Counterion Size Research Articles

Counterion-Dependent Access to Low-Symmetry Lyotropic Sphere Packings of Ionic Surfactant Micelles.

The water-driven self-assembly of homologous dianionic surfactants into lyotropic liquid crystals (LLCs) is investigated, with a focus on understanding how surfactant headgroup and counterion identities guide supramolecular spherical mesophase selection. Using temperature-dependent small-angle X-ray scattering (SAXS), we demonstrate that 2-alkylmalonate surfactants (CnMal-M2) with n = 8 (octyl) or 10 (decyl) and M = K+, Cs+, or (CH3)4N+ form both simple and complex micelle packings. Observed spherical morphologies include body-centered cubic (BCC), hexagonally closest-packed (HCP), and tetrahedrally closest-packed Frank-Kasper (FK) A15 and σ phases (Pm3(-)n and P42/mnm symmetries, respectively). Previously observed in only one other minimally hydrated surfactant, the σ phase is a rare LLC morphology comprising a low-symmetry unit cell containing 30 sub-2-nm quasispherical micelles, each of which belongs to one of five symmetry-equivalent classes with discrete aggregation numbers. Temperature versus water concentration phase maps for CnMal-M2 LLCs reveal that σ-phase formation depends sensitively on the size and polarizability of the surfactant counterion and the length of the surfactant alkyl tail. These observations are rationalized in terms of a delicate interplay between global packing symmetry and local particle symmetry, and the extent to which counterion-headgroup correlations enforce the latter structures in these LLC phases.

Influence of solvent polarization and non-uniform ion size on electrostatic properties between charged surfaces in an electrolyte solution

In this paper, we study electrostatic properties between two similar or oppositely charged surfaces immersed in an electrolyte solution by using the mean-field approach accounting for solvent polarization and non-uniform size effects. Applying a free energy formalism accounting for unequal ion sizes and orientational ordering of water dipoles, we derive coupled and self-consistent equations to calculate electrostatic properties between charged surfaces. Electrostatic properties for similarly charged surfaces depend on the counterion size but not on the coion size. Moreover, electrostatic potential and osmotic pressure between similarly charged surfaces are found to be increased with increasing counterion size. On the other hand, the corresponding ones between oppositely charged surfaces are related to both sizes of positive and negative ions. For oppositely charged surfaces, the electrostatic potential, number density of solvent molecules, and relative permittivity of an electrolyte having unequal ion sizes are not symmetric about the centerline between the charged surfaces. For either case, the consideration of solvent polarization results in a decrease in the electrostatic potential and the osmotic pressure compared to the case without the effect.

Influence of counterion type on dielectric and electrorheological responses of poly(ionic liquid)s

Different from classic water activated polyelectrolytes, poly(ionic liquid)s (PILs) bearing hydrophobic counterions show high electrorheological (ER) response in dry state. This offers opportunity for practical applications of polyelectrolyte-based ER materials in wide temperature region. For guiding the design of PIL-based ER materials with improved performance, understanding the structure-property relationship is very important. In this paper, a series of poly(p-vinylbenzyl trimethylammonium)-based PIL particles with different counteranions ([PVBTMA][X]) were synthesized with the goal of understanding the role of mobile counterions on ER response of PILs. To exclude particle shape effect, the PIL series were controlled to be monodisperse spheres with a similar diameter via microwave-assisted dispersion polymerization. Under electric fields, the ER response of PIL particles when dispersed in insulating oil was investigated by temperature-modulated rheological test. It showed that the ER response of PILs depends on the type of counteranions. The order of magnitude of ER response of PILs with different counteranion is trifluoromethanesulfonate (TfO−) > tetrafluoroborates (BF4−) > bis-(trifluoromethanesulfonylimide) (TFSI−) > hexafluorophosphates (PF6−). The mechanism behind the influence of counterion type on ER response was analysed by dielectric relaxation spectroscopy. It demonstrated that the influence of counteranion type on ER response may be related to the variation of size and plasticization effect of counteranions that has altered the transport dynamic of mobile counterions and ion motion-induced interfacial polarization.

