Molecular dynamics study of the structure organization in a strongly coupled chain of charged particles

The dynamical and equilibrium properties of a strongly coupled chain of charged particles (polyampholyte) submerged in an immobile viscous medium are studied using the molecular dynamics simulations. The polyampholyte relaxes to an equilibrium conformation typically in 300 omega(pe)(-1) due to folding of the chain for low temperatures, and expands several times faster for high temperatures, where omega(pe) is the plasma frequency. Three regimes with distinct conformations as stretched, oblate, and spherical are observed under the Coulomb force at high, medium, and low temperatures, respectively. The change in the conformations is considered to minimize the free energy through the electrostatic potential. The rest-mean-squared size of the polyampholytes in these regimes is scaled, respectively, as R-g similar to-N-1/2, (NT)(1/3), and (NT0.8-1.0)-T-0.3, where N is the number of monomers on the chain and T the temperature. The crossover paint of the regimes is characterized by the unique values of the monomer distance 2R(g)/N-1/3, being insensitive to the length and stiffness of the chain. The present results agree well with the Flory theory in the high and medium temperature regimes. The densely packed state at low temperatures is first obtained here without the use of the lattice model. The transition among the different regimes under the Coulomb force is exactly reversible. However, the transition under the cooperation of the Coulomb force and the attractive short-range force exhibits a hysteresis against successive changes in temperature.