Evolution of Interfacial Hydration Structure Induced by Ion Condensation and Correlation Effects

The human body consists of many different cell types, which all exhibit a cytoskeleton. This cytoskeleton is mainly composed of three different types of protein filaments, as well as cross-linkers and motor proteins. The correct interplay of the protein system is necessary for the proper function of the cytoskeleton. To fully understand the physical and chemical properties it is not only important to understand the function of each protein individually, but also the assembly process needs to be understood, as proteins can miss-fold which sometimes leads to aggregates which are not fully functional anymore. Intermediate filaments (IFs) are one of the three cytoskeletal filaments, which in contrast to the other two, have a hierarchical assembly pathway. In this thesis, the influence of ions on the assembly of vimentin, an IF often found in cells of mesenchymal origin, is investigated. By combining small angle x-ray scattering (SAXS) with microscopy (atomic force microscopy and fluorescence microscopy), information about the cross-section of the filament as well as the overall structure, e.g. single filaments or networks, can be obtained. Before the influence of ions with different valencies could be investigated, a new buffer system was established, which does not complex with the ions used in this study. By adding ions of different valencies to vimentin, it could be shown that the presence of monovalent ions leads to single filaments with a radius of roughly 6 nm, whereas multivalent ions (di-, tri-, and tetravalent) induce network formation and the filaments reach radii of roughly 10 nm before collapsing into very dense and insoluble networks. In addition to static experiments, the assembly process in flow is investigated by combining SAXS with microfluidics. In a first step, a reliable x-ray compatible microfluidic device solely made out of one material is established and characterized using gold colloids. In a second step the very early time points of vimentin assembly in the presence of potassium is observed. The results of this thesis helped to get a better understanding of the influence of ions on the assembly of vimentin IFs. It could be shown that there is a large difference in the assembly of vimentin in the presence of monovalent and multivalent ions. We could furthermore introduce a new x-ray compatible microfluidic device and monitor the first few ms of the assembly process.

Interactions involving charged particles in the presence of multivalent ions are relevant in wide-range of phenomena, including condensation of nucleic acids, cement hardening, or water treatment. Here, we study such interactions by combining direct force measurements with atomic force microscopy (AFM) and aggregation studies with time-resolved light scattering for particles originating from the same colloidal suspension for the first time. Classical DLVO theory is found to be only applicable for monovalent and divalent ions. For ions of higher valence, charge inversion and additional non-DLVO attractive forces are observed. These attractive forces can be attributed to surface charge heterogeneities, which leads to stability ratios that are calculated from direct force measurements to be higher than the experimental ones. Ion-ion correlations are equally important as they induce the charge inversion in the presence of trivalent or tetravalent ions, and they enhance the surface charge heterogeneities. Such heterogeneities therefore play an essential role in controlling interactions in particle suspensions containing multivalent ions.

The rational design of healthier foods to control fat uptake in the diet is an emerging area of research. In order to address these dietary issues effectively one requires an improved understanding of the influence of food structure on the digestion of lipids. Despite the crucial roles played by interfaces in determining the stability of emulsions and the breakdown of emulsion structure on digestion, there are few systematic basic studies dealing directly with the effects of digestion conditions on interfacial structures. A major concern in the attempted manipulation of protein-based interfacial structures is the potential enzymatic breakdown of these structures during digestion. This study has been designed to investigate the effects of the proteolytic enzyme pepsin on β-lactoglobulin surface layers under in vitro gastric digestion. Interfacial dilatational rheology has been used to measure the evolution of the viscoelastic properties of the pre-formed interfacial protein networks. Furthermore, Atomic Force Microscopy (AFM) has been used to visualise directly the effects of degradation on model interfacial protein layers (β-lactoglobulin). The results obtained show that pepsin partially hydrolyses the surface-adsorbed β-lactoglobulin molecules under gastric conditions, but does not break the interconnected surface network. In order to investigate the influence of the presence of surfactants on the enzymatic digestion of emulsions, the effect of addition of the model surfactant Tween 20 on the pepsin-induced changes in interfacial structure under simulated gastric conditions was studied. Modification of the surface conformation of the proteins by surfactant, during ‘orogenic’ displacement, revealed an unexpected synergism which leads to enhanced hydrolysis of the proteins; hence, weakening the network and allowing rupture to occur at lower surface pressures. Such observations are important new generic features of the digestion process which could be potentially manipulated to control the enzyme hydrolysis, allowing rational design of food structures and manipulation of properties such as fat digestion.

