Multiple structural transitions in Langmuir monolayers of charged soft-shell nanoparticles

We have investigated the morphologies of Langmuir layers of charged, polymeric hardcore/interlayer/soft-shell (CIS) nanoparticles spread at the air-water interface. Depending on various mutual interactions, which are correlated to the areal densities of the deposited nanoparticles, we observed ordered patterns of non-dense and closed-packed arrangements of CIS nanoparticle ordering. At low areal densities, we found an almost regular distribution of the charged CIS nanoparticles on the water surface, which resulted from long-range repulsive electrostatic interactions between them. At higher areal densities, domains of more closely packed and ordered nanoparticles were formed, coexisting with regions of randomly and sparsely distributed nanoparticles. We relate these domains to the interplay of electrostatic repulsion and capillary attraction caused by the dipolar character of like-charged particles at the interface, allowing for a characteristic separation distance between nanoparticles of about 3 - 4 times the nanoparticle diameter. At relatively high areal densities, attractive van der Waals forces were finally capable of getting nanoparticles into contact, leading to densely packed patches of hexagonally ordered nanoparticles coexisting with regions of rather well ordered nanoparticles separated by about 1 μm and regions of randomly and sparsely distributed nanoparticles. Intriguingly, upon re-expansion of the area available per nanoparticle, these densely-packed patches disappeared, indicating that steric repulsion due to presence of soft shells as well as longrange electrostatic repulsive forces were strong enough to assure reversibility of the morphological behavior.