Hydration of Cations: A Key to Understanding of Specific Cation Effects on Aggregation Behaviors of PEO-PPO-PEO Triblock Copolymers

Effect of salt addition on a triblock copolymer-zwitterionic surfactant assembly: insight from excited-state proton transfer

Copolymer-surfactant assemblies are frequently utilized across various fields, from medicine to nanotechnology. Understanding the organization of the mixed assemblies in a saline environment will further expand their application horizons, especially under physiological conditions. Excited-state proton transfer (ESPT) can provide insight into the hydration nature and organization of the non-toxic assembly of a triblock copolymer F127 (poly-(ethylene oxide)101 (PEO101)-poly(propylene oxide)56 (PPO56)-PEO101)) and a zwitterionic sulfobetaine surfactant N-dodecyl-N,N-dimethyl-3-ammoniopropane sulfonate (SB12). Here, we present a comprehensive investigation of the compactness and hydration nature of the F127-SB12 mixed assemblies at different salt concentrations using the ESPT of 8-hydroxy pyrene-1,3,6-trisulfonate (HPTS). In the absence of salts, gradual SB12 addition to a premicellar (0.4 mM) or a post-micellar (4 mM) F127 solution leads to an anomalous modulation of the protonated and deprotonated emission bands. The emission intensity ratio (protonated/deprotonated) first increases to a maximum at a particular SB12 concentration (6 mM and 35 mM for the premicellar and post-micellar F127 assemblies, respectively), and then the ratio decreases with a further increase in the surfactant concentration. Since the intensity ratio is an indicator of the retardation of the ESPT process, the mixed micellar configuration displaying a maximum intensity ratio represents the most compact and least hydrated state. Salt addition to this configuration lowers the intensity ratio, signifying an enhanced ESPT process. Dynamic light scattering (DLS) results indicate that the size of the mixed assembly remains almost unaltered with the addition of salts. Thus, salinity enhances the ESPT process inside the F127-SB12 mixed assemblies without significantly altering the hydrodynamic radius.

Conformation and Metal Cation Binding of Zwitterionic Alanine Tripeptide in Saline Solutions by Infrared Vibrational Spectroscopy and Molecular Dynamics Simulations

In this work, linear infrared (IR) spectroscopy and molecular dynamics (MD) simulations were used to examine the interaction of different metal cations (Na⁺, Ca²⁺, Mg²⁺, and Zn²⁺) with backbone (amide C═O) and C-terminal carboxylate (COO^-) groups in zwitterionic alanine tripeptide (Ala3) in aqueous solutions with varying saline concentrations. Circular dichroism spectra and MD results suggest that Ala3 is predominantly in polyproline-II (PP_II) conformation, whose amide-I and asymmetric carboxylate stretching IR vibration signatures are also supported by quantum-chemistry calculations. The zwitterionic form of Ala3 separates the two amide-I modes in frequency, which are weakly coupled modes, as revealed by two-dimensional IR measurement, and can be used to probe backbone-cation interactions at different scenarios (near charged or neutral chemical groups respectively). Cation concentration-dependent IR frequency red shifts in the amide-I mode are seen for both amide-I modes, whereas blue shifts are also seen in the amide-I mode far from the NH₃⁺ group. The observed spectral changes are discussed from the perspective of the salting-in and salting-out abilities of the cations. In addition, all the metal cations studied here (except Zn²⁺) can specifically coordinate to the COO^- group in bidentate and pseudo-bridging forms simultaneously. For Zn²⁺, only the pseudo-bridging form exists. Our results shed light on the macroscopic protein salting-in and salting-out phenomena from the perspective of key chemical bonds in peptides.

Fusion-Induced Growth of Biomimetic Polymersomes: Behavior of Poly(dimethylsiloxane)-Poly(ethylene oxide) Vesicles in Saline Solutions Under High Agitation

Giant unilamellar vesicles serve as membrane models and primitive mockups of natural cells. With respect to the latter use, amphiphilic polymers can be used to replace phospholipids in order to introduce certain favorable properties, ultimately allowing for the creation of truly synthetic cells. These new properties also enable the employment of new preparation procedures that are incompatible with the natural amphiphiles. Whereas the growth of lipid compartments to micrometer dimensions has been well established, growth of their synthetic analogs remains underexplored. Here, the influence of experimental parameters like salt type/concentration and magnitude of agitation on the fusion of nanometer-sized vesicles made of poly(dimethylsiloxane)-poly(ethylene oxide) graft copolymer (PDMS-g-PEO) is investigated in detail. To this end, dynamic light scattering, microscopy, and membrane mixing assays are employed, and the process at different time and length scales is analyzed. This optimized method is used as an easy tool to obtain giant vesicles, equipped with membrane and cytosolic biomachinery, in the presence of salts at physiological concentrations.

