Function Of Salt Concentration Research Articles

Size-Dependent Electrostatic Adsorption of Polymer-Grafted Gold Nanoparticles on Polyelectrolyte Brushes.

Designing a functional surface that selectively adsorbs nanoparticles based on their size and shape is essential for developing an advanced adsorption-based, postsynthesis nanoparticle separation device. We demonstrate selective adsorption of larger nanoparticles from solution onto a polyelectrolyte brush by tuning the salt concentration. Specifically, a positively charged polyelectrolyte brush is created by converting pyridine groups of poly(2-vinylpyridine) to n-methylpyridinium groups using methyl iodide. The adsorption kinetics and thermodynamics of poly(ethylene glycol)-grafted, negatively charged gold nanoparticles (diameters of 12 and 20 nm) were monitored as a function of salt concentration. In a salt-free solution, the polyelectrolyte brush adsorbs gold nanoparticles of both sizes. As the salinity increases, the areal number density of adsorbed nanoparticles monotonically decreases and becomes negligible at high salinity. Interestingly, there is an intermediate range of salt concentrations (i.e., 15-20 mM of NaCl) where the decrease in nanoparticle adsorption is more pronounced for smaller particles, leading to size-selective adsorption of the larger nanoparticles. As a further demonstration of selectivity, the polyelectrolyte brush is immersed in a binary mixture of 12 and 20 nm nanoparticles and found to selectively capture larger particles with ∼90% selectivity. In addition, the size distribution of as-synthesized gold nanoparticles, with an average diameter of 12 nm, was reduced by selectively removing larger particles by exposing the solution to polyelectrolyte brush surfaces. This study demonstrates the potential of a polyelectrolyte brush separation device to remove larger nanoparticles by controlling electrostatic interactions between polymer brushes and particles.

Simulating the Competitive Ion Pairing of Hydrated Electrons with Chaotropic Cations.

Experiments show that the absorption spectrum of the hydrated electron (ehyd-) blue-shifts in electrolyte solutions compared with what is seen in pure water. This shift has been assigned to the ehyd-'s competitive ion-pairing interactions with the salt cation relative to the salt anion based on the ions' positions on the Hofmeister series. Remarkably, little work has been done investigating the ehyd-'s behavior when the salts have chaotropic cations, which should greatly change the ion-pairing interactions given that the ehyd- is a champion chaotrope. In this work, we remedy this by using mixed quantum/classical simulations to analyze the behavior of two different models of the ehyd- in aqueous RbF and RbI electrolyte solutions as a function of salt concentration. We find that the magnitude of the salt-induced spectral blue-shift is determined by a combination of the number of chaotropic Rb+ cations near the ehyd- and the number of salt anions near those cations so that the spectrum of the ehyd- directly reflects its local environment. We also find that the use of a soft-cavity ehyd- model predicts stronger competitive interactions with Rb+ relative to I- than a more traditional hard cavity model, leading to different predicted spectral shifts that should provide a way to distinguish between the two models experimentally. Our simulations predict that at the same concentration, salts with chaotropic cations should produce larger spectral blue-shifts than salts with kosmotropic cations. We also found that at high salt concentrations with chaotropic cations, the predicted blue-shift is greater when the salt anion is kosmotropic instead of chaotropic. Our goal is for this work to inspire experimentalists to make such measurements, which will help provide a spectroscopic means to distinguish between simulations models that predict different hydration structures for the ehyd-.

