Divalent Salt Solutions Research Articles

Solvation Behavior of Elastin-like Polypeptides in Divalent Metal Salt Solutions.

The effects of CaCl2 and MgCl2 on the cloud point temperature of two different elastin-like polypeptides (ELPs) were studied using a combination of cloud point measurements, molecular dynamics simulations, and infrared spectroscopy. Changes in the cloud point for the ELPs in aqueous divalent metal cation solutions were primarily governed by two competing interactions: the cation-amide oxygen electrostatic interaction and the hydration of the cation. In particular, Ca2+ cations can more readily shed their hydration shells and directly contact two amide oxygens by the formation of ion bridges. By contrast, Mg2+ cations were more strongly hydrated and preferred to partition toward the amide oxygens along with their hydration shells. In fact, although hydrophilic ELP V5A2G3 was salted-out at low concentrations of MgCl2, it was salted-in at higher salt concentrations. By contrast, CaCl2 salted the ELP sharply out of solution at higher salt concentrations because of the bridging effect.

Ordered self-assembly of DNA-modified nanoparticles in salt solutions

DNA-modified nanoparticles (DNA-NPs) have been widely applied in medical diagnosis and biosensors. Molecular dynamics (MD) simulation, a common method to track the dynamic changes of ions, nanoparticles and DNA, is able to predict the assembly behavior of DNA-NP, thus providing a fast and efficient method for precise regulation of experiments. At present, the simulation research of the DNA-NP self-assembly mainly focuses on DNA bonding, while the effects of electrostatic interactions between DNA and DNA, ions and ions, ions and DNA on the self-assembly structure and mechanism have not been systematically studied. We have constructed a coarse-grained model of DNA-NP self-assembly in salt solutions to appropriately simulate the experimental conditions. It can well match the directionality and saturation of DNA base pairing, as well as the electrostatic repulsive interactions between DNA, and ion-related effects. How the distribution of ions, electrostatic shielding and DNA hybridization regulate the assembled structure and kinetic properties of DNA-NP in monovalent or divalent salt solutions with different ionic concentrations are deeply investigated. Our simulated results reveal the thermodynamic stability and reversibility of the ordered self-assembly of DNA-NPs, in accordance with experimental results. They also provide a reasonable explanation for the susceptibility of nanoparticles to aggregation at high salt concentration for multivalent salt environments. Our research gives a preliminary attempt for the simulation of DNA-NP self-assembly in salt solution, provides theoretical guidance for designing DNA nanomaterials with controllable structures and properties, and promotes its research progress in the fields of medical diagnosis and biosensors.

Enhanced oil recovery through synergy of the interfacial mechanisms by low salinity water alternating carbon dioxide injection

During this study, a comprehensive investigation of low salinity water alternating CO2 injection was performed for enhanced oil recovery in oil-wet carbonate reservoirs. A synergy of interfacial mechanisms such as IFT, wettability alteration, CO2 solubility, oil swelling, water shielding effect, and rock dissolution was considered in two and three-phase systems. Results showed that the monovalent ions, such as NaCl or KCl, inhibit the dissolution of carbon dioxide in brine in excess of divalent salt solutions, e.g. CaCl2 or MgCl2 due to the salting-out effect. In addition, more water shielding effect was observed in low salinity formation water than it in low salinity seawater. The significant change in the reservoir wettability of oil/brine/CO2 system compared to oil/brine referred to the CO2 solubility in brine which could cause stronger carbonated water in the reservoir. Consequently, the low salinity alternating CO2 injection overpowers the late-production problem that occurred commonly in conventional WAG injection.

