Strength Of Electrostatic Interactions Research Articles

Anion-Dependent Strength Scale of Interactions in Ionic Liquids from X-ray Photoelectron Spectroscopy, Ab Initio Molecular Dynamics, and Density Functional Theory.

Using a combination of experiments and calculations, we have gained new insights into the nature of anion-cation interactions in ionic liquids (ILs). An X-ray photoelectron spectroscopy (XPS)-derived anion-dependent electrostatic interaction strength scale, determined using XPS core-level binding energies for IL cations, is presented here for 39 different anions, with at least 18 new anions included. Linear correlations of experimental XPS core-level binding energies for IL cations with (a) calculated core binding energies (ab initio molecular dynamics (AIMD) simulations were used to generate high-quality model IL structures followed by single-point density functional theory (DFT) to obtain calculated core binding energies), (b) experimental XPS core-level binding energies for IL anions, and (c) other anion-dependent interaction strength scales led to three main conclusions. First, the effect of different anions on the cation can be related to ground-state interactions. Second, the variations of anion-dependent interactions with the identity of the anion are best rationalized in terms of electrostatic interactions and not occupied valence state/unoccupied valence state interactions or polarizability-driven interactions. Therefore, the XPS-derived anion-dependent interaction strength scale can be explained using a simple electrostatic model based on electrostatic site potentials. Third, anion-probe interactions, irrespective of the identity of the probe, are primarily electrostatic, meaning that our electrostatic interaction strength scale captures some inherent, intrinsic property of anions independent of the probe used to measure the interaction strength scale.

Responses of assembled structures of block polyelectrolytes to electrostatic interaction strength.

In this paper, the responses of assembled behaviors of block polyelectrolytes (PEs) to the strength of electrostatic interactions are studied through molecular dynamic simulations. The results show that the assembled structures closely depend on the electrostatic strength. It should be noted that PE coacervation can outweigh the nucleation of hydrophobic blocks and invert the micelle structures at strong electrostatic strengths, leading to the formation of inverted micelles of PE cores and hydrophobic coronas. In the poor solvent condition for neutral block, diverse anisotropic micelles are presented; candy-like conventional micelles of hydrophobic cores and PE patches coexist with inverted candy-like micelles of PE cores and hydrophobic patches and with Janus micelles of semi-neutral aggregate and semi-PE cluster in the presence of divalent and trivalent counterions. The formation of conventional or inverted micelle is largely determined by the type of micellar fusion, which results from the nucleation competition between electrostatic correlation and hydrophobic interaction. The merge of micelles mediated by hydrophobic attraction leads to conventional hydrophobic cores, and the fusion induced by electrostatic correlations results in PE cores micelles. At strong electrostatic strengths, the PE chains exhibit rich conformations at trivalent counterions, ranging from a fully collapsed state to a rod-like state, and parallel alignment of PE chains is found.

Electrostatic interactions between rough dielectric particles.

From colloid suspension to particle aggregation in protoplanetary formation, electrostatic attraction and repulsion between particles is a key mechanism behind the aggregation and clustering of particles. Although most studies have focused on canonical spherical particles, it remains unclear how nonspherical and rough dielectric particles interact and whether the complicated interplay between roughness and charge distribution affects their force couplings. Here a boundary-element method model was leveraged to study electrostatic interactions between charged dielectric particles with modular, axisymmetric surface features. Charge accumulation at convex surface asperities decreases the strength of electrostatic interactions between particles, and the sensitivity of the electrostatic force to the particle surface roughness and orientation is especially apparent at small particle separations. Surface interactions between the particle near-contact regions were isolated to determine the degree that near-contact interactions dictate the relationship between the net electrostatic force and the particle roughness and orientation. A correction factor ΔF is introduced to recover higher order dielectric effects from a low order analytical model. Finally, implications of surface charge asymmetries produced for different particle orientations and surface roughnesses on the long-standing problem of triboelectrification are discussed.

