Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 110, Issue 24, Pages 9674-9679Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1307871110
Keywords
Boltzmann distribution; electrostatic interaction; interfacial phenomena
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Funding
- Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering [DE-FG02-87ER-45331, 100133311-NCSU]
- Marie Curie International Outgoing Fellowship within the European Community Seventh Framework Program [IOF-253079]
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We combine direct surface force measurements with thermodynamic arguments to demonstrate that pure ionic liquids are expected to behave as dilute weak electrolyte solutions, with typical effective dissociated ion concentrations of less than 0.1% at room temperature. We performed equilibrium forcedistance measurements across the common ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(4)mim][NTf2]) using a surface forces apparatus with in situ electrochemical control and quantitatively modeled these measurements using the van der Waals and electrostatic double-layer forces of the DerjaguinLandauVerweyOverbeek theory with an additive repulsive steric (entropic) ionsurface binding force. Our results indicate that ionic liquids screen charged surfaces through the formation of both bound (Stern) and diffuse electric double layers, where the diffuse double layer is comprised of effectively dissociated ionic liquid ions. Additionally, we used the energetics of thermally dissociating ions in a dielectric medium to quantitatively predict the equilibrium for the effective dissociation reaction of [C(4)mim][NTf2] ions, in excellent agreement with the measured Debye length. Our results clearly demonstrate that, outside of the bound double layer, most of the ions in [C(4)mim][NTf2] are not effectively dissociated and thus do not contribute to electrostatic screening. We also provide a general, molecular-scale framework for designing ionic liquids with significantly increased dissociated charge densities via judiciously balancing ion pair interactions with bulk dielectric properties. Our results clear up several inconsistencies that have hampered scientific progress in this important area and guide the rational design of unique, highfree-ion density ionic liquids and ionic liquid blends.
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