In-situ study of the impact of temperature and architecture on the interfacial structure of microgels

TL;DR

This study uses neutron reflectometry and simulations to analyze how temperature and architecture influence the interfacial structure of microgels, revealing their responsiveness and arrangement at air-water interfaces.

Contribution

It provides the first in-situ microscopic characterization of microgel interfacial structures considering temperature and architecture effects.

Findings

Temperature affects only the water-internal part of microgels.

Microgel softness influences protrusion into air, with ultra-low cross-linked microgels forming flat layers.

Standard microgels have a contact angle of a few degrees, while ultra-low cross-linked microgels have zero contact angle.

Abstract

The structural characterization of microgels at interfaces is fundamental to understand both their 2D phase behavior and their role as stabilizers that enable emulsions to be broken on demand. However, this characterization is usually limited by available experimental techniques, which do not allow a direct investigation at interfaces. To overcome this difficulty, here we employ neutron reflectometry, which allows us to probe the structure and responsiveness of the microgels in-situ at the air-water interface. We investigate two types of microgels with different cross-link density, thus having different softness and deformability, both below and above their volume phase transition temperature, combining experiments with computer simulations of realistic in silico synthesized microgels. We find that temperature only affects the portion of microgels in water, while the strongest effect of the microgels softness is observed in their ability to protrude into the air. In particular, standard microgels have an apparent contact angle of few degrees, while ultra-low cross-linked microgels form a flat polymeric layer with zero contact angle. Altogether, this study provides an in-depth microscopic description of how different microgel architectures affect their arrangements at interfaces, and will be the foundation for a better understanding of their phase behavior and assembly. This manuscript has been accepted for publication in Nature Communications (open access). The final version of the manuscript including the Supplementary Information will be available in the future.