Ions and water molecules in an electrolyte solution in contact with charged and dipolar surfaces

Surface tension of aqueous electrolyte solutions. A thermomechanical approach

We determine the surface tension of aqueous electrolyte solutions in contact with non-polar dielectric media using a thermomechanical approach, which involves deriving the stress tensor from the thermodynamic potential of an inhomogeneous fluid. To obtain the surface tension, we calculate both the normal and tangential pressures using the components of the stress tensor, recently derived by us [Y. A. Budkov and P. E. Brandyshev, J. Chem. Phys. 159, 174103 (2023)] within the framework of Wang's variational field theory. Using this approach, we derive an analytical expression for the surface tension in the linear approximation. At low ionic concentrations, this expression represents the classical Onsager-Samaras limiting law. By utilizing only one fitting parameter, which is related to the affinity of anions to the dielectric boundary, we successfully approximated experimental data on the surface tension of several aqueous electrolyte solutions. This approximation applies to both the solution-air and solution-dodecane interfaces, covering a wide range of electrolyte concentrations.

Nanoscale Topography of Anodic TiO2 Nanostructures Is Crucial for Cell-Surface Interactions

Anodic titanium dioxide (TiO2) nanostructures, i.e., obtained by electrochemical anodization, have excellent control over the nanoscale morphology and have been extensively investigated in biomedical applications owing to their sub-100 nm nanoscale topography range and beneficial effects on biocompatibility and cell interactions. Herein, we obtain TiO2 nanopores (NPs) and nanotubes (NTs) with similar morphologies, namely, 15 nm diameter and 500 nm length, and investigate their characteristics and impact on stem cell adhesion. We show that the transition of TiO2 NPs to NTs occurs via a pore/wall splitting mechanism and the removal of the fluoride-rich layer. Furthermore, in contrast to the case of NPs, we observe increased cell adhesion and proliferation on nanotubes. The enhanced mesenchymal stem cell adhesion/proliferation seems to be related to a 3-fold increase in activated integrin clustering, as confirmed by immunogold labeling with β1 integrin antibody on the nanostructured layers. Moreover, computations of the electric field and surface charge density show increased values at the inner and outer sharp edges of the top surfaces of the NTs, which in turn can influence cell adhesion by increasing the bridging interactions mediated by proteins and molecules in the environment. Collectively, our results indicate that the nanoscale surface architecture of the lateral spacing topography can greatly influence stem cell adhesion on substrates for biomedical applications.

Molecularly modulating solvation structure and electrode interface enables dendrite-free zinc-ion batteries

The performance of aqueous Zn ion batteries (AZIBs) is hindered by the uncontrollable growth of Zn dendrites and side reactions at the Zn anode/electrolyte interface. Here, we introduce low-cost glucosamine hydrochloride (GLA) into the ZnSO4 electrolyte system to modulate the Zn anode/electrolyte interface and the solvation structure of Zn2+, which leads to improved reversibility of Zn plating/striping. Through experimental and theoretical analyses, we demonstrate that GLA molecules could adsorp on the Zn metal surface to form a new interface with reduced active water, effectively suppressing water-induced side reactions. Moreover, after adding GLA, the flux of Zn2+ ions is regulated, the desolvation of the primary [Zn(H2O)6]2+ ions is promoted, and the Zn dendrite growth is significantly inhibited. Consequently, superior cyclic stability with a lower voltage hysteresis is simultaneously achieved in a Zn//Zn symmetric cell. When coupled with the Mn3O4 cathode, the fabricated Zn-Mn batteries with the modified ZnSO4 + GLA electrolyte system deliver boosted capacity, improved long-term cycling stability, and better self-discharge performance. This work provides insight into the development of high-efficient and low-cost electrolytes for high-performance Zn-based energy storage devices.

Adhesion dynamics of functionalized nanocarriers to endothelial cells: a dissipative particle dynamics study

Targeted drug delivery to endothelial cells utilizing functionalized nanocarriers (NCs) is an essential procedure in therapeutic and diagnosis therapies. Using dissipative particle dynamics simulation, NCs have been designed and combined with an endothelial environment, such as the endothelial glycocalyx (EG) layer, receptors, water, and cell wall. Furthermore, the energy landscapes of the functionalized NC with the endothelial cell have been analyzed as a function of properties such as the shape, size, initial orientation, and ligand density of NCs. Our results show that an appropriate higher ligand density for each particular NC provides more driving forces than barriers for the penetration of the NCs. Herein we report the importance of shell entropy loss for the NC shape effect on the adhesion and penetration into the EG layer. Moreover, the rotation of the disc shape NC as a wheel during the penetration is an extra driving force for its further inclusion. By increasing the NCs' size larger than the appropriate size for each particular ligand density, due to an increase in the NCs' shell entropy loss, the barriers surpass the driving forces for NC penetration. Furthermore, the parallel orientation provides the NCs with the best penetration capabilities. However, the rotation of the disc shape NCs enhances their diffusion in the perpendicular orientation too. Overall, our findings highlight the crucial role of the shell entropy loss in governing the penetration of NCs. Besides, studying NCs with a homogeneous ligand composition enabled us to cross barriers and probe energetics after the complete inclusion of the NCs.

