Salt Valency Research Articles

Effects of inorganic salt ions on the wettability of deep coal seams: Insights from experiments and molecular simulations

Coal wettability plays a significant role in the efficiency of coalbed methane (CBM) extraction since it determines fluid transport and distribution. Deep coal seam aqueous fluids have a high salinity. Consequently, coal wettability experiments utilizing fresh water are not representative for those coal seams. Therefore, the effect of aqueous fluids with inorganic salt ions on coal wettability needs to be investigated. In this study, both experiments and molecular simulations are conducted to unravel the wetting behaviour and mechanisms of saline fluids on coal surfaces, investigating different fluid salt concentrations, temperatures, valence states, and coal pore sizes. The results reveal that a more saline fluid displays a more hydrophobic behaviour on the coal surface. Microscopically, water molecules have a tendency to assemble around cations and anions as hydration shells and gradually detach from the coal surface, leading to a gradual transition from hydrophilicity to hydrophobicity. A change in wettability with fluid salinity can be caused by two mechanisms. On the one hand, an increase in ion concentration constrains the diffusion of water molecules through hydration, weakening the mobility of water molecules. On the other hand, a higher ion concentration weakens the interaction between water molecules and oxygen-containing functional groups, reducing the affinity of the water molecules with the pore wall. Moreover, at higher temperatures, ion valence states, and for coal with smaller pores, the impact of inorganic salt ions in fluids on the wetting behaviour of coal surfaces becomes more significant. The coal wettability influences the capillary pressure of coal pores. This study provides new insights on the effect of saline fluids on coal wettability which may help address challenges of fracturing fluid loss and water-blocking in CBM production activities.

Salt and Temperature Effects on Xanthan Gum Polysaccharide in Aqueous Solutions.

Xanthan gum (XG) is a carbohydrate polymer with anionic properties that is widely used as a rheology modifier in various applications, including foods and petroleum extraction. The aim was to investigate the effect of Na+, K+, and Ca2+ on the physicochemical properties of XG in an aqueous solution as a function of temperature. Huggins, Kraemer, and Rao models were applied to determine intrinsic viscosity, [η], by fitting the relative viscosity (ηrel) or specific viscosity (ηsp) of XG/water and XG/salt/water solutions. With increasing temperature in water, Rao 1 gave [η] the closest to the Huggins and Kraemer values. In water, [η] was more sensitive to temperature increase (~30% increase in [η], 20-50 °C) compared to salt solutions (~15-25% increase). At a constant temperature, salt counterions screened the XG side-chain-charged groups and decreased [η] by up to 60% over 0.05-100 mM salt. Overall, Ca2+ was much more effective than the monovalent cations in screening charge. As the salt valency and concentration increased, the XG coil radius decreased, making evident the effect of shielding the intramolecular and intermolecular XG anionic charge. The reduction in repulsive forces caused XG structural contraction. Further, higher temperatures led to chain expansion that facilitated increased intermolecular interactions, which worked against the salt effect.

Parallel neural pathways control sodium consumption and taste valence

The hedonic value of salt fundamentally changes depending on the internal state. High concentrations of salt induce innate aversion under sated states, whereas such aversive stimuli transform into appetitive ones under sodium depletion. Neural mechanisms underlying this state-dependent salt valence switch are poorly understood. Using transcriptomics state-to-cell-type mapping and neural manipulations, we show that positive and negative valences of salt are controlled by anatomically distinct neural circuits in the mammalian brain. The hindbrain interoceptive circuit regulates sodium-specific appetitive drive , whereas behavioral tolerance of aversive salts is encoded by a dedicated class of neurons in the forebrain lamina terminalis (LT) expressing prostaglandin E2 (PGE2) receptor, Ptger3. We show that these LT neurons regulate salt tolerance by selectively modulating aversive taste sensitivity, partly through a PGE2-Ptger3 axis. These results reveal the bimodal regulation of appetitive and tolerance signals toward salt, which together dictate the amount of sodium consumption under different internal states.

