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Mineral nanoparticle suspensions with consolidating properties have been successfully applied in the restoration of weathered architectural surfaces. However, the design of these consolidants is usually stone-specific and based on trial and error, which prevents their robust operation for a wide range of highly heterogeneous monumental stone materials. In this work, we develop a facile and versatile method to systematically study the consolidating mechanisms in action using a surface forces apparatus (SFA) with real-time force sensing and an X-ray surface forces apparatus (X-SFA). We directly assess the mechanical tensile strength of nanosilica-treated single mineral contacts and show a sharp increase in their cohesion. The smallest used nanoparticles provide an order of magnitude stronger contacts. We further resolve the microstructures and forces acting during evaporation-driven, capillary-force-induced nanoparticle aggregation processes, highlighting the importance of the interactions between the nanoparticles and the confining mineral walls. Our novel SFA-based approach offers insight into nano- and microscale mechanisms of consolidating silica treatments, and it can aid the design of nanomaterials used in stone consolidation.

Osteoarthritis is a prevalent degenerative joint disease characterized by progressive articular cartilage loss and destruction. The resultant increase in friction causes severe pain. The collagen I matrix (COL I) has been used clinically for cartilage repair; however, how COL I acts at cartilage surfaces is unclear. Here, we studied adsorption and lubrication of synovial fluid components, albumin, γ-globulin, and the phospholipid DPPC, on COL I under physiological conditions using surface plasmon resonance and an in situ sensing surface force apparatus. Our results revealed COL I had poor lubrication ability, a fairly high coefficient of friction (COF, μ = 0.651 ± 0.013), and surface damage under a 7 mN load. DPPC formed an improved lubricating layer on COL I (μ = 0.072 ± 0.016). In sharp contrast, albumin and γ-globulin exhibited poor lubrication with an order of magnitude higher COF but still provided benefits by protecting COL I from wear. Hence, DPPC on COL I may help optimize COL I implantation design.

Lipid bilayer interactions are essential to a vast range of biological functions, such as intracellular transport mechanisms. Surface charging mediated by concentration dependent ion adsorption and desorption on lipid headgroups alters electric double layers as well as van der Waals and steric hydration forces of interacting bilayers. Here, we directly measure bilayer interactions during charge modulation in a symmetrically polarized electrochemical three-mirror interferometer surface forces apparatus. We quantify polarization and concentration dependent hydration and electric double layer forces due to cation adsorption/desorption. Our results demonstrate that exponential hydration layer interactions effectively describe surface potential dependent surface forces due to cation adsorption at high salt concentrations. Hence, electric double layers of lipid bilayers are exclusively dominated by inner Helmholtz charge regulation under physiological conditions. These results are important for rationalizing bilayer behavior under physiological conditions, where charge and concentration modulation may act as biological triggers for function and signaling.

Being able to probe electrolyte structures and ion mobility in nanometer confined cavities/pores is central for the fundamental understanding and steering of various processes in biologic and material systems. Here, we review how force probe experiments were utilized for studying interfacial physics at solid/liquid/solid interfaces. We discuss recent technological achievements and show how micro-to-nano and ultimately subnanometer confinement can be achieved and probed using the surface forces apparatus. We discuss ion-mobility and structuring in confined spaces during reactive and nonreactive conditions. This includes ion-layering and confinement induced effects, such as enhanced reactivity, decreased ion-mobility, electric double layer overlapping and more. We limit the discussion to electrochemical and ionic liquid systems, yet we discuss the broader perspective how to develop the technique further, and how recent advances can already find new exciting applications, across sometimes unexpected fields. These range from studying physiologic processes to technologic application in catalysis, microfluidics, or geology.