Impact of hydrodynamics and rheology of the ion partitioning effect on electrokinetic flow through a soft annulus with a retentive and absorptive wall

The theoretical analysis for the mass transfer process of an oscillatory electroosmotic flow (EOF) in the fractional Jeffrey fluid model is studied through a polyelectrolyte layer (PEL) coated cylindrical annulus with reversible and irreversible wall reactions. The ion partitioning effect is observed due to the difference in permittivity of the PEL and the electrolyte solution, which is accounted for by the Born energy. Considering ion partitioning effects, analytical solutions for induced potential and axial velocity are presented, respectively in both the PEL and electrolyte region from the modified Poisson-Boltzmann equation and the Cauchy momentum equation with a proper constitutive equation, respectively. The Maxwell fluid and classical viscous Newtonian fluid models can be achieved separately by adjusting the relaxation and retardation time in the constitutive equation of this model. The analytical solution of the convection-diffusion equation for solute transport is established in the full domain. The separation of species is found to be dependent mainly on the Damköhler number, absorption parameter, phase partitioning coefficient, etc. It is observed that the osmotic pressure increases with the thickness and fixed charge density of the PEL. The velocity decreases with an increase in the permittivity difference of these layers. Our results suggest that the separation may be achieved through a difference in absorption kinetics.

Determination of elastic parameters of lipid membranes from simulation under varied external pressure

Many cellular processes such as endocytosis, exocytosis, and vesicle trafficking involve membrane deformations, which can be analyzed in the framework of the elastic theories of lipid membranes. These models operate with phenomenological elastic parameters. A connection between these parameters and the internal structure of lipid membranes can be provided by three-dimensional (3D) elastic theories. Considering a membrane as a 3D layer, Campelo et al. [F. Campelo et al., Adv. Colloid Interface Sci. 208, 25 (2014)10.1016/j.cis.2014.01.018] developed a theoretical basis for the calculation of elastic parameters. In this work we generalize and improve this approach by considering a more general condition of global incompressibility instead of local incompressibility. Crucially, we find an important correction to the theory of Campelo et al., which if not taken into account leads to a significant miscalculation of elastic parameters. With the total volume conservation taken into account, we derive an expression for the local Poisson's ratio, which determines how the local volume changes upon stretching and permits a more precise determination of elastic parameters. Also, we substantially simplify the procedure by calculating the derivatives of the moments of the local tension with respect to stretching instead of calculating the local stretching modulus. We obtain a relation between the Gaussian curvature modulus as a function of stretching and the bending modulus, showing that these two elastic parameters are not independent, as was previously assumed. The proposed algorithm is applied to membranes composed of pure dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and their mixture. The following elastic parameters of these systems are obtained: the monolayer bending and stretching moduli, spontaneous curvature, neutral surface position, and local Poisson's ratio. It is shown that the bending modulus of the DPPC/DOPC mixture follows a more complex trend than predicted by the classical Reuss averaging, which is often employed in theoretical frameworks.

Fabrication of Antibacterial TiO2 Nanostructured Surfaces Using the Hydrothermal Method

Implant-associated infections (IAI) are a common cause for implant failure, increased medical costs, and critical for patient healthcare. Infections are a result of bacterial colonization, which leads to biofilm formation on the implant surface. Nanostructured surfaces have been shown to have the potential to inhibit bacterial adhesion mainly due to antibacterial efficacy of their unique surface nanotopography. The change in topography affects the physicochemical properties of their surface such as surface chemistry, morphology, wettability, surface charge, and even electric field which influences the biological response. In this study, a conventional and cost-effective hydrothermal method was used to fabricate nanoscale protrusions of various dimensions on the surface of Ti, Ti₆Al₄V, and NiTi materials, commonly used in biomedical applications. The morphology, surface chemistry, and wettability were analyzed using scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS), and water contact angle analysis. The antibacterial efficacy of the synthesized nanostructures was analyzed by the use of Escherichia coli bacterial strain. XPS analysis revealed that the concentration of oxygen and titanium increased on Ti and Ti₆Al₄V, which indicates that TiO₂ is formed on the surface. The concentration of oxygen and titanium however decreased on the NiTi surface after hydrothermal treatment, and also a small amount of Ni was detected. SEM analysis showed that by hydrothermal treatment alterations in the surface topography of the TiO₂ layer could be achieved. The oxide layer on the NiTi prepared by the hydrothermal method contains a low amount of Ni (2.8 atom %), which is especially important for implantable materials. The results revealed that nanostructured surfaces significantly reduced bacterial adhesion on the Ti, Ti₆Al₄V, and NiTi surface compared to the untreated surfaces used as a control. Furthermore, two sterilization techniques were also studied to evaluate the stability of the nanostructure and its influence on the antibacterial activity. Sterilization with UV light seems to more efficiently inhibit bacterial growth on the hydrothermally modified Ti₆Al₄V surface, which was further reduced for hydrothermally treated Ti and NiTi. The developed nanostructured surfaces of Ti and its alloys can pave a way for the fabrication of antibacterial surfaces that reduce the likelihood of IAI.

