Surface charging and points of zero charge of less common oxides: Beryllium oxide
Zulkifly Abbas. Surface charge density versus pH curves for spherical metal oxide nanoparticles in the size range of nm are calculated at various concentrations of a background electrolyte. The surface charge density of a nanoparticle is found to be highly size-dependent. As the particle diameter drops to below 10 nm there is considerable increase in the surface charge density as compared with the limiting values seen for particles larger than 20 nm.
This increase in the surface charge density is due to the enhanced screening efficiency of the electrolyte solution around small nanoparticles, which is most prominent for particles of diameters less than 5 nm. For example, the surface charge densities calculated for 2 nm particles at 0. These predictions of the dependence of surface charge density on particle size by the CDH-SC theory are in very good agreement with the corresponding results obtained by the MC simulations.
A shift in the pH value of the point of zero charge toward higher pH values is also seen with a decreasing particle size. Introduction Furthermore, it was successfully exploited to synthesize nano- particles of narrow size distribution at pH values farther from Metal oxide nanoparticles are used in a wide range of the pH value at the point of zero charge pHPZC. It is a well-established weathering processes, acid rain drainage, and nucleation from fact that the optical, electronic, and catalytic properties of metal melts and also by exclusion reactions within minerals and oxide nanoparticles are highly size-dependent.
The charged particles in turn less than 20 nm.
There coordinated surface atoms leading to substantially more reactive are some experimental studies in which the surface charging of surface sites. For example, Yamanaka29 reported an enhanced applications. Sonnefeld28 reported higher surface charge This was rationalized by Jolivet et al.
Ridley et al. Thus, it is not the surface tension.
Archive ouverte HAL - Size dependent surface charging of nanoparticles
The latter is an attractive approach to explain possible to infer a general trend from the above-mentioned the still not well understood metastability of nanoparticles. E-mail: zareen chem. Tel: Traditionally, the surface charging of metal oxide particles Fax: In the SC approach, C, Vol. It was found that lateral approach are exclusively based on the Gouy-Chapman GC counterion interactions are necessary to produce the correct model of an electric double layer.
Labbez et al. This SC model has been extensively used to of the calcium silicate hydrate C-S-H and the surface charge interpret the experimental surface charge35,38 as well as ion densities obtained by MC simulation in the framework of adsorption data on various metal oxides.
The silanol groups tion of the GC model can be significantly violated due to the have the same intrinsic dissociation constant Ka0. The latter curvature of the double layer. This problem encountered in was set equal to the first dissociation constant of silicic acid, applying the SC approach to model the surface charging of TiO2 i.
The titration curves were obtained by applying particles less than 10 nm in diameter has been noted by Ridley a grand canonical titration method that enables the calculations et al. In this MC approach, the parameters are the ing the surface charge densities of spherical particles. Thus, from nonlinear PB equation. It was found that with decreasing particle a survey of the literature, we can conclude that the effect of radius the surface charge density increases, but this effect is particle size on surface charging has not been explored in detail small for particles larger than 10 nm in diameter.
Loeb et al. SC 51,52 was developed and successfully applied to predict the Those results reported by Lyklema,41 see Figure 3. Lyklema41 has a different approach as compared to the traditional pKa-based put forward an explanation for such effects; i. Since a nanoparticle in the CDH-SC An important shortcoming of the approaches based on the PB theory is considered to be a sphere, the surface charging of theory is that counterions are considered as point charges. This particles of any size can be predicted by the CDH-SC theory leads to the fact that at sufficiently high charge densities the See Section 2 for details.
In this paper we present the results predicted concentration of counterions near a charged surface of the CDH-SC theory for the surface charging of nanoparticles can be unrealistically high. The Theoretical approaches such as molecular statistics, simulations will serve as a kind of numerical experiment molecular dynamics MD ,47 and Monte Carlo MC 48,49 simula- allowing the predictions of the CDH-SC theory to be tested and tions have also been applied to calculate the surface charging validated.
Rustad and co-workers successfully modeled the experimental surface charge data of goethite43,44 and hematite45,46 particles by molecular statistics calculations. Acidities and proton affinities of surface sites were considered as a sphere having only one type of surface site with first calculated in the gas phase and then converted into solution the sites randomly distributed on the particle surface.
Charging pKa values by using a linear free-energy relationship. In order of an oxide surface is described by a model of the proton to obtain surface charge densities, the basic Stern model of the interaction with the surface sites, in this case a hydroxyl group. MD simulations of the proto- The proton binding at such a site is described by a simple 3D nation of goethite modeled as a planar surface as well as particles harmonic oscillator model in which the interacting proton has of size range nm have shown an enhanced accumulation a specific frequency and interaction energy.
The hydration of of protons around nanoparticles as compared with that of the an oxide surface is described by a layer of water molecules planar surface. As described in our previous paper,52 the Zarzycki48 performed MC simulations for proton and ion best fits to the experimental surface charge data of goethite adsorption on an energetically heterogeneous oxide surface.
In particles were obtained when the thickness of the capacitor layer this MC simulation technique, attractive or repulsive interactions was set equal to 0. The relationship between capacitance between the occupied sites were introduced by a multistate Ising C and thickness of this layer in the CDH-SC theory is given model using a potential introduced by Borkovec. Surface Charging of Nanoparticles J.
This choice is supported by a recent work permittivity of the solvent, and d is the counterion diameter, in which a similar MC approach considering discontinuity i. Thus, a 0. Thus, colloid can be approximated The screening of a charged surface in an electrolyte solution with a uniform high dielectric constant equal to that of the is described by the formation of a diffuse double layer and, at solvent. In the MC approach used in this study the entire system, high surface charge densities, by a condensed counterion IC i. The formation of the IC layer is based spherical cell of radius Rc.
Given the elementary charge e the on a physically sound criterion; i. In the IC layer, ion correlation effects are accounted electrostatic potential energy for via excluded volume due to the finite size of the counterions as well as by a corresponding hole correction to the electrostatic repulsion energy of the counterions. A consequence of the formation of IC layer is that the apparent bare surface charge is as well as short-range hard-core repulsion reduced due to the screening by counterions.
The properties of charged In addition, a one-body external field accounts for the nano- particles immersed in an electrolyte solution, such as the surface particle excluded volume and imposes the cell boundary charge density, effective charge, and thickness of the IC layer, constraint with the boundary acting as a hard wall. In these calculations, only the particle size is varied and the rest of the parameters are exactly the same as in and our previous paper in which surface charge densities of nm goethite particles were calculated in 0. One can note from the last equation that all ions and sites by a capacitor layer of thickness 0.
All calculations by eq 2. Monte Carlo MC Simulations between one ion i and the homogeneously charged surface particle p of total charge Zpe can be expressed by the 3. The Model. For the sake of using same model as that in electrostatic potential energy the CDH-SC theory, in the MC simulations the nanoparticle is considered as a rigid hard sphere with radius Rp bearing implicit sites that gives a smeared out surface charge density.
Hydration of the particle is included via a 0. The electrostatic interactions described above define the Hamiltonian, which forms the basis for a MC simulation of a nanoparticle in salt solution. The standard Figure 1. Surface charge densities predicted by the CDH-SC theory Metropolis algorithm55 is used for ion moves while the are shown for goethite nanoparticles of various diameters immersed in 1 M electrolyte. The model and parameters used in these calculations nanoparticle is kept fixed at the center of the simulation cell.
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