Softness Matters: Effects of Compression on the Behavior of Adsorbed Microgels at Interfaces

TL;DR

This study investigates how the softness of microgel particles influences their structural behavior at interfaces under compression, revealing distinct responses of hard and soft microgels and advancing understanding of their role in stabilizing emulsions.

Contribution

It provides a comprehensive in situ analysis combining neutron reflectometry and simulations to elucidate the impact of particle softness on interfacial structure and behavior under compression.

Findings

Hard microgels are pushed away from the interface upon compression.

Soft microgels remain anchored and are less deformed under compression.

Softness significantly affects the equilibrium phase behavior and stabilization properties.

Abstract

Deformable colloids and macromolecules adsorb at interfaces, as they decrease the interfacial energy between the two media. The deformability, or softness, of these particles plays a pivotal role in the properties of the interface. In this study, we employ a comprehensive \emph{in situ} approach, combining neutron reflectometry with molecular dynamics simulations, to thoroughly examine the profound influence of softness on the structure of microgel Langmuir monolayers under compression. Lateral compression of both hard and soft microgel particle monolayers induces substantial structural alterations, leading to an amplified protrusion of the microgels into the aqueous phase. However, a critical distinction emerges: hard microgels are pushed away from the interface, in stark contrast to the soft ones, which remain steadfastly anchored to it. Concurrently, on the air-exposed side of the monolayer, lateral compression induces a flattening of the surface of the hard monolayer. This phenomenon is not observed for the soft particles as the monolayer is already extremely flat even in the absence of compression. These findings significantly advance our understanding of the pivotal role of softness on both the equilibrium phase behavior of the monolayer and its effect when soft colloids are used as stabilizers of responsive interfaces and emulsions.