Bioactive glass/poloxamer 407 hydrogel composite as a drug delivery system: The interplay between glass dissolution and drug release kinetics

Since patients suffer pain in the post-surgery of bone repair interventions, bioactive glass/hydrogel drug delivery systems containing local anesthetics, such as ropivacaine, could improve patient life quality by combining bone regeneration with anesthetics. However, poloxamer-based hydrogel properties are sensitive to ions, temperature, and water contents and could be structurally influenced by the ionic dissolution products from bioactive glasses of different compositions. Therefore, this study evaluated the interplay between bioactive glass dissolution kinetics and poloxamer 407 properties, establishing a correlation between changes in the hydrogel and drug release kinetics. Three glass compositions were produced, yielding Ca-rich, Na-rich, and an intermediate glass composition. The influence of Ca/Na ratios on the glass structure and dissolution was investigated. Further, the glasses and ropivacaine were incorporated in the poloxamer hydrogel, and the self-assembly ability of poloxamer, the degradation rate, and the drug release kinetics of the composites were evaluated. The results suggested that glass connectivity affected the early-stage of glass dissolution, while sodium mobility influenced the long-term. The dissolution products from the glasses interact with the supramolecular structure of the poloxamer, causing structural changes responsible for hydrogel degradation. Consequently, by changing the Ca/Na ratio in the glasses, it is possible to modulate glass dissolution that, in turn, influences the ropivacaine release. Thus, we propose that the Ca/Na ratio in quaternary bioactive glasses can be used to modulate drug-delivery properties from systems based on bioactive glasses and poloxamer 407.

Supersoft Elastic Bottlebrush Microspheres with Stimuli-Responsive Color-Changing Properties in Brine

Solvent-free supersoft elastomer is highly desirable for building photonic structures with significant stimuli-responsive color changes. We report supersoft elastic porous microspheres with vivid structural colors obtained via self-assembly of amphiphilic bottlebrush block copolymers at the water/oil interface templated by ordered water-in-oil-in-water double emulsions. The porous structure is composed of cross-linked bottlebrush polydimethylsiloxane (PDMS) as the supersoft elastic skeleton and bottlebrush poly(ethylene oxide) (PEO) as the internal responsive layer. The obtained microspheres show large reversible volume changes through well-controlled dehydration or hydration of PEO in response to salt ions in an aqueous environment. As a result, full-spectrum colors are obtained dependent on different salt concentrations. In-situ observation of color reflection of a microsphere indicates a gradual structural transition from the outside to the inside corresponding to migration of water molecules and salt ions. Moreover, rod-like bottlebrush PEO exhibits an anion-induced salting-out behavior different from that of random coil polymers. The significantly responsive behaviors of bottlebrush block copolymer (BBCP) assemblies in the presence of salt ions primarily rely on the supersoft elastic skeleton of the porous structure, providing a facile route to the creation of stimuli-responsive photonic materials by low-cost self-assembly methods.

Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

Ion-Specific Effects of Divalent Ions on the Structure of Polyelectrolyte Brushes

Polyelectrolyte brushes consist of charged polymer chains attached on one end to a surface at high densities. They are relevant for many practical applications ranging from biosensors to drug delivery to colloidal stability. Their structure and functionality can be dramatically influenced by multivalent counterions in the solution environment. In this work, the surface forces apparatus (SFA) and atomic force microscopy (AFM) were used to investigate the effects of three alkaline earth divalent cations, Mg²⁺, Ca²⁺, and Ba²⁺, on the structures of polystyrenesulfonate (PSS) brushes tethered to mica and silicon oxide surfaces. While all these ions caused significant shrinkage of the height of the PSS brushes, strong ion-specific effects were observed. Mg²⁺ and Ca²⁺ caused homogeneous shrinkage; Ba²⁺ led to pinned-micelle like inhomogeneous structures. Isothermal titration calorimetry (ITC) demonstrated that this ion specificity was mainly caused by the difference in binding energy between sulfonate groups and the divalent cations. Considering the abundance of divalent cations in industrial processes, natural environments, and biological systems, the understanding of strong ion-specific effects of divalent counterions is of great importance for theoretical studies and various applications involving polyelectrolytes.

