posted on 2023-04-12, 19:48authored byXin I.
N. Dong, Brian D. Spindler, Minog Kim, Andreas Stein, Philippe Bühlmann
Nanopore-driven sequestration of
ionic liquids from a
silicone
membrane is presented, a phenomenon that has not been reported previously.
Reference electrodes with ionic liquid doped polydimethylsiloxane
(PDMS) reference membranes and colloid-imprinted mesoporous carbon
(CIM) as solid contact are not functional unless special attention
is paid to the porosity of the solid contact. In the fabrication of
such reference electrodes, a solution of a hydroxyl-terminated silicone
oligomer, ionic liquid, cross-linking reagent, and polymerization
catalyst is deposited on top of the carbon layer, rapidly filling
the pores of the CIM carbon. The catalyzed polymerization curing of
the silicone quickly results in cross-linking of the hydroxyl-terminated
polydimethylsiloxane oligomers, forming structures that are too large
to penetrate the CIM carbon pores. Therefore, as solvent evaporation
from the top of freshly prepared membranes drives the diffusional
transport of solvent toward that membrane surface, the solvent molecules
that leave the CIM carbon pores can only be replaced by the ionic
liquid. This depletes the ionic liquid in the reference membrane that
overlies the CIM carbon solid contact and increases the membrane resistance
by up to 3 orders of magnitude, rendering the devices dysfunctional.
This problem can be avoided by presaturating the CIM carbon with ionic
liquid prior to the deposition of the solution that contains the silicone
oligomers and ionic liquid. Alternatively, a high amount of ionic
liquid can be added into the membrane solution to account for the
size-selective sequestration of ionic liquid into the carbon pores.
Either way, a wide variety of ionic liquids can be used to prepare
PDMS-based reference electrodes with CIM carbon as a solid contact.
A similar depletion of the K+ ionophore BME-44 from ion-selective
silicone membranes was observed too, highlighting that the depletion
of active ingredients from polymeric ion-selective and reference membranes
due to interactions with high surface area solid contacts may be a
more common phenomenon that so far has been overlooked.