Surface Charge Mobility Research Articles

Stability of oil-in-water emulsion with mobile surface charge

Experimental and theoretical studies have been carried out for clarifying the effect of the migration of surface charges on the stability of electrically charged oil-in-water (O/W) emulsion droplets. Application of a 10 kHz +20 V/−20 V square wave to a dense oil-in-water emulsion does not accelerate the demulsification, because the migration of surface charges during a half cycle is compensated during the next half cycle. On the other hand, application of a 20 V/0 V square wave of the same frequency shortens the demulsification time from 1 h to only 1 min, because the migration of the charges during a 20 V half cycle is not completely compensated during the next half cycle. This experimental result confirms that the migration of surface charges induced by the approach of emulsion droplets also plays a crucial role in the coalescence of emulsion droplets under no external electric field. The effect of the migration of surface charges on the stability of oil-in-water emulsion systems under no external electric field has been theoretically estimated in the framework of the DLVO theory by calculating the height of energy barrier preventing the coalescence of two-charged oil droplets. The energy is expressed by U = 2 ε l κ ς 2 ( z d 2 e 2 η + ε l κ k T ) e − κ w z d 2 e 2 η ( 1 + e − κ w ) + ε l κ k T ( 1 − e − κ w ) − A H 12 π w 2 where ɛ l is the dielectric constant of water; κ, the Debye–Hückel reciprocal length; w , the separation distance between two oil surfaces; ζ, the zeta potential; ± z d e are electric charges of adsorbed positive and negative ions on the surfaces; kT, the thermal energy; A H, the Hamaker constant; and η, the total number density of the adsorbed positive and negative ions and is equal to the absolute value of ( ɛ l κ/ z d e) ζ if only positive or negative ions are adsorbed. The height of the energy barrier estimated from the above equation is much different from that estimated from a conventional equation that is derived under the assumption that the electrostatic potential between two-charged droplets is given by the superposition of the electrostatic potentials of the isolated droplets.

Polyion Adsorption onto Catanionic Surfaces. A Monte Carlo Study

The adsorption of a single and negatively charged polyion with varying flexibility onto a surface carrying both negative and positive charges representing a charged membrane surface has been investigated by using a simple model employing Monte Carlo simulations. The polyion was represented by a sequence of negatively charged hard spheres connected with harmonic bonds. The charged surface groups were also represented by charged hard spheres, and they were positioned on a hard surface slightly protruding into the solution. The surface charges were either frozen in a liquidlike structure or laterally mobile. With a large excess of positive surface charges, the classical picture of a strongly adsorbed polyion with an extended and flat configuration emerged. However, adsorption also appeared at a net neutral surface or at a weakly negatively charged surface, and at these conditions the adsorption was stronger with a flexible polyion as compared to a semiflexible one, two features not appearing in simpler models containing homogeneously charged surfaces. The presence of charged surface patches (frozen surface charges) and the ability of polarization of the surface charges (mobile surface charges) are the main reasons for the enhanced adsorption. The stronger adsorption with the flexible chain is caused by its greater ability to spatially correlate with the surface charges.

Effects of Deformability, Uneven Surface Charge Distributions, and Multipole Moments on Biocolloid Electrophoretic Migration

Liposomes have been widely used as cellular and bioparticle mimics due to their lipid bilayer structure and relative ease of production and manipulation. Such biocolloids are frequently characterized by capillary electrophoresis (CE), which promises a wealth of information about such properties as surface charge, composition, and rigidity. The applicability of this information is somewhat limited, however, since it is interpreted with colloidal theories that do not account for the unique properties of biocolloids. In this work, the effects of deformability, mobile surface charges, intrinsic polarizability, and uneven surface charge distributions are incorporated into colloidal theories in order to better model the electrophoretic behaviors of liposomes.

