Molecular organisation of water and alcohols at solid-liquid interfaces

Abstract

The organisation and self-assembly of molecules at solid-liquid interfaces is central to numerous natural processes and can be used to create supramolecular architectures with functional applications. Historically, studies into surface-based self-assembly in liquids in ambient conditions are limited to molecules with significant surface interactions predisposed to durably reside at the surface. Outside of extreme conditions, such as low temperatures or under confinement, the self-assembly of small molecules (< 20 atoms) without significant surface interactions remains relatively unexplored.

Here, a joint approach involving atomic force microscopy and molecular dynamics simulations is used to explore the self-assembly of small alcohols and water into supramolecular structures on hydrophobic surfaces in ambient conditions. This self-assembly can occur because of the formation of extended hydrogen bonded networks between the assembling molecules at the interface, enabling the molecules to adsorb as a group. Investigations into this system has led to three, major, novel observations. The first is that graphite catalyses a reaction involving water and volatile organics to produce small quantities of methanol. This reaction is enhanced by applied electric fields and the methanol produced can subsequently self-assembly with the water, thus changing the behaviour of the interfacial liquid. The second is that at hydrophobic interfaces, the structure of small alcohol-water mixtures displays a strong concentration dependence; with alcohol molecules at the surface switching between states of hydrogen bonding with the bulk liquid and with other molecules in the same plane. The final result is that due to the weak molecular surface interactions, the hydrogen bond networks of these group-effect stabilised assemblies can be influenced through multiple approaches to create a wide variety of supramolecular structures. The generality and importance of group-effect self-assembly is demonstrated to be applicable to multiple hydrophobic interfaces.

Overall these results form the foundation for further investigations into small molecule self-assembly, along with having wider implications for other fields including the development of novel carbon-based catalytic materials, studies into transfer properties at electrodes as well as understanding friction and lubrication in many systems.