A mechanism of separation in electrostatic ion chromatography

The retention mechanism of electrostatic ion chromatography (EIC) is currently under debate and is the focus of this paper. A comprehensive set of retention data has been obtained on a C18 column coated with the zwitterionic surfactant 3-(N,N-dimethylmyristylammonio)propanesulfonate used with a range of mobile phases in which both the mobile-phase anion and cation have been varied systematically. Electro-osmotic flow measurements were also obtained on fused-silica capillaries coated with the zwitterion (and also some monofunctional surfactants) and were used to evaluate the nature of the surface charge on the layer of adsorbed surfactant in the presence of various background electrolytes. A new retention mechanism for EIC was developed on the basis of these data. This mechanism proposes that equilibration of the bound zwitterions with a mobile phase containing a suitable electrolyte causes the establishment of a charged layer created by the terminal sulfonate groups of the zwitterion, which acts as a Donnan membrane. The magnitude and polarity of the charge on this membrane depends on the nature of the mobile-phase ions. The Donnan membrane exerts weak electrostatic repulsion or attraction effects on analyte anions. A second component of the retention mechanism is chaotropic interaction of the analyte anion with the quaternary ammonium functional group of the zwitterion. This interaction exerts the major effect on the separation selectivity of EIC, such that analyte anions are eluted in order of increasing chaotropic interactions in accordance with the Hofmeister series.     

Control of electrostatic charge on particles by impact charging

The control of electrostatic charge on particles in gas–solids pipe flow has been studied experimentally and theoretically. Alumina particles of 3.3 μm in count median diameter were dispersed in airflow and pneumatically transported in the dilute phase. Five different materials were used for the transport pipes, and the relationships between the particle charge and the pipe length were obtained. The polarity and the amount of particle charge were found to depend on the pipe material and the length. In order to control the particle charge, a system combining two different pipe materials was proposed depending on the particle-charging characteristics. The charge controlled by this method was in good agreement with the theoretical calculation. Furthermore, it was found that the distribution of particle charge as well as the average can be controlled.     

Process tomography system by electrostatic charge carried by particles

Two methods, the back-projection (BP) method and the least-squares (LS) method, were applied to the electrodynamic tomography system. Electrostatic charges were assumed to exist in the sensing zone, and their images were reconstructed using these methods. The reconstructed images were compared with the original ones. The BP method is accurate in detecting the position of the electrostatic charge and is capable of detecting the size of the object. The BP method could not distinguish two charges at separate points in the sensing zone. The LS method could differentiate two points of charge, but the reconstructed images were of poor quality due to the large pixel size. A reconstruction algorithm combining the two methods produced better results.     

Investigation of granular surface roughness effect on electrostatic charge generation

Electrostatic charge generation is a multivariable and complex issue whose working mechanism has never been fully understood. The objective of this paper is to investigate the effect of granule surface roughness on electrostatic charge generation. Two kinds of granule material, Polyvinyl chloride (PVC) and polypropylene (PP) were used with the granule size of 4 mm diameter, 2 mm height and the shape was cylinder or semi-cylinder. The working surfaces were grounded and roughness ranged from 0.140 to 8.600 μm. It was found that uneven surfaces tended to give rise to voids between two solids, where air stored in the voids was able to accelerate discharging. With the same roughness, PVC tended to generate more electrostatic charge than PP by one order of magnitude. For both materials, electrostatic charge generation first increased with surface roughness and then decreased. The maximum electrostatic charge generated was found to occur when the effects of interaction, contact area and voids discharging were at equilibrium. With the combined effect of humidity, surface roughness and contact area, highest electrostatics generation occurred near the mid-roughness tested in this work. Humidity had more effect on electrostatic charge generation as the granule working surface had lower roughness.     

Inherently tunable electrostatic assembly of membrane proteins

Membrane proteins are a class of nanoscopic entities that control the matter, energy, and information transport across cellular boundaries. Electrostatic interactions are shown to direct the rapid co-assembly of proteorhodopsin (PR) and lipids into long-range crystalline arrays. The roles of inherent charge variations on lipid membranes and PR variants with different compositions are examined by tuning recombinant PR variants with different extramembrane domain sizes and charged amino acid substitutions, lipid membrane compositions, and lipid-to-PR stoichiometric ratios. Rational control of this predominantly electrostatic assembly for PR crystallization is demonstrated, and the same principles should be applicable to the assembly and crystallization of other integral membrane proteins.     

