Hydration and distribution of ions at the mica-water interface

Molecular-scale structures of mica surfaces in electrolyte solutions reveal how ion and interfacial hydration control cation adsorption. Key differences are obtained for Rb+and Sr2+ using resonant anomalous x-ray reflectivity: Rb+ adsorbs in a partially hydrated state and incompletely compensates the surface charge, but Sr2+ adsorbs in both fully and partially hydrated states while achieving full charge compensation. These differences are driven by balancing the energy cost of disrupting ion and interface hydration with the electrostatic attraction between the cation and charged surface.     

Electrostatic behavior of different fines added to a Faraday cup fluidized bed

Experiments were performed in a Faraday cup fluidization column to determine the changes in the electrostatic charges on various fine particles after their addition to gas–solid fluidized beds to better understand their role in influencing electrostatic charge generation/dissipation. The charges transported by different fines (Larostat 519 antistatic agent, glass beads (GB), silver-coated GB, catalyst and silica particles) were determined after their injection into an initially charged bed of much larger mono-sized particles (GB or polyethylene) for a range of relative humidities. Entrained fines carry significant charges out of the column, therefore leaving a net charge behind, with the polarity and quantity of charge depending on the size, physical properties and chemical structure of the particles, and on the moisture content of the fluidizing gas.     

Low-energy electron beam on an insulator surface: Impact of the charging process on the diffraction by mica muscovite

The expected low-energy electron diffraction by an insulator is deduced from the consequence of the surface charge distribution on the diffraction process. If the yield of secondary electron emission is greater than unity, the surface reaches electrostatic equilibrium and charges positively. Then incident electrons are simply accelerated and their wavelength is shortened, so the diffraction condition is modified. We show that this modification is strictly compensated by the deviation of the diffracted backward electron crossing the charged surface. The diffraction pattern displays the same geometry, size and symmetry but the diffracted intensity is modified. Through this process the low energy electron diffraction is shown to be an efficient tool to investigate the charging process induced on insulator surfaces by an electron beam. This is exemplified with the surface of mica muscovite, where we relate the oscillation of the surface charge to the evolution of the intensity of the diffraction spots.     

Charge inversion at minute electrolyte concentrations

Anionic dimyristoylphosphatidic acid monolayers spread on LaCl3 solutions reveal strong cation adsorption and a sharp transition to surface overcharging at unexpectedly low bulk salt concentrations. We determine the surface accumulation of La3+ with anomalous x-ray reflectivity and find that La3+ compensates the lipid surface charge by forming a Stern layer with approximately 1 La3+ ion per 3 lipids below a critical bulk concentration, ct approximately 500 nM. Above ct, the surface concentration of La3+ increases to a saturation level with approximately 1 La3+ per lipid, thus implying that the total electric charge of the La3+ exceeds the surface charge. This overcharge is observed at approximately 4 orders of magnitude lower concentration than predicted in ion-ion correlation theories. We suggest that transverse electrostatic correlations between mobile ions and surface charges (interfacial Bjerrum pairing) may contribute to the charge inversion.     

Counterion release and electrostatic adsorption

The effective charge of a rigid polyelectrolyte (PE) approaching an oppositely charged surface is studied. The cases of a weak (annealed) and strongly charged PE with condensed counterions (such as DNA) are discussed. In the most interesting case of the adsorption onto a substrate of low dielectric constant (such as a lipid membrane or a mica sheet) the condensed counterions are not always released as the PE approaches the substrate, because of the major importance of the image-charge effect. For the adsorption onto a surface with freely moving charges, the image-charge effect becomes less important and full release is often expected.     

Kelvin probe force microscopy and its application

Kelvin probe force microscopy (KPFM) is a tool that enables nanometer-scale imaging of the surface potential on a broad range of materials. KPFM measurements require an understanding of both the details of the instruments and the physics of the measurements to obtain optimal results. The first part of this review will introduce the principles of KPFM and compare KPFM to other surface work function and potential measurement tools, including the Kelvin probe (KP), photoemission spectroscopy (PES), and scanning electron microscopy (SEM) with an electron beam induced current (EBIC) measurement system. The concept of local contact potential difference (LCPD), important for understanding atomic resolution KPFM, is discussed. The second part of this review explores three applications of KPFM: metallic nanostructures, semiconductor materials, and electrical devices.     

