Direct measurements of the pNipam Microgel Counter-ion Cloud via Small-Angle Neutron Scattering

Microgels are formed by cross-linked polymer networks and they are a good model system for soft colloids. However, unlike hard colloids, their behavior is not well understood at high concentrations. pNipam microgels carry charged groups and counter-ions at the periphery due to the initiator used during synthesis, which we have found to play a crucial role for the observed spontaneous deswelling behavior at high concentrations; microgels deswell below random close packing when the percolation of counter-ion clouds induces a suspension osmotic pressure that is greater than the particle bulk modulus[1]. Here, we present the first direct measurements of the counter-ion cloud via small-angle neutron scattering (SANS). Note that the form factor can be decomposed as P(q) = F_p²(q) + 2 F_p(q)F_c(q) + F_c²(q), where F_p(q) and F_c(q) are the scattering amplitudes of the pNIPAM polymer and the counter-ion cloud, respectively. We prepare one suspension with Na⁺ ions and the other with NH₄⁺ ions via dialysis to use the difference in the scattering-length densities of these ions to obtain an augmented scattering signal with structural information about the ion cloud and charged groups on a microgel. Using this method, we show that the counter-ion cloud is indeed located at the particle periphery. This result also corroborates our theory for microgel deswelling at high concentrations. Importantly, our theory relates the particle softness and surface charges with the phase behavior[2]. The findings are crucial for developing a better understanding of soft-deformable colloids at high concentrations and for formulating a model for the phase behavior of microgels that includes observed spontaneous deswelling.

[1]U.Gasser, A. Scotti, and A. Fernandez-Nieves, Physical Review E99, 042602 (2019)

[2]] A. Scotti, U. Gasser, E.S. Herman, J. Han, A. Menzel, L.A. Lyon, and A. Fernandez-Nieves, Phys. Rev. E 96, 032609 (2017)