  Using large-scale molecular dynamics simulations, we investigate the surface
properties of lithium, sodium, and potassium silicate glasses containing 25
mole % of alkali oxide. The comparison of two types of surfaces, a melt-formed
surface (MS) and a fracture surface (FS), demonstrates that the influence of
the alkali modifier on the surface properties depends strongly on the nature of
the surface. The FS exhibits a monotonic increase of modifier concentration
with increasing alkali size while the MS shows a saturation of alkali
concentration when going from Na to K glasses, indicating the presence of two
competing mechanism that influence the properties of a MS. For the FS, we find
that larger alkali ions reduce the concentration of under-coordinated Si atoms
and increase the fraction of two-membered rings, implying an enhanced chemical
reactivity of the surface. For both types of surfaces, the roughness is found
to increase with alkali size, with the effect being more pronounced for the FS
than for the MS. The height-height correlation functions of the surfaces show a
scaling behavior that is independent of the alkali species considered: The ones
for the MS are compatible with the prediction of the frozen capillary wave
theory while the ones for the FS show a logarithmic growth, i.e., on the
nanoscale these surfaces are not self-affine fractals. The influence of the
modifier on the surface properties are rationalized in terms of the interplay
between multiple factors involving the size of the ions, bond strength, and
charge balance on the surface.
