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submitted on 30.09.2019 and posted on 02.10.2019by Azhad U. Chowdhury, Graham J. Taylor, Vera Bocharova, Robert L. Sacci, Yingdong Luo, Ying-Zhong Ma, Stephen A. Sarles, Kunlun Hong, C. Patrick Collier, Benjamin Doughty
Polymer-stabilized liquid-liquid interfaces are an
important and growing class of bioinspired materials that combine the
structural and functional capabilities of advanced synthetic materials with
naturally evolved biophysical systems.
These platforms have the potential to serve as selective membranes for
chemical separations, molecular sequencers, and to even mimic neuromorphic
computing elements. Despite the diversity in function, basic insight into the
assembly of well-defined amphiphilic polymers to form functional structures
remains elusive, which hinders the continued development of these technologies. In this work we provide new mechanistic
insight into the assembly of an amphiphilic polymer-stabilized oil/aqueous
interface, in which the headgroups consist of positively charged
methylimidazolium ionic liquids, and the tails are short, monodisperse oligodimethylsiloxanes
covalently attached to the headgroups. We demonstrate using vibrational sum
frequency generation spectroscopy and pendant drop tensiometery that the composition
of the bulk aqueous phase, particularly the ionic strength, dictates the
kinetics and structures of the amphiphiles in the organic phase as they decorate
the interface. These results show that H-bonding
and electrostatic interactions taking place in the aqueous phase bias the
grafted oligomer conformations that are adopted in the neighboring oil phase. The
kinetics of self-assembly were ionic strength dependent and found to be
surprisingly slow, being composed of distinct regimes where molecules adsorb
and reorient on relatively fast time scales, but where conformational sampling
and frustrated packing takes place over longer timescales. These results set
the stage for understanding related chemical phenomena of bioinspired materials
in diverse technological and fundamental scientific fields and provide a solid
physical foundation on which to design new functional interfaces.