## Chemical and Biological Engineering Publications

#### Campus Units

Chemical and Biological Engineering, Materials Science and Engineering, Physics and Astronomy, Neuroscience, Ames Laboratory

Article

#### Publication Version

Submitted Manuscript

1-14-2020

Langmuir

36

1

311

317

#### DOI

10.1021/acs.langmuir.9b02966

#### Abstract

We use synchrotron X-ray reflectivity and grazing incidence small-angle X-ray scattering to investigate the surface assembly of the polyethylene glycol (PEG) grafted gold nanoparticles (PEG-AuNPs) induced by different salts. We find that NaCl and CsCl behave as many other electrolytes, namely drive the PEG-AuNPs to the vapor/suspension interface to form a layer of single particle depth and organize them into very high quality two-dimensional (2D) hexagonal crystals. By contrast, NaI induces the migration of PEG-AuNPs to the aqueous surface at much higher surface densities than the other salts (at similar concentrations). The resulting 2D ordering at moderate NaI concentrations is very short-ranged and at a higher NaI concentration the high density monolayer is amorphous. Considering NaCl, CsCl and the majority of salts behave similarly, this implicates the anomaly of iodine ion (I$^-$) in this unusual surface population. We argue that the influence of most electrolytes on the PEG corona preserves the polymer in the $\theta$-point with sufficient flexibility to settle into a highly ordered state, whereas I$^-$ has a much more severe effect on the corona by collapsing it. The collapsed PEG renders the grafted AuNP a non-spherical shaped complex that, although packs at high density, cannot organize into a 2D ordered arrangement.

This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Langmuir, copyright © American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/acs.langmuir.9b02966. Posted with permission.

American Chemical Society

en

#### File Format

application/pdf

Supporting Information

COinS