Surface deprotonation

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

In the more alkaline pH-range surface-bound cations may increase the dissolution rate the surface deprotonation increases Coh (cf. Fig. 5.9c) and thus enhances the dissolution. This effect has been observed, above all, with silicates and glasses. 

The effects on the dynamics of photo-injected electrons where not systematically studied, despite scattered reports on the influence of amines, which induce surface deprotonation, and lower surface charge with a resulting negative shift in band edge position and an increase in the open circuit potential, Voc [103], The opposite effect is induced by Li+ ions, which intercalate in the oxide structure. Guanidinium ions increase Voc when used as counterions in place of Li+. Other adsorbing molecules that influence both Voc and short circuit current are polycar-boxylic acids, phosphonic acids, chenodeoxycholate and 4-guanidinobutyric acid. 

The explanation for the obscnre behavior observed in acidic media mnst be songht in the interplay between the dissolved silica and the particle surface. In alkaline solntions, the enhanced charging is evidently due to the overlapping of the surface deprotonation eqnilibrinm and the disintegration of the silica surface. This is observed as a rapid enhancement of the snrface charge density and a continuous enhancement of the potential. However, in acidic solntions the condensation reaction is most probably activated at pH < 6 initiating a competition with the proton adsorption equilibrium. As a result, the surface hydroxyls react with the dissolved silica instead of... 

The surface concentration of the particular surface species (which is equal to the concentration of the precursor of the activated complex) (Eq. 3) can usually be determined from the knowledge of the number of surface sites and the extent of surface protonation or surface deprotonation or the surface concentration of ligands. Surface protonation or deprotonation can be measured from alkali-metric or acidimetric surface titrations, and ligands bound to the surface sites can lie determined analytically, from the change in the concentration of ligands in solution. 

Abstract Enthalpy of charging of hematite surface was determined by calorimetry and also by measuring the dependence of the point of zero charge (p.z.c.) on temperature. The calorimetric experiment was designed in such a way that the point of zero charge was in the middle between the initial and final pH. In that case, the reaction heat corresponds to the difference in standard reaction enthalpies of surface deprotonation andprotonation reactions, without... 

Methylcyclohexanone, pK 20, is typical of a weak acid that undergo H/D exchange. Identify the acidic protons of 2-methylcyclohexanone, i.e., those most susceptible to attack by base, as positions for which the value of the lowest-unoccupied molecular orbital (LUMO) is large. Use a LUMO map (the value of the LUMO mapped onto the electron density surface). Does this analysis correctly anticipate which of the anions obtained by deprotonation of 2-methylcyclohexanone is actually most stable Are any of the other ions of comparable stability, or are they aU much less stable ... 

Figure 5. Cartoon models of the reaction of methanol with oxygen on Cu(llO). 1 A methanol molecule arrives from the gas phase onto the surface with islands of p(2xl) CuO (the open circles represent oxygen, cross-hatched are Cu). 2,3 Methanol diffuses on the surface in a weakly bound molecular state and reacts with a terminal oxygen atom, which deprotonates the molecule in 4 to form a terminal hydroxy group and a methoxy group. Another molecule can react with this to produce water, which desorbs (5-7). Panel 8 shows decomposition of the methoxy to produce a hydrogen atom (small filled circle) and formaldehyde (large filled circle), which desorbs in panel 9. The active site lost in panel 6 is proposed to be regenerated by the diffusion of the terminal Cu atom away from the island in panel 7.

Gold is generally considered a poor electro-catalyst for oxidation of small alcohols, particularly in acid media. In alkaline media, however, the reactivity increases, which is related to that fact that no poisoning CO-hke species can be formed or adsorbed on the surface [Nishimura et al., 1989 Tremihosi-Filho et al., 1998]. Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding to the onset of surface oxidation, emphasizing the key role of surface oxides and hydroxides in the oxidation process. The only product observed upon the electrooxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated form of acetic acid. The lack of carbon dioxide as a reaction product again suggests that adsorbed CO-like species are an essential intermediate in CO2 formation. 

Different orientations for the protonated and deprotonated species as well as a change of the surface coverage with the bulk aqueous pH can further complicate the problem. 

Although the contribution is rather small, the partial discharging of the anesthetics in membranes can be important in the mechanism of the anesthetic action. The most plausible mechanism can be summarized as only a small portion of the cationic species are neutralized (deprotonated) at the bilayer surface and the neutral species are deeply penetrated and widely distributed in the hydrophobic bilayer interior, while the cationic species still remain at the hydrophilic bilayer surface where the hydration is significant. 

Figure 1.15 AdsorptionofcysteineonAu l 1 0. Molecular model shows the deprotonated thiolate surface species, (a) Model of the reconstructed (1 x 2)-Au l 1 0 surface (b-d) show, respectively, dimers of L-cysteine, D-cysteine, and the two together characteristically rotated relative to the (1 1 0) azimuth. (Adapted with permission from Ref. [57], Copyright 2002, Macmillan Publishers Ltd.)...

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