Non-electrostatic Interactions Research Articles

Adhesive Coacervates Driven by Hydrogen-Bonding Interaction.

Coacervation plays a critical role in numerous biological activities such as constructing biological tissues and achieving robust wet adhesion of marine sessile organisms, which conventionally occurs when oppositely charged polyelectrolytes are mixed in aqueous solutions driven by electrostatic attraction. Here, a novel type of adhesive coacervate is reported, driven by hydrogen-bonding interactions, readily formed by mixing silicotungstic acid and nonionic polyethylene glycol in water, providing a new approach for developing coacervates from nonionic systems. The as-prepared coacervate is easily paintable underwater, show strong wet adhesion to diverse substrates, and has been successfully applied as a hemostatic agent to treat organ injuries without displaying hemolytic activity, while with inherent antimicrobial properties thus avoiding inflammations and infections due to microorganism accumulation. This work demonstrates that coacervation can occur in salt-free environments via non-electrostatic interactions, providing a new platform for engineering multifunctional coacervate materials as tissue glues, wound dressings and membrane-free cell systems.

Removal mechanisms of perfluorinated compounds (PFCs) by nanofiltration: Roles of membrane-contaminant interactions

Perfluorinated compounds (PFCs)-membrane interactions play important roles in the removal of PFCs by membrane separation, especially in their adsorption onto membrane interfaces and membrane rejection. In this work, a loose nanofiltration (NF) membrane and a tight NF membrane were used to remove six PFCs that have different head groups and different C-F chains. The roles of typical PFCs-membrane interactions, including electrostatic repulsion, hydrophobic interactions and hydrogen bonding, on membrane adsorption and rejection were quantitatively evaluated. The membrane adsorption capacities to the six PFCs were determined by static adsorption experiments. Influences of the PFCs-membrane interactions mentioned above were quantified with six negatively charged PFCs by comparing the adsorption and rejection at neutral pH with those at the isoelectric points (IEPs) of the two membranes. Results showed that electrostatic repulsion caused that the rejections by the loose membrane and tight membrane increased by 2.2%–36.0%, and 0.8%–7.4%, respectively. Non-electrostatic interactions (hydrophobic interactions and hydrogen bonding) decreased rejections of the tight membrane and the loose membrane by 9.5%–23.6% and 1.2%–10.3%, respectively. Interactions between the PFCs and the membranes were further calculated by density functional theory (DFT). DFT analysis and Gibbs free energy changes suggest that PFCs could interact with membranes in thermodynamics. The head groups are away from the membrane surface because of electrostatic repulsion and the tail groups close to the membrane surface due to hydrophobic interactions and hydrogen bonding.

Effects of Non-Electrostatic Intermolecular Interactions on the Phase Behavior of pH-Sensitive Polyelectrolyte Complexes

Polyelectrolyte complexes (PECs) offer enormous material tunability and desirable functionalities, and consequently have found broad utility in biomedical and material industries. While poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) are a commonly used pairing, various aspects of the phase behavior of PAA–PAH complexes have not been sufficiently quantified. We present a comprehensive experimental study depicting the binodal phase boundaries for the PAA–PAH complexes prepared under acidic, neutral, and basic conditions using thermogravimetric analysis, turbidimetry, and optical microscopy. Under neutral and basic conditions, phase behaviors of the complexes were largely similar to one another and followed general expectations of PEC phase behavior, except for unusually high resistance to disruption of the complex with added salt. Stable complexes are observed up to 4 M NaCl concentrations. Under acidic conditions, strikingly different phase behaviors of the PAA–PAH complexes were observed. The polymer content in the complex phase increased initially, followed by an expected decrease as salt was added to the complexes. This behavior may result from a combination of associative phase separation of PAA and PAH chains, influenced by electrostatic interactions, and segregative phase separation, which can be ascribed to the influence of a combination of the hydrophobic interactions of the aliphatic polymer backbone and the interpolymer hydrogen bonding of un-ionized acrylic monomer units. Our systematic investigations over a range of pH detailing these discrepancies in the PAA–PAH phase behavior are expected to clarify the inconsistencies among the reports in the literature and provide the material design strategies for practical use of the PAA–PAH complexes and multilayer assemblies.

