Electrophoretic Mobility Measurements Research Articles

Influence of surfactants with different ionic character on the structure of poly(acrylic acid) adsorption layer on the activated biocarbons surface – electrokinetic and stability studies

Two samples of activated biocarbons obtained from corncobs and peanut shells precursors were applied for poly(acrylic acid) – PAA macromolecules adsorption from the aqueous solutions containing surfactants with different ionic character (non-ionic Triton X-100, cationic CTAB and anionic SDS). The effects of biocarbon structure, solution pH and surfactant type were examined. The structure of PAA adsorption layer without and with surfactant molecules on the activated carbons surface was described based on the adsorption, electrokinetic and stability results obtained from the spectrophotometric experiments, potentiometric titrations, electrophoretic mobility measurements and stability tests. It was shown that the presence of surfactant influences significantly the adsorption affinity of PAA macromolecules for the micro/mesoporous biocarbons and causes changes in both solid suspensions stability.

Experimental and Theoretical Study of Morphological and Charging Properties of Truncated Octahedron and Cubic Ceria Nanoparticles: Implications for Biomedical Applications

Ceria (CeO2) nanoparticles are known to be very often used in various applications from biomedicine to fuel cells. To optimize the applications, detailed information about the physicochemical properties such as size, shape, and charge of nanoparticles should be available. Therefore, in our study we performed a systematic study of ceria nanoparticles ranging from synthesis to comprehensive experimental and theoretical characterization. We synthesized ceria nanoparticles using two synthesis paths which led to the formation of two types of ceria nanoparticles. The structure and charging properties of both types of ceria nanoparticles were studied by using X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), particle charge detector (PCD) for surface charge density, and a ZetaPlus instrument for electrophoretic mobility measurements. The results suggested that in the case where hydrolysis of Ce(NO3)3 at room temperature was applied nanoparticles with morphology close to a spherical, more exactly truncated octahedron were synthesized. On the other hand, nanoparticles obtained by hydrothermal synthesis had characteristic cube-like morphology. Finally, for more complete understanding and interpretation of the studied system, we prepared a theoretical model based on the classical density functional theory for electrolyte solutions coupled with the surface charge regulation via the law of mass action. Even without using fitting parameters, the theory adequately describes the experimental data. All the results obtained in our study could serve as a basis for obtaining tuned and engineered ceria nanoparticles with optimized physicochemical properties which could lead to the improved applications of the nanotechnology in the biomedical research.

A physico-chemical study of the interaction of ethanolic extracts of propolis with bacterial cells

In the present study, an effort has been made to understand the interaction mode of propolis, a natural substance produced by honey bees, with gram-positive and gram-negative bacterial cells by measuring alterations in cell surface physico-chemical properties following the incubation of the cells with different sub-inhibitory concentrations of this antimicrobial agent. Electrophoretic mobility and surface hydrophobicity measurements revealed for the first time that propolis induced substantial changes in the volumetric charge density, electrophoretic softness and degree of hydrophobicity characterizing the outermost surface layer of cells. These changes, which appear to be dose-dependent, seem to be consistent with the increasing accumulation and penetration of the propolis antimicrobial components through the cells extracellular layer. Moreover, electron microscopy observation and the determination of the cell constituents’ release demonstrated that propolis at sub-bactericidal concentrations already provoked (at least localized) cell wall damage and/or perturbations. These findings thus suggest that the initial mechanism of action of propolis is most likely structural, resulting from sufficient interaction between the different propolis components and bacterial cell wall structures.

Effect of Structural Features of Pectin on Its Complexation with Lysozyme

The interaction between lysozyme (Lys) and pectins with different net and local charges and with block (BD) and random (RD) distributions of methoxyl groups is studied via dynamic light scattering, turbidimetry, high-resolution optical microscopy, and electrophoretic mobility measurements. RD 16.2, RD 38.2, BD 16.9, and BD 33.1 form water-soluble complexes at pH 5.1, ionic strength I = 0.01, and q 3 × 10–3. The corresponding parameters for RD 66.5 and BD 68.2 are higher: q ≅ 3 × 10–3 up to q ≅ 0.01, and q higher than 0.01. Optical microscopy shows that the complexes between Lys and BDs are large (15–20 μm) for all studied values of the degree of methoxylation (DM), while the average size of RD-based complexes is substantially lower (from 0.6 to 3 μm) and depends on the DM. The BD-based complexes have a gel-like morphology irrespective of the DM, whereas particles of RD-based complex have the form of both coacervate drops and gel-like particles, depending on the DM. The threshold value for ionic strength Iset above which Lys does not form complexes with RD and BD is found to be 0.11. That being so, the values for Iset and pHset for the BD/Lys systems are independent of the pectin DM, while for the RD/Lys systems these parameters diminish considerably with an increase in the DM. The dependence of complexation of Lys and pectin on I has a non-monotonic character with a maximum at I = 0.03 and 0.06 for RD and BD, respectively. Our understanding of the effect that the total charge of a pectin molecule and its distribution along the chain has on the complexation of pectin and a protein is important for our ability to predict the stability of the food product structure.

