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

Abstract

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.

Microgels at Interfaces Behave as 2D Elastic Particles Featuring Reentrant Dynamics

“…Experimentally, small microgels-having a diameter ≲200 nm-are the best candidates to interact in this way, since they do not experience long-range attractions due to capillary effects [37]. Indeed, the latter conditions have been widely reported [57][58][59] and found to be relevant only for microgels large enough to induce a local deformation of the water-hexane interface [37]. By contrast, our solvent modeling is aimed essentially at reproducing the surface tension and the microgel solubility, both of which have a direct influence on the conformation of the particle.…”

Single-particle tracking of the formation of a pseudoequilibrium state prior to charged microgel cluster formation at interfaces

“…Cluster formation between two particles initially separated by a distance d > 2a occurs when they are first transported to a common area with a high deviatoric curvature, as observed in We begin by discussing possible mechanisms that contribute to the repulsion potential for the pseudoequilibrium state. When the microgels are separated by more than a particle distance, electrostatic interaction plays a role, as proposed for long-range ordering of soft particles at interfaces 36,37 . Both the submerged and emergent portions of the particle can contribute.…”

“…In contrast, the effective interactions between microgel particles in bulk solvents become more complex as a result of deformation, compression and interpenetration of soft particles and dangling ends, corona-corona, core-corona, core-core, and microgel-counterion interactions need to be considered 35 . Electrostatic repulsion between charged soft-shell/hard-core nanoparticles at the air-water interface has been shown to be responsible for their long-range ordering at low particle densities 36 . This was further supported by a recent computational modeling study that utilized electrostatic repulsion to understand long-range ordering of charged microgels at interfaces that were separated by distances larger than the particle size 37 .…”

How Self‐Assembled Nanodomains Can Impact the Organization of a Phospholipid Monolayer‐Flower‐Like Arrays

“…Two‐dimensional mesoscopic structural patterns such as striped and circular domains are known to form spontaneously in various systems, including nanoparticles, block copolymers and surfactants . Domain shapes and shape transitions have intensively been investigated in phospholipid films, as they pertain to the self‐assembling process of microdomains in plasma membranes, such as functional rafts .…”