title: Self-assembly of nanoparticle amphiphiles with adaptive surface chemistry.
abstract: We investigate the self-assembly of amphiphilic nanoparticles (NPs) functionalized with mixed monolayers of hydrophobic and hydrophilic ligands in water. Unlike typical amphiphilic particles with "fixed" surface chemistries, the ligands used here are not bound irreversibly but can rearrange dynamically on the particles' surface during their assembly from solution. Depending on the assembly conditions, these adaptive amphiphiles form compact micellar clusters or extended chain-like assemblies in aqueous solution. By controlling the amount of hydrophobic ligands on the particles' surface, the average number of nearest neighbors--that is, the preferred coordination number--can be varied systematically from ∼ 1 (dimers) to ∼ 2 (linear chains) to ∼ 3 (extended clusters). To explain these experimental findings, we present an assembly mechanism in which hydrophobic ligands organize dynamically to form discrete patches between proximal NPs to minimize contact with their aqueous surroundings. Monte Carlo simulations incorporating these adaptive hydrophobic interactions reproduce the three-dimensional assemblies observed in experiment. These results suggest a general strategy based on reconfigurable "sticky" patches that may allow for tunable control over particle coordination number within self-assembled structures.
authors.affiliation: Department of Chemical Engineering, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Hee-Young
authors.initials: HY
authors.lastname: Lee
authors.forename: Sun Hae Ra
authors.initials: SH
authors.lastname: Shin
authors.forename: Aaron M
authors.initials: AM
authors.lastname: Drews
authors.forename: Aaron M
authors.initials: AM
authors.lastname: Chirsan
authors.forename: Sean A
authors.initials: SA
authors.lastname: Lewis
authors.forename: Kyle J M
authors.initials: KJ
authors.lastname: Bishop
keywords: Janus particles; hydrophobic interactions; mixed monolayers; patchy particles; surfactants