Influence of salt on the formation and separation of droplet interface bilayers

Phospholipid bilayers are a major component of the cell membrane that is in contact with physiological electrolyte solutions including salt ions. The effect of salt on the phospholipid bilayer mechanics is an active research area due to its implications for cellular function and viability. In this manuscript, we utilize droplet interface bilayers (DIBs), a bilayer formed artificially between two aqueous droplets, to unravel the bilayer formation and separation mechanics with a combination of experiments and numerical modeling under the effects of K+, Na+, Li+, Ca2+, and Mg2+. Initially, we measured the interfacial tension and the interfacial complex viscosity of lipid monolayers at a flat oil–aqueous interface and show that both properties are sensitive to salt concentration, ion size, and valency. Subsequently, we measured DIB formation rates and show that the characteristic bilayer formation velocity scales with the ratio of the interfacial tension to the interfacial viscosity. Next, we subjected the system to a step strain by separating the drops in a stepwise manner. By tracking the evolution of the bilayer contact angle and radius, we show that salt influences the bilayer separation mechanics, including the decay of the contact angle, the decay of the bilayer radius, and the corresponding relaxation time. Finally, we explain the salt effect on the observed bilayer separation by means of a mathematical model comprising the Young–Laplace and evolution equations.

Predicting the impact of salt mixtures on the air-water interfacial behavior of PFAS

Salts are known to have strong impacts on environmental behavior of per- and polyfluoroalkyl substances (PFAS) including air-water interfacial adsorption. Multivalent salts impact interfacial adsorption to a greater extent than monovalent salts. Models to make a priori predictions of PFAS interfacial adsorption in the presence of multiple salts with different ionic charges are needed given the need to predict PFAS environmental fate. This study further develops a mass-action model to predict the interfacial behavior of PFAS as a function of both salt valency and concentration. The model is validated using surface tension data for a series of monovalent and divalent salt mixtures over a wide range of ionic strengths (i.e., from no added salt to 0.5 M) as well as comparison to data from literature. This model highlights the disproportionate impact of multivalent salts on interfacial adsorption and the practical utility of the model for predicting interfacial adsorption in the presence of multiple monovalent and multivalent inorganic salts. Results suggest that failure to account for divalent salt, even when concentrations are much smaller than monovalent salt, under most environmentally relevant aqueous phase conditions will result in significant underpredictions of PFAS interfacial adsorption. Simple examples of PFAS distribution in a range of salt conditions in the vadose zone and in aerated-water treatment reactors highlight the predictive utility of the model.

Effective Repulsion Between Oppositely Charged Particles in Symmetrical Multivalent Salt Solutions: Effect of Salt Valence

Salt ions play critical roles in the assembly of polyelectrolytes such as nucleic acids and colloids since ions can regulate the effective interactions between them. In this work, we investigated the effective interactions between oppositely charged particles in symmetrical (z:z) salt solutions by Monte Carlo simulations with salt valence z ranging from 1 to 4. We found that the effective interactions between oppositely charged particles are attractive for 1:1 and low multivalent salts, while they become apparently repulsive for high multivalent salts. Moreover, such effective repulsion becomes stronger as z increases from 2 to 3, while it becomes weaker when z increases from 3 to 4. Our analyses reveal that the overall effective interactions are attributed to the interplay between ion translational entropy and electrostatic energy, and the non-monotonic salt-valence dependence of the effective repulsions is caused by the rapid decrease of attractive electrostatic energy between two oppositely charged particles with their over-condensed counterions of opposite charges when z exceeds 3. Our further MC simulations show that the involvement of local-ranged counterion–co-ion repulsions can enhance the effective repulsions through weakening the attractive electrostatic energy, especially for higher salt valence.

Facile Monitoring of Water Hardness Levels Using Responsive Complex Emulsions.

The cationic content of water represents a major quality control parameter that needs to be followed by a rapid, on-site, and low-cost method. Herein, we report a novel method for a facile monitoring of the mineral content of drinking water by making use of responsive complex emulsions. The morphology of biphasic oil-in-water droplets solely depends on the balance of interfacial tensions, and we demonstrate that changes in the surfactant effectiveness, caused by variations in the mineral content inside the continuous phase, can be visualized by monitoring internal droplet shapes. An addition of metal cations can significantly influence the surfactant critical micelle concentrations and the surface excess values and therefore induce changes in the effectiveness of ionic surfactants, such as sodium dodecyl sulfate. The morphological response of Janus emulsions droplets was tracked via a simple microscopic setup. We observed that the extent of the droplet response was dependent on the salt concentration and valency, with divalent cations (responsive for water hardness), resulting in a more pronounced response. In this way, Ca2+ and Mg2+ levels could be quantitatively measured, which we showcased by determination of the mineral content of commercial water samples. The herein demonstrated device concept may provide a new alternative rapid monitoring of water hardness levels in a simple and cost-effective setup.