CO2-Reactive Ionic Liquid Surfactants for the Control of Colloidal Morphology.

This article reports on a new class of stimuli-responsive surfactant generated from commercially available amphiphiles such as dodecyltrimethylammmonium bromide (DTAB) by substitution of the halide counterion with counterions such as 2-cyanopyrrolide, 1,2,3-triazolide, and L-proline that complex reversibly with CO2. Through a combination of small-angle neutron scattering (SANS), electrical conductivity measurements, thermal gravimetric analysis, and molecular dynamics simulations, we show how small changes in charge reorganization and counterion shape and size induced by complexation with CO2 allow for fine-tunability of surfactant properties. We then use these findings to demonstrate a range of potential practical uses, from manipulating microemulsion droplet morphology to controlling micellar and vesicular aggregation. In particular, we focus on the binding of these surfactants to DNA and the reversible compaction of surfactant-DNA complexes upon alternate bubbling of the solution with CO2 and N2.

Colloid chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 3. Calculation of electrochemical characteristics of membranes in terms of a homogeneous model

The electrosurface characteristics of nanoporous glass membranes–ion concentrations in pores with taking into account the specificity of counterions, electrokinetically mobile charge, the convective component of pore solution electrical conductivity, electroosmotic mobility of a liquid in the field of streaming potential and ion mobilities in pore space–were calculated within the homogeneous model. The effects of the type of counterion (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium ions), solution concentration, glass composition, and pore size on the equilibrium and transport characteristics of membrane systems have been analyzed. A method for the determining of electrolyte activity coefficients in the membranes has been proposed.

Ultrafast electron diffraction study of single-crystal (EDO-TTF)2SbF6: Counterion effect and dimensionality reduction

Ultrafast electron diffraction is a sensitive tool to directly study molecular dynamics in structural detail. Here, we report the influence of counterion size on the photoinduced insulator-to-metal phase transition in two derivatives of the organic salt (EDO-TTF)2XF6. For X=P, three dominant motions are present and the molecules undergo the transition, whereas in the case of X=Sb, only two dominant motions are found and the molecules do not evolve into the metallic state. This reduction in dimensionality is supported by a novel data analysis method involving singular value decomposition of time-resolved electron diffraction data in reciprocal space.

Monovalent and divalent ion sorption in a cation exchange membrane based on cross-linked poly (p-styrene sulfonate-co-divinylbenzene)

Many fundamentals of ion sorption in ion exchange membranes (IEMs) are not fully understood, and few modern studies focus on multivalent ion sorption, despite the importance of divalent ion transport properties in water purification applications. Here, MgCl2 and CaCl2 sorption in a commercial cross-linked, styrene-divinylbenzene cation exchange membrane (CEM) with fixed sulfonate groups was measured and compared with NaCl, LiCl and KCl sorption in the same material. The data were correlated with counter-ion size and valence, as well as membrane water content. Divalent salts had higher sorption coefficients than monovalent salts, even though membrane water content significantly decreased in the presence of divalent counter-ions. The Donnan potential is weaker in the presence of multivalent counter-ions, which increases co-ion sorption, contributing to the observed increase in salt sorption coefficients for divalent salts. Ion activity coefficients in the membrane were calculated and compared to predictions of the recently proposed Manning-Donnan model. This model showed much better agreement with the experimental data than the ideal Donnan model, suggesting that ion non-ideality in the membrane is important when analyzing ion sorption and transport in IEMs.

Counterion Size and Nature Control Structural and Mechanical Response in Cellulose Nanofibril Nanopapers

Nanopapers formed from aqueous dispersions of cellulose nanofibrils (CNFs) combine stiffness, strength, and toughness. Yet, delicate interactions operate between the CNFs during nanopaper formation and mechanical deformation. We unravel in detail how counterions, being either of the organic alkyl ammonium kind (NR4+) or of the earth metal series (Li+, Na+, Cs+), need to be chosen to achieve outstanding combinations of stiffness, strength, and toughness, extending to previously unreached territories. We relate structure formation processes in terms of colloidal stabilization to nanostructural details such as porosity and ability for swelling, as well as to interfibrillar interactions in bulk and macroscale mechanical properties. We demonstrate that our understanding also leads to new levels of ductility in bioinspired CNF/polymer nanocomposites at high levels of reinforcements. These results contribute to future rational design of CNF-based high-performance materials.