Strong hydration repulsion exists between two negatively charged surfaces in the alkali metal salt solutions, together with the fluid response to the shear of hydration layers, leading to superlubricity. However, whether the multivalent ions can obtain superlubricity has not been revealed yet. Here, we evaluate the lubrication and adsorption properties of multivalent ions at different concentrations between Si3N4 and sapphire surfaces. The divalent and trivalent ions exhibit extremely low friction coefficients of 0.005-0.006 and 0.002-0.004, respectively, under contact pressures above 0.25 GPa, and three trivalent ions can achieve superlubricity at quite low sliding speeds (3.1 mm/s), which is a significant breakthrough for superlubricity under boundary lubrication. Moreover, compared with monovalent ions, divalent ions can reduce surface potential and lower surface charge density even further, and trivalent ions can neutralize the negatively charged ceramic surfaces and even lead to charge inversion due to excess adsorption of the cations, which ensures strong adsorption of hydrated multivalent ions on friction surfaces.

Metal ions play critical roles in RNA structure and function. However, web servers and software packages for predicting ion effects in RNA structures are notably scarce. Furthermore, the existing web servers and software packages mainly neglect ion correlation and fluctuation effects, which are potentially important for RNAs. We here report a new web server, the MCTBI server (http://rna.physics.missouri.edu/MCTBI), for the prediction of ion effects for RNA structures. This server is based on the recently developed MCTBI, a model that can account for ion correlation and fluctuation effects for nucleic acid structures and can provide improved predictions for the effects of metal ions, especially for multivalent ions such as Mg2+ effects, as shown by extensive theory-experiment test results. The MCTBI web server predicts metal ion binding fractions, the most probable bound ion distribution, the electrostatic free energy of the system, and the free energy components. The results provide mechanistic insights into the role of metal ions in RNA structure formation and folding stability, which is important for understanding RNA functions and the rational design of RNA structures.

At biological interfaces, flexible surface structures and mobile water interact with each other to present non-uniform three-dimensional (3D) distributions. In spite of their impact on biological functions, molecular-scale understanding of such phenomena has remained elusive. Here we show direct visualization of such interfacial structures with subnanometer-scale resolution by 3D scanning force microscopy (3D-SFM). We measured a 3D force distribution at an interface between a model biological membrane and buffer solution by scanning a sharp tip within the 3D interfacial space. We found that vertical cross sections of the 3D image taken along a specific lateral direction show characteristic molecular-scale contrasts tilted at 30° to the membrane surface. Detailed analysis of the 3D image reveals that the tilted contrast corresponds to the time-averaged conformation of fluctuating lipid headgroups. On the basis of the obtained results, we discuss the relationships among the hydration structure, headgroup fluctuation, molecular fluidity, and mechanical strength of the membrane. The results demonstrate that 3D-SFM is capable of visualizing averaged 3D distribution of fluctuating surface structures as well as that of mobile water (i.e., hydration structure) at interfaces between biological systems and water.

Understanding the pH-responsive behavior of graphene oxide membrane in removing ions and organic micropollulants

Numerical analysis of the viscosity of ZrF4–BeF2 molten salts for nuclear transmutation using fusion neutron sources

Revealing promotion of interfacial Pt–O–Fe oxygen bridge structure in PtFe/graphene synthesized by a four-electrode electrochemical system on water splitting

We have directly observed reversal of the polarity of charged surfaces in water upon the addition of trivalent and quadrivalent ions using atomic force microscopy. The bulk concentration of multivalent ions at which charge inversion reversibly occurs depends only very weakly on the chemical composition, surface structure, size, and lipophilicity of the ions, but is very sensitive to their valence. These results support the theoretical proposal that spatial correlations between ions are the driving mechanism behind charge inversion.

Forces between positively and negatively charged colloidal particles across aqueous salt solutions containing multivalent ions are measured directly with the atomic force microscope (AFM). The measurements are interpreted quantitatively with Poisson-Boltzmann (PB) theory. Thereby, the surface potentials and regulation properties of the particle surfaces are extracted from symmetric measurements between the same types of particles. This information is used to predict force profiles in the asymmetric situations involving different types of particles without any adjustable parameters. These predictions turn out to be very accurate, which demonstrates that the mean-field PB theory is reliable down to distances of about 5 nm. While various reports in the literature indicate that this theory should fail due to neglect of ion correlations, such effects seem important only at higher concentrations and smaller distances.

