title: 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.
authors.affiliation: Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, 5232 Villigen, Switzerland; School of Physics, Georgia Institute of Technology, Atlanta, GA 30332;
authors.forename: Andrea
authors.initials: A
authors.lastname: Scotti
authors.affiliation: Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, 5232 Villigen, Switzerland; urs.gasser@psi.ch.
authors.forename: Urs
authors.initials: U
authors.lastname: Gasser
authors.affiliation: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332;
authors.forename: Emily S
authors.initials: ES
authors.lastname: Herman
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, GA 30332;
authors.forename: Miguel
authors.initials: M
authors.lastname: Pelaez-Fernandez
authors.affiliation: Laboratory for Macromolecules and Bioimaging, Paul Scherrer Institut, 5232 Villigen, Switzerland, Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, Quanzhou 362200, People's Republic of China;
authors.forename: Jun
authors.initials: J
authors.lastname: Han
authors.affiliation: Laboratory for Macromolecules and Bioimaging, Paul Scherrer Institut, 5232 Villigen, Switzerland.
authors.forename: Andreas
authors.initials: A
authors.lastname: Menzel
authors.affiliation: Schmid College of Science and Technology, Chapman University, Orange, CA 92866.
authors.forename: L Andrew
authors.initials: LA
authors.lastname: Lyon
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, GA 30332;
authors.forename: Alberto
authors.initials: A
authors.lastname: Fernández-Nieves
keywords: SANS; SAXS; crystallization; deswelling; microgels