Revealing and Attenuating the Electrostatic Properties of Tubulin and Its Polymers.

Tubulin is an electrostatically negative protein that forms cylindrical polymers termed microtubules, which are crucial for a variety of intracellular roles. Exploiting the electrostatic behavior of tubulin and microtubules within functional microfluidic and optoelectronic devices is limited due to the lack of understanding of tubulin behavior as a function of solvent composition. This work displays the tunability of tubulin surface charge using dimethyl sulfoxide (DMSO) for the first time. Increasing the DMSO volume fractions leads to the lowering of tubulin's negative surface charge, eventually causing it to become positive in solutions >80% DMSO. As determined by electrophoretic mobility measurements, this change in surface charge is directionally reversible, i.e., permitting control between -1.5 and + 0.2cm2 (Vs)-1 . When usually negative microtubules are exposed to these conditions, the positively charged tubulin forms tubulin sheets and aggregates, as revealed by an electrophoretic transport assay. Fluorescence-based experiments also indicate that tubulin sheets and aggregates colocalize with negatively charged g-C3 N4 sheets while microtubules do not, further verifying the presence of a positive surface charge. This study illustrates that tubulin and its polymers, in addition to being mechanically robust, are also electrically tunable.

Interaction of Ionenes with Lipid Membrane: Unusual Impact of Charge Density.

Synthetic water-soluble polymers are increasingly used for gene delivery, stabilization, and delivery of proteins, and as prospective antimicrobial and antiviral agents. Therefore, study of their interaction with lipid membranes is of special importance. Herein, we studied interaction of aliphatic cationic ionenes (recently tested for gene delivery efficiency) differed in the length of spacer between charged groups (and therefore in charge density) with anionic lipid membrane. A range of approaches such as measurement of particle size and electrophoretic mobility, liposome integrity, ATR-FTIR spectroscopy, isothermal titration calorimetry as well as atomistic molecular modeling was used. Ionene with a spacer of 10 methylene groups has been shown to be incorporated into membrane and interact with its inner hydrophobic part in contrast to ionenes with shorter spacer, which interacted only with outer polar head groups of lipids staying at the water-membrane interface. It affects membrane integrity and results in a different behavior of the polymer-liposome complexes. These findings are relevant for potential biomedical application of ionenes, including creation of composite polymer-liposome systems for drug delivery.

Adsorption kinetic of myoglobin on mica and silica – Role of electrostatic interactions

Adsorption kinetics of myoglobin molecules on mica and silica was studied using the atomic force microscopy (AFM), the colloid enhancement and the quartz microbalance (QCM) methods. Measurements were carried out for the NaCl concentration of 0.01 and 0.15 M as a function of pH comprising pH 7.4 stabilized by the PBS buffer. The electrophoretic mobility measurements enabled to derive the molecules zeta potential as a function of pH. The isoelectric point appearing at pH 5, is lower than that predicted from the theoretical calculations of the nominal dissociation charge. The AFM investigations confirmed that myoglobin molecules irreversibly adsorb at pH 3.5 yielding well-defined layers of single molecules. These layers were characterized using the colloid enhancement method involving polymer microparticles for pH range 3–9. The microparticle deposition kinetics was adequately interpreted in terms of a hybrid random sequential adsorption model. It is confirmed that the myoglobin layers exhibit a negligible zeta potential at pH equal to 5 in accordance with the electrophoretic mobility measurements. Analogous adsorption kinetic measurements were performed for the silica substrate using QCM and AFM. It is observed that myoglobin molecules irreversibly adsorb at pH 3.5 forming stable layers of single molecules. On the other hand, its adsorption kinetics at larger pHs was much slower exhibiting a poorly defined maximum coverage. This was attributed to aggregation of the myoglobin solutions due to their vanishing charge. The kinetic QCM runs were adequately interpreted in terms of a theoretical model combining the Smoluchowski aggregation theory with the convective diffusion mass transfer theory.

