capillary, electrostatic, van der Waals, fluctuation (thermodynamic Casimir force), and solvation interactions. We restrict the discussion to spherical objects and neglect the gravitational force due to the nanoparticles' size. The curves (HTred, LTblue) seemed to move towards longer distances when adding ions to the subphase. Thus, particles were located further away in the histograms in Figure . The minimum for the long-range attraction seemed to be deeper, as big "ice-floe" like aggregates appeared directly after the deposition of PSiFe at KCl-containing subphases, as evidenced by BAM (Figure ). However, in classical DLVO theory, adding ions should screen the interaction, shifting the curves towards smaller distances between particles. Thus, other forces not described in DLVO theory were most likely involved in the system.