Controlled-release kinetics measurement

Simplified Procedure for Measuring Controlled-Release Kinetics... 

The relevance of the control of the reverse micelles and dependence of the release kinetics on the reverse-micelles concentration and size was demonstrated. The amount of solubilized water in the oil phase, as a function of Span concentration, was measured (after centrifugation). It was found that water is present only in very minor quantities when the siliconic emulsifier is employed by itself. The water concentration increased as the amounts of Span 80 were raised (87). These findings are also in good correlation with the release rates and the lag time. 

Cervera et al. [222] studied the controlled release of phenytoin, which is an anticonvulsant drug for the treatment of epilepsy, from nanostructured Ti02 reservoirs. The y loaded the Ti02 reservoirs with 5 wt% phenytoin, and studied the release kinetics in a pH 7.2 buffer by measuring the ultraviolet-visible (UV-vis) absorbance spectrum as a function of time. The authors also explored the relationship between the phenytoin release rate and the properties of the Ti02 nanomaterials used in the preparation of the reservoir. The results showed that the reservoirs are capable of releasing phenytoin for more than 45 days, and that the release kinetics are characterized by two regimes an initial fast release and a subsequent slow release. The slow release rate is independent of time and showed... 

Accumulatory pressure measurements have been used to study the kinetics of more complicated reactions. In the low temperature decomposition of ammonium perchlorate, the rate measurements depend on the constancy of composition of the non-condensable components of the product mixture [120], The kinetics of the high temperature decomposition [ 59] of this compound have been studied by accumulatory pressure measurements in the presence of an inert gas to suppress sublimation of the solid reactant. Reversible dissociations are not, however, appropriately studied in a closed system, where product readsorption and diffusion effects within the product layer may control, or exert perceptible influence on, the rate of gas release [121]. 

Such systems have the experimental advantage that kinetic data may be obtained by gravimetric or evolved gas pressure measurements. However, these data must be interpreted with care, since gas release is not necessarily concurrent with the solid—solid interaction but may, in principle, be a distinct rate process under independent kinetic control and occur either before or after reaction between the solids. Possible mechanisms to be considered, therefore, include the following. 

The study and control of a chemical process may be accomplished by measuring the concentrations of the reactants and the properties of the end-products. Another way is to measure certain quantities that characterize the conversion process, such as the quantity of heat output in a reaction vessel, the mass of a reactant sample, etc. Taking into consideration the special features of the chemical molding process (transition from liquid to solid and sometimes to an insoluble state), the calorimetric method has obvious advantages both for controlling the process variables and for obtaining quantitative data. Calorimetric measurements give a direct correlation between the transformation rates and heat release. This allows to monitor the reaction rate by observation of the heat release rate. For these purposes, both isothermal and non-isothermal calorimetry may be used. In the first case, the heat output is effectively removed, and isothermal conditions are maintained for the reaction. This method is especially successful when applied to a sample in the form of a thin film of the reactant. The temperature increase under these conditions does not exceed IK, and treatment of the experimental results obtained is simple the experimental data are compared with solutions of the differential kinetic equation. 

Kinetic simulation methods are used as advisory controls in potentially thermally hazardous batch amination reactions of nitroaromatic compounds. Time—temperature process data are fed as input to a kinetic simulation computer program which calculates conversions, heat release and pressure profdes. Results are compared continuously on-line with measured batch data to detect any deviations from normal operating conditions. 

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