Divalent Lead in Aqueous Solution Changes the Surface Morphology of Dolomite and Inhibits Dissolution

In groundwater systems, heavy metal ions as solutes (e.g., Pb) can adsorb onto the surface of calcite group rocks and influence their dissolution processes. The dolomite surface was examined using field emission scanning electron microscopy (SEM) and various characterization tools and the changes in water chemistry indexes were reviewed throughout the dissolution process. Pb adsorption on the dolomite surface was evident after 15 days of exposure to 1 mg/L or 50 mg/L divalent Pb salt solutions; the Pb surface phase was mainly PbCO3 with an octahedral ligand structure. SEM images show that dolomite in divalent Pb salt solutions can lead to the surface morphology exhibiting curved dissolution steps. In the closed system with Pb (1 mg/L), the total alkalinity and conductivity of the solution were lower than in the Pb-free system, and the pH difference was slight, indicating that the dibasic metal ion Pb inhibited the dolomite dissolution process. Combined with the composition of the final solid phase, it is suggested that the dolomite surface preferentially adsorbs Pb2+ on the active sites and that the newly grown solid phase is PbCO3 possesses an octahedral ligand structure. Part of the surface-active site is occupied, resulting in a change in the dissolution profile, and thus preventing further development of the dissolution profile. Ultimately, the entire dolomite surface area is prevented from dissolution. The results of this study suggest that Pb2+ may be an effective inhibitor of dolomite dissolution and may help to further refine the geological carbon sink.

Hybrid NF and UF membranes tailored using quaternized polydopamine for enhanced removal of salts and organic pollutants from water

In this work, a quaternized polydopamine (QSiPD) nanohybrid was synthesized via condensation reaction between the silicon hydroxyl moieties of the hydrolyzed QSiP and hydroxyl moieties of polydopamine (PDA) and applied for the first time as a single additive to Poly(ether sulfone) (PES) ultrafiltration (UF) and loose nanofiltration (NF) membranes made via non-solvent induced phase separation (NIPS). QSiPD was added at a maximum loading of 8 and 20 wt% in UF and NF membranes, respectively. The nanohybrid material and membranes were thoroughly characterized. The hydrophilicity, surface charge, and pore structures of the hybrid membranes were progressively tuned with the increase of the QSiPD loading. The UF membrane water permeability of the membrane with 2 wt% QSiPD was as high as ~559 L m−2 h−1 bar−1. The hybrid UF membranes also exhibited superior antifouling activity with a BSA rejection rate of ~99 % uncompromised by the high membrane flux. The hybrid NF membranes showed significant rejection (45–67 %) of several monovalent (NaCl) and divalent (CaCl2, MgSO4, Na2SO4) salt solutions and an outstanding rejection of Congo red dye (>99.9 %). Furthermore, both hybrid UF and NF membranes showed a strong antibacterial activity against E. coli.

Effects of the Temperature and Salt Concentration on the Structural Characteristics of the Protein (PDB Code 1BBL).

The effect of the temperature and salt solution on the structural characteristics of the protein 1BBL was investigated by molecular dynamics simulations. The paper presents simulation results regarding the non-bonded energy and the structural stability of the protein immersed in salt solutions with different concentrations and temperatures. Our work demonstrates that the electrostatic potential energy and van der Waals energy of the system show the opposite changes with the influence of the external environment. Since the electrostatic potential energy changes more obviously, it is dominated in the non-bonding interactions. The structural parameters, such as the root mean square deviation and the radius of gyration, increased initially and decreased afterward with the increase of the salt concentration. The protein presented the loose structure with a relative low stability when it was immersed in a monovalent solution with a salt concentration of 0.8 mol/L. The salt concentration corresponding to the maximum value of structural parameters in the monovalent salt solution was double that in the divalent salt solution. It was also concluded that the protein presented a compact and stable structure when immersed in salt solutions with a high concentration of 2.3 mol/L. The analysis of the root mean square deviation and root mean square fluctuation of the protein sample also exhibited that the structural stability and chain flexibility are strongly guided by the effect of the temperature. These conclusions help us to understand the structural characteristics of the protein immersed in the salt solutions with different concentrations and temperatures.