Study of the effects of mineral salts on the biofilm formation on polypropylene fibers using three quantification methods.

The microbial biofilms are ubiquitous in nature and represent important biological entities that affect various aspects of human life. As such, they attracted considerable attention during last decades, with the factors affecting the biofilm development being among the frequently studied topics. In our work, the biofilm was cultivated on the surface of polypropylene fibers in a nutrient medium inoculated by the suspension of two unsterile soils. The effects of ionic strength and valence of salt on the amount of the produced biofilm and on composition of biofilm microbial communities were investigated. The effect of valence was significant in some OTUs: Arthrobacter/Pseudarthrobacter/Paenarthrobacter and Bacillus with positive response to monovalent salt (KCl) and Streptomyces, Lysinibacillus, Pseudomonas, and Ensifer with positive response to divalent salt (MgSO4). The significant preference for a certain concentration of salts was observed in the case of OTUs Agrobacterium, Bacillus (both 100mM), and Brevundimonas (30mM). A new quantification method based on measuring of oxidizable organic carbon in biofilm biomass, based on dichromate oxidation, was used. We compared the results obtained using this method with results of crystal violet destaining and measuring of extracted DNA concentration as proxies of the biofilm biomass. The dichromate oxidation is simple, inexpensive, and fast, and our results show that it may be more sensitive than crystal violet destaining. The highest biomass values tended to associate with high concentrations of the divalent salt. This trend was not observed in treatments where the monovalent salt was added. Our data confirm the importance of inorganic ions for biofilm composition and biomass accumulation.

Viscosity Behavior of P(DAC-AM) with Serial Cationicity and Intrinsic Viscosity in Inorganic Salt Solutions.

The poly(acryloyloxyethyl trimethyl ammonium chloride–co–acrylamide), P(DAC-AM), is a kind of cationic polyelectrolyte usually applied in a solution form, and its performance is affected by its structure and the environment where it is used. In particular, its viscosity properties in salt solutions are directly related to its efficacy in various applications, and the performance is one of the most important solution properties. Therefore, in this paper, the effects of the salt concentration and valence of seven kinds of inorganic salts, NaCl, LiCl, KCl, MgCl2, AlCl3, Na2SO4, and Na3PO4, on the values of apparent viscosity (ηa) of P(DAC-AM) samples with cationicity of 10%, 50%, and 90%, and intrinsic viscosity ([η]) of 5, 10, and 15 dL/g were investigated. The ηa was determined using a rotational viscometer. The interaction mechanism between the polymers and salt ions was also investigated. The results showed that depending on the salt concentration, the ηa firstly decreased sharply to the inflection point which indicated the minimum volume of the molecule shrinking, and then either maintained the value unchanged or increased. The salt concentration corresponding to the inflection point decreased with the increase of the salt ion valence but with the reduction of the cationicity of the polymer. The ηa at the inflection point increased as the [η] of the polymer grew. This indicated that the salt concentration and the salt ion valence had a notable impact on the stretch of the cationic polymer molecule in the salt solutions. It was discovered that the phenomenon of the increase of the ηa of P(DAC-AM) samples in the multivalent salt solutions after the inflection point was caused by not only the increase of the ηa of the complexes formed from the pure salts, but also the viscosity resistance of the charge and volume between the polymer molecules and salt ions, as well as the complexes themselves. The linear relationship between the increased ηa and the salt concentration, representing the interaction both among the complexes themselves and between the polymer and complexes, was obtained. Furthermore, the interaction model between the salt ions and P(DAC-AM) molecules in a wide range of salt concentrations was illustrated.

