Charge distribution model, adsorption

In the recent CD [94] (charge distribution) model the charge of the specifically adsorbed ions is distributed between the surface plane and another electrostatic plane (whose distance from the surface corresponds roughly to the /3-plane distance), thus, the center of charge of specifically adsorbed ions is located between these two planes. The CD concept was originally introduced to model adsorption of... 

FIGURE 12.15 The adsorption of PO4" by monodomainic goethite used by Hiemstra and Van Riemsdijk (1996) and that of Bowden et aL (1980). The lines are calculated with the charge distribution model and are consistent with the surface species observed by cylindrical internal reflection-Fourier transform infrared spectroscopy. (Reprinted from Journal of Colloid and Interface Science, 179, Hiemstra, T. and Van Riemsdijk, W. H. 488-508. Copyright 1996, with permission from Elsevier.)... 

The distribution of charges on an adsorbate is important in several respects It indicates the nature of the adsorption bond, whether it is mainly ionic or covalent, and it affects the dipole potential at the interface. Therefore, a fundamental problem of classical electrochemistry is What does the current associated with an adsorption reaction tell us about the charge distribution in the adsorption bond In this chapter we will elaborate this problem, which we have already touched upon in Chapter 4. However, ultimately the answer is a little disappointing All the quantities that can be measured do not refer to an individual adsorption bond, but involve also the reorientation of solvent molecules and the distribution of the electrostatic potential at the interface. This is not surprising after all, the current is a macroscopic quantity, which is determined by all rearrangement processes at the interface. An interpretation in terms of microscopic quantities can only be based on a specific model. 

Charge distribution multisite complexation model (CD-MUSIC) A surface complexation model for explaining ion adsorption on the surfaces of adsorbents. Hiemstra and van Riemsdijk (1999) used the model to explain the adsorption of arsenate oxyanions on goethite. 

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

However, the possibilities to adjust the course of model uptake curves by the selection of the simple model of specific adsorption (electrostatic position and the number of protons released per one specifically adsorbed cation or per one molecule of weak acid) are limited. It should be emphasized that the CD model was introduced quite recently, and in most publications summarized in Tables 4.1 and 4.2 only the choice between two electrostatic positions (inner- and outer-sphere) was considered. Thus, the ability to find a simple model properly simulating the actual uptake curves was even more limited than nowadays. But even when the charge distribution concept is taken into account, it often happens that the simple models fail to properly reflect the experimentally observed effects of the pH and ionic strength on the specific adsorption. This problem has been solved by... 

Weerasooriya. R., Aluthpatabendi, I), and Tobschall, H.J., Charge distribution multi-site complexation (CD-MUSIC) modeling of Pb(II) adsorption on gibbsite. Colloids Surf. A. 189, 131, 2001. 

Weng, L. et al.. Ligand and charge distribution (LCD) model for the description of fulvic acid adsorption to goethite, J. Colloid Interf. Sci., 302,442, 2006. 

The pioneering finite temperature Monte Carlo study [282] of the orientational disordering of commensurate and incommensurate monolayers of CO on graphite was based on empirical potentials and 64 molecules in a rectangular periodic cell. The CO-surface interactions were modeled with the Fourier representation [324, 326], and the necessary Lennard-Jones parameters were obtained from a fit to the measured isosteric heats of adsorption [287]. The nonelectrostatic CO-CO intermolecular interactions were based on Buckingham-type potentials as parameterized in Refs. 238 and 287. The electrostatics was represented by a three-site point-charge distribution located on the molecular axis [282]. The chosen values yield a reasonable representation of the moments up to the hexadecapole the dipole moment, however, is larger than the experimental value. These interactions... 

Theoretical methods are required to derive structural information from spectroscopic data, which usually concern measurements of electronic features. Because of the availability of large and efficient computer power and the current state of the art of theoretical chemistry, electronic structure calculations on model systems of relevance to experimental studies can be made. In addition, the catalytic chemist needs insight into the factors that determine the transition-state potential energy surface of reacting molecules. Also methods are needed to predict the geometry of the adsorption site as a function of metal surface composition or charge distribution in the zeolite. These methods will be extensively discussed in the next chapters. 

Spectroscopy has provided a progressive flow of information concerning the binding mechanism(s) of ions and its complex structure. Qualitative and quantitative information from spectroscopy can be included in ion adsorption modelling, connecting molecular details to macroscopic measurements. Various examples of the application of the charge distribution approach to the adsorption of cat- and anions are given. 

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