Sr2+ and Ba2+ salts induced conformational structure of sodium polyacrylate PAA investigated by molecular dynamics simulations

The shrinkage of sodium-poly(acrylate) PAA in divalent salt (strontium chloride (SrCl2) and barium chloride (BaCl2) solutions is investigated by atomistic molecular dynamics (MD) simulations to study the salt effect on PAA structure. The salt concentration (Cs) was varied in the range of 0 < Cs < 1 M for fully charged PAA. The PAA radius-of-gyration (Rg) decreases with Cs in the presence of both the divalent salts in qualitative agreement with experiments. The PAA chain stiffness calculated in persistence length (Lp) decreases over the entire range of Cs. At lower Cs, the PAA chains don't form aggregate, while at higher salt concentrations, PAA chains undergo self-association in both the divalent salts. The PAA-Water H-bonds decreases with Cs significantly in the presence of BaCl2 as compared to SrCl2. The PAA-Na+ radial distribution function (RDF) shows a decrease in its coordination number with Cs. The distribution of salt-ions around PAA shows that the Ba2+ condensation onto PAA is more significant than Sr2+ because of a greater coordination number for the former than the latter. Overall, the present study significantly advanced the molecular-level understanding of PAA microstructure in the dilute divalent metal salt solution.

Non-swellability of polyelectrolyte gel in divalent salt solution due to aggregation formation

The swelling behavior of polyelectrolyte gels depends on their ambient conditions owing to the ionic pressure of the Gibbs‒Donnan equilibrium. In a polyelectrolyte gel with a high fixed ion density, the counterion condensation effect influences the Gibbs‒Donnan equilibrium. In particular, the mechanism of the divalent salt effect remains unclear due to its complexity. This study investigated the swelling properties of polyelectrolyte gels in a divalent CaCl 2 salt solution using model polyelectrolyte gels called tetra-polyacrylic acid-polyethylene glycol gels (Tetra-PAA-PEG gels). In [Ca 2+ ] < 0.1 mM, the swelling ratios increased with increasing pH. With increasing Ca 2+ ions concentration, the gel showed the non-swellability independent of pH, which cannot be described by the simple mean-field theories. The viscoelastic and small-angle X-ray scattering measurements indicate that an aggregation formation due to the divalent salt induces an anomaly even in the model system. These findings will help understand the swelling behavior of polyelectrolyte gels in divalent salt systems. • Design of a model polyelectrolyte gel with alternating neutral and charged sequences. • Non-swellability was achieved in divalent salt solution even using the model system. • Well-disperse sphere structures were constructed by divalent salt. • Divalent salt-induced structural change resulting into non-swellability.

Stimuli responsive self-assembled structural aggregates of ionic liquid based surfactants as the membrane free microreactors for dyes sequestration and drug encapsulation

Judiciously designed surfactant based protocell models such as micelles, vesicles, gels and coacervates with higher thermal stability, low-toxicity and bio-compatibility found applications in various new age technologies. In our attempt to obtain the membrane free microreactors for accumulating dyes and biomolecules within a single system, herein we had designed ionic liquid (ILs) based simple coacervates. These coacervates were formed by merely changing solution pH of ester functionalized imidazolium and morpholinium based surface active ILs with varied alkyl chain length and anions. On reaching the neutral pH, the large isotropic (micellar and vesicular) phase de-mixes into two conjugate phases; the dense coacervate phase in equilibrium with the dilute phase. Stability of these membrane free microreactors were studied in the presence of mono and divalent salt solution, viscous solution, higher temperature and acidic as well as alkaline solutions for their realistic applications. These newly formulated microreactors were investigated for the membrane free transport of neutral, cationic, and anionic dyes under non-equilibrium conditions. The same coacervate droplets further show excellent encapsulation for the anticancer hydrophobic drug, curcumin; opening an all new avenue in drug delivery. Presently investigated environmentally benign simple coacervates could replace their costly and inefficient natural analogous for multi-purpose applications including biomimetic operations.

Molecular insights on the influence of temperature and metal ions on the hydration of kaolinite (001) surface

ABSTRACT In the present research, various divalent and monovalent metal salt solutions (NaCl, KCl, MgCl2 and CaCl2) were added into the kaolinite-water system to explore its impact on the surface hydration of kaolinite. The hydration layers’ density and adsorption energy between kaolinite and water molecules were gradually decreased with an increase in temperature from 280 to 330 K. The addition of metal ions had a great influence on the hydration of the kaolinite surface. While the adsorption energy and structure of hydration layers in the kaolinite-water-ion system was unchanged when the ion concentration reaches 0.8 M. The adsorption energy between kaolinite and divalent ions was greater than between kaolinite and monovalent ions.

