We combine small-angle scattering experiments and simulations to investigate the internal structure and interactions of composite poly­(N-isopropylacrylamide)–poly­(ethylene glycol) (PNIPAM–PEG) microgels. At low temperatures the experimentally determined form factors and the simulated density profiles indicate a loose internal particle structure with an extended corona that can be modeled as a starlike object. With increasing temperature across the volumetric phase transition, the form factor develops an inflection that, using simulations, is interpreted as arising from a conformation in which PEG chains are incorporated in the interior of the PNIPAM network. This gives rise to a peculiar density profile characterized by two dense, separated regions, at odds with configurations in which the PEG chains reside on the surface of the PNIPAM core. The conformation of the PEG chains also have profound effects on the interparticle interactions: Although chains on the surface reduce the solvophobic attraction typically experienced by PNIPAM particles at high temperatures, PEG chains inside the PNIPAM network shift the onset of attractive interaction at even lower temperatures. Our results show that by tuning the morphology of the composite microgels, we can qualitatively change both their structure and their mutual interactions, opening the way to explore new collective behaviors of these objects.

Hypothesis: Soft microgel colloids can be densely packed since particle networks can compress and interpenetrate. This evolution of the particle's internal structure associated with packing is expected to determine the linear viscoelastic properties and the yielding behavior of dense suspensions of microgel colloids. Experiments: We investigated the volume fraction-dependent linear and non-linear rheological response of suspensions of soft core-shell particles formed by a poly(Nisopropylacrylamide) (PNIPAM) microgel core and a thin poly(ethylene glycol) (PEG) shell. Findings: The linear viscoelasticity of suspensions reveals a transition from a fluid to a jammed glass state. Increasing volume fraction within the jammed state, the linear storage modulus and the yield stress show distinct regimes associated with the evolution of particle contacts, which involve progressive compression and interpenetration of the shell and core. The yielding of jammed suspensions occurs in two-steps: At small strains jammed cages are rearranged, while full disentanglement of interpenetrating networks only occurs at large deformations and results in fluidization. Yield strains and stresses increase with increasing shear rate or frequency, suggesting a progressive dominance of the timescale associated with shear over that associated with the internal dynamics of the system.

The Ewald method has been the cornerstone in molecular simulations for modeling electrostatic interactions of charge-stabilized many-body systems. In the late 1990s, Wolf and collaborators developed an alternative route to describe the long-range nature of electrostatic interactions; from a computational perspective, this method provides a more efficient and straightforward way to implement long-range electrostatic interactions than the Ewald method. Despite these advantages, the validity of the Wolf potential to account for the electrostatic contribution in charged fluids remains controversial. To alleviate this situation, in this contribution, we implement the Wolf summation method to both electrolyte solutions and charged colloids with moderate size and charge asymmetries in order to assess the accuracy and validity of the method. To this end, we verify that the proper selection of parameters within the Wolf method leads to results that are in good agreement with those obtained through the standard Ewald method and the theory of integral equations of simple liquids within the so-called hypernetted chain approximation. Furthermore, we show that the results obtained with the original Wolf method do satisfy the moment conditions described by the Stillinger–Lovett sum rules, which are directly related to the local electroneutrality condition and the electrostatic screening in the Debye–Hückel regime. Hence, the fact that the solution provided by the Wolf method satisfies the first and second moments of Stillinger–Lovett proves, for the first time, the reliability of the method to correctly incorporate the electrostatic contribution in charge-stabilized fluids. This makes the Wolf method a powerful alternative compared to more demanding computational approaches.

The reactive Monte Carlo (RxMC) method was proposed to describe the sorption of gases in solid materials due to the chemical reaction A + B ⇌ C. Two models were used to simulate the solid; the first model considered simulations with rigid particles in the solid whereas in the second model the particles were allowed to vibrate inside the solid with a given spring constant, i.e. an Einstein solid was used to simulate the substrate. In both models not only physisorption but also chemisorption of the fluid was observed. Sorption curves, at different spring constants, were simulated and it was noted that sorption was always enhanced with the Einstein solid model. Moreover, an inverse dependent function of the spring constant with the temperature was found. Finally, the second model might be used to explain the unusual sorption behavior observed in actual experimental reactions such as CO2 + Li2O ⇌ Li2CO3.