Macroions in salty water with multivalent ions: giant inversion of charge

Screening of a strongly charged macroion by oppositely charged colloidal particles, micelles, or short polyelectrolytes is considered. Because of strong lateral repulsion such multivalent counterions form a strongly correlated liquid at the surface of the macroion. This liquid provides correlation-induced attraction of multivalent counterions to the macroion surface. As a result even a moderate concentration of multivalent counterions in the solution inverts the sign of the net macroion charge. We show that at high concentration of monovalent salt the absolute value of inverted charge can be larger than the bare one. This giant inversion of charge can be observed in electrophoresis.     

Screened electrostatics of charged particles on a water droplet

We study the electrostatic properties of charged particles trapped at an interface in a water-in-oil microemulsion. The electrostatic potential and the counterion distribution in the water droplet are given in terms of the ratio of the Debye screening length κ^-1 and the droplet radius R. In the limit R→∞ we recover the well-known results for a flat interface. Finite-size corrections are obtained in terms of the small parameter 1/κR. Part of the counterions spread along the interface and form a charged layer of one Debye length thickness. In particular, there is a uniform surface charge contribution. We derive explicit expressions for the electric field, the mobile charge density, and the charge-induced pressure on the interface.     

On the determination of vertical profiles of temperature, charge concentration, and potential in the surface layer of an aircraft

A “3/2 law” law has been established for the vertical charge concentration profile (i.e., the particle concentration is proportional to temperature to a power of 3/2). The expression is derived for the electric field potential produced by charged particles at the upper boundary of the viscous sublayer, as well as for the density current produced by charged particles moving in the air flow streamlining an aircraft. It is shown that all these parameters increase with altitude and assume maximum values at the turbulent layer surface.     

随活性剂浓度变化的分离压对垂直液膜排液过程的影响

针对含不溶性活性剂的垂直液膜排液过程,基于文献实验结果进一步完善了受活性剂浓度影响的分离压(disjoining pressure)模型,应用润滑理论建立了液膜厚度、活性剂浓度和液膜表面速度的演化方程组,通过数值计算分析了在不同分离压作用下含不溶性活性剂液膜的演化特征.结果表明,垂直液膜的排液过程通常经历两个阶段:首先是厚膜阶段,此时重力对排液过程起主导作用.在随后的薄膜阶段,毛细作用和分离压作用影响逐渐增大,其中分离压将控制液膜的演化历程.分离压对垂直液膜排液过程的影响与活性剂类型及活性剂浓度与静电作用力的关联强度密切相关.当分离压与活性剂浓度正相关时,随斥力关联系数α增大,液膜的排液和变薄过程得以减缓,由此增强了液膜稳定性;当分离压与活性剂浓度负相关时,随斥力关联系数α绝对值增大,液膜排液过程加速,由此加大液膜失稳的风险.     

Non-monotonic swelling of a macroion due to correlation-induced charge inversion

It is known that a large, charged body immersed in a solution of multivalent counterions may undergo charge inversion as the counterions adsorb to its surface. We use the theory of charge inversion to examine the case of a deformable, porous macroion which may adsorb multivalent ions into its bulk to form a three-dimensional strongly-correlated liquid. This adsorption may lead to non-monotonic changes in the size of the macroion as multivalent ions are added to the solution. The macroion first shrinks as its bare charge is screened and then reswells as the adsorbed ions invert the sign of the net charge. We derive a value for the outward pressure experienced by such a macroion as a function of the ion concentration in solution. We find that for small deviations in the concentration of multivalent ions away from the neutral point (where the net charge of the body is zero), the swollen size grows parabolically with the logarithm of the ratio of multivalent ion concentration to the concentration at the neutral point.     

