Origin of the dielectric decrement of salt water*

The dielectric constant is one of the most important properties of salt water, which determines the Coulomb interactions between the solution components and therefore regulates the microstructures and the physicochemical properties of the solution. Since the last century, it has been widely reported in experiments that the dielectric constant of salt solutions decreases nonlinearly with increasing solute concentration, a phenomenon called dielectric decrement. In this work, we study the dielectric decrement using the advanced deep potential long range (DPLR) method, which enables efficient ab initio-level simulations. Moreover, DPLR includes long-range electrostatic interactions explicitly, which is essential for the correct description of long-range dipole-dipole interactions. The computed dielectric constant agrees well with experimental data. The detailed analyses indicate that the dielectric decrement is mostly due to the loss of correlation between water molecular dipoles, which is caused by ions' disruption to the tetrahedral hydrogen-bond network of water. The anti-correlation between water molecular dipoles in the ionic first solvation shells (FSSs) contributes to a steep and linear drop in the correlation factor. Whereas the nonlinear decrement originates from the reduced cross-dipolar correlation between water molecules within ionic FSSs with that outside the FSSs. Surprisingly, the dipolar correlation among water molecules outside ionic FSSs have a negligible effect on the dielectric decrement.