Nonlocal dielectric response of aqueous electrolytes and decay behavior of ionic correlations

We present molecular dynamics simulation results for the static nonlocal dielectric function, ɛ(k), and ion-ion correlations in aqueous electrolytes across various concentrations, analyzed within the framework of statistical mechanical theory. We investigate the complicated behavior of ɛ(k) and its influence on the screening patterns of ionic electric fields. At large to moderate values of the wave number k, ɛ(k) is primarily governed by the molecular structure of water, even at concentrations up to 1 M. However, at higher concentrations, ionic structuring begins to diminish water's influence. In agreement with the theoretical analysis presented, both Debye-type plain exponential and exponentially decaying oscillatory contributions to the electrostatic interactions are obtained for low electrolyte concentrations of NaCl in water. The latter originate from the water structure, while the former can be quantitatively described up to at least 2 M solutions by using a Debye length calculated with a concentration-dependent "dielectric constant" obtained by simulations. The theoretical basis for the use of such a "constant" is examined. At higher concentrations, there are only oscillatory contributions to the interactions. In our simulations, we do not have long-range monotonic interactions of the type reported in surface force measurements at concentrations above 1 M NaCl in water, often referred to as "anomalous underscreening." Instead, we find a Debye-type decay, modified by an electrolyte-concentration-dependent effective dielectric constant. This behavior is seen up to at least 2 M concentrations, with decay lengths significantly shorter than those associated with underscreening.