Effect of valency of cation on micellization behaviour of pluronic mixed micelle F127 and L64

In this work the effect of mono, di- and trivalent ions on the micellization and clouding behavior of mixed micellar system has been reported. A mixed pluronic micellar system constituting 0.5 wt%/0.5 wt% of pluronic F127 and L64 has been used for the investigation. The UV and Fluorescence studies were carried out using eosin yellow dye as the probe. Both studies indicate that the mixed micellar system displayed the spectral properties intermediate between the neat pluronic solutions. Also, it was observed that the spectral changes occurring were in the order trivalent > di valent > monovalent cations. The order was not strictly followed in the cloud point observation. With minimum discrepancies the clouding behavior of mixed pluronic followed the order Al3+>Mg2+>K+. Dynamic light scattering studies suggested that the addition of salt changed the particle size of the mixed micelles. This salt effect can be better explained considering the small particle size of cations, such as Na+, Mg2+, and Al3+. There was increase in diameter of mixed micelle from 42.81 nm to 54.99 nm for Na+, to 45.11 nm for Mg2+ and to 46.46 nm for Al3+. The infrared studies also indicated the spectral change of mixed micelle due to salt addition. Different ions affect the dehydration of the PPO core differently. To the best of our knowledge this is the first report of salt and cationic valence effect on mixed micellar system.

Aggregation dynamics of charged peptides in water: Effect of salt concentration.

Extensive molecular dynamics simulations have been employed to probe the effects of salts on the kinetics and dynamics of early-stage aggregated structures of steric zipper peptides in water. The simulations reveal that the chemical identity and valency of cation in the salt play a crucial role in aggregate dynamics and morphology of the peptides. Sodium ions induce the most aggregated structures, but this is not replicated equivalently by potassium ions which are also monovalent. Divalent magnesium ions induce aggregation but to a lesser extent than that of sodium, and their interactions with the charged peptides are also significantly different. The aggregate morphology in the presence of monovalent sodium ions is a compact structure with interpenetrating peptides, which differs from the more loosely connected peptides in the presence of either potassium or magnesium ions. The different ways in which the cations effectively renormalize the charges of peptides are suggested to be the cause of the differential effects of different salts studied here. These simulations underscore the importance of understanding both the valency and nature of salts in biologically relevant aggregated structures.

Effect of salt valency and concentration on shear and extensional rheology of aqueous polyelectrolyte solutions for enhanced oil recovery

The injection of polymer solutions into an oil basin can lead to enhanced oil recovery (EOR) by increasing the microscopic sweep of the reservoir, improving the water-oil motility ratio, and thus leading to greater yield from oil fields. In this contribution, we characterize both shear and extensional rheological response of aqueous solutions of partially hydrolyzed polyacrylamide (HPAM), the most commonly used polymer for EOR, for velocity gradients in both the flow direction (extensional) and perpendicular to flow (shear) arise in EOR applications. As HPAM is a charged polymer, to better emulate the environment in oil basins, the rheological response was investigated in presence of salt, sodium chloride, and calcium chloride, with concentrations 3.7 × 10−4 − 1.5 M, as a function of polymer molecular weight (2–10 million g/mol) and concentration (0.005–0.3 wt%). The extensional relaxation times and extensional viscosity are measured using dripping-onto-substrate (DoS) rheometry protocols, and a commercial shear rheometer was utilized for characterizing the shear rheology response. The polyelectrolyte solutions formed by HPAM exhibit shear thinning in steady shear, but show strain hardening in response to extensional flow. Even though an increase in monovalent salt concentration leads to a decrease in both shear viscosity and extensional relaxation times, an increase in divalent salt concentration leads to an increase in extensional viscosity and relaxation time, implying that ion coordination can play a role in the presence of multivalent ions.

Antibacterial oxidized starch/ZnO nanocomposite hydrogel: Synthesis and evaluation of its swelling behaviours in various pHs and salt solutions

