title: Electrolyte-Mediated Assembly of Charged Nanoparticles.
abstract: Solutions at high salt concentrations are used to crystallize or segregate charged colloids, including proteins and polyelectrolytes via a complex mechanism referred to as "salting-out". Here, we combine small-angle X-ray scattering (SAXS), molecular dynamics (MD) simulations, and liquid-state theory to show that salting-out is a long-range interaction, which is controlled by electrolyte concentration and colloid charge density. As a model system, we analyze Au nanoparticles coated with noncomplementary DNA designed to prevent interparticle assembly via Watson-Crick hybridization. SAXS shows that these highly charged nanoparticles undergo "gas" to face-centered cubic (FCC) to "glass-like" transitions with increasing NaCl or CaCl2 concentration. MD simulations reveal that the crystallization is concomitant with interparticle interactions changing from purely repulsive to a "long-range potential well" condition. Liquid-state theory explains this attraction as a sum of cohesive and depletion forces that originate from the interelectrolyte ion and electrolyte-ion-nanoparticle positional correlations. Our work provides fundamental insights into the effect of ionic correlations in the salting-out mechanism and suggests new routes for the crystallization of colloids and proteins using concentrated salts.
authors.affiliation: Materials Science and Engineering Department, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: Sumit
authors.initials: S
authors.lastname: Kewalramani
authors.affiliation: Materials Science and Engineering Department, Northwestern University, Evanston, Illinois 60208, United States; Instituto de Física, Universidad Autónoma de San Luis Potosí, Àlvaro Obregón 64, 78000 San Luis Potosí, San Luis Potosí, Mexico.
authors.forename: Guillermo I
authors.initials: GI
authors.lastname: Guerrero-García
authors.affiliation: Materials Science and Engineering Department, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: Liane M
authors.initials: LM
authors.lastname: Moreau
authors.affiliation: Materials Science and Engineering Department, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: Jos W
authors.initials: JW
authors.lastname: Zwanikken
authors.affiliation: Materials Science and Engineering Department, Northwestern University, Evanston, Illinois 60208, United States; Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
authors.forename: Chad A
authors.initials: CA
authors.lastname: Mirkin
authors.affiliation: Materials Science and Engineering Department, Northwestern University, Evanston, Illinois 60208, United States; Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States; Physics and Astronomy Department, Northwestern University, Evanston, Illinois 60208, United States.
authors.forename: Monica
authors.initials: M
authors.lastname: Olvera de la Cruz
authors.affiliation: Materials Science and Engineering Department, Northwestern University, Evanston, Illinois 60208, United States; Physics and Astronomy Department, Northwestern University, Evanston, Illinois 60208, United States.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Bedzyk