On the low temperature anomalies in the properties of the electrochemical interface.: A non-local free-energy density functional approach

The restricted primitive model has proved to be a useful system to describe the behaviour of electrical double layers. In this model, ions are represented by charged hard spheres of equal diameter and the solvent is represented by a uniform dielectric constant. Classical Gouy-Chapman's theory, and its modification by Stern, always predicts a monotonically decreasing capacitance for this system when the fluid's temperature is increased. Similar results are given by the mean spherical approximation. These predictions are in qualitative agreement with experiment for dissolved electrolytes, but disagree with molten salt experiments where capacitance increases with temperature. Additionally, recent Monte Carlo (MC) simulations for this model show that at very low temperatures, the capacitance of the interface, near its point of zero charge, increases with increasing temperature for both diluted and highly concentrated salts. In this work we apply a particular model of a non-local free-energy density functional theory to study the capacitance of the electrical interface. In our calculations we considered symmetrical 1:1 systems for both diluted electrolytes and highly concentrated salts at very low electrode surface charge. Density functional theory agrees very well with MC results for capacitance at high temperature, but fails to predict a positive slope for this property at low temperatures. Comparison of theoretical density profiles with MC results allows the exploration of possible causes of failure.