Effects of counterion size and backbone rigidity on the dynamics of ionic polymer melts and glasses.

It is well-known that the nature and size of the counterions affect the ionic conductivity and glass transition temperature of ionic polymers in a significant manner. However, the microscopic origin of the underlying changes in the dynamics of chains and counterions is far from completely understood. Using coarse-grained molecular dynamics simulations of flexible and semi-flexible ionic polymers, we demonstrate that the glass transition temperature of ionic polymeric melts depends on the size of monovalent counterions in a non-monotonic manner. The glass transition temperature is found to be the highest for the smallest counterions and decreases with an increase in the counterion radii up to a point, after which the glass transition temperature increases with a further increase in the radii. This behavior is because the counterions have significant effects on the coupled dynamics of the charges on the chains and counterions. In particular, increase in the radii of the counterions leads to strongly coupled dynamics between the charges on the chains and the counterions. The static dielectric constant of the polymer melts also has a significant effect on the coupling and the glass transition temperature. The glass transition temperature is predicted to decrease with an increase in the dielectric constant. This, in turn, leads to an increase in the diffusion constant of the counterions at a given temperature. Backbone rigidity is shown to increase the glass transition temperature and decrease the coupling. Furthermore, faster counterion dynamics is predicted for the melts of semi-flexible chains in comparison with flexible chains at the same segmental relaxation time. As the semi-flexible chains tend to have a longer segmental relaxation time, semi-flexible polymers with high dielectric constants are predicted to have diffusion constants of counterions comparable with flexible polymers.

Nanocomposites made from thermoplastic waterborne polyurethane and colloidal silica. The influence of nanosilica type and amount on the functional properties

Abstract Novel waterborne polyurethane/silica nanocomposites were prepared via blending of two aqueous dispersions. Polyurethane water dispersion (PUD) was synthesized from polycarbonate diol (PC), 1,6-diisocyanatohexane (HDI) and butane-1,4-diol (BD) in order to provide solely linear character of PU chain. Two types of commercial colloidal silica, Ludox AS and Ludox TMA, differing in the size, shape and type of counter ions were used. The nanocomposites were made from negatively charged PUD and aqueous nanofiller sol at concentrations 5, 32 and 50 wt% of nanosilica. The films were obtained after slow drying of the water mixtures and characterized by AFM, SEM, tensile testing, DMTA, TG and water uptake measurements. It was found that all films are thermoplastics materials, reinforced by hydrogen bonds between silica particles and the polymer matrix, soluble in organic solvents and possible to re-production of PUD. Ludox AS was smaller than TMA and revealed better improvement of PUD-based films due to higher physical cross-linking density. It was observed that film containing 32 wt% of silica Ludox AS showed ten-times increasing Young’s modulus with slight improvement of elongation at break and two-times higher melting temperature in comparison to the neat PU matrix. Moreover sample with 5 wt% of AS nanofiller loading revealed excellent water resistance (swelling 0.8%). The obtained PU/silica nanocomposites have promising applications in recyclable environment-friendly waterborne coatings.

Madelung and Hubbard interactions in polaron band model of doped organic semiconductors.

The standard polaron band model of doped organic semiconductors predicts that density-of-states shift into the π–π* gap to give a partially filled polaron band that pins the Fermi level. This picture neglects both Madelung and Hubbard interactions. Here we show using ultrahigh workfunction hole-doped model triarylamine–fluorene copolymers that Hubbard interaction strongly splits the singly-occupied molecular orbital from its empty counterpart, while Madelung (Coulomb) interactions with counter-anions and other carriers markedly shift energies of the frontier orbitals. These interactions lower the singly-occupied molecular orbital band below the valence band edge and give rise to an empty low-lying counterpart band. The Fermi level, and hence workfunction, is determined by conjunction of the bottom edge of this empty band and the top edge of the valence band. Calculations are consistent with the observed Fermi-level downshift with counter-anion size and the observed dependence of workfunction on doping level in the strongly doped regime.