DNA adsorption and desorption on mica surface studied by atomic force microscopy

The adsorption of DNA molecules on mica surface and the following desorption of DNA molecules at ethanol-mica interface were studied using atomic force microscopy. By changing DNA concentration, different morphologies on mica surface have been observed. A very uniform and orderly monolayer of DNA molecules was constructed on the mica surface with a DNA concentration of 30 ng/μL. When the samples were immersed into ethanol for about 15 min, various desorption degree of DNA from mica (0-99%) was achieved. It was found that with the increase of DNA concentration, the desorption degree of DNA from the mica at ethanol-mica interface decreased. And when the uniform and orderly DNA monolayers were formed on the mica surface, almost no DNA molecule desorbed from the mica surface in this process. The results indicated that the uniform and orderly DNA monolayer is one of the most stable DNA structures formed on the mica surface. In addition, we have studied the structure change of DNA molecules after desorbed from the mica surface with atomic force microscopy, and found that the desorption might be ascribed to the ethanol-induced DNA condensation.     

H2O在Al(111)表面吸附的量子化学研究

用量子化学从头算方法，以原子族Ａｌ１０模拟表面，研究了水在Ａｌ（１１１）表面上不同吸附拉的吸附情况，计算得到了稳定的吸附构型和结合能，结果表明：顶位是其最佳吸附位，而且水在表面能以两种取向被吸附，距表面较远时，Ｈ端靠近表面，然后跨过一能垒到达最佳吸附位，此时氧端靠近表面。在吸附过程中，水向表面转移电荷，导致表面功函降低，在氧原子不加极化函数进，水分子的二次轴垂直于表面时能量最低；当考虑中氧的ｄ轨道     

Debye-Hückel theory for slab geometries

The electrostatic interaction of charged particles through or at a low-dielectric slab, such as a lipid bilayer immersed in water or a self-assembled monolayer (SAM) on a metal substrate, is considered theoretically in the presence of salt within the Gaussian approximation using a generalized Green's formalism. A number of separate situations are discussed: i) The presence of a low-dielectric slab leads to pronounced interactions of a single charge with the slab via the formation of polarization surface charges. For SAMs on metal substrates, there is an intricate crossover from image-charge attraction to the metal substrate (for large distances) to image-charge repulsion from the SAM (for small distances) with a stable minimum at a distance of roughly 20 times the thickness of the hydrophobic film. For bilayers in water, the interaction of a single charge is always repulsive. ii) The surface potential of a SAM is calculated for the case when the hydrophobic layer contains dipole moments, which might explain the recently observed long-ranged repulsion of hydrophobic scanning tips from PEG-terminated SAMs on gold. iii) The interaction between charged particles through the bilayer is weakened. Oppositely charged particles still attract each other through the membrane. The free-energy minimum occurs as a result of the competition between self-repulsion from the slab and interparticle attraction and is located at a separation from the membrane surface which equals 15 times the membrane thickness. iv) Surface charges on the two surfaces of a bilayer attract each other through the bilayer unless the surface charge densities are the same, even if the signs are the same. v) All these effects are strongly influenced by the presence of salt. Received 25 January 2000     

Visualization of human plasma fibrinogen adsorbed on highly oriented pyrolytic graphite by scanning probe microscopy

Human plasma fibrinogen (HPF) was observed by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) conducted in non-contact mode. The HPF was adsorbed on a highly oriented pyrolytic graphite (HOPG) substrate as single molecules, as aggregated bundles, and as aggregated fibers. Topographic and phase images confirmed structural changes in the HPF after exposure to air, while topographic and KPFM images confirmed fibers with the width of a single HPF molecule. Additionally, KPFM confirmed the surface potential difference between the HPF and the HOPG, and periodical potential drop reflecting the E and D domains in the fiber.     

