Low Dielectric Constant Media Research Articles

Equilibrium Studies on Pd(II)–Amine Complexes with Bio-Relevant Ligands in Reference to Their Antitumor Activity

This review article presents an overview of the equilibrium studies on Pd-amine complexes with bio-relevant ligands in reference to their antitumor activity. Pd(II) complexes with amines of different functional groups, were synthesized and characterized in many studies. The complex formation equilibria of Pd(amine)2+ complexes with amino acids, peptides, dicarboxylic acids and DNA constituents, were extensively investigated. Such systems may be considered as one of the models for the possible reactions occurring with antitumor drugs in biological systems. The stability of the formed complexes depends on the structural parameters of the amines and the bio-relevant ligands. The evaluated speciation curves can help to provide a pictorial presentation of the reactions in solutions of different pH values. The stability data of complexes with sulfur donor ligands compared with those of DNA constituents, can reveal information regarding the deactivation caused by sulfur donors. The formation equilibria of binuclear complexes of Pd(II) with DNA constituents was investigated to support the biological significance of this class of complexes. Most of the Pd(amine)2+ complexes investigated were studied in a low dielectric constant medium, resembling that of a biological medium. Investigations of the thermodynamic parameters reveal that the formation of the Pd(amine)2+ complex species is exothermic.

Effect of polyol type on the structure and properties of lecithin liposomes prepared using the polyol dilution method

The polyol dilution (PD) method is an approach for preparing liposomes by pre-mixing a lipid/polyol binary mixture, followed by hydration, and is widely used in the cosmetics industry. In this method, polyols play an important role in the formulation design and application, however, the effect of the polyol type on liposomes (PD-liposomes) is yet to be understood. In this study, we investigated the effect of three types of polyols—glycerol (Gly), propylene glycol (PG), and 1,3-butylene glycol (BG)—on the physicochemical properties of PD-liposomes composed of hydrogenated egg yolk lecithin (EL). The particle size of PD-liposomes was controlled by adjusting the type and concentration of the polyols. Furthermore, the addition of polyols improved the dispersion stability, indicating that PD-liposomes are suitable for industrial applications. The morphology of lipid/polyol binary mixture was the determining factor of the liposome structure, and the formation region of unilamellar vesicles (ULVs) and multilamellar vesicles (MLVs) was closely related to the dielectric constant of the dispersion medium. ULVs were formed using a high dielectric constant dispersion medium, such as Gly, and MLVs were formed using a low dielectric constant dispersion medium, such as PG and BG at high concentrations. This study showed that the addition of polyols improves the phase homogeneity of the bilayer and that PG or BG is more effective than Gly. The control of the homogeneity of the bilayer phase is an effective technique to prevent leakage and control the sustained release of the active ingredient; thus, this method enhances the efficacy of the active ingredient. This study is the first to systematically elucidate the control mechanisms of the physicochemical properties of PD-liposomes, including the particle size, dispersion stability, shape, and phase state of the bilayer. Our findings will contribute to the wide range of applications of PD-liposomes.

Probing Electrolyte Influence on CO2 Reduction in Aprotic Solvents

Selective CO2 capture and electrochemical conversion are important tools in the fight against climate change. Industrially, CO2 is captured using a variety of aprotic solvents due to their high CO2 solubility. However, most research efforts on electrochemical CO2 conversion use aqueous media and are plagued by competing hydrogen evolution reaction (HER) from water breakdown. Fortunately, aprotic solvents can circumvent HER, making it important to develop strategies that enable integrated CO2 capture and conversion. However, the influence of ion solvation and solvent selection within nonaqueous electrolytes for efficient and selective CO2 reduction is unclear. In this work, we show that the bulk solvation behavior within the nonaqueous electrolyte can control the CO2 reduction reaction and product distribution occurring at the catalyst–electrolyte interface. We study different tetrabutylammonium (TBA) salts in two electrolyte systems with glyme ethers (e.g., 1,2 dimethoxyethane or DME) and dimethyl sulfoxide (DMSO) as a low and high dielectric constant medium, respectively. Using spectroscopic tools, we quantify the fraction of ion pairs that forms within the electrolyte. Also, we show how ion pair formation is prevalent in DME and is dependent on the anion type. More importantly, we show that as ion pair formation decreases within the electrolyte, CO2 current densities increase, and a higher CO Faradaic efficiency is observed at low overpotentials. Meanwhile, in an electrolyte medium where the ion pair fraction does not change with the anion type (such as in DMSO), a smaller influence of solvation is observed on CO2 current densities and product distribution. By directly coupling bulk solvation to interfacial reactions and product distribution, we showcase the importance and utility of controlling the reaction microenvironment in tuning the electrocatalytic reaction pathways. Insights gained from this work will enable novel electrolyte designs for efficient and selective CO2 conversion to desired fuels and chemicals.

