Proton dynamics in protic ionic liquids and plastic crystals, including 40 new pK

Project: Research project

Description

This is a proposal for support of new research in the field of protic ionic liquids (PILs). With two major reviews in Chemical Reviews already, it might seem like PILs is approaching a mature field, but we argue here that there exist extremely wide domains of variable proton activity ionic liquids, extending over some 40 orders of magnitude below that in hydroxide ionic liquids, and that are still quite uncharted. We cite the case of di-substituted amides which range from those near the extremely basic NH2- (38, superbase) to those near CF3SO2)2N- (-18, a superacid) with intermediate cases that have the same pKa as (OH-) and H3O+, respectively. We describe their distinct mode of preparation and propose studies of their properties, and their limitations and possible applications.
After a background section in which we describe our recent application of lithium amide in converting new solid acids (of record-breaking proton conductivity) to lithium ion-conducting plastic crystals (also of very high, liquid-like conductivity), and many other achievements of a concluding ARO grant, we propose a variety of new research objectives in addition to the study of basic/superbasic PILS referred to above.
(i) One is the expanded study of what we describe as protonic liquids as opposed to protic ionic liquids. These are polyprotic ionic liquids that contain anions in different states of protonation between which protons can pass with smaller barriers than those characterizing normal vehicular migration. They manifest superprotonic behavior on a Walden plot but, for full understanding, require additional characterization by NMR transport studies which we propose to carry out.
(ii) Another is based on the expectation that there should exist alternatively structured anions that have shapes and interactions that are similar to those of the plastic crystal solid acids and lithium ion conductors described above, and will lead to discovery of new solid acids of properties rivalling or exceeding those already found but which have improved stabilities and other properties that might make them serve even better as fuel cell or battery electrolytes.. Alternatively they could be used as suitable mixture components to stabilize the existing materials against detrimental phase changes that can reduce the conductivities.
(iii) A third deals with the identification and evaluation of an interesting case encountered in a study of the narrow acidity range in which stock anhydrous acids can go from non-protonating to a given base, to fully protonating, as judged by large 15N chemical shifts on sensitive aromatic nitrogen bases like 2 methyl pyridine. One case with intermediate 15N shift was identified, raising the question of the speciation in liquid states of systems that might lie at the midpoint of the titration sigmoid. Are they simply mixtures of original acid and base, together with a significant number of ionic products from successful transfers, or are they a sea of H-bonded dipoles? We have proposals to elucidate this question using both short and long time scale measurements, relative to the NMR time-scale..
In each of the above, there is a role for each of the complimentary expertizes of the three principal investigators who specialize in synthesis and calorimetry, electrical relaxation and nuclear magnetic resonance, respectively, and can offer highly specialized equipment to the investigations, i.e. it is a truly collaborative study.

StatusActive
Effective start/end date3/1/192/29/20

Funding

  • DOD-ARMY: Army Materiel Command (AMC): $160,482.00

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Ionic Liquids
Protons
Plastics
Crystals
Acids
Lithium
Nuclear magnetic resonance
Amides
Anions
Fuel cells
Liquids
Ions
Proton conductivity
Protonation
Chemical shift
Calorimetry
Titration
Acidity
Electrolytes
Nitrogen