Critical Salt Concentration Research Articles

Sodium chloride-programmed phase transition of β-conglycinin/lysozyme electrostatic complexes from amorphous precipitates to complex coacervates

The associative phase separation of biomacromolecules can produce both liquid-like coacervates or solid-like precipitates. In this study, we found that sodium chloride (NaCl) can programme the phase transition of β-conglycinin/lysozyme (β-CG/LYS) complexes. NaCl reduced the ζ-potential of both β-CG and LYS. Their complex coacervate was formed at pHs 6 and 7 with 5–80 mM NaCl and at pH 8 with 40–80 mM NaCl. Unlike the high critical salt concentration and entropy gain driving force of polyelectrolyte-based complex coacervation, 100 mM NaCl almost completely inhibited β-CG/LYS complexation, and the exothermic enthalpy change was the main driving force for β-CG/LYS complex formation. Confocal laser scanning microscopy with fluorescein isothiocyanate-labeled proteins demonstrated dynamic protein exchange in coacervate droplets, similar to that in polyelectrolyte-based complex coacervates.

Application of DLVO Modeling to Study the Effect of Silica Nanofluid to Reduce Critical Salt Concentration in Sandstones

Critical salt concentration (CSC) is the minimum salt concentration of injected water used for different oil recovery operations, below which fines migration initiates within sandstone reservoirs having clay contents and could be one of the potential causes of formation damage. This paper estimated CSC for Berea sandstone-NaCl brine system experimentally and by using DLVO modeling based on Van der Waals and electric double layer (EDL) surface forces. Furthermore, the effectiveness of silica nanofluid to reduce CSC and control fines migration was investigated. At different salinities of the injected fluid, the experimental CSC was determined by performing corefloods and analyzing effluent turbidity and absorbance. Attractive (van der Waals) and repulsive (EDL) forces were estimated by DLVO theory to predict the value of CSC. The experimental data were used to validate the model. The application of silica nanoparticles (NPs) was studied by injecting 0.1 wt% silica nanofluid and the resultant reduction in CSC was observed. During the injection of brines at different salinities ranging from 0.2 M to 0.05 M into the sandstone porous media, fines production was detected in the effluent after the injection of 0.1M NaCl solution, indicating that the CSC was 0.1M for this sand-fine-brine (SFB) system. The zeta potentials for SFB systems of different salinities were measured by zeta-sizer and were in the range of -32 mV to -24 mV. DLVO model based on attractive and repulsive forces was then used to predict the CSC, at which the total DLVO energy shifts from negative to positive and was around 0.1M for NaCl solution. The model prediction was in close agreement with the experimental results. Both the experiment and DLVO model showed the efficiency of silica nanofluid to mitigate fines migration as no fines production was observed for 0.1M NaCl case after the treatment with nanofluid. The estimation of CSC is critical to avoid formation damage during many oil recovery processes, included but not limited to low salinity waterflooding and alkaline flooding. The utilization of nanofluid provided promising results in controlling fines migration and reducing CSC. The proposed model based on DLVO theory can work as a useful tool to predict CSC without the need for extensive experimental work.

Complex Coacervation of Polymerized Ionic Liquids in Non-aqueous Solvents.

Oppositely charged polymerized ionic liquids (PILs) were used to form complex coacervates in two different organic solvents, 2,2,2-trifluoroethanol (TFE) and hexafluoro-2-propanol (HFIP), and the corresponding phase diagrams were constructed using UV-vis, NMR, and turbidity experiments. While previous studies on complex coacervates have focused almost exclusively on aqueous environments, the use of PILs in the current work enabled studies in solvents with substantially lower dielectric constants (27.0 for TFE, 16.7 for HFIP). The critical salt concentration required to induce complete miscibility was roughly 2-fold larger in HFIP compared with TFE, and two different PIL complexes, solidlike precipitates and liquidlike coacervates, were found in both systems. This study provides insight into the effects of low-dielectric-constant solvents on complex coacervation, which has not been widely studied because of the limited solubility of conventional polyelectrolytes in these media.