The electrolytes in layered charged interfaces modulate the total pressure by ion condensation

We determined with Canonical Monte Carlo simulations the spatial distribution of counterions released from discretely charged interfaces which form layered dielectric media, as simple models of either an aqueous thin film or a lipid membrane. The ions in a medium of low dielectric constant (membrane) experience a substantial condensation onto the charged interfaces forming a quasi-two-dimensional ionic double layer which induces attraction between the equally charged bounding surfaces. When the dielectric constant is increased (case of a thin film) there is a shift in the sign of the interaction between the interfaces, changing from attractive at short distances of their separation, towards repulsive for larger distances. At very high dielectric constant the force-distance curve always shows repulsion.

(Invited) Polymer Simulations Under External Fields

In order to design next generation functional polymers, not only the structure but the dynamics under external field should be understood. In this paper we report our simulation approach to investigate the structure and dynamics. Polymer brush is used for the model system of very low friction such as synovial joints. Recently, polymer brush is focused as a functional binder of sodium ion secondary battery. Since sodium ions are hard to make passive film, or SEI (solid electrolyte interphase) layer on the negative electrode, functional polymer binder help to stabilize the atmosphere around the electrode. We simulate the ionic conductivity of polyelectrolyte binder system under external electric field. The conductivity depend on the polymer structure, such as hardness and linear charge density of the polymer chain. The other important function of polymers are the fluid dynamics effect of polymers under external fluid field. As a model system of VII (viscosity index improver) polymers, we simulate the dynamics under shear filed. The dynamics of polymers are studied for bulk system and the confined system. We made simulation algorithm. The Brownian motion of polymer segments are treated by Langevin Dynamics, and the solvent flow is treated by Lattice Boltzmann Method, in order to obtain both high computational efficiency and precise dynamics. We succeed to reproduce the flow dynamics of polymers under shear. Moreover, the model is extended to include long-range interactions of functional groups. In low dielectric constant medium such as lubricant oils, the functional groups interact as ionic groups in high dielectric constant medium such as waters. The specific results shows that the colloidal science in oils have many problems to be solved.

Humidity Influence on Optical Properties of Nanowire Colloids with Modulated Visual Response to Electrostatic Charge

Electric field created by the nearby electrostatic charge orients the colloidal nanowires in the medium of low dielectric constant, causing a visible color change. We show that the kinetics of subsequent color restoration correlates well to ambient relative humidity. Presented principle could be used for detecting humidity, which affects net surface electrostatic charge possibly via chemical reactions.

Solvent Dependence of Double Proton Transfer in the Formic Acid-Formamidine Complex: Path Integral Molecular Dynamics Investigation.

Solvent dependence of double proton transfer in the formic acid-formamidine (FA-FN) complex at room temperature was investigated by means of ab initio path integral molecular dynamics (AIPIMD) simulation with taking nuclear quantum and thermal effects into account. The conductor-like screening model (COSMO) was applied for solvent effect. In comparison with gas phase, double proton delocalization between two heavy atoms (O and N) in FA-FN were observed with reduced proton transfer barrier height in low dielectric constant medium (<4.8). For dielectric constant medium at 4.8, the chance of finding these two protons are more pronounced due to the solvent effect which completely washes out the proton transfer barrier. In the case of higher dielectric constant medium (>4.8), the ionic species becomes more stable than the neutral ones and the formate anion and formamidium cation are thermodynamically stable. For ab initio molecular dynamics simulation, in low dielectric constant medium (<4.8) a reduction of proton transfer barrier with solvent effect is found to be less pronounced than the AIPIMD due to the absence of nuclear quantum effect. Moreover, the motions of FA-FN complex are significantly different with increasing dielectric constant medium. Such a difference is revealed in detail by the principal component analysis.

