I. INTRODUCTION A phenomenon of critical adsorption occurs when a fluid is brought to its bulk critical point in the presence of an attractive substrate or wall, for example, along the critical isochore. The wall causes a perturbation of the relevant order parameter profile to extend over a distance ∼ ξ$_{b}$ , the bulk correlation length, from the surface [1, 2]. Close to criticality, where ξ$_{b}$ ∼| ( T − T$_{c}$ ) /T$_{c}$ | − ν ( ν is the critical exponent), the influence of the wall extends to macroscopic distances. As a result the amount adsorbed (adsorption Γ) diverges as τ = ( T − T$_{c}$ ) /T$_{c}$ → 0. Here we focus on the equivalent phenomenon that occurs for binary liquid mixtures near their consolute points. In these systems, generically there is a preferential adsorption of one component of the mixture on the wall surfaces, resulting in the divergence of the relative adsorption near the critical point. Critical adsorption was much studied both theoretically and experimentally. Of interest has been the scaling prediction by Fisher and de Gennes [3] that Γ should have a power law dependence on τ , with a universal exponent which does not depend on the details of the specific system. Also, this phenomenon is of significant practical importance, e.g., for the use of supercritical fluids in micro- and nanofluidics [4] and as solvents in colloidal suspensions [5]. In colloidal suspensions one usually encounters the presence of ions. This may be due to the dissociation of a salt-free solvent, e.g., in binary solvents containing water, or due to dissolved salt. Ions influence the electrostatic interactions between the charged colloids leading to screening effects [6]. If there is a chemical preference of ions for one component of the solvent, then larger amount of ions may be dissolved in regions where the concentration of the preferred component of the solvent is larger. For this reason critical adsorption in binary solvents may affect the distribution of ions near charged surfaces. This effect in turn may influence the screening of electrostatic interactions between charged colloidal particles. Such a scenario is relevant for an experimental system used recently to investigate the effective forces acting on spherical particles close to a substrate immersed in a near-critical water- lutidine mixture [7, 8]. The aim of that study was to measure directly a critical Casimir force between a single spherical colloid and a flat surface. Polystyrene particles with high surface charge density were used as colloids, and silica glass treated chemically to achieve the desired adsorption properties, which also carried some surface charge, was used as the substrate. 2