Temperature and electrostatics effects on charged poly(N-isopropylacrylamide) microgels at the interface

Abstract

Thermoresponsive microgels based on poly-N-isopropylacrylamide (PNIPAM) have many biomedical applications owing to their biocompatibility and their Volume Phase Transition Temperature (VPTT) around the physiological temperature. Understanding the surface structure of PNIPAM microgels in monolayers is important in terms of fundamental science and towards their use in thermoresponsive emulsions. However, to date there are no reported studies of PNIPAM monolayers above and below the VPTT. In addition, the role of electrostatics in surface films of charged microgels remains controversial. Hence, in the present paper, we study the surface conformation of charged PNIPAM at different swelling states. Shrunken PNIPAM microgels in bulk show higher effective charge than swollen microgels. In agreement with this, we find that monolayers of shrunken PNIPAM particles are compressed more easily than that of swollen PNIPAM particles. The role of electrostatic interactions is further addressed by modifying the concentration of electrolyte, which alters the effective charge but retains the particle size of microgels. We find that screening the effective charge of the microgel has a strong impact on the lateral packing of the monolayer displacing the compression isotherms to higher normalised areas. Moreover, the displacement correlates with the changes in the effective charge of the microgels induced by the electrolyte. Hence, we prove that beyond the different swelling ratio, electrostatics can modulate the interactions between charged microgel particles at the interface.