The role of acid-base effects on particle charging in apolar media

Mesoscale Acid-Base Complexes Display Size-Associated Photophysical Property and Photochemical Activity

The properties of single molecules and molecular aggregates can differ dramatically, leading to a long-standing interest in mesoscale aggregation processes. Herein, a series of acid-base molecular complexes is developed by using a tetraphenylethylene-backboned fluorophore, and investigated the photophysical properties and photochemical activities at different aggregation length scales. This fluorophore, with two basic diethylamine groups and two acidic tetrazole groups, exhibits sparse solubility due to multivalent interactions that cause infinite aggregation. The addition of a third acid leads to the formation of fluorophore/acid complexes with good dispersibility and colloidal stability. This assembly process can be controlled by the use of different acids and their stoichiometry, resulting in aggregates ranging in size from a few to hundreds of nanometers. A crystalline structure is obtained to illustrate the complex properties of the acid-base network. Unlike the single molecule, these complexes show a trend of size-related properties for photoluminescence efficiency and photochemical activity. As the amount of acid added increases, the size of the complexes decreases, the aggregation effect of the complexes on fluorescence emission increases, and the rates of the oxidative photocyclization and photodecomposition slow down. This work may help to understand size-controlled molecular materials at the mesoscale for functional design.

Adjusting the charging behavior of TiO2 with basic surfactants in an apolar medium for electrophoretic displays

Electrophoretic displays (EPDs) are attracting attention as potential candidates for information display due to their eye-friendly nature, environmental friendliness and bistability. However, their response speed, which is closely related to the charging behavior of electrophoretic particles, is still inadequate for practical applications. Herein, five basic surfactants were employed to adjust the particle charge of titanium dioxide (TiO2) in the apolar medium Isopar L. Particle charge is strongly related to the effective surfactant coverage on surface sites, dominated by the interaction between anchoring groups and solvation chains. As a result, the electrophoretic mobility of TiO2 could be tuned between -8.09 × 10-10 and +2.26 × 10-10 m2 V-1 s-1. Due to the increased particle charge, TiO2 particles could be well dispersed in Isopar L with the assistance of S17000, T151 and T154. A black-white dual particle electrophoretic system with 2.0% (w/v) S17000 was constructed to obtain EPD devices. The EPD device gained a maximum white-and-black-state reflectivity of 41.79%/0.56% and a peak contrast ratio of 74.15. Its response time could be reduced to as low as 166.7 ms, which outperforms the majority of other black-white EPD devices.

Electrostatic Stabilization of Nano Liquid Metals in Doped Nonpolar Liquids

Liquid metals and alloys are attracting renewed attention owing to their potential for application in various advanced technologies. Eutectic gallium-indium (EGaIn) has been focused on in particular because of its integrated advantages of high conductivity, low melting point, and low toxicity. In this study, the colloidal behavior of nano-dispersed EGaIn in nonpolar oils is investigated. Although the nonpolar oil continuous phase is commonly considered to be free of electric charges, electrostatic repulsion appears to be crucial in the colloidal stabilization of the nano-dispersed EGaIn phases, the modulation of which is possible by doping the oil phases with different types of oil-soluble surfactants. The qualitative correlation between the observed colloidal stabilities and the "zero field" particle mobilities inferred from the field-dependent electrophoretic mobilities indicates that the electric charging of EGaIn particles in surfactant-doped nonpolar oils is a static phenomenon that is maintained in equilibrium, rather than a solely field-induced process. A systematic investigation of the charging properties of these unique biphasic particles, consisting of the liquid Ga-In bulk and the solid Ga₂ O₃ surface that formed spontaneously, reveals the complicated system-dependent nature of the charging mechanisms mediated by ionic and nonionic surfactants in nonpolar media.

The Effects of Ethanol Concentration and of Ionic Strength on the Zeta Potential of Titania in the Presence of Sodium Octadecyl Sulfate

Sodium octadecyl sulfate (C18H37SO4Na) induces a negative zeta potential of metal oxides at very low surfactant concentrations as compared with shorter-chained sodium alkyl sulfates. The problem of low solubility of sodium octadecyl sulfate in water was solved by the addition of the surfactant to dispersions as ethanolic stock solution, but then the presence of ethanol in dispersions was inevitable. We demonstrate that the concentration of ethanol (up to 5% by mass) in a dispersion containing titania (TiO2) and sodium octadecyl sulfate has an insignificant effect on the zeta potential of particles. We further demonstrate that the shifts in the IEP of titania induced by the presence of sodium octadecyl sulfate are independent of the NaCl concentration. The results obtained in this study can be generalized for 1-1 salts other than NaCl, for metal oxides other than titania, for organic co-solvents other than ethanol, and for sparingly soluble ionic surfactants other than sodium octadecyl sulfate.