Computational and Experimental Study of a Sustainable Lithium Acetate and Urea-Based Electrolyte for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most widely used energy storage systems in portable devices, electric vehicles, large-scale energy storage power, and other applications. The optimal performance and safety of LIBs are mainly determined by the electrolyte, which facilitates the transport of Li+ ions and promotes the formation of the SEI layer. Since commercial LIBs are predominantly composed of organic-based electrolytes, these present elevated costs, safety risks, and environmental hazards due to their elevated flammability and fluorinated composition. To address these concerns, aqueous electrolytes have been explored as safer alternatives to organic solvents. However, the use of aqueous LIB electrolytes is restricted due to the narrow electrochemical stability window of water (ESWH2O ≈ 1.23 V). This problem is usually overcomed by using superconcentrated electrolytes, including "water-in-salt" and "water-in-bisalt," which extend the ESW. However, their practicality remains uncertain due to the high concentration of fluorinated salts, which leads to increased cost and toxicity.1 In this study, we employed computational and experimental methods to investigate the use of non-fluorinated aqueous electrolytes prepared by combining different ratios of LiCH3COOH and a water-urea mixture of fixed composition (LiCH3COO-H2O-CO(NH2)2) for their use as an electrolyte for LIBs. ESWs were determined by cyclic voltammetry using GCE, Pt, and Al as working electrodes. The electrolyte with the highest ESW (3.2 V) was probed to be compatible with commercial electrodes such as TiO2, LTO, LMO, and LFP, and was used to assemble a LIB cell with LTO and LMO as electrodes. This cell presented an average output voltage of 2.3 V and a specific capacity of 105 mAh/g over 200 cycles at a current density of 0.5 A/g. Furthermore, the thermodynamic and kinetic factors associated with the electrochemical stability of the electrolyte have been studied using DFT and molecular dynamics (MD), respectively. Adiabatic redox potentials of the electrolyte species obtained by DFT show calculated ESWs higher than 4.5 V. Transport properties of the species in the electrolyte were studied via molecular dynamics and these show that the diffusion of Li+ is comparable to the diffusion observed in organic electrolytes.2 Structurally, we also observed that urea disrupts the Li-water structure, leading to a decrease in the coordination number of water molecules to Li+ (CNLi-H2O = 1.4). This result has been experimentally confirmed using NIR and Raman spectroscopy. Overall, these results demonstrate the effectiveness of employing a lithium acetate-urea-based aqueous electrolyte as an alternative to organic electrolytes for the development of more secure, environmentally friendly, and sustainable LIBs.1. Brown, J.; Grimaud, A. With Only a Grain of Salt. Nat. Energy 2022, 7 (2), 126–127.2. Galvez-Aranda, D. E.; Seminario, J. M. Ion Pairing, Clustering and Transport in a LiFSI-TMP Electrolyte as Functions of Salt Concentration Using Molecular Dynamics Simulations. J. Electrochem. Soc. 2021, 168 (4), 040511.

Frequency-Dependent Microelectrophoresis Study of Colloids with Tunable Surface Charge.

Nonaqueous poly(methyl methacrylate) (PMMA) colloidal suspensions in a solvent that is simultaneously matched in both density and refractive index have been important for real-space studies of colloidal self-assembly, but their complex electrostatic character remains largely unexplored. Electrophoresis is a powerful tool for determining the surface potential and charge of the colloidal suspension; however, because of refractive index matching, standard electrophoresis measurements are not feasible. We carry out microscope-based microelectrophoresis measurements on PMMA colloids in cyclohexyl bromide and cis-trans decalin to measure particle charge as a function of salt concentration in both DC and frequency-variable AC fields. The colloid charge depends on salt concentration and reverses sign near 0.35 μM, providing evidence that solution ions are actively modifying the colloid surface. The frequency dependence of the electrophoretic mobility yields the characteristic time scale for electric double-layer polarization and provides intriguing evidence for Manning condensation and polyion formation.

Water and ions in electrified silica nano-pores: a molecular dynamics study.

Solid-liquid interfaces (SLIs) are ubiquitous in science and technology from the development of energy storage devices to the chemical reactions occurring in the biological milieu. In systems involving aqueous saline solutions as the liquid, both the water and the ions are routinely exposed to an electric field, whether the field is externally applied, or originating from the natural surface charges of the solid. In the current study a molecular dynamics (MD) framework is developed to study the effect of an applied voltage on the behaviour of ionic solutions located in a ∼7 nm pore between two uncharged hydrophilic silica slabs. We systematically investigate the dielectric properties of the solution and the organisation of the water and ions as a function of salt concentration. In pure water, the interplay between interfacial hydrogen bonds and the applied field can induce a significant reorganisation of the water orientation and densification at the interface. In saline solutions, at low concentrations and voltages the interface dominates the whole system due to the extended Debye length resulting in a dielectric constant lower than that for the bulk solution. An increase in salt concentration or voltage brings about more localized interfacial effects resulting in dielectric properties closer to that of the bulk solution. This suggests the possibility of tailoring the system to achieve the desired dielectric properties. For example, at a specific salt concentration, interfacial effects can locally increase the dielectric constant, something that could be exploited for energy storage.