Effect of adjusting pH and chondroitin sulfate on the formation of curcumin-zein nanoparticles: Synthesis, characterization and morphology

This study is aiming to investigate the stabilizing effect of chondroitin sulfate (CS) on the preparation of curcumin nanoparticles (NPs). The results showed that adding CS before the anti-solvent process of zein (Z) at pH7 could fabricate most stable NPs (Cur/CS/Z-pH7) with particle size of 197 ± 5 nm and zeta-potential of −48.4 ± 1.9 mV. The pH had a significant effect on the fabrication of NPs. Cur/CS/Z-pH7 was more stable than Cur/CS/Z-pH3, while Cur/Z-CS-pH3 was more stable than the Cur/Z-CS-pH7. According to the results of XRD, FTIR, DSC and CD, CS can form irreversible macromolecular complexes with zein through non-electrostatic interactions during the anti-solvent process (Cur/CS/Z-pH7, Cur/CS/Z-pH3). However, CS was adsorbed on the surface of Zein NPs by electrostatic interaction (Cur/Z-CS-pH7, Cur/Z-CS-pH3), when CS was added after anti-solvent process of zein. These results illustrated that it is better to add CS before the anti-solvent process of zein at neutral pH.

Adsorption Behavior of Polyelectrolyte onto Alumina and Application in Ciprofloxacin Removal.

This study aims to investigate the adsorption behavior of a strong polyelectrolyte poly(styrenesulfonate) (PSS) onto alumina particles. Adsorption of PSS onto positively charged alumina surface increased with increasing ionic strength, indicating that non-electrostatic and electrostatic interaction controlled the adsorption. The removal of an emerging antibiotic ciprofloxacin (CFX) from water environment using PSS-modified alumina (PMA) was also studied. The removal of CFX using PMA was much higher than that using alumina particles without PSS modification in all pH ranges of 2–11. The removal of CFX reached 98% under the optimum conditions of pH 6, contact time of 120 min, adsorbent dosage of five milligrams per milliliter and ionic strength 104-M NaCl. The adsorption isotherms of CFX at different salt concentrations fit well with a two-step adsorption model, while the adsorption kinetic fit well with a pseudo-second-order model with a good correlation coefficient (R2 > 0.9969). The CFX-removal from a hospital wastewater using PMA was more than 75%. Our study demonstrates that adsorption of PSS onto alumina to modify the particle surface is important to form a novel adsorbent PMA for CFX-removal from water environments.

Adsorption of Poly(acrylic acid) onto Negatively Charged Polystyrene Sulfate Latex Particles by Means of Particle Tracking of Brownian Motion, Electrophoretic Mobility and Fourier Transform Infrared Spectroscopy

The adsorption of poly(acrylic acid) (PAA5K) with negative charge on the highly negatively charged polystyrene sulfate latex (PSL) particles at various ionic strengths were investigated in the present study. The adsorption of PAA was promoted with high ionic strength while the driving forces were mainly controlled by the non-electrostatic interactions including hydrogen bondings, dipole–ions and dipole–dipole. The adsorbed layer thickness of PAA on PSL and electrophoretic mobility of PSL in the presence of PAA increased with increasing ionic strength. In case of adsorption of 5 ppm PAA, the adsorbed layer thickness increased slightly about 2 times corresponding with 41.3 to 80.4 nm while the electrophoretic mobility of PSL changed from –4.87 to –6.31 μm cm/(V s) with increasing 100 times of the ionic strength from 0.1 to 10 mM KCl. The change of surface functional groups of PSL after PAA adsorption was thoroughly determined by Fourier transform infrared (FTIR) spectroscopy. The distinction between the electrostatic and the non-electrostatic interaction which contributed to the adsorption of PAA on the PSL particles and the adsorbed polymer conformations at different salt concentrations was clearly clarified. We demonstrate that the adsorbed PAA conformations were more vertically swollen at the PSL solid–solution interface.

