Counterion\nDistribution in the Stern Layer on Charged\nSurfaces

Counterion adsorption at the solid–liquid interface\naffects\nnumerous applications. However, the counterion adsorption density\nin the Stern layer has remained poorly evaluated. Here we report the\ndirect determination of surface charge density at the shear plane\nbetween the Stern layer and the diffuse layer. By the Grahame equation\nextension and streaming current measurements for different solid surfaces\nin different aqueous electrolytes, we are able to obtain the counterion\nadsorption density in the Stern layer, which is mainly related to\nthe surface charge density but is less affected by the bulk ion concentration.\nThe charge inversion concentration is further found to be sensitive\nto the ion type and ion valence rather than to the charged surface,\nwhich is attributed to the ionic competitive adsorption and ion–ion\ncorrelations. Our findings offer a framework for understanding ion\ndistribution in many physical and chemical processes where the Stern\nlayer is ubiquitous.