The Nanostructured Future of Electrolytes

Lithium-ion and lithium-metal batteries with cobalt-free high-energy cathodes, as well as the newly emerging batteries employing post-lithium ions as charge carriers, are a few topics tackled in recent years for the development of batteries with higher energy density, lower cost, and improved sustainability. While all of them could (and will) be improved over time, electrolytes are prominent because of their ease of adapting to current technologies. Conventional organic electrolytes are usually flammable, toxic, and harmful to humans and the environment. For these reasons, over the years, several alternatives have been proposed. Ionic Liquids1 are promising from a safety point of view, being non-volatile and non-flammable, but they are usually fluorinated to meet specific performance standards. Another sustainable and cost-efficient approach is represented by the Water-in-salt electrolytes (WiSEs)2,3, given that the used salt is sustainable itself. Finally, the newest approach is to tackle the solubility properties of salts to obtain locally concentrated electrolytes using non-solvating solvents4. For all of the proposed new technologies, their impressive properties seem to emerge from the peculiar structural organizations of the species in the system. Therefore, employing experimental (SAXS and Raman) and computational (MD and DFT) techniques to comprehensively understand how the chemical species interact in such systems, is mandatory to further progress in the research.Figure 1: a) Snapshot of the simulation box of a Locally Concentrated electrolyte. The ionic liquid comprise the worm-like structures, the solvent is only depicted on the left side as a transparent medium. b) coordination numbers as a function of salt concentration in a Water-in-salt electrolyte. c) X-ray scattering patterns for ionic liquid electrolytes and locally concentrated ionic liquid electrolytes at two different temperatures (20 °C top, and -20 °C bottom).References1 J. S. Wilkes, A short history of ionic liquids—from molten salts to neoteric solvents, Green Chemistry, 2002, 4, 73–80.2 L. Suo, F. Han, X. Fan, H. Liu, K. Xu and C. Wang, ‘water-in-Salt’ electrolytes enable green and safe Li-ion batteries for large scale electric energy storage applications, J Mater Chem A Mater, 2016, 4, 6639–6644.3 J. Han, A. Mariani, S. Passerini and A. Varzi, Energy Environ Sci, 2023, https://doi.org/10.1039/D2EE03682G4 X. Liu, A. Mariani, H. Adenusi and S. Passerini, Angewandte Chemie - International Edition, 2023,https://doi.org/10.1002/ange.202219318. Figure 1

Thin-Film Electrodes Based on Conjugated Polyelectrolytes for High-Performance Pseudocapacitance in Aqueous Media

Conjugated polyelectrolytes (CPEs) are an emerging class of materials that exhibits competitive pseudocapacitive performances in their hydrogel form due to their mixed electronic/ionic conductive nature. Interest in developing solid-state electrodes based on CPEs for practical applications is needed. However, the water-soluble nature of the CPEs hinders their thin-film stability in aqueous media. Herein, we report that the slow-dried drop-casted CPE-K thin films formed a solid-state electrode insoluble in aqueous electrolyte solutions when the salt concentration is higher than 0.1 M. XRD measurements revealed that slow-dried, drop-casted CPE-K film possesses a more ordered macromolecular structure when compared to its pre-deposited powder form. DLS measurements revealed that the extent of CPE-K aggregation increases as a function of salt concentration, which enhances the stability of the films in the electrolyte solutions. In addition, AFM imaging suggests well-ordered and uniform nanostructures. As a result, it is possible to use CPE-K thin-film as a pseudocapacitive electrodes capable to maintain 85% (71 F g-1 ) of its original capacitance (84 F g-1) at a specific current of 500 A g-1. In addition, CPE-K film electrodes exhibit good cycling stability where a capacitance retention of 93% after 100,000 cycles at a current density of 35 A g-1 was obtained. To our knowledge, such high charge/discharge rates and stabilities are greater than those reported for state-of-the-art CP in any form. Further electrochemical characterization revealed that surface-controlled pseudocapacitive behavior dominates the charge-storing mechanism of the CPE-K electrodes. In addition, our electrochemical analysis suggests that facile ionic diffusion attributed to the presence of an ionic lattice in the ordered structure of the CPE-K films is responsible for the unprecedented charging and cycling performances. These findings provide a new strategy to obtain solid-state electrodes based on CPEs without the need for binders or additives. Additional advantages such as water processability, low cost, and bio-compatibility may allow CPE-K to be a sustainable solution for the future of electrochemical energy storage.

