Charge affinity and solvent effects in numerical simulations of ionic microgels

Abstract

Ionic microgel particles are intriguing systems in which the properties of thermo-responsive polymeric colloids are enriched by the presence of charged groups. In order to rationalize their properties and predict the behaviour of microgel suspensions, it is necessary to develop a coarse-graining strategy that starts from the accurate modelling of single particles. Here, we provide a numerical advancement of a recently-introduced model for charged co-polymerized microgels by improving the treatment of ionic groups in the polymer network. We investigate the thermoresponsive properties of the particles, in particular their swelling behaviour and structure, finding that, when charged groups are considered to be hydrophilic at all temperatures, highly charged microgels do not achieve a fully collapsed state, in favorable comparison to experiments. In addition, we explicitly include the solvent in the description and put forward a mapping between the solvophobic potential in the absence of the solvent and the monomer–solvent interactions in its presence, which is found to work very accurately for any charge fraction of the microgel. Our work paves the way for comparing single-particle properties and swelling behaviour of ionic microgels to experiments and to tackle the study of these charged soft particles at a liquid–liquid interface.

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

“…This result can be rationalized by considering the electric charge brought by initiator molecules. Indeed, in agreement with previous studies on charged microgels ( 33 , 39 ), a shift of to higher values is found when the amount of charges increases. In addition, we observe that, for all studied samples, the VPT transition is encountered at a lower temperature, by C, for with respect to .…”

“…In all cases, the decrease of at the minimum is also accompanied by its decrease at small , indicating that the growth of is not compensated by that of under swollen conditions. At large , most curves tend to recover the HS value, except for the most charged microgels, which are expected to completely collapse at much larger effective temperatures ( 33 , 39 ), beyond those explored in the present simulations.…”

“…To address these gaps, we recently developed a computational method ( 30 ) to assemble disordered networks with desired cross-linker concentration and a core–corona structure that closely reproduces experimental behavior ( 13 , 31 ). After imposing the correct internal structure, we extended our method to properly include the presence of charged monomers with explicit counterions ( 32 ), again validating our results in the presence of explicit solvent and comparing with available experiments ( 33 ). For these reasons, we are now in the condition to carefully assess the effect of initiator charges on the deswelling mechanism of pNIPAM microgels across the VPT.…”

“…In the presence of charges, our treatment explicitly includes the presence of counterions, as these turn out to be crucial to provide a proper description of the local inhomogeneities arising in the network ( 32 ). In addition, charged monomers do not change their affinity to the solvent as we vary the temperature in order to properly capture the experimental picture ( 33 ). A detailed description of the model is provided in Materials and Methods .…”

“…Here and are constants defining the functional shape of the potential ( 59 ). We showed in a previous work ( 33 ) that, for a correct description of charge effects, only neutral monomers should interact with the additional potential. Instead, charged monomers retain a solvophilic character at all temperatures, i.e., for the interaction among charged beads and other charged or neutral ones we always set .…”