Microencapsulation of vitamin E by gelatin-high/low methoxy pectin complex coacervates: Effect of pH, pectin type, and protein/polysaccharide ratio

In this study, gelatin (GA), high-methoxyl pectin (HMP) or low-methoxyl pectin (LMP) were used to prepare complex coacervates, and effect of pectin type, pH, and gelatin-pectin ratio on formation and physiochemical properties of complex coacervates were deeply investigated. Furthermore, vitamin E (VE) was encapsulated using GA-HMP/LMP complex coacervates. The GA-HMP/LMP complex coacervates and GA-HMP/LMP-VE microcapsules were optimized and characterized. At the gelatin-pectin ratios of 2:1 and 3:1 and pH of 4.0, the GA-HMP and GA-LMP had the maximum yield of 54.67 ± 1.82% and 59.33 ± 1.49%, and turbidity of 1.33 ± 0.02 and 1.30 ± 0.04, respectively, and GA-HMP/LMP were close to zero potential at this point, indicating that the strength of electrostatic interaction was strongest. The GA-LMP and GA-HMP displayed spherical and irregular short-rod like appearance, respectively, and more aggregates and denser network were formed at gelatin-pectin ratio of 3:1, especially in GA-HMP3:1. The formation of GA-HMP/LMP complex coacervates was driven by electrostatic interaction, and GA-HMP3:1 had the optimum thermal stability. Except GA-HMP2:1-VE, the prepared and optimized VE mirocapsules by GA-LMP and GA-LMP complex coacervates showed spherical state and uniform particle size. The GA-HMP2:1-VE and GA-HMP3:1-VE possess the highest hygroscopicity and wettability of 109.08 ± 7.66 s and 26.67 ± 0.85 g/100g, respectively. The GA-HMP3:1-VE microcapsules had the highest encapsulation efficiency of 82.67 ± 1.77 %. Results of differential scanning calorimetry and release behavior in hot-water indicated that the microcapsules had excellent thermal stability. Furthermore, the GA-HMP and GA-LMP made VE stable in gastric stages and sustainable release in intestinal stages.

Vapor-liquid equilibrium for binary systems containing n-alkanes and cycloalkanes

In this study, VLE data for five binary systems comprising n-octane, n-nonane, methylcyclopentane, and methylcyclohexane were measured using the total pressure method at 393.15 K, 433.15 K, and 473.15 K. The NRTL-HOC and the UNIQUAC-HOC models were used to successfully correlate all VLE data and determine binary interaction parameters. No azeotropic behavior was observed in any of the binary systems under the operating conditions. The predictive UNIFAC-DMD and COSMO-SAC models were implemented to predict binary VLE phase diagram and unveiled the mechanism of the separation efficiency of alkanes–cycloalkanes. Through the examination of the COSMO-SAC based molecular surface charge profile (σ-profile), the variance in non-hydrogen bonding segments within the σ-profiles of the analyzed alkanes determines both the strength of electrostatic interactions and the temperature-dependent VLE separation behaviors. The results indicate that σ-profile could be suggested as a good indicator to assess operating conditions of the alkanes VLE separation.

Effect of chitosan and chitosan oligosaccharide on the interaction and in vitro gastric digestive behavior of whey protein isolate

Carbohydrate-protein interactions can improve the properties and texture of proteins. The objective of this study was to investigate the effects of pH and preheating to 90 °C on the interaction between chitosan (CS)/chitosan oligosaccharide (COS) and whey protein isolate (WPI). Additionally, by simulating in vitro gastric digestion to evaluate the digestive behavior of the CS/COS-WPI complexes when applied to protein-supplemented beverages. The results demonstrated that only CS exhibited greater efficacy in preventing the denaturation and aggregation of WPI caused by heat treatment at pH 4.0. As pH continued to expand to 6.0, changes in the strength of electrostatic interactions and the tertiary structure of proteins influenced the formation of the complex system structure. The in vitro digestion experiments revealed that CS and COS inhibition of WPI digestion in the gastric resulted in an opposite trend with an increasing ratio. We hope that this work will offer promising strategies for developing protein-enrich beverages.

Sliding Dynamics of a Small Charged Ring Chain on the Diblock Polyelectrolyte in Poly[2]catenane in the Presence of Counterions.

