On the asymptotic limit of steady state Poisson--Nernst--Planck equations with steric effects

Abstract: When ions are crowded, the effect of steric repulsion between ions becomes significant and the conventional Poisson--Boltzmann (PB) equation (without steric effect) should be modified. For this purpose, we study the asymptotic limit of steady state Poisson--Nernst--Planck equations with steric effects (PNP-steric equations). By the assumptions of steric effects, we transform steady state PNP-steric equations into a PB equation with steric effects (PB-steric equation) which has a parameter $\Lambda$ and positive constants $\lambda_i$'s (depend on the radii of ions and solvent molecules). The nonlinear term of PB-steric equation is mainly determined by a Lambert type function which represents the concentration of solvent molecules. As $\Lambda=0$, the PB-steric equation becomes the conventional PB equation but as $\Lambda>0$, a large $\Lambda$ makes the steric repulsion (between ions and solvent molecules) stronger. This motivates us to find the asymptotic limit of PB-steric equation as $\Lambda$ goes to infinity. Under the Robin (or Neumann) boundary condition, we prove theoretically and numerically that the PB-steric equation has a unique solution $\phi_\Lambda$ which converges to solution $\phi^*$ of a modified PB (mPB) equation as $\Lambda$ tends to infinity. Our results show that the limiting equation of PB-steric equation (as $\Lambda$ goes to infinity) is a mPB equation which has the same form (up to scalar multiples) as those mPB equations in \cite{1942bikerman,1997borukhov,2007kilic,2009li,2009li2,2013li,2011lu}. Therefore, the PB-steric equation can be regarded as a generalized model of mPB equations.