Influence of Humectants on the Thermotropic Behaviour and Nanostructure of Fully Hydrated Lecithin Bilayers

Humectants are used widely in topical formulations as they provide cosmetic and health benefits to skin. Of particular interest to our laboratories is the interaction of humectants in phospholipid based topical skin care formulations. This study probed the effects of three exemplary humectants on a fully hydrated lecithin system (DPPC) by use of X-ray scattering and differential scanning calorimetry. While the three humectants affected the nanostructure of 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC, bilayers in a similar manner; leading to an increased membrane order, differences in the effect on the thermal behaviour of DPPC suggest that betaine and sarcosine interacted via a different mechanism compared to acetic monoethanolamide, AMEA. At concentrations above 0.4M, betaine and sarcosine stabilised the gel phase by depletion of the interfacial water via the preferential exclusion mechanism. At the same time, a slight increase in the rigidity of the membrane was observed with an increase in the membrane thickness. Overall, the addition of betaine or sarcosine resulted in an increase in the pre- and main transition temperatures of DPPC. AMEA, on the other hand, decreases both transition temperatures and although the interlamellar water layer was also decreased, there was evidence from the altered lipid chain packing, that AMEA molecules are present also at the bilayer interface, at least at high concentrations. Above the melting point in the fluid lamellar phase, none of the humectants induced significant structural changes, neither concerning the bilayer stacking order nor its overall membrane fluidity.

Dielectric Properties of Phosphatidylcholine Membranes and the Effect of Sugars

Simple carbohydrates are associated with the enhanced risk of cardiovascular disease and adverse changes in lipoproteins in the organism. Conversely, sugars are known to exert a stabilizing effect on biological membranes, and this effect is widely exploited in medicine and industry for cryopreservation of tissues and materials. In view of elucidating molecular mechanisms involved in the interaction of mono- and disaccharides with biomimetic lipid systems, we study the alteration of dielectric properties, the degree of hydration, and the rotational order parameter and dipole potential of lipid bilayers in the presence of sugars. Frequency-dependent deformation of cell-size unilamellar lipid vesicles in alternating electric fields and fast Fourier transform electrochemical impedance spectroscopy are applied to measure the specific capacitance of phosphatidylcholine lipid bilayers in sucrose, glucose and fructose aqueous solutions. Alteration of membrane specific capacitance is reported in sucrose solutions, while preservation of membrane dielectric properties is established in the presence of glucose and fructose. We address the effect of sugars on the hydration and the rotational order parameter for 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphocholine (POPC) and 1-stearoyl-2-oleoyl-sn-glycero-3- phosphocholine (SOPC). An increased degree of lipid packing is reported in sucrose solutions. The obtained results provide evidence that some small carbohydrates are able to change membrane dielectric properties, structure, and order related to membrane homeostasis. The reported data are also relevant to future developments based on the response of lipid bilayers to external physical stimuli such as electric fields and temperature changes.

On the Role of Electrostatic Repulsion in Topological Defect-Driven Membrane Fission

Within a modified Langevin Poisson–Boltzmann model of electric double layers, we derived an analytical expression for osmotic pressure between two charged surfaces. The orientational ordering of the water dipoles as well as the space dependencies of electric potentials, electric fields, and osmotic pressure between two charged spheres were taken into account in the model. Thus, we were able to capture the interaction between the parent cell and connected daughter vesicle or the interactions between neighbouring beads in necklace-like membrane protrusions. The predicted repulsion between them can facilitate the topological antidefect-driven fission of membrane daughter vesicles and the fission of beads of undulated membrane protrusions.

The Effect of the Osmotically Active Compound Concentration Difference on the Passive Water and Proton Fluxes across a Lipid Bilayer

The molecular details of the passive water flux across the hydrophobic membrane interior are still a matter of debate. One of the postulated mechanisms is the spontaneous, water-filled pore opening, which facilitates the hydrophilic connection between aqueous phases separated by the membrane. In the paper, we provide experimental evidence showing that the spontaneous lipid pore formation correlates with the membrane mechanics; hence, it depends on the composition of the lipid bilayer and the concentration of the osmotically active compound. Using liposomes as an experimental membrane model, osmotically induced water efflux was measured with the stopped-flow technique. Shapes of kinetic curves obtained at low osmotic pressure differences are interpreted in terms of two events: the lipid pore opening and water flow across the aqueous channel. The biological significance of the dependence of the lipid pore formation on the concentration difference of an osmotically active compound was illustrated by the demonstration that osmotically driven water flow can be accompanied by the dissipation of the pH gradient. The application of the Helfrich model to describe the probability of lipid pore opening was validated by demonstrating that the probability of pore opening correlates with the membrane bending rigidity. The correlation was determined by experimentally derived bending rigidity coefficients and probabilities of lipid pores opening.

