Table ). The shift was non-linear. Isotherms recorded at 0.01 M KCl were only slightly shifted compared to pure water. The results show that in the presence of 0.1 M KCl, the interaction distance between PSiFe particles increased significantly compared to water, as seen by the shifts in surface areas at various pressures. At a low surface pressure of 0.3 mN/m, the surface area nearly doubled at both low (LT) and high temperatures (HT), indicating a broader spread of particles. This effect was even more pronounced at higher surface pressures (5 and 10 mN/m). At 5 mN/m, the surface area increased by a factor of 3.56 at LT and 2.29 at HT, and at 10 mN/m, by 5.04 at LT and 2.63 at HT compared to water. These significant shifts at LT suggest a substantial impact of KCl on particle spacing (Table ). This was counterintuitive as salt addition causes the screening of electrostatic interactions and often an aggregation of nanoparticles, as well as the salt-out effect that should work relatively weaker but accordingly. Here, however, the separation between particles increased. Like pure water, the isotherms recorded at the subphase containing KCl, but at different temperatures, were shifted, with particles at lower temperatures occupying more area (Figure and). This was again counterintuitive, as thermal expansion usually causes an opposite effect. For 0.1 M KCl and 500 μl of applied PSiFe, the area change was from around 66% (at 5 mN/m) to around 80% (at 10 mN/m). At KCl subphases, the changes were much larger than the geometrical limit related to changes in PNIPAM conformation, which could be around 10-20%. Surprisingly, at 0.1 M KCl, increasing the amount of PSiFe did not result in the shift of the isotherms when recorded with respect to the area of the trough (Figure ). That means that similar amounts of particles were present at the surface irrespective of the volume of PSiFe deposited. Consequently, unlike expected for stable Langmuir films, the isotherms did not overlap when expressed in cm 2 /mg (or nm 2 /NP or μm 2 /NP). However, experiments performed at the ultrapure water subphase showed that the films scaled linearly with subsequently added deposits of PSiFe. The films were stable with a constant percentage decrease, probably due to the deposition method or partial desorption, as PSiFe particles are relatively hydrophilic. The compression isotherms were used to calculate surface compressional moduli (Figure ). Davies and Rideal, this corresponded to the liquid expanded phase. This was similar to the case of pure water. However, for the films compressed at pure water, the increases in compressional moduli were monotonous. At KCl, the rheological properties of the film were different. Several local maxima were observed. The maximum in surface compressional modulus curves usually appears before the collapse. Thus, we concluded that the compression caused some PSiFe particles to be ejected from the interface (either toward the air or into the water subphase). This explained the observations that there was a limit of particles that could be present at 0.1 M KCl subphase. Excess of PSiFe above this limit was expelled from the interface.