"GAP" between traditional ionic liquids and molten salts

ECTP2014 - 20th European Conference on Thermophysical Properties
Plenary Lecture - PL_4 – 4th Sept. 09:00-09:45, Auditorium
Graphical Abstract
Upon what may exist in the "Gap" between traditional Ionic Liquids and
Molten Salts
Luís Paulo Rebelo, Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa,
Portugal.
ECTP2014 - 20th European Conference on Thermophysical Properties
Abstract
Upon what may exist in the “Gap” between traditional Ionic Liquids and
Molten Salts
Luís Paulo N. Rebelo*, José Nuno Canongia Lopes
Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa,
Av. República, Apt. 127, 2781-901 Oeiras, Portugal
*[email protected]
Aprotic Ionic Liquids, i.e. liquid salts, are generally defined as substances exclusively composed of
anions and cations (typically, not exclusively though, organic cations and inorganic anions)
presenting melting temperatures below a commonly-used, conventional temperature of 100 ºC.
They are widely perceived as “technologically clean” solvents, owing to their general nonflammability, negligible vapour pressure even at moderately-high temperatures, [1] recyclability, as
well as their tuneable physical and chemical properties. In turn, the latter is a consequence of the
high number of possible one-to-one, one-to-two, two-to-two, etc., anion-cation combinations, each
of which with possible distinct structures and diverse chemical functionalities. Typically, when
long alkyl chains are incorporated either in the cation and/or anion these liquid salts tend to selfaggregate into nanosized polar and non-polar domains. Thus, Coulombic forces (and sometimes Hbonding) prevail in the first of these domains, while dispersion interactions (Van der Waals-type)
predominate in the latter case. All these characteristics make them unique materials with yet an
immense territory for exploring both the fundamental details of the (doubly, dual) nature [2] of
their interactions and their potential applications.
In “contrast”, molten salts are usually understood as constituted by combination(s) of small-sized
inorganic anions with inorganic cations. The quasi-exclusive presence of long-range Coulombic
forces is paramount to rationalize their easy crystallization, and, thus, their very high melting
temperatures.
In this work, we intend to bridge the gap between these two, apparently, distinct class of
compounds. In short, "Is There a Continuum between Ionic Liquids and Inorganic Salts?".
To this end, we explore (i) new types of anion-cation combinations, (ii) mixtures of ionic liquids
with molten salts, and (iii) signs/evidence that, typically, either the role or impact of the presence of
a given anion is distinct from that of a given, isoelectronic cation. Therefore, “Mind the Gap!”
[1]
[2]
M. J. Earle et al., Nature, 2006, 439, 831-834.
L.P.N. Rebelo et al., Accounts Chem. Res., 2007, 40, 1114-1121.