Self-Diffusion of Ionic Surfactants and Counterions in Premicellar and Micellar Solutions of Sodium,Lithium and Cesium Dodecyl Sulfates as Studied by NMR-Diffusometry

Self-diffusion of surfactants and counterions in premicellar and micellar solutions of sodium, lithium and cesium dodecyl sulfates has been examined by nuclear magnetic resonance (NMR)-diffusometry. Self-diffusion of surfactants obeys as a whole the well-known two-site exchange model in which surfactants diffuse as micelles and non-micellized molecules in monomer and dimer forms. To explain self-diffusion of counterions, the model which takes into account their diffusive motion about the surface of micelles is proposed. It is shown that this phenomenon contributes considerably to charge transfer in micellar solutions. Estimation of surface diffusion of counterions about the micellar surface is made on the basis of experimental results on self-diffusion of surfactants and counterions, the data obtained on critical micelle concentration and the degree of counterion binding.     

Investigation of scaling laws in a turbulent boundary layer flow with adverse pressure gradient using PIV

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient at Reynolds number up to Re_θ = 10, 000. We combine large-scale particle image velocimetry (PIV) with microscopic PIV for measuring the near wall region including the viscous sublayer. We investigate scaling laws for the mean velocity and for the total shear stress in the inner part of the boundary layer. In the inner part the mean velocity can be fitted by a log-law. In the outer part of the inner layer the log-law ceases to be valid. Instead, a modified log-law provides a good fit, which is given in terms of the pressure gradient parameter and a parameter for the mean inertial effects. Finally we describe and assess a simple quantitative model for the total shear stress distribution which is local in wall-normal direction without streamwise history effects.     

A Theoretical Model to Explain the Mechanism of Electromagnetic Wave Propagation Along Cylindrical Micelles

A theoretical model to explain the mechanism of the electromagnetic wave propagation in the quasi two-dimensional layer of counterions adjacent to the surface of a charged cylindrical membrane is presented. By using Maxwell and hydrodynamic equations with appropriate boundary conditions, general expression of dispersion relation is obtained for the electromagnetic wave with mixed TE and TM modes.     

Model of a Surface Liquid-Like Layer of Ice

The previously developed model of an intermediate phase of ice with a liquid system of hydrogen bonds has been applied to describe a quasiliquid surface layer. This approach leads to a two-component model of a quasiliquid ice surface layer. In the outer part of the layer, both the proton and oxygen sublattices are melted, constituting water or the Thomson sublayer. In the inner part of the layer, only the proton sublattice is melted, whereas the oxygen sublattice holds its structure (the liquid state of the system of hydrogen bonds or the Faraday sublayer). The proposed model gives correct-in-magnitude estimates of various physical characteristics of the layer, explains the contradiction between the Faraday and Thomson hypotheses, and is consistent with recent experimental results and numerical studies.     

强流脉冲电子束诱发温度场及表面熔坑的形成

在强流脉冲电子束表面改性实验的基础上，通过数值计算方法对铝和钢的温度场和熔化过程 进行模拟，给出了最先熔化的位置、形成熔孔的最大深度以及表面层下熔化的最大深度. 通 过铝和钢的熔化潜热温度补偿的数值模拟结果和实验结果的对比，揭示了亚表层率先升温及 熔化从而通过表层向外喷发的熔坑的形成机制. 关键词： 强流脉冲电子束 热传导 数值模拟 熔孔     

A Theoretical Model to Explain the Mechanism of Electromagnetic Wave Propagation Along Cylindrical Micelles

A theoretical model to explain the mechanism of the electromagnetic wave propagation in the quasi two-dimensional layer of counterions adjacent to the surface of a charged cylindrical membrane is presented. By using Maxwell and hydrodynamic equations with appropriate boundary conditions, general expression of dispersion relation is obtained for the electromagnetic wave with mixed TE and TM modes.     

Charged double layer at the boundary between a symmetric plasma and a wall

It is shown that the features of formation and structure of a stationary charged layer at the interface between a symmetric plasma and a wall depend to a considerable extent on the ratios τ_± (for both plasma components) of the thermal velocity to the velocity of the plasma flow at the beginning of the layer, as well as on the polarity and magnitude of electric potential φ_W of the wall. The conditions under which a double layer-type structure can be formed in the near-wall layer of the symmetric plasma are formulated. The charge double layer contains two sublayers with opposite polarities of charge for a fixed polarity of the wall. The charge polarity of sublayer adjoining the plasma is opposite to the polarity of the wall, while the sublayer adjoining the wall has a charge of the same polarity as that of the wall. The ranges of symmetric plasma parameters for which the double layer exists are determined. It is found that a structure of this type can be formed nor for all parameters of the plasma, for which a charged layer exists; the possibility of formation of this structure depends on τ_± as well as on the sign of the potential of the wall.     

