Effect of Interfacial Ion Structuring on Range and Magnitude of Electric Double Layer, Hydration, and Adhesive Interactions between Mica Surfaces in 0.05–3 M Li⁺ and Cs⁺ Electrolyte Solutions

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

Angstrom-Resolved Real-Time Dissection of Electrochemically Active Noble Metal Interfaces

Electrochemical solid|liquid interfaces are critically important for technological applications and materials for energy storage, harvesting, and conversion. Yet, a real-time Angstrom-resolved visualization of dynamic processes at electrified solid|liquid interfaces has not been feasible. Here we report a unique real-time atomistic view into dynamic processes at electrochemically active metal interfaces using white light interferometry in an electrochemical surface forces apparatus. This method allows simultaneous deciphering of both sides of an electrochemical interfacethe solution and the metal sidewith microsecond resolution under dynamically evolving reactive conditions that are inherent to technological systems <i>in operando.</i> Quantitative <i>in situ</i> analysis of the potentio­dynamic electrochemical oxidation/reduction of noble metal surfaces shows that Angstrom thick oxides formed on Au and Pt are high-<i>ik</i> materials; that is, they are metallic or highly defect-rich semiconductors, while Pd forms a low-<i>ik</i> oxide. In contrast, under potentiostatic growth conditions, all noble metal oxides exhibit a low-<i>ik</i> behavior. On the solution side, we reveal hitherto unknown strong electrochemical reaction forces, which are due to temporary charge imbalance in the electric double layer caused by depletion/generation of charged species. The real-time capability of our approach reveals significant time lags between electron transfer, oxide reduction/oxidation, and solution side reaction during a progressing electrode process. Comparing the kinetics of solution and metal side responses provides evidence that noble metal oxide reduction proceeds <i>via</i> a hydrogen adsorption and subsequent dissolution/redeposition mechanism. The presented approach may have important implications for designing emerging materials utilizing electrified interfaces and may apply to bioelectro­chemical processes and signal transmission

Real-Time Multiple Beam Interferometry Reveals Complex Deformations of Metal–Organic-Framework Crystals upon Humidity Adsorption/Desorption

Gas adsorption in metal–organic-frameworks (MOFs) can dramatically affect the size of the crystals (expansion and/or shrinkage) or lead to distortion of their porous structure/framework. This can strongly affect mechanical properties of MOFs potentially leading to loss or improvement of the performance of applications such as membranes, filters, or sensors. Here, we utilize white light multiple-beam-interferometry (MBI) in a surface forces apparatus (SFA) to measure in real-time the deformations taking place in HKUST-1 crystals during humidity adsorption/desorption cycles. MBI provides a real-time measurement of crystal deformations during guest molecule uptake with msecs time and Å distance resolution. We find unusual and unexpected dynamic and nonmonotonic deformation behavior upon humidity loading in HKUST-1 crystals, which we attributed to the gradual filling of the different adsorption sites in the MOF crystal framework. Also, the effect of the external mechanical pressure applied to hydrated/dehydrated crystals strongly affects the flexibility of HKUST-1 crystals and hence their deformation characteristics during guest molecule loading. Our results may have important implications for better understanding/modeling of the effect of gas adsorption on the mechanical properties of MOFs. MBI may be extended not only to other MOFs but also to any other guest–molecule systems in which adsorbate/adsorbent interactions affect structure–property relationships

Real-Time Multiple Beam Interferometry Reveals Complex Deformations of Metal–Organic-Framework Crystals upon Humidity Adsorption/Desorption

Gas adsorption in metal–organic-frameworks (MOFs) can dramatically affect the size of the crystals (expansion and/or shrinkage) or lead to distortion of their porous structure/framework. This can strongly affect mechanical properties of MOFs potentially leading to loss or improvement of the performance of applications such as membranes, filters, or sensors. Here, we utilize white light multiple-beam-interferometry (MBI) in a surface forces apparatus (SFA) to measure in real-time the deformations taking place in HKUST-1 crystals during humidity adsorption/desorption cycles. MBI provides a real-time measurement of crystal deformations during guest molecule uptake with msecs time and Å distance resolution. We find unusual and unexpected dynamic and nonmonotonic deformation behavior upon humidity loading in HKUST-1 crystals, which we attributed to the gradual filling of the different adsorption sites in the MOF crystal framework. Also, the effect of the external mechanical pressure applied to hydrated/dehydrated crystals strongly affects the flexibility of HKUST-1 crystals and hence their deformation characteristics during guest molecule loading. Our results may have important implications for better understanding/modeling of the effect of gas adsorption on the mechanical properties of MOFs. MBI may be extended not only to other MOFs but also to any other guest–molecule systems in which adsorbate/adsorbent interactions affect structure–property relationships

Influence of Molecular Dipole Orientations on Long-Range Exponential Interaction Forces at Hydrophobic Contacts in Aqueous Solutions

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