The importance of water on molecular ion structuring and charging mechanism of solid interfaces in room temperature ionic liquid (RTIL) is unclear and has been largely ignored. Water may alter structures, charging characteristics, and hence performance at electrified solid/RTIL interfaces and is utilized in various fields including energy storage, conversion, or catalysis. Here, atomic force microscopy and surface forces apparatus experiments are utilized to directly measure how water alters the interfacial structuring and charging characteristics of [C2mim][Tf2N] on mica and electrified gold surfaces. On hydrophilic and ionophobic mica surfaces, water‐saturated [C2mim][Tf2N] dissolves surface‐bound cations, which leads to high surface charging and strong layering. In contrast, layering of dry RTIL at weakly charged mica surfaces is weakly structured. At electrified, hydrophobic, and ionophilic gold electrodes, significant water effects are found only at positive applied electrochemical potentials. Here, the influence of water is limited to interactions within the RTIL layers, and is not related to a direct electrosorption of water on the polarized electrode. More generally, the results suggest that effects of water on interfacial structuring of RTIL strongly depend on both (1) surface charging mechanism and (2) interfacial wetting properties. This may greatly impact utilization/design of RTILs and surfaces for interface‐dominated processes.

Ions and water structuring at charged-solid/electrolyte interfaces and forces arising from interfacial structuring in solutions above 100 mM concentrations dominate structure and functionality in many physiological, geological, and technological systems. In these concentrations, electrolyte structuring occurs within the range of molecular dimensions. Here, we quantitatively measure and describe electric double layer (EDL) and adhesive interactions at mica-interfaces in aqueous CsCl and LiCl solutions with concentrations ranging from 50 mM to 3 M. Complementarily, using atomic force microscopy and surface forces apparatus experiments we characterize concentration-dependent stark differences in the inner and outer EDL force profiles, and discuss differences between the used methods. From 50 mM to 1 M concentrations, interactions forces measured in CsCl-solutions exhibit strong hydration repulsions, but no diffuse EDL-repulsions beyond the Stern layer. In confinement the weakly hydrated Cs(+) ions condensate into the mica-lattice screening the entire surface charge within the Stern layer. In contrast, strongly hydrated Li(+) ions only partially compensate the surface charge within the Stern layer, leading to the formation of a diffuse outer double layer with DLVO behavior. Both LiCl and CsCl solutions exhibit oscillatory ion-hydration forces at surface separations from 2.2 nm to 4-8 Å. Below 4-8 Å the force profiles are dominated in both cases by forces originating from water and/or ion confinement at the solid/electrolyte/solid interface. Adhesive minima and their location vary strongly with the electrolyte and its concentration due to specific ion correlations across the interface, while dispersion forces between the surfaces are overpowered. Highly concentrated 3 M solutions exhibit solidification of the inner EDL structure and an unexpected formation of additional diffuse EDL forces with an increasing range, as recently measured in ionic liquids. Our results may have important implications for understanding and modeling of interaction forces present in static and dynamic systems under physiological and high salt conditions.

Unravelling the complexity of the macroscopic world relies on understanding the scaling of single-molecule interactions towards integral macroscopic interactions. Here, we demonstrate the scaling of single acid-amine interactions through a synergistic experimental approach combining macroscopic surface forces apparatus experiments and single-molecule force spectroscopy. This experimental framework is ideal for testing the well-renowned Jarzynski's equality, which relates work performed under non-equilibrium conditions with equilibrium free energy. Macroscopic equilibrium measurements scale linearly with the number density of interfacial bonds, providing acid-amine interaction energies of 10.9±0.2 kT. Irrespective of how far from equilibrium single-molecule experiments are performed, the Jarzynski's free energy converges to 11±1 kT. Our results validate the applicability of Jarzynski's equality to unravel the scaling of non-equilibrium single-molecule experiments to scenarios where large numbers of molecules interacts simultaneously in equilibrium. The developed scaling strategy predicts large-scale properties such as adhesion or cell-cell interactions on the basis of single-molecule measurements.