In this study, we investigate the sliding dynamics of small charged ring chains along the rigid central cyclic diblock polyelectrolyte of AnBn in radial charged poly[2]catenane in the presence of counterions using molecular dynamics simulations and the Lifson-Jackson formula, and our aim is to study the effects of electrostatical interaction strength, the size of the charged small ring chain, and the rigid block length of the diblock polyelectrolyte on the sliding dynamics of a small ring chain threaded on the rigid diblock polyelectrolyte. The mean-square displacement g3(t) of a small ring chain sliding along the rigid diblock polyelectrolyte of A10B10 exhibits oscillating behavior at short time scales for the moderate electrostatical interaction strength, while for the weak or strong electrostatic interactions, it is normal subdiffusion at short time scales. For n = 1, the diffusion coefficient D of the small ring chain sliding along the rigid diblock polyelectrolyte of A1B1 decreases monotonically as the relative electrostatic interaction strength A increases from A = 0.25-4. However, for n ≠ 1, the diffusion coefficient D of the small ring chain sliding along the rigid diblock polyelectrolyte of AnBn first decreases and then increases with the increase of A, and the nonmonotonous relationship between D and A becomes more obvious for larger n. In view of the free energy potential, the sliding diffusion of a small ring chain is governed by both the width of the free energy potential well and the height of the free energy potential barrier. According to the potential of mean force (PMF) of the small ring chain sliding along the rigid diblock polyelectrolyte, we find that our results are in good agreement with the theoretical analysis using the Lifson-Jackson formula. These results may help us to understand the diffusion motion of a ring chain in radial poly[n]catenanes from a fundamental point of view and control the sliding dynamics in molecular designs.

Near-Unity Emitting, Widely Tailorable, and Stable Exciton Concentrators Built from Doubly Gradient 2D Semiconductor Nanoplatelets.

The strength of electrostatic interactions (EIs) between electrons and holes within semiconductor nanocrystals profoundly affects the performance of their optoelectronic systems, and different optoelectronic devices demand distinct EI strength of the active medium. However, achieving a broad range and fine-tuning of the EI strength for specific optoelectronic applications is a daunting challenge, especially in quasi two-dimensional core-shell semiconductor nanoplatelets (NPLs), as the epitaxial growth of the inorganic shell along the direction of the thickness that solely contributes to the quantum confined effect significantly undermines the strength of the EI. Herein we propose and demonstrate a doubly gradient (DG) core-shell architecture of semiconductor NPLs for on-demand tailoring of the EI strength by controlling the localized exciton concentration via in-plane architectural modulation, demonstrated by a wide tuning of radiative recombination rate and exciton binding energy. Moreover, these exciton-concentration-engineered DG NPLs also exhibit a near-unity quantum yield, high photo- and thermal stability, and considerably suppressed self-absorption. As proof-of-concept demonstrations, highly efficient color converters and high-performance light-emitting diodes (external quantum efficiency: 16.9%, maximum luminance: 43,000 cd/m2) have been achieved based on the DG NPLs. This work thus provides insights into the development of high-performance colloidal optoelectronic device applications.

Formation mechanism of cold-set bitter apricot kernel protein isolate-gellan gum binary gels with excellent freeze-thaw stability

Binary gels with various microstructures, including apricot kernel protein isolate-gellan gum gels with high freeze-thaw stability, were obtained by combining transglutaminase covalent cross-linking and gellan gum modification. Adding 0–0.1% gellan gum (w/w) can form continuous protein network structures dominated by covalent networks and disulfide bonds. In addition to covalent cross-linking, the increase in the strength of hydrophobic and electrostatic interactions at gellan gum concentrations below 0.2% (w/w) facilitated the formation of a bicontinuous network (AGG3). When excess gellan gum was added (0.03%, w/w), gellan gelation occurred in the system, leading to the formation of heterogeneous aggregated structures (AGG4). Confocal laser microscopy revealed a dense porous structure in the continuous protein network after freeze-thaw treatment, and AGG4 showed heterogeneous and loose aggregated microstructure, while the bicontinuous network structure was maintained. The water holding capacity of AGG3 was increased by 21.36% after freezing compared with the gels without gellan gum, and AGG3 maintained high hardness and springiness. Low-field nuclear magnetic resonance and magnetic resonance imaging showed that the bicontinuous network was more conducive to the tethering ability and uniform distribution of hydrogen protons than the continuous network structure of proteins. Therefore, AGG3 exhibited excellent freeze-thaw stability.

