Orientational Ordering Of Water Dipoles Research Articles

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.

Differential Capacitance of Electric Double Layer – Influence of Asymmetric Size of Ions, Thickness of Stern Layer and Orientational Ordering of Water Dipoles

The mean-field theoretical model of the electric double layer which takes into account the asymmetric finite size of anions and cations and the orientational ordering of water dipoles in the Stern and the diffuse layers is described together with a short description of the main concepts and a brief review of the literature in the theory of the electric double layer. As an example of the application of the described mean-field lattice model of the electric double layer, the influence of different sizes of anions and cations, the influence of the thickness of the Stern layer and the influence of the orientational ordering of water molecules on the asymmetric, bimodal camel-like dependence of differential capacitance on the surface potential is theoretically considered. The presented theoretical model of the electric double layer is flexible enough to be in the future extended to more complicated multicomponent systems with molecules of different sizes and the orientational ordering of molecules.

Asymmetric Finite Size of Ions and Orientational Ordering of Water in Electric Double Layer Theory Within Lattice Model.

In the present review, a brief historical survey of the mean-field theoretical description of Electric Double Layer (EDL) is presented. Special attention is devoted to asymmetric finite size of ions and orientational ordering of water dipoles. A model of Wicke and Eigen, who were first to explicitly derive the ion distribution functions for finite size of ions, is discussed. Arguments are given in favour of changing the recently adopted name of the mean-field EDL model for finite size of ions from Bikerman model to Bikerman-Wicke-Eigen model. Theoretically predicted asymmetric and symmetric camel-like shape of the voltage dependence of the differential capacitance is also discussed.

Influence of solvent polarization and non-uniform ion size on electrostatic properties between charged surfaces in an electrolyte solution

In this paper, we study electrostatic properties between two similar or oppositely charged surfaces immersed in an electrolyte solution by using the mean-field approach accounting for solvent polarization and non-uniform size effects. Applying a free energy formalism accounting for unequal ion sizes and orientational ordering of water dipoles, we derive coupled and self-consistent equations to calculate electrostatic properties between charged surfaces. Electrostatic properties for similarly charged surfaces depend on the counterion size but not on the coion size. Moreover, electrostatic potential and osmotic pressure between similarly charged surfaces are found to be increased with increasing counterion size. On the other hand, the corresponding ones between oppositely charged surfaces are related to both sizes of positive and negative ions. For oppositely charged surfaces, the electrostatic potential, number density of solvent molecules, and relative permittivity of an electrolyte having unequal ion sizes are not symmetric about the centerline between the charged surfaces. For either case, the consideration of solvent polarization results in a decrease in the electrostatic potential and the osmotic pressure compared to the case without the effect.

Electrokinetic Properties of TiO2 Nanotubular Surfaces.

Surface charge is one of the most significant properties for the characterisation of a biomaterial, being a key parameter in the interaction of the body implant with the surrounding living tissues. The present study concerns the systematic assessment of the surface charge of electrochemically anodized TiO2 nanotubular surfaces, proposed as coating material for Ti body implants. Biologically relevant electrolytes (NaCl, PBS, cell medium) were chosen to simulate the physiological conditions. The measurements were accomplished as titration curves at low electrolytic concentration (10−3 M) and as single points at fixed pH but at various electrolytic concentrations (up to 0.1 M). The results showed that all the surfaces were negatively charged at physiological pH. However, the zeta potential values were dependent on the electrolytic conditions (electrolyte ion concentration, multivalence of the electrolyte ions, etc.) and on the surface characteristics (nanotubes top diameter, average porosity, exposed surface area, wettability, affinity to specific ions, etc.). Accordingly, various explanations were proposed to support the different experimental data among the surfaces. Theoretical model of electric double layer which takes into account the asymmetric finite size of ions in electrolyte and orientational ordering of water dipoles was modified according to our specific system in order to interpret the experimental data. Experimental results were in agreement with the theoretical predictions. Overall, our results contribute to enrich the state-of-art on the characterisation of nanostructured implant surfaces at the bio-interface, especially in case of topographically porous and rough surfaces.

Effect of orientational ordering of water dipoles on stratification of counterions of different size in multicomponent electrolyte solution near charged surface - a mean field approach

We theoretically studied electric double layer by using the mean-field approach including the non-uniform size effect and the orientational ordering of water dipoles in electrolyte solution. Performing the minimization of the free energy, the resulting ion and water distribution functions are determined numerically in the process of solving the Poisson's equation. The effect of non-uniform ionic size and orientational ordering of water dipoles on stratification of counterions of different size in multicomponent electrolyte solution near the charged surface are presented and discussed. Spatial dependence of relative permittivity and differential capacitance of electric double layer are also calculated for binary electrolyte solution and then compared with the predictions of the previous models. Effect of bulk concentration of counterions and ion size effects on surface charge density, excess portion of counterions, relative permittivity and differential capacitance are studied for multicomponent electrolyte solution in detail.

