Salt Concentration Dependence Research Articles

Ion Conductivity in Salt-Doped Polymers: Combined Effects of Temperature and Salt Concentration.

We construct a coarse-grained molecular dynamics model based on poly(ethylene oxide) and lithium bis(trifluoromethane)sulfonimide salt to examine the combined effects of temperature and salt concentration on the transport properties. Salt doping notably slows the dynamics of polymer chains and reduces ion diffusivity, resulting in a glass transition temperature increase proportional to the salt concentration. The polymer diffusion is shown to be well represented by a modified Vogel-Fulcher-Tamman (M-VFT) equation that accounts for both the temperature and salt concentration dependence. Furthermore, we find that, at any temperature, the concentration dependence of the conductivity is well described by the product of its infinite dilution value and a correction factor accounting for the reduced segmental mobility with increasing salt concentration. These results highlight the important role of polymer segmental mobility in the salt concentration dependence of ion conductivity for temperatures near and above the glass transition.

A Unified Description of Salt Effects on the Liquid-Liquid Phase Separation of Proteins.

Protein aggregation via liquid-liquid phase separation (LLPS) is ubiquitous in nature and is intimately connected to many human diseases. Although it is widely known that the addition of salt has crucial impacts on the LLPS of proteins, full understanding of the salt effects remains an outstanding challenge. Here, we develop a molecular theory that systematically incorporates the self-consistent field theory for charged macromolecules into the solution thermodynamics. The electrostatic interaction, hydrophobicity, ion solvation, and translational entropy are included in a unified framework. Our theory fully captures the long-standing puzzles of the nonmonotonic salt concentration dependence and the specific ion effect. We find that proteins show salting-out at low salt concentrations due to ionic screening. The solubility follows the inverse Hofmeister series. In the high salt concentration regime, protein continues salting-out for small ions but turns to salting-in for larger ions, accompanied by the reversal of the Hofmeister series. We reveal that the solubility at high salt concentrations is determined by the competition between the solvation energy and translational entropy of the ion. Furthermore, we derive an analytical criterion for determining the boundary between the salting-in and salting-out regimes, which is in good agreement with experimental results for various proteins and salt ions.

Non‐monotonic salt concentration dependence of inverted electrokinetic flow

AbstractModeling of electrokinetic flows is crucial to understand numerous phenomena associated with electrochemistry, biophysics, and colloidal science. Here, we incorporate the modified Gaussian renormalized fluctuation theory into transport equations for electrolyte solutions to study the ion‐correlation‐induced inversion of electrokinetic flows, also known as charge inversion. We are able to capture the non‐monotonic dependence of inverted streaming current and reversed electrophoretic mobility on salt concentration. By analyzing the double‐layer structure, we elucidate that this non‐monotonicity is a consequence of the competition between spatially varying ion correlations and the translational entropy of the ions. We find that for practical values of surface charge densities, the excluded volume effect does not play any significant role. In a significant improvement over existing theories, our theoretical predictions are in quantitative agreement with experimental measurements for charge inversion in trivalent salts.

Dual Nanofriction Force Microscopy/Fluorescence Microscopy Imaging Reveals the Enhanced Force Sensitivity of Polydiacetylene by pH and NaCl.

Polydiacetylene (PDA) is a popular mechanochromic material often used in biosensing. The effect of its headgroup-headgroup interactions on thermochromism such as pH or salt concentration dependency has been extensively studied before; however, their effect on mechanochromism at the nanoscale is left unstudied. In this work, nanofriction force microscopy and fluorescence microscopy were combined to study the effect of pH and ionic strength on the polydiacetylene (PDA) force sensitivity at the nanoscale. We found that the increase in pH from 5.7 to 8.2 caused an 8-fold enhancement in force sensitivity. The elevation of NaCl concentration from 10 to 200 mM also made the PDA 5 times more force-sensitive. These results suggest that the PDA force sensitivity at the nanoscale can be conveniently enhanced by "pre-stimulation" with pH or ionic strength.