Negatively charged chitosan nanoparticles prepared by ionotropic gelation for encapsulation of positively charged proteins

The nanoprecipitation of hydrogel nanoparticles by complex coacervation is investigated through a systematic study of the popular chitosan-polyphosphate pair of polyelectrolytes with opposite charges at pH 4. Polyphosphates of varying molar masses and electrical charges are investigated as alternatives to the commonly used tripolyphosphate, so as to assess the influence of the strength of electrostatic interactions on the fabrication possibility, the size of hydrogel particles, and their overall charge. Sodium hexametaphosphate and sodium polyphosphate allow the manufacture of such nanoparticles with either a positive or a negative charge, depending on the chitosan/polyphosphate ratio and the order of mixing. The classical way of mixing by pouring the polyphosphate solution into the chitosan solution yields microparticles. Inverting the order of mixing by pouring the chitosan solution into the polyphosphate solution allows the precipitation of negatively charged nanoparticles with diameters in the range 100–200 nm. Such charge inversion of the chitosan into negative is not possible with the common TPP. It was achieved using sodium hexametaphosphate and sodium polyphosphate having a larger negative charge. Charge inversion of chitosan allows an efficient encapsulation of positively charged proteins with an improved encapsulation efficiency than in the usual TPP-based coacervate. The encapsulation of the bovine serum albumin at pH 4 is given as a case study of a positively charged protein.

Self-Assembly of Charged Diblock Copolymers with Reduced Backbone Polarity

Charged block copolymers that can self-assemble into a host of nanostructures offer a great opportunity as next-generation battery electrolytes with outstanding ionic conductivity and mechanical robustness. The impact of ions on the self-assembly of charged block copolymers, however, remains to be fully understood. In this article, we report the phase behavior of charged-neutral block copolymers where a relatively nonpolar matrix was employed in the charged block to boost the strength of electrostatic interaction. The phase behavior was established using small-angle X-ray scattering (SAXS). We found that the overall shape of the phase boundary between ordered and disordered states is asymmetric, tilting toward the charged block-lean side of the phase portrait. However, the composition windows of the ordered phases, such as lamellae and hexagonally packed cylinders observed in this study, are comparable to those of neutral diblock copolymers and are not obviously affected by the incorporation of charges. The results obtained in this work provide insight into the impact of ions on the self-assembly of charged block copolymers as well as the design of nanostructured polymer electrolytes.

Capturing the free energy of transition state stabilization: insights from the inhibition of mandelate racemase.

Mandelate racemase (MR) catalyses the Mg2+-dependent interconversion of (R)- and (S)-mandelate. To effect catalysis, MR stabilizes the altered substrate in the transition state (TS) by approximately 26 kcal mol-1 (-ΔGtx), such that the upper limit of the virtual dissociation constant of the enzyme-TS complex is 2 × 10-19 M. Designing TS analogue inhibitors that capture a significant amount of ΔGtx for binding presents a challenge since there are a limited number of protein binding determinants that interact with the substrate and the structural simplicity of mandelate constrains the number of possible isostructural variations. Indeed, current intermediate/TS analogue inhibitors of MR capture less than or equal to 30% of ΔGtx because they fail to fully capitalize on electrostatic interactions with the metal ion, and the strength and number of all available electrostatic and H-bond interactions with binding determinants present at the TS. Surprisingly, phenylboronic acid (PBA), 2-formyl-PBA, and para-chloro-PBA capture 31-38% of ΔGtx. The boronic acid group interacts with the Mg2+ ion and multiple binding determinants that effect TS stabilization. Inhibitors capable of forming multiple interactions can exploit the cooperative interactions that contribute to optimum binding of the TS. Hence, maximizing interactions with multiple binding determinants is integral to effective TS analogue inhibitor design. This article is part of the theme issue 'Reactivity and mechanism in chemical and synthetic biology'.

Physical Adsorption of Graphene Oxide onto Polymer Latexes and Characterization of the Resulting Nanocomposite Particles.