We show how the colloidal-probe technique, which is based on force measurements made with the atomic force microscope, can be used to accurately determine the charging parameters of water-solid interfaces. Besides yielding accurate values of the double-layer or diffuse-layer potential, the method also allows reliable determination of the charge regulation properties of the surfaces. The latter can be quantified with a regulation parameter, which is essential to properly describe forces between interfaces, especially in asymmetric situations when one of the interfaces is charged and the other one is close to neutral. The technique relies on a highly charged probe particle, for which the charging properties are accurately determined by interpreting the double-layer contribution of the measured force profiles in the symmetric sphere-sphere geometry with Poisson-Boltzmann (PB) theory. Once the probe particle is calibrated, this particle is used to measure the force profile between an unknown substrate in the asymmetric sphere-sphere or sphere-plane geometry. From this profile, the diffuse-layer potential and regulation parameter of the substrate can be again determined with PB theory. The technique is highly versatile, as it can be used for a wide variety of substrates, including colloidal particles and planar substrates. The technique is also applicable in salt solutions containing multivalent ions. The current drawbacks of the technique are that it can only be applied up to moderately high salt levels, typically to 10 mM, and only for relatively large particles, typically down to about 1 μm in diameter. How the technique could be extended to higher salt levels and smaller particle size is also briefly discussed.

The behavior of ions in liquids confined between macromolecules determines the outcome of many nanoscale assembly processes in synthetic and biological materials such as colloidal dispersions, emulsions, hydrogels, DNA, cell membranes, and proteins. Theoretically, the macromolecule-liquid boundary is often modeled as a dielectric interface and an important quantity of interest is the ionic structure in a liquid confined between two such interfaces. The knowledge gleaned from the study of ionic structure in such models can be useful in several industrial applications, such as in the design of double-layer supercapacitors for energy storage and in the extraction of metal ions from wastewater. In this article, we compute the ionic structure in a model system of electrolyte confined by two planar dielectric interfaces using molecular dynamics simulations and liquid state theory. We explore the effects of high electrolyte concentrations, multivalent ions, dielectric contrasts, and external electric field on the ionic distributions. We observe the presence of non-monotonic ionic density profiles leading to a layered structure in the fluid which is attributed to the competition between electrostatic and steric (entropic) interactions. We find that thermal forces that arise from symmetry breaking at the interfaces can have a profound effect on the ionic structure and can oftentimes overwhelm the influence of the dielectric discontinuity. The combined effect of ionic correlations and inhomogeneous dielectric permittivity significantly changes the character of the effective interaction between the two interfaces.

Increasing demand of high energy density and high capacity batteries require development beyond Li-ion technology and innovations in electrodes and electrolytes, alike. An approach to achieve energy density higher than the existing Li-ion batteries is to replace monovalent Li ion by multivalent ions such as Mg, Ca. The ability of Mg batteries to provide much higher volumetric capacity, particularly on the anode side where the Mg metal can theoretically provide 3833 mA h/cc as compared to the Li counterpart graphite (∼ 800 mA h/cc) at a lower cost makes the technology an attractive candidate for future batteries. However, development and commercialization of Mg batteries require not only improved electrode discovery and development but also novel electrolytes, which are compatible with the Mg metal as conventional electrolytes fail to penetrate the Mg metal passivation layer. Thus a fundamental understanding of molecular level properties of these electrolytes is required to improve the electrochemical stability and the charge transfer properties. An automatic High-throughput infrastructure has been constructed for the electrolyte genome project supported by the US Joint Center for Energy Storage Research (JCESR)2. In this work, we present classical molecular dynamics simulations coupled with ab initio calculations for Mg salts in various solvents. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode1. We elucidate systematic correlations between molecular level interactions and composite electrolyte properties, such as electrochemical stability, solvation structure, and dynamics. We find that Mg electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. Specifically, we observe that, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg2+ → Mg+), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. Specifically, TFSI− exhibits a significant bond weakening while paired with the transient, partially reduced Mg+. Furthermore, we observe that higher order glymes as well as DMSO improve the solubility of Mg salts, but only the longer glyme chains reduce the dynamics of the ions in solution. This information provides critical design metrics for future electrolytes as it elucidates a close connection between bulk solvation and cathodic stability as well as the dynamics of the salt.

This work described the preparation of membranes based on aromatic polysulfones through the phase-inversion method induced by a nonsolvent, generating the phase separation (NIPS) process. Three new techniques, including the nano iron acid etching method, base hydrolysis method of crosslinked polymers, and base hydrolysis method of a reactive component in a binary polymer blend, were developed for pore creation on membranes. The modified polymers and obtained membranes were carefully characterized. The uniform pores were successfully created by base hydrolysis of the crosslinked polymers and obtained at the size of the crosslinker. Moreover, homogeneous pores were created after base hydrolysis of the membranes prepared from binary polymer blends due to the internal changes in the polymer structure. The separation performance of membranes was tested with different inorganic salt solutions and compared with commercially known membranes. These new membranes exhibited high water flux (up to 3000 L/m−2·h−1 at 10 bar and at 25 °C) and reasonable rejections for monovalent (21–44%) and multivalent ions (18–60%), depending on the different etching of the hydrolysis times. The comparison of these membranes with commercial ones confirmed their good separation performance and high potential application for water treatment applications.