Understanding modulation of liquid molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials, such as in sensors, nanofluidic devices, and biological systems. Here, we use force measurements by atomic force microscopy (AFM) to directly measure hydrodynamic, solvation, and frictional forces along the graphene plane in seven liquids. The results show that the greater slip lengths correlate with the interfacial ordering of the liquid molecules, which suggests that the ordering of the liquid forming multiple layers promotes slip. This phenomenon appears to be more relevant than solely the wetting behavior of graphene or the solid−liquid interaction energy, as traditionally assumed. Furthermore, the slip boundary condition of the liquids along the graphene plane is sensitive to the substrate underneath graphene, indicating that the underlying substrate affects graphene's interaction with the liquid molecules. Because interfacial slip can have prominent consequences on the pressure drop, on electrical and diffusive transport through nanochannels, and on lubrication, this work can inspire innovation in many applications through the modulation of the substrate underneath graphene and of the interfacial ordering of the liquid.

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILswith ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectivelyon electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene’s potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene’s quantum capacitance (C Q ) to the total interfacial capacitance (C int ) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (C IL ∼ 80 μF cm–2), so that C Q governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (C IL ∼ 6–9 μF cm–2), which governs C int already at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

Ionic liquids (ILs) are organic salts that remain liquid in absence of a solvent over a wide range of temperatures, often at room temperature. This chapter summarizes the progress in understanding colloidal interactions mediated by ILs and their electrical double layer (EDL) based on experimental observations and theory. It is well known that short-range oscillatory forces in ILs originate from the overscreening provided by ion layers that accumulate close to the charged surface. In contrast, the origin of the more surprising long-range decaying force is not well understood yet. There is experimental and theoretical evidence opposing the originally proposed dilute behavior of ILs, arising from either ion pair formation or solvent/voids/alkyl tails being the effective charge carrier. Here, we overview experiments and theory that supports an alternative explanation of this long-range force based on ion aggregation.