Surfaces In Electrolyte Solutions Research Articles

Measurement of Surface Diffusion at the Electrochemical Interface by In Situ Linear Optical Diffraction.

A new in situ method for measuring the surface diffusion rates of adsorbates on electrode surfaces in electrolyte solution is presented. The method is based on the generation of a periodic spatial modulation of the adsorbate coverage via interfering laser pulses and subsequent monitoring of the diffusion-induced decay of this pattern using the optical diffraction signal of a second laser. Proof-of-principle measurements of the surface diffusion of adsorbed sulfur on Pt(111) electrodes in 0.1 M H2SO4 indicate potential- and coverage-dependent diffusion constants that are significantly higher than those of sulfur on Pt(111) under vacuum conditions.

A singular behavior at the electrolytes solution surfaces: Experimental and simulation investigation over an extended range of temperature

The aqueous electrolytes solutions have an essential role in a variety of systems including natural, biological, and industrial processes. The vital role in human bio-activity and the enhanced oil recovery are two familiar examples with the features that mainly root from the special organization of ions in the solution surface. Despite research works that have been reported on the electrolyte's solution surface, the dependence of surface properties on the thermodynamic state and the chemical nature of constituting phases is still an open field of ongoing research. In this study, the surface properties of halides (NaCl, KBr, and KI) aqueous solutions at the interface with vapor-saturated and at n-octane phase are investigated by surface tension measurement and molecular dynamics simulation. Surface entropy, surface enthalpy, and surface excess concentration (Γ) are explored in a wider range of temperature (up to 343.15 K) than those already have been reported. The surface tension measured under vapor-saturation conditions decreases with temperature and increases with concentration linearly, though shows some characteristics fluctuations. For both (NaCl and KI) systems, variation of Γ versus temperature peculiarly shows a maximum centered in the range of 308.15–313.15 K at all concentrations. These peculiar singularities involve a wealth of information related to the alteration of the surface structure that roots from the nature of Cl− and I− anions at vapor-saturation. To decipher this peculiarity for microscopic details and to account for the factors underlying this singularity, molecular dynamics simulation of NaCl, KBr, and KI aqueous solution was performed over the range 298.15–343.15 K. The simulated bulk and surface molecular structure, as well as the bulk transport properties of 1.0 M (mol/lit) NaCl, KBr, and KI aqueous solution, confirm a singular behavior in the range of 308.15–313.15 K which accounts for the behavior and trend of thermodynamic and surface excess concentration we measured experimentally as a function of temperature. Considering halides salts are taking the main part in the human body intervascular fluid, this discovery also presents a baseline that allows unraveling the effect of temperature fluctuations on the distribution of the excess concentration at the fluid/cell tissue interface.

Imaging Ion Pairs Forming Structural Arrangements in Interfacial Regions.

A technique to image ion pairs in solution is reported. We investigated structural and dynamic properties of ion-pair distributions deposited on highly oriented pyrolytic graphite (HOPG) surfaces in electrolyte solutions. Atomic force microscopy images of HOPG immersed in NaCl and KCl solutions display regular arrangements on top of the hexagonal carbon rings forming the HOPG atomic structure. These arrangements are the result of the low value of the aqueous interfacial dielectric constant (εr ≈ 3–11). The measured ion-pair radius is a function of the salt present in the solution; for KCl, the ion-pair radius is equal or smaller than 0.42 nm; for NaCl, the ion-pair radius is 0.36 nm. A comparison of these values with their crystalline lattice dimensions indicates that both KCl and NaCl ion pairs in solution at the HOPG/solution interfacial region exist as tight contact ion pairs in quasistationary distributions. The NaCl ion-pair distribution forms an aligned arrangement, and the KCl distribution is formed by intercalated pairs.

The influence of temperature on the charging of polytetrafluoroethylene surfaces in electrolyte solutions

The surface properties of Polytetrafluoroethylene (PTFE) depend on the pH and the ionic strength of the electrolyte solution. The point of zero charge (pHpzc) and the isoelectric point (pHiep) of PTFE were found to be 2.9 and 3.2 at 25 °C, respectively. The electrophoretic mobility at pH > pHiep indicates that the PTFE particles are negatively charged in the neutral pH region. In absence of surface functional groups, this observation can be explained by the distribution of OH− and H+ ions between the PTFE interface and the remaining solution. A thermodynamic model of ion distribution and the temperature dependency of the electroneutrality points which enables evaluation of the thermodynamic parameters is proposed. The enthalpies of interfacial reactions were indirectly determined by measuring the temperature dependency of the electroneutrality points by potentiometric mass titration and streaming current measurements. It was found that the exchange of the H+ and OH− ions between the interfacial region and bulk of the solution is an endothermic reaction. The specific enthalpy of dilution of PTFE dispersions was found to be pH dependent, with the lowest value at pH ≈ 3, i.e. in the zero surface charge region.