Preprint version of the open access article Dworschak, D., Cheng, H.-W., Ku, C.-S., Chiang, C.-Y., Lin, C.-H., & Valtiner, M. (2021). Comparison of elemental resolved non-confined and restricted electrochemical degradation of nickel base alloys. Corrosion Science, 190, 109629. https://doi.org/10.1016/j.corsci.2021.109629, Passive film properties of nickel base alloys (NBAs) have been studied extensively, yet elemental resolved dissolution currents under corrosive conditions are less well studied. Here, we compare elemental dissolution currents during anodic polarisation and repassivation under crevice and freely-exposed conditions for various NBAs using an ICP-MS flow-cell approach. With a new sample design we can track communication of a crevice environment with the electrolyte, providing insight into crevice solution chemistry and solubility of passive film-forming elements. Data indicates Mo can only form stable precipitate layers under openly corroding conditions. Local elemental depletion was further examined by XPS and nanometer resolved XRF.

Adhesive and repulsive, nm-range surface forces acting between mineral grains control colloidal stability and mineral aggregation but less is known about how these forces are affected by surface reactivity and to what extent these grain-scale forces can influence various deformation processes in rocks. In this experimental work, we explore and quantify the surface forces acting between dynamic mineral surfaces that undergo recrystallization or are chemically reactive in contact with water or aqueous salt solutions. Our experimental setup consists of the surface forces apparatus (SFA) coupled with the multiple beam interferometry (MBI). This setup can excellently reproduce a typical grain contact geometry with nanometer-thin water films confined between contacting mineral grains over relatively large, micron-sized contact areas. Owing to the use of MBI, both surface growth or dissolution processes can be monitored during force measurements in real-time. As such, SFA can provide information about the links between surface reactivity and adhesive or repulsive surface forces. Using the examples of force measurements between recrystallizing or chemically reactive mineral surfaces such as carbonates, hydroxides, and silicates, we comment on the relationship between the measured surface forces and surface reactivity. We link our findings with the observed changes in mineral phases, surface topographies, or surface roughness. We also comment on how the micron-scale confinement in the SFA affects the growth and dissolution processes in contrast to less confined regions. The magnitude of the forces associated with dynamic mineral surfaces and the potential significance of these forces to macroscopic deformation processes and cohesion in rocks are discussed.

The surface forces apparatus (SFA) was developed in the late 1960s as a powerful tool for investigating molecular interactions across apposing surfaces including the first measurement of van der Waals forces and interactions in biologic and liquid media. However, the SFA has two major disadvantages. First, it traditionally uses white light interference between back-silvered muscovite mica surfaces to measure distances and to infer forces from distance shifts during interaction of two surfaces. Hence, distance shifts and force measurement are not decoupled. Second, productive SFA interferometers are so far limited to measuring across mica versus mica or mica versus metal. Direct gold-gold configurations were suggested in the late 1990s but not experimentally achieved as proof-of-principle until recently using a templating technique. In this work, we show how we solve these two disadvantages. First, we present a new SFA design that decouples force and distance measurements with similar resolution. The presented SFA design is inexpensive and can be home-built with mostly commercially available parts. Second, we present an alternative physical vapor deposition approach to construct a stable gold-gold interferometer and demonstrate its performance showing hydrophobic interactions, bubble formation, hemifusion of bilayers, and friction experiments. The presented system is easy to use. The obtained results show excellent reproducibility, indicating that the designed SFA and the three-mirror gold-gold interferometer functions as well as or even better than the traditional interferometer configurations used in SFA. This opens SFA to a wide range of options for various possible applications. Specifically, the gold-gold configuration allows a broad range of surface modifications for studying biophysical interactions as demonstrated in this work.

Understanding elemental corrosion currents and visualizing corroding topographies provide a detailed insight into corrosion mechanisms at the nano-scale. Here, we develop a strategy to understand the elemental composition, corrosion resistivity and local stability of passive materials. Specifically, we utilize a pulse voltammetry approach in a novel electrochemical AFM cell and complement this data by real-time dissolution currents based on spectro-electrochemical online analysis in an ICP-MS flow cell. We study the oxide properties and their protective behaviour, when formed under different applied potentials using alloy 600 as model sample. Both AFM and ICP-MS data show that passive films formed on alloy 600 at around +0.3 to +0.4~V in neutral 1 mM NaCl solution are most stable during anodic corrosion at +1.0~V, while AFM further demonstrates that local dissolution occurs, indicating locally varying defect levels in the passive film. In combination of both techniques, our approach provide real-time elementally resolved and localized information of passive film quality under corrosive conditions, and it may prove useful for other corroding materials.