The influence of hydrocarbon, fluorinated and silicone surfactants on the adsorption, stability and electrokinetic properties of the κ-carrageenan/alumina system

Over recent years, the use and development of surfactants and biopolymers has been increased. The interactions between oppositely charged polymers and surfactants lead to the formation of the polymer-surfactant complexes. In this work we focused on studying if different types of cationic surfactants such as hydrocarbon, fluorinated and silicone ones can interact with natural polysaccharide - κ-carrageenan. The conducted tensiometric studies have proven that carrageenan forms the complexes with all of the studied surfactants. In the following part we have studied if and how the pre-formed complexes adsorb on the alumina surface. The conducted adsorption experiments indicated that the polymer-surfactant complexes adsorb on the alumina surface via polymer part, and that the process is governed by the electrostatic and non-electrostatic interactions. The results show that the adsorption of carrageenan increased in the presence of the studied surfactants, likewise the adsorption of the surfactants increased in the presence of carrageenan. Moreover, the influence of the hydrocarbon, fluorinated and silicone surfactants on stability and the zeta potential of the carrageenan/alumina suspensions were studied. The addition of carrageenan to the alumina suspensions leads to the increased stability due to the combination of the electrosteric and depletion stabilizations, whereas the presence of surfactants influenced stability only to a small extent. The obtained results have important implications for stabilization of colloidal systems and preparation of different functionalized materials. The findings of our research are very important in the field of preparation of different colloidal dispersions such as cosmetics, pharmaceuticals, and other useful (bio-)materials.

Surface Modified Laterite Soil with an Anionic Surfactant for the Removal of a Cationic Dye (Crystal Violet) from an Aqueous Solution

The work aims to study the removal of crystal violet (CV) using laterite soil with surface modification by surfactant (SML). Surface modification of laterite soil was conducted by pre-adsorption of sodium dodecyl sulfate (SDS) at pH 4 and low ionic strength to enhance removal of CV. The effective conditions for CV removal through adsorption technique using SML were optimized and found to be contact time 60 min, pH 6, adsorbent dosage 5 mg/mL, and 5 mM NaCl as background electrolyte. The highest removal of CV using SML reached to 86.5% under optimum conditions. We used Fourier transform infrared spectroscopy (FT-IR) to evaluate the change of surface vibrational groups of laterite after SDS pre-adsorption and after CV adsorption while the different charged surface was determined by ζ potential measurements. The CV adsorption onto SML increased when increasing ionic strength from 1 to 10 mM. Nevertheless, at high ionic strength, this trend is reversal due to desorption of SDS from laterite surfaces. Adsorption isotherms of CV onto SML at different NaCl concentrations were tried to fit by Langmuir, Freundlich, and a two-step adsorption models. The adsorption kinetics were in good agreement with pseudo-second-order model. The removal efficiency of CV after four regenerations still reached higher than 85%. On the basis of adsorption isotherms, charged surface change by ζ potential and surface modification by FT-IR, we suggest that CV adsorption onto SML was induced by both non-electrostatic and electrostatic interactions. We also demonstrate that SML is a novel, reusable, and low-cost adsorbent for cationic dye removal from aqueous solution.

Interfacial structure and differential capacitance of ionic liquid/graphite interface: A perturbed-chain SAFT density functional theory study

The perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) was combined with the electrostatic free energy from the mean spherical approximation (MSA) theory, and applied to represent densities of pure imidazolium ionic liquids (ILs) with anion [BF4]−. The PC-SAFT parameters of cations were linearized with their molar mass and obtained by simultaneously fitting the model predictions to experimental densities of some ILs. The PC-SAFT-MSA model provides accurate correlations and predictions of densities comparing with experimental data. Then a classical density functional theory (DFT) was developed based on PC-SAFT-MSA. The DFT model was applied to explore the structure and differential capacitance of the electrical double layer (EDL) in ILs on graphite. The model predicts similar density profiles for both cation and anion on a neutral surface, and layered structure with alternating layers of cations and anions on a charged surface. The charge inversion phenomena were also studied based on ion distributions. We further studied the effects of the alkyl chain length, temperature and non-electrostatic solid-fluid interactions on the differential capacitance of the EDL. The model provides bell-shaped differential capacitance curves. The peak positions of differential capacitance curves shift toward positive potentials as ions size asymmetry increases in agreement with previous experiments and simulations studies. The maximum capacitance decreases with increasing alkyl chain length as well as increasing temperature.