Elucidation of the pH-Dependent Electric Double Layer Structure at the Silica/Water Interface Using Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

The silica/water interface is one of the most abundant charged interfaces in natural environments, and the elucidation of the water structure at the silica/water interface is essential. In the present study, we measured the interface-selective vibrational (χ(2)) spectra in the OH stretch region of the silica/water interface in a wide pH range of pH 2.0-12.0 while changing the salt concentration by heterodyne-detected vibrational sum-frequency generation spectroscopy. With the help of singular value decomposition analysis, it is shown that the imaginary part of the χ(2) (Imχ(2)) spectra can be decomposed into the spectra of the diffuse Gouy-Chapman layer (DL) and the compact Stern layer (SL), which enables us to quantitatively analyze the spectra of DL and SL separately. The salt-concentration dependence of the DL spectra at different pH values is analyzed using the modified Gouy-Chapman theory, and the pH-dependent surface charge density and the pKa value (4.8 ± 0.2) of the silica/water interface are evaluated. Furthermore, it is found that the pH-dependent change of the SL spectra is quantitatively explained by three spectral components that represent the three characteristic water species appearing in different pH regions in SL. The quantitative understanding obtained from the analysis of each spectral component in the Imχ(2) spectra provides a clear molecular-level picture of the electric double layer at the silica/water interface.

C-B-A Test of DNA Force Fields.

The DNA duplex may be locally strongly bent in complexes with proteins, for example, with polymerases or in a nucleosome. At such bends, the DNA helix is locally in the noncanonical forms A (with a narrow major groove and a large amount of north sugars) or C (with a narrow minor groove and a large share of BII phosphates). To model the formation of such complexes by molecular dynamics methods, the force field is required to reproduce these conformational transitions for a naked DNA. We analyzed the available experimental data on the B-C and B-A transitions under the conditions easily implemented in modeling: in an aqueous NaCl solution. We selected six DNA duplexes which conformations at different salt concentrations are known reliably enough. At low salt concentrations, poly(GC) and poly(A) are in the B-form, classical and slightly shifted to the A-form, respectively. The duplexes ATAT and GGTATACC have a strong and salt concentration dependent bias toward the A-form. The polymers poly(AC) and poly(G) take the C- and A-forms, respectively, at high salt concentrations. The reproduction of the behavior of these oligomers can serve as a test for the balance of interactions between the base stacking and the conformational flexibility of the sugar-phosphate backbone in a DNA force field. We tested the AMBER bsc1 and CHARMM36 force fields and their hybrids, and we failed to reproduce the experiment. In all the force fields, the salt concentration dependence is very weak. The known B-philicity of the AMBER force field proved to result from the B-philicity of its excessively strong base stacking. In the CHARMM force field, the B-form is a result of a fragile balance between the A-philic base stacking (especially for G:C pairs) and the C-philic backbone. Finally, we analyzed some recent simulations of the LacI-, SOX-4-, and Sac7d-DNA complex formation in the framework of the AMBER force field.

Capturing coacervate formation and protein partition by molecular dynamics simulation.

Biomolecules localize and function in microenvironments where their local concentration, spatial organization, and biochemical reactivity are regulated. To compartmentalize and control the local properties of the native microenvironment, cellular mimics and artificial bioreactors have been developed to study the properties of membraneless organelles or mimic the bio-environment for life origin. Here, we carried out molecular dynamics simulation with the Martini 3.0 model to reproduce the experimental salt concentration and pH dependency of different complex coacervates. We showed that coacervates inside vesicles are able to change their shape. In addition, we used these coacervate systems to explore the partitioning of the ubiquitous cytoskeletal protein actin and found that actin spontaneously partitions to all the coacervate peripheries. Therefore, we believe that our study can provide a better understanding of the versatile coacervate platform, where biomolecules partition and gather to fulfill their biological duties.

Cylindrical brushes with ionized side chains: Scaling theory revisited.

We revisit the classic scaling model of a cylindrical polyelectrolyte (PE) brush focusing on molecular brushes with stiff backbones and dispersions of polymer-decorated nanorods. Based on the blob representation we demonstrate that similarly to the case of planar PE brushes, separation of intra- and intermolecular repulsions between charges leads to novel scaling regimes for cylindrical PE brushes in salt-added solution and a sharper decrease in its thickness versus salt concentration dependence. These theoretical predictions may inspire further comprehensive experimental research and computer simulations of synthetic and biopolyelectrolyte cylindrical brushes.

Rectified and Salt Concentration Dependent Wetting of Hydrophobic Nanopores.

Nanopores lined with hydrophobic groups function as switches for water and all dissolved species, such that transport is allowed only when applying a sufficiently high transmembrane pressure difference or voltage. Here we show a hydrophobic nanopore system whose wetting and ability to transport water and ions is rectified and can be controlled with salt concentration. The nanopore we study contains a junction between a hydrophobic zone and a positively charged hydrophilic zone. The nanopore is closed for transport at low salt concentrations and exhibits finite current only when the concentration reaches a threshold value that is dependent on the pore opening diameter, voltage polarity and magnitude, and type of electrolyte. The smallest nanopore studied here had a 4 nm diameter and did not open for transport in any concentration of KCl or KI examined. A 12 nm nanopore was closed for all KCl solutions but conducted current in KI at concentrations above 100 mM for negative voltages and opened for both voltage polarities at 500 mM KI. Nanopores with a hydrophobic/hydrophilic junction can thus function as diodes, such that one can identify a range of salt concentrations where the pores transport water and ions for only one voltage polarity. Molecular dynamics simulations together with continuum models provided a multiscale explanation of the observed phenomena and linked the salt concentration dependence of wetting with an electrowetting model. Results presented are crucial for designing next-generation chemical and ionic separation devices as well as understanding fundamental properties of hydrophobic interfaces under nanoconfinement.