Dispersion of thulium-yttria nanoparticles to build up smart structures

The development of new materials for radiation dosimetry is a challenge to assure quality improvement practices related to radiation protection concept. On this context, colloidal stability provides conditions to build up smart structured materials from bottom-up perspective. The present work reports zeta potential characterization of thulium-yttria nanoparticles in aqueous medium. Thulium-yttria nanoparticles formed by a relative low temperature hydrotermal synthesis were characterized by XRD, PCS, and SEM. The stability of particles in aqueous medium was evaluated by electrophoretic mobility measurements, followed by zeta potential calculation. The results revealed that the isoelectric point of thulium-yttria suspensions shifted in accordance with thulium concentration from pH 8.5 (“pure” yttria) to pH 9.2 (2at.%Tm). Besides, most suspensions could be stabilized at pH 10.5, presenting zeta potential values around 30 mV. These results are substantial parameters to advance toward new materials for radiation dosimetry.

Molybdenite Flotation in the Presence of a Polyacrylamide of Low Anionicity Subjected to Different Conditions of Mechanical Shearing

Polyacrylamides (PAM) are used as flocculants in the copper-molybdenum mining industry to improve the settling rate of flotation tailings. These types of reagents are recirculated to flotation in the water that is recovered from the thickeners, and as a result of this practice they can cause depression of flotation of some important minerals such as molybdenite. The objective of this work was to study the effect of a PAM of 11.9% of degree of anionicity on the flotation of molybdenite. The effect of the anionic PAM on molybdenite flotation was studied under different levels (three) of flocculant mechanical shearing. The flotation data was discussed along with intrinsic viscosity, adsorption, induction time, and electrophoretic mobility measurements. It was found that the non-sheared PAM (NS–PAM) had the strongest depressing effect on molybdenite flotation, followed by the moderately sheared PAM (MS–PAM). The depressing effect of the strongly sheared PAM (SS–PAM) was negligible. The flotation data correlated with the induction time measurements that showed that molybdenite became more hydrophilic in the presence of the NS–PAM. The SS–PAM has no effect on the induction times. Further work is ongoing to understand the effect of PAM on molybdenite flotation in the presence of clay minerals and different aqueous media.

Aggregation and transport behavior of goethite colloids as affected by dissolved organic matter and pH: Electrostatic vs. hydrophilic interactions

Wettable surfaces in aqueous media experience hydrophilic repulsion forces, which have been thoroughly examined for interactions between biocolloids. However, there is no systematic assessment yet on the effects of hydrophilic repulsion on aggregation and transport behavior of soil mineral colloids. The aim of this study was to fill that gap in knowledge, with an emphasis on whether hydrophilic or electrostatic forces govern the respective interactions. For that, the interactions of hydrophilic goethite colloids were examined by batch aggregation experiments and colloid breakthrough experiments using columns filled with hydrophilic quartz sand. To induce electrostatic interactions ranging from attractive to repulsive, colloid zeta potential was modified via changes in dissolved organic matter concentration and pH. Classic DLVO and the extended DLVO (XDLVO) theory that includes hydrophilic interaction energies were applied using electrophoretic mobility measurements and sessile drop contact angle data. Results showed that for each case in which hydrophilic repulsion was predicted to be stronger than electrostatic attraction, the latter always controlled the colloid aggregation and transport behavior in the experiment. The likely reason was that the irregular shape and surface roughness of the colloids, as shown by SEM images, rendered the short-range hydrophilic repulsion ineffective, so that the more long-range electrostatic interactions predominated. We conclude that in contrast to biocolloids, hydrophilic repulsion did not have a relevant effect on the interactions between soil mineral colloids. Hence, our study clearly shows that for the aggregation and mobility of soil mineral colloids, soil parameters affecting their surface charge are more important than parameters affecting their wettability.

