quantitatively denote the sizes of the respective chains, and the system is characterized by its free energy, calculated within the uniform spherical expansion approximation. A comparison of the entropy gain of counterions freed due to complexation and enthalpy change due to the reorganization of ion-pairs of two types-the monomer-counterion and oppositely charged monomer-monomer identifies the entropy to be the major driving force for the process. Addition of salt weakens the drive for complexation. The free energy of the complex is monotonically downhill at modest physical conditions.