The effects of alkylammonium counterions on the aggregation of fluorinated surfactants and surfactant ionic liquids

The effects of organic counterions with varying carbon number on surfactant aggregation have been analysed by coupling perfluorooctanoate surfactant anions with various alkylammonium counterions. Both the degree of substitution (primary to tertiary) and alkyl chain length (0–3 carbons) of the counterions were varied to provide a comprehensive matrix of geometries and lipophilicities. Surface activity was measured using pendant drop tensiometry, while temperature-controlled small-angle neutron scattering was used to probe changes in aggregation morphology. It was found that the use of such alkylammonium counterions resulted in a strong preference for bilayer formation even at low surfactant concentration (<2wt%), when compared to simple inorganic counterions such as sodium which favour near-spherical micelles. At increased temperatures, some counterions led to unique phase behaviour wherein a transition between two structurally different lamellar phases is seen, rationalised as a transition into a microscopic phase separation wherein a surfactant-rich lamellar phase coexists with a dilute micellar phase. The results indicate that aggregation is controlled by a delicate balance of counterion size, hydrophilicity and diffuseness of charge, providing new methods for the subtle control of surfactant solutions.

Studies of Fundamental Charge Transfer Processes in Organic Radical-Containing Polymer Films Using Spectroelectrochemical Techniques

The electrochemical redox reaction of stable organic polymer materials with unpaired electrons is a relatively new and exciting area of battery research. Specifically, stable nitroxyl radical-containing polymers have displayed excellent characteristics as the cathode-active material and have presented themselves as a low-cost and environmentally friendly alternative to the current Li-ion battery technologies. However, with the advancement of organic-radical battery technology there is a lack of fundamental understanding of the dynamics of charge transfer processes, electronic and ionic, within organic radical-containing polymers. In order to elucidate these mechanisms further, we have conducted extensive spectroscopic and spectroelectrochemical investigations of macromolecular systems containing nitroxide radicals. We have employed spectroelectrochemical experiments for our study of stable radical polymer systems whereby, kinetic information was determined, specifically rates of charge transfer and ion diffusion within our polymer systems. A key factor determining the success of such experiments is that the material of interest exhibits a strong absorption coefficient and allows for measurement in the concentration regime conducive for electrochemical studies (typically up to 10 mM). For this reason, of the available stable nitroxyl radical systems, we have chosen first to study the oxidation of a nitronyl nitroxide derivative. To simulate an electrode architecture, spectroelectrochemical studies were performed in the solid state, and the efficacy of charge transfer was investigated as a function of film thickness, counter-ion size, and radical polymer composition in “neat” polymer films. The addition of semiconducting carbon nanotubes at various loadings was also explored as changes to film conductivity has significant implications for rates of ion transport.

Effect of counterion size on wormlike micelles formed by a C22-tailed anionic surfactant

Wormlike micelles (WLMs), generally formed by a surfactant in the presence of a hydrotrope, have gained much interest over last decades; however, less emphasis was thrown on investigating the influence of hydrotrope size on the morphology of WLMs, and WLMs based long-chain anionic surfactants (>C18) have been seldom noticed. In the present work, rheology, phase observation and cryo-TEM imaging were used to get insight into the dependence of morphology and rheological properties of WLMs formed by a C22-tailed anionic surfactant, sodium erucate (NaOEr), on hydrodynamic volume of hydrotrope by comparing tetramethylammonium (TMA+) and tetrabutylammonium (TBA+). It was found on one hand, TBA+ presents superior ability over TMA+ to reduce Krafft point of NaOEr; on the other hand, the impact of hydrophobe size on rheological responses varies with NaOEr concentration. Comparatively, TMA+ is more effective in inducing micellar growth at low NaOEr concentration; while TBA+ predominates at high NaOEr concentration. The molecular interaction between NaOEr and TBA+ shows high dependency on NaOEr concentration. The hydrophobicity and the volume factor of TBA+ alternately influence the micellar arrangement. When at high NaOEr concentration, the hydrophobicity plays a critical role only and the volume factor begins to dominate at low concentration.