pH dependent electrical properties of the inner- and outer- leaflets of biomimetic cell membranes

Composition and asymmetry of lipid membranes provide a means for regulation of trans-membrane permeability of ions and small molecules. The pH dependence of these processes plays an important role in the functioning and survival of cells. In this work, we study the pH dependence of membrane electrical resistance and capacitance using electrochemical impedance spectroscopy (EIS), surface plasmon resonance (SPR) and neutron reflectometry (NR) measurements of biomimetic tethered bilayer lipid membranes (tBLMs). tBLMs were prepared with single-component phospholipid compositions, as well as mixtures of phospholipids (phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingomyelin and cholesterol) that mimic the inner- and outer- leaflets of plasma cell membranes. We found that all studied tBLMs have a resistance maximum at pHs near the pK_as of the phospholipids. SPR and NR indicated that surface concentration of phospholipids and the thickness of the hydrophobic part of the membrane did not change versus pH. We postulate that these maxima are the result of protonation of the phosphate oxygen of the phospholipids and that hydronium ions play a major role in the conductance at pHs < pK_as while sodium ions play the major role at pHs > pK_as. An additional sharp resistance maximum of the PE tBLMs found at pH 5.9 and most likely represents the phosphatidylethanolamine's isoelectric point. The data show the key roles of the characteristic parts of phospholipid molecules: terminal group (choline, carboxyl, amine), phosphate, glycerol and ester oxygens on the permeability and selectivity of ions through the membrane. The interactions between these groups lead to significant differences in the electrical properties of biomimetic models of inner- and outer- leaflets of the plasma cell membranes.

Mechanical and Electrical Interaction of Biological Membranes with Nanoparticles and Nanostructured Surfaces

In this review paper, we theoretically explain the origin of electrostatic interactions between lipid bilayers and charged solid surfaces using a statistical mechanics approach, where the orientational degree of freedom of lipid head groups and the orientational ordering of the water dipoles are considered. Within the modified Langevin Poisson–Boltzmann model of an electric double layer, we derived an analytical expression for the osmotic pressure between the planar zwitterionic lipid bilayer and charged solid planar surface. We also show that the electrostatic interaction between the zwitterionic lipid head groups of the proximal leaflet and the negatively charged solid surface is accompanied with a more perpendicular average orientation of the lipid head-groups. We further highlight the important role of the surfaces’ nanostructured topography in their interactions with biological material. As an example of nanostructured surfaces, we describe the synthesis of TiO₂ nanotubular and octahedral surfaces by using the electrochemical anodization method and hydrothermal method, respectively. The physical and chemical properties of these nanostructured surfaces are described in order to elucidate the influence of the surface topography and other physical properties on the behavior of human cells adhered to TiO₂ nanostructured surfaces. In the last part of the paper, we theoretically explain the interplay of elastic and adhesive contributions to the adsorption of lipid vesicles on the solid surfaces. We show the numerically predicted shapes of adhered lipid vesicles corresponding to the minimum of the membrane free energy to describe the influence of the vesicle size, bending modulus, and adhesion strength on the adhesion of lipid vesicles on solid charged surfaces.

Effect of Bjerrum pairs on the electrostatic properties of an electrolyte solution near charged surfaces: a mean-field approach

In this paper, we investigate the consequences of ion association, coupled with the considerations of finite size effects and orientational ordering of Bjerrum pairs as well as ions and water molecules, on the electric double layer near charged surfaces. Based on the lattice statistical mechanics accounting for finite sizes and dipole moments of ions, Bjerrum pairs and solvent molecules, we consider the formation of Bjerrum pairs and derive the mathematical expressions for Bjerrum pair number density as well as cation/anion number density and water molecule number density. We reveal several significant phenomena. Firstly, it is shown that our approach naturally yields the equilibrium constant for dissociation-association equilibrium between Bjerrum pairs and ions. Secondly, at low surface charge densities, an increase in the bulk concentration of Bjerrum pairs enhances the permittivity and decreases the differential capacitance. Next, for the cases where Bjerrum pairs in an alcohol electrolyte solution have a high value of dipole moment, the Bjerrum pair number density increases with decreasing distance from the charged surface, and the differential capacitance and permittivity are high compared to those for the cases with lower values of Bjerrum-pair dipole moments. Finally, we show that the difference in the concentration and dipole moment of Bjerrum pairs can lead to some variation in osmotic pressure between two similarly charged surfaces.

