The role of ions in the self-healing behavior of soft particle suspensions

Abstract

Impurities in crystals generally cause point defects and can even suppress crystallization. This general rule, however, does not apply to colloidal crystals formed by soft microgel particles [Iyer ASJ, Lyon LA (2009) Angew Chem Int Ed 48:4562-4566], as, in this case, the larger particles are able to shrink and join the crystal formed by a majority of smaller particles. Using small-angle X-ray scattering, we find the limit in large-particle concentration for this spontaneous deswelling to persist. We rationalize our data in the context of those counterions that are bound to the microgel particles as a result of the electrostatic attraction exerted by the fixed charges residing on the particle periphery. These bound counterions do not contribute to the suspension osmotic pressure in dilute conditions, as they can be seen as internal degrees of freedom associated with each microgel particle. In contrast, at sufficiently high particle concentrations, the counterion cloud of each particle overlaps with that of its neighbors, allowing these ions to freely explore the space outside the particles. We confirm this scenario by directly measuring the osmotic pressure of the suspension. Because these counterions are then no longer bound, they create an osmotic pressure difference between the inside and outside of the microgels, which, if larger than the microgel bulk modulus, can cause deswelling, explaining why large, soft microgel particles feel the squeeze when suspended with a majority of smaller particles. We perform small-angle neutron scattering measurements to further confirm this remarkable behavior.oint defects in crystalline materials disrupt the crystal structure and often prevent crystallization. This is the case, for instance, when large particles are introduced in a crystal of smaller particles. Bragg illustrated the consequences of this disruption using soap bubbles (1). In metal melts, a size mismatch of 15% between the atoms suppresses crystallization (2), and, in hard spheres, which constitute an important model system for condensed matter, a polydispersity above 12% also prevents crystallization (3). Furthermore, the polydispersity in hard sphere crystals does not exceed 5.7%, due to local segregation of dissimilar particles during crystallization (4, 5).Remarkably, these restrictions do not necessarily apply to suspensions of soft microgels, which are cross-linked polymer particles immersed in a solvent that can exist in either swollen or deswollen states, depending on external conditions like temperature (6) and pH (7). Indeed, microgel suspensions containing a small fraction of larger particles can crystallize without defects by shrinking the larger particles to a size that is identical to that of the smaller and more abundant microgel particles (8). This deswelling was hypothesized to result either from the direct interaction between large and small particles or from the osmotic pressure exerted by the small microgels on the larger ones (2). Here, we show that none of these effects drive this proces...

Spontaneous deswelling of microgels controlled by counterion clouds

“…Recently, we have presented a model for this selective deswelling at high concentrations [14] and have shown that the freezing point of polydisperse and bidisperse pNIPAM suspensions is directly linked to particle deswelling [6]. Our findings suggest that the counterions associated to the charged groups at the periphery of pNI-PAM microgels and the osmotic pressure they exert at high particle concentrations are the key to the deswelling behavior.…”

“…When these overlap-regions take most of the space between particles and percolate through the suspension volume, the previously bound counterions become free to explore the space between the particles, hence contributing to the osmotic pressure of the suspension which, therefore, increases. In our previous work [14], we showed with simple Monte-Carlo simulations that the osmotic pressure due to those counterions outside the particles is indeed higher than that inside the particles for φ 0.5. The counterion concentration inside the particles does not increase with φ, as the electrostatic conditions there are mostly unaffected provided the configuration of fixed charges in the particle periphery stays fixed.…”

“…We have studied pNIPAM microgels containing 2.7 wt% of crosslinker N,N'-methylene-bis(acrylamide) (BIS) with hydrodynamic radii in the range from 110 nm to 190 nm. The details of the particle synthesis are described in the supporting information (SI) of our previous work [14].…”

“…It is this ∆Π that is responsible for the spontaneous particle deswelling whenever ∆Π is comparable or exceeds the bulk modulus, K, of the particles [14]. At this point, the particles shrink until the elastic stress of the network restores mechanical equilibrium.…”

Determination of Internal Density Profiles of Smart Acrylamide-Based Microgels by Small-Angle Neutron Scattering: A Multishell Reverse Monte Carlo Approach

“…core-shell. 9,13 Microgel particles are very promising with respect to applications in biomedical devices 14,15 , as sensors 16,17 , for drug delivery [18][19][20][21][22][23] , as functionalized and possibly double responsive microgels for nanoreactors [24][25][26][27] , and in colloidal crystals [28][29][30][31][32][33][34] .…”