Solubility of FeCO3 in aqueous NaCl, and aqueous HCl

FeCO3 solubility plays a crucial role in the rate of carbon sequestration and corrosion processes. We present an extensive dataset on precipitation of the solid–liquid equilibria in FeCO3-NaCl-H2O, and FeCO3-HCl-H2O studied at 25–80 °C. 136 new data points on FeCO3 solubility in aqueous NaCl solutions, and 63 data points on the solubility in aqueous HCl solutions are presented. The temperature has a minor impact on the FeCO3 solubility in solutions with NaCl and HCl. FeCO3 is ten times more soluble in HCl than in NaCl solutions. The dissolution rate of FeCO3 in NaCl solutions decreased as a function of salt concentrations and increased with increased temperature. The dissolution rate of FeCO3 in HCl was much larger compared to dissolution in NaCl solutions. The Extended UNIQUAC model was able to calculate FeCO3 solubility. The new FeCO3 solubility data create the foundation for a better understanding of the consequences of CO2 corrosion.

Effective interactions and phase behavior of protein solutions in the presence of hexamine cobalt(III) chloride

It is well established that deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) exhibit a reentrant condensation (RC) phase behavior in the presence of the trivalent hexamine cobalt(III) cations (Hac) which can be important for their packing and folding. A similar behavior can be observed for negatively charged globular proteins in the presence of trivalent metal cations, such as Y3+ or La3+. This phase behavior is mainly driven by charge inversion upon an increasing salt concentration for a fixed protein concentration (cp). However, as Hac exhibits structural differences compared to other multivalent metal cations, with six ammonia ligands (NH3) covalently bonded to the central cobalt atom, it is not clear that Hac can induce a similar phase behavior for proteins. In this work, we systematically investigate whether negatively charged globular proteins β-lactoglobulin (BLG), bovine serum albumin (BSA), human serum albumin (HSA) and ovalbumin (OVA) feature Hac-induced RC. Effective protein–protein interactions were investigated by small-angle X-ray scattering. The reduced second virial coefficient (B2/B2HS) was obtained as a function of salt concentration. The virial coefficient analysis performed confirms the reentrant interaction (RI) behavior for BLG without actually inducing RC, given the insufficient strengths of the interactions for the latter to occur. In contrast, the strength of attraction for BSA, HSA and OVA are too weak to show RC. Model free analysis of the inverse intensity 1/Iq→0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1/I\\left( {q \ o 0} \\right)$$\\end{document} also supports this finding. Looking at different q-range by employing static (SLS) and dynamic light scattering experiments, the presence of RI behavior can be confirmed. The results are further discussed in view of metal cation binding sites in nucleic acids (DNA and RNA), where Hac induced RC phase behavior.Graphical abstract

Unveiling the Layered Structure of Sulfobetaine Polymer Brushes through Bimodal Atomic Force Microscopy