Modulation of electroosmotic flow through electrolyte column surrounded by a dielectric oil layer

The present study deals with the electroosmotic flow (EOF) through a column of an aqueous solution (medium of high dielectric constant) surrounded by an oil layer (low dielectric constant medium). The dielectric oil layer contains solubilized surfactants bearing additional mobile charges and the electrolyte medium comprises of binary symmetric electrolyte ions. A mathematical model based on Poisson-Boltzmann equation for induced potential and Stokes equation for fluid flow is adopted. The model assumes that the oil-electrolyte interface bears a nonzero surface charge and the electrolyte ions can penetrate across the interface. A semi-analytical method has been developed to study the modulation of EOF by considering the effect of partition coefficient of the electrolyte ions. The effect of concentration of mobile charges inside the oil layer and the interfacial charges on the modulation of EOF through such a system is investigated for a wide range of bulk electrolyte concentration.

Preorganized Hydrogen Bond Donor Catalysts: Acidities and Reactivities.

Measured DMSO pKa values for a series of rigid tricyclic adamantane-like triols containing 0-3 trifluoromethyl groups (i.e., 3(0)-3(3)) are reported. The three compounds with CF3 substituents are similar or more acidic than acetic acid (pKa = 13.5 (3(1)), 9.5 (3(2)), 7.3 (3(3)) vs 12.6 (HOAc)), and the resulting hydrogen bond network enables a remote γ-trifluoromethyl group to enhance the acidity as well as one located at the α-position. Catalytic abilities of 3(0)-3(3) were also examined. In a nonpolar environment a rate enhancement of up to 100-fold over flexible acyclic analogs was observed presumably due to an entropic advantage of the locked-in structure. Gas-phase acidities are found to correlate with the catalytic activity better than DMSO pKa values and appear to be a better measure of acidities in low dielectric constant media. These trends are reduced or reversed in polar solvents highlighting the importance of the reaction environment.

Interactions between Pairs of Charges Buried in the Hydrophobic Interior of a Protein are Unexpectedly Weak

Internal ion pairs buried in the hydrophobic protein interior are essential for many important biochemical processes, including H+ transport, e- transfer, ion homeostasis, and catalysis. Despite the importance of these pairs, their properties remain poorly understood. It has been suggested that, for some systems, medium or long-range Coulomb interactions between buried groups could play a role in determining biological function. In principle, this should only be possible if the protein interior behaves like a medium of low dielectric constant, as assumed in most electrostatics models. These motifs are then of special interest not only for their functional roles, but also as probes to examine the balance between Coulomb and hydration energies experienced by buried charges. As the distance between the internal charges increases, the balance of these energies will disfavor burying the groups in the charged state, eliminating any Coulomb interaction between them. To examine the balance between Coulomb and dehydration energies in the protein interior we engineered a series of double variants in a highly stable variant of staphylococcal nuclease. Each variant included an internal histidine paired with either an aspartate or glutamate buried at various internal positions throughout the protein interior. This set of proteins used to probe the distance dependence of potential Coulomb interactions. No significant electrostatic coupling was observed for pairs that interacted through medium- or long-range Coulomb interactions. When the distance between the groups was short, favorable coupling energies were only observed when the side chains of the pair could achieve a geometry favorable for a H-bond; spatial proximity alone was insufficient to create favorable Coulomb interactions. Simple electrostatics models that describe ionized states as point charges interacting through space are unlikely to be able to reproduce these data.

Evaluation of Heme Peripheral Groups Interactions in Low-Dielectric Constant Media

In this study, we measured the contributions of the ionization of the heme propionates to the reduction potentials of heme b and heme a (bis)N-methylimidazole complexes in various low-dielectric constant conditions. Additionally, we measured the effects of H-bond to the heme a formyl group on the reduction potential of the heme. The performed electrochemical measurements show that ionization of the heme propionates lead to the largest redox change in dichloromethane with no electrolyte. The measured reduction potential changes for heme b and heme a were - 55 and −47 mV (±10 mV) per ionized propionate, respectively. For heme a, the study demonstrates how the dielectric constant of the medium is important in the magnification of the αpKa upon redox-linked ionization of the heme propionates and their roles in the proton pump of cytochrome c oxidase. Additionally, we carried out a detailed study on the H-bonding properties of heme a model compounds (copper mono- and di-acetyl porphyrins) and the effects of the dielectric constant of the solvent on the measured dissociation constant (Kd) between various H-bond donors and the copper porphyrins. Our measurements show that H-bonds between the copper porphyrins and TFA or phenol are significant in benzene and dichloromethane, but extremely weak in the presence of acetonitrile. This study shows how the dielectric constant of the medium also plays a role in modulating the properties of heme a through H-bonding of its formyl group.

