Surface groups, protonable

Quantum Chemistry Surface groups Proton affinities... 

The photodecomposition of isopropyl alcohol on silica gel produces a seven-line spectrum having a hyperfine separation of 20.7 G and an amplitude ratio of 1 6.7 20.2 31 21.1 7.4 1.5 (68). This spectrum was attributed to SiOCMe2 formed from the ether surface groups. In addition to this spectrum the spectrum of the methyl radical was also observed. Irradiation of adsorbed tert-butyl alcohol produced a three-line spectrum which was attributed to SiOMe2OCH2 (68). Apparently the splitting from the methyl protons was too small to be observed. 

Attention was paid early on to solution pH, and in particular, to a surface — bulk proton balance. Various models of hydroxyl chemistry have been developed in colloid science literature [21], Perhaps the simplest and most common model assumes a single type of OH group and amphoteric behavior (i.e., one set of Kx and K2 from Figure 6.1). More complicated models invoke multiple OH groups and proton affinity distributions [22]. It will be demonstrated below that the simpler type has worked well for the revised physical adsorption (RPA) model. 

Hiemstra et al. (1989) have elaborated on a multisite proton adsorption model taking into account the various types of surface groups intrinsic log K values for the protonation of various types of surface groups can be estimated with this model. 

Similarly, surface protonation tends to increase the dissolution rate, because it leads to highly polarized interatomic bonds in the immediate proximity of the surface central ions and thus facilitates the detachment of a cationic surface group into the solution. On the other hand, a surface coordinated metal ion, e.g., Cu2+ or Al3+, may block a surface group and thus retard dissolution. An outer-sphere surface complex has little effect on the dissolution rate. Changes in the oxidation state of surface central ions have a pronounced effect on the dissolution rate (see Chapter 9). 

Different modifications of hydrous oxides, even if present in solution with the same surface area concentrations, are characterized by significantly different reactivities (e.g., dissolution rate). This depends above all on the different coordination geometry of the surface groups. For a given pH (on surface protonation) the reactivity of a Fem-center is likely to increase with the number of terminal ligands (Wehrli et al., 1990), i.e., groups such as -Fe-OH are less acid and react faster than... 

The monoprotic surface group model can be described in terms of the more familiar diprotic surface group model. In the diprotic model, the surface is thought of as an ensemble of Ng diprotic surface groups, which, under the condition of zero protonic charge, are occupied by Ns protons. 

Molecular modeling of PT at dense interfacial arrays of protogenic surface groups in PEMs needs ab initio quantum mechanical calculations. In spite of fhe dramafic increase in computational capabilihes, it is still "but a dream" to perform full ab initio calculations of proton and water transport within realistic pores or even porous networks of PEMs. This venture faces two major obstacles structural complexity and the rarity of proton transfer events. The former defines a need for simplified model systems. The latter enforces the use of advanced compufahonal techniques that permit an efficient sampling of rare evenfs. ... 

Venema, P. Hiemstra,T. Weidler, P.G. van Riemsdijk,W.A. (1998) Intrinsic proton affinity of reactive surface groups of metal (hy-dr)oxides Application to iron (hydrjoxides. J. Colloid Interface Sci. 198 282-295 Venenna, P. Hiemsta, T. Van Riemsdijk,W.A. 

Another possible mechanism is the ionization or dissociation of a surface group (e.g., dissociation of a proton from a carboxylic group, namely, —COOH - —COO- + H +, which leaves the surface with a negative charge). 

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