Effects of Amino Acid Side-Chain Length and Chemical Structure on Anionic Polyglutamic and Polyaspartic Acid Cellulose-Based Polyelectrolyte Brushes.

We used atomistic molecular dynamics (MD) simulations to study polyelectrolyte brushes based on anionic α,L-glutamic acid and α,L-aspartic acid grafted on cellulose in the presence of divalent CaCl2 salt at different concentrations. The motivation is to search for ways to control properties such as sorption capacity and the structural response of the brush to multivalent salts. For this detailed understanding of the role of side-chain length, the chemical structure and their interplay are required. It was found that in the case of glutamic acid oligomers, the longer side chains facilitate attractive interactions with the cellulose surface, which forces the grafted chains to lie down on the surface. The additional methylene group in the side chain enables side-chain rotation, enhancing this effect. On the other hand, the shorter and more restricted side chains of aspartic acid oligomers prevent attractive interactions to a large degree and push the grafted chains away from the surface. The difference in side-chain length also leads to differences in other properties of the brush in divalent salt solutions. At a low grafting density, the longer side chains of glutamic acid allow the adsorbed cations to be spatially distributed inside the brush resulting in a charge inversion. With an increase in grafting density, the difference in the total charge of the aspartic and glutamine brushes disappears, but new structural features appear. The longer sides allow for ion bridging between the grafted chains and the cellulose surface without a significant change in main-chain conformation. This leads to the brush structure being less sensitive to changes in salt concentration.

Diffusion through soil-bentonite backfill from a constructed vertical cutoff wall

Diffusion coefficients were measured for soil-bentonite (SB) backfill sampled from a constructed vertical cutoff wall in Pennsylvania, USA. The backfill consisted of lean clay (the base soil) combined with bentonite-water slurry containing 5–6% sodium bentonite, resulting in a total bentonite content of ~1%. Ten dialysis leaching tests (DLTs) were conducted to measure apparent diffusion coefficients (Da) for chloride (Cl−) diffusion through the lean clay-bentonite backfill. In addition, model sand-bentonite backfills (total bentonite content = 4.7%) were prepared in the laboratory, and eight DLTs were performed for comparison with the field-sampled backfill and the diffusion literature. Both backfill types were tested with monovalent and divalent salt solutions (KCl and CaCl2), over a wide range of concentrations (60–350 mM). Despite the difference in bentonite content, values of Da were similar for the two backfills. Changes in Da with increasing average Cl− concentration were relatively minor, increasing by less than half an order of magnitude over the wide range of salt solutions, likely due to the low percentages of bentonite (<5%) in both backfills. Thus, for SB backfills with bentonite contents <5%, Da appears to be relatively insensitive to backfill composition, salt type and concentration.

Fabrication of a charged PDA/PEI/Al2O3 composite nanofiltration membrane for desalination at high temperatures

Ceramic membranes with high thermal and chemical resistances play an important role in nanofiltration process under harsh conditions. However, the application of ceramic membranes for salt filtration is challenging due to the limitation of pore size and surface charge. Herein, composite ceramic nanofiltration membranes were prepared by the codeposition of polydopamine and polyethyleneimine (PDA/PEI), with subsequent cross-linking by glutaraldehyde (GA). Ceramic ultrafiltration membranes served as supports, endowing the composite membranes with high mechanical stabilities and good hydrophilicity. A significant improvement in the rejection of salt solutions was achieved after modification due to the strong positive surface charge and small membrane pore size. The polydopamine/PEI modification layer crosslinked by glutaraldehyde and bonded to the surface of ceramic membrane through covalent bond show good thermal resistance. At high temperatures, the composite ceramic nanofiltration membranes still have a good rejection performance for divalent salt solutions. Overall, this study offers a composite ceramic nanofiltration membrane for desalination at high temperature, allowing a sustainable chemical process.