Temperature dependence and short-range electrolytic interactions within the e-PPC-SAFT framework

Understanding and modelling electrolyte systems is of interest both in an industrial context and from a fundamental point of view. Their importance is related to their key role played in traditional and emerging industries, and to the ubiquitous presence of saline solutions in biological, chemical, and geological environments. The development of electrolyte equations of state has advanced greatly in recent years, with the aim to describe the properties of electrolyte systems using a limited number of parameters, in a way that allows describing accurately the equilibrium properties of these solutions. Here, we explore the temperature and salt concentration dependence of various PPC-SAFT-based models, using simple alkali halide salts (primarily NaCl). The dispersion and association free-energy terms characteristic of SAFT-like equations are compared for the description of the ion-solvent non-Coulombic interactions. Several formulations of the dielectric constant are also investigated. In all models the MSA term is used to account for the electrostatic interactions between the ions and the Born term is added to model the ion-solvent electrostatic interactions. The molecular model parameters are determined by comparison to experimental enthalpy of solution, activity coefficient, osmotic coefficient and apparent molar volume data.We find that equivalent results can be obtained with or without incorporating a explicit salt concentration dependence in the treatment of the dielectric constant; the most accurate results (smallest average absolute relative deviation) when comparing to experimental the data chosen are found in the case that salt concentration is not explicitly considered. For each approach, the most sensitive parameters are identified, which makes it possible to reduce the number of adjustable parameters without significantly affecting the overall quality of the model. We find that both the approach incorporating a dispersion term and that using an association term in the treatment of short-range interactions yield similar results. Unfortunately, none of the models can describe accurately and with physically meaningful parameters the correct low-temperature trend of the mean ionic activity coefficient observed experimentally.

Salt Concentration-Dependence of Negatively Capacitive Currents Associated with Dissolution of Solver by Anodic Stripping Voltammetry

Salt Concentration-Dependence of Negatively Capacitive Currents Associated with Dissolution of Solver by Anodic Stripping Voltammetry

Mechanisms of ion transport in lithium salt‐doped polymeric ionic liquid electrolytes at higher salt concentrations

AbstractWe used atomistic simulations to study the mechanisms of ion transport in salt‐doped polymeric ionic liquid systems at higher salt concentrations. Consistent with the experimental observations, our simulations indicate that at higher salt concentrations, the anion mobilities become lower than that of the lithium cations. Further, the anion mobilities become relatively insensitive to the salt concentration, while the mobilities of lithium increase with increasing salt concentration. We rationalize the results for the anion mobilities by considering the fractions of anions which are exclusively coordinated with the polycations (Type1); co‐coordinated with cations and lithium (Type2); and those exclusively coordinated with lithium (Type3). By considering the coordination characteristics of the different types of anions and their hopping motions, we demonstrate that the net anion mobilities results from a compensation effect of the salt concentration dependence of the mobilities of the different anions. With respect to the mobilities of the lithium ions, we demonstrate that the latter moves primarily by a structural diffusion mechanism involving refreshing of the solvation shell during hopping. Further, for the majority of the lithium ions, the solvation shell is shown to be comprised of co‐coordinated Type2 anions, and that the number of polycations and the unique polymer chains involved in such coordination decreases with increasing salt concentration. Such changes are shown to weaken the solvation shell around the lithium, thereby facilitating faster ion motion. Together, our results suggest that systems in which the anion which exhibits a stronger coordination to the polycation in comparison to that of the lithium can facilitate higher transference numbers without a concomitant reduction in the mechanical strength.

Simulation Study of the Conformational Properties of Diblock Polyelectrolytes in Salt Solutions.