Many zwitterionic polymer brushes exhibit highly stimuli-responsive properties stemming from the strong dipole and electrostatic interaction of their building blocks. Here, we showed how a combination of two atomic force microscopy (AFM) modes can reveal the layered structure of poly(sulfobetaine methacrylate) brushes synthesized by surface-initiated atom-transfer radical polymerization. Due to polydispersity and anti-polyelectrolyte effect, a diffused layer emerges on top of a condensed layer of the brush as a function of salt concentration. The amplitude-modulation mode of the AFM, owing to the tip’s dynamic motion, can only probe the more stable condensed layer near the substrate, whereas the force-spectroscopic mode with its high sensitivity can accurately detect the diffused layer and hence determine the total brush thickness. Infrared spectroscopy and quartz crystal microbalance monitoring revealed the strong ion-screening effect and higher brush hydration propensity of multivalent ions. Different cation valencies also showed subtle effects on the dimensionality of the layered structure. Our results highlight the usefulness of AFM in revealing various contextual phenomena that arise from the unique properties of zwitterionic polymers.

Simple Coacervation of Guanidinium-Containing Polymers Induced by Monovalent Salt

Liquid–liquid phase separation (LLPS) of macromolecules, called coacervation, is induced by non-covalent intermolecular associations in aqueous solutions, which is frequently observed in biological processes including cellular compartmentation, signaling, and regulation. Recently, associative interactions between π-conjugated residues have emerged to induce LLPS, along with electrostatic interaction. An arginine residue having a positively charged guanidinium group plays a pivotal role in protein phase behavior because the orientational amphiphilicity of guanidinium groups allows face-to-face π–π stacking despite Coulombic repulsion. In this study, we investigate the simple coacervates of guanidinium-functionalized polyelectrolytes (i.e., poly-arginine) in aqueous media as a function of salt concentration, salt type, and temperature to understand the exclusive role of π-conjugated moieties. Contrary to ammonium-functionalized polyelectrolyte (i.e., poly-lysine) solutions, guanidinium-functionalized polyelectrolyte solutions become turbid by adding monovalent salts and exhibit the upper critical solution temperature (UCST) behavior; the critical temperature is harnessed by salt concentration and salt type. Although a π–π stacked guanidinium pair can exist at a lower salt concentration, LLPS occurs when the number of intermolecular guanidinium pairs goes beyond a critical value to produce the network structure. Furthermore, the enthalpically favored π–π interaction directly affects the bulk rheological behavior and the interfacial property of the coacervates. Our findings provide insights into the underlying interactions of protein phase separation and shed new light on the critical role of π–π stacking in the biological process and material design.

Coarse-grained intrinsically disordered protein simulations with explicit ions parameterized by salting-out constants.

Implicit solvent coarse-grained models have emerged as an effective method for simulating intrinsically disordered proteins (IDPs) due to the long length and time-scale need to effectively simulate their equilibrium behavior. These coarse-grained methods typically rely on Debye-Hückel theory to explain the effects of salt in the simulations. However, Debye-Hückel electrostatics are only accurate over a limited range of ionic strengths and are also unable to capture ion-type specific effects, typically called Hofmeister effects that arise at higher salt concentrations. To address this, we developed an explicit ion model of potassium chloride that can interface with the otherwise implicit solvent hydrophobicity scale model for intrinsically disordered protein. Parameters for the explicit ion model were determined by matching the salting-out constants of several small peptide-like molecules. Salting-out constants were determined in the simulation by creating a system with an ion concentration gradient via an external potential and performing umbrella sampling along the concentration gradient. The remaining amino acids for which there is no data for the salting-out constant are determined using a scaled Kapcha-Rosky hydrophobicity scale determined from the umbrella sampling simulations. The parameters are validated against a set FRET data from a diverse set of linker peptides at several salt concentrations. This explicit ion model is able to capture the changes in protein expansion as a function of salt concentration at concentration above 0.5 M where Debye-Hückel screened electrostatics predict no change in protein expansion.

Direct Correlation between Short-Range Vibrational Spectral Diffusion and Localized Ion-Cage Dynamics of Water-in-Salt Electrolytes.