Suction and crack propagation in GCLs subjected to drying and wetting in CaCl2-solutions

The current study addresses the cracking and self-healing capacity of Geosynthetic clay liners (GCLs) subjected to drying and wetting in a divalent salt solution. Commercially available GCLs, initially saturated under a load of 3.92 kPa, were stepwise dried for different durations in an oven at 30 °C and rewetted afterwards in deionised water and divalent salt solutions (CaCl2) of different molarities (0.05 mol/l, 0.5 mol/l and 0.05 mol/l prehydrated). The evolution of cracks and their patterns were studied by analysing X-ray images. In parallel, the water retention behavior was tested on the raw bentonite using micro-cells and a chilled-mirror hygrometer. The morphology of the crack patterns in the GCLs was highly affected by the pore fluid, which was reasoned by reduced tensile strength caused by the salt induced aggregation. The ability to retain water at a given suction was found to be higher for the samples subjected to CaCl2-solutions in comparision to the sample saturated with deionised water. However, a calculation of the osmotic suction caused by the additional CaCl2 in the porewater shows that the matric suction of the samples subjeted to CaCl2 decreases. The crack intensitiy factor (CIF) followed a similar trend and three different drying regimes were identified.

Translocation of a Polyelectrolyte through a Nanopore in the Presence of Trivalent Counterions: A Comparison with the Cases in Monovalent and Divalent Salt Solutions.

A polyelectrolyte threading through a nanopore in a trivalent salt solution is investigated by means of molecular dynamics simulations under a reflective wall boundary. By varying the chain length N and the strength E of the driving electric field applied inside the pore, the translocation time is carefully calculated to get rid of the bouncing effect because of the boundary. The results are analyzed under the scaling form ⟨τ⟩ ∼ NαE–δ and four driving force regimes; namely, the unbiased, the weakly driven, the strongly driven trumpet, and the strongly driven isoflux regime, are distinguished. The exponents are calculated in each regime and compared with the cases in the monovalent and divalent salt solutions. Owing to strong condensation of counter ions, the changes of the exponents in the force regimes are found to be nontrivial. A large increase in translocation time can be, however, achieved as the driving field is weak. The variations of the chain size, the ion condensation, and the effective chain charge show that the process is proceeded in a quasi-equilibrium way in the unbiased regime and deviated to exhibit strong nonequilibrium characteristics as E increases. Several astonishing scaling behaviors of the waiting time function, the translocation velocity, and the diffusion properties are discovered in the study. The results provide deep insights into the phenomena of polyelectrolyte translocation in various salt solutions at different driving forces.

Mixed brushes consisting of oppositely charged Y‐shaped polymers in salt free, monovalent, and divalent salt solutions

AbstractThe conformation and the internal stratification of mixed brushes formed from oppositely charged Y(−) and Y(+)‐shaped chains in salt free, monovalent, and divalent salt solutions were studied by means of molecular dynamics simulations using the primitive model. Scaling relations of mixed brush height with respect to the grafting surface per chain, the ratio of the total positive to the total negative charge of polyelectrolyte chains, and salt concentrations were obtained. The simulations predicted that mixed brushes show a unique response to divalent salt (1:2) solutions. For symmetric brushes having the same spacer lengths, number of chains and charged units fractions the increase of the salt concentration leads to the enrichment of the outer brush surface with Y(+) units and the lamella microphase separation. For asymmetric brushes in high salt concentration cylindrical domain microphases are formed.

Mechanisms of Surface Charge Modification of Carbonates in Aqueous Electrolyte Solutions

