Long-range electrostatic screening in ionic liquids
#MMPMID26040001
Gebbie MA
; Dobbs HA
; Valtiner M
; Israelachvili JN
Proc Natl Acad Sci U S A
2015[Jun]; 112
(24
): 7432-7
PMID26040001
show ga
Electrolyte solutions with high concentrations of ions are prevalent in
biological systems and energy storage technologies. Nevertheless, the high
interaction free energy and long-range nature of electrostatic interactions makes
the development of a general conceptual picture of concentrated electrolytes a
significant challenge. In this work, we study ionic liquids, single-component
liquids composed solely of ions, in an attempt to provide a novel perspective on
electrostatic screening in very high concentration (nonideal) electrolytes. We
use temperature-dependent surface force measurements to demonstrate that the
long-range, exponentially decaying diffuse double-layer forces observed across
ionic liquids exhibit a pronounced temperature dependence: Increasing the
temperature decreases the measured exponential (Debye) decay length, implying an
increase in the thermally driven effective free-ion concentration in the bulk
ionic liquids. We use our quantitative results to propose a general model of
long-range electrostatic screening in ionic liquids, where thermally activated
charge fluctuations, either free ions or correlated domains (quasiparticles),
take on the role of ions in traditional dilute electrolyte solutions. This
picture represents a crucial step toward resolving several inconsistencies
surrounding electrostatic screening and charge transport in ionic liquids that
have impeded progress within the interdisciplinary ionic liquids community. More
broadly, our work provides a previously unidentified way of envisioning highly
concentrated electrolytes, with implications for diverse areas of inquiry,
ranging from designing electrochemical devices to rationalizing electrostatic
interactions in biological systems.
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