Charge Interactions Modulate the Encounter Complex Ensemble of Two Differently Charged Disordered Protein Partners of KIX.

Disordered proteins play important roles in cell signaling and are frequently involved in protein-protein interactions. They also have a larger proportion of charged and polar residues than their folded counterparts. Here, we developed a structure-based model and applied molecular dynamics simulations to examine the presence and importance of electrostatic interactions in the binding processes of two differently charged intrinsically disordered ligands of the KIX domain of CBP. We observed non-native opposite-charged contacts in the encounter complexes for both ligands with KIX, and this may be a general feature of coupled folding and binding reactions. The ensemble of successful encounter complexes is a diverse set of structures, and in the case of the highly charged ligand, this ensemble was found to be malleable with respect to ionic strength. There are only minor differences between encounter complex ensembles for successful and unsuccessful collisions with no key interactions that appear to make the process far more productive. The energy landscape at this early stage in the process does not appear highly funneled. Strikingly we observed many native interactions that appear to reduce chances of an encounter complex being productive. Instead it appears that collectively non-native electrostatic interactions in the encounter complex increase the likelihood of productivity by holding the proteins together long enough for folding to take place. This mechanism is more effective for the more highly charged ligand.

Adsorption characteristic of ciprofloxacin antibiotic onto synthesized alpha alumina nanoparticles with surface modification by polyanion

In the present study, we reported the first adsorption characteristic of Ciprofloxacin (CFX) antibiotic onto synthesized alumina nanoparticles with surface modification by a polyanion. Strong polyanion poly(styrenesulfonate) (PSS), was used for surface modification of synthesized alumina nanoparticles. The alumina which was structural alpha phase, was confirmed by X-ray diffraction (XRD). The functional vibration groups of AlO and OH was characterized by Fourier transform infrared spectroscopy (FT-IR). The sphere morphology and particle size of about 40 nm was evaluated by Transmission electron microscopy (TEM), while the specific surface area of 6.08 m2/g was determined by Brunauer–Emmett–Teller (BET) method. Surface modification of nanoalumina with PSS enhanced a significant increase in CFX removal from aqueous solution. Some important parameters which influenced to the CFX removal including pH, contact time and adsorbent dosage, were systematically optimized. Under experimentally selected parameters, the removal efficiency and adsorption capacity of CFX using PSS-modified nanoalumina (PMNA) reached maximum of 97.8% and 34.5 mg/g, respectively. Adsorption isotherms of CFX onto PMNA were fitted well by the two-step adsorption model while adsorption kinetics of CFX followed the pseudo-second order. Based on the change in surface charge monitored by zeta potential, surface modification by FT-IR and adsorption isotherms, we suggest that CFX adsorption onto PMNA was induced by both electrostatic and non-electrostatic interactions but electrostatic attraction was dominant. The PMNA were reusable with high removal efficiency for CFX >96%. Removal efficiency for CFX in an actual hospital wastewater sample using PMNA achieved over 75%. Our results indicate that PMNA is a new and excellent adsorbent for antibiotic removal in wastewater treatment.

Effects of Surface Transition and Adsorption on Ionic Liquid Capacitors.

Room-temperature ionic liquids (RTILs) are synthetic electrolytes with electrochemical stability superior to that of conventional aqueous-based electrolytes, allowing a significantly enlarged electrochemical window for application as capacitors. In this study, we propose a variant of an existing RTIL model for solvent-free RTILs, accounting for both ion-ion correlations and nonelectrostatic interactions. Using this model, we explore the phenomenon of spontaneous surface charge separation in RTIL capacitors and find that this transition is a common feature for realistic choices of the model parameters in most RTILs. In addition, we investigate the effects of asymmetric preferential ion adsorption on this charge separation transition and find that proximity of the transition in this case can result in greatly enhanced energy storage. Our work suggests that differential chemical treatment of electrodes can be a simple and useful means for optimizing energy storage in RTIL capacitors.

