The electrical double layer for a fully asymmetric electrolyte around a spherical colloid: An integral equation study

The hypernetted chain/mean spherical approximation (HNC/MSA) integralequation is obtained and solved numerically for a totally asymmetric primitivemodel electrolyte around a spherical macroparticle. The ensuing radialdistribution functions show a very good agreement when compared to our MonteCarlo and molecular dynamics simulations for spherical geometry and withrespect to previous anisotropic reference HNC calculations in the planar limit.We report an analysis of the potential vs charge relationship, radialdistribution functions, mean electrostatic potential and cumulative reducedcharge for representative cases of 1:1 and 2:2 salts with a size asymmetryratio of 2. Our results are collated with those of the Modified Gouy-Chapman(MGC) and unequal radius Modified Gouy-Chapman (URMGC) theories and with thoseof HNC/MSA in the restricted primitive model (RPM) to assess the importance ofsize asymmetry effects. One of the most striking characteristics found isthat,\textit{contrary to the general belief}, away from the point of zerocharge the properties of an asymmetric electrical double layer (EDL) are notthose corresponding to a symmetric electrolyte with the size and charge of thecounterion, i.e. \textit{counterions do not always dominate}. This behaviorsuggests the existence of a new phenomenology in the EDL that genuinely belongsto a more realistic size-asymmetric model where steric correlations are takeninto account consistently. Such novel features can not be described bytraditional mean field theories like MGC, URMGC or even by enhanced formalisms,like HNC/MSA, if they are based on the RPM.Comment: 29 pages, 13 figure