Yielding and rheopexy of aqueous xanthan gum solutions

Xanthan gum (XG) is widely used in cosmetic and pharmaceutic products (creams, pastes) and in oil industry (drilling fluids) as a stabilizing and/or thickening agent. In literature, its rheological behavior is mainly presented as that of a shear-thinning or a yield stress fluid. Here, in order to clarify this rheological behavior, we study in detail the flow characteristics during continued flow under given conditions (i.e., controlled stress) for a mass concentration ranging from 0.2 to 5%. We are thus able to identify the apparent flow curve of the material after a short flow duration and the flow curve in steady state (i.e., after a long flow duration). The validity of this flow curve, determined from standard rheometry, is confirmed by magnetic resonance velocimetry. These materials start to exhibit a yield stress behavior beyond some critical xanthan or salt concentration. In that case, a significant increase (by a factor up to 5) of the apparent viscosity is observed during flow in some range of stresses, before reaching a steady state. This original rheopectic effect might be due, after some time of flow associated with deformation and reconfiguration of the XG molecules, to the progressive formation of intermolecular links such as hydrogen bonds and/or intermolecular association due to acetate residues.

Switchable [formula omitted]-lactoglobulin (BLG) adsorption on protein resistant oligo (ethylene glycol) (OEG) self-assembled monolayers (SAMs)

HypothesisAlthough protein adsorption at an interface is very common and important in biology and biotechnology, it is still not fully understood – mainly due to the intricate balance of forces that ultimately control it. In food processing (and medicine), controlling and manipulating protein adsorption, as well as avoiding protein adsorption (biofilm formation or membrane fouling) by the production of protein-resistant surfaces is of substantial interest. A major factor conferring resistance towards protein adsorption to a surface is the presence of tightly bound water molecules, as is the case in oligo ethylene glycol (OEG)-terminated self-assembled monolayers (SAMs). Due to strong attractive protein-protein and protein-surface interactions observed in systems containing trivalent salt ions, we hypothesize that these conditions may lead to a breakdown of protein resistance in OEG SAMs. ExperimentsWe studied the adsorption behavior of BLG in the presence of a lanthanum(III) chloride (LaCl3) at concentrations of 0, 0.1, 0.8 and 5.0 mM on normally protein resistant triethylene glycol-termianted (EG3) SAMs on a gold surface. We used quartz-crystal microbalance with dissipation (QCM-D) and neutron reflectivity (NR) to characterize the morphology of the interfacial region of the SAM. FindingsWe demonstrate that the protein resistance of the EG3 SAM breaks down beyond a threshold salt concentration c∗ and mirrors the bulk behaviour of this system, showing reduced adsorption beyond a second critical salt concentration c∗∗. These results demonstrate for the first time the controlled switching of the protein-resistant properties of this type of SAM by the addition of trivalent salt.

Deciphering the Role of π-Interactions in Polyelectrolyte Complexes Using Rationally Designed Peptides.

Electrostatic interactions, and specifically π-interactions play a significant role in the liquid-liquid phase separation of proteins and formation of membraneless organelles/or biological condensates. Sequence patterning of peptides allows creating protein-like structures and controlling the chemistry and interactions of the mimetic molecules. A library of oppositely charged polypeptides was designed and synthesized to investigate the role of π-interactions on phase separation and secondary structures of polyelectrolyte complexes. Phenylalanine was chosen as the π-containing residue and was used together with lysine or glutamic acid in the design of positively or negatively charged sequences. The effect of charge density and also the substitution of fluorine on the phenylalanine ring, known to disrupt π-interactions, were investigated. Characterization analysis using MALDI-TOF mass spectroscopy, H NMR, and circular dichroism (CD) confirmed the molecular structure and chiral pattern of peptide sequences. Despite an alternating sequence of chirality previously shown to promote liquid-liquid phase separation, complexes appeared as solid precipitates, suggesting strong interactions between the sequence pairs. The secondary structures of sequence pairs showed the formation of hydrogen-bonded structures with a β-sheet signal in FTIR spectroscopy. The presence of fluorine decreased hydrogen bonding due to its inhibitory effect on π-interactions. π-interactions resulted in enhanced stability of complexes against salt, and higher critical salt concentrations for complexes with more π-containing amino acids. Furthermore, UV-vis spectroscopy showed that sequences containing π-interactions and increased charge density encapsulated a small charged molecule with π-bonds with high efficiency. These findings highlight the interplay between ionic, hydrophobic, hydrogen bonding, and π-interactions in polyelectrolyte complex formation and enhance our understanding of phase separation phenomena in protein-like structures.