Bio and aqueous applications of ionic liquids (IL) such as catalysis in micelles formed in aqueous IL solutions or extraction of chemicals from biologic materials rely on surface-active and self-assembly properties of ILs. Here, we discuss qualitative relations of the interfacial and bulk structuring of a water-soluble surface-active IL ([C8MIm][Cl]) on chemically controlled surfaces over a wide range of water concentrations using both force probe and X-ray scattering experiments. Our data indicate that IL structuring evolves from surfactant-like surface adsorption at low IL concentrations, to micellar bulk structure adsorption above the critical micelle concentration, to planar bilayer formation in ILs with

The molecular structure at charged solid/liquid interfaces is vital for many chemical or electrochemical processes, such as adhesion, catalysis or the stability of colloidal dispersions. How cations influence structural hydration forces and interactions across negatively charged surfaces have been studied in great detail. However, how anions influence structural hydration forces on positively charged surfaces is much less understood. Here we report force versus distance profiles on freshly cleaved mica using Atomic Force Microscopy with silicon tips. We characterize steric anion hydration forces for a set of common anions (Cl-, ClO4-, NO3-, SO42- and PO43-) at pH ~ 1 in pure acids, where protons are the co ions. Solutions containing anions with low hydration energies exhibit repulsive structural hydration forces, indicating significant ion and/or water structuring within the first 1-2 nm on a positively charged surface. We attribute this to specific adsorption effects within the Stern layer. In contrast, ions with high hydration energies show exponentially repulsive hydration forces, indicating less degree of structuring within the Stern layer. The presented data demonstrates that anion hydration forces in the inner double layer are comparable to cation hydration forces, and that they qualitatively correlate with hydration free energies. This work may contribute to understanding interaction processes where positive charge is screened by anions within an electrolyte.

Preprint version of the open access article Merola, C., Cheng, H., Dworschak, D., Ku, C., Chiang, C., Renner, F. U., & Valtiner, M. (2019). Nanometer Resolved Real Time Visualization of Acidification and Material Breakdown in Confinement. Advanced Materials Interfaces, 6(10), 1802069. https://doi.org/10.1002/admi.201802069, Localized surface reactions in confinement are inherently difficult to visualize in real-time. Herein multiple-beam-interferometry (MBI) is extended as a real-time monitoring tool for corrosion of nanometer confined bulk metallic surfaces. The capabilities of MBI are demonstrated, and the initial crevice corrosion mechanism on confined nickel and a Ni75Cr16Fe9 model material is compared. The initiation of crevice corrosion is visualized in real time during linear sweep polarization in a 1 × 10−3 m NaCl solution. Pre- and post-experiment analysis is performed to complementarily characterize the degraded area with atomic force microscopy (AFM), optical microscopy, nano-Laue diffraction (nano-LD), scanning electron microscopy (SEM)/electron backscatter diffraction (EBSD), and X-ray photoelectron spectroscopy (XPS). Overall, Ni75Cr16Fe9 displays a better corrosion resistance; however, MBI imaging reveals 200 nm deep severe localized corrosion of the alloy in the crevice opening. Chromium rich passive films formed on the alloy contribute to accelerated corrosion of the confined alloy by a strongly acidifying dissolution of the passive film in the crevice opening. Nano-LD further reveals preferential crystallographic defect and corrosion migration planes during corrosion. MBI provides nanometer accurate characterization of topologies and degradation in confined spaces. The technique enables the understanding of the initial crevice corrosion mechanism and testing modeling approaches and machine-learning algorithms.

