Rapid dissolution

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

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. 

Medical Uses. Citric acid and citrate salts are used to buffer a wide range of pharmaceuticals at their optimum pH for stabiUty and effectiveness (65—74). Effervescent formulations use citric acid and bicarbonate to provide rapid dissolution of active ingredients and improve palatabiUty. Citrates are used to chelate trace metal ions, preventing degradation of ingredients. Citrates are used to prevent the coagulation of both human and animal blood in plasma and blood fractionation. Calcium and ferric ammonium citrates are used in mineral supplements. 

The aluminum alloys most commonly bonded are 2024 bare, 2024 clad and 7075 bare. Clad 7075 was also used extensively in early bonded structure but was largely abandoned after service performance demonstrated that it was susceptible to rapid dissolution or corrosion of the clad layer. Naturally aged tempers such as T3, in particular 2024-T3 because of its widespread use, are restricted to bonding with adhesives that cure at 250°F or below in order to avoid adversely affecting the temper. Various other alloys and tempers are bonded to a lesser extent, though the dominance of 7075 and 2024 is decreasing as higher-performance alloys and tempers are adopted. 

The examples already discussed lead to the conclusion that any reaction of a metal with its environment must be regarded as a corrosion process irrespective of the extent of the reaction or of the rates of the initial and subsequent stages of the reaction. It is not illogical, therefore, to regard passivity, in which the reaction product forms a very thin protective film that controls rate of the reaction at an acceptable level, as a limiting case of a corrosion reaction. Thus both the rapid dissolution of active titanium in 40% H2SO4 and the slow dissolution of passive titanium in that acid must be... 

Attack on the substrate by contact with Mg(OH>2 and Ca(OH)2 (calcareous scale) can also cause deplatinisation to occur. Anodes located close to the cathode or operating at high current densities can lead to a rapid build up of calcareous deposit, the major constituents of which are Mg(OH)2 and Ca(OH>2. The alkaline conditions so generated can lead to rapid dissolution of the platinum. The calcareous deposit can be removed by washing with dilute nitric acid. 

When using this approach to polymer solubility, we need to remember that the basis is thermodynamics. In other words, this approach gives information about the energetics of solubility, but does not give any insight in the kinetics of the process. In order to promote rapid dissolution, it may be more helpful to employ a solvent that is less good thermodynamically, but that consists of small, compact molecules that readily diffuse into the polymer and hence dissolve the polymer more quickly. 

The promise of the isolation and production of therapeutic polypeptides and proteins demands that for treatment of a chronic disease state an oral delivery system be developed which will protect these valuable agents from the hostile gastric environment. Subsequently, the drugs will have to be completely released in the intestine, preferably in a state that will enhance their rapid dissolution and transport across the gut wall minimizing interaction with intestinal proteases. 

Thus, as the pH increases, the dissolution rate of a weak base decreases. Referring to Table 2, we can see that, for the weak acid tolbutamide, the dissolution rate increases as pH is increased, as predicted by Eq. (3). Additionally, for the weak base tetracycline, as predicted by Eq. (4), the dissolution rate decreases as pH is increased. Thus far, the more rapid dissolution of the salt forms of these drugs and the direction of change of the dissolution rate with pH have been accounted for with Eqs. (1) to (4). However, there are six possible dissolution rate... 

The rapid dissolution class boundary is defined in terms of the in vitro dissolution being greater than 85% in 30 min in 900 mL aqueous media at pH 1, 4.5, and 6.8, using USP Apparatus I (100 rpm) or Apparatus II [50 rpm (revolutions/min)] [104]. A similar guideline has been introduced in the European Union [105]. Examples of molecules from the various four classes are presented in Fig. 2.9 [102,104]. 

Working co-operatively with others, we have found some indication that certain alilphatic linkages between aromatic nucleii are involved in the rapid dissolution of coal. The absolute aliphatic hydrogen content as determined by P. Solomon using FTIR (22) shows a very good linear relationship with conversion of coal in 3 minutes to pyridine soluble materials (Figure 14a). 