ZnO Nanowire Transistors

ZnO nanowire field-effect transistors (FETs) were fabricated and studied in vacuum and a variety of ambient gases from 5 to 300 K. In air, these n-type nanowire transistors have among the highest mobilities yet reported for ZnO FETs (mu(e) = 13 +/- 5 cm(2) V(-1) s(-1)), with carrier concentrations averaging 5.2 +/- 2.5 x 10(17) cm(-3) and on-off current ratios ranging from 10(5) to 10(7). Four probe measurements show that the resistivity of the Ti/Au-ZnO contacts is 0.002-0.02 Omega.cm. The performance characteristics of the nanowire transistors are intimately tied to the presence and nature of adsorbed surface species. In addition, we describe a dynamic gate effect that seems to involve mobile surface charges and causes hysteresis in the transconductance, among other effects.

Hydrodynamics and electrokinetics of spherical liposomes with coatings of terminally anchored poly(ethylene glycol): Numerically exact electrokinetics with self-consistent mean-field polymer

A detailed theoretical model is presented to interpret electrokinetic experiments performed on colloids with uncharged polymer layers. The methodology removes many of the degrees of freedom that otherwise have to be accounted for by adopting multiple empirical fitting parameters. Furthermore, the level of detail provides a firm basis for future studies examining liposome surface chemistry and charge, surface-charge mobility, and the dynamics of adsorbed polymer on fluidlike membranes. The model predictions are compared with experimental measurements of the electrophoretic mobility of stealth liposomes with molecular weights of terminally anchored poly(ethylene glycol) (PEG) in the range 0.35-10 kg mol(-1) [J.A. Cohen and V.A. Khorosheva, Colloids Surf. A 195, 113 (2001)]. The experimental data are interpreted by drawing upon self-consistent mean-field calculations of the polymer segment density distributions and numerically exact solutions of the governing transport equations [R.J. Hill, D.A. Saville, and W.B. Russel, J. Colloid Interface Sci. 258, 56 (2003)]. The approach leads to excellent agreement between theory and experiment with one adjustable parameter--the hydrodynamic size (Stokes radius) a(s) approximately equal to 0.175 A of the statistical PEG segments with (Kuhn) length l=7.1 A . The remarkably small Stokes radius is demonstrated to be consistent with other applications of the well-known Debye-Brinkman model and, consequently, this work reveals important limitations of the mean-field hydrodynamic model. Despite such limitations, the "full" electrokinetic model is robust in its predictive capacity. The molecular weights of the terminally anchored PEG span the range where the coatings undergo a transition from mushroomlike to brushlike conformations, and the hydrodynamic size and electrophoretic mobility of the liposomes are demonstrated to be sensitive to the PEG chain length and the effects of double-layer polarization.

Laser pump and X-ray probe surface photovoltage spectroscopy on Si(111)

Laser pump and X-ray probe core-level photoemission experiments probe surface photovoltage transients on p-type Si(111) surfaces. The data are consistent with a picture where the dynamics of mobile surface charge dominate the photovoltage shift, with changes in the surface-states charge density of only secondary importance. A value for the equilibrium band bending is determined, which suggests that a residual oxide layer reduces the density of surface states.

Size-dependent mobile surface charge model of cell electrophoresis

A model that accurately predicts the effects of cellular size and electric field strength on electrophoretic mobility has been developed. Previous models have predicted that electrophoretic mobility (EPM) is dependent only on cell surface charge, bath viscosity and ionic strength of the electrolyte. However, careful analysis of experimental data from the literature shows that these models do not accurately depict the relationship between chemically determined surface charge and observed mobility. We propose a new model that accounts for electrically driven redistribution of mobile surface charge islands, such as the recently proposed lipid raft structures. This model predicts electrophoretic mobility as a function of a new dimensionless quantity, A, that incorporates the cell radius, the electric field strength, and the average diameter of charged membrane complexes.

Electrofriction and dynamic stern layers at planar charged surfaces.

Using dynamic simulations, the electrophoretic mobility of counterions at a substrate with fixed or mobile surface charges under the action of a lateral electric field is studied. The lateral charge inhomogeneity and corrugation of the substrate is taken into account. Because of the pronounced electrofriction between counterions and surface ions, a large fraction of counterions is practically immobilized for highly charged substrates. This explains the experimentally observed saturation of the electrophoretic mobility of charged particles in the limit of high surface charge density.

A surface potential change in the membranes of frog skeletal muscle is associated with excitation-contraction coupling.