Dispersed liquid crystals as pH-adjustable antimicrobial peptide nanocarriers

HypothesispH-responsive nanocarriers have the potential to provide targeted delivery of antimicrobial peptides (AMPs) to sites of bacterial infection with typically abnormal pH levels in the body. However, the local pH of the infected sites varies substantially among different infection-related diseases, calling for the development of delivery systems capable of targeting local pathological conditions in an adjustable pH range. ExperimentsIn this study, a highly versatile pH-responsive nanocarrier platform, based on dispersions of oleic acid (OA) and glycerol monooleate (GMO) self-assemblies with the human cathelicidin AMP LL-37, was designed and characterized. FindingsA detailed pH-composition phase diagram was constructed from small angle X-ray scattering and cryogenic transmission electron microscopy data. In addition, the protonation state and apparent pKa of OA embedded in these nano-self-assemblies were investigated by electrophoretic mobility measurements at different pHs and found to be strongly dependent on nanocarrier composition. By varying composition of these nanocarriers, the apparent pKa of embedded OA molecules could be tuned from 7.8 to 6.3, shifting the range of nanocarriers' pH-response. The study advances our fundamental understanding of self-assembly and pH-responsiveness in lipid-peptide systems containing monounsaturated long-chain fatty acids. The results may guide the future design of highly adaptable nanocarriers for patient-optimized pH-targeted AMP delivery.

Investigating the binding measurements of human α-acid glycoprotein with chlorambucil and dacarbazine in the presence of imidazolium based -ionic liquid by affinity capillary electrophoresis

Human alpha (α1)-acid glycoprotein (AGP) is an acute phase protein whose plasma concentration increases several-folds in the presence of various diseases. The variability in AGP plasma concentration is expected to have a huge impact on the drug binding equilibrium. Therefore, a precise measurement of AGP-drug binding is of great demand for drug development. In the current study, an ionic liquid-based aqueous two-phase system combined with affinity capillary electrophoresis (ILATPS/ACE) was utilised in order to improve the accuracy of AGP-drug binding analysis through the measurements of electrophoretic mobilities. The utilisation of ILATPS has shown to have a positive impact on the stability of AGP activity solution during the storage for an extended period of time. In addition, the effect of various alkyl chains (C2-C10) of imidazolium-based ILs with concentrations ranging between 10.00 and 1000.0 μmol L−1 on the AGP binding with the anti-cancer drugs chlorambucil (CHL) and dacarbazine (DAC) was examined by the system developed (ILATPS/ACE). A 100.00 μmol L−1 1-ethyl-3-methylimidazolium chloride (EMImCl) prepared in the physiological buffer conditions containing AGP (5.00–100.00 µmol L−1) has provided an accurate apparent binding constant of 1.99 ± 0.11 and 6.95 ± 0.14 L mmol−1 with CHL and DAC respectively. Apart from the ACE analysis, EMImCl/phosphate buffer solution was found to be a distinguished system that could lengthen the stability of AGP activity for a period of time reaching 90 days during the solution storage at 4.00 °C. This effect is thought to be due to the easy conversion of one-phase EMImCl/phosphate buffer/AGP at the ambient lab temperature into the two-phase solution at refrigerator temperature, 4.00 °C, and vice versa. Therefore, the ILATP/ACE system could be used to enhance the accuracy for other AGP-drug bindings with a fast, easy to use, and cost-effective analysis.

Adenine Derivatives for Regenerable Antibacterial Surface Applications Based on A−T Base Pairing

AbstractSurface contamination is a major concern in the design, fabrication, and application of biomaterials. In this work, a series of new adenine derivatives were synthesized in a three‐step method with the goal of protecting the functional materials against microbial adhesion. Initially, 9‐(chloroalkyl)‐9H‐purin‐6‐amine compounds were synthesized from adenine. Then, these compounds were reacted with potassium thiocyanate or potassium selenocyanate. In the last step, adenine derivatives incorporating a tetrazole ring were synthesized via the cycloaddition of sodium azide with thiocyano or selenocyano derivatives. The antimicrobial activity of the compounds was evaluated by using the minimal inhibitory concentration method. Furthermore, the effect of the compounds on pBR322 plasmid DNA was studied using gel electrophoretic mobility measurements. The antimicrobial assay showed that some of the synthesized compounds exerted vigorous antibacterial and antifungal activities. Further experiments indicated that seleno‐adenine derivatives have higher antimicrobial and DNA effect than other derivatives. The surfaces activated by the adenine derivative demonstrated antibacterial activity resisting bacterial attachment in order to remove dead bacteria from the surface. All results showed that some of these adenine derivatives have an antibacterial activity and the potential for further applications, for example as a smart surface coating that prevents bacterial adhesion to biomaterials.

