Aqueous-phase solvation water dissociation

Nanoparticles have different morphologies than flat, bulk surfaces. Perez et al. have considered the activation of water and COads + OHads reactions on Pt and PtRu clusters including the effects of solvation." They found that the presence of under-coordinated Ru adatoms on the Pt cluster surfaces enhances the production of OHads from water compared to Ru alloyed into the nanoparticle surfaces. More significantly, they found that the presence of an aqueous environment simulated by up to six water molecules dramatically stabilized the transition state and products of the reactions. For example, in a gas-phase environment they calculated a water dissociation barrier of 20 kcal/mol whereas in the solvated environment the barrier was reduced to 4.5 kcal/mol on the alloy surface. The barrier for water dissociation on the Ru adatom in the aqueous environment was only 0.9 kcal/mol. Although their results are for an adatom on a near flat (111) surface, they may have significance in describing the catalytic properties of undercoordinated Ru atoms at edge and corner sites on nanoparticles. 

The increase in Kow for unsaturated phases reflects a decreasing dioxin content with time in the water phase. This is thought to be due to the transfer of octanol solvated dioxin into the water phase at the start of the equilibration while the two phases are in the process of reaching mutual saturation. The octanol solvate would be unstable in the aqueous phase, however, preferring to dissociate into octanol and water solvated dioxin. A mean value of the partition coefficient was calculated from measurements in which the systems were thought to be at equilibrium. Table VI compares our mean experimental value with other values found in the literature. 

Following a similar approach, a Born-Haber cycle can be used to approximate the ability of other transition metal surfaces to activate water in the aqueous phase from the energetics of water activation in the vapor phase. This is quite useful since the vapor phase calculations are much less computationally intensive. The Born-Haber cycle for such a reaction scheme is given in Figure 19.3. The heterolytic activation of water over a metal surface is directly tied to the homolytic dissociation of water (Eq. 19.1) on that surface and the ease with which it can form protons from adsorbed hydrogen (Eq. 19.3). The specific steps in the Born cycle presented in Figure 19.3 include (1) the dissociation of H2O in the vapor phase to form OH(avapor phase)], (2) the desorption of H(ad) into the gas phase as H- [AE = Eb(n gas phase)], 0) the ionization of H to form H+ + e [A = E(h. ionization)], (4) the solvation of H [AE = E (h+solvation)], and (5) the capture of the electron by the metal surface [AE = — ]. The overall reaction energy for heterolytic aqueous-phase water activation, A , . , (aqueous phase), is ... 

The H entry into a metal fiom an aqueous electrolyte is believed to involve the same surface-bulk transfer step as in the gas phase, but the preliminary adsorption step is a more complex process because more H sources are involved in aqueous solution, allowing more possible H surface reactions, and also because of the specificity of the electrolyte-metal interface. Whereas H adsorption in the gas phase occurs by dissociative adsorption of gaseous H2 on the free sites of a bare metallic surface, H adsorption in aqueous solution may occur either chemically by dissociation of dissolved H2 or electrochemically from solvated (hydrated) protons or water molecules it takes place on a hydrated surface and thus implies the displacement of adsorbed water molecules or specifically adsorbed ions and local reorganization of the double layer [20] competition with the adsorption of oxygen species formed from the dissociation of water may also occur [21-23], The adsorbed H layer is also in interaction with surrounding water molecules, i.e., it is hydrated [8c,24,25],... 

Substituting in equation 11 the known experimental parameters for phenol dissociation (AG, = 13.8 kcalmol" calculated from the ground-state equilibrium constant, pX, = 10.0), AGt((PhO ) — (PhOH)) of the phenolate/phenol system is about —76 kcalmoH, which is about 10% less than the accepted value for the electrostatic solvation energy of the chloride anion in water, AGe(Cr) = —85 kcalmol". These simple considerations imply that the AGt((PhO ) — (PhOH)) contribution to the overall free energy of solvation is largely electrostatic, and that relatively small differences in the gas-phase proton affinity of the base and in specific solvent-solute interactions of the photoacid and the base determine the relatively narrow (in free-energy units) acidity scale in aqueous solution. It... 

It should also be mentioned that the Gibbs energy of ion transfer can be affected by complexation phenomena at the liquid liquid boundary. A classical example is given by the work of Koryta [25], who studied the transfer of from water to nitrobenzene assisted by dibenzo-18-crown-6. The complexation step decreases the energy of solvation of the cation in the organic phase, decreasing the formal transfer potential as defined in Eq. (5). Various mechanisms have been proposed for assisted ion-transfer processes, namely, aqueous complexation followed by transfer, transfer followed by complexation in the organic phase, transfer by interfacial complexation, or transfer by interfacial dissociation [26,27]. 

The hydrogen ion H" " cannot exist as a free species in condensed phases its hydration has long fascinated chemists and physicists. Existence of the hydrated proton was first postulated to explain the catalytic effect of the proton in esterification and later to rationalize the conduction of aqueous sulphuric acid solutions , The concept of electrolytic dissociation and consequent conduction in aqueous solutions is a forerunner of the modern notion of the salts themselves as solid electrolytes in the absence of any solvating medium. The parallel is particularly clear for strong mineral acid hydrates where several acid/water compositions of ionic character exist, many of which are proton conducting, and in which proton hydrates and H502 have been identified . 

Compounds with the ionic bond (salts) that form in the solid state the ion crystalline lattice dissociate to ions. Being dissolved, acids and bases undergo complete or partial dissociation where a noticeable chemical interaction of ions with solvents occurs. Each ion in the solvent, e.g., in water, is surrounded by the dense solvate shell of polar molecules. This shell appears due to the ion-dipole interaction. Solvation is manifested, first, in that the dissolution of a salt in H2O is accompanied by a decrease in the volume and, second, liberation of a great amount of heat. This is seen from the AH values where the ion from the gas phase is transfoied to an aqueous solution (A//Li+ = A/fp-, AH in kj/mol)... 

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