Ion Clusters and Networks in Water-in-Salt Electrolytes

Water-in-salt electrolytes (WiSEs) are a class of super-concentrated electrolytes that have shown much promise in replacing organic electrolytes in lithium-ion batteries. At the extremely high salt concentrations of WiSEs, ionic association is more complicated than the simple ion pair description. In fact, large branched clusters can be present in WiSEs, and past a critical salt concentration, an infinite percolating ionic network can form spontaneously. In this work, we simplify our recently developed thermodynamic model of reversible ionic aggregation and gelation, tailoring it specifically for WiSEs. Our simplified theory only has a handful of parameters, all of which can be readily determined from simulations. Our model is able to quantitatively reproduce the populations of ionic clusters of different sizes as a function of salt concentration, the critical salt concentration for ionic gelation, and the fraction of ions incorporated into the ionic gel, as observed from molecular simulations of three different lithium-based WiSEs. The extent of ionic association and gelation greatly affects the effective ionic strength of solution, the coordination environment of active cations that is known to govern the chemistry of the solid-electrolyte interface, and the thermodynamic activity of all species in the electrolyte.

Temperature-Responsive Peptide-Nucleotide Coacervates.

Coacervates are a type of liquid–liquid phase separated (LLPS) droplets that can serve as models of membraneless organelles (MLOs) in living cells. Peptide–nucleotide coacervates have been widely used to mimic properties of ribonucleoprotein (RNP) granules, but the thermal stability and the role of base stacking is still poorly understood. Here, we report a systematic investigation of coacervates formed by five different nucleoside triphosphates (NTPs) with poly-l-lysine and poly-l-arginine as a function of temperature. All studied combinations exhibit an upper critical solution temperature (UCST), and a temperature-dependent critical salt concentration, originating from a significant nonelectrostatic contribution to the mixing free energy. Both the enthalpic and entropic parts of this nonelectrostatic interaction decrease in the order G/A/U/C/T, in accordance with nucleobase stacking free energies. Partitioning of two dyes proves that the local hydrophobicity inside the peptide–nucleotide coacervates is different for every nucleoside triphosphate. We derive a simple relation between the temperature and salt concentration at the critical point based on a mean-field model of phase separation. Finally, when different NTPs are mixed with one common oppositely charged peptide, hybrid coacervates were formed, characterized by a single intermediate UCST and critical salt concentration. NTPs with lower critical salt concentrations can remain condensed in mixed coacervates far beyond their original critical salt concentration. Our results show that NTP-based coacervates have a strong temperature sensitivity due to base stacking interactions and that mixing NTPs can significantly influence the stability of condensates and, by extension, their bioavailability.

Phase Behavior of Partially Charged Polyelectrolyte Solutions with Salt: A Theoretical Study

AbstractWeakly (or partially) charged polyelectrolytes represent an important subset of the polyelectrolyte family. For instance, polycarboxylate‐based superplasticizers, or PCEs, are comb‐shaped polyelectrolytes possessing anionic backbones grafted with neutral side chains. PCEs play an important role in modern concrete production. However, their behavior in salt solutions remain poorly understood. The present work builds upon a recently developed liquid‐state theory for fully charged polyelectrolyte solutions and extends it to describe the phase behavior and salt partitioning of PCEs in a 2:1 salt solution. Previous studies have shown that there can be an additional short‐range attraction, often referred to as the “calcium‐binding” interaction between calcium ions and the negatively‐charged carboxylate groups of PCEs. Such a calcium‐binding interaction and how its strength affects the phase behavior are investigated. It is found that increasing the calcium‐binding strength expands the phase‐separated region and increases the critical extra salt concentration, and it also leads to a wider phase‐separated region for salting‐out and salting‐in phenomena. The structural parameters of PCEs also affect the phase behavior. Increasing the side‐chain length shrinks the phase‐separated region, while increasing the acid‐to‐ether ratio expands the phase‐separated region. Those results may find applications in molecular design of weakly charged polyelectrolytes like PCEs.

