Inhibition, dissolution rate

Specific adsorption of ligands can enhance or inhibit dissolution rates by altering the strength and lability of Me-0 lattice bonds. Salicylate, oxalate, and citrate promote the dissolution of alumina (40). In the presence of ligand (L) the dissolution rate becomes (7 ) , ... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Various devices can be used to determine the kinetics and rates of chemical weathering. In addition to the batch pH-stats, flow through columns, fluidized bed reactors and recirculating columns have been used (Schnoor, 1990). Fig. 5.15a illustrates the fluidized bed reactor pioneered by Chou and Wollast (1984) and further developed by Mast and Drever (1987). The principle is to achieve a steady state solute concentration in the reactor (unlike the batch pH-stat, where solute concentrations gradually build up). Recycle is necessary to achieve the flow rate to suspend the bed and to allow solute concentrations to build to a steady state. With the fluidized bed apparatus, Chou and Wollast (1984) could control the AI(III) concentration (which can inhibit the dissolution rate) to a low level at steady state by withdrawing sample at a high rate. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

For some metallic electrodes, such as transition metals, metal ions dissolve directly from the metallic phase into acidic solutions tiiis direct dissolution of metal ions proceeds at relatively low (less anodic) electrode potentials. The direct dissolution of metal ions is inhibited by the formation of a thin oxide film on metallic electrodes at higher (more anodic) electrode potentials. At still higher electrode potentials this inhibitive film becomes electrochemically soluble (or apparently broken down) and the dissolution rate of the metal increases substantially. These three states of direct dissolution, inhibition by a film, and indirect dissolution via a film (or a broken film) are illustrated in Fig. 11-9. 

They may accelerate or retard the process. Additives may act in solution (via com-plexation), but more often adsorb on the oxide and either raise or lower the energy of attachment between the surface ions and those of the interior. In extreme cases, adsorbed additives may inhibit dissolution. pH has a strong influence on the dissolution of iron oxides. At atmospheric pressure, dissolution of well crystalline Fe " oxides requires a pH of <1 even at 70 °C. The high affinity of protons with structural 0 assists the release of iron particularly at low pH. It is the release of the cation, rather than the anions which is likely to be rate limiting. pH also influences the electrochemical surface potential and hence redox processes. The surface potential is determined largely by surface charge, which in turn, depends upon pH (see Chap. 10). 

In acidic solutions, the corrosion rate is relatively high. Studies on cadmium monocrystals and polycrystals in acidic chloride solutions revealed anodic dissolution independent of the crystallographic orientation the dissolution rate was controlled by the mass transport of CdCl" ions [331]. The inhibitive influence of adsorbed organic substances, for example, alcohols [332], phenotiazine [333], and some polymers (e.g. poly (vinyl alcohol), poly(acrylic acid), sodium polyacrylate. 

Silicon is an alloy in the Cu cladding being stripped and, hence, Si concentration is proportional to Cu concentration. The gelatinous silicic acid tended to coat the solid oxalic acid particles and inhibit the rate of dissolution of the crystals at higher Cu/Si concentrations. 

There are numerous accounts in the literature of increased bioavailability in animals when changing the solid state. Kato and Kohetsu (1981) showed that form II amobarbital is more rapidly absorbedn vivo than form I. Dissolution rate experiments in water at(3"Showed a 1.6 times faster dissolution ratei vitro for form II compared to form I. Yokoyama et al. (1981) found that form III of 6-mercaptopurine was 1.5 times as bioavailable in rabbits as form I. It was six to seven times as soluble as the form I polymorph in studies by Kuroda et al. (1982). Kokubu et al. (1987) examined the therapeutic effect of different polymorphs of cimetidine in inhibition of ulcers in the rat. Pharmacokinetic studies found that form C was 1.4-1.5 times as bioavailable as forms A and B. This translated into a greater protection against stress ulceration, as shown in Table 19.4. The effec of form C was signiLcant compared to forms A, B, and D, which were all equivalent. 

Specifically adsorbed cations and anions may lower reductive dissolution rates by blocking oxide surface sites or by effecting release of Mn(II) into solution. Stone and Morgan (1984a) found that PO4- considerably inhibited the reductive dissolution of Mn(III/IV) oxides by hydroquinone. For example, addition of 10-2 M PO4- at pH 7.68 resulted in the dissolution rate being only 25% of the rate in the absence of PO4-. The dissolution rate was affected more by PO than by Ca2+. 

Mg2+ influences calcite dissolution rates the same way, but not to the same extent as Ca2+. The inhibition effects of Mg2+ can be described in terms of a Langmuir adsorption isotherm. Sjoberg (1978) found he could model results for the combined influences of Ca2+ and Mg2+ in terms of site competition consistent with ion exchange equilibrium. The inhibition effects of Mg2+ in calcite powder runs increase with increasing Mg2+ concentration and as equilibrium is approached. 

In wet FGD processes, either DA or limestone slurry, the combined effects of calcium and magnesium actually determine the limestone dissolution rate. Sjoberg s results(fi) indicated that Ca2+ can inhibit the CaCO dissolution rate much more effectively than Mg2+ by the same surface adsorption phenomenon. The combined effects of Ca2+ and Mg2+ can be described as competitive adsorption, and the limestone surface will act as an ion-exchanger. The fraction of surface occupied by adsorbed Ca2+ and Mg2+ can be expressed as ... 

Equation (7) indicates that the relative effectiveness of Mg2+ and Ca2+ in inhibiting the limestone dissolution rate depends on the ratio of Mg2+ concentration to Ca2+ concentration. On the other hand, the sensitivity of limestone dissolution rate to the Mg2+ concentration is determined by the Ca2+ concentration. As indicated by Equation (7), when the minimum ratio ( Mg/0Ca) 0 5 is required... 

Ligands and metal complexes present in aqueous systems in contact with natural oxides can affect their dissolution either by promoting or inhibiting it. For example, some metal—EDTA complexes react with Fe2C>3 and dissolve it, producing [Fe(III)EDTA]. Other minerals like Co(III)OOH and Mn(III)OOH reductively dissolve by oxidizing ligands and metal complexes. Dissolution rates can... 

However, the photochemistry itself does not make a relief image. Rather it is used to modify the solubility of the polymeric binder. The diazoquinone compounds used in resists are referred to as dissolution inhibitors or photoactive components (PAC s). The addition of a diazoquinone molecule dramatically inhibits the dissolution rate of a thin film of a novolac resin. Upon exposure, the dissolution rate of the novolac based resist is considerably faster than the rate for the novolac alone. The accelerated dissolution rate may be caused by formation of acid eind its subsequent ionization during development or by enhauiced diffusion of the developer into the coating because of changes caused by the formation and fate of the nitrogen (2). 

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