showing that water comprises ∼ 90% of the complex. In our simulations, mass fraction is not well defined because we are using an implicit solvent. Therefore, we used volume fractions, which can be assumed proportional to the mass fractions by a multiplicative factor given by ratio of mean mass densities of water and the wet complex. As wet complex is mostly composed of water, the above factor should be close to unity, although its accurate value cannot be determined. Given this inherent uncertainty, we interpret the comparison of partition coefficients in Figure as a sufficient concurrence between the experiment and simulation. We reiterate that absolute quantitative agreement between the two can be in principle reached by fine-tuning of model parameters, but this is not the main objective of our study. We will comment on this point in more dedail in Section Sec. 3.3. Finally, the experiments also show, that the ratio between the polymer concentrations in the interpolyelectrolyte complex and supernatant is ≈ 200 (Sec. S3 in the ESI), hence validating our assumption of near-to-none polymer in the supernatant.