Measurement of electrostatic dipoles and net charge on particles suspended in air

Explanations of electrostatic behaviour of individual particulates have always invoked net electrostatic charge and neglected any fixed dipoles arising from nonuniform distribution of charge on their surface. Here for the first time, techniques are described to measure both net charge of each particle and its dipole moment by sampling, photography, and movement analysis of particles suspended in air. In this study, spherical dielectric particles were subjected to a non-uniform electric field (around a probe) and their trajectory was followed with a multi-flash high-speed video camera. These first results support the existence of fixed or “permanent” dipoles, on tribocharged particles.     

FCI — Field charge interaction program

A computer simulation code FCI for electron beam devices has been developed. This program solves the self-consistent interaction problem between charge and electromagnetic field. A simple example is given.     

Effect of electrostatic charge of particles on hydrodynamics of gas-solid fluidized beds

The aim of this work was to investigate effect of electrostatic charge of particles on the fluidization hydrodynamics. Behavior of bubbles in beds of polyethylene particles was studied through analysis of pressure fluctuations in the frequency domain. Fluidized beds of uncharged, pre-charged and bed-charged particles were used in the experiments. Results revealed that in the bed of pre-charged particles, compared to uncharged experiments, particle-particle repulsive force increases the bed voidage and reduces equilibrium bubble size while the transition velocity to turbulent fluidization is decreased. In the case of bed-charged particles, at low gas velocities bubble fraction is greater compare to the other cases due to faster bubble coalescence in the presence of particle-wall attractive electrostatic force. Electrostatic charge of bulk increases by increasing the gas velocity. At high gas velocities, the repulsion force between highly charged particles overcomes the particle-wall effect on bubble formation and reduces the bubble size to less than in uncharged experiments. Accumulation of particles near the wall in the bed od bed-charged particles affects the hydrodynamics in two ways: first it accelerates bubble growth via bubble coalescence at low gas velocities, second it limits the bubble growth and reduces the transition velocity to turbulent regime to a value less than for pre-charged particles.     

Determination of effective protein charge by capillary electrophoresis: effects of charge regulation in the analysis of charge ladders

Protein charge ladders are an effective tool for measuring protein charge and studying electrostatic interactions. However, previous analyses have neglected the effects of charge regulation, the alteration in the extent of amino acid ionization associated with differences between the pH at the protein surface and in the bulk solution. Experimental data were obtained with charge ladders constructed from bovine carbonic anhydrase. The protein charge for each element in the ladder was calculated from the protein electrophoretic mobility as measured by capillary electrophoresis using the hindrance factor for a hard sphere with equivalent hydrodynamic radius. The protein charge was also evaluated theoretically from the amino acid sequence by assuming a Boltzmann distribution in the hydrogen ion concentration. The calculations were in excellent agreement with the data, demonstrating the importance of charge regulation on the net protein charge. These results have important implications for the use of charge ladders to evaluate effective protein charge in solution.     

Local charge transport in two-dimensional PbSe nanocrystal arrays studied by electrostatic force microscopy

Two-dimensional PbSe nanocrystal arrays on silicon nitride membranes were investigated using electrostatic force microscopy (EFM) and transmission electron microscopy (TEM). Changes in lattice and transport properties upon annealing in a vacuum were revealed. Local charge transport behavior was directly imaged by EFM and correlated to nanopatterns observed with TEM. Charge transport through nanochannels in complex two-dimensional nanocrystal networks was identified. Our results demonstrate the importance of measurements of local transport details complementary to the conventional current-voltage (I-V) measurements.     

Influence of the size and charge of nonstoichiometric silver sulfide nanoparticles on their interaction with blood cells

Silver sulfide (Ag₂S) nanoparticles synthesized using different precursors have been characterized by dynamic light scattering measurements and high-resolution transmission electron microscopy. In addition to Ag₂S nanoparticles, we have detected Ag₂S/Ag heterostructures. Using optical microscopy, we have examined interaction of the nanoparticles with red cells of peripheral blood. The results of the interaction have been shown to depend on the particle size and charge. A red cell solution containing large, negatively charged particles coagulated, whereas small, positively charged Ag₂S nanoparticles were concentrated around red cells.     

Protein aggregation in high-performance liquid chromatography: hydrophobic interaction chromatography of beta-lactoglobulin A

Aggregation of beta-lactoglobulin A under acidic buffer conditions was studied in hydrophobic interaction chromatography. At high ammonium sulfate concentrations, pH 4.5 and 4 degrees C, UV chromatograms revealed a maximum of three peaks for beta-lactoglobulin A concentrations greater than 5 mg/mL, suggesting three distinct aggregate species. The size of the smallest aggregate (tetramer) and its stoichiometric relationship to the other two aggregates (octamer and dodecamer) were determined from the chromatographic data and a simple mass balance model. These stoichiometries agreed with those determined in a separate study by on-line low-angle laser light scattering. In addition, the association constants describing the formation of octamer from two tetramer molecules and the formation of dodecamer from the octameric and tetrameric species were found to be (2.4 +/- 0.5) X 10(4) M-1 and (3.3 +/- 0.8) X 10(3) M-1, respectively. Analysis of the beta-lactoglobulin A system is based on a model in which aggregates form in solution upon injection before adsorbing to the column matrix. The column retains those species formed in solution and induces little change in the relative amounts of each species. These results illustrate another example by which multiple peaks can arise in high-performance liquid chromatography, beyond the previously described studies of protein conformational changes during chromatography.     