Multiple beam interferometry (MBI) evolved as a powerful tool for the simultaneous evaluation of thin film thicknesses and refractive indices in Surface Forces Apparatus (SFA) measurements. However, analysis has relied on simplifications for providing fast or simplified analysis of recorded interference spectra. Here, we describe the implementation of new optics and a generalized fitting approach to 4 × 4 transfer matrix method simulations for the SFA. Layers are described by dispersive complex refractive indices, thicknesses, and Euler angles that can be fitted, providing modeling for birefringent or colored layers. Normalization of data by incident light intensities is essential for the implementation of a fitting approach. Therefore, a modular optical system is described that can be retrofit to any existing SFA setup. Real-time normalization of spectra by white light is realized, alignment procedures are considerably simplified, and direct switching between transmission and reflection modes is possible. A numerical approach is introduced for constructing transfer matrices for birefringent materials. Full fitting of data to the simulation is implemented for arbitrary multilayered stacks used in SFA. This enables self-consistent fitting of mirror thicknesses, birefringence, and relative rotation of anisotropic layers (e.g., mica), evaluation of reflection and transmission mode spectra, and simultaneous fitting of thicknesses and refractive indices of media confined between two surfaces. In addition, a fast full spectral fitting method is implemented for providing a possible real-time analysis with up to 30 fps. We measure and analyze refractive indices of confined cyclohexane, the thickness of lipid bilayers, the thickness of metal layers, the relative rotation of birefringent materials, contact widths, as well as simultaneous fitting of both reflection and transmission mode spectra of typical interferometers. Our analyses suggest a number of best practices for conducting SFA and open MBI in an SFA for increasingly complex systems, including metamaterials, multilayered anisotropic layers, and chiral layers. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

The interfacial decomposition products forming the so-called solid-electrolyte interphase (SEI) significantly determine the destiny of a Li-ion battery. Ultimate knowledge of its detailed behavior and better control are required for higher rates, longer life-time, and increased safety. Employing an electrochemical surface force apparatus, it is possible to control the growth and to investigate the mechanical properties of an SEI in a lithium-ion battery environment. This new approach is here introduced on a gold model system and reveals a compressible film at all stages of SEI growth. The demonstrated methodology provides a unique tool for analyzing electrochemical battery interfaces, in particular in view of alternative electrolyte formulations and artificial interfaces. B.M. acknowledges travel support by InnoEnergy Ph.D. School. The authors thank Carsten Dosche for additional XPS support. Furthermore, the funding of the Escalab250 Xi XPS spectrometer at Carl von Ossietzky University of Oldenburg by the Deutsche Forschungsgemeinschaft (INST 184/106-1 FUGG and INST 184/144-1 FUGG) is acknowledged. B.M. and H.W.C. performed the SFA experiments. C.M. and Q.H. helped with the experiments and produced thin films. B.M., Q.H., and M.O. provided XPS measurements. M.V. and F.U.R. conceived and designed the experiments. M.S. designed and carried out theoretical kinetic analysis. All authors analyzed data, discussed the results, co-wrote the paper, and commented on the manuscript.

Understanding of electrical double layers is essential to all electrochemical devices, particularly at high charge carrier concentrations. Using a combined approach (surface force apparatus, zeta potential, infrared spectroscopy), we propose a model for the interfacial structure of triglyme electrolytes on muscovite mica. In contact with the pure triglyme, a brush-like polymeric structure grows on the mica surface. When lithium triflate is present in the triglyme, this structure is suppressed by anion adsorption and an extended double layer is formed. A surprising result of great fundamental significance is that the effective screening length measured by surface force apparatus at considerable lithium triflate concentrations (above 0.2 M) is substantially higher than expected from the Debye–Huckel theory. This suggests a high degree of complex salt association as a novel characteristic feature of salt-containing electrolytes.

