  In this work, we perform a comparative study of the size dependence of
diffusion of charged and neutral solutes in water. The neutral solute in water
shows a nonmonotonicity in the size dependence of diffusion. This is usually
connected to the well known Levitation effect where it is found that when
solute diffuses through the transient solvent cages then for attractive
solute-solvent interaction and for a particular size of the solute there is a
force balance which leads to the maximum in diffusion. Similar maximum in
diffusion of charged solutes has also been observed and connected to Levitation
effect. However, earlier studies of ionic diffusion connects this
nonmonotonicity to the interplay between hard sphere repulsion and Coulombic
attraction. In this work, we show that although the size dependence of both
charged and neutral solutes have a nonmonotonicity, there is a stark difference
in their behaviour. For charged solute with increase in attraction the maximum
shifts to higher solute sizes and has a lower value whereas for neutral solute
it remains at the same place and has a higher value. We show by studying the
ionic and non-ionic part of the potential that for larger solutes it is the
nonionic part which dominates and for smaller solutes the ionic part and the is
a transition between them. As the charge on the solute increases, this
transition takes place at larger solute sizes which leads to the shift in the
diffusivity maxima and reduction of the peak value. We show that although the
charged solutes also explore the solvent cage even before we reach the size
which Levitates due to Coulombic attraction the diffusion value drops. Thus the
origin of diffusivity maxima in charged and neutral solute diffusion is
different.