The molecular dynamics simulations of a "water-in-salt" electrolyte, lithium bis(trifluoromethyl sulfonyl) imide (LiNTf2), with a varying concentration range of 3 to 20 m were performed to establish a direct connection between a dynamic property like the ion-cage lifetime with the short-range vibrational stretching frequency shift of the used probe, HOD. The properties reported here are compared to that obtained from experiments performed at the same concentrations. The time-series wavelet transform was adopted as a preferable mathematical tool for calculating the instantaneous fluctuating frequencies of the probe O-D stretch mode and the concentration-dependent vibrational stretch spectral signature based on the variable functions associated with a particular chemical bond derived from classical molecular dynamics trajectories. The decay time constants of frequency fluctuations and the lifetime of the ion cage (τIC) were estimated as a function of salt concentration. Herein, we emphasize the correlation between the slowest time constant (τ3) of the decay of O-D stretch frequency fluctuations and the timescales associated with the lifetime of ion cages (τIC). The results exhibit that the existing relationships were also concentration-dependent. Therefore, this study highlights the connection between the ionic motions that regulate the overall system dynamics with the short-range vibrational frequency shift of the used probe, which was used similar to experiments. It also provides an understanding of the interionic interactions and the dynamical and spectral properties of the electrolytic mixtures. We establish a direct correlation between short-range frequency profile and localized ion-cage lifetime, which can fill the gap of understanding between viscosity, vibrational frequency, and ion-cage dynamics of electrolytes.

QENS study on local segmental dynamics of polyelectrolytes in complex coacervates

The segmental dynamics of complex coacervates were investigated as a function of salt concentration and a charge type using quasi-elastic neutron scattering (QENS). The coacervates were prepared by mixing two oppositely charged polyether-based polyelectrolyte solutions functionalized for cationic ammonium and guanidinium groups or anionic sulfonate groups. For the QENS time scale, the intrinsic ion pairs were considered effective cross-links, and therefore the dynamics of chain segments between the intrinsic junctions were probed. We observed that as salt concentration increases, both the number of mobile chain segments and their segmental dynamics increase for ammonium-based coacervates where the electrostatic interaction primarily governs the phase behavior. However, the guanidinium-based coacervates show slow and nearly identical segmental relaxation independent of the salt concentration, which is potentially attributed to the non-electrostatic interactions. The combination of the chemical structure of ionic moieties, salt concentration, and temperature plays a significant role to determine polymer chain dynamics mainly affected by the strength of ionic bonding.

Modification at the C2'-O-Position with 2-Methylbenzothiophene Induces Unique Structural Changes and Thermal Transitions on Duplexes of RNA and DNA.

Oligonucleotides can be chemically modified for a variety of applications that include their use as biomaterials, in therapeutics, or as tools to understand biochemical processes, among others. This work focuses on the functionalization of oligonucleotides of RNA and DNA (12- or 14-nucleotides long) with methylbenzothiophene (BT), at the C2′-O-position, which led to unique structural features. Circular dichroism (CD) analyses showed that positioning the BT units on one strand led to significant thermal destabilization, while duplexes where each strand contained 4-BT rings formed a distinct arrangement with cooperativity/interactions among the modifications (evidenced from the appearance of a band with positive ellipticity at 235 nm). Interestingly, the structural arrays displayed increased duplex stabilization (>10 °C higher than the canonical analogue) as a function of [Na+] with an unexpected structural rearrangement at temperatures above 50 °C. Density functional theory–polarizable continuum model (DFT-PCM) calculations were carried out, and the analyses were in agreement with induced structural changes as a function of salt content. A model was proposed where the hydrophobic surface allows for an internal nucleobase rearrangement into a more thermodynamically stable structure, before undergoing full denaturation, with increased heat. While this behavior is not common, B- to Z-form duplex transitions can occur and are dependent on parameters that were probed in this work, i.e., temperature, nature of modification, or ionic content. To take advantage of this phenomenon, we probed the ability of the modified duplexes to be recognized by Zα (an RNA binding protein that targets Z-form RNA) via electrophoretic analysis and CD. Interestingly, the protein did not bind to canonical duplexes of DNA or RNA; however, it recognized the modified duplexes, in a [monovalent/divalent salt] dependent manner. Overall, the findings describe methodology to attain unique structural motifs of modified duplexes of DNA or RNA, and control their behavior as a function of salt concentration. While their affinity to RNA binding proteins, and the corresponding mechanism of action, requires further exploration, the tunable properties can be of potential use to study this, and other, types of modifications. The novel arrays that formed, under the conditions described herein, provide a useful way to explore the structure and behavior of modified oligonucleotides, in general.