Polymer/graphene oxide (GO) nanocomposite particles were prepared via heteroflocculation between 140–220 nm cationic latex nanoparticles and anionic GO nanosheets in either acidic or basic conditions. It is demonstrated that nanocomposite particles can be formed using either poly(2-vinylpyridine)-b-poly(benzyl methacrylate) (P2VP–PBzMA) block copolymer nanoparticles prepared by reversible-addition chain-transfer (RAFT)-mediated polymerization-induced self-assembly (PISA), or poly(ethylene glycol)methacrylate (PEGMA)-stabilized P2VP latexes prepared by traditional emulsion polymerization. These two latexes are different morphologically as the P2VP–PBzMA block copolymer latexes have P2VP steric stabilizer chains in their corona, whereas the PEGMA-stabilized P2VP particles have a P2VP core and a nonionic steric stabilizer. Nevertheless, both the P2VP–PBzMA and PEGMA-stabilized P2VP latexes are cationic at low pH. Thus, the addition of GO to these latexes causes flocculation to occur immediately due to the opposite charges between the anionic GO nanosheets and cationic latexes. Control heteroflocculation experiments were conducted using anionic sterically stabilized poly(potassium 3-sulfopropyl methacrylate)-b-poly(benzyl methacrylate) (PKSPMA–PBzMA) and nonionic poly(benzyl methacrylate) (PBzMA) nanoparticles to demonstrate that polymer/GO nanocomposite particles were not formed. The degree of flocculation and the strength of electrostatic interaction between the cationic polymer latexes and GO were assessed using disc centrifuge photosedimentometry (DCP), transmission electron microscopy (TEM), and UV–visible spectrophotometry. These studies suggest that the optimal conditions for the formation of polymer/GO nanocomposite particles were GO contents between 10% and 20% w/w relative to latex, with the latexes containing P2VP in their corona having a stronger electrostatic attraction to the GO sheets.

Explicit characterization of counterion dynamics around a flexible polyelectrolyte.

The article presents a comprehensive study of counterion dynamics around a generic linear polyelectrolyte chain with the help of coarse-grained computer simulations. The ion-chain coupling is discussed in the form of binding time, mean-square displacement (MSD) relative to the chain, local ion transport coefficient, and spatiotemporal correlations in the effective charge. We have shown that a counterion exhibits subdiffusive behavior 〈δR^{2}〉∼t^{δ}, δ≈0.9 w.r.t. chain's center of mass. The MSD of ions perpendicularly outward from the chain segment exhibits a smaller subdiffusive exponent compared to the one along the chain backbone. The effective diffusivity of ion is the lowest in chain's close proximity, extending up to the length-scale of radius of gyration R_{g}. Beyond R_{g} at larger distances, they attain diffusivity of free ion with a smooth cross-over from the adsorbed regime to the free ion regime. We have shown that the effective diffusivity drastically decreases for the multivalent ions, while the crossover length scale remains the same. Conversely, with increasing salt concentration the coupling-length scale reduces, while the diffusivity remains unaltered. The effective diffusivity of adsorbed-ion reveals an exponential reduction with electrostatic interaction strength. We further corroborate this from the binding time of ions on the chain, which also grows exponentially with the coupling strength of the ion-polymer duo. Moreover, the binding time of ions exhibits a weak dependence with salt concentration for the monovalent salt, while for multivalent salts the binding time decreases dramatically with concentration. Our work also elucidates fluctuations in the effective charge per site, where it exhibits strong negative correlations at short length-scales.

Membrane Potentials Trigger Molecular-Scale Rearrangements in the Outer Membrane of Gram-Negative Bacteria.

