**4. On the Role of Electrostatic Interactions**

Electrostatic interactions between the charged surface and the electrolyte solution result in the formation of the electric double layer (EDL) [38,69–77]. In an EDL, ions with an electric charge of the opposite sign compared to the charged surface (counterions) are accumulated close to the charged surface, and the ions with a charge of the same sign as the surface (coions) are depleted from this region [69–71,78–81]. Due to a nonhomogeneous distribution of ions in the EDL, the electric field strength is screened at larger distances from the charged surface. Due to high magnitudes of the electric field in the EDL, the water dipoles near the charged surface are strongly oriented (Figure 7) [38,82–90].

In the past, the first theoretical description of the EDL was introduced by Helmholtz [91,92], who assumed that a single layer of counterions forms at the charged surface. Later, the spatial distribution of point-like ions in the vicinity of the charged surface was described by the Boltzmann distribution function for the counter-ions and co-ions in the Poisson equation [69,70] within the so-called classical Poisson–Boltzmann (PB) model [69,70]. The PB model neglects the finite size of molecules and considers the relative permittivity as a constant throughout the whole electrolyte solution, i.e., PB approach neglects the spatial dependence of relative permittivity. A constant relative permittivity is a relatively good approximation for small magnitudes of surface charge density, but not for higher magnitudes of surface charge density where a substantial decrease in relative permittivity due to the strong orientational ordering of water dipoles in the vicinity of the charged surface was predicted [38,86,88]. In addition, the PB model also does not take into account the spatially dependent volume electric charge distribution in the zwitterionic lipids headgroup region, which depends on the interaction with neighboring charged bodies and also on the composition of the electrolyte solution [38,39,43].

The finite size of ions in the theoretical description of the EDL was first incorporated by Stern [78] with the so-called distance of closest approach and later developed further by [71,79–81]. Their work was further improved by numerous theoretical studies and simulations taking into account the asymmetry of the size of the ions, direct interactions between ions, orientational ordering of water dipoles, discrete charge distribution of the surface, quantum mechanical approach, etc. [1,72–74,76,84–86,88,90,93–115]. The physical properties of the EDL are crucial in understanding the interaction between charged surfaces in electrolyte solutions [34,38,43,116–124].
