Minimum dissolution rate

Tseng et al. [164] suecessfully used UNIFAC to optimize polymer-solvent interactions in three-solvent systems, determining polymer activity as a function of the solvent eomposition. The composition yielding the minimum in polymer aetivity was taken as the eriterion for optimum interaetion, and it eompared well with experimental measurements of dissolution rate and solution clarity. Better agreement was obtained using UNIFAC than using solubility parameter methods. 

Studies on hot water tank enamelsin media of varying pH demonstrate a minimum corrosion rate at pH value of 4. In citric acid (pH 2), IR measurements indicate that ion exchange is the principal mode of corrosion. Distilled water (pH 7) showed evidence of a bulk dissolution mechanism with no silica enrichment of the surface layer. In neutral solutions, the first stage of attack is leaching of alkali ions, raising the pH of solution, which subsequently breaks down the glass network of the acidic oxides. 

In addition, assuming that the rate-determining step is the bulk diffusion (i.e.,ka/(DJt) lholds), we can derive the minimum dissolution current observed after the fluctuation-diffusion current, that is,... 

Ca,Gd,Ce,Hf)2(Ti,Mo)207 in neutral to basic solutions. Near steady-state Mo concentrations (0-4 ppb range) yield apparent dissolution rates of 2-6 x 10 3 g/m2/d for both samples at pH = 6-8 and flow rate = 2 mL/d. In total, the results for (Ca,Gd,Ce,Hf)2(Ti,Mo)207 indicate a weak pH dependence with a minimum at pH = 7. Similar pyrochlore-rich ceramics were studied by Hart et al. (2000), who showed that the release rate of Pu dropped from approximately 10 3 g/m2/d to 10 5 g/m2/d or less after nearly one year in pure water at 90 °C (Fig. 7). The release rates of U and Gd in these experiments were higher than Pu by factors of about 10 and 100, respectively. 

Drug release profiles from the tablets in various dissolution media are shown in Fig. 2. In all cases the release rates decreased initially from the control (distilled water) as electrolyte concentration increased, until a minimum release rate was obtained. As the electrolyte concentration further increased the release rates similarly increased until a burst release occurred. These initial decreases in release rates were probably coincident with a decrease in polymer solubility, in that as the ionic strength of the dissolution medium is increased the cloud point is lowered towards 37°C. It may be seen from Table 5 that minimum release rates occurred when the cloud point was 37°C. At this point the pore tortuosity within the matrix structure should also be at a maximum. It is unlikely to be an increase in viscosity that retards release rates since Ford et al. [1] showed that viscosity has little effect on release rates. Any reduction in hydration, such as that by increasing the concentration of solute in the dissolution media or increasing the temperature of the dissolution media, will start to prevent gelation and therefore the tablet will cease to act as a sustained release matrix. 

It could be shown [5] that the dissolution of polymers depends on the type of ions present in the dissolution medium. Dissolution is base catalyzed and can be described by the Bronstedt dissolution law [38], At a given pH, a linear relationship exists between the logarithm of the dissolution rate and the pKa of the acidic component of the salt present in the dissolution medium. Cellulose acetate phosphate, especially, showed a strong dependency of the dissolution rate on the type of ions added. Sodium chloride prevented the dissolution of some polymers, because the base catalysis was at a minimum level. 

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... 

Reduction of particle size. In the case of glass, solid solutions, and amorphous dispersions, particle size is reduced to a minimum level. This can result in an enhanced dissolution rate due to an increase in both the surface area solubilization. 

The measurement of IDR forms an important part of preformulation studies as they permit direct solubility and dissolution rate comparisons between different drug candidates, salts, polymorphic and pseudopolymorphic forms. The measurements are relatively straightforward to carry out but require a minimum of about 200mg of sample per determination. Production of the pellet can occasionally result in a pressure-induced polymorphism change the new polymorph may have different dissolution properties. Therefore, the presence or absence of a new polymorph should be verified by IR spectroscopy or by Differential Scanning Calorimetry (DSC). 

H+ and OH-, which weaken and break the metal cation framework bonds then using the above calculations showing the distribution of surface species, basalt dissolution is promoted by the absorption of H + to A1 and Fe sites (mainly Al) in acidic solutions, whereas it is controlled by the adsorption of OH- at Si sites in alkaline solutions. The combination of the two distinct types of surface sites, Al and Fe on one hand, and Si on the other hand, results in a dissolution rate minimum at a pH value between the pHzpc of the two groups of oxide components. 

Salt Formation. Salt formulation is one of the most commonly used approaches to deal with Class II compounds as a way to enhance drug solubility and dissolution rate. Salt selection largely depends on piiTa. It is generally accepted that a minimum difference of 3 units between the value of the group and... 

The proportionality factor would include, at a minimum, the rate constant for the reaction, and the relative surface area of the mineral, i.e., the mineral surface area per unit volume of solution. We might also add a term, v, to account for cases in which the dissolution of one mole of solid phase gives rise to v moles of the component we are measuring. For example, if we measured the solubility of Mg2SiC>4 by measuring aqueous Mg, v = 2. 

Solubility of calcite is closely associated with its dissolution rate and also changes within a very broad range depending on water composition. Its minimum value provide hydration and dissociation ... 

The rate of hydrolysis of Si(OR)4 (Equation 18.1) exhibits a minimum at pH 7, and increases exponentially at either lower or higher pH. In contrast, the rate of silane condensation (Equations 18.2 and 18.3) exhibits a minimum at pH 2, and a maximum around pH 7 where Si02 solubility and dissolution rates are maximized. 

The dissolution rates of the la-lc-S02 terpolymer films were also measured under the same conditions (bake 130 °C for 60 sec, developer 0.21 N TMAH) and plotted in Figure 5 as a function of the terpolymer composition. As the norbomene Ic bearing a butyl ester group is introduced into the polymer, the dissolution rate becomes exponentially smaller. The dissolution rate for la/lc=l/l is ca. 170 A/sec and the terpolymers with more than 55 mol% butyl ester do not dissolve at all in this developer. It thus appears that the concentration of the fluoroalcohol unit in the terpolymer must be below 45 % for an unexposed resist film to provide minimum thinning in the industry-standard 0.26 N TMAH solution. 

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