Oxides, surface charge

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

While surface charge is by no means the only factor responsible for electrolyte adsorption (94), particularly organic electrolytes (9, 27), the extent of adsorption of the less specifically adsorbed species, such as the simple aquo ions and, for example, primary amines and alkyl sulfonates, decreases rapidly when the sign of the oxides surface charge is changed to that of the sorbing species (6, 10). 

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

The samples taken to the experiments were not purified with the method advised by Parfitt [165]. The Parfit s opinion on acidity or alkalinity of the hydroxyl group of both form of the titania comes from the position of the pHpzc. This value is determined by the adsorption/desorption reactions of the proton and adsorption of the background electrolyte ions. The oxide surface charge depends on the alkali-acid properties of the hydroxyl groups and confine only to the analysis of the acid properties of the oxide surface group, do not allow to predict the shift of pHpzc properly. 

An important aspect of this reaction is the way in which reactants are concentrated near the oxide surface. The free, ionized carboxylate group of MPT - provides a basis for specific adsorption, through complex formation with surface Al centers. Monophenyl terephthalate also experiences favorable electrostatic attraction towards the positive-charged oxide surface. The positive Al oxide surface charge and extent of MPT adsorption both diminish as the pH is decreased (Stone, 1989a). The extent of MPT-adsorption also decreases as the ionic strength is increased, an indication that the surface complex is outer sphere rather than inner sphere (Hayes et al.,... 

As expected, both the extent of reactant adsorption and hydrolysis rate decrease substantially as the ionic strength is increased. At high ionic strength, counter ions of the supporting electrolyte accumulate in the diffuse layer, shielding the oxide surface charge and lessening the accumulation of MPT and OH at the Al oxide/water interface. 

Specifically adsorbing anionic species (such as maleate) and natural organic matter lower the extent of MPT - adsorption and overall rates of hydrolysis by blocking surface sites and by lowering the oxide surface charge (Stone, 1989b). 

Stack, A.G., Higgins, S.R., and Eggleston, C.M., Point of zero charge of a corundum-water interface probed with optical second harmonic generation (SHG) and atomic force microscopy (AFM). New approaches to oxide surface charge, Geochim. Cosmochim. Acta, 65, 3055, 2001. 

Ostomel, T.A. et al.. Metal oxide surface charge mediated hemostasis, Langmuir, 23, 11233, 2007. 

Dobson, K.D., Connor, P.A., and McQuillan, A.J., Monitoring hydrous metal oxide surface charge and adsorption by STIRS, Langmuir, 13, 2614, 1997. 

Proton adsorption/desorption kinetics may be studied by pressure-jump type techniques. Protonation is usually very fast deprotonation may be slower but time scales of a few tens of seconds are not exceeded For practical purposes, the oxide surface charge can be considered as being instantaneously established on contact with the metal-containing solution. 

T. A. Ostomel, Q. Shi, P. K. Stoimenov, and G. D. Stucky, 2007, Metal oxide surface charge mediated hemostasis, Langmuir 23, 11233-11238 J-H. Park, L. Gu, G. von Maltzahn, E. Ruoslahti, S. N. Bhatia, and M. J. Sailor, 2009, Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nature... 

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