Influence Of Surface Charge Density Research Articles

Heterogeneous CNF/MoO3 nanofluidic membranes with tunable surface plasmon resonances for solar-osmotic energy conversion.

Two-dimensional (2D) nanofluidic membranes are competitive candidates for osmotic energy harvesting and have been greatly developed. However, the use of diverse inherent characteristics of 2D nanosheets, such as electronic or optoelectronic properties, to achieve intelligent ion transport, still lacks sufficient exploration. Here, a cellulose nanofiber/molybdenum oxide (CNF/MoO3) heterogeneous nanofluidic membrane with high performance solar-osmotic energy conversion is reported, and how surface plasmon resonances (SPR) regulate selective cation transport is revealed. The SPR of amorphous MoO3 endows the heterogeneous nanofluidic membranes with tunable surface charge and good photothermal conversion. Through DFT calculations and finite element modeling, the regulation of electronic and optoelectronic properties on the surface of materials by SPR and the influence of surface charge density and temperature gradient on ion transport in nanofluidic membranes are demonstrated. By mixing 0.01/0.5 M NaCl solutions using SPR and photothermal effects, the power density can achieve a remarkable value of ≈13.24 W m-2, outperforming state-of-the-art 2D-based nanofluidic membranes. This work first reveals the regulation and mechanism of SPR on ion transport in nanofluidic membranes and systematically studies photon-electron-ion interactions in nanofluidic membranes, which could also provide a new viewpoint for promoting osmotic energy conversion.

Charged Lipids Modulate the Phase Separation in Multicomponent Membranes.

Phase separation in lipid membranes controls the organization of membrane components and thus regulates membrane-mediated processes. Membrane phase behavior is influenced by the molecular properties of its components and their relative concentrations. Charged lipid species are among the most essential components of lipid membranes, and their impact on the membrane phase behavior is yet to be fully understood. Aiming to provide insight into this impact, this paper investigates how the presence and amount of anionic and cationic lipids affect the phase behavior of multicomponent membranes. Membranes of ternary composition DOPC:DPPC:Chol with two distinct molar ratios were used to test the hypothesis that inclusion of charged lipids with saturated tails, beyond a certain concentration, would impede phase separation in an otherwise phase-separating membrane. Fluorescence microscopy examination of electroformed giant liposomes revealed that when more than half of DOPC in the examined mixtures was replaced with DOPA or DOTAP, phase separation in liposomes was somewhat suppressed, and this effect increased with increasing charged lipid content. This effect depended on the membrane surface charge density as the half-maximal effect was observed at around 0.0072 C Å-2 in all examined cases. The phase-separation suppressing effect of DOPA was neutralized when oppositely charged lipid DOTAP was included in the mixture. Likewise, presence of divalent cation Ca2+ in the solution neutralized the impact of negatively charged DOPA. These results underline the detrimental influence of surface charge density on membrane phase behavior. More importantly, these findings suggest that the charged lipid content in membranes may be a regulator of their phase behavior and open new opportunities for the design of synthetic lipid membranes.

Enhanced salt rejection of hydroxylated BaTiO3/Ti3C2Tx@polyvinyl alcohol composite nanofiltration membranes under electrochemical assistance

The influence of surface charge density of nanofiltration (NF) membranes could due to enhanced electrostatic interaction through electrically assisted. In this study, the hydroxylated BaTiO3/Ti3C2Tx (HBT)@polyvinyl alcohol (PVA) (HBTA) composite NF membranes were prepared by crosslinking HBT on the surfaces of NF membranes. The optimized HBTA-0.06 sample exhibited a water contact angle of 43.7°, zeta potential of −13.73 mV (pH = 6.5), average effective pore size of 0.319 nm, and pure water permeance of 55.1 L·m–2·h–1. At an applied electric field of 3 V, the rejection rates of LiCl, NaCl, KCl, RbCl, and MgCl2 determined for HBTA-0.06 reached 73.3%, 60.1%, 53.6%, 49.4%, and 59.8%, respectively. HBTA-0.06 has good stabilization properties that are especially advantageous for water purification, indicating good long-term filtration stability within 168 h. Furthermore, HBT doping significantly increased the charge density of the HBTA membranes and the rejection efficiency of chloride salts, both chlorine radicals (·Cl) and hydroxyl radicals (·OH) generation pathways were proposed under electrochemical assistance. Hence, electrical assistance represents a novel and effective technique for enhancing the salt rejection properties of multifunctional composite NF membranes.

Influence of Cellulose Charge on Bacteria Adhesion and Viability to PVAm/CNF/PVAm-Modified Cellulose Model Surfaces.