Grand-canonical simulation of DNA condensation with two salts, effect of divalent counterion size.

The problem of DNA- DNA interaction mediated by divalent counterions is studied using a generalized grand-canonical Monte-Carlo simulation for a system of two salts. The effect of the divalent counterion size on the condensation behavior of the DNA bundle is investigated. Experimentally, it is known that multivalent counterions have strong effect on the DNA condensation phenomenon. While tri- and tetra-valent counterions are shown to easily condense free DNA molecules in solution into toroidal bundles, the situation with divalent counterions is not as clear cut. Some divalent counterions like Mg(+2) are not able to condense free DNA molecules in solution, while some like Mn(+2) can condense them into disorder bundles. In restricted environment such as in two dimensional system or inside viral capsid, Mg(+2) can have strong effect and able to condense them, but the condensation varies qualitatively with different system, different coions. It has been suggested that divalent counterions can induce attraction between DNA molecules but the strength of the attraction is not strong enough to condense free DNA in solution. However, if the configuration entropy of DNA is restricted, these attractions are enough to cause appreciable effects. The variations among different divalent salts might be due to the hydration effect of the divalent counterions. In this paper, we try to understand this variation using a very simple parameter, the size of the divalent counterions. We investigate how divalent counterions with different sizes can lead to varying qualitative behavior of DNA condensation in restricted environments. Additionally, a grand canonical Monte-Carlo method for simulation of systems with two different salts is presented in detail.

Conformational Behavior of a Single Polyelectrolyte Chain with Bulky Counterions

We report a dissipative dynamic simulation study of a polyelectrolyte chain conformational behavior with small and bulky counterions. The size of the small counterions was equal to the size of a monomer unit, while the bulky counterions were modeled as big spherical particles with twice the radius of a monomer unit. Two cases of the charge location within the bulky counterions were studied: (i) in the center (symmetrical case) or (ii) on the counterion surface (asymmetrical case). The strength of electrostatic interactions, λ, as well as the fraction of charged beads on the polymer chain, f, was varied. At high charge fractions the chains with both small counterions and symmetrical bulky counterions collapsed at large λ. In contrast, the chain with asymmetrical bulky counterions did not collapse at all; instead, it adopted swollen conformations with counterions strongly attached to the chain backbone. At low charge fractions and high λ the multiplet structures observed in the systems with small and symmet...

Nanoparticles incorporating pH-responsive surfactants as a viable approach to improve the intracellular drug delivery.

The pH-responsive delivery systems have brought new advances in the field of functional nanodevices and might allow more accurate and controllable delivery of specific cargoes, which is expected to result in promising applications in different clinical therapies. Here we describe a family of chitosan-TPP (tripolyphosphate) nanoparticles (NPs) for intracellular drug delivery, which were designed using two pH-sensitive amino acid-based surfactants from the family N(α),N(ε)-dioctanoyl lysine as bioactive compounds. Low and medium molecular weight chitosan (LMW-CS and MMW-CS, respectively) were used for NP preparation, and it was observed that the size distribution for NPs with LMW-CS were smaller (~168 nm) than that for NPs prepared with MMW-CS (~310 nm). Hemolysis assay demonstrated the pH-dependent biomembrane disruptional capability of the constructed NPs. The nanostructures incorporating the surfactants cause negligible membrane permeabilization at pH7.4. However, at acidic pH, prevailing in endosomes, membrane-destabilizing activity in an erythrocyte lysis assay became evident. When pH decreased to 6.6 and 5.4, hemolytic capability of chitosan NPs increased along with the raise of concentration. Furthermore, studies with cell culture showed that these pH-responsive NPs displayed low cytotoxic effects against 3T3 fibroblasts. The influence of chitosan molecular weight, chitosan to TPP weight ratio, nanoparticle size and nature of the surfactant counterion on the membrane-disruptive properties of nanoparticles was discussed in detail. Altogether, the results achieved here showed that by inserting the lysine-based amphiphiles into chitosan NPs, pH-sensitive membranolytic and potentially endosomolytic nanocarriers were developed, which, therefore, demonstrated ideal feasibility for intracellular drug delivery.

