Multiple Structural Transitions in Langmuir Monolayers of Charged Soft-Shell Nanoparticles

We have investigated the morphologies of Langmuir layers of
charged, polymeric hard-core/interlayer/soft-shell 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
nondense and closed-packed arrangements of core/interlayer/shell (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 making nanoparticles to come in contact with each other, 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 the presence of soft shells as well as long-range
electrostatic repulsive forces were strong enough to assure reversibility of the morphological behavior.