Reactivity in confinement is central to a wide range of applications and systems, yet it is notoriously difficult to probe reactions in confined spaces in real time. Using a modified electrochemical surface forces apparatus (EC-SFA) on confined metallic surfaces, we observe in situ nano- to microscale dissolution and pit formation (qualitatively similar to previous observation on nonmetallic surfaces, e.g., silica) in well-defined geometries in environments relevant to corrosion processes. We follow “crevice corrosion” processes in real time in different pH-neutral NaCl solutions and applied surface potentials of nickel (vs. Ag|AgCl electrode in solution) for the mica–nickel confined interface of total area ∼0.03 mm2. The initial corrosion proceeds as self-catalyzed pitting, visualized by the sudden appearance of circular pits with uniform diameters of 6–7 μm and depth ∼2–3 nm. At concentrations above 10 mM NaCl, pitting is initiated at the outer rim of the confined zone, while below 10 mM NaCl, pitting is initiated inside the confined zone. We compare statistical analysis of growth kinetics and shape evolution of individual nanoscale deep pits with estimates from macroscopic experiments to study initial pit growth and propagation. Our data and experimental techniques reveal a mechanism that suggests initial corrosion results in formation of an aggressive interfacial electrolyte that rapidly accelerates pitting, similar to crack initiation and propagation within the confined area. These results support a general mechanism for nanoscale material degradation and dissolution (e.g., crevice corrosion) of polycrystalline nonnoble metals, alloys, and inorganic materials within confined interfaces.

We present a study focused on characterizing the interaction forces between mica surfaces across solutions containing star-shaped polymers with cationic ends. Using the surface forces apparatus, we show that the interaction forces in pure water between surfaces covered with the polymers can be adequately described by the dendronized brush model. In that framework, our experimental data suggest that the number of branches adsorbed at the surface decreases as the concentration of polymer in the adsorbing solution increases. The onset of interaction was also shown to increase with the concentration of polymer in solution up to distances much larger than the contour length of the polymer, suggesting that the nanostructure of the polymer film is significantly different from that of a monolayer. High compression of the polymer film adsorbed at low polymer concentration revealed the appearance of a highly structured hydration layer underneath the polymer layer. These results support that charged polymer chains do not necessarily come into close contact with the surface even if strong electrostatic interaction is present. Altogether, our results provide a comprehensive understanding of the interfacial behavior of star-shaped polymers and reveal the unexpected role of hydration water in the control of the polymer conformation.

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to tailor performance of ILs in applications such as super-capacitors, catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement, surface charging and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, at high surface charging and/or in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and dominated by bulk structuring. Our results demonstrate that boundary conditions such as charging, confinement and doping by impurities have decisive influence on structure formation of ILs at interfaces. As such, these results have important implications for understanding the behavior of solid/IL interfaces as they significantly extend previous interpretations.

A facile organic-solution method was developed for the synthesis of two-dimensional cuprous nanostructures. Ribbons as thin as 50 nm were successfully prepared by dissolving CuCl in ethylene glycol before raising the solution temperature to 150°C in air. Transmission electron microscopic studies revealed that the ribbon nanostructures obtained were polycrystalline, with nanocrystals present in the structures mostly less than 25 nm. Selective-area electron diffraction patterns taken from the ribbon nanostructures indicated that the chemical composition of the nanocrystals was Cu2O, though X-ray photoelectron spectrometric analysis showed that the nanostructures also contained the Cu2+ phase. Growth factors including the molecular structure of the solvent and the counter-ion of copper in the precursor that may affect the formation of polycrystalline nanoribbons were examined. More importantly, the detail of chemistry involved in the step-by-step, dimensional growth of copper-based nanostructures in ethylene glycol is presented at the molecular level for the first time using the growth of the Cu2O nanoribbon as an example. Ethylene glycol chelates Cu2+, which is produced from Cu+ undergoing disproportionation reactions, to form tetragonally elongated glycolates. A sequence of nucleophilic substitutions then takes place to bond glycolates together to yield stripe-like polymers before the polymers aggregate via van der Waals force into ribbon nanostructures. The Cu0 produced from the disproportionation reaction is crystallized out within the polymers and oxidized at elevated temperature by the dissolved O2 in the solution to form Cu2O nanocrystals.

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to design performance of ILs in emerging technologies including super-capacitors, heterogeneous catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is insignificant and not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, and at high surface charging in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and bulk structuring also dominates interface structures in confinement. Our results demonstrate that boundary conditions such as charging, confinement and the presence of impurities drive structure formation of ILs at interfaces, while at low surface charge solvophobic interactions have a much weaker influence on interface structuring than generally believed. These results have important implications for understanding solid/IL interfaces and clarify previous interpretations.

