Surface Dissolution

The interfacial properties of colloidal suspensions are determined by the chemical reactions (e.g. protonation) and adsorption of solutes. Additionally, the interface can be affected by the dissolution-precipitation equilibrium of the particle phase. This is because precipitation changes the surface morphology (Vigil et al. 1994) or leads to phase transition (Lefevre et al. 2002 Carrier et al. 2007). In addition, dissolution means degradation of the particles and may result in the loss of the finest particle fractions (i.e. Ostwald ripening). For this reason, it is necessary to understand the factors governing the dissolution of solid particles. 

Solubility is a material property only at the lower nanoscale does particle size matter (cf. Sect. 3.1.4). The dissolution of oxides strongly depends on the reducing 

The released metal ions are covered by a hydration shell, the size of which depends on the valency of the ion and the size of the nucleus. The hydrate complexes [M(H20) ] of multi-valent ions can deprotonate (i.e. they act as acids), which affects the chemical equilibrium of the reaction (3.21). As a result, the apparent solubility is a function of the pH-value. Deprotonation leads to a transformation of the hydrate complexes into hydroxide complexes [M(OH) ] . The neutral hydroxide complexes are hardly soluble. The total amount of dissolved material and its composition can be calculated from the acid-base-equilibria of the complex ions and the pH. In general, the lower the main group of the metal is, the higher the oxide solubility. 

Equation (3.21) cannot be applied to the oxides of metalloids and nonmetals. In that case, the dissolution reaction produces acid molecules with oxyanions. Deprotonisation of these acids eventually causes a pH-dependency of the solubility, 

For instance, the dissolution reaction of amorphous silica can be written as a synthesis of orthosilicic acid Si02(s) + 2 H2O ++ H4S104 (pKdigs = 2.7, Gunnarsson and Amorsson 2000). At pH-values above 8, the H4Si04 molecules dissociate into H3Si04 and H2Si04 and, thus, affect the dissolution equilibrium. Hence, the solubility of silica in a basic environment exceeds that of the solubility in neutral milieu considerably (approx, by factor 2 at pH 10 and by factor 50 at pH 11, cf. Fig. 3.11 Her 1979, pp. 47 and 123). 

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The collagen shield, fabricated from procine scleral tissue, is a spherical contact lens-shaped film whose thickness can be made to vary from 0.027 to 0,071 mm. It has a diameter of 14.5 mm and a base curve of 9 mm. Once the shield is hydrated by tear fluid and begins to dissolve, it softens and conforms to the corneal surface. Dissolution rates can be varied from 2 to as long as 72 hr by exposing the shields to ultraviolet radiation in order to achieve varying degrees of crosslinking. 

At point B, the steps disappear due to surface dissolution. This is the deliquescence point. Source Ref. 78. 

Fujishima A, Sugiyama E, Honda K (1971) Photosensitized electrolytic oxidation of iodide ions on cadmium sulfide single crystal electrode. Bull Chem Soc Japan 44 304 Inoue T, Watanabe T, Fujishima A, Honda K, Kohayakawa K (1977) Suppression of surface dissolution of CdS photoanode by reducing agents. J Electrochem Soc 124 719-722 Elhs AB, Kaiser SW, Wrighton MS (1976) Visible light to electrical energy conversion. Stable cadmium sulfide and cadmium selenide photoelectrodes in aqueous electrolytes. J Am Chem Soc 98 1635-1637... 

Wan L-J, Moriyama T, Ito M, Uchida H, Watanabe M. 2002. In situ STM imaging of surface dissolution and rearrangement of a Pt-Fe alloy electrocatalyst in electrol3de solution. Chem Commun 1 58 59. 

The formation of carbon over Ni, Fe, and Co has been extensively studied, both for catalytic applications " and for dusting or dry corrosion, the problem of pitting when steels are exposed to hydrocarbons at high temperatures. Recently, the properties of Ni for forming carbon have even been proposed for use in the manufacture of carbon nanofibers. The mechanism on each of these metals, shown diagrammatically in Figure 8a, involves deposition of a carbon source onto the metal surface, dissolution of the carbon into the bulk of the metal, and finally precipitation of carbon as a fiber... 

