Absorption rate coefficient

Absorption rate coefficients and permeability values obtained through different systems, i.e. in vitro or in situ, finally are determined with the objective in mind of predicting the fraction of a dose that will be absorbed through the intestine when the drug is orally administered. In order to use the data obtained in those systems it is essential to validate them properly. The present section reviews the capability of the parameters representatives of the absorption determining with different techniques to predict in vivo drug absorption in absence of any limited step (as dissolution or solubility) by means of the previously described model and Eq. (16). In particular, the absorption rate constant obtained with an in situ close loop assay, permeability through Caco-2 cell lines, and PAM PA models have been examined. 

For the correlations, the absorption rate coefficients, ka, obtained in situ were transformed into permeability values as mentioned above. 

From these equations one also finds the rate coefficient matrix for themial radiative transitions including absorption, induced and spontaneous emission in a themial radiation field following Planck s law [35] ... 

Figure B2.5.7 shows the absorption traces of the methyl radical absorption as a fiinction of tune. At the time resolution considered, the appearance of CFt is practically instantaneous. Subsequently, CFl disappears by recombination (equation B2.5.28). At temperatures below 1500 K, the equilibrium concentration of CFt is negligible compared witli (left-hand trace) the recombination is complete. At temperatures above 1500 K (right-hand trace) the equilibrium concentration of CFt is appreciable, and thus the teclmique allows the detennination of botli the equilibrium constant and the recombination rate [54, M]. This experiment resolved a famous controversy on the temperature dependence of the recombination rate of methyl radicals. Wliile standard RRKM theories [, ] predicted an increase of the high-pressure recombination rate coefficient /r (7) by a factor of 10-30 between 300 K and 1400 K, the statistical-adiabatic-chaunel model predicts a...

The diffusion coefficient depends upon the characteristics of the absorption process. Reducing the thickness of the surface films increases the coefficient and correspondingly speeds up the absorption rate. Therefore, agitation of the Hquid increases diffusion through the Hquid film and a higher gas velocity past the Hquid surface could cause more rapid diffusion through the gas film. 

Noryl is a rigid dimensionally stable material. Dimensional stabiUty results from a combination of low mold shrinkage, low coefficient of thermal expansion (5.9 x 10 per° C), good creep resistance (0.6—0.8% in 300 h at 13.8 MPa (2000 psi)), and the lowest water absorption rate of any of the engineering thermoplastics (0.07% in 24 h at room temperature). Noryl resins are completely stable to hydrolysis. They are not affected by aqueous acids or bases and have good resistance to some organic solvents, but they are attacked by aromatic or chlorinated aUphatic compounds. 

Effects of High Solute Concentrations on Ug and As discussed previously, the stagnant-film model indicates that fcc should be independent of ysM and/cc should be inversely proportional to The data of Vivian and Behrman [Am. Tn.st. Chem. Eng. J., 11, 656 (1965)] for the absorption of ammonia from an inert gas strongly suggest that the film model s predicted trend is correct. This is another indication that the most appropriate rate coefficient to use is fcc. nd the proper driving-force term is of the form (y — yd ysM-... 

The gas-phase rate coefficient fcc is not affecded by the fact that a chemic reaction is taking place in the liquid phase. If the liquid-phase chemical reaction is extremely fast and irreversible, the rate of absorption may be governed completely by the resistance to diffusion in the gas phase. In this case the absorption rate may be estimated by knowing only the gas-phase rate coefficient fcc of else the height of one gas-phase transfer unit Hq =... 

Whenever these conditions on the ratio yjy apply, the design can be based upon the physical rate coefficient /cg or upon the height of one gas-phase mass-transfer unit He- The gas-phase mass-transtor hmited condition is approximately vahd, for instance, in the following systems absorption oi NH3 into water or acidic solutions, vaporization of water into air, absorption of H9O into concentrated sulfuric acid solutions, absorption of SO9 into alkali solutions, absorption of H9S from a dllute-... 

Reaction between an absorbed solute and a reagent reduces the equilibrium partial pressure of the solute, thus increasing the rate of mass transfer. The mass-transfer coefficient hkewise is enhanced, which contributes further to increased absorption rates. Extensive theoretical analyses of these effects have been made, but rather less experimental work and design guidehnes. 

The results of Massimilla et al., 0stergaard, and Adlington and Thompson are in substantial agreement on the fact that gas-liquid fluidized beds are characterized by higher rates of bubble coalescence and, as a consequence, lower gas-liquid interfacial areas than those observed in equivalent gas-liquid systems with no solid particles present. This supports the observations of gas absorption rate by Massimilla et al. It may be assumed that the absorption rate depends upon the interfacial area, the gas residence-time, and a mass-transfer coefficient. The last of these factors is probably higher in a gas-liquid fluidized bed because the bubble Reynolds number is higher, but the interfacial area is lower and the gas residence-time is also lower, as will be further discussed in Section V,E,3. 

The results reported for beds of small particles (1 mm diameter and less) are in substantial agreement on the fact that the presence of solid particles tends to decrease the gas holdup and, as a consequence, the gas residencetime. This fact may also support the observations of gas absorption rate by Massimilla et al. (Section V,E,1) if it is assumed that a decrease of absorption rate caused by a decrease of residence time outweighs the increase of absorption rate caused by increase of mass-transfer coefficient arising from the increase in bubble Reynolds number. These results on gas holdup are in... 