A contact-active antibacterial approach based on the physical adsorption of a cationic polyelectrolyte onto the surface of a cellulose material is today regarded as an environment-friendly way of creating antibacterial surfaces and materials. In this approach, the electrostatic charge of the treated surfaces is considered to be an important factor for the level of bacteria adsorption and deactivation/killing of the bacteria. In order to clarify the influence of surface charge density of the cellulose on bacteria adsorption as well as on their viability, bacteria were adsorbed onto cellulose model surfaces, which were modified by physically adsorbed cationic polyelectrolytes to create surfaces with different positive charge densities. The surface charge was altered by the layer-by-layer (LbL) assembly of cationic polyvinylamine (PVAm)/anionic cellulose nanofibril/PVAm onto the initially differently charged cellulose model surfaces. After exposing the LbL-treated surfaces to Escherichia coli in aqueous media, a positive correlation was found between the adsorption of bacteria as well as the ratio of nonviable/viable bacteria and the surface charge of the LbL-modified cellulose. By careful colloidal probe atomic force microscopy measurements, it was estimated, due to the difference in surface charges, that interaction forces at least 50 nN between the treated surfaces and a bacterium could be achieved for the surfaces with the highest surface charge, and it is suggested that these considerable interaction forces are sufficient to disrupt the bacterial cell wall and hence kill the bacteria.

Influence of surface charge density on ligand-metal bonding: A DFT study of NH3 and HCOOH on Mg (0 0 0 1) surface

Activation of the ligand binding to substrate is a highly desirable, yet challenging reaction for biomolecular coating on the biomedical material surface. One advantage of this process is their modular construction on the surface as the biomolecular linkers, allowing for flexible design and detailed control of certain properties. In this work, we studied the binding properties of NH3 and HCOOH on Mg (0 0 0 1) surface and tried to understand the effect of alloy elements in the binding process. First principles within density functional theory (DFT) has been used to investigate the adsorption of NH3 and HCOOH molecules on Mg (0 0 0 1) and Zn-(Y-) doped surfaces. The electron redistribution, effect of alloy element and changes in the density of states of interaction systems are analyzed. Our results revealed that NH3 and HCOOH interact with the Mg (0 0 0 1) surface through dative bond with the adsorption energy (Eads) −0.70 and −0.64 eV, and the Zn/Y alloy element can improve the adsorption process which accompanied by charge-transfer between substrate and adsorbate. Calculated results of the nudged elastic band (NEB) show that the energy barrier of 0.14 eV need to be overcome for NH3 from the neighbor site to Y atom. Our findings provide insight for the binding mechanism of ligand on Mg and Mg-based alloy surfaces and also point out some directions for future experimental efforts.

Estimation of aspect ratio of cellulose nanocrystals by viscosity measurement: influence of surface charge density and NaCl concentration

Cellulose nanocrystals (CNCs) with similar size and various surface charge densities were prepared by sulfuric acid hydrolysis and NaOH desulfation. The influence of surface charge density and NaCl concentration on the intrinsic viscosity of CNC suspensions and predicted aspect ratio were investigated by Ubbelohde viscometer. With decreased CNC surface charge density, the intrinsic viscosity initially decreased due to the electric double layers on the CNC surface and subsequently increased due to CNC aggregation. To screen electroviscous effect, NaCl was added into CNC suspensions. With increased NaCl concentration, the intrinsic viscosity of CNC suspensions first decreased and then increased. The aspect ratios of CNCs predicted by Batchelor equation from the minimum intrinsic viscosity were consistent with that measured by transmission electron microscopy. Suspensions of CNCs with higher surface charge density needed less NaCl to obtain minimum intrinsic viscosity. The NaCl content that should be added to the suspension to predict the actual physical aspect ratio of CNC can be estimated by Debye–Huckel theory, assuming that the Debye length is equal to the CNC diameter.

Influence of Surface Charge Density and Morphology on the Formation of Polyelectrolyte Multilayers on Smooth Charged Cellulose Surfaces.

To clarify the importance of the surface charge for the formation of polyelectrolyte multilayers, layer-by-layer (LbL) assemblies of polydiallyldimethylammonium chloride (pDADMAC) and polystyrenesulfonate (PSS) have been investigated on cellulose films with different carboxylic acid contents (20, 350, 870, and 1200 μmol/g) regenerated from oxidized cellulose. The wet cellulose films were thoroughly characterized prior to multilayer deposition using quantitative nanomechanical mapping (QNM), which showed that the mechanical properties were greatly affected by the degree of oxidation of the cellulose. Atomic force microscopy (AFM) force measurements were used to determine the surface potential of the cellulose films by fitting the force data to the DLVO theory. With the exception of the 1200 μmol/g film, the force measurements showed a second-order polynomial increase in surface potential with increasing degree of oxidation. The low surface potential for the 1200 μmol/g film was attributed to the low degree of regeneration of the cellulose film in aqueous media due to increasing solubility with increasing charge. The multilayer formation was characterized using a quartz crystal microbalance with dissipation (QCM-D) and stagnation-point adsorption reflectometry (SPAR). Extensive deswelling was observed for the charged films when pDADMAC was adsorbed due to the reduced osmotic pressure when ions inside the film were released, and the 1:1 charge compensation showed that all the charges in the films were reached by the pDADMAC. The multilayer formation was not significantly affected by the charge density above 350 μmol/g due to interlayer repulsions, but it was strongly affected by the salt concentration during the layer build-up.