1. Voltage changes and intramembrane charge movements in the transverse tubule membranes (T-system) of frog fast twitch muscle fibres were compared using the potentiometric dye WW-375 and a Vaseline-gap voltage clamp. As shown previously, the potentiometric dye reports a dynamic surface potential change that occurs on the myoplasmic face of the T-system membranes when the macroscopic potential applied across the surface membrane exceeds the mechanical threshold (about -60 mV). 2. The voltage dependence of the extra surface potential change and charge movement were found to be similar. Both activated with a sigmoid voltage dependence centred around -35 to -40 mV, and saturated at voltages above 0 mV. Both processes inactivated upon sustained depolarization, with a mid-point for inactivation of -40 mV. 3. Pharmacological agents which alter charge movement and excitation-contraction (E-C) coupling altered the non-linear surface potential change in a parallel manner. Perchlorate, which potentiates charge movement and E-C coupling, slowed the activation and deactivation of both charge movement and the non-linear surface potential change at voltages above -40 mV, and shifted the voltage dependence of both processes by 13 14 mV to more negative voltages. Dantrolene, which depresses charge movement and E-C coupling, shifted the voltage dependence of both processes to more positive voltages. Nifedipine, which suppresses charge movement and E-C coupling, reduced the magnitude of both charge movement and the non-linear surface potential change. 4. The non-linear surface potential change remained after the sarcoplasmic reticulum (SR) was depleted of Ca2+, suggesting that it is not a consequence of Ca2+ release. 5. These results suggest that the non-linear surface potential change is closely associated with movements of the voltage sensor (dihydropyridine (DHP) receptor) that control E-C coupling and/or signal transduction across the triadic junction. We propose that the movement of charged intramembrane domains of the DHP receptor which generate charge movement drive a subsequent movement of charged intracellular molecular domains that move within about 1 nm of the T-system membrane to generate a measurable change in surface charge. For example, the postulated mobile surface charges could be on an intracellular domain of the voltage sensor or closely associated protein, or could be a charged molecular domain of a protein that associates/dissociates with T-system membrane or DHP receptor during E-C coupling.

Soil-borne mobile colloids as influenced by water flow and organic carbon

Paucity of understanding mechanisms relevant to the generation of subsurface mobile colloids is a major limitation to our current knowledge of colloid-facilitated contaminant transport. To evaluate the roles of natural organic materials and pore water velocity on mobile colloid generation, colloids generated from 14-m{sup 3} lysimeters containing reconstructed soil profiles were collected and characterized. Colloids generated during low flow rates were 1030% less abundant, contained at least 65% more iron oxides and gibbsite, were 80% smaller, and had 40% greater electrophoretic mobility than colloids generated during higher flow rates. Quartz, kaolinite, and hydroxy-interlayered vermiculite were enriched by at least 32% in colloids generated during faster flow rates. Mobile colloid surface charge was greatly enhanced by organic carbon (OC) coatings. Concentrations of OC associated with mobile colloids were higher than or equal to the OC concentrations existing in the bulk soils from which the mobile colloids were derived. The profound effects of pore water flow rate and OC on mobile colloid generation introduces complexity to this potentially critical, yet poorly understood, component of subsurface contaminant transport. 41 refs., 8 figs., 1 tab.

Deposition and imaging of localized charge on insulator surfaces using a force microscope

A force microscope has been used in a new application to deposit and image localized surface charge on insulators. The lateral resolution for imaging surpasses that of currently available techniques. By applying voltage pulses to an etched nickel microscope tip, micron-sized regions of approximately 2×10−16 C were created on polymethylmethacrylate and single-crystal sapphire surfaces. After depositing the charge, high-contrast images of the charged region were obtained as contours of constant force gradient. The contrast was observed to decay over approximately 1 h, providing evidence for surface charge mobility. The minimum detectable surface charge was estimated to be on the order of 100 electrons.