Static polarizability effects on counterion distributions near charged dielectric surfaces: A coarse-grained Molecular Dynamics study employing the Drude model

Coarse-grained implicit solvent Molecular Dynamics (MD) simulations have been used to investigate the structure of the vicinal layer of polarizable counterions close to a charged interface. The classical Drude oscillator model was implemented to describe the static excess polarizability of the ions. The electrostatic layer correction with image charges (ELCIC) method was used to include the effects of the dielectric discontinuity between the aqueous solution and the bounding interfaces for the calculation of the electrostatic interactions. Cases with one or two charged bounding interfaces were investigated. The counterion density profile in the vicinity of the interfaces with different surface charge values was found to depend on the ionic polarizability. Ionic polarization effects are found to be relevant for ions with high excess polarizability near surfaces with high surface charge.     

Ground-state correlation energy of counterions at a charged planar wall: Gibbs-Bogoliubov lower-bound approach

Recent simulation results imply the lowering of the ground-state correlation energy per counterion at a charged planar wall, compared with that of the 2D and 3D one-component plasma systems. Our aim is to correctly evaluate the ground-state energy of strongly-coupled counterion systems by considering a quasi-2D bound state where bound counterions are confined to a layer of molecular thickness. We use a variational approach based on the Gibbs-Bogoliubov inequality for the lower-bound free energy so that the liquid-state theory can be incorporated into the formulations. The soft mean spherical approximation demonstrates that the lowered ground-state energy can be reproduced by the obtained analytical form of a quasi-2D bound state.     

Simple model for attraction between like-charged polyions

We present a simple model for the possible mechanism of appearance of attraction between like charged polyions inside a polyelectrolyte solution. The attraction is found to be short ranged, and exists only in the presence of multivalent counterions. It is produced by the correlations in layers of condensed counterions surrounding each polyion and is only weakly temperature dependent. We find the attraction to be maximum at zero temperature and dimish as the temperature is raised. The attraction is only possible if the number of condensed counterions exceeds the threshold, , where is the valence of counterions and Z is the polyion charge. Received 10 March 1999 and Received in final form 20 April 1999     

Off-centre charge model of the planar electric double layer for asymmetric 2:1/1:2 valencies

ABSTRACT

Grand canonical Monte Carlo simulation results are reported for the structure and capacitance of a planar electric double layer containing off-centre charged rigid sphere cations and centrally charged rigid sphere anions. The ion species are assigned asymmetric valencies, +2:?1 and +1:?2, respectively, and set in a continuum dielectric medium (solvent) characterised by a single relative permittivity. An off-centre charged ion is obtained by displacing the ionic charge from the centre of the sphere towards its surface, and the physical double layer model is completed by placing the ionic system next to a uniformly charged, non-penetrable, non-polarizable planar electrode. Structural results such as electrode-ion singlet distribution functions, ionic charge density and orientation profiles are complemented by differential capacitance results at electrolyte concentrations of 0.2?mol/dm³ and 1?mol/dm³, respectively, and for various displacements of the cationic charge centre. The effect of asymmetry due to off-centre cations and valency asymmetry on the double layer properties is maximum for divalent counterions and when the cation charge is closest to the hard sphere surface.     