The structural complexity of the cell envelope of Gram-negative bacteria limits the fabrication of realistic models of bacterial cell membranes. A vertical Langmuir-Blodgett withdrawing was used to deposit a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) monolayer on the Au(111) surface. The second leaflet composed of di[3-deoxy-D-manno-octulosonyl]-lipid A (KLA) was deposited using Langmuir-Schaefer transfer. The use of an electrode material as a support for the POPE-KLA bilayer allowed electrochemical control of the membrane's stability, compactness, and structure. Capacitance-potential curves showed a typical pattern for the supported lipid bilayers electrochemical characteristic. The minimum membrane capacitance was ∼4 μF cm-2 and did not change in the following desorption-adsorption cycles, indicating the presence of a stable bilayer structure with an asymmetric composition of both leaflets. However, at a molecular scale, as elucidated in spectroelectrochemical experiments, large differences in the response of both leaflets to electric potentials were observed. The acyl chains in POPE and KLA existed in a liquid state. The quantitative analysis of the CH stretching modes indicated potential-driven reorientations in the hydrophobic fragment of the bilayer, already in the adsorbed state. To assign observed rearrangements to POPE and KLA lipids in both leaflets, per-deuterated d31-POPE was transferred into the inner leaflet. Since no potential-dependent changes of the CD2 stretching modes in the d31-POPE-KLA bilayer were observed, reorientations in the acyl chain region were assigned to the KLA molecules. Mg2+ ions were bound to the polar head groups of KLA. The strength of electrostatic interactions in the polar head group region of KLA was dependent on the direction of the electric field. At negative electric potentials, the binding of divalent cations weakened, which gave the KLA molecules increased orientational flexibility. This behavior in electric fields is peculiar for the outer membrane and indicates that the microbial cell membranes have different electrochemical properties than phospholipid bilayers.

Contrasting effects of microplastic aging upon the adsorption of sulfonamides and its mechanism

The aging behavior of microplastics (MPs) is unavoidable in the natural environment, but the mechanisms underlying the differences in MP aging affecting the adsorption performance of different MPs remain unclear. In this work, the adsorption characteristics of sulfonamides (SAs) on virgin MPs (thermoplastic polyurethanes (TPUs), polyamides (PAs)) and aged MPs were determined under simulated conditions (ultraviolet (UV), ultraviolet combined hydrogen peroxide (UV + H2O2)). Contrasting effects of MP aging on SA adsorption were observed. That is, the adsorption capacities of aged PAs were decreased from 14.45% to 58.53%, whereas those of aged TPUs were increased from 13.93% to 171.30%. To interpret this interesting phenomenon, the structural characteristics of the tested MPs were determined, and the results indicated that the surface structures of MPs changed with aging, which resulted in differences in hydrophobicity and surface charge and a significant difference in the concentrations of oxygen-containing functional groups (P < 0.05). Furthermore, density functional theory (DFT) results revealed that the equilibrium configurations and the interactions (H bond, electrostatic effect, van der Waals (vdW) forces were significantly influenced by the change in the concentration of oxygen-containing functional groups of MPs (P < 0.05), with the strength of H bonds increasing for aged TPUs-SAs and the strength of electrostatic interactions decreasing for aged PA-SAs. Our findings are the first to provide a theoretical basis for an improved understanding of the role of aging in SAs adsorption onto MPs.

Molecular Dynamics Simulations to Explore the Structure and Rheological Properties of Normal and Hyperconcentrated Airway Mucus.

We develop the first molecular dynamics model of airway mucus based on the detailed physical properties and chemical structure of the predominant gel-forming mucin MUC5B. Our airway mucus model leverages the LAMMPS open-source code [https://lammps.sandia.gov], based on the statistical physics of polymers, from single molecules to networks. On top of the LAMMPS platform, the chemical structure of MUC5B is used to superimpose proximity-based, non-covalent, transient interactions within and between the specific domains of MUC5B polymers. We explore feasible ranges of hydrophobic and electrostatic interaction strengths between MUC5B domains with 9 nanometer spatial and 1 nanosecond temporal resolution. Our goal here is to propose and test a mechanistic hypothesis for a striking clinical observation with respect to airway mucus: a 10-fold increase in non-swellable, dense structures called flakes during progression of cystic fibrosis disease. Among the myriad possible effects that might promote self-organization of MUC5B networks into flake structures, we hypothesize and confirm that the clinically confirmed increase in mucin concentration, from 1.5 to 5 mg/mL, alone is sufficient to drive the structure changes observed with scanning electron microscopy images from experimental samples. We post-process the LAMMPS simulated datasets at 1.5 and 5 mg/mL, both to image the structure transition and compare with scanning electron micrographs and to show that the 3.33-fold increase in concentration induces closer proximity of interacting electrostatic and hydrophobic domains, thereby amplifying the proximity-based strength of the interactions.

