Squeezing Out Interfacial Solvation: The Role of Hydrogen-Bonding in the Structural and Orientational Freedom of Molecular Self-Assembly

Abstract

Bioinspired membrane molecules with improved physical properties and enhanced stability can serve as functional models for conventional lipid or amphiphilic species. Importantly, these molecules can also provide new insights into emergent phenomena that manifest during self-assembly at interfaces. Here, we elucidate the structural response and mechanistic steps underlying the self-assembly of the amphiphilic, charged oligodimethylsiloxane imidazolium cation (ODMS-MIM+) at the air–aqueous interface using Langmuir trough methods with coincident surface-specific vibrational sum-frequency generation (SFG) spectroscopy. We find evidence for a new compression-induced desolvation step that precedes commonly known disordered-to-ordered phase transitions to form nanoscopic assemblies. The experimental data was supported by atomistic molecular dynamics (MD) simulations to provide a detailed mechanistic picture underlying the assembly and the role of water in these phase transitions. The sensitivity of the hydrophobic ODMS tail conformations to compressionowing to distinct water–ODMS interactions and tail–tail solvation propertiesoffers new strategies for the design of interfaces that can be further used to develop soft-matter electronics and low-dimensional materials using physical and chemical controls.

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces

“…To observe analogous features with the other cations, we need to multiply the RDFs by 100, as shown in Figure a. This result shows that Li + directly interacts with the tail, pinning it to the interface, in a manner that is analogous to previously reported H-bonding interactions at air/aqueous interfaces decorated with ODMS-MIM + . The binding of Li + to the tail parallels the interactions found in solutions, , polymers, battery materials, − and small molecules , where solvation and pairing interactions of the Li + cation by oxygen moieties were found.…”

“…This is due to increased charge screening and ion pairing interactions of the Cl – anion with the MIM + headgroup. These interactions allow for a higher packing density of headgroups at the surface, forcing the tails to adopt a more upright orientation or pack into smaller volumes (i.e., mushroom-like structures). ,, If cation effects were negligible, one would expect similar results from the three salt-added systems; however, we clearly find that the SFG response and spectral features vary based on the type of cations (Figure ), indicating a specific cation effect in assembly. Since SFG probes asymmetry at the interface, these more tightly packed centrosymmetric structures lead to a decreased SFG response in the SSP spectra, compared to the salt free system.…”

“…This result shows that Li + directly interacts with the tail, pinning it to the interface, in a manner that is analogous to previously reported H-bonding interactions at air/aqueous interfaces decorated with ODMS-MIM + . 35 The binding of Li + to the tail parallels the interactions found in solutions, 41,42 polymers, 58 battery materials, 59−63 and small molecules 45,46 where solvation and pairing interactions of the Li + cation by oxygen moieties were found. Similarly, recent work has shown how contact ion pairs are preferable in Li + binding to sulfate headgroups, yet they possess weak thermodynamic binding affinities relative to larger cations.…”

“…However, noting that the tail is capable of H-bonding with water, and in the absence of a solvating oil, the tails pack to form liquid crystallike structures. 35 As such, there is an opportunity to use cation−tail interactions to stabilize different structures beyond those defined by the headgroup and its associated interactions.…”

“…For the case of ODMS-MIM + , assumptions regarding limited cation partitioning to the interface and associated electrostatic repulsion from the positively charged headgroups at the surface would suggest a negligible effect of the cations on assembly. However, noting that the tail is capable of H-bonding with water, and in the absence of a solvating oil, the tails pack to form liquid crystal-like structures . As such, there is an opportunity to use cation–tail interactions to stabilize different structures beyond those defined by the headgroup and its associated interactions.…”