Coarse-grained molecular dynamics simulations are performed to understand the behavior of diblock polyelectrolytes in solutions of divalent salt by studying the conformations of chains over a wide range of salt concentrations. The polymer molecules are modeled as bead spring chains with different charged fractions and the counterions and salt ions are incorporated explicitly. Upon addition of a divalent salt, the salt cations replace the monovalent counterions, and the condensation of divalent salt cations onto the polyelectrolyte increases, and the chains favor to collapse. The condensation of ions changes with the salt concentration and depends on the charged fraction. Also, the degree of collapse at a given salt concentration changes with the increasing valency of the counterion due to the bridging effect. As a quantitative measure of the distribution of counterions around the polyelectrolyte chain, we study the radial distribution function between monomers on different polyelectrolytes and the counterions inside the counterion worm surrounding a polymer chain at different concentrations of the divalent salt. Our simulation results show a strong dependence of salt concentration on the conformational properties of diblock copolymers and indicate that it can tune the self-assembly behaviors of such charged polyelectrolyte block copolymers.

Diffusion of nucleotide excision repair protein XPA along DNA by coarse-grained molecular simulations* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11974173 and 11774158) and the HPC center of Nanjing University.

Protein XPA plays critical roles in nucleotide excision repair pathway. Recent experimental work showed that the functional dynamics of XPA involves the one-dimensional diffusion along DNA to search the damage site. Here, we investigate the involved dynamical process using extensive coarse-grained molecular simulations at various salt concentrations. The results demonstrated strong salt concentration dependence of the diffusion mechanisms. At low salt concentrations, the one-dimensional diffusion with rotational coupling is the dominant mechanism. At high salt concentrations, the diffusion by three-dimensional mechanism becomes more probable. At wide range of salt concentrations, the residues involved in the DNA binding are similar and the one-dimensional diffusion of XPA along DNA displays sub-diffusive feature. This sub-diffusive feature is tentatively attributed to diverse strengths of XPA–DNA interactions. In addition, we showed that both binding to DNA and increasing salt concentration tend to stretch the conformation of the XPA, which increases the exposure extent of the sites for the binding of other repair proteins.

Kinetic Microscale Thermophoresis for Simultaneous Measurement of Binding Affinity and Kinetics.

Microscale thermophoresis (MST) is a versatile technique to measure binding affinities of binder–ligand systems, based on the directional movement of molecules in a temperature gradient. We extended MST to measure binding kinetics as well as binding affinity in a single experiment by increasing the thermal dissipation of the sample. The kinetic relaxation fingerprints were derived from the fluorescence changes during thermodynamic re‐equilibration of the sample after local heating. Using this method, we measured DNA hybridization on‐rates and off‐rates in the range 104–106 m −1 s−1 and 10−4–10−1 s−1, respectively. We observed the expected exponential dependence of the DNA hybridization off‐rates on salt concentration, strand length and inverse temperature. The measured on‐rates showed a linear dependence on salt concentration and weak dependence on strand length and temperature. For biomolecular interactions with large enthalpic contributions, the kinetic MST technique offers a robust, cost‐effective and immobilization‐free determination of kinetic rates and binding affinity simultaneously, even in crowded solutions.

Kinetic Microscale Thermophoresis for Simultaneous Measurement of Binding Affinity and Kinetics

AbstractMicroscale thermophoresis (MST) is a versatile technique to measure binding affinities of binder–ligand systems, based on the directional movement of molecules in a temperature gradient. We extended MST to measure binding kinetics as well as binding affinity in a single experiment by increasing the thermal dissipation of the sample. The kinetic relaxation fingerprints were derived from the fluorescence changes during thermodynamic re‐equilibration of the sample after local heating. Using this method, we measured DNA hybridization on‐rates and off‐rates in the range 104–106 m−1 s−1 and 10−4–10−1 s−1, respectively. We observed the expected exponential dependence of the DNA hybridization off‐rates on salt concentration, strand length and inverse temperature. The measured on‐rates showed a linear dependence on salt concentration and weak dependence on strand length and temperature. For biomolecular interactions with large enthalpic contributions, the kinetic MST technique offers a robust, cost‐effective and immobilization‐free determination of kinetic rates and binding affinity simultaneously, even in crowded solutions.

Reprogramming Virus Coat Protein Carboxylate Interactions for the Patterned Assembly of Hierarchical Nanorods

The self-assembly system of the rod-shaped tobacco mosaic virus (TMV) has been studied extensively for nanoscale applications. TMV coat protein assembly is modulated by intersubunit carboxylate groups whose electrostatic repulsion limits the assembly of virus rods without incorporating genomic RNA. To engineer assembly control into this system, we reprogrammed intersubunit carboxylate interactions to produce self-assembling coat proteins in the absence of RNA and in response to unique pH and ionic environmental conditions. Specifically, engineering a charge attraction at the intersubunit E50-D77 carboxylate group through a D77K substitution stabilized the coat proteins assembly into virus-like rods. In contrast, the reciprocal E50K modification alone did not confer virus-like rod assembly. However, a combination of R46G/E50K/E97G substitutions enabled virus-like rod assembly. Interestingly, the D77K substitution displays a unique pH-dependent assembly-disassembly profile, while the R46G/E50K/E97G substitutions confer a novel salt concentration dependency for assembly control. In addition, these unique environmentally controlled coat proteins allow for the directed assembly and disassembly of chimeric virus-like rods both in solution and on substrate-attached seed rods. Combined, these findings provide a controllable means to assemble functionally discrete virus-like rods for use in nanotechnology applications.

