Density functional theory for charged fluids

An improved density functional theory (DFT) for an inhomogeneous charged system (including electrolyte and/or polyelectrolyte) is proposed based on fundamental measure theory, thermodynamic perturbation theory and mean-spherical approximation. Our DFT combines the existing treatment of hard-sphere contributions using fundamental measure theory (FMT) with a new treatment of the electrostatic correlations for the non-bonded ions and chain connectivity that are approximated by employing a first-order Taylor expansion, with the reference fluid density determined using the technique from Gillespie et al. [D. Gillespie et al., J. Phys.: Condens. Matter, 2002, 14, 12129]. We show that the first-order Taylor expansion for the non-bonded electrostatic correlations yields numerically comparable results to the more involved second-order expansion. Furthermore, we find that the existing treatment of the chain connectivity correlation predicts a spurious layer-by-layer phase at moderately large Bjerrum lengths, which is avoided in our new treatment. These simplifications and improvements should significantly facilitate the implementation and reduce the computational cost.