In this study, oxidized starch hydrogels have been fabricated, and then ZnO nanoparticles were added to swollen oxidized starch hydrogels through the in-situ process. The purpose of this work was to study the effect of ZnO nanoparticles on swelling behavior of oxidized starch hydrogels, as well as investigation their potential to be used in the antibacterial applications. The obtained results showed that the swelling behavior of the nanocomposite hydrogels was dependent on pH conditions. And at pH 7 the highest swelling was observed for samples because of the carboxylate anions created from samples constituent. The ZnO nanoparticles formation in the hydrogels was established with FT-IR spectroscopy, X-ray diffraction and scanning electron microscopy (SEM). SEM micrographs showed the construction of ZnO nanoparticles with a size range of 35–70 nm within the hydrogel matrix. Also, the swelling behaviours of the nanocomposite hydrogels were studied in several pH values and salt solutions. The swelling capacity of the ZnO nanocomposite hydrogels was reliant on the abundance of the zinc oxide nanoparticles in the oxidized starch hydrogels matrix. Furthermore, these oxidized starch/ZnO nanocomposite hydrogels showed smart swelling behaviours in NaCl, CaCl2 and AlCl3 aqueous solutions and their swelling ratio reduced with a growth of the salt concentration and valence of the cations. The swelling capacity for the resulted compounds in diverse salt solutions with the equal concentration was in order of NaCl > CaCl2 > AlCl3. Also, the antibacterial activities of the ZnO nanocomposite hydrogels were proven against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The nanocomposite hydrogels confirmed fine antibacterial properties. The developed oxidized starch/ZnO nanocomposite hydrogels have the potential to be used for biomedical use.

Interfacial properties of polymeric complex coacervates from simulation and theory

Polymeric complex coacervation occurs when two oppositely charged polyelectrolytes undergo an associative phase separation in aqueous salt solution, resulting in a polymer-dense coacervate phase and a polymer-dilute supernatant phase. This phase separation process represents a powerful way to tune polymer solutions using electrostatic attraction and is sensitive to environmental conditions such as salt concentration and valency. One area of particular research interest is using this to create nanoscale polymer assemblies, via (for example) block copolymers with coacervate-forming blocks. The key to understanding coacervate-driven assembly is the formation of the interface between the coacervate and supernatant phases and its corresponding thermodynamics. In this work, we use recent advances in coacervate simulation and theory to probe the nature of the coacervate-supernatant interface. First, we show that self-consistent field theory informed by either Monte-Carlo simulations or transfer matrix theories is capable of reproducing interfacial features present in large-scale molecular dynamics simulations. The quantitative agreement between all three methods gives us a way to efficiently explore interfacial thermodynamics. We show how salt affects the interface, and we find qualitative agreement with literature measurements of interfacial tension. We also explore the influence of neutral polymers, which we predict to drastically influence the phase behavior of coacervates. These neutral polymers can significantly alter the interfacial tension in coacervates; this has a profound effect on the design and understanding of coacervate-driven self-assembly, where the equilibrium structure is tied to interfacial properties.

Influence of salt valence on the rectification behavior of nanochannels

Taking account of the influence of electroosmotic flow, the behavior of the ion current rectification of a charged conical nanochannel is studied theoretically focusing on the effect of ionic valence. A continuum-based model comprising coupled Poisson-Nernst-Planck (PNP) equations for the ionic mass transport and Navier-Stokes equations for the hydrodynamic field is adopted. We show that if the bulk salt concentration is fixed, the behavior of the current-voltage curve depends highly on the ionic valence, which arises from the difference in ionic strength and ion diffusivity. As the bulk salt concentration varies, the rectification factor shows a local maximum, and the bulk salt concentration at which it occurs depends upon the salt valence: the higher the valence the lower that concentration. However, regardless of the salt valence, the ionic strength at which that local maximum occurs is essentially the same, implying that the thickness of electric double layer is the key factor. Due to the difference in ionic diffusivity, the magnitude of the rectification factor depends upon the type of salt. For example, the rectification factor of KCl is larger than that of KNO3. The qualitative behavior of the ion current rectification of a positively charged conical nanochannel is similar to that of a negatively charged nanochannel.

Role of Salt Valency in the Switch of H-NS Proteins between DNA-Bridging and DNA-Stiffening Modes

