Relative dissolution rate, function

Figure 9. The relative dissolution rate, R/Rf)J as a function of pH. Dashed lines were calculated by using the equilibrium and surface complex formation constants for pH 2S at 10r2 atm = /SO/ / = 10 1 M and--------------= /H2P047 =...

Figure 9. The relative dissolution rate, R/Rf>, as a function of pH. Dashed lines were calculated by using the equilibrium and surface complex formation con-...

The solubility of silica is also important. Figure 48.7 shows the relative dissolution rate for aqueous silicates as a function of pH [23]. The rate is slow at low pH so there is little bond redistribution. SAXS studies coupled with computer simulations have shown that under these conditions reaction-limited cluster-cluster aggregation is... 

This aromatic alcohol has been an effective preservative and still is used in several ophthalmic products. Over the years it has proved to be a relatively safe preservative for ophthalmic products [138] and has produced minimal effects in various tests [99,136,139]. In addition to its relatively slower rate of activity, it imposes a number of limitations on the formulation and packaging. It possesses adequate stability when stored at room temperature in an acidic solution, usually about pH 5 or below. If autoclaved for 20-30 minutes at a pH of 5, it will decompose about 30%. The hydrolytic decomposition of chlorobutanol produces hydrochloric acid (HC1), resulting in a decreasing pH as a function of time. As a result, the hydrolysis rate also decreases. Chlorobutanol is generally used at a concentration of 0.5%. Its maximum water solubility is only about 0.7% at room temperature, which may be lowered by active or excipients, and is slow to dissolve. Heat can be used to increase dissolution rate but will also cause some decomposition and loss from sublimation. Concentrations as low as 0.125% have shown antimicrobial activity under the proper conditions. 

It is suggested that the anodic dissolution will be inhibited if the adsorbed anion and the reaction intermediate are stable and hardly dissolve in aqueous solution. On the contrary, if the reaction intermediate is relatively unstable and readily dissolves into aqueous solution, the anion will function as an electrocatalyst accelerating the metal dissolution rate. It is now common knowledge that hydroxide ions, OH, catalyze the anodic dissolution of metallic iron and nickel in acid solution [81,82]. It is also known that chloride ions inhibit the anodic dissolution of iron in acidic solution [83]. No clear-cut understanding is however seen in literature on why hydroxide ions catalyze but chloride ions inhibit the anodic dissolution of iron, even though the two kinds of anions are in the same group of hard base. We assume that the hardness level in the Lewis base of adsorbed anions will be one of the most effective factors that determine the catalytic activity of the adsorbates. Further clarification on the catalytic characteristics will require a quantum chemical approach to the adsorption of these anions on the metal surface. 

Excess concentrations were plotted and found to be fit relatively well as a function of depth by a second-order polynomial (Fig. 18, Table V). Insertion of these functions into Eq. (6.9) results in an estimate of a constant dissolution rate for Fe in each modeled interval. These range over... 

Nucleation of protein crystals typically requires extremely high supersaturation levels. Studies of protein nucleation are limited, with most efforts focused on light scattering as a tool to detect nucleation. Feher and Kam s work set the tone for much of the work that followed (Feher and Kam 1985). They model nucleation in a classical fashion, as a cooperative step-by-step addition of monomers to a cluster. Light scattering is utilized to follow the cluster size distribution as a function of time and solution variables, which yield estimates for the relative forward (cluster growth) and reverse (cluster dissolution) rates of monomer addition. Certainly, the protein crystal nucleation is an area that deserves additional study. 

These relationships are illustrated in Fig. 6 in which the variation of coating weight, crystal size, the relative surface coverage, and the rate of substrate dissolution were measured as a function of time for the phosphatation of steel in a laboratory trication phosphatation bath. The partial free metal surface ( -9) decreases as the average crystal size (di) and coating mass increase. The dissolution rate of the metal rjiss (here defined as a negative value) drops toward zero at the end of the reaction. 

Another example is the very slight delamination that occurs when a thin copper layer is overcoated with an organic coating such as a photoresist and the system is made anodic. The rate of disbonding is a function of the applied potential and hence the rate of dissolution of the copper beneath the coating. Anodic delamination occurs very slowly relative to cathodic delamination at equal potential differences from the corrosion potential. 

CMP processes for oxide planarization (ILD and STI) rely on slurry chemistry to hydrolyze and soften the Si02 surface. Mechanical abrasion then controls the actual material removal. Thus, the key process output control variables (i.e., removal rate and nonuniformity) are strong functions of the mechanical properties of the system, namely, the down force and the relative velocity between the pad and the wafer. Metal CMP processes such as copper CMP rely more on chemical oxidation and dissolution of the metal than mechanical abrasion to remove the metal overburden. Consequently, careful control of the chemistry of the CMP process is more important for these CMP processes than it is for oxide CMP. Thus, CMP tools and processes optimized for ILD may not be optimal for metal CMP and vice versa. 

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