Influence of liposomes composition on their stability during the nebulization process by vibrating mesh nebulizer

In this study, three different molecules (cholesterol, phosphatidic acid, and polyethylene glycol) were used for the stabilization of liposomes during the nebulization process. The purpose of this article is to answer the question of whether the change in the composition of liposomes affected the parameters of generated aerosol and whether the nebulization process affected observed properties of liposomes. Firstly, liposomes with different composition were prepared and their properties were checked by dynamic and electrophoretic light scattering. The membrane properties were measured by fluorescence spectroscopy - especially generalized polarization (Laurdan) and anisotropy (Diphenylhexatriene). The same characteristic of liposomes was measured after the nebulization by vibrating mesh nebulizer. Cholesterol was capable of liposome stabilization because of increased membrane fluidity. The membrane properties of the outer and inner parts were not influenced by the nebulization process. Electrostatic stabilization was successful for the lowest concentration of phosphatidic acid, but after the nebulization process the hydration of the membrane outer part was changed. Higher amount of PEG needs to be added for successful steric stabilization. The nebulization process of the two lowest concentrations of PEG slightly influenced immobilized water and the rigidity of inner part of the membrane (especially around the phase transition temperature).

Continuum analysis to assess field enhancements for tailoring electroporation driven by monopolar or bipolar pulsing based on nonuniformly distributed nanoparticles

Recent reports indicate that nanoparticle (NP) clusters near cell membranes could enhance local electric fields, leading to heightened electroporation. This aspect is quantitatively analyzed through numerical simulations whereby time dependent transmembrane potentials are first obtained on the basis of a distributed circuit mode, and the results then used to calculate pore distributions from continuum Smoluchowski theory. For completeness, both monopolar and bipolar nanosecond-range pulse responses are presented and discussed. Our results show strong increases in TMP with the presence of multiple NP clusters and demonstrate that enhanced poration could be possible even over sites far away from the poles at the short pulsing regime. Furthermore, our results demonstrate that nonuniform distributions would work to enable poration at regions far away from the poles. The NP clusters could thus act as distributed electrodes. Our results were roughly in line with recent experimental observations.

Electric Double Layer and Orientational Ordering of Water Dipoles in Narrow Channels within a Modified Langevin Poisson-Boltzmann Model

The electric double layer (EDL) is an important phenomenon that arises in systems where a charged surface comes into contact with an electrolyte solution. In this work we describe the generalization of classic Poisson-Boltzmann (PB) theory for point-like ions by taking into account orientational ordering of water molecules. The modified Langevin Poisson-Boltzmann (LPB) model of EDL is derived by minimizing the corresponding Helmholtz free energy functional, which includes also orientational entropy contribution of water dipoles. The formation of EDL is important in many artificial and biological systems bound by a cylindrical geometry. We therefore numerically solve the modified LPB equation in cylindrical coordinates, determining the spatial dependencies of electric potential, relative permittivity and average orientations of water dipoles within charged tubes of different radii. Results show that for tubes of a large radius, macroscopic (net) volume charge density of coions and counterions is zero at the geometrical axis. This is attributed to effective electrolyte charge screening in the vicinity of the inner charged surface of the tube. For tubes of small radii, the screening region extends into the whole inner space of the tube, leading to non-zero net volume charge density and non-zero orientational ordering of water dipoles near the axis.

Magneto-mechanical actuation of barium-hexaferrite nanoplatelets for the disruption of phospholipid membranes

HYPOTHESIS

The magneto-mechanical actuation (MMA) of magnetic nanoparticles with a low-frequency alternating magnetic field (AMF) can be used to destroy cancer cells. So far, MMA was tested on different cells using different nanoparticles and different field characteristics, which makes comparisons and any generalizations about the results of MMA difficult. In this paper we propose the use of giant unilamellar vesicles (GUVs) as a simple model system to study the effect of MMA on a closed lipid bilayer membrane, i.e., a basic building block of any cell.

EXPERIMENTS

The GUVs were exposed to barium-hexaferrite nanoplatelets (NPLs, ~50 nm wide and 3 nm thick) with unique magnetic properties dominated by a permanent magnetic moment that is perpendicular to the platelet, at different concentrations (1-50 µg/mL) and pH values (4.2-7.4) of the aqueous suspension. The GUVs were observed with an optical microscope while being exposed to a uniaxial AMF (3-100 Hz, 2.2-10.6 mT).

FINDINGS

When the NPLs were electrostatically attached to the GUV membranes, the MMA induced cyclic fluctuations of the GUVs' shape corresponding to the AMF frequency at the low NPL concentration (1 µm/mL), whereas the GUVs were bursting at the higher concentration (10 µg/mL). Theoretical considerations suggested that the bursting of the GUVs is a consequence of the local action of an assembly of several NPLs, rather than a collective effect of all the absorbed NPLs.

Structure of the Electrical Double Layer Revisited: Electrode Capacitance in Aqueous Solutions

The structure of the electrical double layer at the interface of planar electrodes and aqueous solutions is investigated. Electrical impedance spectroscopy is used to measure the impedance of aqueous solutions of sodium chloride and two different surfactants over a wide range of concentrations. The electrode capacitance is directly inferred from the admittance spectra as well as by regression of the impedance spectra to an equivalent circuit. It is found that the electrode capacitance remains on the same order of magnitude over the entire range of investigated concentrations. This is contradictive to the predictions of the Gouy-Chapman-Stern theory which predicts that, at low concentrations, the electrode capacitance should be determined by the diffuse layer. It is concluded that the Stern layer capacitance always dominates the electrode capacitance, even at very low concentrations, and the establishment of a diffuse layer capacitance requires an ionic strength of around 1 mM.