Molecular dynamics study of homogeneous and inhomogeneous phase in charged colloids: The influence of surface charge density

Compared with previous Mote Carlo (MC) simulations with 432 particles, molecular dynamics (MD) simulations with much larger number of particles have been carried out to investigate the dynamic process of the structural ordering and voids formation in charge stabilized colloidal suspensions. Sogami and Ise (SI) potential which has a long-range attraction is used to represent the interaction between colloidal particles. As increasing the surface charge density on the colloidal particles, the data obtained from the simulations, such as the crystallization, bcc–fcc phase transition, homogeneous to inhomogeneous transition and the voids formation, are in agreement with previous observations of MC simulations and experiments. The effects of particle number used in the simulations are studied in detail. MD simulations in highly charged colloidal system with small sizes show very few crystallized particles, in accord with the results of MC simulations. However, the structure in the system with larger number of particles is always the voids coexisting crystallites instead of a glasslike or disordered inhomogeneous phase, indicating that the glasslike or disordered phase region obtained at very high charge density in small system is an artifact produced by very limited number of particles used in the simulations. Therefore, SI potential is not applicable for explaining the reentrant transition of highly charged colloidal systems.

Charge Transport Characteristics in Boron-Doped Silicon Nanowires

We report the charge transport and inferred surface depletion characteristics of silicon nanowires (Si NWs) with diameters of 90-170 nm after boron doping to 8 times 1017 and 4 times 1019 cm-3 by a proximity diffusion doping technique. Four-probe current-voltage measurements were performed to obtain the NW resistivity, and the electrically active dopant concentration and surface oxide charge density were extracted by varying the NW diameter. The Ti/Au to Si NW contact resistance and specific contact resistivity were also obtained, and specific contact resistivities as low as 2 times 10-5 Omega middot cm2 were achieved. The derived parameters for these ex situ boron-doped Si NWs agree reasonably well with the expected characteristics and earlier reported results for in situ boron-doped Si NWs. Interface charge creates a surface depletion region in p-type Si NWs, which decreases the conducting area of the NW. This effect increases the NW resistance and becomes increasingly significant with decreasing dopant concentration and NW diameter. A simple method is presented to estimate the relative influence of surface charge density on electrical transport in NWs for this case.

Influence of surface charge density on the plasmon resonance modes in gold nanoellipsoid

The charge distribution on the surface of gold nanoellipsoid is different from direction. This anisotropic charge density can further influence the collective behavior of surface electrons. Therefore, ellipsoidal gold nanoparticles exhibit two surface plasmon bands corresponding to transverse and longitudinal resonance, respectively. Theoretical calculation shows a nonlinear relationship between the absorption maximum of the longitudinal plasmon resonance and the mean aspect ratio, which is in agreement with the experimental results.

Influence of Surface Charge Density on the Electrochemically Derived Surface Roughness of Bi Electrodes

Electrical double-layer properties of variously treated Bi(001) and Bi(111) electrodes were studied in terms of the Debye length dependent roughness theory (i.e., nonlinear Poisson-Boltzmann theory), recently developed by Daikhin et al. The geometrical roughness factor, root mean square (rms) departure of the surface from flatness, the medium lateral correlation length values have been obtained. According to the experimental data and results of theoretical calculations, the surface roughness increases in the order of Bi electrodes: electrochemically polished single-crystal Bi(111) at the temperature of liquid nitrogen Bi(111) etched Bi(111) etched Bi(111) etched Bi(001) plane. This order of the electrode surface roughness is in agreement with the ex situ AFM data, i.e., with the rms roughness values. At small electrolyte concentrations the deviation of experimental roughness function vs. inverse Debye length curves from the theoretically calculated ones has been observed, which is explained by the influence of energetic inhomogeneity of the polycrystalline surfaces, i.e., the nonequal surface charge density values at the various homogeneous regions exposed at the macropolycrystalline electrode surface, on the interfacial capacitance values. © 2003 The Electrochemical Society. All rights reserved.