Flow microcalorimetry: Experimental development and application to adsorption of heavy metal cations on silica

Sorption of metal ions at the oxide mineral-water interfaces is a complex process involving many various contributions that can be explained using thermodynamics. The aim of this study is to obtain experimental thermodynamic data on adsorption of two heavy metal ions (Cd(II) and Pb(II)) on macroporous silica. Thermal signals of adsorption are studied by flow microcalorimetry which has been preferred because physico-chemicals conditions (pH, equilibrium concentration,…) can be controlled (the routine configuration was optimized in order to get a very stable pressure baseline and avoid important fluctuations in the determination of heat). Mechanisms driving the adsorption have been explained. The calculation of the effective charge of ions determined from the speciation diagrams and of the surface charges shows that the interactions between the two metals and the silica surface are mainly electrostatic. The differential enthalpies of adsorption Δ_adsH have been experimentally measured. The heat of cadmium adsorption is low, endothermic and quantitatively equivalent to that of desorption. In the case of lead, the adsorption is athermal. Free energies and entropic effects related to cation adsorption have then been deduced according to the Gibbs’law. The entropy is positive during the adsorption process and at this temperature (298 K) is quite equivalent to free energy. This entropy is due to modification of hydration shell of the ions during their insertion into the interfacial region.     

Effective Electrostatic Interactions Between Two Overall Neutral Surfaces with Quenched Charge Heterogeneity Over Atomic Length Scale

Using Monte Carlo results as a reference, a classical density functional theory (CDFT) is shown to reliably predict the forces between two heterogeneously charged surfaces immersed in an electrolyte solution, whereas the Poisson–Boltzmann (PB) theory is demonstrated to deteriorate obviously for the same system even if the system parameters considered fall within the validity range of the PB theory in the homogeneously charged surfaces. By applying the tested CDFT, we study the effective electrostatic potential of mean force (EPMF) between two face–face planar and hard surfaces of zero net charge on which positive and negative charges are separated and considered to present as discontinuous spots on the inside edges of the two surfaces. Main conclusions are summarized as follows: (i) strength of the EPMF in the surface charge separation case is very sensitively and positively correlated with the surface charge separation level and valency of the salt ion. Particularly, the charge separation level and the salt ion valency have a synergistic effect, which makes high limit of the EPMF strength in the surface charge separation case significantly go beyond that of the ideal homogeneously charged surface counterpart at average surface charge density similar to the average surface positive or negative charge density in the charge separation case. (ii) The surface charge distribution patterns mainly influence sign of the EPMF: symmetrical and asymmetrical patterns induce repulsive and attractive (at small distances) EPMF, respectively; but with low valency salt ions and low charge separation level the opposite may be the case. With simultaneous presence of both higher valency cation and anion, the EPMF can be repulsive at intermediate distances for asymmetrical patterns. (iii) Salt ion size has a significant impact, which makes the EPMF tend to become more and more repulsive with the ion diameter regardless of the surface charge distribution patterns and the valency of the salt ion; whereas if the 1:1 type electrolyte and the symmetrical patterns are considered, then the opposite may be the case. All of these findings can be explained self-consistently from several perspectives: an excess adsorption of the salt ions (induced by the surface charge separation) serving to raise the osmotic pressure between the plates, configuration fine-tuning in the thinner ion adsorption layer driven by the energy decrease principle, direct Coulombic interactions operating between charged objects on the two face-to-face plates involved, and net charge strength in the ion adsorption layer responsible for the net electrostatic repulsion.     

Instability of a bubble in a liquid dielectric in an external electrostatic field

The sizes, charges, and number of daughter bubbles emitted during the development of instability with respect to the polarization charge in a uniform electrostatic field of a gas bubble in a liquid dielectric are found on the basis of the Onsager principle of minimum energy dissipation for nonequilibrium processes. Zh. Tekh. Fiz. 69, 10–13 (November 1999)     

Electrical characterisation of local electronic properties of self-assembled semiconductor nanostructures using AFM

Scanning capacitance microscopy (SCM) and Kelvin probe force microscopy (KPFM) have been used to characterise self-assembled Ge nanoislands embedded in silicon. High contrast in capacitance and in surface potential are found between areas with nanostructures and areas without any nanostructures. The local dC/dV spectroscopy shows flat band voltage shift attributed to the presence of electric charges in the nanostructures. The KPFM contrast has been correlated with the band structure offsets between the nanostructure and the matrix barrier. Effects of the charging have been measured from the dC/dV curve and discussed in terms of the wetting layer influence that contributes to the escape of the charges when percolation of charges is observed.     

