Without the knowledge of interfacial
chemistry, however, creating functional bilayers from natural or
synthetic molecules involves a certain degree of mystery. Chemical
species interacting in a beaker of solution may or may not form analogous
membranes with selective properties, such as the capability to store or
filter sensory impulses that make up the nondigital
language of neuromorphic computing.
“To be able to train molecules for
specific purposes and unlock new functionalities, we need to understand
what is happening on a molecular level during self-assembly,” Collier
For the experiment, researchers chose an
oligomer, a small polymer variant with a similar structure to natural
lipids, and used surface spectroscopy methods to probe the molecular
monolayer — one side of a bilayer — formed between water and oil.
The ORNL team is one of only a few
groups that has probed the liquid-liquid interface, an important area of
research, but understudied because of technical challenges.
“Our goal was to investigate how the
asymmetry at the oil-water interface causes species to adsorb differently,
to pack and order into a functional design,” Doughty said.
The studied oligomer is an amphiphilic
molecule, meaning parts of its structure are hydrophobic while others are
hydrophilic. When samples stabilized in oil are introduced into a water-based
solution, the molecules self-assemble in response to their mixed
attraction and repulsion to water.
Like goes to like — the oligomers’
slightly charged polar heads want to be in the water phase, which is also
polar, and the nonpolar tails want to be in the oil phase, which is not.
“Being able to observe in real time how
these molecules arrange at a varied interface is a broadly applicable
fundamental scientific accomplishment,” Doughty said.
As shown in the animation, the charged
oligomer heads home in on the water phase; but the flexible tails coil up
in the oil when they have room to spare, or tighten to accommodate
neighbors as the interface becomes crowded.
“We discovered that adjusting the ions,
or charged particles, in the water phase aided in the formation of
well-defined interfaces, with oligomers taking on more tightly coiled
structures,” Doughty said.
Too few ions and the tails spread out
loosely, leaving gaps; too many, and they squeeze in, ballooning from the
“The findings point to approaches for
modifying the size and shape of monolayers, and — at the next stage —
enabling bilayers with asymmetrical designs, just like natural lipids,”
Collier said. “The work brings us a step closer to unlocking new
potentials in biomaterials.”
Tailoring surfaces on a molecular level
to design new materials opens possibilities not only for biocomputing but also broadly for chemical
separations, sensing and detection.
“Observing the liquid-liquid interface
helps us understand the chemistry that drives all of these technologies,”
The journal article is published as
“Insight into the Mechanisms Driving the Self-Assembly of Functional
Interfaces: Moving from Lipids to Charged Amphiphilic Oligomers.”
The research was supported by ORNL’s
Laboratory Directed Research and Development Program. Measurements and
materials synthesis performed by collaborators were supported by the DOE
Office of Science and the National Science Foundation. A portion of
the research was conducted at the Center for Nanophase Materials