Operational Limits in Processes with Water, Salt, and Short-Chain Alcohol Mixtures as Aqueous Two-Phase Systems and Problems in Its Simulation

The low critical solution temperature (LCST) and minimum salt concentration that cause phase splitting in water and short-chain alcohol mixtures were determined experimentally. The short-chain alcohols, which are completely miscible with water (methanol, ethanol, 1-propanol, 2-propanol, and tert-butanol) and most of the common salts (those with Na+, K+, Ca2+, and NH4+ cations and Cl–, SO42–, CO32–, and NO3– anions), were studied. Experimentally determined data increased future development opportunities of column sequences and process design using aqueous two-phase systems (ATPSs) by establishing alcohols, some salts, and their operational limits. An example using these systems in phase transition extraction (PTE) separation processes was proposed and discussed. Finally, the use of different thermodynamic models, such as the extended UNIQUAC for electrolytes and the electrolyte nonrandom two-liquid (NRTL) model, for equilibrium calculations and the LCSTs necessary for simulation of these extraction processes with ATPS systems were analyzed.

A nonlinear evolution of the structure, morphology, and optical properties of PbS–CdS films with cadmium nitrate in the reaction mixture

Herein, we describe the nonlinear processes for the formation of thin films of the PbS-CdS system using chemical bath deposition with a gradual change in the cadmium nitrate content in the reaction mixture. The morphology of films was studied via scanning electron microscopy and atomic force microscopy. The mechanism for the formation of thin-film compounds can be considered as cluster-particle aggregation (diffusion-limited aggregation). X-ray diffraction confirmed the formation of single-phase layers of substitutional B1-type CdxPb1-xS solid solutions (space group Fm3[combining macron]m) when the concentration of cadmium nitrate in the reaction bath increased up to [Cd(NO3)2] = 0.08 M. The maximum cadmium content in CdxPb1-xS solid solutions was determined to be x = 0.094. At the concentration of [Cd(NO3)2] = 0.10 M, a two-phase film was formed, where the film consisted of crystallites of cubic CdxPb1-xS with x = 0.071 (lower compared to the film obtained at [Cd(NO3)2] = 0.08 M) and fine-crystalline hexagonal B4-type Cd1-δS (space group P63mc). The texture of grains forming films was observed; where a predominant orientation with the (111) plane along substrate in PbS changed to the (200) plane in CdxPb1-xS films, the portion of (200) oriented grains increased with an increase in cadmium nitrate content up to [Cd(NO3)2] = 0.08 M, and at 0.10 M of cadmium nitrate, a radical change in the type of texture to the (111) type occurred. The concentration [Cd(NO3)2] = 0.10 M is called the critical concentration, where under this condition, the deposition process occurs due to the excess Gibbs energy. The higher cadmium content x in films determined by the energy-dispersive X-ray analysis and Auger spectroscopy compared with that estimated from the crystal lattice parameter is associated with the presence of an additional amorphous CdS phase formed as a sublayer distributed in intercrystalline spaces and island formations. Optical studies showed a nonlinear change in the band gap (Eg) of obtained films from 0.53 to 0.76 eV, whereas at the critical cadmium salt concentration (0.10 M), two crystalline phases with Eg equal to 0.73 and 2.47 eV were observed.

PH-Dependent complexation and polyelectrolyte chain conformation of polyzwitterion-polycation coacervates in salted water.

The phase behavior and chain conformational structure of biphasic polyzwitterion-polyelectrolyte coacervates in salted aqueous solution are investigated with a model weak cationic polyelectrolyte, poly(2-vinylpyridine) (P2VP), whose charge fraction can be effectively tuned by pH. It is observed that increasing the pH leads to the increase of the yielding volume fraction and the water content of dense coacervates formed between net neutral polybetaine and cationic P2VP in contrast to the decrease of critical salt concentration for the onset of coacervation, where the P2VP charge fraction is reduced correspondingly. Surprisingly, a single-molecule fluorescence spectroscopic study suggests that P2VP chains upon coacervation seem to adopt a swollen or an even more expanded conformational structure at higher pH. As the hydrophobicity of P2VP chains is accompanied by a reduced charge fraction by increasing the pH, a strong pH-dependent phase and conformational behaviors suggest the shift of entropic and enthalpic contribution to the underlying thermodynamic energy landscape and chain structural dynamics of polyelectrolyte coacervation involving weak polyelectrolytes in aqueous solution.