In-situ oil presence sensor using simple-structured upward open-channel microbial fuel cell (UOC-MFC)

Existing oil leakage detection approaches on the ocean or on the land have suffered from shortcomings like low-sensitivity, environment-vulnerability and high cost and energy consumption. A unique simple-structured upward open-channel microbial fuel cell (UOC-MFC) was developed to solve these challenges, in which the open-channel rolled cathode vertically floated on the electrolyte solution surface and simultaneously contacted with both air and water. Oil presence covered the cathode surface, blocked the oxygen diffusion on the cathode and thus caused the prompt drop of the potential readings of the UOC-MFC sensors. The oil shock tests using engine oil (EO) showed that the potential decrease and the response time exhibited excellent linear relationships (R2 > 0.99) with the EO amount, indicating a good sensitivity. Due to the separation of the anode and the invading oil, the UOC-MFC sensor recovered its external potential within 2.3 h after the EO shock tests, indicating a good reusability. The power density of the UOC-MFC was 450 mW/m3 under normal condition and dropped to 207 mW/m3 after EO shock due to the internal resistance increasing from 1.29 to 3.80 kΩ. The UOC-MFC integrated with a power management system (PMS) theoretically supported a voltage meter and data transmitter. This study well demonstrated the great potential of the UOC-MFC for in-situ detecting oil presence in a real-time self-sustained mode.

Understanding Interactions between Clay and Model Coal Surfaces in Electrolyte Solutions by a Quartz Crystal Microbalance with Dissipation Study

Clay slime coating is known to have a negative impact on coal flotation. It is therefore of great importance to study interactions between clay and coal under flotation conditions. In this study, the effect of electrolytes on the interactions of montmorillonite (MMT) clays with carbon or asphaltenes on sensor surfaces of quartz crystal microbalance with dissipation (QCM-D) as model coal was investigated. Ion beam sputtering deposition and spin coating methods were used to prepare carbon and asphaltene (a class of natural polyaromatic compounds) surfaces. The prepared surfaces were characterized by Raman spectroscopy. QCM-D was used to determine in situ interactions between MMT clays and model coal surfaces in electrolyte aqueous solutions. The MMT clays were shown to deposit significantly on the model coal surfaces only in the presence of NaCl or CaCl2 as a result of the compression of electrical double layers. Moreover the enhanced deposition by calcium ions is stronger than by sodium ions. The results f...

Trends in mica–mica adhesion reflect the influence of molecular details on long-range dispersion forces underlying aggregation and coalignment

Oriented attachment of nanocrystalline subunits is recognized as a common crystallization pathway that is closely related to formation of nanoparticle superlattices, mesocrystals, and other kinetically stabilized structures. Approaching particles have been observed to rotate to achieve coalignment while separated by nanometer-scale solvent layers. Little is known about the forces that drive coalignment, particularly in this "solvent-separated" regime. To obtain a mechanistic understanding of this process, we used atomic-force-microscopy-based dynamic force spectroscopy with tips fabricated from oriented mica to measure the adhesion forces between mica (001) surfaces in electrolyte solutions as a function of orientation, temperature, electrolyte type, and electrolyte concentration. The results reveal an ∼60° periodicity as well as a complex dependence on electrolyte concentration and temperature. A continuum model that considers the competition between electrostatic repulsion and van der Waals attraction, augmented by microscopic details that include surface separation, water structure, ion hydration, and charge regulation at the interface, qualitatively reproduces the observed trends and implies that dispersion forces are responsible for establishing coalignment in the solvent-separated state.

CALCULATION METHOD OF REAL ACTIVITY COEFFICIENTS OF IONS BY EXAMPLE OF HYDROGEN AND CHLORINE IONS IN AQUEOUS HCl SOLUTIONS