Counterion distribution in a spherical charged sparse brush

The counterion distribution within a spherical polyelectrolyte sparse brush was measured by small-angle X-ray scattering using contrast variation with different counterions by means of ion dialysis. The brush was made by self-association of charged diblock copolymers. Thanks to the contrast variation method, we were able to separate the signal due to the monomers and the signal due to the counterions. At a small length scale, it is demonstrated that the system behaves as independent charged rods whose counterion distribution follows the Poisson-Boltzmann model. Received 14 February 2001 and Received in final form 2 May 2001     

Equation of state for a coarse-grained DPPC monolayer at the air/water interface

Pulmonary surfactant, a complex mixture of phospholipids and proteins, secreted by the type II epithelial cells in the lungs, is crucial to reducing the effort required for breathing. A lack of adequate amounts of pulmonary surfactant in underdeveloped lungs of premature infants results in infant respiratory distress syndrome (RDS). Surfactant replacement therapy (SRT) is the approved method of mitigating the effects of RDS. The development of new SRT regimens requires a fundamental understanding of the links between surfactant biochemistry and functionality. We use a coarse-grained (CG) model to predict the surface pressure–surface concentration relationship (equation of state) for pure DPPC, which is a major component of endogenous and synthetic pulmonary surfactant mixtures. We show that the model can be efficiently used to predict the equation of state in excellent agreement with experimental results. We also study the structure of the monolayer as a function of the surface tension of the system. We show that a decrease in the applied surface tension results in an increase in order in the head group region and a decrease in order in the tail region of DPPC. This technique may be useful in the prediction of equations of state for surfactant replacements.     

Effect of counterions on the swelling of spherical polyelectrolyte brushes

We investigate the swelling of colloidal spherical polyelectrolyte brushes in the presence of different counterions. The colloidal particles consist of a solid poly(styrene) core of ca. 100 nm diameter onto which linear polyelectrolyte chains are chemically grafted. Two types of polyelectrolyte chains have been used here: The cationic polyelectrolyte poly(2-(acryloyl)ethyltrimethylammonium chloride)) (PATAC) and the anionic poly(styrenesulfonate) (PSS). Both systems are dispersed in water and the degree of swelling of the surface layer is studied by dynamic light scattering. Adding more and more salt leads to a strong shrinking of the surface layer as expected for polyelectrolyte brushes. It is shown that data obtained at low ionic strength can be collapsed on suitable master curves for monovalent and divalent counterions, respectively. For some ions, however, high salt concentrations may lead to a re-swelling of the brush layer in case of the cationic systems. This points to specific interactions of the counterions with the PATAC chains. This strong specific interaction between the counterions and the attached polyelectrolyte may even lead to flocculation of the particles at intermediate salt concentration. Surprisingly, for iodide and magnesium counterions the solubility increases again if the salt concentration is raised to 1 mol/l. Hence, specific interaction leads to salting-out effects as well as to salting-in effects for these colloidal particles. All specific effects seen at high concentrations of added salt can be explained by the increase of the reduced excluded-volume parameter which is due to the adsorption of salt ions.     

The distribution of counterions around synthetic rod-like polyelectrolytes in solution

An investigation of the radial distribution of the counterions of a synthetic rodlike polyelectrolyte in aqueous solution is presented. The cationic polyelectrolyte used here has a poly(p-phenylene) backbone. For typical molecular weights the macroion comprises approximately one persistence length (ca. 20 nm) and effects of finite stiffness may be disregarded. Each repeating unit bears four charges which leads to a charge parameter of ξ = 6.65. The distribution of the iodide counterions around this highly charged macroion is studied by small-angle X-ray scattering (SAXS) in dilute aqueous solution. These investigations are supplemented by measurements using anomalous small-angle X-ray scattering (ASAXS) that furnishes additional information about the contrast of the macroion. Data taken at high scattering angles give indication for contributions caused by the longitudinal fluctuations of the counterions. After correction for this effect the experimental results are compared to intensities calculated by use of the Poisson-Boltzmann (PB)-cell model. It is found that the PB-cell model describes the corrected data at intermediate and high scattering angles. Deviations at low scattering angle are attributed to the mutual interaction of the rod-like polyelectrolyte that can be described in terms of an effective structure factor. Data taken at lowest scattering angles point to a weak attraction between the rod-like macroions. Received: 27 July 2001 and Received in final form 27 March 2002