Spectroelectrochemical Studies of Charge Transfer Processes in Stable Nitroxyl Radical-Containing Polymers

The electrochemical redox reaction of stable organic polymer materials with unpaired electrons is a relatively new and exciting area of battery research. Specifically, stable nitroxyl radical-containing polymers have displayed excellent characteristics as the cathode-active material and have presented themselves as a low-cost and environmentally friendly alternative to the current Li-ion battery technologies. However, with the advancement of organic-radical battery technology there is a lack of fundamental understanding of the dynamics of charge transfer processes, electronic and ionic, within organic radical-containing polymers. In order to elucidate these mechanisms further, we have conducted extensive spectroscopic and spectroelectrochemical investigations of macromolecular systems containing nitroxide radicals. We have employed potential modulated spectroelectrochemical techniques for our study of stable radical polymer systems whereby, kinetic information can be determined, specifically rates of charge transfer and ion diffusion within our polymer systems. A key factor determining the success of such experiments is that the material of interest exhibits a strong absorption coefficient and allows for measurement in the concentration regime conducive for electrochemical studies (typically up to 10 mM). For this reason, of the available stable nitroxyl radical systems, we have chosen first to study the oxidation of a nitronyl nitroxide derivative. To simulate an electrode architecture, spectroelectrochemical studies were performed in the solid state, and the efficacy of charge transfer was investigated as a function of film thickness, counter-ion size, and radical polymer composition. As we have developed a synthetic process to produce ordered systems of stable organic radicals in the form of fixed polymer brushes grown from transparent conducting substrates, the effects of nano- and mesoscale ordering of the polymer film on charge transfer dynamics will also be presented.

Syntheses, structures and anion exchange properties of accommodative silver chains using a positively charged and flexible ligand

Eight silver(I) coordination polymers were synthesized and structurally characterized. Direct reaction of various Ag(I) starting materials with a positively charged and flexible ligand, 1,3-di(2-picolyl)imidazolium (L) in different salts, including L-Cl, L-ClO4 and L-PF6, gave compounds [AgL(NO3)2] (1), [AgL(CH3CN)](BF4)2 (2), [Ag2L2(NO3)2](NO3)Cl (3), [AgL(CH3CN)](PF6)2·0.5CH3CN (4), [AgL(NO3)](PF6) (5), [AgL](NO3)(PF6) (6), [AgL(CH3CN)2](ClO4)(PF6)·CH3CN (7), and [AgL](ClO4)2·CH3CN (8). Structural analysis revealed that ligand L has three kinds of conformations in these structures. As benefited by the flexible feature of L, the silver chains in 1–8 are different and are depended on the shape and the size of counter anions. Anion exchange studies showed that crystals of 4, 6 and 8 had anion exchange properties, and the channel size in the corresponding solid state structures played a crucial role. These results highlighted the significance of positively charged and flexible ligand L in the construction of various silver chain structures with different conformations and thus packing diagrams for anion exchange studies.

The interaction of electric field and hydrostatic pressure in an electrical double layer: A simple “first principle” model that accounts for the finite sizes of counterions

A simple model describing the influence of ion size in the electrical double layer (EDL) near a highly charged plane is proposed here. This model is based on the Poisson–Boltzmann equation with a single additional term representing the mechanical response of bulky ions to hydrostatic pressure. This pressure is produced by Coulomb forces, and increases to several kilobars in the vicinity of a highly charged plane. Numerical simulations demonstrate close packing as a limit for counterion concentrations. Differential capacity reaches maximum at 0.1–0.3V and remains reasonably small in wide range of potentials.