In this paper, we proposed a calculating method of the real individual activity coefficients. To calculate the energy of the ion, transfer through the interface we offered to use the value of the electrolyte solution surface potential, and to use electronegativity elements values for accounting contribution of the charge transfer energy between the surface and the bulk solution. To verify the proposed equation, the activity coefficients of hydrogen and chlorine ions in an aqueous HCl solutions were calculated over a wide range of electrolyte concentrations. The Debye-Hückel equation within 2nd approximation describes satisfactorily the dependence of the activity coefficient on the electrolyte concentration for anions, but not reproduce the minimum on the curve for cations. This dependence for the cation H⁺ is most extremal, and namely it is the reason of the choice of HCl for the model calculation. The calculated values were compared with the data obtained from the results of determining the activity coefficients by experimental method of "vertical jet" [1]. The comparison showed that our proposed equation reproduces the character of the concentration dependence of real activity coefficients not for anion only but for cation as well. Previously we found, that at low concentrations of solutes up to 0.1 mol/kg the values of some thermodynamic characteristics are strongly determined by not the volume concentration of the electrolyte, but the restructuring of the solvent structure in the surface layer of the solution, and therefore, the changing the energy state of the molecules at the interphase surface. Hence, the number of cations and anions at the surface in the areas of high and low bulk concentrations of electrolyte also differ greatly. Obviously, the small variation between calculated and experimental values of the real activity coefficients may be connected with necessity of using at calculation the bulk and surface concentration of ions. For citation:Fedorovа A.A., Sharonov N.Yu., Filippov D.V. Calculation method of real activity coefficients of ions by example of hydrogen and chlorine ions in aqueous HCl solutions. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 3. P. 29-35.

Preparation of titanium dioxide thin films by indirect-electrodeposition

Titanium oxide films were prepared on a quartz glass substrate by the cathodic galvanostatic electrolysis of a solution of titanium bis(ammonium lactato)dihydroxide and ammonium nitrate at 323K using a stainless steel electrode as a “dummy electrode” located in the vicinity (within ~1mm) of the substrate. This novel film preparation method, named “indirect-electrodeposition” is effective for nonconductive substrates without catalysts or reducing agents. During indirect electrodeposition, the films are deposited on the area of the substrate above the electrolyte solution surface. The deposited film was identified as TiO2 by X-ray photoelectron spectroscopy. The amorphous as-deposited film was converted to the anatase-type crystalline TiO2 phase by calcination at 723K. Optical bandgap of the film was evaluated with Kubelka-Munk function from diffuse reflection spectra.

Surface charge mapping with a nanopipette.

Nanopipettes are emerging as simple but powerful tools for probing chemistry at the nanoscale. In this contribution the use of nanopipettes for simultaneous surface charge mapping and topographical imaging is demonstrated, using a scanning ion conductance microscopy (SICM) format. When a nanopipette is positioned close to a surface in electrolyte solution, the direct ion current (DC), driven by an applied bias between a quasi-reference counter electrode (QRCE) in the nanopipette and a second QRCE in the bulk solution, is sensitive to surface charge. The charge sensitivity arises because the diffuse double layers at the nanopipette and the surface interact, creating a perm-selective region which becomes increasingly significant at low ionic strengths (10 mM 1:1 aqueous electrolyte herein). This leads to a polarity-dependent ion current and surface-induced rectification as the bias is varied. Using distance-modulated SICM, which induces an alternating ion current component (AC) by periodically modulating the distance between the nanopipette and the surface, the effect of surface charge on the DC and AC is explored and rationalized. The impact of surface charge on the AC phase (with respect to the driving sinusoidal signal) is highlighted in particular; this quantity shows a shift that is highly sensitive to interfacial charge and provides the basis for visualizing charge simultaneously with topography. The studies herein highlight the use of nanopipettes for functional imaging with applications from cell biology to materials characterization where understanding surface charge is of key importance. They also provide a framework for the design of SICM experiments, which may be convoluted by topographical and surface charge effects, especially for small nanopipettes.

Surface Force Measurements between Titanium Dioxide Surfaces Prepared by Atomic Layer Deposition in Electrolyte Solutions Reveal Non-DLVO Interactions: Influence of Water and Argon Plasma Cleaning

Surface force measurements between titania surfaces in electrolyte solutions have previously revealed an unexplained long-range repulsive force at high pH, not described by Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. Here, the surface forces between titania surfaces produced by atomic layer deposition (ALD) and cleaned using a variety of methods have been measured to determine the influence of the cleaning protocol on the measured forces and test the hypothesis that water plasma cleaning of the surface results in non-DLVO forces at high pH. For argon plasma and water plasma cleaned surfaces, a diffuse double layer repulsion and van der Waals attraction is observed near the isoelectric point. At high pH, the force remained repulsive up until contact, and no van der Waals attraction or adhesion was observed. Differences in the measured forces are explained by modification of the surface chemistry during cleaning, which alters the density of charged groups on the surface, but this cannot explain the observed disagreement with DLVO theory at high pH.