Newton and Baker, using wavelengths of 737 m/i or 648 m/i (U(IV) absorption), have obtained similar overall rate coefficients to those obtained by polaro-graphy . However, they have found U(VI) to retard the disproportionation reaction due to the presence of an additional process... 

The reaction between complexes of the type Fe(H20)5X and V(H20)6 has been investigated by a flow method by measurements at the absorption maximum of the former species . Rate coefficients are recorded in Table 3 when... 

The reaction was followed by means of the strong absorption of the Os(II) complex at 480 m/i. Unlike the Tl(riI) + Fe(II) system, there is a slight increase in rate as the hydrogen-ion concentration is increased. The kinetic data were interpreted on the basis that both Tl and TIOH react with Os(bipy)3 (with rate coefficients and respectively). At 24.5 °C and ju = 2.99 M, kj = 36.0 l.mole. see and= 14.7 l.mole sec corresponding activation energies are 6.90 and 11.5 kcal.mole" The latter values are considerably smaller than those for the T1(III) + T1(I) exchange and for the Tl(III)- -Fe(II) reaction . On the other hand, all three reactions are subject to retardation by Cl ions. 

The complex has been separated by ion exchange and characterised by direct analysis . The complex has a distinctive absorption spectrum (Fig. 11), quite unlike that of Np(V) and Cr(III). The rate coefficient for the first-order decomposition of the complex is 2.32 x 10 sec at 25 °C in 1.0 M HCIO. Sullivan has obtained a value for the equilibrium constant of the complex, K = [Np(V) Cr(III)]/[Np(V)][Cr(III)], of 2.62 + 0.48 at 25 °C by spectrophotometric experiments. The associated thermodynamic functions are AH = —3.3 kcal. mole" and AS = —9.0 cal.deg . mole . The rates of decay and aquation of the complex, measured at 992 m/t, were investigated in detail. The same complex is formed when Np(VI) is reduced by Cr(II), and it is concluded that the latter reaction proceeds through both inner- and outer-sphere paths. It is noteworthy that the substitution-inert Rh(lII), like Cr(III), forms a complex with Np(V) °. This bright-yellow Np(V) Rh(III) dimer has been separated by ion-exchange... 

Regime Conditions Expression for absorption rate or enhancement coefficient... 

The rate-limiting step in the absorption of those compounds that readily penetrate the intestinal membrane (i.e., have a large permeability coefficient) may be the rate at which blood perfuses the intestine. However, absorption will be independent of blood flow for those compounds that are poorly permeable. Extensive studies have illustrated this concept in rats [106,107]. The absorption rate of tritiated water, which is rapidly absorbed from the intestine, is dependent on intestinal blood flow, but a poorly absorbed compound, such as ribitol, penetrates the intestine at a rate independent of blood flow. In between these two extremes are a variety of intermediate compounds whose absorption rate is dependent on blood flow at low flow rates but independent of blood flow at higher flow rates. By altering blood flow to the intestine of the dog, as blood flow decreased the rate of sulfaethidole absorption also decreased [108]. These relationships are illustrated in Fig. 16. 

Membrane uptake of nonionized solute is favored over that of ionized solute by the membrane/water partition coefficient (Kp). If Kp = 1 for a nonionized solute, membrane permeability should mirror the solute ionization curve (i.e., membrane permeability should be half the maximum value when mucosal pH equals solute pKa). When the Kp is high, membrane uptake of nonionized solute shifts the ionization equilibrium in the mucosal microclimate to replace nonionized solute removed by the membrane. As a result, solute membrane permeability (absorption rate) versus pH curves are shifted toward the right for weak acids and toward the left for weak bases (Fig. 7). 

Figure 7 (Left panel) Relative absorption rate for a weak acid (pKa = 3) as a function of mucosal pH for increasing barrier (membrane) permeability (Pb) with fixed unstirred aqueous layer permeability (Pul). X = pHinflectionpoint in Eq. (4). (Right panel) Partition coefficient-dependent absorption rates for salicylic acid and the weak base ephedrine. (From Ref. 19.)...

The fluorescence observed when 3-hydroxyflavone is excited by the absorption of light is explained (Sengupta and Kasha, 1979) by Fig. 11. Measurement of the rise-time of fluorescence is thought to provide an estimate of the rate coefficient for proton transfer within the excited state (48). However, the value of the rate coefficient k 108s 1 (Strandjord et al., 1983) is much lower than previous estimates for similar processes in other... 

The mechanisms for the NMHCs (except DMS) required to fully characterise OH chemistry were extracted from a recently updated version of the Master Chemical Mechanism (MCM 3.0, available at http //mcm.leeds.ac.uk/MCM/). The MCM treats the degradation of 125 volatile organic compounds (VOCs) and considers oxidation by OH, NO3, and O3, as well as the chemistry of the subsequent oxidation products. These steps continue until CO2 and H2O are formed as final products of the oxidation. The MCM has been constructed using chemical kinetics data (rate coefficients, branching ratios, reaction products, absorption cross sections and quantum yields) taken from several recent evaluations and reviews or estimated according to the MCM protocol (Jenkin et al., 1997, 2003 Saunders et al., 2003). The MCM is an explicit mechanism and, as such, does not suffer from the limitations of a lumped scheme or one containing surrogate species to represent the chemistry of many species. 

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