Interactions of Monovalent and Divalent Cations at Palmitoyl-Oleoyl-Phosphatidylcholine Interface

Li⁺ is a biologically active and medically important cation. Experiments show that Li⁺ modulates some phospholipid bilayer properties in a manner similar to divalent cations, rather than other monovalent cations. We previously performed a comparative simulation study of the interaction of several monovalent cations with palmitoyl-oleoyl-phosphatidylcholine bilayers and reported that Li⁺ exhibited the highest association with lipids and formed a unique tetrahedral coordinated structure with lipid head groups. Here we extend these studies to two biologically important divalent cations, Mg²⁺ and Ca²⁺, and observe that, just like monovalent cations, Mg²⁺ and Ca²⁺ reduce bilayer areas and increase chain order. Bilayer area changes induced by cations are strongly correlated with the amount of charge inside the headgroup region; however, Mg²⁺ and Li⁺ are clear outliers. At the same time though, Mg²⁺ adsorption in the bilayer is the smallest among all cations, which is in contrast to Li⁺ that binds strongly to lipids. In fact, in contrast to all other cations, Mg²⁺ remains fully hydrated in the lipid headgroup region. However, Li⁺ and Mg²⁺ share high overlap between their inner-shell coordination topologies. This suggests that Li⁺ can structurally replace Mg²⁺, which is bound to other biomolecules with up to fourfold coordination, provided such replacement is energetically feasible. We compute structural topologies and compare them quantitatively using a new weighted-graphs-based method. Finally, we find that the specificity of cation interaction with lipid head groups exhibit consistent trend with the solvation shell energetics of ions in lipid headgroup and bulk water regions.

Grand-canonical approach to density functional theory of electrocatalytic systems: Thermodynamics of solid-liquid interfaces at constant ion and electrode potentials

Properties of solid-liquid interfaces are of immense importance for electrocatalytic and electrochemical systems, but modeling such interfaces at the atomic level presents a serious challenge and approaches beyond standard methodologies are needed. An atomistic computational scheme needs to treat at least part of the system quantum mechanically to describe adsorption and reactions, while the entire system is in thermal equilibrium. The experimentally relevant macroscopic control variables are temperature, electrode potential, and the choice of the solvent and ions, and these need to be explicitly included in the computational model as well; this calls for a thermodynamic ensemble with fixed ion and electrode potentials. In this work, a general framework within density functional theory (DFT) with fixed electron and ion chemical potentials in the grand canonical (GC) ensemble is established for modeling electrocatalytic and electrochemical interfaces. Starting from a fully quantum mechanical description of multi-component GC-DFT for nuclei and electrons, a systematic coarse-graining is employed to establish various computational schemes including (i) the combination of classical and electronic DFTs within the GC ensemble and (ii) on the simplest level a chemically and physically sound way to obtain various (modified) Poisson-Boltzmann (mPB) implicit solvent models. The detailed and rigorous derivation clearly establishes which approximations are needed for coarse-graining as well as highlights which details and interactions are omitted in vein of computational feasibility. The transparent approximations also allow removing some of the constraints and coarse-graining if needed. We implement various mPB models within a linear dielectric continuum in the GPAW code and test their capabilities to model capacitance of electrochemical interfaces as well as study different approaches for modeling partly periodic charged systems. Our rigorous and well-defined DFT coarse-graining scheme to continuum electrolytes highlights the inadequacy of current linear dielectric models for treating properties of the electrochemical interface.

Liquid crystalline droplets in aqueous environments: electrostatic effects

We demonstrate the strong impact of electrostatic properties on radial-bipolar structural transitions in nematic liquid crystal (LC) droplets dispersed in different aqueous environments. In the experimental part of the study, we systematically changed the electrostatic properties of both LC droplets and aqueous solutions. Mixtures of nematics were studied by combining LC materials with negative (azoxybenzene compounds) and strongly positive (cyanobiphenyl) dielectric anisotropy. The aqueous solutions were manipulated by introducing either polyvinyl alcohol, glycerol, electrolyte or amphiphilic anionic surfactant SDS into water. In the supporting theoretical study, we identified the key parameters influencing the dielectric constant and the electric field strength of aqueous solutions. We also estimated the impact of different electrolytes on the Debye length at the LC-aqueous interface. The obtained results are further analysed for chemical and biological sensing applications.

Diffuse double-layer structure in mixed electrolytes considering ions as dielectric spheres

The structure of the diffuse part of the electric double layer at solid-electrolyte solution interfaces is examined using a theoretical model that takes into account the finite ion size by modeling the solution as a suspension of polarizable insulating spheres in water. This formalism is applied to mixed electrolyte solutions using the "Boublik-Mansoori-Carnahan-Starling-Leland" (BMCSL) theory for the steric interactions among ions. It is shown that the ionic size differences have a strong bearing on the diffuse part of the electric double-layer structure of these systems. Moreover, for strong potential values, the different size-related effects become important even for binary electrolyte solutions due to the presence of H⁺ and OH^- ions that are substantially smaller than hydrated ions originated from salt dissociation. The obtained results display some of the qualitative features observed in experiments on aqueous systems that are generally interpreted in terms of totally different mechanisms.