Polarity-Dependent Adsorption of Inverse Micelles

The adsorption of charged inverse micelles at the electrode-liquid interface has an important effect on field screening and on the voltage drop over diffuse double layers. Recently, we analyzed the behavior of inverse micelles in a nonpolar liquid close to this electrode-liquid interface. For the fluorocarbon/surfactant system under study, we are in the limit of slow adsorption and negligible desorption of inverse micelles on the electrodes. Upon applying a voltage step, this results in a measurable Stern layer buildup in the time range of hours clearly distinguishable from the diffuse double layer buildup, which happens in less than 1 s. This Stern layer buildup manifests itself by a shift in the voltage drop from the diffuse double layer to the Stern layer until the voltage drop over the Stern layers reaches the applied voltage, leaving a zero bulk field without the diffuse double layer. New measurements of the transients of Stern layer buildup show that the buildup of charges in the Stern layer is more complex. We explain the observed transient behavior by introducing an asymmetry in the adsorption rate of charged inverse micelles. We provide an equivalent electrical network, an analytical solution to explain the behavior in more detail, and simulations within the diffuse double layer limit for a range of adsorption rates.

Charge and Electrical Double Layer Formation in a Nonpolar Solvent Using a Nonionic Surfactant

In this work, we study the charge formation and the characteristics of the electrical double layer in a nonpolar medium using electrical impedance spectroscopy. To stabilize the free ionic species, a nonionic surfactant is added to the system. The conductivity and permittivity of the medium are obtained from high- to medium-frequency impedance data. Based on the correlation between (viscosity-adjusted) conductivity and surfactant concentration, we conclude that charge formation occurs due to a disproportionation mechanism. We accordingly estimate the concentration of the charge carriers in the sample and the Debye length of the diffuse double layer. The capacitance of the electrical double layer can be extracted from the low-frequency impedance data. We use this data to calculate the electrode distance of an equivalent parallel-plate capacitor. It is found that this distance is on the order of magnitude of Angstroms, indicating that the measured electrical double-layer capacitance is in fact the Stern layer capacitance.

Surfactant concentration modulates the motion and placement of microparticles in an inhomogeneous electric field

This study examined the effects of surfactants on the motion and positioning of microparticles in an inhomogeneous electric field. The microparticles were suspended in oil with a surfactant and the electric field was generated using sawtooth-patterned electrodes. The microparticles were trapped, oscillating, or attached to the electrodes. The proportion of microparticles in each state was defined by the concentration of surfactant and the voltage applied to the electrodes. Based on the trajectory of the microparticles in the electric field, we developed a new physical model in which the surfactant adsorbed on the microparticles allowed the microparticles to be charged by contact with the electrodes, with either positive or negative charges, while the non-adsorbed surfactant micellizing in the oil contributed to charge relaxation. A simulation based on this model showed that the charging and charge relaxation, as modulated by the surfactant concentration, can explain the trajectories and proportion of the trapped, oscillating, and attached microparticles. These results will be useful for the development of novel self-assembly and transport technologies and colloids sensitive to electricity.

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.

Proton transfer in nonpolar solvents: an approach to generate electrolytes in aprotic media

Stabilizing charged species in nonpolar solvents is challenging due to their low dielectric constant. As a contrast to formally ionic electrolytes, two series of acidic "potential" electrolytes have been developed in this study. These can be ionized by combining them stoichiometrically with a small molecule base in a typical nonpolar solvent, n-dodecane. The electrolytic conductivity of solutions of bis(2-ethylhexyl)phosphoric acid as mixtures with linear and branched dioctylamines and trioctylamines was measured, and the solutions were found to become increasingly conductive as the concentration increased, demonstrating that proton transfer occurred between the two species. Linear octylamines were found to be most effective at deprotonation. An acid-tipped poly(lauryl methacrylate) polymer (PLMA48-COOH) was also studied to give a polymer soluble in n-dodecane with a single ionizable group located precisely at the end of the polymer chain. Trioctylamine could successfully deprotonate this acid group. Even in an aprotic solvent, the transfer of protons between acidic and basic moieties is a useful method for controlling the properties of dissolved molecules.

Contact Charge Electrophoresis: Fundamentals and Microfluidic Applications

Contact charge electrophoresis (CCEP) uses steady electric fields to drive the oscillatory motion of conductive particles and droplets between two or more electrodes. In contrast to traditional forms of electrophoresis and dielectrophoresis, CCEP allows for rapid and sustained particle motions driven by low-power dc voltages. These attributes make CCEP a promising mechanism for powering active components for mobile microfluidic technologies. This Feature Article describes our current understanding of CCEP as well as recent strategies to harness it for applications in microfluidics and beyond.