Experimental investigations of rock-fluid interactions in low salinity water injection for enhanced oil recovery in Shaly carbonate oil reservoirs

ABSTRACT Water flooding has regularly been acknowledged as a promising method to maintain reservoir pressure during oil production. The foremost challenge that comes with this approach is the increase in scale potential due to the incompatibility between the injected water and the in-place water due to their diverse physical and chemical properties. In this paper, the thermodynamic conditions for the formation damage caused by the inorganic deposits, swelling of the clay minerals, and their migration through porous media was experimentally investigated, and then the effect of these deposits on the permeability of Shaly-carbonate core samples was investigated. Additionally, the effect of salinity of the injected water on the swelling and non-swelling clay minerals were investigated, and the critical salt concentration (CSC) of the injected water was experimentally and mathematically estimated. The results indicated that the deposition of calcium sulfate ions causes the highest damage factor (up to 60%) with the lowest pore volume (P.V.) of the injected water (7 P.V.), as mostly non-removable damage. Also, the CSC of the injected water reached to the swelling clay inorganics was higher than that for the non-swelling clay minerals. Eventually, the second type of CSCs of the injected waters containing the ions Na+, Na++Ca2+, and Ca2+ were experimentally obtained as 0.5, 0.05, and 0.01 and mathematically estimated as 0.35, 0.06, and 0.01, respectively.

Influence of polycation/cation competition on the aggregation threshold of magnetic nanoparticles

Precise control of the aggregation of magnetic nanoparticles in aqueous environment is a key to their successful application in the biological and medical fields. We investigate here the critical aggregation threshold of PAA-coated maghemite nanoparticles in aqueous solution, in the presence of ionenes, polycations with a regular, tunable and pH-insensitive chain charge density, and a variety of ammonium-based salts. Aggregation of the nanoparticles is followed by a combination of dynamic light scattering and light absorbance measurements, along a dilution pathway from initially dispersed systems at high salt concentration (3 M). The position of the aggregation threshold, or critical salt concentration, is found to depend on two parameters: volume of the salt cation (Vcation) and volume of the charged monomer unit of the polycation (Vmono+, reflecting the polycation charge density). Combining these two parameters, a master curve emerges, showing that the critical salt concentration follows in a linear fashion the ratio of the above two volumes, Vcation/Vmono+. Interpreted within the theory of Collins and coworkers, the master curve is a clear demonstration of the importance of local ion-pairing in governing the stability of the system, which remains unaccounted for in the continuum electrostatic theories attempting to describe polyelectrolyte complex formation. Moreover, such a master curve provides a very simple rule for the control of interactions between charged polyelectrolytes moderated by salts.

Fine Migration Control in Sandstones: Surface Force Analysis and Application of DLVO Theory.

Formation damage caused by fine migration and straining is a well-documented phenomenon in sandstone reservoirs. Fine migration and the associated permeability decline have been observed in various experimental studies, and this phenomenon has been broadly explained by the analysis of surface forces between fines and sand grains. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory is a useful tool to help understand and model the fine release, migration, and control phenomena within porous media by quantifying the total interaction energy of the fine–brine–rock (FBR) system. Fine migration is mainly caused by changes in the attractive and repulsive surface forces, which are triggered by mud invasion during drilling activity, the utilization of completion fluid, acidizing treatment, and water injection into the reservoir during secondary and tertiary recovery operations. Increasing pH and decreasing water salinity collectively affect the attractive and repulsive forces and, at a specific value of pH, and critical salt concentration (CSC), the total interaction energy of the FBR system (VT) shifts from negative to positive, indicating the initiation of fine release. Maintaining the system pH, setting the salinity above the CSC, tuning the ionic composition of injected water, and using nanoparticles (NPs) are practical options to control fine migration. DLVO modeling elucidates the total interaction energy between fines and sand grains based on the calculation of surface forces of the system. In this context, zeta potential is an important indicator of an increase or decrease in repulsive forces. Using available data, two correlations have been developed to calculate the zeta potential for sandstone reservoirs in high- and low-salinity environments and validated with experimental values. Based on surface force analysis, the CSC is predicted by the DLVO model; it is in close agreement with the experimental value from the literature. The critical pH value is also estimated for alkaline flooding. Model results confirm that the application of NPs and the presence of divalent ions increase the attractive force and help to mitigate the fine migration problem. Hence, a new insight into the analysis of quantified surface forces is presented in current research work by the practical application of the DLVO theory to model fine migration initiation under the influence of injection water chemistry.