Evaluation of the interfacial tension of binary surfactant mixtures and crude oil using the response surface method

Since it is possible to improve the surface activity of surfactants and mixing them to reduce their critical micelle concentration (CMC), this study is aimed to evaluate the binary mixture of different surfactants types including cationic quaternary ammonium surfactant (CTAB), anionic surfactant (AOT) and nonionic dispersing agent (Tween 80) with a cationic imidazolium-based ionic liquid (ILs) including 1-dodecyl-3-methyl imidazolium chloride ([C12mim]Cl). To optimize the interfacial tension (IFT) of crude oil/surfactant solutions and to evaluate the IFT trend as a function of salt concentrations, a central composite design approach (CCD) is used as the design of experiment approach (DoE) using the response surface methodology (RSM). In this way, effects of surfactants concentrations (170–430 ppm), salt concentration (650–38350 ppm), types of surfactants (Tween 80, CTAB, AOT), and volume ratio of [C12mim]Cl: surfactants (1: 2, 1:1, and 2: 1) are being investigated. The statistical data and the interaction of factors is investigated using analysis of variance (ANOVA) approach based on the rotatable CCD design and a quadratic model using total experimental IFT data points of 117. The excellent coefficient of determination (R2 = 0.9984) and adjusted R2 (0.9980) reveal the accurate IFT values prediction using RSM. According to the DoE approach, optimal conditions are obtained with CTAB surfactant with a concentration of less than 200 ppm with a slight change as the salt concentration and its volume ratio with IL is changed.

Calculating PFAS interfacial adsorption as a function of salt concentration using model parameters determined from chemical structure

Per- and polyfluoroalkyl substances (PFAS) are widely-detected environmental contaminants known to concentrate at surfaces and interfaces. Many of the most commonly-detected PFAS function as ionic surfactants under environmental conditions. The interfacial behaviors of ionic surfactants, including PFAS, are strongly dependent on salt concentration and composition, with interfacial affinity potentially varying by orders of magnitude for the same compound under different conditions. The work described here presents a tool for predicting the salt-dependent adsorption of PFAS compounds based entirely on chemical structure, something of great value for predicting the real-world environmental behavior of many of the large numbers of PFAS compounds for which experimental data are not available. The approach combines two different previously-developed models, one a mass-action model designed to predict the effects of salts on interfacial adsorption of ionic PFAS (the UNSW-OU salt model), and the second a group-contribution model designed to predict interfacial adsorption of PFAS in the absence of salt based on chemical structure. The challenge of combining the two models comes from the fact that both are based on different isotherms. The salt model can produce sigmoidal isotherms under salt-limited conditions (an isotherm shape that is supported by experimental evidence), while the group-contribution model can generate Langmuir parameters from calculations based on chemical structure. Equations were derived to determine salt model isotherm parameters from Langmuir parameters (either from the group-contribution model or experimental sources) by matching surface tension curves in the vicinity of the concentration of highest second derivative. Refined group-contribution model parameters were determined based on data from an additional 40 surface tension curves to allow improved structure-based predictions for important classes of PFAS that were not sufficiently well-represented in the original model. The resulting equations provide a tool allowing quantitative predictions of PFAS behavior under realistic environmental conditions for compounds for which little or no experimental data are available.