Clustering and separation of hydrophobic nanoparticles in lipid bilayer explained by membrane mechanics

Small hydrophobic gold nanoparticles with diameter lower than the membrane thickness can form clusters or uniformly distribute within the hydrophobic core of the bilayer. The coexistence of two stable phases (clustered and dispersed) indicates the energy barrier between nanoparticles. We calculated the distance dependence of the membrane-mediated interaction between two adjacent nanoparticles. In our model we consider two deformation modes: the monolayer bending and the hydroxycarbon chain stretching. Existence of an energy barrier between the clustered and the separated state of nanoparticles was predicted. Variation analysis of the membrane mechanical parameters revealed that the energy barrier between two membrane embedded nanoparticles is mainly the consequence of the bending deformation and not change of the thickness of the bilayer in the vicinity of nanoparticles. It is shown, that the forces between the nanoparticles embedded in the biological membrane could be either attractive or repulsive, depending on the mutual distance between them.

Protein interactions with layers of TiO2 nanotube and nanopore arrays: Morphology and surface charge influence

In the present work we investigate the key factors involved in the interaction of small-sized charged proteins with TiO₂ nanostructures, i.e. albumin (negatively charged), histone (positively charged). We examine anodic nanotubes with specific morphology (simultaneous control over diameter and length, e.g. diameter - 15, 50 or 100nm, length - 250nm up to 10μm) and nanopores. The nanostructures surface area has a direct influence on the amount of bound protein, nonetheless the protein physical properties as electric charge and size (in relation to nanotopography and biomaterial's electric charge) are crucial too. The highest quantity of adsorbed protein is registered for histone, for 100nm diameter nanotubes (10μm length) while higher values are registered for 15nm diameter nanotubes when normalizing protein adsorption to nanostructures' surface unit area (evaluated from dye desorption measurements) - consistent with theoretical considerations. The proteins presence on the nanostructures is evaluated by XPS and ToF-SIMS; additionally, we qualitatively assess their presence along the nanostructures length by ToF-SIMS depth profiles, with decreasing concentration towards the bottom.

STATEMENT OF SIGNIFICANCE

Surface nanostructuring of titanium biomedical devices with TiO₂ nanotubes was shown to significantly influence the adhesion, proliferation and differentiation of mesenchymal stem cells (and other cells too). A high level of control over the nanoscale topography and over the surface area of such 1D nanostructures enables a direct influence on protein adhesion. Herein, we investigate and show how the nanostructure morphology (nanotube diameter and length) influences the interactions with small-sized charged proteins, using as model proteins bovine serum albumin (negatively charged) and histone (positively charged). We show that the protein charge strongly influences their adhesion to the TiO₂ nanostructures. Protein adhesion is quantified by ELISA measurements and determination of the nanostructures' total surface area. We use a quantitative surface charge model to describe charge interactions and obtain an increased magnitude of the surface charge density at the top edges of the nanotubes. In addition, we track the proteins presence on and inside the nanostructures. We believe that these aspects are crucial for applications where the incorporation of active molecules such as proteins, drugs, growth factors, etc., into nanotubes is desired.

Internal configuration and electric potential in planar negatively charged lipid head group region in contact with ionic solution

The lipid bilayer composed of negatively charged lipid 1-palmitoyl-3-oleoyl-sn-glycero-3-phosphatidylserine (POPS) in contact with an aqueous solution of monovalent salt ions was studied theoretically by using the mean-field modified Langevin-Poisson-Boltzmann (MLPB) model. The MLPB results were tested by using molecular dynamic (MD) simulations. In the MLPB model the charge distribution of POPS head groups is theoretically described by the negatively charged surface which accounts for negatively charged phosphate groups, while the positively charged amino groups and negatively charged carboxylate groups are assumed to be fixed on the rod-like structures with rotational degree of freedom. The spatial variation of relative permittivity, which is not considered in the well-known Gouy-Chapman (GC) model or in MD simulations, is thoroughly derived within a strict statistical mechanical approach. Therefore, the spatial dependence and magnitude of electric potential within the lipid head group region and its close vicinity are considerably different in the MLPB model from the GC model. The influence of the bulk salt concentration and temperature on the number density profiles of counter-ions and co-ions in the lipid head group region and aqueous solution along with the probability density function for the lipid head group orientation angle was compared and found to be in qualitative agreement in the MLPB and MD models.