Modular Fabrication of Intelligent Material-Tissue Interfaces for Bioinspired and Biomimetic Devices

One of the goals of biomaterials science is to reverse engineer aspects of human and nonhuman physiology. Similar to the body's regulatory mechanisms, such devices must transduce changes in the physiological environment or the presence of an external stimulus into a detectable or therapeutic response. This review is a comprehensive evaluation and critical analysis of the design and fabrication of environmentally responsive cell-material constructs for bioinspired machinery and biomimetic devices. In a bottom-up analysis, we begin by reviewing fundamental principles that explain materials' responses to chemical gradients, biomarkers, electromagnetic fields, light, and temperature. Strategies for fabricating highly ordered assemblies of material components at the nano to macro-scales via directed assembly, lithography, 3D printing and 4D printing are also presented. We conclude with an account of contemporary material-tissue interfaces within bioinspired and biomimetic devices for peptide delivery, cancer theranostics, biomonitoring, neuroprosthetics, soft robotics, and biological machines.

Efficacy of several additives to modulate the phase behavior of biomedical polymers: A comprehensive and comparative outlook

Several new classes of polymeric materials are being introduced with unique properties. Thermoresponsive polymers (TRPs) are one of the most fascinating and emerging class of biomaterials in biomedical research. The design of TRPs with good response to temperature and its ability to exhibit coil to globular transition behavior near to physiological temperature made them more promising materials in the field of biomaterials and biomedicines. Instead of numerous studies on TRPs, the mechanistic interplay among several additives and TRPs is still not understood clearly and completely. The lack of complete understanding of biomolecular interactions of various additives with TRPs is limiting their applications in interdisciplinary science as well as pharmaceutical industry. There is a great need to provide a collective and comprehensive information of various additives and their behavior on widely accepted biopolymers, TRPs such as poly(N-isopropylacrylamide) (PNIPAM), poly(vinyl methyl ether) (PVME), poly(N-vinylcaprolactum) (PVCL) and poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEG-PPG-PEG) in aqueous solution. Obviously, as the literature on the influence of various additives on TRPs is very vast, therefore we focus our review only on these four selected TRPs. Additives such as polyols, methylamines, surfactants and denaturants basically made the significant changes in water structure associated to polymer via their entropy variation which is the direct influence of their directly or indirectly binding abilities. Eventually, this review addresses a brief overview of the most recent literature of applications based phase behavior of four selected TRPs in response to external stimuli. The work enhances the knowledge for use of TRPs in the advanced development of drug delivery system and in many more pharmaceutical applications. These kinds of studies provide powerful impact in exploring the utility range of polymeric materials in various field of science.

Structure and rheology of liquid crystal hydroglass formed in aqueous nanocrystalline cellulose suspensions

HYPOTHESIS

Liquid crystal hydroglass (LCH) is a biphasic soft material with flow programmable anisotropy that forms via phase separation in suspensions of charged colloidal rods upon increases in ionic strength. The unique structure and rheology of the LCH gel formed using nanocrystalline cellulose (NCC) is hypothesised to be dependent on colloidal stability that is modulated using specific ion effects arising from Hofmeister phenomena.

EXPERIMENTS

LCHs are prepared in NCC suspensions in aqueous media containing varying levels of sodium chloride (NaCl) or sodium thiocyanate (NaSCN). The NCC suspensions are characterised using rheology and structural analysis techniques that includes polarised optical microscopy, zeta potential, dynamic light scattering and small-angle X-ray scattering.

FINDINGS

The two salts have a profound effect on the formation process and structure of the LCH. Differences in network density and size of the liquid crystal domains are observed within the LCH for each of the salts, which is associated with the strength of interaction between NCC particles during LCH formation. In comparison to Cl^- at the same salinity, the chaotropic nature of the weakly hydrated SCN^- enhances colloidal stability by rendering NCC particles more hydrated and repulsive, but this also leads to weaker gel strength of the LCH. The results suggest that salts are a means in which to control the formation, structure and rheology of these anisotropic soft materials.

