Specific Ion Effects on the Assembly of Ionic Amphiphilic Oligomers Elucidated by Spectroscopy and Neutron Reflectivity

Advances in building two-terminal memory elements from soft materials have resulted in new opportunities for the development of artificial neural networks. Bilayers of ionic amphiphilic oligomers (oligodimethylsiloxane-methylimidazolium cation, ODMS-MIM⁺) show short-term synaptic behaviors analogous to those consisting of phospholipid molecules. The advantages of using OMDS-MIM⁺ bilayers rather than lipid bilayers in designing soft-matter memristive and memcapacitive devices include not only improved reliability and stability of the material, but also their salt-dependent electrochemical properties at surfaces and interfaces. To better understand how salt affects the assembly of ODMS-MIM⁺ oligomers into bilayer membranes at the molecular level, we employed surface-active vibrational sum frequency generation (vSFG) spectroscopy on Langmuir-Blodgett (LB) films to give insights into the orientation and (re)arrangement of the molecules at the interface. Neutron reflectometry (NR), a complementary characterization technique to vSFG spectroscopy, provided information about the thickness, roughness, and scattering length density of the LB films. These studies have shown that the nanostructures of charged, amphiphilic OMDS-MIM⁺bilayers are very sensitive to the types of salt present and the overall ionic strength of the system. This information will enable for the development of refined protocols for the construction of enhanced neuromorphic devices based on ionic, amphiphilic oligomers.