Abstract

At the interface with solids, liquid tend to behave differently than in bulk. The interaction of the liquid molecules with the solid and their loss of configurational entropy often results in this interfacial liquid being more ordered and less mobile than its bulk counterpart. The nanoscale organisation and dynamics of interfacial liquids is key to countless processes from friction and lubrication to protein function, heterogeneous catalysis, crystal growth and molecular self-assembly. Experimentally little is know the behaviour of the interfacial liquid at the molecular level, partly for lack of technique able to gather in-situ local information, including over inhomogeneous interfaces. Atomic force microscopy (AFM) can in principle overcome this difficulties and provide sub-nanometre maps of the solvation landscape and the local solid-liquid affinity [1]. Because the measurement is dynamical [2] more information can be derived about the nanoscale flow of the liquid parallel to the solid. Here I present studies investigating the unusual behaviour of water at several hard and soft interfaces using a combination of high-resolution AFM and molecular dynamics simulations. On hard surfaces such as rocks, hydration water can drive the self-assembly of counter-ions [3]. Single ion-tracking and nano-shearing experiments reveals a dynamics largely dominated by (glassy) hydration water for adsorbed ions at hydrophilic interfaces. Interestingly, similar effects can be seen at soft biological interfaces, where they may have important implications for the function of the biomolecules. Interfacial self-assembly of the liquid itself can also be achieved, for example when using homogenous solutions comprising two pure liquids such as water and alcohol. The resulting solid-like nanostructures are remarkably stable and comprise both types of molecules. These structures can be exploited for controlled self-assembly and the development of functional interfaces.

[1] K. Voitchovsky, et al., Nature Nanotechnology 5, 401-405 (2010) [2] Ortiz-Young et al, Nature Communications 4 2482 (2013) [3] Ricci et al, Nature Communications 5 4400 (2014)