Overcharging and reentrant condensation of thermoresponsive ionic microgels

Truzzolillo, Domenico; Sennato, Simona; Sarti, S.; Casciardi, Stefano; Bazzoni, Chiara; Bordi, F.

doi:10.1039/c7sm02357j

Cited by 22 publications

(66 citation statements)

References 83 publications

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“…The trend can be rationalised by thinking of the microgels only in physical terms (ignoring polymer-solvent interactions which are not yet dominant): as the particles become smaller, they also become more compact and hence somewhat less penetrable. A further change in interactions at high T is however hinted by the present results, as the simple repulsive model that we have adopted shows increasing deviations at the highest studied T = 30 • C, approaching the VPTT at 32 • C. Close to the VPTT, a much more careful evaluation, also in terms of charge effects which could become important as shown by our and others preliminary measurements [43], will be required. For the examined T -interval, the present findings clearly show that the variation of volume fraction that is obtained by changing T , a commonly used method in experiments to efficiently explore a larger portion of the phase diagram, should be done with caution, as doing so significantly affects the effective interactions between the particles.…”

Section: Discussionsupporting

confidence: 53%

A new look at effective interactions between microgel particles

et al. 2018

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Thermoresponsive microgels find widespread use as colloidal model systems, because their temperature-dependent size allows facile tuning of their volume fraction in situ. However, an interaction potential unifying their behavior across the entire phase diagram is sorely lacking. Here we investigate microgel suspensions in the fluid regime at different volume fractions and temperatures, and in the presence of another population of small microgels, combining confocal microscopy experiments and numerical simulations. We find that effective interactions between microgels are clearly temperature dependent. In addition, microgel mixtures possess an enhanced stability compared to hard colloid mixtures - a property not predicted by a simple Hertzian model. Based on numerical calculations we propose a multi-Hertzian model, which reproduces the experimental behavior for all studied conditions. Our findings highlight that effective interactions between microgels are much more complex than usually assumed, displaying a crucial dependence on temperature and on the internal core-corona architecture of the particles.

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Section: Discussionsupporting

confidence: 53%

A new look at effective interactions between microgel particles

et al. 2018

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“…Though since the pioneering work of Pelton and co-workers [ 30 ], a large increase of microgel electrophoretic mobility is known to exist above the volume phase transition temperature of the microgels due to the ionic initiators anchored to the polymer network, PNIPAm microgels have been often considered as neutral colloids, since their inner structure does not contain ionizable monomers. Only very recently few works pointed out that the residual charge borne by PNIPAm microgels synthesized via FRP can affect their behavior in aqueous media [ 31 , 32 ] and in presence of oppositely charged polymers [ 32 ]. Whether PNIPAm microgels synthesized via FRP behave like neutral colloids in water and how their VPT affects their electrostatics are still debated issues.…”

Section: Introductionmentioning

confidence: 99%

“…In a recent work [ 32 ] we have shown that PNIPAm microgels can electrostatically adsorb short polylysine polymers, that the amount of adsorbed polymer largely increases above the VPT and that such an adsorption triggers, though only very close to the critical temperature, both microgel reentrant condensation and overcharging, two phenomena characterizing charged colloids and occurring only in the presence of particles with sufficiently high charge densities.…”

Section: Introductionmentioning

confidence: 99%

The Double-Faced Electrostatic Behavior of PNIPAm Microgels

et al. 2021

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PNIPAm microgels synthesized via free radical polymerization (FRP) are often considered as neutral colloids in aqueous media, although it is well known, since the pioneering works of Pelton and coworkers, that the vanishing electrophoretic mobility characterizing swollen microgels largely increases above the lower critical solution temperature (LCST) of PNIPAm, at which microgels partially collapse. The presence of an electric charge has been attributed to the ionic initiators that are employed when FRP is performed in water and that stay anchored to microgel particles. Combining dynamic light scattering (DLS), electrophoresis, transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments, we show that collapsed ionic PNIPAm microgels undergo large mobility reversal and reentrant condensation when they are co-suspended with oppositely charged polyelectrolytes (PE) or nanoparticles (NP), while their stability remains unaffected by PE or NP addition at lower temperatures, where microgels are swollen and their charge density is low. Our results highlight a somehow double-faced electrostatic behavior of PNIPAm microgels due to their tunable charge density: they behave as quasi-neutral colloids at temperature below LCST, while they strongly interact with oppositely charged species when they are in their collapsed state. The very similar phenomenology encountered when microgels are surrounded by polylysine chains and silica nanoparticles points to the general character of this twofold behavior of PNIPAm-based colloids in water.

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“…Indeed, even those constituted by PNIPAM only (often considered as neutral microgels) show interesting features, particularly above the VPT temperature, which suggest the presence of charge effects. 10,15 In addition, microgels consisting of two different interpenetrated networks, made of PNIPAM and PAAc respectively, have recently gathered a lot of attention because of their suitability to study the problem of fragility in structural glasses. 16,17 From the theoretical point of view, several investigations of the swelling of charged microgels, mostly relying on a mean-field treatment of the polymer network based on the Flory-Rehner theory, 18 have been reported.…”

Section: Introductionmentioning

confidence: 99%

Numerical insights on ionic microgels: structure and swelling behaviour

et al. 2019

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Recent progress has been made in the numerical modelling of neutral microgel particles with a realistic, disordered structure. In this work we extend this approach to the case of co-polymerised microgels where a thermoresponsive polymer is mixed with acidic groups. We compare the cases where counterions directly interact with microgel charges or are modelled implicitly through a Debye-Hückel description. We do so by performing extensive numerical simulations of single microgels across the volume phase transition varying the temperature and the fraction of charged monomers. We find that the presence of charges considerably alters the microgel structure, quantified by the monomer density profiles and by the form factors of the microgels, particularly close to the volume phase transition (VPT). We observe significant deviations between the implicit and explicit models, with the latter comparing more favourably to available experiments. In particular, we observe a shift of the VPT temperature to larger values as the amount of charged monomers increases. We also find that below the VPT the microgel-counterion complex is almost neutral, while it develops a net charge above the VPT. Interestingly, under these conditions the collapsed microgel still retains a large amount of counterions inside its structure. Since these interesting features cannot be captured by the implicit model, our results show that it is crucial to explicitly include the counterions in order to realistically model ionic thermoresponsive microgels.

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Overcharging and reentrant condensation of thermoresponsive ionic microgels

Cited by 22 publications

References 83 publications

A new look at effective interactions between microgel particles

A new look at effective interactions between microgel particles

The Double-Faced Electrostatic Behavior of PNIPAm Microgels

Numerical insights on ionic microgels: structure and swelling behaviour

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