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Journal ArticleDOI

Effects of counterion size on the attraction between similarly charged surfaces.

24 Nov 2010-Journal of Chemical Physics (American Institute of Physics)-Vol. 133, Iss: 20, pp 204901-204901
TL;DR: The Wang-Landau sampling Monte Carlo (MC) simulation method is applied and it is found that for large Ξ and small ion radius, there is a global equilibrium distance D̃=D̃(eq)=2(1+R̃), correctly giving the expected value at the point counterion limit.
Abstract: Interaction between similarly charged surfaces can be attractive at high electrostatic coupling constants Ξ = l(B)Z(2)/μ(GC), where l(B) is the Bjerrum length, μ(GC) the Gouy-Chapman length, and Z the valency of counterions. While this effect has been studied previously in detail, as a function of surface charge density and valency of the pointlike counterions, much less is known about the effect of counterion size. We apply the Wang-Landau sampling Monte Carlo (MC) simulation method to compute the free energy F as a function of the scaled distance between the plates D=D/μ(GC) for a range of Ξ and scaled counterion radii R=R/μ(GC). We find that for large Ξ and small ion radius, there is a global equilibrium distance D=D(eq)=2(1+R), correctly giving the expected value at the point counterion limit. With increasing R the global minimum in F(D) changes to a metastable state and finally this minimum vanishes when R reaches a critical value, which depends on Ξ. We present a state diagram indicating approximate boundaries between these three regimes. The Wang-Landau MC method, as it is applied here, offers a possibility to study a wide spectrum of extended problems, which cannot be treated by the use of contact value theorem.

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Citations
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Posted Content
TL;DR: This two-part series considers steric effects on diffuse charge dynamics (in the absence of electro-osmotic flow) and two simple models for the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage.
Abstract: The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatic forces. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt, which are often exceeded, e.g. in microfluidic pumps and electrochemical sensors. Since the 1950s, several modifications of the PB equation have been proposed to account for the finite size of ions in equilibrium, but in this two-part series, we consider steric effects on diffuse charge dynamics (in the absence of electro-osmotic flow). In this first part, we review the literature and analyze two simple models for the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance typically varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. In PB theory, the capacitance blows up exponentially with voltage, but steric effects actually cause it to decrease above a threshold voltage where ions become crowded near the surface. Other nonlinear effects in PB theory are also strongly suppressed by steric effects: The net salt adsorption by the double layers in response to the applied voltage is greatly reduced, and so is the tangential "surface conduction" in the diffuse layer, to the point that it can often be neglected compared to bulk conduction (small Dukhin number).

603 citations

Journal ArticleDOI
TL;DR: The solvation of biomolecules with a computational biophysics view toward describing the phenomenon is discussed, and the main focus lies on the computational aspect of the models.
Abstract: An understanding of molecular interactions is essential for insight into biological systems at the molecular scale. Among the various components of molecular interactions, electrostatics are of special importance because of their long-range nature and their influence on polar or charged molecules, including water, aqueous ions, proteins, nucleic acids, carbohydrates, and membrane lipids. In particular, robust models of electrostatic interactions are essential for understanding the solvation properties of biomolecules and the effects of solvation upon biomolecular folding, binding, enzyme catalysis, and dynamics. Electrostatics, therefore, are of central importance to understanding biomolecular structure and modeling interactions within and among biological molecules. This review discusses the solvation of biomolecules with a computational biophysics view toward describing the phenomenon. While our main focus lies on the computational aspect of the models, we provide an overview of the basic elements of biomolecular solvation (e.g. solvent structure, polarization, ion binding, and non-polar behavior) in order to provide a background to understand the different types of solvation models.

174 citations


Additional excerpts

  • ...…2005 ; Wen & Tang, 2004) and like-charge attraction (Angelini et al., 2003 ; Kim et al., 2008 ; Mukherjee, 2004 ; Netz & Naji, 2004 ; Pietronave et al., 2008 ; Podgornik & Dobnikar, 2001 ; Qiu et al., 2010 ; Todd et al., 2008 ; Zelko et al., 2010) that can be important for highly charged systems....

