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 represe nted by charged hard spheres of equal diameter and the solvent is represent ed by a uniform dielectric constant. Classical Gouy-Chapman's theory, and i ts modification by Stern, always predicts a monotonically decreasing capaci tance for this system when the fluid's temperature is increased. Similar re sults are given by the mean spherical approximation. These predictions are in qualitative agreement with experiment for dissolved electrolytes, but di sagree with molten salt experiments where capacitance increases with temper ature. Additionally, recent Monte Carlo (MC) simulations for this model sho w that at very low temperatures, the capacitance of the interface, near its point of zero charge, increases with increasing temperature for both dilut ed and highly concentrated salts. In this work we apply a particular model of a non-local free-energy density functional theory to study the capacitan ce of the electrical interface. In our calculations we considered symmetric al 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 pre dict a positive slope for this property at low temperatures. Comparison of theoretical density profiles with MC results allows the exploration of poss ible causes of failure.