Inoue T, Watanabe T, Fujishima A, Honda K, Kohayakawa K (1977) Suppression of Surface Dissolution of CdS Photoanode by Reducing Agents. J Electrochem Soc 124 719-722... 

Oxidations for varying times showed systematic variations in the U +/U + ratios that fell into three values stable for significant periods of time (Fig. 5). These ratios of U +/U + = 0.5, 1.0 and 2.0 are believed to correspond to the presence of U3O7 (U02 U02 U03), U205 (U02 U03), and U30g (U02-U03-U03), respectively. The surface film composition is controlled by various rates of surface dissolution, oxygen diffusion into the UO2 and precipitation. Film dissolution is believed to occur upon oxidation to UO3. Because no appreciable layer of UO3 was found on the surface, it is concluded that dissolution of UO3 occurs at a rate faster than solid-state oxidation. 

Figure 4.11 Bulk and surface dissolution of biodegradable polymers...

The main properties of the solid are the texture, the nature of functional groups (e.g., the number and strength of the acidic and basic centers,the isoelectric point), the presence of exchangeable ions, and the reactivity (surface dissolution in acidic or basic solution, etc.). 

El-Sabawi, D., Price, R., Edge, S., and Young, P. M. (2006), Novel temperature controlled surface dissolution of excipient particles for carrier based dry powder inhaler formulations, Drug Dev. Ind. Pharm., 32,243-251. 

In the equations given above a is a dimensionless constant (different for each equation) ft is the molar volume of the grains k is Boltzmann s constant 17 is the viscosity of the bonding phases and c represents the surface dissolution velocity under a unit driving force. After Pharr and Ashby,s... 

Surface dissolution occurs between particle impacts. 

In the past, the practice has been to take a sample from any depth in a large metal or (better) plastic container and then transfer the sample to another, usually plastic, container for subsequent analysis by appropriate analytical methods. Obviously, a metal container will contribute to the trace metal content of the sample, and even plastic containers will cause problems. Trace analysis studies have shown that plastic or glass sample containers can both absorb trace metal ions from the sample and/or contribute other metal ions to solution by surface dissolution 12, 13), Thus, the sample cannot be analyzed accurately because of the time-dependent effects on concentration which are related simply to the nature of the container and the conditions used to store the sample. 

Five samples were cleaned, dried and weighed (with 1 mg accuracy) for the determination of RS unprotected surface dissolution in the corrosion product stripping solution, containing the hydrochloric acid. After 10 min dripping in the hydrochloric acid solution they were rinsed at room temperature with water, acetone and dried before weighing. The average mass loss was 2 mg and its standard deviation was 0.9 mg. 

NMR exchange experiments for the study of surface dissolution species in solution-aged metaphosphate glass has been reported. It has been demonstrated that use of CP allows the resonances of phosphate tetrahedral species within the hydrated dissolution surface to be selectively and cleanly edited from the bulk unaged phosphate species. Incorporating the CP-editing into a 2D RFDR exchange experiment also has allowed the local spatial connectivity between these surface dissolution phosphate species to be directly addressed. 

Also useful are the measurements of zeta potential. Particles in suspensions typically acquire a surface charge by the adsorption of ions on the surface, dissolution of the material, chemical reaction, or preferential adsorption of a specific additive or impurity ions from the solution. Surface charge of colloidal particles can be inferred through the measurement of the zeta potential (the particle... 

The results of this investigation show that CaCC>3 dissolution is controlled by mass transfer and not surface reaction kinetics. Buffer additives such as adipic acid enhance mass transfer by increasing acidity transport to the limestone surface. Dissolution is enhanced at low sulfite concentration but inhibited at high sulfite concentration, indicating some kind of surface adsorption or crystallization phenomenon. The rate of dissolution is a strong function of pH and temperature as predicted by mass transfer. At high CO2 partial pressure, the rate of dissolution is enhanced due to the CO2 hydrolysis reaction. 

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