  The author used synchrotron x-ray reflectivity to study the ion-size effect
for alkali ions (Na$^+$, K$^+$, Rb$^+$, and Cs$^+$), with densities as high as
$4 \times 10^{18}- 7 \times 10^{18}$ m$^{-2}$, suspended above the surface of a
colloidal solution of silica nanoparticles in the field generated by the
surface electric-double layer. According to the data, large alkali ions
preferentially accumulate at the sol's surface replacing smaller ions, a
finding that qualitatively agrees with the dependence of the Kharkats-Ulstrup
single-ion electrostatic free energy on the ion's radius.