Amorphous form provide the most rapid dissolution and the most often increased solubility by supersaturation however, practical usefulness is limited by stability issues, including transformation of the solid state form. 

These dyes have affinity for one or, usually, more types of hydrophobic fibre and they are normally applied by exhaustion from fine aqueous dispersion. Although pure disperse dyes have extremely low solubility in cold water, such dyes nevertheless do dissolve to a limited extent in aqueous surfactant solutions at typical dyeing temperatures. The fibre is believed to sorb dye from this dilute aqueous solution phase, which is continuously replenished by rapid dissolution of particles from suspension. Alternatively, hydrophobic fibres can absorb disperse dyes from the vapour phase. This mechanism is the basis of many continuous dyeing and printing methods of application of these dyes. The requirements and limitations of disperse dyes on cellulose acetate, triacetate, polyester, nylon and other synthetic fibres will be discussed more fully in Chapter 3. Similar products have been employed in the surface coloration of certain thermoplastics, including cellulose acetate, poly(methyl methacrylate) and polystyrene. 

Explanation Iodine is sparingly soluble in water but undergoes rapid dissolution in the presence of potassium iodide due to the formation of the corresponding triiodide ion ... 

A bio waiver is applicable for drug products meeting the BCS Class 1 criteria, HS/HP/RD (Rapid Dissolution). 

Anodic oxide formation suggests itself as a passivating mechanism in aqueous electrolytes, as shown in Fig. 6.1a. However, pore formation in silicon electrodes is only observed in electrolytes that contain HF, which is known to readily dissolve Si02. For current densities in excess of JPS a thin anodic oxide layer covers the Si electrode in aqueous HF, however this oxide is not passivating, but an intermediate of the rapid dissolution reaction that leads to electropolishing, as described in Section 5.6. In addition, pore formation is only observed for current densities below JPS. Anodic oxides can therefore be excluded as a possible cause of pore wall passivation in PS layers. Early models of pore formation proposed a... 

The Committee for Proprietary Medicinal Products [8] applied the BCS, with certain requirements, to dispense with bioequivalency tests if the active pharmaceutical ingredient is class I and the in vitro dissolution of the finished dosage form is fast [9], An active substance is considered highly soluble if the amount contained in the HDS of an IR product is dissolved in 250 ml of each of three buffers within the range of pH 1-8 at 37°C (e.g., pH 1.0, 4.6, and 6.8). There should be linear and complete absorption, which indicates HP to reduce the possibility of an IR dosage form influencing the bioavailability [8], The similarity of the dissolution profiles of the test and reference products is demonstrated in each of three buffers within the range of pH 1-8 at 37°C (e.g., pH 1.0,4.6, and 6.8). If there is rapid dissolution of the product, where at least 85% of the active substance is dissolved within 15 min, no further comparison of the test and reference is required. Further requirements include that excipients be well established and have no interaction with the pharmacokinetics of the active substance and that the method of manufacture of finished product... 

Stimulation of saliva production is under sympathetic and parasympathetic control. Parasympathetic stimulation produces a serous watery secretion, whereas sympathetic stimulation produces much thicker saliva. Drug delivery systems, therefore, should not be placed over a duct or adjacent to a salivary duct, as this may dislodge the retentive system or may result in excessive wash-out of the drug or rapid dissolution/erosion of the delivery system making it difficult to achieve high local drug concentrations. If a retentive system is placed over salivary ducts, the reduced salivary flow rate may produce less or no mucus which is required for the proper attachment of a mucoadhesive delivery device. 

A reaction occurring at an interface of two phases. Some heterogeneous processes also display homogeneous aspects for example, a particular reaction may occur in only one of the system s phases (e.g., the rapid dissolution of a gas into a liquid followed by a particular reaction). See Fractal Reaction Kinetics... 

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