Magnitude Of Surface Charge Density Research Articles

Pore connectivity effects on the internal surface electric charge of mesoporous silica

Nano-scale confinements within mesoporous systems develop overlapping electric double layers (EDL) such that the existing theoretical models cannot predict the electric potential distributions and resulting surface charges. In addition, ionic conditions undergo local variation through connections between the pore voids and pore throats. For the first time in literature, we studied the charging behavior of mesoporous silica in terms of the pore to throat size ratio (Rpt) to characterize the pore connectivity effects, in addition to porosity (є) and pore size (H). Both local and average surface charge densities inside mesoporous silica were examined by varying these parameters systematically. Results showed that the magnitude of surface charge density decreased with increasing EDL overlap and decreasing connectivity effects. We formulized this behavior and developed an extended model to predict mesoporous silica’s internal charge as a function of porosity, pore size, and pore to throat size ratio.

Image-induced overcharging in the weakly charged surfaces

Electrostatic interactions at dielectric boundaries can bring about novel properties which are vital not only to understand the complex behavior at biological interfaces but also to design desirable interfacial processes for a broad variety of practical applications. Here, we aim to provide comprehensive information by Monte Carlo simulations about the annihilation and formation of the image-induced overcharging effect, a new class of charge accumulation phenomena resulting from an excess of adsorbed ions on the like-charged surfaces. It is observed that whether the image-charge interactions are repulsive or attractive, the buildup of overcharging depends critically on both the concentration and the charge-asymmetry of electrolytes. On the other hand, overcharging is gradually cancelled with an increase in the size of ions and the concentration of monovalent salt as well as in the magnitude of surface charge density. Also, our simulations show that the image-induced overcharging effect cannot promote charge reversal. These findings are especially relevant to colloidal stability and dispersibility, and will hopefully serve as a foundation for the development of adequate theories.

Effect of thermodiffusion on pH-regulated surface charge properties of nanoparticle.

Surface properties of nanoparticle are of high importance in the field of biotechnology, drug delivery and micro/nanofabrication. In this article, we developed a comprehensive theoretical model and subsequently solved that numerically to study the effect of thermodiffusion of ions on surface charge properties of nanoparticle. The theoretical study has been done considering silica nanoparticle for two aqueous solutions NaCl and KCl. The effect of solution pH in conjunction with nanoparticle temperature on surface charge density has been obtained for different salt concentrations (1, 10 and 100 mM) and nanoparticle size (diameter of 2 and 100 nm). It is observed from the results that with increasing temperature of the nanoparticle, the negative surface charge density gets higher due to increasing thermodiffusion effect. It is also found out that the magnitude of surface charge density is higher for KCl solution than NaCl solution under same condition which is attributed mostly due to less thermodiffusion of counterions for KCl than NaCl. Present study also shows that magnitude of surface charge density decreases with increasing nanoparticle size until it reaches a limiting value (called critical size) above which the effect of nanoparticle size on surface charge density is insignificant.

The coupling of surface charge and boundary slip at the solid–liquid interface and their combined effect on fluid drag: A review

Fluid drag of micro/nano fluidic systems has inspired wide scientific interest. Surface charge and boundary slip at the solid–liquid interface are believed to affect fluid drag. This review summarizes the recent studies on the coupling of surface charge and slip, and their combined effect on fluid drag at micro/nano scale. The effect of pH on surface charge of borosilicate glass and silica surfaces in deionized (DI) water and saline solution is discussed using a method based on colloidal probe atomic force microscopy (AFM). The boundary slip of various oil–solid interfaces are discussed for samples with different degrees of oleophobicity prepared by nanoparticle-binder system. By changing the pH of solution or applying an electric field, effect of surface charge on slip of a smooth hydrophobic octadecyltrichlorosilane (OTS) in DI water and saline solution is studied. A theoretical model incorporating the coupling relationship between surface charge and slip is used to discuss the combined effect of surface charge-induced electric double layer (EDL) and slip on fluid drag of pressure-driven flow in a one-dimensional parallel-plates microchannel. A theoretical method is used to reduce the fluid drag. The studies show that the increasing magnitude of surface charge density leads to a decrease in slip length. The surface charge results in a larger fluid drag, and the coupling of surface charge and slip can further increase the fluid drag. Surface charge-induced EDLs with asymmetric zeta potentials can effectively reduce the fluid drag.

Water desalination by a designed nanofilter of graphene-charged carbon nanotube: A molecular dynamics study

Using molecular dynamics simulations, we show that a designed graphene-charged carbon nanotube (CNT) membrane can act as a nanofilter with the efficiency more than 90%, to separate Na+/Mg+2/Fe+3 and Cl− ions from NaCl, MgCl2 and FeCl3 solutions. It is observed that there are two significant factors in ion separation process: the magnitude of charge density which covers the surface of the CNT and the velocity of the movable wall. The ion separation is improved by increasing the magnitude of surface charge density and decreasing the velocity of the movable wall. Also, it is found that, reducing the positive surface charge of the CNT will increase the percentage of passed water molecules through the CNT.

Electrophoretic mobility of latex spheres in the presence of divalent ions: experiments and modeling

Electrophoretic mobilities (EPM) of negatively charged latex spheres were measured as a function of salt type and salt concentration. The measured values of EPM were analyzed using a standard electrokinetic model that includes double layer relaxation and the Poisson–Boltzmann model of diffuse double layer. Calculated values of EPM were in good agreement with experimental data taken in simple 1:1 (KCl) and 1:2 (Na2SO4) electrolyte solutions without using any fit parameters. For 2:1 electrolytes (CaCl2 and MgCl2), however, the magnitude of EPM calculated by the model was higher than the measured values of EPM at higher electrolyte concentrations. The difference between measured and calculated EPM was reduced by assuming the distance of slipping plane x s = 0.25 nm or by assuming the decrease of the magnitude of surface charge density from −0.07 to −0.025 C/m2. These are probably due to the accumulation of divalent counterions in the vicinity of a particle’s surface.