Synthesis and electrokinetics of cationic spherical nanoparticles in salt-free non-polar media

Cationic diblock copolymer nanoparticles have been prepared in n-dodecane via polymerization-induced self-assembly (PISA). A previously reported poly(stearyl methacrylate)-poly(benzyl methacrylate) (PSMA-PBzMA) PISA formulation (Chem. Sci. 2016, 7, 5078-5090) was modified by statistically copolymerizing an oil-soluble cationic methacrylic monomer, (2-(methacryloyloxy)ethyl)trimethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, with either SMA or BzMA, to produce either charged shell or charged core nanoparticles. The electrokinetics were studied as a function of many variables (function of volume function, particle size, solvent viscosity, and number of ions per chain). These data are consistent with electrophoresis controlled by counterion condensation, which is typically observed in salt-free media. However, there are several interesting and unexpected features of interest. In particular, charged shell nanoparticles have a lower electrophoretic mobility than the equivalent charged core nanoparticles, and the magnitude of the electrophoretic mobility increases as the fraction of cationic stabilizer chains in the shell layer is reduced. These results show that cationic PSMA-PBzMA spheres provide an interesting new example of electrophoretic nanoparticles in non-polar solvents. Moreover, they should provide an ideal model system to evaluate new electrokinetic theories.

Charge transport by inverse micelles in non-polar media

Charged inverse micelles play an important role in the electrical charging and the electrodynamics of nonpolar colloidal dispersions relevant for applications such as electronic ink displays and liquid toner printing. This review examines the properties and the behavior of charged inverse micelles in microscale devices in the absence of colloidal particles. It is discussed how charge in nonpolar liquids is stabilized in inverse micelles and how conductivity depends on the inverse micelle size, water content and ionic impurities. Frequently used nonpolar surfactant systems are investigated with emphasis on aerosol-OT (AOT) and poly-isobutylene succinimide (PIBS) in dodecane. Charge generation in the bulk by disproportionation is studied from measurements of conductivity as a function of surfactant concentration and from generation currents in quasi steady-state. When a potential difference is applied, the steady-state situation can show electric field screening or complete charge separation. Different regimes of charge transport are identified when a voltage step is applied. It is shown how the transient and steady-state currents depend on the rate of bulk generation, on insulating layers and on the sticking or non-sticking behavior of charged inverse micelles at interfaces. For the cases of AOT and PIBS in dodecane, the magnitude of the generation rate and the type of interaction at the interface are very different.

Electrokinetics and behavior near the interface of colloidal particles in non-polar dispersions

The electrokinetics and charging of nonpolar colloidal dispersions subjected to a voltage are investigated by electric current and optical measurements. From electric current measurements in response to an alternating triangular voltage with a peak value of a few hundred volts, we find that polystyrene toner particles are compacted near the electrodes and their charge increases by more than a factor of 20. The important increase of charge is interpreted by a mechanism in which counter charges, which are originally at the particle surface, are desorbed. Optical measurements performed under a dc voltage of the order of a few hundred volts demonstrate that the charge of the particles can again decrease or even be inverted. These phenomena are attributed to the movement of counter charged species from the interface layers onto the surface of the particles. The findings of this study are relevant for electrophoretic displays and liquid toner printing.

Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality

Surface charging phenomena in nonpolar dispersions are exploited in a wide range of industrial applications, but their mechanistic understanding lags far behind. We investigate the surface charging of a variety of polymer particles with different surface functionality in alkane solutions of a custom-synthesized and purified polyisobutylene succinimide (PIBS) polyamine surfactant and a related commercial surfactant mixture commonly used to control particle charge. We find that the observed electrophoretic particle mobility cannot be explained exclusively by donor-acceptor interactions between surface functional groups and surfactant polar moieties. Our results instead suggest an interplay of multiple charging pathways, which likely include the competitive adsorption of ions generated among inverse micelles in the solution bulk. We discuss possible factors affecting the competitive adsorption of micellar ions, such as the chemical nature of the particle bulk material and the size asymmetry between inverse micelles of opposite charge.

Celebrating Soft Matter's 10th Anniversary: Influencing the charge of poly(methyl methacrylate) latexes in nonpolar solvents

Sterically-stabilized poly(methyl methacrylate) (PMMA) latexes dispersed in nonpolar solvents are a classic, well-studied system in colloid science. This is because they can easily be synthesized with a narrow size distribution and because they interact essentially as hard spheres. These PMMA latexes can be charged using several methods (by adding surfactants, incorporating ionizable groups, or dispersing in autoionizable solvents), and due to the low relative permittivity of the solvents (εr ≈ 2 for alkanes to εr ≈ 8 for halogenated solvents), the charges have long-range interactions. The number of studies of these PMMA particles as charged species has increased over the past ten years, after few studies immediately following their discovery. A large number of variations in both the physical and chemical properties of the system (size, concentration, surfactant type, or solvent, as a few examples) have been studied by many groups. By considering the literature on these particles as a whole, it is possible to determine the variables that have an effect on the charge of particles. An understanding of the process of charge formation will add to understanding how to control charge in nonaqueous solvents as well as make it possible to develop improved technologically relevant applications for charged polymer nanoparticles.