An electric double layer of colloidal particles in salt-free concentrated suspensions including non-uniform size effects and orientational ordering of water dipoles.

The response of a suspension under a variety of static or alternating external fields strongly depends on the equilibrium electric double layer that surrounds the colloidal particles in the suspension. The theoretical models for salt-free suspensions can be improved by incorporating non-uniform size effects and orientational ordering of water dipoles neglected in previous mean-field approaches, which are based on the Poisson-Boltzmann approach. Our model including non-uniform size effects and orientational ordering of water dipoles seems to have quite a promising effect because the model can predict the phenomena like a heavy decrease in relative permittivity of the suspension and counterion stratification near highly charged colloidal particles. In this work we numerically obtain the electric potential, the counterion concentration and the relative permittivity around a charged particle in a concentrated salt-free suspension corrected by non-uniform size effects and orientational ordering of water dipoles. The results show the worth of such corrections for medium to high particle charges at every particle volume fraction. We conclude that non-uniform size effects and orientational ordering of water dipoles are necessary for the development of new theoretical models to study non-equilibrium properties in concentrated colloidal suspensions.

Experimental data and theoretical modelling on the electrokinetics of porous TiO2-coatings for body implants

Event Abstract Back to Event Experimental data and theoretical modelling on the electrokinetics of porous TiO2-coatings for body implants Martina Lorenzetti1, Mukta Kulkarni2, Ekaterina Gongadze2 and Aleš Iglič2 1 Jožef Stefan Institute, Department for Nanostructured Materials, Slovenia 2 University of Ljubljana, Faculty of Electrical Engineering, Slovenia Introduction: Even though titanium and its alloys are generally good materials for metallic implants, they lack per se in terms of biocompatibility, due to their intrinsic inertness. One of the strategies to improve their surface properties is to apply a coating, able to ensure a beneficial, pre-conditioning film of ions and proteins which will support the further cell adhesion and wound healing. Thus, it is fundamental to study the electrokinetic behaviour of the implant surfaces, in connection with surface topography and wettability, in order to predict a priori the influence of the material properties on the events occurring at the bio-interface [1][2]. Materials and Methods: Electrochemical anodization (EA) was applied to grow TiO2-nanotubes (NTs), forming dense coatings on titanium foils [3]. The NTs characterisation was carried out in terms of morphology (scanning electron microscopy, SEM), wetting by sessile drop contact angle, and zeta potential (ZP) by streaming potential technique. The latter method allowed the quantification of surface charge in different electrolytic solutions and different ionic strengths, relevant for the biological assessment. Titration curves (ZP vs. pH) were systematically obtained in sodium chloride, phosphate buffer saline, artificial saliva, and Dulbecco’s Modified Eagle Medium. The experimental data were analysed and interpreted by a theoretical mean-field model of electric double layer which takes into account the orientational ordering of water dipoles and the asymmetric finite size of hydrated anions and cations in electrolyte solution [4]. Additionally, the effect of the ionic strength on ZP was quantitatively modelled by fitting the experimental data. Results and Discussion: Tailoring the synthesis parameters applied during the EA, different NTs morphologies and porosity were obtained, which resulted in distinctive surface properties (i.e. high exposed surface area and wettability). The ZP dependence on the pH revealed that, even though all the surfaces were negatively charged at physiological pH in the biologically-relevant electrolytes, ZP differed in magnitude. The isoelectric point shifted by mean of the present ions nature (valence, ionic and hydrated radius, affinity with the surface). The ZP dependence on the electrolytic concentration was also evaluated (Figure 1). All these effects were computed by theoretical models, resulting in equations which represent the behaviour of the electrical double layer around the surface. Moreover, the effect of hydrophilicity, dipole interactions, and surface exposed area at the top-edge of NTs on ZP was also considered. Figure 1. ZP dependence of the TiO2-NTs array (15 nm diameter) on the concentration of NaCl saline solution. Conclusion: To our best knowledge, for the first time a systematic electrokinetic study of NTs-coated surfaces for body implants was carried out in terms of surface charge in biologically-relevant electrolytes. The results were supported by theoretical calculations, so that equations specific to our system, but applicable also to other nanoporous structures, were designed. The outcomes broad the scenario to the characterization of the bio-interface at the implant surface. This work was supported by the Slovenian Research Agency (project grant numbers P2-0084 and P2-0232).

Asymmetric size of ions and orientational ordering of water dipoles in electric double layer model - an analytical mean-field approach

Electric double layer is theoretically described within the mean-field approach by taking into account the orientational ordering of water dipoles and asymmetric size of cations and anions. Analytical expressions for the spatial distribution of ions and water dipoles are derived. The effect of asymmetric ionic size on accumulation of counterions and partial depletion of water molecules near the charged surface, on spatial dependence of relative permittivity and on differential capacitance of electric double layer are presented.