Anion Specificity in Dimethyl Sulfoxide-Water Mixtures Exemplified by a Thermosensitive Polymer

In the present work, we have investigated the anion-specific upper critical solution temperature (UCST) behavior of polymer-supported borinic acid (PBA) in dimethyl sulfoxide-water (DMSO-H₂O) mixtures. An inverted V-shaped series CH₃COO^- < Cl^- < salt-free > NO₃^- > ClO₄^- > SCN^- is observed in terms of the anion-specific UCST of PBA in the DMSO-H₂O mixtures. Both direct anion-polymer interactions and indirect solvent-mediated anion-polymer interactions are involved in the specific anion effect on the UCST behavior of PBA. The direct binding of anions to the PBA surface generates a salting-in effect on PBA, causing the UCST for the different types of anions to increase from chaotropic to kosmotropic anions due to the stronger binding of the more chaotropic anions. On the other hand, the indirect anionic polarization of hydrogen bonding between PBA and DMSO molecules also produces a salting-in effect on PBA, leading the UCST for the different types of anions to increase from kosmotropic to chaotropic anions because of the stronger capability of the more kosmotropic anions to polarize the hydrogen bonding. Thus, the dominating anion-PBA interactions change from the direct anion binding to the indirect anionic polarization of hydrogen bonding as the anions change from chaotropes to kosmotropes. The observed inverted V-shaped series suggests that the specific anion effect on the UCST behavior of PBA in the DMSO-H₂O mixtures is determined by the combined effects of the binding of anions to the PBA surface and the anionic polarization of hydrogen bonding between PBA and DMSO molecules.

Influence of additives on thermoresponsive polymers in aqueous media: a case study of poly(N-isopropylacrylamide)

Thermoresponsive polymers (TRPs) in different solvent media have been studied over a long period and are important from both scientific and technical points of view.

Strong Stretching of Poly(ethylene glycol) Brushes Mediated by Ionic Liquid Solvation

We have measured forces between mica surfaces coated with a poly(ethylene glycol) (PEG) brush solvated by a vacuum-dry ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide, with a surface forces apparatus. At high grafting density, the solvation mediated by the ionic liquid causes the brush to stretch twice as much as in water. Modeling of the steric repulsion indicates that PEG behaves as a polyelectrolyte; the hydrogen bonding between ethylene glycol and the imidazolium cation seems to effectively charge the polymer brush, which justifies the strong stretching. Importantly, under strong polymer compression, solvation layers are squeezed out at a higher rate than for the neat ionic liquid. We propose that the thermal fluctuations of the PEG chains, larger in the brush than in the mushroom configuration, maintain the fluidity of the ionic liquid under strong compression, in contrast to the solid-like squeezing-out behavior of the neat ionic liquid. This is the first experimental study of the behavior of a polymer brush solvated by an ionic liquid under nanoconfinement.

Ion specificities of artificial macromolecules

Artificial macromolecules are well-defined synthetic polymers, with a relatively simple structure as compared to naturally occurring macromolecules. This review focuses on the ion specificities of artifical macromolecules. Ion specificities are influenced by solvent-mediated indirect ion-macromolecule interactions and also by direct ion-macromolecule interactions. In aqueous solutions, the role of water-mediated indirect ion-macromolecule interactions will be discussed. The addition of organic solvents to aqueous solutions significantly changes the ion specificities due to the formation of water-organic solvent complexes. For direct ion-macromolecule interactions, we will discuss specific ion-pairing interactions for charged macromolecules and specific ion-neutral site interactions for uncharged macromolecules. When the medium conditions change from dilute solutions to crowded environments, the ion specificities can be modified by either the volume exclusion effect, the variation of dielectric constant, or the interactions between ions, macromolecules, and crowding agents.

Effect of ionic strength on intra-protein electron transfer reactions: The case study of charge recombination within the bacterial reaction center