A Quantitative Investigation of the Open‐Circuit Photovoltage at the Semiconductor/Liquid Interface

A quantitative analysis of the theoretical value for the open‐circuit photovoltage, , of a semiconductor/liquid junction reveals that control of bulk carrier transport properties is crucial to interpreting the observables at the semiconductor/liquid interface. Use of characterized semiconductor samples yields quantitative agreement between the maximum theoretical and experimentally observed values for both n‐Si and p‐Si surfaces in nonaqueous solvents. This accord between theory and experiment rules out deleterious effects of charged surface states on the of these interfaces. Lower than ideal values in other systems might reflect poor diffusion lengths in the semiconductor, classical tunneling over the barrier, or the effects of surface states. The observation of large photovoltages from n‐ and p‐type‐based semiconductor interfaces (n‐Si, p‐Si, , ) in the same solvent is used to rule out a fixed density of charged surface states as the mechanism for obtaining constant photovoltages at these junctions. Direct support for this interpretation is obtained by techniques which verify the presence of mobile surface charge on p‐type Si cathode surfaces in the inversion condition. Thus, control and investigation of bulk semiconductor properties that has been eminently significant to the understanding of p‐n junction solar cells is also crucial to developing a rational understanding of the observables at the semiconductor/liquid interface.

The surface-charge transistor

The surface-charge transistor (SCT) is a new active semiconductor device that can be used to store, transport, and control the transport of nonequilibrium surface charge. In its simplest (discrete) form, it consists of two adjacent storage electrodes and an overlapping control electrode. All of these electrodes are separated from each other and the semiconductor surface by a thin insulating film. This structure will provide storage and controlled transport of mobile surface charge along the semiconductor surface. Data are presented for both the quasi-static characteristics of this device and its transient response. The quasi-static data are compared with an analysis based on the solution of Poisson's equation for the device, and the measured transient response is compared with a numerical solution of the equation of motion for surface-charge transport. The accuracy of these models is attested to by the agreement between theory and experiment. The fabrication of surface-charge transistor structures is also described.

Experimental study of semiconductor surface conductivity

In a first part a method permitting the direct measurement of surface charge mobility, derived from the action of a magnetic field on surface conductivity, is presented and compared to indirect methods. Applied to Si and Ge this measurement shows that the rate of variation of surface conductivity, lower than might be expected from purely electrostatic consideration, is not produced by a trapping of carriers by surface states. It is a consequence of the decreasing of surface mobility with increasing electric field in surface inversion layers. This decrease is less rapid than expected from Schrieffer's theory. In a second part the various time constants of variation of surface conductivity are examined. The long time constant variations in semiconductors covered by a dielectric layer can be interpreted by the influence of mobile charges in the bulk or at the surface of the dielectric. Short time constant variations, in the case of inversion layers conductivity depends on the generation rate of carriers in space charge regions as described by Shockley. Theoretical models are proposed with experimental verification.

Surface dependence of Ge high-frequency high-gain transistors

Low-frequency base current in high-frequency high-gain p-n-p germanium-diffused base transistors can be shifted reversibly at constant voltage and temperature by surface treatment that is believed to change the moisture content of the surface oxide. Changes by a factor of 10 are found with baking in dry oxygen-free hydrogen to increase base current and then washing in water to decrease it. Collector current is nearly unchanged by the surface treatment. The theory of Sah, Noyce and Shockley has been applied to the surface of the emitter barrier. The temperature dependence of the base current at constant emitter-to-base voltage ( V EB ) is in good agreement with the one-half energy-gap dependence predicted by the theory. It is concluded that base current in these transistors at low frequency at and below room temperature is due mainly to recombination at surface centers within the emitter space-charge region. The base current under reverse Veb increases with baking in agreement with the changes observed under forward bias. The reverse current dependence on V EB deviates from the theory. This is attributed to the effect of mobile surface charge. The baking step appears to increase the number of recombination centers without changing the types of centers. In the single-trap approximation, the recombination level is located about 75 mV from the midgap, with a trapping cross-section ratio of order 10 4–10 5. Measurements of junction capacities are interpreted as showing increased positive surface charge in the baked state. Collector current depends on emitter-base voltage and temperature, as expected from ideal p-n-junction theory. It appears that Sah, Noyce and Shockley current at the emitter-barrier surface can account for the temperature dependence and the fall-off with reduction in emitter current of the low-frequency current gain of many types of high-frequency high-gain Ge transistors.