Asymmetric electrostatic properties of an electric double layer: a generalized Poisson-Boltzmann approach taking into account non-uniform size effects and water polarization

We theoretically study electrostatic properties of electric double layer using a generalized Poisson-Boltzmann approach taking into account the orientational ordering of water dipoles and the excluded volume effect of water molecules as well as those of positive and negative ions with different sizes in electrolyte solution.Our approach enables one to predict that the number densities of water molecules, counterions and coions and the permittivity of electrolyte solution close to a charged surface, asymmetrically vary depending on both of sign and magnitude of the surface charge density and the volume of counterion. We treat several phenomena in more detail. Firstly, an increase in the volume of counterions and an increase in the surface charge density can cause the position of the minimum number density of water molecules to be farther from the charged surface. Secondly, width of the range of voltage in which the properties at the charged surface symmetrically vary decreases with increasing bulk salt concentration. In addition, we show that the excluded volume effect of water molecules and the orientational ordering of water dipoles can lead to early onset and lowering of the maximum of electric capacitance according to surface voltage. Our approach and results can be applied to describing electrostatic properties of biological membranes and electric double layer capacitor for which excluded volume effects of water molecules and ions with different sizes may be important.

Spatial variation of permittivity of an electrolyte solution in contact with a charged metal surface: a mini review

Contact between a charged metal surface and an electrolyte implies a particular ion distribution near the charged surface, i.e. the electrical double layer. In this mini review, different mean-field models of relative (effective) permittivity are described within a simple lattice model, where the orientational ordering of water dipoles in the saturation regime is taken into account. The Langevin–Poisson–Boltzmann (LPB) model of spatial variation of the relative permittivity for point-like ions is described and compared to a more general Langevin–Bikerman (LB) model of spatial variation of permittivity for finite-sized ions. The Bikerman model and the Poisson–Boltzmann model are derived as limiting cases. It is shown that near the charged surface, the relative permittivity decreases due to depletion of water molecules (volume-excluded effect) and orientational ordering of water dipoles (saturation effect). At the end, the LPB and LB models are generalised by also taking into account the cavity field.

Decrease of permittivity of an electrolyte solution near a charged surface due to saturation and excluded volume effects

The dipole moment of a water molecule in liquid water differs from that of an isolated one because each molecule is further polarized by the electric field of its neighbours. In this work a formula for the spatial dependence of the relative permittivity of an electrolyte near a highly charged surface is obtained in which the mutual influence of the water molecules is taken into account by means of the cavity field. The orientational ordering of water dipoles is considered in the saturation regime. It is predicted that the relative permittivity of an electrolyte solution near the highly charged surface (i.e. in saturation regime) may be substantially decreased due to orientational ordering of water (saturation effect) and depletion of water molecules (excluded volume effect) due to accumulation of counterions.

Langevin Poisson-Boltzmann equation: point-like ions and water dipoles near a charged surface

Water ordering near a charged membrane surface is important for many biological processes such as binding of ligands to a membrane or transport of ions across it. In this work, the mean-field Poisson-Boltzmann theory for point-like ions, describing an electrolyte solution in contact with a planar charged surface, is modified by including the orientational ordering of water. Water molecules are considered as Langevin dipoles, while the number density of water is assumed to be constant everywhere in the electrolyte solution. It is shown that the dielectric permittivity of an electrolyte close to a charged surface is decreased due to the increased orientational ordering of water dipoles. The dielectric permittivity close to the charged surface is additionally decreased due to the finite size of ions and dipoles.

Enhanced Orientational Ordering of Water Dipoles in Uniaxially Stretched Hydrogels

The electrostatics and density correlations of dipolar solvent molecules in weakly charged polyelectrolyte solutions and charged gels are studied using a field-theoretical approach. For miscible dipolar solvent mixtures, an exact expression for the effective dielectric permittivity is obtained on the mean-field level, which depends upon the individual volume of each solvent species before mixing and the final volume of the mixture. If the effect of volume change is small during mixing, the dielectric permittivity is approximately equal to the volume-averaged dielectric permittivity of the individual components of the dipolar solvent mixture. It is found that the electrostatic interaction between the dipolar solvent molecules or any charged species in the polyelectrolyte solution is screened by both counterions (and co-ions if salt is added) and polyions, and there exists a total screening length for any charged species in the system which contains the contribution from the polyions. The screening from the polyions is as important as that from the counterions on the length scale of the polymer coil size. For uniaxially stretched charged gels, it is shown that there is an enhanced orientational ordering of the dipolar solvent molecules along the stretching direction, and the correlation length (r(cor)) of this enhanced orientational ordering is about the product of the total screening length (xi(-1)) due to the counterions and polyions times the square root of the dielectric constant of the medium, that is, r(cor)approximately equal to (epsilon(r)xi(-1))1/2.