The unsatisfactory behaviour of the solid electrolyte interphase (SEI) on silicon-based anodes still prevents the commercial introduction of silicon in lithium-ion batteries – a milestone which would be a huge leap forward towards a sustainable energy use.[1] Our group has recently developed an electrolyte mixture improving the cycling performance of a silicon-based lithium-ion battery. For understanding the stabilizing effect of this novel electrolyte on the SEI, advanced characterization techniques had to be employed, such as a modified Surface Forces Apparatus (SFA) and synchrotron-based Hard X-ray Photoelectron Spectroscopy (HAXPES). So far, the SFA was used to research fundamental biologic and solution based interfacial processes.[2] Here we present a detailed investigation of the SEI on a silicon anode by designing a battery SFA cell for in-situ electrochemical measurements. This enabled us to monitor the SEI thickness during the different stages of the SEI formation on both silicon and graphite surfaces. Additionally, we report the mechanical properties of the SEI for different electrolytes which can also be addressed by the SFA setup. HAXPES data collected on a modified SEI on silicon based anodes reveal a complex nanostructure and provide an additional insight into the stabilization mechanism of the electrolyte mixture. Coin-cell testing confirm the stabilizing effect of the novel electrolyte mixture on the battery performance. The electrochemical battery SFA will be an important tool for evaluating novel electrolyte compositions and their effect on the mechanical properties and growth mechanism of the interfacial SEI. The introduction of this in-situ battery test cell for SFA opens the way for both fundamental interfacial insights as well as applied material studies in battery research. REFERENCES 1. E.C. Evarts, To the limits of lithium, Nature, 2015, 526, 593-595. 2. J. Israelachvili et al., Recent advances in the surface forces apparatus (SFA) technique, Rep. Prog. Phys., 2010, 73, 1-16. Figure 1

Corrosion of nickel and its alloys in confined areas remains a serious concern for the safety of structural materials. We uniquely combined white light interferometry and microscopy with high spatial and time resolution for a real-time study of the corrosion mechanism of confined nickel surfaces in alkali metal chloride (e.g., NaCl) electrolytes (c = 1 – 100 mM) at pH = 7. A mica surface is used as crevice former in the electrochemical surface forces apparatus to establish 600-1000 µm2 confined dry areas on extended nickel surfaces. NaCl is the wetting medium of the confined zones and the electrolyte layer spreads out along all the surface. The nickel surfaces form interfacial oxides films at the beginning when no electrochemical potential is applied. Subsequent application of an electrochemical potential ramp from -400 mV to +800 mV initiates severe corrosion of the formed crevices. Surprisingly, the corrosion inside the crevices proceeds as preferential and self-catalyzed “pitting” corrosion (inhomogeneous localized dissolution with the formation of pits) within in the confined zone. Moreover, different behavior of the Nickel corrosion is observed depending on the concentration: inside the confined zone at low concentration (10mM). Our data reveal a surprising and detailed view into the initial corrosion mechanism of nickel surfaces in confined zones, and may contribute to a deeper understanding and, ultimately, prevention of localized corrosion of other metallic surfaces. A double-layer theory coupled with diffusion-limited effects provides a plausible explanation for the observed phenomena.

Electrolyte decomposition and the formation of the solid-electrolyte interface SEI depends critically on the used active materials, electrolytes or electrolyte additives. A better knowledge of the initial steps of SEI formation is thus necessary to cope with new active anode materials or new electrolytes. The initial wetting as well as the further SEI growth are critical for the stability of the applied battery system. Here we present the use of different surface sensitive techniques to address the SEI formation. Next to Synchrotron-based high-energy or hard x-ray XPS (HAXPES) or AFM we belief that studies employing a surface force apparatus (SFA) may play a more prominent role in the future. We report first results on using an electrochemical SFA setup which was modified for addressing battery interfaces. The SFA technique detects changes in layer thickness together with the mechanical behavior of the growing surface films. The potential-induced SEI formation was followed first on Au, but also the initial wetting behavior of a EC/DEC electrolyte on model electrodes such as mica, gold or graphene was studied providing an unprecedented insight in ultrathin wetting layer formation under confined conditions.

