Extracting local surface charges and charge regulation behavior from atomic force microscopy measurements at heterogeneous solid-electrolyte interfaces

We present a method to determine the local surface charge of solid–liquid interfaces from Atomic Force Microscopy (AFM) measurements that takes into account shifts of the adsorption/desorption equilibria of protons and ions as the cantilever tip approaches the sample. We recorded AFM force distance curves in dynamic mode with sharp tips on heterogeneous silica surfaces partially covered by gibbsite nano-particles immersed in an aqueous electrolyte with variable concentrations of dissolved NaCl and KCl at pH 5.8. Forces are analyzed in the framework of Derjaguin–Landau–Verwey–Overbeek (DLVO) theory in combination with a charge regulation boundary that describes adsorption and desorption reactions of protons and ions. A systematic method to extract the equilibrium constants of these reactions by simultaneous least-squared fitting to experimental data for various salt concentrations is developed and is shown to yield highly consistent results for silica-electrolyte interfaces. For gibbsite-electrolyte interfaces, the surface charge can be determined, yet, an unambiguous identification of the relevant surface speciation reactions is not possible, presumably due to a combination of intrinsic chemical complexity and heterogeneity of the nano-particle surfaces

Nanometer-Resolved Operando Photo-Response of Faceted BiVO₄ Semiconductor Nanoparticles

Photo(electro)catalysis with semiconducting nanoparticles (NPs) is an attractive approach to convert abundant but intermittent renewable electricity into stable chemical fuels. However, our understanding of the microscopic processes governing the performance of the materials has been hampered by the lack of operando characterization techniques with sufficient lateral resolution. Here, we demonstrate that the local surface potentials of NPs of bismuth vanadate (BiVO4) and their response to illumination differ between adjacent facets and depend strongly on the pH of the ambient electrolyte. The isoelectric points of the dominant {010} basal plane and the adjacent {110} side facets differ by 1.5 pH units. Upon illumination, both facets accumulate positive charges and display a maximum surface photoresponse of +55 mV, much stronger than reported in the literature for the surface photo voltage of BiVO4 NPs in air. High resolution images reveal the presence of numerous surface defects ranging from vacancies of a few atoms, to single unit cell steps, to microfacets of variable orientation and degree of disorder. These defects typically carry a highly localized negative surface charge density and display an opposite photoresponse compared to the adjacent facets. Strategies to model and optimize the performance of photocatalyst NPs, therefore, require an understanding of the distribution of surface defects, including the interaction with ambient electrolyte.</p

Non-equilibrium configurations of swelling polymer brush layers induced by spreading drops of weakly volatile oil

Polymer brush layers are responsive materials that swell in contact with good solvents and their vapors. We deposit drops of an almost completely wetting volatile oil onto an oleophilic polymer brush layer and follow the response of the system upon simultaneous exposure to both liquid and vapor. Interferometric imaging shows that a halo of partly swollen polymer brush layer forms ahead of the moving contact line. The swelling dynamics of this halo is controlled by a subtle balance of direct imbibition from the drop into the brush layer and vapor phase transport and can lead to very long-lived transient swelling profiles as well as non-equilibrium configurations involving thickness gradients in a stationary state. A gradient dynamics model based on a free energy functional with three coupled fields is developed and numerically solved. It describes experimental observations and reveals how local evaporation and condensation conspire to stabilize the inhomogeneous non-equilibrium stationary swelling profiles. A quantitative comparison of experiments and calculations provides access to the solvent diffusion coefficient within the brush layer. Overall, the results highlight the - presumably generally applicable - crucial role of vapor phase transport in dynamic wetting phenomena involving volatile liquids on swelling functional surfaces

Electrically controlled localized charge trapping at amorphous fluoropolymer-electrolyte interfaces

Charge trapping is a long-standing problem in electrowetting-on-dielectric (EWOD), causing reliability reduction and restricting its practical applications. Although this phenomenon has been investigated macroscopically, the microscopic investigations are still lacking. In this work, the trapped charges are proven to be localized at three-phase contact line region by using three detecting methods -- local contact angle measurements, electrowetting (EW) probe, and Kelvin Probe Force Microscopy (KPFM). Moreover, we demonstrate that this EW-induced charge trapping phenomenon can be utilized as a simple and low-cost method to deposit charges on fluoropolymer surfaces. Charge density near the three-phase contact line up to 0.46 mC/m2 and the line width with deposited charges ranging from 20 to 300 micrometer are achieved by the proposed method. Particularly, negative charge densities do not degrade even after harsh testing with a water droplet on top of the sample surfaces for 12 hours, as well as after being treated by water vapor for 3 hours. These findings provide an approach for applications which desire stable and controllable surface charges

Response of crude oil deposited organic layers to brines of different salinity:An atomic force microscopy study on carbonate surfaces

The various microscopic processes that take place during enhanced oil-recovery upon injecting low salinity brines are quite complex, particularly for carbonate reservoirs. In this study, we characterize the in-situ microscopic responses of the organic layers deposited on flat Iceland spar calcite surface to brines of different salinity using Atomic force Microscopy (AFM). Organic layers were deposited from crude oil at the end of a two-step aging procedure. AFM topography images reveal that the organic layers remain stable in high-salinity brines and desorb upon exposure to low-salinity brines. In addition, the organic layers swell in low-salinity brines, and the stiffness of the organic layers is found to directly proportional to the brine salinity. These observations are explained in terms of ‘salting-out’ effects, where the affinity of organic layers to solvent molecules increases upon reducing the brine salinity. The swelling and desorption of organic materials provide access for the brine to mineral surface causing dissolution and change in wetting properties of the surface. Our results show the significance of de-stabilizing the organic layer on rock surfaces in order to design any successful improved oil recovery (IOR) strategy