Adsorption of polyelectrolytes at an oppositely charged surface

We develop a scaling theory of polyelectrolyte adsorption at an oppositely charged surface. At low surface charge densities, the thickness of the adsorbed layer is determined by the balance between electrostatic attraction to the charged surface and chain entropy. At high surface charge densities, it is determined by the balance between electrostatic attraction and short-range monomer-monomer repulsion. These different stabilizing mechanisms result in the nonmonotonic dependence of the layer thickness on the surface charge density.     

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100 nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.     

基于AFM的固液界面表面电荷密度测量方法

固液界面的表面电荷会影响微纳流体系统的流体阻力,因此如何测量固液界面的表面电荷密度以及分析表面电荷的产生机理对于研究表面电荷对流体阻力的影响具有较大的意义。提出了一种基于接触式AFM的固液界面表面电荷密度测量方法。基于该方法测量了浸在去离子水和0.01 mol/L的NaCl溶液中的高硼硅玻璃和二氧化硅样本的表面电荷密度,并研究了溶液pH值对表面电荷的影响。研究结果表明高硼硅玻璃和二氧化硅由于表面硅烷基的电离带负电。溶液pH值和离子浓度的增加都会增加浸在去离子水和0.01 mol/L的NaCl溶液中高硼硅玻璃和二氧化硅的表面电荷密度的绝对值。     

Self-consistent field theory of adsorption of flexible polyelectrolytes onto an oppositely charged sphere

The adsorption of flexible polyelectrolyte （PE） with the smeared charge distribution onto an oppositely charged sphere immersed in a PE solution is studied numerically with the continuum self-consistent field theory. The power law scaling relationships between the boundary layer thickness and the surface charge density and the charge fraction of PE chains revealed in the study are in good agreement with the existing analytical result. The curvature effect on the degree of charge compensation of the total amount of charges on the adsorbed PE chains over the surface charges is examined, and a clear understanding of it based on the dependences of the degree of charge compensation on the surface charge density and the charge fraction of PE chains is established.     

Experimental and Theoretical Studies on the Interaction between Isonicotinic Acid Molecules and Silver Nanoclusters

The present paper describes the studies on the adsorption behavior and charge transfer from isonicotinic acid to silver nanoparticles with experiment and theory. Compared with UV–Visible adsorption spectrum, the adsorption spectrum of Ih-Ag₁₃ cluster was quite good agreed with that of silver colloidal nanoparticle. So that one Ag₁₃ cluster as a substrate was used to simulate Raman frequencies of the adsorption configuration. Here, it is demonstrated the calculated Raman spectra are in good agreement with experimental results. The analysis of Mulliken charge was obtained by density functional theory, which indicated the charge characteristics of Ag₁₃ nanoparticle. Once isonicotinic acid molecules were adsorbed on sliver clusters, the charges transfer from isonicotinic acid to silver clusters, so that the surface charges of silver clusters are uneven.     

Water contact angles on charged surfaces in aerosols

Interactions between water and solid substrates are of fundamental importance to various processes in nature and industry. Electric control is widely used to modify interfacial water, where the influence of surface charges is inevitable. Here we obtain positively and negatively charged surfaces using LiTaO₃ crystals and observe that a large net surface charge up to 0.1 C/m² can nominally change the contact angles of pure water droplets comparing to the same uncharged surface. However, even a small amount of surface charge can efficiently increase the water contact angle in the presence of aerosols. Our results indicate that such surface charges can hardly affect the structure of interfacial water molecular layers and the morphology of the macroscopic droplet, while adsorption of a small amount of organic contaminants from aerosols with the help of Coulomb attraction can notably decrease the wettability of solid surface. Our results not only provide a fundamental understanding of the interactions between charged surfaces and water, but also help to develop new techniques on electric control of wettability and microfluidics in real aerosol environments.