title: Ionic liquids behave as dilute electrolyte solutions.
abstract: 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 force-distance measurements across the common ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][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 Derjaguin-Landau-Verwey-Overbeek theory with an additive repulsive steric (entropic) ion-surface 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 [C4mim][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 [C4mim][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, high-free-ion density ionic liquids and ionic liquid blends.
authors.affiliation: Materials Department and Department of Chemical Engineering, University of California, Santa Barbara, CA 93106, USA.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Gebbie
authors.forename: Markus
authors.initials: M
authors.lastname: Valtiner
authors.forename: Xavier
authors.initials: X
authors.lastname: Banquy
authors.forename: Eric T
authors.initials: ET
authors.lastname: Fox
authors.forename: Wesley A
authors.initials: WA
authors.lastname: Henderson
authors.forename: Jacob N
authors.initials: JN
authors.lastname: Israelachvili
mesh_terms: D000465:Algorithms; D055664:Electrochemical Techniques / Q000295:instrumentation / Q000379:methods; D004573:Electrolytes / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry; D052578:Ionic Liquids / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D008956:Models, Chemical; D012996:Solutions / Q000737:chemistry*; D055672:Static Electricity; D013449:Sulfonamides / Q000737:chemistry; D013499:Surface Properties; D013816:Thermodynamics
chemical_list: C493485:1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide; D004573:Electrolytes; D007093:Imidazoles; D052578:Ionic Liquids; D007477:Ions; D012996:Solutions; D013449:Sulfonamides
keywords: Boltzmann distribution; electrostatic interaction; interfacial phenomena