Adsorption and Assembly of Ions and Organic Molecules at Electrochemical Interfaces: Nanoscale Aspects

We describe the history of electrochemical scanning tunneling microscopy (STM) and advances made in this field during the past 20 years. In situ STM allows one to monitor various electrode processes, such as the underpotential deposition of copper and silver ions; the specific adsorption of iodine and sulfate/bisulfate ions; electrochemical dissolution processes of silicon and gold single-crystal surfaces in electrolyte solutions; and the molecular assembly of metalloporphyrins, metallophthalocyanines, and fullerenes, at atomic and/or molecular resolution. Furthermore, a laser confocal microscope, combined with a differential interference contrast microscope, enables investigation of the dynamics of electrochemical processes at atomic resolution.

Interaction forces between muscovite and silica surfaces in electrolyte solutions measured with AFM

Interaction forces between a silica colloidal sphere and a muscovite flat surface in electrolyte solutions were directly measured with an atomic force microscope (AFM). The results showed a significant impact of time, electrolyte concentration and solution pH on both long-range (non-contact) and adhesion (pull-off) force. A strong long-range repulsive force was observed under conditions of lower electrolyte concentration and higher solution pH, while a weak long-range attractive force was observed in the higher electrolyte concentration and lower pH solutions. With the electrolyte concentration increasing, the interaction forces decreased from strong repulsive force to strong attractive force. The measured long-range forces were monotonically repulsive at pH 5.8-10.2 and changed in a small scale. However, when the solution pH decreased to 3.4, a weak attractive force was observed at a separation distance of 22-32 nm. In low electrolyte concentration and pH solutions, the adhesion force between the muscovite and silica is large. With increasing the electrolyte concentration and solution pH, the adhesion force decreased and became relatively stable at last. The measured interaction forces were fitted well with the classical DLVO theory.

The bulk electroviscous effect

Electroviscous stresses arise as hydrodynamic flows disturb the ionic (Debye) clouds that screen charged surfaces in electrolyte solutions. The contribution thereof to the effective bulk viscosity (also known as the second or volume viscosity) of two-phase suspensions is quantified here. Specifically, the bulk viscosity of two model suspensions is calculated: (1) a dilute dispersion of rigid charged spherical particles immersed in a compressible electrolyte that undergoes uniform dilatation and (2) a dilute suspension of charged gas bubbles expanding uniformly in an incompressible electrolyte. In both cases, it is assumed that the fluid flow only slightly drives the Debye cloud out of equilibrium, which formally requires that the ratio of the ion diffusion to flow time scales—a Peclet number—is small. For a suspension of rigid particles, the electroviscous contribution to the effective bulk viscosity is proportional to the particle volume fraction and decreases monotonically as the ratio of the particle radius to the Debye length increases. Similar behavior is well known for the electroviscous contribution to the effective shear viscosity of a dilute hard-sphere suspension; a quantitative comparison between the bulk and shear viscosities is made. In contrast, the electroviscous contribution to the bulk viscosity of a dilute suspension of bubbles is independent of the bubble volume fraction and attains a finite value in the limit of vanishing Debye length.

Ionic Enhancement of Silica Surface Nanowear in Electrolyte Solutions

The nanoscale wear and friction of silica and silicon nitride surfaces in aqueous electrolyte solutions were investigated by using sharp atomic force microscope (AFM) cantilever tips coated with silicon nitride. Measurements were carried out in aqueous solutions of varying pH and in monovalent and divalent cation chloride and nitrate solutions. The silica surface was shown to wear strongly in solutions of high pH (≈11.0), as expected, but the presence of simple cations, such as Cs(+) and Ca(2+), was shown to dramatically effect the wear depth and friction force for the silica surface. In the case of monovalent cations, their hydration enthalpies correlated well with the wear and friction. The weakest hydrated cation of Cs(+) showed the most significant enhancement of wear and friction. In the case of divalent cations, a complex dependence on the type of cation was found, where the type of anion was also seen to play an important role. The CaCl(2) solution showed the anomalous enhancement of wear depth and friction force, although the solution of Ca(NO(3))(2) did not. The present results obtained with an AFM tip were also compared with previous nanotribology studies of silica surfaces in electrolyte solutions, and possible molecular mechanisms as to why cations enhance the wear and friction were also discussed.