A high density 3-aminopropyltriethoxysilane grafted pumice-derived silica aerogel as an efficient adsorbent for ibuprofen: Characterization and optimization of the adsorption data using response surface methodology

Abstract Pumice-derived silica aerogel (PDSA) was successfully synthesized as an efficient adsorbent for ibuprofen removal. A multilayer of highly stable and high density amine groups was successfully grafted on the PDSA surface using a nonpolar solvent and tetraethyl orthosilicate as a co-hydrolyzer, at 80 °C, under reflux. The amine grafted pumice-derived silica aerogel (AMPDSA) was utilized for efficient removal of ibuprofen. The characterization study by FTIR, XRD, FESEM, and TEM showed that the prepared AMPDSA had excellent adsorption activity and ability. The morphology of AMPDSA is characterized as amorphous phases, homogeneous, spherical particles containing silica, with a particle size of less than 25 nm and 407 m2/g specific surface areas and a well-defined pearl-like morphology. The isothermal data were correlated with a number of two-and-three parameters theoretical models. The Khan equation was the best fit among them giving a maximum adsorption capacity of 39.95 mg/g. The optimization study using the central composite design methodology by invoking the values of the analysis of variance, multiple R2, adjusted R2, and the Durbin–Watson statistic has ascertained a good fit between the adsorption data and the second order polynomial equation. This methodology predicted 100% removal of ibuprofen at conditions of contact time 150 min, 0.5 g/L of AMPDSA dose, 6.53 mg/L of initial ibuprofen concentration and pH close to 7. The results show that hydrophobic interactions, electrostatic and non-electrostatic interactions, such as hydrogen bonding control ibuprofen adsorption mechanism. Through the obtained results, the prepared AMPDSA can be used as an inexpensive and highly efficient adsorbent in the field of removal and remediation of pharmaceutical compounds.

A unified analytical theory of heteropolymers for sequence-specific phase behaviors of polyelectrolytes and polyampholytes.

The physical chemistry of liquid-liquid phase separation (LLPS) of polymer solutions bears directly on the assembly of biologically functional dropletlike bodies from proteins and nucleic acids. These biomolecular condensates include certain extracellular materials and intracellular compartments that are characterized as "membraneless organelles." Analytical theories are a valuable, computationally efficient tool for addressing general principles. LLPS of neutral homopolymers is quite well described by theory, but it has been a challenge to develop general theories for the LLPS of heteropolymers involving charge-charge interactions. Here, we present a theory that combines a random-phase-approximation treatment of polymer density fluctuations and an account of intrachain conformational heterogeneity based on renormalized Kuhn lengths to provide predictions of LLPS properties as a function of pH, salt, and charge patterning along the chain sequence. Advancing beyond more limited analytical approaches, our LLPS theory is applicable to a wide variety of charged sequences ranging from highly charged polyelectrolytes to neutral or nearly neutral polyampholytes. This theory should be useful in high-throughput screening of protein and other sequences for their LLPS propensities and can serve as a basis for more comprehensive theories that incorporate nonelectrostatic interactions. Experimental ramifications of our theory are discussed.

Protein retention in dextran-grafted cation exchange chromatography: The influence of pHs, counterions and polymer structure

