SCHEDULED MAINTENANCE
Underscreening in concentrated electrolytes: re-entrant swelling in polyelectrolyte brushes†
‡a
Gareth R.
Elliott,
a
Andrew R. J.
Nelson,
b
Anton P.
Le Brun,
b
Grant B.
Webber,
a
Stuart W.
Prescott,
c
Vincent S. J.
Craig,
d
Erica J.
Wanless
a
and
Joshua D.
Willott
‡*a
Abstract
Hypersaline environments are ubiquitous in nature and are found in myriad technological processes. Recent empirical studies have revealed a significant discrepancy between predicted and observed screening lengths at high salt concentrations, a phenomenon referred to as underscreening. Herein we investigate underscreening using a cationic polyelectrolyte brush as an exemplar. Poly(2-(methacryloyloxy)ethyl)trimethylammonium (PMETAC) brushes were synthesised and their internal structural changes and swelling response was monitored with neutron reflectometry and spectroscopic ellipsometry. Both techniques revealed a monotonic brush collapse as the concentration of symmetric monovalent electrolyte increased. However, a non-monotonic change in brush thickness was observed in all multivalent electrolytes at higher concentrations, known as re-entrant swelling; indicative of underscreening. For all electrolytes, numerical self-consistent field theory predictions align with experimental studies in the low-to-moderate salt concentration regions. Analysis suggests that the classical theory of electrolytes is insufficient to describe the screening lengths observed at high salt concentrations and that the re-entrant polyelectrolyte brush swelling seen herein is consistent with the so-called regular underscreening phenomenon.
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