Self-Assembly of Charged Oligomeric Bilayers at the Aqueous/Organic Interface Regulated by Ion-Pair Associations
Phospholipid bilayer membranes show promise as biomolecular soft materials that mimic the ability of living systems to sense, respond and learn but are fragile. Amphiphilic charged oligomers (oligodimethylsiloxane-methylimidazolium cation, ODMS-MIM(+)), assembled into bilayers at the oil-aqueous interfaces of droplet interface bilayers (DIBs), possessed similar size and functionality as phospholipid bilayers, but were stable. The ionic liquid headgroups (MIM(+)) of the oligomers were covalently bound to short-chain hydrophobic tails (ODMS). Bilayer self-assembly was influenced both by the charged headgroups, constrained to two-dimensional diffusion at the liquid-liquid interface, which formed electric double layers in the aqueous phase, and the tails in the organic phase. Bilayers formed spontaneously at low ionic strength but required an external voltage to form at higher ionicities. This switch in assembly behavior was due to ion-pairing of the MIM(+) headgroups with chloride ions, resulting in an increase in the density of the charged headgroups at the interface and the ODMS hydrophobic tails in the oil phase as they were covalently grafted to the headgroups. Chain overlap led to repulsive disjoining pressures between droplets due to osmotic stress. The applied voltage caused an attractive electrocompressive stress that overcame the repulsion, enabling bilayer formation. Bilayer assembly at high ionic strength, while requiring a voltage to initiate, was irreversible, and the resulting membrane was considerably more stable than those formed at lower values of the ionic strength. This switching of assembly behavior can be exploited as an additional mechanism for short-term synaptic plasticity in neuromorphic device applications using soft materials.