Ion-size effect at the surface of a silica hydrosol.

Using synchrotron x-ray reflectivity, I studied the ion-size effect for alkali ions (Na(+), K(+), Rb(+), and Cs(+)), with densities as high as 4x10(18)-7x10(18) m(-2), suspended above the surface of a colloidal solution of silica nanoparticles in the field generated by the surface electric-double layer. I found that large alkali ions preferentially accumulate and replace smaller ones at the surface of the hydrosol, a result qualitatively agreeing with the dependence of the Kharkats-Ulstrup single-ion electrostatic free energy on the ion's radius.