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.

The interface with the electrolyte is a decisive feature in the design of composite battery electrodes, which typically contain active material particles, conductive material, and polymer binder. A detailed understanding of the electrolyte layering and wetting of materials such as graphene, graphites, metals such as aluminum or even gold, silicon, and metal oxides are ultimately important for improving crucial lithium-ion battery mechanisms. In particular the solid electrolyte interphase (SEI) and lithium-ion mobility at electrode interfaces are still not well understood. Here we use the (Electrochemical) Surface Forces Apparatus (SFA) (1) to address the wetting behavior of nano-scale-confined lithium-ion battery electrodes by battery electrolytes. After its introduction in the 1970s by Israelachvili and Adams (2), the SFA has been used to investigate interfacial phenomena in fields varying from geoscience to microbiology (3). Figure 1 shows a detail of the SFA-setup that was used for the experiments. White light is thereby guided through a flat contact area of two semi-transparent SFA disks, causing interference patterns which are analyzed using a spectrometer. Monitoring the pattern throughout the experiments allows to follow electrolyte wetting behavior in the confined contact area. Some of the investigated surfaces were mica, gold and graphene. For instance for graphene, a unique combination of rigid and mobile lithium-ion containing electrolyte layers was observed. Figure 2 shows a typical population mapping of a SFA-experiment which focused on the initial wetting behavior of a battery electrolyte (EC/DEC - LiPF6) on dry mica, gold and graphene surfaces. After an initial drop of about 1.8 Å in the inter-mirror distance, which is due to the excretion of a monolayer of adsorbed water from the initially dry interface, the wetting process of the electrolyte was only in the case of graphene able to overcome the contact force. Similar observations were made when nano-confined gaps were wetted with ionic liquids. Molecular beam interferometry is a versatile and powerful tool for studying such nanoscaled wetting processes using battery electrodes including ionic liquids or organic solvents. REFERENCES 1. M. Valtiner et al., Langmuir, 28, 13080 (2012) 2. J. N. Israelachvili and G. E. Adams, Nature, 262, 774 (1976) 3. S. Raman, M. Valtiner, et al., Nature Communications, 5, 5539 (2014) Figure 1

Structure and dynamics at electrochemical metal/liquid interfaces determine lifetime and stability of advanced and structural materials, and are fundamental to corrosive processes. Here, we present recent experiments with the newly developed and unique electrochemical surface force apparatus1,2, which allow us to unravel electrochemical oxidation and reduction processes in real-time at well-defined and nano-confined electrodes. In particular, we monitor metal-oxide thin film formation with Å-accuracy during potentiostatic and potentiodynamic polarizations. In confined spaces we find unexpectedly strong electrochemical depletion forces acting between apposing surfaces during active electrochemical processes. Depending on the conditions, these forces are repulsive and can strongly overpower attractive Van der Waals forces, and the associated electric-field gradients might have significant effects in corrosive processes occurring in nano-confined spaces (such as crevices or pits). In addition, using the EC-SFA we have, for the first time, monitored oxide growth and reduction on noble metals (Au, Pt, Pd and Ir) in sulfuric acid solutions, as well as Nickel and Aluminum corrosion in confinement situations, with Ångstrom resolution in thickness and in real time with milliseconds resolution. Our experiments reveal stark differences of oxide thickness and its electronic properties during potentio-dynamic and potentio-static oxide growth, which will be discussed in the presentation. 1. Valtiner, M. et al. The Electrochemical Surface Forces Apparatus: The Effect of Surface Roughness, Electrostatic Surface Potentials, and Anodic Oxide Growth on Interaction Forces, and Friction between Dissimilar Surfaces in Aqueous Solutions. Langmuir 2012, 28(36), 13080-13093. 2. Israelachvili, J. N. et al The Intersection of Interfacial Forces and Electrochemical Reactions. J. Phys. Chem B, 2013, 117 (51), 16369–16387.