The inherent physical, optical and conductivity properties of ionic liquid - polymeric membranes; a self indicating, simultaneous response upon coordination to transition metal ions
Kavanagh, Andrew and Hilder, Matthius and Clark, Noel and Diamond, Dermot and Radu, Aleksandar (2010) The inherent physical, optical and conductivity properties of ionic liquid - polymeric membranes; a self indicating, simultaneous response upon coordination to transition metal ions. In: Macro 2010: 43rd IUPAC World Polymer Congress, 11-16 July 2010, Glasgow, Scotland.
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Ionic Liquids (IL’s) are organic salts that are liquid at room temperature. Typically, they contain a bulky,
asymmetric organic cation and a small inorganic/organic anion held together via weak electrostatic
interactions, which prevents them from forming a structured lattice.
They exhibit many favourable physical and chemical characteristics, which has led to their use in a variety
of analytical techniques 1. All IL’s display a measured ionic conductivity, plus liquid properties such as
density and viscosity can be controlled by correct choice and/or chemical functionalisation of the ion pair 2.
Ion Selective Electrodes (ISE’s) utilise polymeric membranes for the detection of a target ion in trace
amounts. They typically require a polymer and plasticizer, plus also an ion-exchanging salt and an
ionophore which will selectively bind to a target ion 3. The response of a target ion binding to an ionophore
can be monitored electrochemically, or optically if the ion-ionophore complex produces a colour.
There are several reports in which IL’s plasticize both polyvinylchloride (PVC) and polymethylmethacrylate
(PMMA) based membranes for use in ISE’s 4. Furthermore IL’s also act as ion-exchangers, thereby
eliminating the need for facilitated transport of target ions from aqueous to organic phase.
We will describe the work done to date on the IL trihexyltetradecylphosphonium dicyanamide [P6,6,6,14][DCA]
and its use in polymeric membranes typical of ISE’s. Once solidified into a PVC membrane, [P6,6,6,14][DCA]
can not only act as the plasticizer and ion-exchanger, but also as the ionophore, producing a colorimetric
response upon coordination to Cu2+ (yellow), Co2+ (blue), but also both ions simultaneously (green).
The multifunctionality of [P6,6,6,14][DCA] leads to a dramatic simplification of membrane components,
producing a system capable of a self-indicating, simultaneous, colorimetric response.
As well as a selective optical response, we have also explored the possibility to see if the inherent
conducting properties of these membranes can be exploited. Radio frequency (RF) detection provides a
technique which can monitor the conductivity of a sample wirelessly, but also has the required sensitivity
and is non-invasive on the sample.
RF can not only discriminate between the coordinated and non-coordinated membranes, but also between
the individual co-ordinated membranes. The resultant downward trend in conductivity from Blank > Cobalt
> Mixture > Copper has been validated by Electrochemical Impedance Spectroscopy (EIS) and by portable
X-Ray Fluorescence (XRF).
XRF shows that the results obtained from RF and EIS are directly related to the binding selectivity of the
ligand [DCA]-. We have observed the highest binding levels for membranes exposed to Cu2+ ions, thereby
producing the lowest RF conductivity signal and the highest impedance values. The opposite applies for
Co2+, we have observed lower binding values, conversely producing the highest co-ordinated RF signal
and the lowest coordinated impedance values.
IL’s have been shown to bind to a variety of heavy metal ions 5, lanthanide ions6 plus important target
analytes such as CO2
7 . If a change in conductivity can be presumed upon binding to the analyte, then the
inherent conductivity of IL’s could be exploited in future electrochemical sensing.
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