Tuning the Polarity of Dinitrile-Based Electrolyte Solutions for CO2 Electroreduction on Copper and Copper-Based Catalysts

Accumulating carbon dioxide (CO2) concentrations in the atmosphere due to anthropogenic sources has become an increasing threat to the environment because of the resulting effects of global warming. It has therefore become imperative to develop and utilize alternative sustainable sources for fuels and feedstock chemicals. CO2 electrochemical reduction (CO2ER) has been identified as a promising process which can convert CO2 to added value chemicals, such as hydrocarbons. In combination with the use of renewable energy resources, the technology has potential to be a net zero carbon process. Commonly, aqueous electrolytes are used for the reaction such as bicarbonate or phosphate-buffered saline solutions. However, these exhibit low CO2 solubilities, which hinders the performance of the reaction by limiting mass transfer. Organic solvent electrolytes possess relatively higher CO2 solubilities. These are also able to suppress the competing hydrogen evolution reaction (HER) due to lowered proton concentrations. Organic nitrile-solvent-based electrolytes, more commonly studied for Li-ion batteries, are known to possess high thermal and electrochemical stability. Furthermore, the polarity and CO2 miscibility can be tuned in dinitrile solvents by variation of the chain length. As such, the effect of chain length was explored on organic dinitrile-based solvent electrolytes for the application of CO2 electroreduction. The selected dinitrile solvents were Adiponitrile (n=4), Suberonitrile (n=6), and Sebaconitrile (n=8), where n indicates the number of methylene groups in the chain. These dinitriles were also compared to Acetonitrile, a commonly used organic mononitrile solvent. Conductivity was first studied as a function of salt concentration and temperature for all solvents. Then, CO2 solubility was evaluated for the different solvents. Finally, electrochemical tests utilizing copper and copper-based bimetallic catalysts were conducted. The catalysts explored consisted of synthesized copper (I) oxide (Cu2O) and copper-silver (CuAg) nanoparticles, obtained through a chemical reduction method in water using sodium borohydride (NaBH4) at ambient conditions. These particles were evaluated against commercial Cu2O and CuAg nanoparticle counterparts. The Cu-based nanoparticles were characterized using XRD, TEM, EDX, and EELS techniques. The obtained results are summarized and discussed.

3D-printed SAXS chamber for controlled in situ dialysis and optical characterization.

3D printing changes the scope of how samples can be mounted for small-angle X-ray scattering (SAXS). In this paper a 3D-printed X-ray chamber, which allows for in situ exchange of buffer and in situ optical transmission spectroscopy, is presented. The chamber is made of cyclic olefin copolymers (COC), including COC X-ray windows providing ultra-low SAXS background. The design integrates a membrane insert for in situ dialysis of the 100 µl sample volume against a reservoir, which enables measurements of the same sample under multiple conditions using an in-house X-ray setup equipped with a 17.4 keV molybdenum source. The design's capabilities are demonstrated by measuring reversible structural changes in lipid and polymer systems as a function of salt concentration and pH. In the same chambers optical light transmission spectroscopy was carried out measuring the optical turbidity of the mesophases and local pH values using pH-responsive dyes. Microfluidic exchange and optical spectroscopy combined with in situ X-ray scattering enables vast applications for the study of responsive materials.

Effect of counter-ion on rheological properties of mixed surfactant solutions

The effects of the charge, size and concentration of counter-ions (Na+, NH4+, K+, Mg2+, Ca2+ and Al3+) on the rheological properties of 300 mM SLES+CAPB surfactant solutions are studied and compared with results available in the literature for the effects of Na+ and K+ on SLES solution. The results show that all counter-ions studied are able to induce formation of wormlike micelles in a certain concentration range – as a consequence, the solution viscosity passes through a high maximum as a function of salt concentration. The salt concentration leading to maximum in the solution viscosity decreases with increasing the charge density of the counter-ion, defined as the ratio of the charge of the counterion and its hydrated radius, Z/R. The maximum viscosity in the salt curve decreases with increasing the hydrated radius of the counter-ion. The addition of CAPB to SLES leads to about 10-fold higher viscosity and to higher maximum viscosity in the presence of K+ as compared to Na+, whereas the opposite trend was observed for SLES alone. These effects are attributed to the interplay between the anionic and zwitterionic surfactants, and the electrolyte in the mixed SLES+CAPB system.