Electrolyte solutions with high concentrations of ions are prevalent in biological systems and energy storage technologies. Nevertheless, the high interaction free energy and long-range nature of electrostatic interactions makes the development of a general conceptual picture of concentrated electrolytes a significant challenge. In this work, we study ionic liquids, single-component liquids composed solely of ions, in an attempt to provide a novel perspective on electrostatic screening in very high concentration (nonideal) electrolytes. We use temperature-dependent surface force measurements to demonstrate that the long-range, exponentially decaying diffuse double-layer forces observed across ionic liquids exhibit a pronounced temperature dependence: Increasing the temperature decreases the measured exponential (Debye) decay length, implying an increase in the thermally driven effective free-ion concentration in the bulk ionic liquids. We use our quantitative results to propose a general model of long-range electrostatic screening in ionic liquids, where thermally activated charge fluctuations, either free ions or correlated domains (quasiparticles), take on the role of ions in traditional dilute electrolyte solutions. This picture represents a crucial step toward resolving several inconsistencies surrounding electrostatic screening and charge transport in ionic liquids that have impeded progress within the interdisciplinary ionic liquids community. More broadly, our work provides a previously unidentified way of envisioning highly concentrated electrolytes, with implications for diverse areas of inquiry, ranging from designing electrochemical devices to rationalizing electrostatic interactions in biological systems.

Strong and particularly long ranged (>100 nm) interaction forces between apposing hydrophobic lipid monolayers are now well understood in terms of a partial turnover of mobile lipid patches, giving rise to a correlated long-range electrostatic attraction. Here we describe similarly strong long-ranged attractive forces between self-assembled monolayers of carboranethiols, with dipole moments aligned either parallel or perpendicular to the surface, and hydrophobic lipid monolayers deposited on mica. We compare the interaction forces measured at very different length scales using atomic force microscope and surface forces apparatus measurements. Both systems gave a long-ranged exponential attraction with a decay length of 2.0 ± 0.2 nm for dipole alignments perpendicular to the surface. The effect of dipole alignment parallel to the surface is larger than for perpendicular dipoles, likely due to greater lateral correlation of in-plane surface dipoles. The magnitudes and range of the measured interaction forces also depend on the surface area of the probe used: At extended surfaces, dipole alignment parallel to the surface leads to a stronger attraction due to electrostatic correlations of freely rotating surface dipoles and charge patches on the apposing surfaces. In contrast, perpendicular dipoles at extended surfaces, where molecular rotation cannot lead to large dipole correlations, do not depend on the scale of the probe used. Our results may be important to a range of scale-dependent interaction phenomena related to solvent/water structuring on dipolar and hydrophobic surfaces at interfaces.

Crevice corrosion (CC) of metals remains a serious concern for structural materials. Yet a real-time in situ visualization of corrosion, and its inhibition within a confined geometry, remains challenging. Here, we present how multiple-beam interferometry in a Surface Forces Apparatus can be utilized to directly visualize corrosion processes in real-time and with Angstrom resolution within welldefined confinement geometries. We use atomically smooth muscovite mica surfaces to form round-shaped ∼1000 μm² crevices on aluminum. After exposure to NaCl solutions we can detect and track active sites of aluminum corrosion within this confined geometry. CC of aluminum randomly initiates in the confined crevice mouth, where the distance between apposing surfaces is between 20-300 nm. We can directly track oxide dissolution/formation, and corrosion-rates as well as their retardation due to sodium vanadate inhibitors present in solution. Formation of a compacted oxide layer effectively inhibits CC in 5 mM NaCl solutions with 2.5 mM of added NaVO3, while inhibition rapidly breaks down at chloride concentrations above 50 mM. Breakdown of the inhibition-layers is initiated by rapid dissolution of the protective oxide within the confined zone. Our technique may be adapted for monitoring CC, corrosion inside of crack-tips, and evaluation of inhibitor efficiencies in a variety of metals.

We review recent developments in experimental techniques that simultaneously combine measurements of the interaction forces or energies between two extended surfaces immersed in electrolyte solutions-primarily aqueous-with simultaneous monitoring of their (electro)chemical reactions and controlling the electrochemical surface potential of at least one of the surfaces. Combination of these complementary techniques allows for simultaneous real time monitoring of angstrom level changes in surface thickness and roughness, surface-surface interaction energies, and charge and mass transferred via electrochemical reactions, dissolution, and adsorption, and/or charging of electric double layers. These techniques employ the surface forces apparatus (SFA) combined with various "electrochemical attachments" for in situ measurements of various physical and (electro)chemical properties (e.g., cyclic voltammetry), optical imaging, and electric potentials and currents generated naturally during an interaction, as well as when electric fields (potential differences) are applied between the surfaces and/or solution-in some cases allowing for the chemical reaction equation to be unambiguously determined. We discuss how the physical interactions between two different surfaces when brought close to each other (<10 nm) can affect their chemistry, and suggest further extensions of these techniques to biological systems and simultaneous in situ spectroscopic measurements for chemical analysis.