This function is monotonic and qualitatively accounts for the shape and curvature of the P (ϕ mon ) dependence, as explained in Ref. The final step of the protocol is the assessment of self-consistency. As noted above, we require the interpolyelectrolyte complex at ϕ 0 mon to be a homogeneous single-phase system. For that reason we analyze the spherically averaged structure factors, S(k), between monomeric units, displayed in Figure ). If the system should demix to form polymer-rich and polymer-poor domains, the structure factor would diverge in the limit of small wavevectors, k → 0. 113 In Figure ) for c NaCl ≈ 156mM we indeed see such demixing peak in S(k) of the system with mean volume fraction 0.60ϕ 0 mon , indicating the separation. On the contrary, S(k) of the interpolyelectrolyte complex with mean volume fraction of ϕ 0 mon does not feature such a peak, indicating a stable homogeneous phase, coexisting with the supernatant. We can observe similar hallmarks of demixing at very high salt concentrations, c NaCl ≈ 1529mM, as testified by the peak in Figure )