It is a common believe that intra-protein electron transfer (ET) involving reactants and products that are overall electroneutral are not influenced by the ions of the surrounding solution. The results presented here show an electrostatic coupling between the ionic atmosphere surrounding a membrane protein (the reaction center (RC) from the photosynthetic bacterium Rhodobacter sphaeroides) and two very different intra-protein ET processes taking place within it. Specifically we have studied the effect of salt concentration on: i) the kinetics of the charge recombination between the reduced primary quinone acceptor QA(-) and the primary photoxidized donor P(+); ii) the thermodynamic equilibrium (QA(-)↔QB(-)) for the ET between QA(-) and the secondary quinone acceptor QB. A distinctive point of this investigation is that reactants and products are overall electroneutral. The protein electrostatics has been described adopting the lowest level of complexity sufficient to grasp the experimental phenomenology and the impact of salt on the relative free energy level of reactants and products has been evaluated according to suitable thermodynamic cycles. The ionic strength effect was found to be independent on the ion nature for P(+)QA(-) charge recombination where the leading electrostatic term was the dipole moment. In the case of the QA(-)↔QB(-) equilibrium, the relative stability of QA(-) and QB(-) was found to depend on the salt concentration in a fashion that is different for chaotropic and kosmotropic ions. In such a case both dipole moment and quadrupole moments of the RC must be considered.

Cation-sensitive compartmentalization in metallacarborane containing polymer nanoparticles

The inner structure of hybrid nanoparticles based on metallacarborane complexation with diblock copolymer PEO–POX is sensitive to alkaline cations.

Anion-dipole interactions make the homopolymers self-assemble into multiple nanostructures

Anion-dipole interactions can make homopolymers self-assemble like an amphiphilic block copolymer. Generally, common homopolymers cannot self-assemble into multiple nanostructures. Here, it is reported that anion-dipole interactions can enable a number of homopolymers to achieve a variety of self-assembly behaviors in aqueous solution. Such interactions and self-assembly features have been exclusively reserved for amphiphilic (block) polymers until now.

Influence of Ag+ interaction on 1D droplet array spacing and the repulsive forces between stimuli-responsive nanoemulsion droplets

This paper reports results on the effect of interaction of Ag(+) on 1D droplet array spacing and the repulsive forces between stimuli-responsive nanoemulsion droplets, stabilized with an anionic surfactant--sodium dodecyl sulfate--and a diblock polymer--poly(vinyl alcohol)-vinyl acetate. The repulsive interaction is probed by measuring the in-situ equilibrium force-distance in the presence of Ag(+) using the magnetic chaining technique. At a constant static magnetic field, emulsion droplets form 1D array that diffract visible light. A large blue-shift in the diffracted light is observed in the presence of interacting Ag(+) because of the reduction in the interdroplet spacing within the 1D array. The in-situ equilibrium force-distance measurement results show that the onset of repulsions and magnitude of repulsive forces are strongly influenced by the presence of Ag(+) in ppb levels. This suggests that the Ag(+) ions screen the surface charges through the formation of both Stern and diffuse electric double layer and produces a dramatic blue-shift in surfactant-stabilized emulsion, whereas a dramatic conformational change in the adsorbed polymer layer causes a reduction in the 1D array spacing in the diblock polymer stabilized emulsion. The force-distance results are compared with the predictions of electrical double-layer and repulsive steric forces. The droplet array shows an excellent selectivity to Ag(+) due to the strong interaction of Ag(+) with the stabilizing moieties at the oil-water interface. The possible mechanisms of interaction of Ag(+) with surfactant and polymer are discussed. The dramatic decrease in the 1D array spacing in the presence of Ag(+) may find promising practical applications in the development of optical sensors for selective detection of cations with ultrahigh sensitivity.

Specific cation effects on hemoglobin aggregation below and at physiological salt concentration

Turbidity titrations are used to study the ion specific aggregation of hemoglobin (Hb) below and physiological salt concentration in the pH range 4.5-9.5. At a salt concentration 50 mM cations promote Hb aggregation according to the order Rb(+) > K(+) ~ Na(+) > Cs(+) > Li(+). The cation series changes if concentration is increased, becoming K(+) > Rb(+) > Na(+) > Li(+) > Cs(+) at 150 mM. We interpret the puzzling series by assuming that the kosmotropic Li(+) will bind to kosmotropic carboxylates groups-according to the law of matching water affinities (LMWA)-whereas the chaotropic Cs(+) will bind to uncharged protein patches due to its high polarizability. In fact, both mechanisms can be rationalized by invoking previously neglected ionic nonelectrostatic forces. This explains both adsorption to uncharged patches and the LMWA as a consequence of the simultaneous action of electrostatic and dispersion forces. The same interpretation applies to anions (with chaotropic anions binding to chaotropic amine groups). The implications extend beyond hemoglobin to other, still unexplained, ion specific effects in biological systems.