The influence of different types of salts (NaCl, CaCl 2 , MgCl 2 , NaHCO 3 , and Na 2 SO 4 ) on the surface characteristics of unconditioned calcite and dolomite particles, and conditioned with stearic acid, was investigated. This study used zeta potential measurements to gain fundamental understanding of physico-chemical mechanisms involved in surface charge modification of carbonate minerals in the presence of diluted salt solutions. By increasing the salt concentration of divalent cationic salt solution (CaCl 2 and MgCl 2 ), the zeta potential of calcite particles was altered, resulting in charge reversal from negative to positive, while dolomite particles maintained positive zeta potential. This is due to the adsorption of potential-determining cations (Ca 2 + and Mg 2 + ), and consequent changes in the structure of the diffuse layer, predominantly driven by coulombic interactions. On the other hand, chemical adsorption of potential-determining anions (HCO 3 − and SO 4 2 − ) maintained the negative zeta potential of carbonate surfaces and increased its magnitude up to 10 mM, before decreasing at higher salt concentrations. Physisorption of stearic acid molecules on the calcite and dolomite surfaces changed the zeta potential to more negative values in all solutions. It is argued that divalent cations (Ca 2 + and Mg 2 + ) would result in positive and neutral complexes with stearic acid molecules, which may result in strongly bound stearic acid films, whereas ions resulting in negative mineral surface charges (SO 4 2 − and HCO 3 − ) will cause stearic acid films to be loosely bound to the carbonate mineral surfaces. The suggested mechanism for surface charge modification of carbonates, in the presence of different ions, is changes in both distribution of ions in the diffuse layer and its structure as a result of ion adsorption to the crystal lattice by having a positive contribution to the disjoining pressures when changing electrolyte concentration. This work extends the current knowledge base for dynamic water injection design by determining the effect of salt concentration on surface electrostatics.

Effects of Divalent Salts on the Interfacial Activity of the Mixed Surfactants at the Water/Model Oil Interface

AbstractIn our previous report, the mixed cationic/anionic surfactant system consisting of N‐dodecyl‐N‐methylpyrrolidinium bromide (L12) and sodium dodecyl sulfate (SDS) showed good interfacial tension (IFT) reduction of water/model oil (Vtoluene:V n‐decane = 1:1). In the present study, the effects of divalent salts (MgCl2 or CaCl2) on the interfacial activity were systematically evaluated. The additional Mg2+ ions greatly reduced the IFT to an ultralow value, whereas Ca2+ ions caused the generation of the precipitates and resulted in increased IFT values. The precipitates disappeared in binary divalent salt solutions, and the IFT values remained at a low level. Based on the valence, polarizability, and hydrated radius of the ions, we proposed a model to explain the abnormal changes. The effects of NaCl and temperature were investigated to further verify our proposed mechanism. Moreover, the additional divalent salts obviously enhanced the stability of L12/SDS stabilized emulsions.

Fractionation of mono- and divalent ions by capacitive deionization with nanofiltration membrane

In this study, we demonstrated the influence of adding nanofiltration membranes to a CDI cell, when desalinating mono- and divalent salt solutions both theoretically and experimentally, and proposed a new procedure for the separation of mono- and divalent ions. It is revealed that the distinction of the diffusion coefficient between mono- and divalent ions is the key to the effective separation. Combined with the appropriate time of adsorption (1.5 h), the concentration ratio of NaCl to MgSO4 can be increased by a factor of 3, from a mixture with equal amount of NaCl and MgSO4.

Discriminative ionic capabilities on hydrogen-bond transition from the mode of ordinary water to (Mg, Ca, Sr)(Cl, Br)2 hydration

It has been a long pursuit to discriminate the ionic roles of mono- and di-valent salt solutions in modulating the hydrogen bonding network and solution properties. We attended this issue by examining the effect of concentrated YX2 (YMg, Ca, Sr; XCl, Br) solvation on O:HO bonds transition from the mode of ordinary water to hydration in terms of the number fraction fYX2(C) and the segmental O:HO bond phonon stiffness shift Δω(C) with C being the solute concentration. The invariant dfY(C) / dC at C ≤ <0.05 suggests that the small Y2+ forms a constantly-sized hydration droplet with weak responding to interference of other ions because its hydrating H2O dipoles screen mostly its electric field. However, the number inadequacy of the highly-ordered hydrating H2O dipoles partially screens the large X−. The X− ↔ X− electrostatic repulsion weakens its electric field. The concentration-trend consistency of the fYX2(C), the solution conductivity σYX2(C), and surface stress (contact angle) θYX2(C) for YX2 solutions clarifies their common origin of ionic polarization. However, the Jones–Dale notion disobedience of the viscosity ηYX2(C) suggests the dominance of the inter-ion repulsion.