Synergistic effect of silica nanofluid and biosurfactant on bitumen recovery from unconventional oil

In this paper, the nanofluid (bionanofluid) was prepared by dispersing SiO2 nanoparticles (NPs) in water (biosurfactant-sophorolipid) solution. The nanofluid (bionanofluid) stability was measured through zeta potential measurement, DLS (dynamic light scattering) measurement and steady-state fluorescence measurement. The critical salt concentration (CSC) for bionanofluids was 3%. The results indicated that sophorolipid could improve the nanofluids stability in liquid, and the steady-state fluorescence measurement and zeta potential illustrated the sophorolipid adhered on the surface of the SiO2 nanoparticles, and then the steric hindrance between different nanoparticles increased significantly. It was also found that the silica nanoparticles and biosurfactant had obvious synergistic effect on bitumen recovery from unconventional oils. The nanofluids and bionanofluids viscosities increased with high nanoparticles concentration and high salinities. The bionanofluid alter the solid wettability from oil-wet to inter medium-wet, the contact angle (oil-minerals) increased from 58.5° (in deionized water) to 87.6° (bionanofluid solution). The bionanofluid could reduce the oil-water interfacial tension, and the interfacial tension was lower than the surfactant solution used only. The toluene/deionized water interfacial tension was 36.1 mN/m, when deionized water was replaced by the sophorolipid solution, the interfacial tension was 8.4 mN/m, but the interfacial tension of toluene/bionanofluid solution was decreased to 7.1 mN/m. The similar downward path of interfacial tension occurred when the oil phases were cyclohexane, n-heptane and bitumen. The oil/water interfacial tension decreased with higher surfactant concentration and nanoparticles concentration. In the end, the structural disjoining pressure was calculated to explain the synergistic effect. The structural adjoining pressure is proportional to the nanofluid velocity of enhancing oil (V C ). V C increased with the decrease diameter of nanofluids, but V C decreased with the decrease of effective nanofluids concentrations. The bionanofluids could decrease the nanofluids diameter and increase the nanofluids effective concentrations, leading to the increasing of V C ,structural adjoining pressure and the oil recovery.

The effect of CO2-enriched water salinity on enhancing oil recovery and its potential formation damage: an experimental study on shaly sandstone reservoirs

Many experimental investigations on carbonated water injection (CWI) have shown an increase in oil recovery which CWI is defined as the process of injecting CO2-saturated water in oil reservoirs as a displacing fluid. In every enhanced oil recovery method, the potential formation damage of the injected fluid is considered. This is due to the fact that the injection of incompatible fluids often causes clay swelling and fines migration and thus impairs the formation permeability. Permeability reduction by clay particles mostly depends on its distribution which can be pore lining, pore bridging, dispersed or combination of these causing pore blocking or pore-throat diameter reduction. Besides, fine migration is considered as an important mechanism of recovery improvement during injection of low-salinity water in sandstone oil reservoirs. The present paper investigates the impact of injection of carbonated water and brines with the different salt concentrations on oil recovery and formation damage focusing on permeability variation. The investigation has been done on 12 relatively homogeneous clay-containing sandstone cores, while the compositions of the injection water were varied from 40,000 to 1000 ppm, at 176° F and 2000 psi. The amount of recovery improvement and permeability drop recorded in all tests and the fine effluent of two experiments were analysed using XRD, one for CWI and one for WF (water flooding). In all salinities, CWI has shown more oil recovery improvement than conventional water. CWI of 40,000 ppm showed the minimum permeability reduction of 6 percent, while the highest permeability was obtained by injection of water with 1000 ppm. Maximum ultimate oil recoveries of 61.2% and 42% were achieved by 1000 ppm both for CWI and WF, respectively. In comparison with brine injection, CWI resulted in more permeability drop in salinity above critical salt concentration (CSC), while below CSC, WF has caused more formation damage than CWI. Experimental results also showed that fine migration was the main reason behind formation damage. It was also revealed that permeability was significantly reduced due to fine production in the effluent.