Spatial segregation of mixed-sized counterions in dendritic polyelectrolytes

Langevin dynamics simulations are utilized to study the structure of a dendritic polyelectrolyte embedded in two component mixtures comprised of conventional (small) and bulky counterions. We vary two parameters that trigger conformational properties of the dendrimer: the reduced Bjerrum length, lambda _B^*, which controls the strength of electrostatic interactions and the number fraction of the bulky counterions, f_b, which impacts on their steric repulsion. We find that the interplay between the electrostatic and the counterion excluded volume interactions affects the swelling behavior of the molecule. As compared to its neutral counterpart, for weak electrostatic couplings the charged dendrimer exists in swollen conformations whose size remains unaffected by f_b. For intermediate couplings, the absorption of counterions into the pervaded volume of the dendrimer starts to influence its conformation. Here, the swelling factor exhibits a maximum which can be shifted by increasing f_b. For strong electrostatic couplings the dendrimer deswells correspondingly to f_b. In this regime a spatial separation of the counterions into core–shell microstructures is observed. The core of the dendrimer cage is preferentially occupied by the conventional ions, whereas its periphery contains the bulky counterions.

Temporal Changes in Interparticle Interactions Drive the Formation of Transiently Stable Nanoparticle Precipitates.

We demonstrate the power of fine-tuned interparticle interactions, emanating from appropriately functionalized surfaces, in creating self-assembled structures that show a transient switching between completely precipitated and redispersed stages of nanoparticles (NPs). The pH-dependent temporal changes in the strength of electrostatic interactions are explored to unveil a transient self-assembly response in plasmonic NPs. The assembly process was triggered by the electrostatic attraction between positively charged gold NPs (AuNPs) and an aggregating agent, ethylenediaminetetraacetic acid (EDTA). The autonomous changes in the pH and ionic strength of the solution, under the influence of atmospheric CO2, weaken the aggregating ability of EDTA and initiate the complete disassembly of [+] AuNP-EDTA precipitates. The use of a nondestructive mode of autonomous disassembly helped in achieving some of the desirable feats in the field of transient self-assembly such as easy removal of waste, formation of a transiently stable precipitate state, and negligible dampness in redispersion. The chemical strategy adopted in the present work, to introduce transientness, can act as a generic tool in creating the next generation of complex matter.

The removal selectivity of heavy metal cations in micellar-enhanced ultrafiltration: A study based on critical micelle concentration

Micellar enhanced ultrafiltration (MEUF) is a process to remove heavy metal by complexing it with the counterionic surfactant micelle, and the complex formed was subsequently removed using ultrafiltration. The surfactant is usually added in excess, which renders high rejection (low selectivity) for all metal cations in the mixture. This work emphasises MEUF conducted at surfactant dosage around the critical micelle concentration (CMC). Few studies revealed that the presence of counterions could lower the CMC of a charged surfactant. The magnitude of reduction depends on the strength of electrostatic interaction between the counterions and surfactant. Such observation suggests that the MEUF process's selectivity could be altered based on the dosage of surfactant and the types of cations presented in the solution. This work started with determining CMC of sodium dodecyl sulfate (SDS) in the presence of single or binary metal cations at varying pH (pH 3, pH 10 and without pH adjustment). The lowest CMC (4.00 mM) was obtained in the solution containing 100 ppm copper (II) at pH 3, about 42% lower compared to CMC of SDS obtained in pure water without pH adjustment (7.00 mM). Then, the filtration was performed using 7 mM of SDS, under varying pH (pH 3, pH 10 and no pH adjustment), initial metal concentration of 100 mg/L, pressure of 2.5 bar using polyethersulfone (PES) membrane of 10 k Da molecular weight cut-off (MWCO). Overall, the highest metal rejection was obtained at pH 10, with 66% of copper and 26% of chromium (III) was rejected. The highest selectivity of copper (II) over chromium (III) was observed at neutral pH. Such findings could be due to the higher diffusivity of copper cations over chromium cations in aqueous since copper cations can reach the micelle surface and form metal-micelle complex faster than chromium cations.