Browsing by Author "Maafi, W."
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Item Open Access Modeling and Elucidation of the Kinetics of Multiple Consecutive Photoreactions AB4 (4Φ) With Φ-order Kinetics. Application to the Photodegradation of Riboflavin.(Elsevier, 2016-10-06) Maafi, W.; Maafi, MounirNew semi-empirical rate-law system of equations is proposed for the first time for consecutive photoreactions that involve up to 4 photoreaction steps, AB4(4Φ). The equation system was developed, tested, and validated against synthetic kinetic traces generated by fifth-order Runge-Kutta calculations. The model accurately fitted the kinetic traces of Riboflavin photodegradation in ethanol which decomposes via the AB2(2Φ) mechanism involving 2 consecutive photoreaction steps. A kinetic elucidation methodology useful for consecutive photoreactions was also proposed to determine all the kinetic parameters and reaction attributes defining AB2(2Φ) reactions. The quantum yields of photodegradation, determined for wavelengths in the visible region 400-480 nm, ranged from 0.005 to 0.00756 and 0.0012 to 8 10−5 for the first and second photoreaction steps, respectively. They were found to increase with wavelength in defined sigmoid functions. For this monochromatic irradiation range, riboflavin proved to be a useful actinometer. Finally, a photodegradation scale based on pseudo-rate-constant values was also proposed for drugs. This scale (including 4 groups) is thought to contribute to rationalizing photodegradation testing and might prove useful in categorizing drugs' photodegradation reactivity.Item Open Access Modelling nifedipine photodegradation, photostability and actinometric properties.(Elsevier, 2013-08-14) Maafi, W.; Maafi, MounirBackground The photodegradation of drugs obeying unimolecular mechanisms such as that of Nifedipine (NIF) were usually characterized in the literature by zero-, first- and second-order kinetics. This approach has been met with varying success. This paper addresses this issue and proposes a novel approach for unimolecular photodegradation kinetics. The photodegradation of the cardiovascular drug Nifedipine is investigated within this framework. Methods Experimental kinetic data of Nifedipine photodegradation were obtained by continuous monochromatic irradiation and DAD analysis. Fourth-order Runge-Kutta calculated kinetic data served for the validation of the new semi-empirical integrated rate-law model proposed in this study. Results A new model equation has been developed and proposed which faithfully describes the kinetic behaviour of NIF in solution for non-isosbestic irradiations at wavelengths where both NIF and its photoproduct absorb. NIF absolute quantum yield values were determined and found to increase with irradiation wavelength according to a defined sigmoid relationship. The effects of increasing NIF or excipients’ concentrations on NIF kinetics were successfully modelled and found to improve NIF photostability. The potential of NIF for actinometry has been explored and evaluated. A new reaction order (the so-called -order) has been identified and specifically proposed for unimolecular photodegradation reactions. Conclusion The semi-empirical and integrated rate-law models facilitated reliable kinetic studies of NIF photodegradation as an example of AB(1) unimolecular reactions. It allowed filling a gap in kinetic studies of drugs since, thus far, thermal first-order or a combination of first- and zero order kinetic equations were generally applied for drug photoreactions in the literature. Also, a new reaction order, the “-order”, has been evidenced and proposed as a specific alternative for photodegradation kinetics.Item Open Access Montelukast photodegradation: Elucidation of Ф-order kinetics, determination of quantum yields and application to actinometry.(Elsevier, 2014-05-15) Maafi, W.; Maafi, MounirA recently developed F–order semi–emperical integrated rate–law for photoreversible AB(2F) photoreversible reactions has been successfully applied to investigate Montelukast sodium (Monte) photodegradation kinetics in ethanol. The model equations also served to propose a new stepwise kinetic elucidation method valid for any AB(2F) system and its application to the determination of Monte forward (ΦA→Blirr) and reverse (ΦB→Alirr ) quantum yields at various irradiation wavelengths. It has been found that ΦA→Blirr undergoes a 15–fold increase with wavelength between 220 and 360 nm, with the spectral section 250 – 360 nm representing Monte effective photodegradation causative range. The reverse quantum yield values were generally between 12 and 54 % lower than those recorded for ΦA→Blirr, with the trans–isomer (Monte) converts almost completely to its cis–counterpart at high irradiation wavelengths. Furthermore, the potential use of Monte as an actinometer has been investigated and an actinometric method was proposed. This study demonstrated the usefulness of Monte for monochromatic light actinometry for the dynamic range 258 – 380nm.Item Metadata only Φ-order kinetics of photoreversible-drug reactions(Elsevier, 2014-08) Maafi, Mounir; Maafi, W.