Modification of electrostatic charge on inhaled carrier lactose particles by addition of fine particles

Triboelectrification affects particle adhesion and agglomeration and hence the formulation, manufacture, and use of dry powder inhaler (DPI) devices. Electrostatic charge measurement of two component mixes of spray-dried or crystalline lactose fine particles (< 10 microns) 0, 5, 10, 15, 20, and 30% w/w with spray-dried or crystalline lactose 63-90 microns, respectively, has been undertaken using a system incorporating pneumatic transport of the mixed powders to a stainless steel cyclone charging device. The magnitude of charge on the mixes was shown to decrease with increased fine particle content, and there was no significant difference in charge for each concentration between spray-dried and crystalline lactose. Both the variation of charge and powder adhesion to the cyclone surface increased with increase in fine particle content. The proportion of fine particles in carrier systems in DPIs may thus have an important role where triboelectrification is involved.     

Template-synthesized nanotubes through layer-by-layer assembly under charge interaction

Aqueous polymer nanotubes can be assembled by combining the layer-by-layer (LbL) assembly and template technique under charge interaction. This method allows lots of species, especially in an aqueous system to form a tubular structure in the pores of the template. The tubes functions can be readily modified by introducing various functional components. Such assembled nanotubes are often mechanically stable and highly flexible. They have also numerous potential applications in delivering materials.     

Measuring the size and charge of single nanoscale objects in solution using an electrostatic fluidic trap

Measuring the size and charge of objects suspended in solution, such as dispersions of colloids or macromolecules, is a significant challenge. Measurements based on light scattering are inherently biased to larger entities, such as aggregates in the sample, because the intensity of light scattered by a small object scales as the sixth power of its size. Techniques that rely on the collective migration of species in response to external fields (electric or hydrodynamic, for example) are beset with difficulties including low accuracy and dispersion-limited resolution. Here, we show that the size and charge of single nanoscale objects can be directly measured with high throughput by analysing their thermal motion in an array of electrostatic traps. The approach, which is analogous to Millikan's oil drop experiment, could in future be used to detect molecular binding events with high sensitivity or carry out dynamic single-charge resolved measurements at the solid/liquid interface.     

Influence of scale on electrostatic forces and torques in AC particulate electrokinetics

Dielectrophoretic forces and torques move and manipulate biological cells, typically of the order of 10 mum ( approximately 10(-5) m) in diameter and ordinarily suspended in aqueous liquids, using electrodes with dimensions around 100 mum ( approximately 10(-4) m). The ability to exploit these same electromechanical effects for particles below 1 mum, that is, <10(-6) m, creates opportunities for remote manipulation and handling of subcellular components, biological macromolecules, and DNA. In this paper, Trimmer's bracket notation is adapted for systematic examination of the scaling laws governing electrokinetic behaviour. The purpose is to shed light on how critical performance measures relevant to the laboratory on a chip are affected by reducing particle sizes and electrode dimensions into the nanometre range. The scaling methodology facilitates consideration of the effect of electrode structure and particle size reduction on voltage, electric field, heating, and response time. Particles with induced moments, dipolar and quadrupolar, as well as permanent dipoles are examined. Separate consideration is given to electrical torque and its application in electrorotation and particle alignment. An eventual goal of these scaling studies is to identify the lower limit on the size of particles that can be manipulated effectively using electrokinetic phenomena.     

Extended thermodynamic approach to ion interaction chromatography

The chromatographic behavior of charged analytes in ion interaction chromatography (IIC) is theoretically investigated. The chemical modifications of the stationary and mobile phases in the presence of ion interaction reagent (IIR) are theoretically shown to change the partition coefficient for charged molecules. The most reliable literature experimental results concerning retention behavior of charged molecules in IIC were used to test the new theory. Retention equations are compared with those that can be obtained from the most important retention models in IIC. The present exhaustive retention model, which is well-founded in physical chemistry, goes further than the previous ones whose retention equations can be viewed as limiting cases of the present theory. The present extended thermodynamic approach reduces to stoichiometric or electrostatic retention models if the surface potential or pairing equilibria are respectively neglected. Moreover, it is able to quantitatively explain experimental evidences that cannot be rationalized by the existing retention models.