Analysis of colloids .9. Investigation of the electrical double layer of colloidal inorganic nanometer-particles by size-exclusion chromatography

In a liquid containing ions an electrical double layer is formed close to any charged solid surface. Thus a charged dispersed species wears a ''coat'' of ions. In size-exclusion chromatography (SEC) of charged colloidal particles the effective size including this electrical double layer has to be considered. For the investigation of the effective size crystalline colloids with rigid core are advantageous, as their core diameters can be determined by electron microscopy. According to the Gouy-Chapman model the thickness of the double layer is inverse proportional to the square root of the ionic strength of the liquid. Thus the particles investigated (CdS colloids) were chromatographed on the same SEC column with eluents with different ionic strength. It was found that the retention times increased with increasing concentration and valency of the electrolyte present in the eluent. It could be shown that adsorption or repulsion did not play a significant role for this retention time shift as long as the electrolyte content is not too high, The layer thickness and its ionic strength dependence, derived from the SEC experiments with CdS particles in the low nm range were found in good agreement with theory. The sum of diameter of the particle core and the Debye length 1/kappa (as predicted from theory) agreed with effective particle size measured chromatographically.