Palladium(II) Complexes Containing Mixed Nitrogen-Sulphur Donor Ligands: Interaction of [Pd(Methionine Methyl Ester)(H2O)2](2+) with Biorelevant Ligands.

Pd(MME)Cl2 complex (MME = methionine methyl ester) was synthesised and characterized by physicochemical measurements. The reaction of [Pd(MME)(H2O)2]2+ with amino acids, peptides, or dicarboxylic acids was investigated at 25°C and 0.1 M ionic strength. Amino acids and dicarboxylic acids form 1 : 1 complexes. Peptides form both 1 : 1 complexes and the corresponding deprotonated amide species. The stability of the complexes formed was determined and the binding centres of the ligands were assigned. Effect of solvent on the stability constant of Pd(MME)-CBDCA complex, taken as a representative example, shows that the complex is more favoured in a medium of low dielectric constant. The concentration distribution diagrams of the complexes were evaluated.

Evaluation of a novel tripodal diglycolamide for actinide extraction: Solvent extraction and SLM transport studies

A novel tripodal diglycolamide (T-DGA) extractant in a diluent mixture (10:1 n-dodecane– iso-decanol) was evaluated for the extraction of actinide ions such as UO 2 2+, Np(IV), Pu(IV), and Am(III) and fission product element ions such as Cs + and Sr 2+ from acidic feed solutions. Conditions for quantitative extraction and stripping were found out for the actinide ions other than UO 2 2+ which is poorly extracted. Solvent extraction studies were carried out at varying concentrations of the extractant and nitric acid using Am(III) and were compared with the results obtained with TODGA ( N, N, N′, N′-tetraoctyldiglycolamide), a representative diglycolamide extractant proposed for actinide partitioning. The extracted species was found out to be Am(NO 3) 3·2T-DGA (o) at higher T-DGA concentrations, while mixed 1:1 and 1:2 species were observed at lower T-DGA concentrations. The prevalence of 1:2 species in the present case as compared to the 1:1 species reported in a similar system was attributed to the low dielectric constant diluent medium used in the present study. Supported liquid membrane studies were also carried out using the T-DGA under varying feed acidities, carrier extractant concentration and compared with TODGA under identical conditions. The transport rates are dependent on the strippant used in the receiver phase and a 0.01 M EDTA solution (pH 3.0) was found to yield quantitative transport of Am(III) using T-DGA as compared to dilute nitric acid, which was efficiently used for the TODGA system in an earlier study. The transport efficiency of the carrier solvent system was Am(III) > Pu(IV) ≫ Np(IV) > UO 2 2+. The diffusion parameters were calculated for the T-DGA system and compared with the experimentally determined parameters.

Flux decline during cross flow membrane filtration of electrolytic solution in presence of charged nano-colloids: A simple electrokinetic model

An electrokinetic transport based approach for quantification of reversible flux decline due to the concentration polarization of an electrolyte solution in presence of charged colloids is presented. The model envisions the electrolyte transport across a charged cake or gel layer as transport of ions through charged cylindrical capillaries. This model is coupled with the standard theory of concentration polarization during cross flow membrane filtration. The analysis is carried out entirely in terms of generalized, non-dimensional variables. A dimensionless group termed as the scaled gel layer resistance evolves from the analysis, which accounts for the electrical properties of the charged nano-colloids and the electrolyte solution. A parametric study is performed to elucidate the coupled influence of mass transfer, membrane resistance, gel resistance, and electrical properties of the gel-electrolyte polarized layer. The effects of these parameters are examined on the filtration performance through the model equations.

40.1: Invited Paper: Dyed Polymeric Microparticles for Colour Rendering in Electrophoretic Displays

AbstractRealising bright and vivid full colour performance in electrophoretic displays presents a significant challenge to display architects and material designers. A range of dyed polymeric microparticles with tunable size, charge and colour have been developed. When formulated as colloidal dispersions in low dielectric constant media, the particles are electrophoretically active and are suitable for use in full colour or monochrome electrophoretic displays.

Simple electrostatic model applicable to biomolecular recognition.

