The role of charges on the interfacial properties of microgels has been discussed controversially in the literature. In this work, we resolve the contradiction by combining compression isotherms and deposition experiments with atomic force microscopy imaging and computer simulations. We demonstrate that charges do affect the compressibility of microgels and that the influence is even more intricate than stated before. At low compression, charged microgels indeed need to be compressed further than uncharged ones to reach the same surface pressure. However, at high compression, the behavior reverses and charged microgels become less compressible than their uncharged counterparts. We show that, depending on the degree of compression, the compressibility is dominated either by in-plane or out-of-plane electrostatic interactions. Furthermore, we identify the size of the investigated microgels to play a critical role and to be responsible for the contrasting findings reported before. Our results highlight the characteristic distinction of soft polymer microgels from the behavior of charged rigid particles.
Maximilian M. Schmidt, Steffen Bochenek, Alexey A. Gavrilov, Igor I. Potemkin, and Walter Richtering
Langmuir 2020, DOI: 10.1021/acs.langmuir.0c02081