Influence of surface charge density on protein adsorption on polymeric nanoparticles: analysis by two-dimensional electrophoresis

Plasma protein adsorption is regarded as a key factor for the in vivo organ distribution of intravenously administered colloidal drug carriers, and strongly depends on their surface characteristics, e.g. surface hydrophobicity or charge. A range of polymeric nanoparticles with a steep variation of the surface charge density was synthesized as model drug carriers. Physicochemical parameters, i.e. particle size, surface charge density, hydrophobicity and surface topography were determined. Two-dimensional electrophoresis (2-DE) was employed for determination of particle interactions with human plasma proteins. Increasing surface charge density showed an increase in plasma protein adsorption, but did not show differences in the detected protein species. For the first time it was possible to show plasma protein adsorption patterns on a range of nanoparticles with variation of only one parameter, i.e. the charge, while size and surface hydrophobicity remain practically unchanged. The knowledge about the interactions of proteins with particulate surfaces can be exploited for the future controlled design of colloidal drug carriers and possibly in the controlled creation of biocompatible surfaces of other devices that come into contact with proteins (e.g. microparticles and implants).

Preparation and properties of polystyrene model colloids: II. Effect of surface charge density on coagulation behavior

The influence of surface charge density on the stability of model polymer colloids was studied through the coagulation kinetics of monodisperse polystyrene latexes in which particle size and surface charge density were independent variables. The time dependence of light scattering intensity was used to measure the stability ratios of the colloids as a function of electrolyte concentration. These were used to calculate the Stern potential and the Hamaker constant by means of the method suggested by Reerink and Overbeek. The calculated Stern potential increases systematically with increasing surface charge density at constant latex particle size. The difference between this Stern potential and the surface potential calculated from the Gouy Chapman equation was found to depend on the surface charge density of the particles. A high surface charge density produces a larger potential drop through the Stern layer. The calculated value of the Hamaker “constant” increased regularly with increasing surface charge density. Variation of the calculated Hamaker constant was attributed to interactions other than the van der Waals attractive force and the electrostatic repulsive force involved in the colloidal system.

The influence of surface charge density on the fracture of zinc single crystal electrodes

This study suggests that variation in the charge density on metal electrode surfaces may affect the plasticity and fracture of model metals such as zinc. It seems likely, moreover, that double layer phenomena may play a role in the environmental embrittlement of other metals and alloys as well. In this view, one might anticipate that phenomena such as stress corrosion cracking -- in which case electrical double layers are clearly present -- may be sensitive, in part, to the charge density on the metal surface. This may be especially true in the range of potentials for a particular system where film formation is unlikely -- i.e., cathodic potentials, for example, at which premature failure is usually attributed to hydrogen embrittlement. It should be appreciated that changes in the charge density on a metal surface may be induced by means other than by applied electrode potentials. If hydrogen behaves as an electropositive adsorbate at the surface of the metal, a change in the charge density will again occur by the transfer of charge from hydrogen to the metal surface atoms. Hence, the model shown in Fig. 3 may be relevant, for example, to the atomistics of hydrogen embrittlement as well as to liquid metal embrittlement.

The Influence of Surface Charge Density of Phosphatides on the Binding of Some Cations

Abstract Surface potentials, ψg, of mixtures of synthetic phosphatides dispersed in uni-univalent electrolyte have been calculated from the limiting molecular areas through use of the Gouy equation. Electrophoretic mobilities of the dispersed particles were measured and used to calculate ζ potentials from the Helmholtz-Smoluchowski equation. In analogy with several structurally related systems, the ratio ζ/ψg was found to be unity at low surface charge density (σ l 20,000 e.s.u. cm-2), decreasing progressively at higher surface charge density to a minimum of 0.58. The ζ potential at low surface charge density is appreciably affected by the composition of the fatty acyl substituents to the extent that this determines the cross-sectional area per net negative charge in the bilayer. Charge reversal concentrations have been measured for various metal ions on several phosphatide surfaces differing in charge density. Inserting the conditions for charge reversal into the Stern equation, the standard free energy of adsorption for each cation was calculated and used to obtain a value for the logarithm of the apparent association constant, K'. At an initial surface charge density below 20,000 e.s.u. cm-2, the sequence of values of log K' was Ag l Ba l Sr l Ni l Mg l Ca l Co l Zn l Cu l Mn l Pb l Cd l La l Ce l Th l UO2. Comparison of log K' for magnesium, calcium, strontium, and barium as a function of surface charge density suggests that the larger cations are able to form ion triplets at high surface charge density while the smaller cations are unable to do so. The high affinity of UO22+ for phosphatide surfaces is also considered. The results obtained are discussed in terms of their potential applicability to several problems of current biochemical interest.