Dynamic Coupling in Unentangled Liquid Coacervates Formed by Oppositely Charged Polyelectrolytes.

We develop a scaling theory that predicts the dynamics of symmetric and asymmetric unentangled liquid coacervates formed by solutions of oppositely-charged polyelectrolytes. Symmetric coacervates made from oppositely-charged polyelectrolytes consist of polycations and polyanions with equal and opposite charge densities along their backbones. These symmetric coacervates can be described as mixtures of polyelectrolytes in the quasi-neutral regime with a single correlation length. Asymmetric coacervates are made from polycations and polyanions with unequal charge densities. The difference in charge densities results in a double semidilute structure of asymmetric coacervates with two correlation lengths, one for the high-charge-density and the other for the low-charge-density polyelectrolytes. We predict that the double-semidilute structure in asymmetric coacervates results in a dynamic coupling which increases the friction of the high-charge-density polyelectrolyte. This dynamic coupling increases the contribution to the zero-shear viscosity of the high-charge-density polyelectrolyte. The diffusion coefficient of the high-charge-density polyelectrolyte is predicted to depend on the concentration and degree of polymerization of the low-charge-density polyelectrolyte in the coacervate if the size of the low-charge-density polymer is smaller than the correlation length of the high-charge-density polymer. We also predict a non-monotonic salt concentration dependence of the zero-shear viscosity of asymmetric coacervates.

Freezing point depression and freeze-thaw damage by nanofluidic salt trapping

A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryo-tolerance in biology is usually attributed to "anti-freeze" proteins, and yet massive supercooling ($< -40^\circ$C) is also possible in porous media containing only simple aqueous electrolytes. For concrete pavements, the common wisdom is that freeze-thaw damage results from the expansion of water upon freezing, but this cannot explain the large pressures ($> 10$~MPa) required to damage concrete, the observed correlation between pavement damage and de-icing salts, or the damage of cement paste loaded with benzene (which contracts upon freezing). In this Letter, we propose a different mechanism -- nanofluidic salt trapping -- which can explain the observations, using simple mathematical models of dissolved ions confined to thin liquid films between growing ice and charged surfaces. Although trapped salt lowers the freezing point, ice nucleation in charged pores causes enormous disjoining pressures via the rejected ions, until their removal by precipitation or surface adsorption at a lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the non-monotonic salt-concentration dependence of freeze-thaw damage in concreter and provides a general framework to understand the origins of cryo-tolerance.

Polyethylene oxide‐Li6.5La3Zr1.5Ta0.5O12 hybrid electrolytes: Lithium salt concentration and biopolymer blending

AbstractHybrid electrolytes are developed to meet the requirements of safety, performance, and manufacturing for electrolytes suitable for Li‐ion batteries with Li‐anodes. Recent challenges—in addition to these key properties—emphasize the importance of sustainability. While compromising between these three objectives, the currently available materials are still well below the targeted goals. Three important issues for the design of hybrid electrolytes are (i) the role of the morphology and surface state of the ceramic particles in the polymer matrix, (ii) the dependence of salt concentration and ionic conductivity and, (iii) the effects of substituting part of the polyethylene oxide (PEO), with biopolymers. Electrolyte films were prepared from PEO, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta), and biopolymers with varying contents of these components by a solution casting method. The films were analyzed with respect to structural and microstructural characteristics by DSC, Raman spectroscopy, and SEM. Ionic conductivity was evaluated by electrochemical impedance spectroscopy. Most interesting, when comparing films with LLZO:Ta versus without, the content of LiTFSI required for the maximum conductivity in the respective systems is different: a higher LiTFSI concentration is required for the former type. Overall, addition of LLZO:Ta as well as partial substitution of PEO by chitosan mesylate or cellulose acetate decrease the ionic conductivity. Thus—at least in the present approaches—a loss in performance is the drawback from attempts to enhance the safety by LLZO:Ta additions and sustainability by biopolymer blending of hybrid electrolytes.