Wettability studies of topologically distinct titanium surfaces

Biomedical implants made of titanium-based materials are expected to have certain essential features including high bone-to-implant contact and optimum osteointegration, which are often influenced by the surface topography and physicochemical properties of titanium surfaces. The surface structure in the nanoscale regime is presumed to alter/facilitate the protein binding, cell adhesion and proliferation, thereby reducing post-operative complications with increased lifespan of biomedical implants. The novelty of our TiO2 nanostructures lies mainly in the high level control over their morphology and roughness by mere compositional change and optimisation of the experimental parameters. The present work focuses on the wetting behaviour of various nanostructured titanium surfaces towards water. Kinetics of contact area of water droplet on macroscopically flat, nanoporous and nanotubular titanium surface topologies was monitored under similar evaporation conditions. The contact area of the water droplet on hydrophobic titanium planar surface (foil) was found to decrease during evaporation, whereas the contact area of the droplet on hydrophobic nanorough titanium surfaces practically remained unaffected until the complete evaporation. This demonstrates that the surface morphology and roughness at the nanoscale level substantially affect the titanium dioxide surface-water droplet interaction, opposing to previous observations for microscale structured surfaces. The difference in surface topographic nanofeatures of nanostructured titanium surfaces could be correlated not only with the time-dependency of the contact area, but also with time-dependency of the contact angle and electrochemical properties of these surfaces.

Origin of 1/f noise in hydration dynamics on lipid membrane surfaces

Water molecules on lipid membrane surfaces are known to contribute to membrane stability by connecting lipid molecules and acting as a water bridge. Although water structures and diffusivities near the membrane surfaces have been extensively studied, hydration dynamics on the surfaces has remained an open question. Here we investigate residence time statistics of water molecules on the surface of lipid membranes using all-atom molecular dynamics simulations. We show that hydration dynamics on the lipid membranes exhibits 1/f noise. Constructing a dichotomous process for the hydration dynamics, we find that residence times in each state follow a power-law with exponential cutoff and that the process can be regarded as a correlated renewal process where interoccurrence times are correlated. The results imply that the origin of the 1/f noise in hydration dynamics on the membrane surfaces is a combination of a power-law distribution with cutoff of interoccurrence times of switching events and a long-term correlation between the interoccurrence times. These results suggest that the 1/f noise attributed to the correlated renewal process may contribute to the stability of the hydration layers and lipid membranes.

Titanium nanostructures for biomedical applications

Titanium and titanium alloys exhibit a unique combination of strength and biocompatibility, which enables their use in medical applications and accounts for their extensive use as implant materials in the last 50 years. Currently, a large amount of research is being carried out in order to determine the optimal surface topography for use in bioapplications, and thus the emphasis is on nanotechnology for biomedical applications. It was recently shown that titanium implants with rough surface topography and free energy increase osteoblast adhesion, maturation and subsequent bone formation. Furthermore, the adhesion of different cell lines to the surface of titanium implants is influenced by the surface characteristics of titanium; namely topography, charge distribution and chemistry. The present review article focuses on the specific nanotopography of titanium, i.e. titanium dioxide (TiO2) nanotubes, using a simple electrochemical anodisation method of the metallic substrate and other processes such as the hydrothermal or sol-gel template. One key advantage of using TiO2 nanotubes in cell interactions is based on the fact that TiO2 nanotube morphology is correlated with cell adhesion, spreading, growth and differentiation of mesenchymal stem cells, which were shown to be maximally induced on smaller diameter nanotubes (15 nm), but hindered on larger diameter (100 nm) tubes, leading to cell death and apoptosis. Research has supported the significance of nanotopography (TiO2 nanotube diameter) in cell adhesion and cell growth, and suggests that the mechanics of focal adhesion formation are similar among different cell types. As such, the present review will focus on perhaps the most spectacular and surprising one-dimensional structures and their unique biomedical applications for increased osseointegration, protein interaction and antibacterial properties.

Interplay of electrostatics and lipid packing determines the binding of charged polymer coated nanoparticles to model membranes

Cationic nanoparticles show larger penetration within well-packed zwitterionic lipid bilayer.

Scaling behaviour for the water transport in nanoconfined geometries

The transport of water in nanoconfined geometries is different from bulk phase and has tremendous implications in nanotechnology and biotechnology. Here molecular dynamics is used to compute the self-diffusion coefficient D of water within nanopores, around nanoparticles, carbon nanotubes and proteins. For almost 60 different cases, D is found to scale linearly with the sole parameter θ as D(θ)=D_B[1+(D_C/D_B−1)θ], with D_B and D_C the bulk and totally confined diffusion of water, respectively. The parameter θ is primarily influenced by geometry and represents the ratio between the confined and total water volumes. The D(θ) relationship is interpreted within the thermodynamics of supercooled water. As an example, such relationship is shown to accurately predict the relaxometric response of contrast agents for magnetic resonance imaging. The D(θ) relationship can help in interpreting the transport of water molecules under nanoconfined conditions and tailoring nanostructures with precise modulation of water mobility.

A precise control of water transport in different configurations is crucial for many technological applications. Chiavazzo et al. define a dimensionless scaling parameter to fully describe the water self-diffusion coefficient in various nanoconfined geometries using molecular dynamic simulations.