Effect of temperature on the association behavior in aqueous mixtures of an oppositely charged amphiphilic block copolymer and bile salt

The association in aqueous mixtures of a thermoresponsive cationic diblock copolymer composed of poly(N-isopropylacrylamide) (PNIPAM) and poly(3-acrylamidopropyl)-trimethylammonium-chloride (PAMPTMA(+)) and the oppositely charged bile salt sodium deoxycholate (NaDC) is investigated at different compositions by light and X-ray scattering, calorimetry, and electrophoretic mobility measurements. Clouding reveals aggregation upon heating. The addition of NaDC to the copolymer solution lowers the temperature of the transition and increases its cooperativity. At high temperature and low NaDC fractions, mixed aggregates with a dehydrated PNIPAM-rich interior and a PAMPTMA(+)-rich shell partially neutralized by DC– anions are formed. At high NaDC fractions, the aggregates present internal regularly spaced segregated nanoregions of dehydrated PNIPAM and PAMPTMA(+)/DC– (microphase separation). The results suggest that the mixed aggregates have appealing composition-controlled thermoresponse. The system phase separates at body temperature and the highest NaDC fractions investigated, meaning in conditions accomplished when the use of the polymer as a bile salt sequestrant is hypothesized.

Determination of Protein Charge in Aqueous Solution Using Electrophoretic Light Scattering: A Critical Investigation of the Theoretical Fundamentals and Experimental Methodologies.

Studies are reported of the measurement of electrophoretic mobilities of bovine serum albumin (BSA) in aqueous potassium chloride solutions as a function of ionic strength and pH using electrophoretic light scattering (ELS). It is demonstrated that the use of palladium or platinum electrodes should be avoided and that platinized platinum electrodes are necessary to avoid interference from unwanted electrochemical phenomena at the electrode-liquid interface. Potentiometric acid titration was performed to quantify the amount of protonic charge per protein molecule at the same pH values as the electrophoretic mobility measurements. It is shown that appropriate selection of an electrokinetic model yields excellent agreement between predicted and experimental electrophoretic mobilities across the ranges of pH and ionic strength studied in accordance with the protonic charge values obtained by titration. The experimental results are explained in terms of protonation, chloride counterion binding, and protein molecule permeability. This work highlights specific requirements of using ELS for confident analysis of proteins in aqueous solutions.

Characterization of Human Glioblastoma versus Normal Plasma-Derived Extracellular Vesicles Preisolated by Differential Centrifugation Using Cyclical Electrical Field-Flow Fractionation.

Although many properties for small extracellular vesicles (sEVs, formerly termed "exosomes") isolated at ∼100 000g are known, a wide range of values are reported for their electrophoretic mobility (EM) measurements. This paper reports for the first time the effect of dilution on the EM of U87 glioblastoma cell-derived and plasma-derived sEVs and medium size EVs (mEVs, commonly termed "oncosomes") preisolated by differential centrifugation. Furthermore, the effect of resalting on the EM of sEVs and mEVs was evaluated. The EM of U87 sEVs and U87 mEVs showed an increase as the salt concentration decreased to 0.005% of the initial salt concentration. However, for the plasma sEVs and plasma mEVs, the electrophoretic mobility increased as the salt concentration decreased to 0.01% of the initial salt concentration and then increased to its initial value when the salt concentration decreased to 0.005% of the initial salt concentration. For both U87 and plasma sEVs and mEVs, the EM remained almost constant when the concentration of the particles changed and the salt concentration was kept the same as its initial value. This indicates that the EM of EVs is only a function of the salt concentration of the buffer and is independent of the concentration of the particles. The sEVs and mEVs were separated with cyclical ElFFF for the first time. The results indicate that ElFFF was able to fractionate the EVs, and a crescent-shaped trend was found for the retention time when the applied AC voltage was altered (increased).

How Low Can You Go? Molecular Details of Low-Charge Nanoemulsion Surfaces.

Negative charge accumulation at aqueous-hydrophobic interfaces and its pH-dependent behavior are routinely ascribed to special adsorption properties of hydroxide ions. Mounting experimental and computational evidence, however, indicates that this negative charge accumulation is the result of surface-active impurities. If true, these impurities would obfuscate our fundamental understanding of the molecular structure and bonding environment at aqueous-hydrophobic interfaces. In this work, we describe the preparation and characterization of bare low-charge nanoemulsions (LCNEs), nanosized droplets of oil-absent emulsifiers. Electrophoretic mobility measurements of LCNE droplets in varying pH environments suggest that trace surface-adsorbed impurities are contributing to the lingering negative surface charge that leads to their marginal stability. We then use vibrational sum-frequency scattering spectroscopy to support this claim and to study the molecular structure and bonding environment of the interfacial aqueous and hydrophobic phases on both the LCNE surface and the surface of nanoemulsions with increasing amounts of adsorbed surfactants. For LCNE samples, our results show that interfacial water bonds more strongly to the oil phase on the droplet surface compared to similar planar interfaces. Interfacial oil molecules are found to orient mostly parallel to the bare droplet surface and reorganize upon surfactant adsorption. In summation, the results reported here provide a new look at the molecular structure and bonding of bare nanoemulsion surfaces and contribute to our evolving understanding of bare aqueous-hydrophobic interfaces.