Polymer-grafted ion exchange adsorbents were of great interest for the development of high-performance protein chromatography in biopharmaceutical and related fields. In this work, protein retention was systematically investigated in ion exchange chromatography packed respectively with dextran-grafted cation exchange adsorbents containing sulphopropyl (SP) ligand, SP Sepharose XL and Capto S, and non-grafted cation exchange adsorbent, SP Sepharose FF, using five proteins. With an increase of buffer pHs, retention factors of proteins decreased among all the adsorbents, demonstrating the dominant role of electrostatic interaction for protein binding on cation exchange adsorbents. The evidences further revealed that the scattered positive charges on the surface of protein molecules, rather than net charge of protein molecule, determined protein retention on cation exchange adsorbent. Likely, counterions including NH4+, K+, Na+ and Mg2+ exhibited distinct influence on protein retention. It was well ascribed to solvent-mediated indirect ion–macromolecule interactions and direct ion–macromolecule interactions. Compared with SP Sepharose FF, polymer structure in dextran-grafted cation exchange adsorbents ultimately brought about different ligand distributions and smaller pore sizes, thereby regulating protein retention in cation exchange chromatography. By comparing the retention of myoglobin and β-lactoglobulinB in SP Sepharose XL and Capto S, we reasonably speculated that the enhancement of non-electrostatic interaction caused by reducing the space arm length was a major reason for an increasing retention factor of myoglobin in Capto S. The results in this research help us understand adsorption mechanism of protein in polymer-grafted adsorbents and give scientific guidance for the development of chromatographic materials.

Electrostatically Driven Complex Coacervation and Amyloid Aggregation of Tau Are Independent Processes with Overlapping Conditions

Amyloid aggregation of the microtubule binding protein tau is a hallmark of many neurodegenerative diseases. Recently, tau has been found to undergo liquid-liquid phase separation (LLPS) by an electrostatically driven complex coacervation (CC) mechanism near physiological conditions. Although LLPS and aggregation have been shown to simultaneously occur under certain common conditions, it is unclear whether LLPS promotes aggregation of tau, or whether they are two independent processes. In this study, we address this question by combining multiple biochemical and biophysical assays in vitro. We investigate the impacts of LLPS-CC on cofactor-induced tau aggregation by evaluating the conformation of tau, kinetics of aggregation, and fibril quantity. We showed that none of these properties are influenced directly by LLPS-CC and that LLPS-CC and cofactor-induced aggregation of tau merely occur under overlapping conditions of enhanced intermolecular interactions and localization but are two independent processes. We furthermore showed that tau LLPS can be driven by nonelectrostatic interaction using high-salt concentrations. Under these conditions, LLPS strongly correlated with increased aggregation propensity. Whether LLPS of tau formed under different conditions or of different constituents may actively promote aggregation of tau remains an open question, but this study shows that the readily accessible electrostatically driven condensation of tau into LLPS in and of itself is not sufficient to promote aggregation.

How electrostatic and non-electrostatic interactions play a role in water wettability of possible nanostructure surfaces

The present paper investigated the effects of the electrostatic and non-electrostatic interactions on the water wettability of a surface. Based on this aim, examinations are performed on the molybdenum disulfide (MoS2) as possible nanostructure surface. The results obtained through calculating the water contact angle on the MoS2 surface indicate which this surface is a weak hydrophobic substrate. The present simulations illustrate that the electrostatic interactions have little impact on the wettability amount of the MoS2 substrate. However, the molybdenum disulfide is composed of two charged parts with significant values. In addition, it is observed that the relation between density, van der Waals and electrostatic interactions is different from each other. It have been observed that the van der Waals interactions have direct relationship with water density while there is not anything between the electrostatic interactions and water density peak. The Obtained results via simulation demonstrate that the effectiveness of electrostatic interactions on the wettability depends on the sigma in the Lennard–Jones equation. In the other words, the role of electrostatic and van der Waals interactions on the wettability are not indissociable from each other and this method is sufficient in nanostructure systems.Graphic abstract

Analytical strong-stretching theory of polyelectrolyte brushes loaded with charged nanoparticles