An exact, analytic solution for a simple electrostatic model applicable to biomolecular recognition is presented. In the model, a layer of high-dielectric constant material (representative of the solvent, water), whose thickness may vary separates two regions of low-dielectric constant material (representative of proteins, DNA, RNA, or similar materials), in each of which is embedded a point charge. For identical charges, the presence of the screening layer always lowers the energy compared to the case of point charges in an infinite medium of low-dielectric constant. Somewhat surprisingly, the presence of a sufficiently thick screening layer also lowers the energy compared to the case of point charges in an infinite medium of high-dielectric constant. For charges of opposite sign, the screening layer always lowers the energy compared to the case of point charges in an infinite medium of either high or low dielectric constant. The behavior of the energy leads to a substantially increased repulsive force between charges of the same sign. The attractive force between charges of opposite signs is weaker than in an infinite medium of low dielectric constant material but stronger than in an infinite medium of high dielectric constant material. The presence of this behavior, which we name asymmetric screening, in the simple system presented here confirms the generality of the behavior that was established in a more complicated system of an arbitrary number of charged dielectric spheres in an infinite solvent.

Dyed polymeric microparticle colloid dispersions for full colour electrophoretic displays

Realizing bright and vivid full colour performance in electrophoretic displays presents a significant challenge to display architects and material designers. A range of dyed polymeric microparticles with tunable size, charge and colour have been developed. When formulated as colloidal dispersions in low dielectric constant media, the particles are electrophoretically active and are suitable for use in full colour or monochrome electrophoretic displays.

Direct Measurement of Photoinduced Charge Separation Distances in Donor−Acceptor Systems for Artificial Photosynthesis Using OOP-ESEEM

The distance over which two photogenerated charges are separated in electron donor-acceptor systems for artificial photosynthesis depends on the structure of the system, while the lifetime of the charge separation and, ultimately, its ability to carry out useful redox chemistry depend on the electronic coupling between the oxidized donor and reduced acceptor. The radical ions produced by charge separation are frequently delocalized over the pi systems of the final oxidized donor and reduced acceptor, so that there is often significant uncertainty as to the average distance between the separated charges, especially in low dielectric constant media, where the Coulomb attraction of the ions may be significant and the charge distribution of the ions may be distorted, so that the average distance between them may be shorter than that implied by their chemical structures. The charge separation distances between photogenerated radical ions in three donor-acceptor molecules having different donor-acceptor distances were measured directly from their dipolar spin-spin interactions using out-of-phase electron spin echo envelope modulation (OOP-ESEEM). The measured distances in toluene at 85 K compare favorably to the calculated distances between the centroids of the spin distributions of the radical ions within the radical ion pairs. These results show that despite the intrinsically nonpolar nature of medium, the spin (and charge) distributions of the RPs are not significantly distorted by Coulomb attraction over these long distances. This study shows that OOP-ESEEM is well-suited for probing the detailed structural features of charge-separated intermediates that are essential to understanding how to design molecular structures that prolong and control charge separation for artificial photosynthesis.

Coupled donors in quantum dots: Quantum size and dielectric mismatch effects

Spatial and dielectric confinement modulations of the spontaneous emission rates, transition energies, and charge-density distributions of a singly ionized double donor system $({D}_{2}^{+})$ in a spherical quantum dot are calculated within the framework of the effective-mass envelope function approximation. Dipole moments, energy splittings, transition moments, electron-density distributions, and spontaneous emission rates involving bonding and antibonding lowest-lying molecular states are addressed for different dielectric environments, quantum dot radii, and relative locations of the coupled impurities inside the dot. The results indicate that the donor molecule behaves as heteropolar when the spatial confinement breaks the inversion symmetry, which is paralleled by a strong reduction in the excited-state radiative lifetime. Dielectric confinement, acting on a larger length scale than spatial confinement, may recover the bulklike homopolar character when the dot is embedded in a low dielectric constant medium. In the weak spatial confinement regime, dielectric effects can increase the corresponding bulk radiative lifetimes significantly and simultaneously modulate the charge-density distribution.

Theory of dielectrically induced surface excitonic states in spherical quantum dots

The formation of quantum dot (QD) excitonic surface states induced by dielectric mismatch is theoretically explored in spherical nanocrystals embedded in very high and in very low permittivity media. It is found that the transition from volume to surface exciton states $(V\ensuremath{\rightarrow}S)$ always parallels a sudden drop of exciton brightness if the QD is embedded in low dielectric constant media. This is not the case of a QD buried in high permittivity media. In this case, the $V\ensuremath{\rightarrow}S$ transition is monitored by a reduction in exciton brightness or not depending on the ${m}_{h}^{*}∕{m}_{e}^{*}$ ratio between the effective masses of electron and hole. The presence of a hydrogenic donor impurity at the QD center can drastically reduce the electron-hole density overlap and thus the excitonic binding energy and the drop of brightness that parallels the formation of surface states.