Polyelectrolyte Adsorption onto Oppositely Charged Interfaces: Image-Charge Repulsion and Surface Curvature

We analyze theoretically the influence of low-dielectric boundaries on the adsorption of flexible polyelectrolytes onto planar and spherical oppositely charged surfaces in electrolyte solutions. We rationalize to what extent polymer chains are depleted from adsorbing interfaces by repulsive image forces. We employ the WKB (Wentzel-Kramers-Brillouin) quantum mechanical method for the Green function of the Edwards equation to determine the adsorption equilibrium. Scaling relations are determined for the critical adsorption strength required to initiate polymer adsorption onto these low-dielectric supports. Image-force repulsion shifts the equilibrium toward the desorbed state, demanding larger surface charge densities and polyelectrolyte linear charge densities for the adsorption to take place. The effect is particularly pronounced for a planar interface in a low-salt regime, where a dramatic change in the scaling behavior for the adsorption-desorption transition is predicted. For the adsorbed state, polymers with higher charge densities are displaced further from the interface by image-charge repulsions. We discuss relevant experimental evidence and argue about possible biological applications of the results.

Effect of Counterion and Configurational Entropy on the Surface Tension of Aqueous Solutions of Ionic Surfactant and Electrolyte Mixtures

In order to clarify the adsorption behavior of cationic surfactants on the air/aqueous electrolyte solution surface, we derived the theoretical equation for the surface tension. The equation includes the electrical work required for charging the air/water surface and the work attributable to the configurational entropy in the adsorbed film. By fitting the equation to the experimental data, we determined the binding constant between adsorbed surfactant ion and counterion, and found that the bromide ions, rather than the chloride ions, are preferentially adsorbed by the air/water surface. Furthermore, it was suggested that the contribution of configurational entropy to the surface tension is predominant in the presence of electrolytes because of the increase in the surface density of surfactant molecules associated with decreasing the repulsive interaction between their hydrophilic groups.

Mapping charge-mosaic surfaces in electrolyte solutions using surface charge microscopy

A significant limitation of electrokinetic measurements is that only an average value of the zeta potential/streaming potential is measured—regardless of whether the surface charge distribution is homogeneous or otherwise. However, in real-world situations, nearly all solids (and liquids) of technological significance exhibit surface heterogeneities. To detect heterogeneities in surface charge, analytical tools which provide accurate and spatially resolved information about material surface potential—particularly at microscopic and sub-microscopic resolutions—are needed. A novel AFM-based technique for mapping surface charge domains on heterogeneous surfaces, which we call Surface Charge Microscopy ( SCM), was recently introduced by our research team. It relies on recording colloidal force curves over multiple locations on the substrate surface using small probes. The surface charge characteristics of the heterogeneous substrate are determined from the recorded colloidal force curves, allowing for the surface charge variation to be mapped. In this communication, we briefly review the SCM technique. Examples of results of measurements of the surface interaction forces that were recorded between a silicon nitride AFM cantilever and a multi-phase volcanic rock and heterogeneous surface of bitumen are also given.

Single Rising Bubble Motion in Aqueous Solution of Electrolyte

Effects of electrolytes on bubble coalescence are investigated experimentally from fluid dynamic point of view. Collisions of a bubble with free surface in two kinds of liquids, namely, ultrapure water and sodium chloride solution, are observed using a high speed camera. It is found that the repetitive number of bubble bounces on a free surface in electrolyte solution is larger than that in ultrapure water when the equivalent bubble radii are the same. This result qualitatively shows that electrolyte works to prevent bubbles from coalescing. It is also revealed that the bubbles can coalesce if their Weber numbers, which are based on the radius of curvature of the front side of the bubble and the approach velocity just before the collision with the free surface, are less than a critical Weber number. Furthermore, the critical Weber number decreases with the increase in the concentration of the electrolyte solutions despite the fluid dynamic characteristics, such as bubble rise velocity and bubble shape, are not affected by the addition of the electrolytes. It is concluded that Weber number is the most important parameter to specify the effects of electrolytes on bubble coalescence.

AFM STUDY OF THE EVOLUTION OF DOUBLE LAYER ON SiO2 SURFACE AND SELF-ASSEMBLY MONOLAYER INDUCED BY THE POLARIZATION WITH DC VOLTAGES

AFM was used to study the evolution of double layer on SiO 2 surface and self-assembly monolayer induced by the polarization with DC voltages. Approach force curves were recorded when external DC voltages were applied between solution and SiO 2 or aminopropyltriethoxysilane (APTES) modified SiO 2 surfaces in electrolyte solution. The results showed that the reversing of tip–surface interaction forces between attraction and repulsion could take place only by adjusting DC voltages. It is very useful for biotechnology because the adsorption of biomolecules could be controllable by DC voltages. A model was proposed to explain the behavior of double layer under DC voltages.