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Journal ArticleDOI
TL;DR: It is suggested that osteoblasts are most strongly bound along the sharp convex edges or spikes of nanorough titanium surfaces where the magnitude of the negative surface charge density is the highest and it is plausible that nanorough regions of titanium surfaces with sharp edges and spikes promote the adhesion of osteoblast.
Abstract: This work considers the adhesion of cells to a nanorough titanium implant surface with sharp edges. The basic assumption was that the attraction between the negatively charged titanium surface and a negatively charged osteoblast is mediated by charged proteins with a distinctive quadrupolar internal charge distribution. Similarly, cation-mediated attraction between fibronectin molecules and the titanium surface is expected to be more efficient for a high surface charge density, resulting in facilitated integrin mediated osteoblast adhesion. We suggest that osteoblasts are most strongly bound along the sharp convex edges or spikes of nanorough titanium surfaces where the magnitude of the negative surface charge density is the highest. It is therefore plausible that nanorough regions of titanium surfaces with sharp edges and spikes promote the adhesion of osteoblasts.

168 citations


Cites background from "Effects of counterion size on the a..."

  • ...The origin of this effect may be bridging and direct interaction forces.(22,29,34,35) Accordingly, it was indicated recently that an increase in the negative net charge of a titanium surface promotes the fibronectin-mediated binding of osteogenic cell receptors....

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  • ...However, since the surface charge density at very sharp edges of a nanorough titanium surface could be very high, due to the dramatic increase of the corresponding coupling constant, monovalent cations could also not be excluded as bridging ions.(34) To conclude, our suggestion is that the increased surface charge density and corresponding electric field strength at the highly curved edges of a nanorough titanium surface is important for the efficient adhesion of cells....

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Journal ArticleDOI
G. S. Manning1
TL;DR: The suggestion is that the DNA is attracted to the virtual polyelectrolytes that may be located in the nanoslit where floor meets walls, and the detailed calculations needed to document this suggestion are presented.
Abstract: There is abundant experimental evidence suggesting the existence of attractive interactions among identically charged polyelectrolytes in ordinary salt solutions. The presence of multivalent counterions is not required. We review the relevant literature in detail and conclude that it merits more attention than it has received. We discuss also some recent observations of a low ionic strength attraction of negatively charged DNA to the region of a negatively charged glass nanoslit where the floor of the nanoslit meets the walls, again in the absence of multivalent ions. On the theoretical side, it has become clear that purely electrostatic interactions require the presence of multivalent counterions if they are to generate like-charge attraction. Any theory of like-charge attraction in the absence of multivalent counterions must therefore contain a non-electrostatic component. We point out that counterion condensation theory, which has predicted like-charge polyelectrolyte attraction in an intermediate range of distances in ordinary 1:1 salt conditions, contains both electrostatic and non-electrostatic elements. The non-electrostatic component of the theory is the modeling constraint that the counterions fall into two explicit populations, condensed and uncondensed. As reviewed in the paper, this physically motivated constraint is supported by strong experimental evidence. We proceed to offer an explanation of the nanoslit observations by showing in an idealized model that the line of intersection of two intersecting planes is a virtual polyelectrolyte. Since we have previously developed a counterion condensation theory of attraction of two like-charged polyelectrolytes, our suggestion is that the DNA is attracted to the virtual polyelectrolytes that may be located in the nanoslit where floor meets walls. We present the detailed calculations needed to document this suggestion: an extension of previous theory to the case of polyelectrolytes with like but not identical charges; the demonstration of counterion condensation on a plane with bare charge density greater than an explicitly exhibited critical value; a calculation of the free energy of the plane; a calculation of the interaction of a line charge polyelectrolyte with a like-charged plane; and the detailed demonstration that the line of intersection of two planes is a virtual polyelectrolyte.

117 citations

Journal ArticleDOI
TL;DR: In this article, the Langevin-Poisson-Boltzmann (LPB) and Langevin -Bikerman (LBP) models were used to model the electrolyte-charged surface interface and the space dependence of relative permittivity.

63 citations

References
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Journal ArticleDOI
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Journal ArticleDOI
TL;DR: The thermodynamic consequences of electrostatic correlations in a variety of systems ranging from classical plasmas to molecular biology are reviewed.
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988 citations