Non-invasive targeted iontophoretic delivery of cetuximab to skin

ABSTRACTBackground: Cetuximab (CTX) is a glycosylated anti-EGFR monoclonal antibody of great interest in the treatment of non-melanoma skin cancers. Its intravenous administration is associated with severe side effects. This is the first report on the noninvasive iontophoretic-targeted topical delivery of CTX to skin.Methods: Iontophoretic transport of CTX (0.5 mA/cm2) was studied as a function of formulation pH (4, 5.5 and 7) and duration of current application (2, 4 and 8 h). CTX cutaneous biodistribution was determined; electrotransport mechanisms and penetration pathways were investigated.Results: Electrophoretic mobility measurements of CTX isoforms and co-iontophoresis of acetaminophen at each pH demonstrated that CTX electrotransport was due to electroosmosis: despite an ~8-fold reduction in charge, CTX skin deposition was greater at pH 7 than pH 4 (8.974 ± 1.952 and 0.482 ± 0.165 μg/mm3) – consistent with the increased electroosmotic flow at pH 7. Iontophoresis of an Alex488-CTX conjugate showed that skin penetration occurred by the intercellular and follicular routes. Therapeutic concentrations of CTX in the viable epidermis, upper dermis and lower dermis were achieved following iontophoresis for 2, 4 and 8 h, respectively.Conclusion: The results demonstrate the topical delivery of a 152 kDa monoclonal antibody into skin in a targeted, controlled and entirely noninvasive manner.

Nanoparticle charging with mixed reverse micelles in apolar media

The charging of colloidal particles using mixed surfactants in apolar media is investigated. Previous work has shown that one can predict the sign and magnitude of particle charge by comparing the acid-base properties of an oil-soluble surfactant to the acid-base properties of a specific particle. The present work sought to tune the acid-base properties by using mixed surfactants. Unexpected mineral oxide particle charging results were obtained however, from mixing the acidic surfactant sorbitan monooleate (SPAN 80), with the basic surfactant polyisobutylene succinimide (OLOA 11000), requiring further investigation into the mixed reverse co-micelle particle charging mechanism. Conductometric titrations and small-angle neutron and x-ray scattering measurements showed that the surfactant mixtures formed spherical reverse co-micelles. Quartz crystal microbalance adsorption measurements demonstrated that both surfactants can adsorb to the particle surface. Lastly, electrophoretic mobility measurements showed that the surfactant mixture produced negative particle charging for nearly all surfactant ratios, and a greater magnitude of negative charging than that produced by the OLOA 11000 surfactant alone. It was discovered that in the mixed surfactant system studied, the basic surfactant would charge the hydroxyl groups on both the oxide surface, and the adsorbed acidic surfactant head groups. The results showed that particle charging behavior in apolar media using mixed micelles requires knowledge beyond that for the individual surfactants and their proportions in the mixture.

Charging of Oxide Nanoparticles in Media of Intermediate Dielectric Constant.

The use of surfactants to charge colloidal particles in solvents of intermediate dielectric constants (5 < ε < 40) is investigated. While particle charging mechanisms in aqueous (ε = 80) and apolar (ε < 5) media are well understood, the interplay of these different charging mechanisms, which can all occur in solvents of intermediate dielectric constants (sometimes referred to as "leaky dielectrics"), remains to be fully explored. Conductometric titrations determining the critical micelle concentration (CMC) of the surfactant (aerosol-OT) confirm the existence of reverse micelles in intermediate dielectrics and show that as the solvent dielectric constant decreases, the CMC decreases as well. Electrophoretic mobility measurements of three oxide particles (SiO2, TiO2, and MgO) highlight various charging mechanisms that arise from particle-solvent, particle-surfactant, and solvent-surfactant interactions in a solvent series of alcohols and ketones. The results show that a combination of donor-acceptor particle-solvent interactions, surfactant ion adsorption, and reverse micelle-mediated acid-base interactions can all charge oxide particles in intermediate dielectrics. Furthermore, the results show that the dielectric constant of the solvent affects the relative magnitudes of each charging mechanism.