Multiphase Complex Coacervate Droplets.

Liquid–liquid phase separation plays an important role in cellular organization. Many subcellular condensed bodies are hierarchically organized into multiple coexisting domains or layers. However, our molecular understanding of the assembly and internal organization of these multicomponent droplets is still incomplete, and rules for the coexistence of condensed phases are lacking. Here, we show that the formation of hierarchically organized multiphase droplets with up to three coexisting layers is a generic phenomenon in mixtures of complex coacervates, which serve as models of charge-driven liquid–liquid phase separated systems. We present simple theoretical guidelines to explain both the hierarchical arrangement and the demixing transition in multiphase droplets using the interfacial tensions and critical salt concentration as inputs. Multiple coacervates can coexist if they differ sufficiently in macromolecular density, and we show that the associated differences in critical salt concentration can be used to predict multiphase droplet formation. We also show that the coexisting coacervates present distinct chemical environments that can concentrate guest molecules to different extents. Our findings suggest that condensate immiscibility may be a very general feature in biological systems, which could be exploited to design self-organized synthetic compartments to control biomolecular processes.

Conductance Interplay in Ion Concentration Polarization across 1D Nanochannels: Microchannel Surface Shunt and Nanochannel Conductance.

Ion concentration polarization has received much interest in the past decade for lab-on-a-chip applications, primarily preconcentration of biomolecules and water desalination. Studying the basic phenomenon in microfluidics has also generated new knowledge, which could be pivotal in the design of novel devices. Many studies, however, have focused on designs featuring nanoslits/slots or surface-patterned ion-selective membranes whereas the characteristics of 1D nanochannels are still lacking. Here, we report on ion concentration polarization across highly ordered 1D nanochannels in isolation as well as in array formation. Intriguingly, the nanochannels in isolation exhibit a linear current-voltage characteristic at low salt concentrations despite the confirmed presence of ion-depletion zone, which is associated with the diffusion-limited transport and the consequent nonlinearity in the classical sense. The characteristic in array formation breaks away from the linearity with a peculiar dip in current for a critical salt concentration in the dilute limit. We describe these findings based on the interplay between the nanochannel conductance and the conductance of the neighboring microchannel walls (the so-called surface shunt). Also, the nanochannel transport is identified with the mobility of protons more closely than that of salt cations. We attribute fast transport to phosphorus-doped silicate glass, the nanochannel material known to have very fine pores likely to be populated with protons as a result of moisture and carbon dioxide adsorption from the air. The nanochannels possess a tubular profile 70 nm in nominal diameter and fabricated through thermal reflow of doped glass on silicon without using advanced lithography.

Ion Content of Polyelectrolyte Complex Coacervates and the Donnan Equilibrium

Oppositely charged polyelectrolytes in solution spontaneously associate into hydrated complexes or coacervates, PECs. The morphology, stability, and properties of PECs depend strongly on their ion content, which moderates the “sticky” reversible interactions between Pol+ and Pol– oppositely charged repeat units. Here, it is shown that the distribution of ions between a PEC and the aqueous solution in which it is immersed is accurately predicted by the Donnan equilibrium. For ideal, stoichiometric mixing of polyelectrolytes, corresponding to an enthalpy of complexation ΔHPEC → 0, the salt, MA, concentration inside the PEC, [MA]PEC, is equal to the solution salt concentration, [MA]s. Isothermal calorimetry measurements along a Hofmeister series show that if mixing is exothermic, [MA]PEC < [MA]s, while for endothermic association of Pol+ and Pol–, [MA]PEC > [MA]s. A set of simple self-consistent expressions illustrate PEC salt response without consideration of net Coulombic or electrostatic forces between charged species. ΔHPEC exactly predicts deviations from ideal Donnan equilibria, which are connected to the equilibria between associated or intrinsic pairs of Pol+Pol– and extrinsic Pol+A– and Pol–M+ pairs, where counterions compensate polyelectrolyte charges. The equilibrium constant Kpair for Pol+Pol– pair formation is shown to be proportional to the volume charge density of the hydrated, ion-free complex. Kpair may also be used to estimate the critical salt concentration at which polyelectrolytes completely dissociate.