Diffuse Layer Models

This page presents a more in-depth description of the diffuse layer models implemented in Environ, along with all revelant input parameters.

Explicit Charge

Environ has the ability to represent external charges of various types. As with most of the examples on this page, we will assume a setup consisting of some noble metal slab (i.e. a two-dimensional system) that can possess some surface charge. To compensate we then consider the addition of countercharge, either explicitly or via an electrolyte solution. Explicit charges come in a variety of implemented forms depending on the specified dimensionality. Suppose we want a point charge electron at some arbitrary position. One could enter the following into the Environ input file,

EXTERNAL_CHARGES (bohr)
-1 0. 0. 0. 1.0 0 0

Remember to update the env_external_charges parameter appropriately (here we could have something like

&ENVIRON
   env_external_charges = 1

This places a point-charge (since the dimensionality is set to 0), at the origin. The charge is given a negative value whose magnitude equals that of an electron. The spread is set to 1.0. The dimensionality is given as 0, and the axis setting is irrelevent but set to 0 anyway. The definition of the spread is based off the following 1D definition for the gaussian,

\[ae^{\frac{(x-b)^2}{c^2}}\]

Note the lack of the factor of two that some definitions use. In three dimensions, the equation is effectively the same, only radial around the 3-dimensional point specified in the input. The gaussian is normalized to the specified charge. This convention has a lot of transferability, and by simply changing the dimensionality of the input, can be extended to line charges, and planar charges. For these objects, the axis is relevant. For a line charge, the chosen axis corresponds to a the axis parallel to the line charge, and for a plane, it corresponds to the axis normal to the plane. Note that this definition clearly does not account for all possible definitions of line and plane charge distributions, but this can be achieved instead by changing the atomic positions of the slab.

For an example of plane countercharges, see Example: Charge Distributions.

For models with charge distribution, the free energy functional can be written as

\[F^{\text{PC}}[\rho(\mathbf{r}), \phi(\mathbf{r}] = \int\left[-\frac{\epsilon(\mathbf{r})}{8\pi}\lvert\nabla\phi(\mathbf{r})\rvert^2 + \rho(\mathbf{r})\phi(\mathbf{r}) + \rho^{\text{ions}}(\mathbf{r})\phi(\mathbf{r})\right]d\mathbf{r}\]

where \(\rho(\mathbf{r})\) is the total charge density of the solute, \(\phi(\mathbf{r})\) is the electrostatic potential, and \(\rho^{\text{ions}}(\mathbf{r})\) is the external charge density that mimics the counterion accumulation.

Poisson-Boltzmann model

The Poisson-Boltzmann model accounts for the chemical potential and the entropy of the ions in solution. Adding these consequently explicit concentration-dependent terms results in a different free energy functional than before,

\[ \begin{align}\begin{aligned}F^{\text{PC}}[\rho(\mathbf{r}), \phi(\mathbf{r}, \{c_i(\mathbf{r}\}] = \int\left[-\frac{\epsilon(\mathbf{r})}{8\pi}\lvert\nabla\phi(\mathbf{r})\rvert^2 + \rho(\mathbf{r})\phi(\mathbf{r}) + \rho^{\text{ions}}(\mathbf{r})\phi(\mathbf{r})\right.\\\left.-\sum^{\text{p}}_{i=1}\mu_i(c_i(\mathbf{r})-c_i^0)-T(s[\{c_i(\mathbf{r})\}]-s[\{c_i^0\}])\right]d\mathbf{r}.\end{aligned}\end{align} \]

Here, \(\mu_i\) is the chemical potential of the ith electrolyte species and T is the temperature, that is included as part of the entropy term.

We assume that these electrolytes have point-charge, and there is ideal mixing, leading us to the following entropy expression,

\[s[\{c_i\}] = -k_B\sum^{\text{p}}_{i=1}c_i(\mathbf{r})\ln\frac{c_i(\mathbf{r})}{\gamma(\mathbf{r})}\]

where \(\gamma(\mathbf{r})\) is the exclusion function and sets the boundary between the electrolyte and solute regions.

The functional above is minimized with respect to the concentrations in order to find the equilibrium for electrolyte concentrations. This leads to the well-known Poisson-Boltzmann equation, which can be numerically solved using Newton’s iterative algorithm and a preconditioned conjugate gradient algorithm, which works with linear equations.

There are times when it is beneficial to approximate the Poisson-Boltzmann equation, which is non-linear and thus non-trivial to solve. For \(z_i\phi(\mathbf{r}) \ll k_BT\), that is, when the electrostatic potential is low, or in the high-temperature limit, one can approximate the Poisson-Boltzmann by its linear form, which is derived by Taylor expanding the appropriate terms in the full Poisson-Boltzmann equation. The result is a less computationally expensive approach to representing the electrolyte, while still maintaining the same parameter set.

In the case where we deal with linear slabs with two-dimensional periodicity, a different approximation can be used. The idea here is that the one-dimensional Poisson-Boltzmann equation can be analytically solved and thus this result can be applied as a PBC (periodic boundary condition) correction. This approach is also known as the Gouy-Chapman Stern model for an electrolyte, and compares well to the numerical approach for our examples.

Modified Poisson-Boltzmann model

The Poisson-Boltzmann can be improved by dropping the assumpion of point-like ions. This assumption leads to an overestimate of the electrolyte countercharge accumulation at electrode surfaces, and thus, by accounting for the steric repulsion between ions, which opposes the electrostatic attraction between the counterions and the electrode surface, one can improve on the full Poisson-Boltzmann model. The size-modified Poisson-Boltzmann (MPB) can be derived from the same free energy functional as before, only with a modified entropy expression

\[ \begin{align}\begin{aligned}s[\{c_i\}] = -k_B\sum^{\text{p}}_{i=1}c_i(\mathbf{r})\ln\frac{c_i(\mathbf{r})}{c_{\text{max}}\gamma(\mathbf{r})}\\-k_B\left(c_{\text{max}}\gamma(\mathbf{r} - \sum^{\text{p}}_{i=1}c_i(\mathbf{r})\right)\ln\left(1 - \sum^{\text{p}}_{i=1}\frac{c_i(\mathbf{r})}{c_{\text{max}}\gamma(\mathbf{r})}\right).\end{aligned}\end{align} \]

The idea is to essentially impose a space dependent maximum ionic concentration, and the result is a better representation of the electrolyte, verified by a comparison to experimental differential capacitance.

Additional Interactions

Environ

Environ has implemented all of the above models in a modular way that allows one to mix and match models and correction methods where reasonable.