This work investigates the interactions of H-NS proteins and bacterial genomic DNA through computer simulations performed with a coarse-grained model. The model was developed specifically to study the switch of H-NS proteins from the DNA-stiffening to the DNA-bridging mode, which has been observed repeatedly upon addition of multivalent cations to the buffer but is still not understood. Unraveling the corresponding mechanism is all the more crucial, as the regulation properties of H-NS proteins, as well as other nucleoid proteins, are linked to their DNA-binding properties. The simulations reported here support a mechanism, according to which the primary role of multivalent cations consists in decreasing the strength of H-NS/DNA interactions compared to H-NS/H-NS interactions, with the latter ones becoming energetically favored with respect to the former ones above a certain threshold of the effective valency of the cations of the buffer. Below the threshold, H-NS dimers form filaments, which stretch along the DNA molecule but are quite inefficient in bridging genomically distant DNA sites (DNA-stiffening mode). In contrast, just above the threshold, H-NS dimers form three-dimensional clusters, which are able to connect DNA sites that are distant from the genomic point of view (DNA-bridging mode). The model provides clear rationales for the experimental observations that the switch between the two modes is a threshold effect and that the ability of H-NS dimers to form higher order oligomers is crucial for their bridging capabilities.

Physical Polyurethane Hydrogels via Charge Shielding through Acids or Salts.

Physical hydrogels with tunable stress-relaxation and excellent stress recovery are formed from anionic polyurethanes via addition of acids, monovalent salts, or divalent salts. Gel properties can be widely adjusted through pH, salt valence, salt concentration, and monomer composition. We propose and investigate a novel gelation mechanism based on a colloidal system interacting through charge repulsion and chrage shielding, allowing a broad use of the material, from acidic (pH 4-5.5) to pH-neutral hydrogels with Young's moduli ranging from 10 to 140 kPa.

Effect of aqueous electrolyte concentration and valency on contact angle on flat glass surfaces and inside capillary glass tubes

Multiphase flow in underground formations is controlled to a large extent by capillary forces that depend on the interfacial tension, contact angle and pore diameter. The interfacial tension and contact angle depend mainly on the type of brine and its concentration. In this study, the static contact angles on flat glass surfaces have been explored as a function of brine concentrations, while the static contact angles inside glass micro-tubes have been studied as a function of pore diameter and brine concentrations for monovalent (NaCl, KCl) and divalent (MgCl2, CaCl2) brine solutions. The concentrations ranged from 0.001 to 6 M under ambient conditions, depending on the investigated salt, while the inner diameters of the glass micro-tubes ranged from 100 to 1000 μm. A linear correlation (h = 0.3249 × d – 4.6203) between the glass micro-tube inner diameter and the meniscus height was obtained for the above solutions for concentration of less than or equal to 1 m. This correlation can be used with the Cheong equation to calculate the contact angle of the investigated solutions when only the micro-tube pore diameter is known. The results also showed that salt concentration and valency have more influence on contact angles on flat glass substrates than inside micro-sized glass tubes. In general, the contact angles on a flat glass surface increased with increasing concentration. On the other hand, the contact angles inside glass micro-tubes showed no variation with increasing concentration but increased slightly as the capillary inner diameters decreased. The difference might be associated with the curvature of the three-phase contact line. The contact angles on flat glass surfaces (≈63–90°) were much higher than those inside the glass capillaries (≈24–33°).

Tuning chain interaction entropy in complex coacervation using polymer stiffness, architecture, and salt valency

Theory and simulation demonstrate how molecular features can be used to design the phase behavior of polymeric complex coacervates.

Rheological properties of N-[(2-hydroxyl)-propyl-3-trimethyl ammonium] chitosan chloride

N-[(2-hydroxyl)-propyl-3-trimethyl ammonium] chitosan chloride (HTCC) was synthesized using 1-allyl-3-methylimidazole chloride (AmimCl) as a homogeneous green reaction media. The effects of polymer concentration, temperature, cation valence, anion type and concentration of inorganic salt, and degree of substitution (DS) on the rheological properties of HTCC were investigated. The apparent viscosity indicated that HTCC solution was a typical pseudoplastic fluid, which could be described by Ostwald de Waele and Cross-empirical models. The viscoelastic properties showed the formation of gel-like structures. Temperature showed slight effect on the apparent viscosity of HTCC solution, while the DS of HTCC showed remarkable effect on the apparent viscosity. At a low shear rate, the apparent viscosities of HTCC solution increased firstly with an increase of cNaCl from 0 to 40mmol/L, and then decreased with further increasing cNaCl to 60mmol/L. At a high shear rate, the opposite trends were observed. The variation of HTCC molecular construction under different inorganic salt conditions was confirmed by scanning electron microscopy, meanwhile, the variation mechanism was proposed.