Nanoparticles can be used to tune the properties of polyelectrolyte brushes, and polyelectrolyte brushes can be used to control the interaction between nanoparticles and substrates. In the present paper, we investigate the polyelectrolyte brushes immersed in a nanoparticle solution within the analytical strong-stretching theoretical framework. The theoretical model does not take the excluded volume interaction between any two components into account. When there is no nanoparticle loaded, the polyelectrolyte brush is assumed to be an osmotic brush. Local electroneutral approximation is assumed to be still valid after the nanoparticles have been loaded. The loaded nanoparticles are not big enough to deform the grafted polyelectrolyte chains laterally. Analytical formulae for density profiles of each component and brush thickness are derived. The loaded nanoparticles always compress the polyelectrolyte brush. By analyzing the limiting case, a scaling-type diagram for behaviors of the nanoparticle-loading polyelectrolyte brush is constructed. Two characteristic nanoparticle controlling regimes are shown. When the charge of the nanoparticle, <i>Z</i>, is not very large, charged nanoparticles penetrate into the brush and the brush thickness is scaled by <inline-formula><tex-math id="M5">\begin{document}$H \sim (Z\varPhi)^{-1/3}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200432_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200432_M5.png"/></alternatives></inline-formula>, where <inline-formula><tex-math id="M6">\begin{document}$\varPhi$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200432_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200432_M6.png"/></alternatives></inline-formula> is the nanoparticle volume fraction. When the nanoparticle charge <i>Z</i> is large enough, nanoparticles are mainly distributed outside the brush and the brush thickness is scaled by <inline-formula><tex-math id="M7">\begin{document}$H \sim (Z\varPhi)^{-1}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200432_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200432_M7.png"/></alternatives></inline-formula>. In the former case, the Coulombic repulsion between the grafted polyelectrolyte chains is screened by the counterions and the nanoparticles, and the brush behavior is determined by the balance between the chain elasticity and the osmotic pressure of the counterions and the nanoparticles. In the latter case, the electrostatic screening is executed by the counterions, and the chain elasticity is balanced by the osmotic pressure of the counterions. The two regimes are divided into subregimes which are dominated respectively by electrostatic or non-electrostatic interaction. The effects of size polydispersity of the nanoparticles are also investigated. It is found that the behaviors of the grafted polyelectrolyte chains are mainly determined by the ratio between the first two moments of the nanoparticle size distribution function. The polyelectrolyte brush is compressed more by the polydispere nanoparticles than by the monodisperse ones. Possible improvement in the present theory is discussed in the conclusion section.

How charge regulation and ion-surface affinity affect the differential capacitance of an electrical double layer.

The differential capacitance of an electrical double layer is a topic of great importance to develop more efficient and environment-friendly energy storage devices: electric double layer supercapacitors. In addition to the bare electrostatic interactions, recent experimental and computational studies suggest that electrodes covered by ionizable groups do interact selectively with specific ion types, an effect that can increase the maximal conductivity and voltage of a supercapacitor. Inspired by this, in the present work we investigate how ion-specific non-electrostatic interactions modify the differential capacitance of a flat electrode whose surface is covered by ionizable groups subject to a charge regulation process. The incorporation of hydration interactions by means of ion-specific Yukawa potential into the Poisson-Boltzmann theory allows our model to describe different scenarios of ion-surface affinity and, hence, the selective depletion or accumulation of specific ion types close to a charged surface. We obtained larger capacitance values when considering electrodes that favor the accumulation of cations and the depletion of anions.

Chemical physics of whey protein isolate in the presence of mucin: From macromolecular interactions to functionality

Oral processing, textural perception and functionality of colloidal foods are strongly influenced by the interactions between the salivary mucins and the food proteins. This work studies the physico-chemical aspects of mixtures of a typical food protein, whey protein isolate (WPI) and mucin. Phase separations result from aggregation between the two components at pH 7 and at pH 3. ζ-potential and fluorimetry data show that electrostatics contribute to entropically-driven interactions at pH 3, while at pH 7, two different non-electrostatic interactions, an entropically-driven and an enthalpically-driven one lead to aggregation and phase separation. Substitution of WPI with increasing mucin concentrations at pH 7 results in a marked increase of the shear viscosity in comparison with pH 3. Mucin enhances the extensional viscosity in a similar fashion, e.g. the incorporation of mucin into a WPI system at 6:4 ratio increases the extensional viscosity ≥ 3-fold (0.27˗0.85 Pa s) and ≥2-fold (0.38˗0.89 Pa s) at pH 3 and pH 7, respectively. These results indicate a notable increase of the extensional over shear viscosity ratio (Trouton's ratio). The above highlight the effect of the molecular-level interactions between food and salivary macromolecules on phase behavior and flow during oral processing.