Besides the aggregation being thermodynamically favorable in both C4 and C8 ionic liquids, there are energy barriers for the dimer association larger than the thermal energy except for the hydrophilic NPs in C4 (Figure top). The increase of the cation alkyl group from butyl to octyl not only dramatically increases the barrier prior to the contact minimum but also results in a complex pattern for the long-range behavior of the pmf, with multiple maxima and minima. It is important to notice that the direct interaction between NPs is given by a Lennard-Jones potential (Equation S1 in Supporting Information), hence, this is a short-range interaction and becomes negligible for r>6 nm, implying that those multiple barriers are due to solvent-mediated interactions. To characterize those, the structure of the solvent around NPs was analyzed using the radial distribution function of the anion, the cation polar head and the cation hydrophobic tail in model systems containing only one NP in the solvent. The radial distribution function is given by the density of a species of interest at a spherical shell of radius r AB around the NP center of mass divided by the average density in the whole system (Equation ). Figure shows the results for the solvent organization around NPs in C4. Around the hydrophobic particle, as expected, there is an excess of cation apolar tail (green curve in g(r) and green spheres in the structure representations at Figure bottom) and this is followed by a region rich in the polar portion of the ionic liquid (red and blue curves and spheres). The opposite is found for the hydrophilic particle, which prefers to interact with the polar portions of the ionic liquid. However, as noticed by the radial distribution function curves, the solvent organization around the hydrophilic NP is weaker than around the hydrophobic one. As discussed above, this does not imply in a stronger NP-solvent interaction for the hydrophobic one, instead, the weaker organization around the hydrophilic NP results from the strength of the ionic interactions between the cation head and the anion, which made those groups less available to solvate the hydrophilic particle.