Ion adsorption-induced wetting transition in oil-water-mineral systems

The relative wettability of oil and water on solid surfaces is generally governed by a complex competition of molecular interaction forces acting in such three-phase systems. Herein, we experimentally demonstrate how the adsorption of in nature abundant divalent Ca2+ cations to solid-liquid interfaces induces a macroscopic wetting transition from finite contact angles (≈10°) with to near-zero contact angles without divalent cations. We developed a quantitative model based on DLVO theory to demonstrate that this transition, which is observed on model clay surfaces, mica, but not on silica surfaces nor for monovalent K+ and Na+ cations is driven by charge reversal of the solid-liquid interface. Small amounts of a polar hydrocarbon, stearic acid, added to the ambient decane synergistically enhance the effect and lead to water contact angles up to 70° in the presence of Ca2+. Our results imply that it is the removal of divalent cations that makes reservoir rocks more hydrophilic, suggesting a generalizable strategy to control wettability and an explanation for the success of so-called low salinity water flooding, a recent enhanced oil recovery technology

Absence of anomalous underscreening in highly concentrated aqueous electrolytes confined between smooth silica surfaces

Recent surface forces apparatus experiments that measured the forces between two mica surfaces and a series of subsequent theoretical studies suggest the occurrence of universal underscreening in highly concentrated electrolyte solutions. We performed a set of systematic Atomic Force Spectroscopy measurements for aqueous salt solutions in a concentration range from 1 mM to 5 M using chloride salts of various alkali metals as well as mixed concentrated salt solutions (involving both mono- and divalent cations and anions), that mimic concentrated brines typically encountered in geological formations. Experiments were carried out using flat substrates and submicrometer-sized colloidal probes made of smooth oxidized silicon immersed in salt solutions at pH values of 6 and 9 and temperatures of 25 °C and 45 °C. While strong repulsive forces were observed for the smallest tip-sample separations, none of the conditions explored displayed any indication of anomalous long range electrostatic forces as reported for mica surfaces. Instead, forces are universally dominated by attractive van der Waals interactions at tip-sample separations of ≈2 nm and beyond for salt concentrations of 1 M and higher. Complementary calculations based on classical density functional theory for the primitive model support these experimental observations and display a consistent decrease in screening length with increasing ion concentration

Nanostructuration contrôlée de films de polymères

Il est bien connu que la structuration de surface dans la nature est très importante et ses applications très répandues. Des modèles simples, comme les vagues de surface, sont indispensables dans certains processus naturels et peuvent avoir une application directe aux innovations technologiques. Dans cette thèse, j'étudie les nouvelles méthodes de structuration et de processus de formation de structures contrôlées dans des films minces de polymères, en particulier à des températures inférieures à leur transition vitreuse. J'ai trouvé que la surface de polystyrène vitreux peut être reconstruite à température ambiante, par application directe ou indirecte d'un champ électrique, suggérant fortement qu’une couche de mobilité accrue existe à la surface de ce polymère vitreux. De plus grâce à cette thèse, nous présentons une nouvelle méthode pour induire et contrôler des structures submicroniques sur des substrats hydrophobes en une seule étape de traitement simple, basée sur le traitement du substrat avec une solution aqueuse dégazée. Cette nanostructuration est le résultat d'adsorption des espèces chargées proches sur la surface hydrophobe des polymères créant un champ électrique élevé, ce qui, combiné avec la mobilité de la surface du polymère, induit la déformation du substrat polymère. Comme l'étude directe des propriétés spécifiques de cette région, proche de la surface libre de films minces de polymères, est très rare en raison de la limite des techniques expérimentales appropriées, j'ai réalisé une étude approfondie de la relaxation temporelle des surfaces polymères préalablement structurées par les méthodes décrites ci-dessus.It is well known that the importance and the applications of surface structuration in Nature and in technology are widespread. Simple patterns, such as surface waves, are indispensable in some natural processes and may have direct application to technological innovations. In this thesis I investigate novel methods of structuring and control structure formation process in thin polymer films, particularly at temperatures lower than their glass transition. We have found that the surface of glassy polystyrene can be reconstructed at room temperature either by direct or indirect application of an electric field, strongly suggesting that a layer of enhanced mobility indeed exists at the surface of this glassy polymer. Additionally through this thesis we present a novel developed way to induce and control submicron structures on hydrophobic substrates by a single, simple treatment step based on treating the substrate with degassed aqueous solution. This nanostructuration is the result of close adsorption of charged species on the hydrophobic polymeric surface building a high electric field, which, combined with the mobility of the polymer surface, induces the deformation of the polymer substrate. Since, the direct study of properties of this specific near free surface region of thin polymer films is very rare due to the limited suitable experimental techniques; we have completed an extensive study of influence of supporting substrate and the temporal relaxation of previously polymer structured surfaces by above described methods