Approximate Analytical Expression for Surface Potential as a Function of Surface Charge Density

The surface potential is derived as a function of surface charge density for a nonplanar solid surface immersed in an arbitrary a:b electrolyte solution. This relation is essential to the evaluation of the basic thermodynamic properties of an electrical double layer. The approximate analytical expression is based on a perturbation method characterized by the following ratio: radius of a surface/thickness of a double layer. An attempt is also made to take the inverse of the surface-charge density-surface potential relation for a spherical surface immersed in both 1:1 and 1:2 electrolyte solutions as reported by H. Ohshima, T. W. Healy, and L. R. White ( J. Colloid Interface Sci. 90, 17, 1982). For a 1:1 electrolyte solution, the maximum deviation from the exact value is on the order of 3% for a double layer of thickness smaller than one half the radius of a surface, and this is almost independent of the magnitude of surface charge density. For a 1:2 electrolyte solution, the maximal deviation appears to increase slightly with surface charge density. At a relatively low surface charge density, the deviation is on the order of 3% for a double layer having thickness less than one-half the radius of a sphere, and it is on the order of 5% at a relatively high surface charge density.

Surface charges, the heterogeneous lateral distribution of the two photosystems, and thylakoid stacking

Spinach grown in light which preferentially excites Photosystem II (PSII-light) had increased amounts of PS I reaction centres on a chlorophyll (Chl) basis, a higher Chl a/Chl b ratio but less extensive thylakoid stacking, when compared with growth in light preferentially favouring excitation of PS I (PSI-light). Surprisingly, despite the marked difference in the extent of thylakoid stacking, the overall surface charge densities of thylakoid membranes were similar for plants grown in PSII- and PSI-light. The amounts of PS II reaction centres, cytochrome f and adenosine triphosphate synthase were also similar on a Chl basis for the two growth light qualities. Thylakoids of the Chl-b-less barley mutant had a smaller magnitude of surface charge density, although the extent of thylakoid stacking (Goodchild, D.J., Highkin, H.R. and Boardman, N.K. (1966) Exp. Cell Res. 43, 684–688) was much smaller, compared with wild-type barley. The results are interpreted in terms of the possible mechanisms governing the heterogeneous lateral distribution of the two photosystems. They suggest that PS I is laterally segregated to non-appressed thylakoid membrane domains because of the excess negative charges it carries on the outer membrane surface, whereas PS II is mainly sequestered into the appressed regions because of an association with its Chl a/b-proteins which appear to contribute more to van der Waals attraction than hitherto recognized.

In-situ insulator surface charge measurements in dielectric bridged vacuum gaps using an electrostatic probe

Surface charge measurements on alumina and polymer insulators were carried out after stressing them with DC voltages in a high vacuum. The order of magnitude of surface charge density was found to be the same for materials with supposedly widely varying secondary-emission yields. Surface coatings on alumina insulators reduced charge accumulation because of increased surface conductivity and/or reduced secondary-emission yield, which led to significant improvement in voltage hold-off for alumina ceramics. Removing the cathode triple junction from the main body of the cylindrical insulator, reducing the X-ray activity in the gap, or relieving the stress at the critical junction did not significantly alter the surface charge characteristics of cylindrical insulators. Wet hydrogen firing of plain alumina reduced the voltage hold-off by 25% without altering the surface charge density. It is postulated that the charging of insulators in bridged vacuum gaps with DC stresses is due to internal secondary emission produced by ionization of the lattice in the surface layer of the insulating material, by primary electrons injected at the cathode triple junction. This mechanism of charge production differs from the current models where charging is believed to occur due to electrons hopping along the surface/vacuum interface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Modification of electric field at the solid insulator–vacuum interface arising from surface charges on the solid insulator

The surface of a solid insulator in vacuum (and in a high gas pressure) becomes electrically charged when subjected to a high voltage stress. The surface charge density is proportional to the applied voltage. The magnitude of the surface charge density depends on the secondary electron emission characteristic, the geometrical shape, and the material of the insulator. The field enchancement, contributing to a lower withstand voltage of solid insulators in vacuum (and in high pressure gases), due to the presence of surface charge is computed along the surface of the solid insulator and at the triple electrode-solid insulator-vacuum junction near the anode and cathode. Different patterns of surface charge density distribution has been considered in order to evaluate their effects on the field enhancement. The polarity, the magnitude and the shape of the distribution of the surface charge density has been found to have a considerable effect on the field enhancement. The observed influence of the charge distribution patterns on the electric field suggests that in order to obtain the experimentally observed field enhancements, the insulator surface must possess a positive surface charge except in the region close to the anode, where it is negatively charged. The electric field and the potential distributions of the interfacial boundary between the solid insulator and vacuum is computed for different realistic surface charge distributions. The computed electric field at the triple junction for varying values of the dc applied field is then compared with the measured values reported in the literature. Good agreement is obtained. The effects of the magnitude of surface charge density, the applied field and the length of the solid insulator on the field enhancement at the triple junctions are also investigated. Nine different solid insulator materials having a relative permittivity in the range 2.1–13 and electron impact energies in the range 20–60 eV are examined. The effect of the surface charge on the withstand voltage of the insulating vacuum gap bridged by a solid insulator is discussed.