Factors Affecting Diffusivity

Another important factor affecting diffusion rate is the solubihty of the gas in blood. When the solnbihty of the gas in blood is large, the diffusion rate is fast and the gas is removed qnickly from the alveoli. In this case, the limiting factor in dehvery of gas to the blood is the rate of supply of gas to the alveoli. When this solnbihty is small, the diffnsion rate is slow thus blood flow (i.e., cardiac output) rather than minnte volume becomes the rate-limiting factor in toxicity. 

Kaneto et al.523) have made measurements on the diffusion of lead perchlorate in polythiophene by following the colour change. They found a diffusion coefficient which varied from 10-1° to I0-12 cm2 s 1, depending upon the applied potential. The complexities introduced by morphological heterogeneity, counter-ion motion and solvent effects mean that further studies will be required to determine the relative importance of factors affecting diffusion in these materials. 

A review on drying, including factors affecting diffusion coefficients, is... 

The equilibrium constant K was determined for various values of temperature T. Then, using Equation 5.33, the partial pressure of magnesium vapor (pMg) was calculated at various temperatures by keeping partial pressure of carbon monoxide gas [ppoj = 1- The calculations showed that p g was about 10 atm. at 1500°C and about 0.1 atm. at 1700°C. Fortunately, for the refractory in service, two factors mitigate the anticipated Mg evaporation rate (a) mass transport is impeded by slag and other factors affecting diffusion [74], and (b) much of the Mg vapor apparently reoxidizes within the refractory [75] either at lower temperature behind the hot face or upon... 

Several factors affect the bandshapes observed ia drifts of bulk materials, and hence the magnitude of the diffuse reflectance response. Particle size is extremely important, siace as particle size decreases, spectral bandwidths generally decrease. Therefore, it is desirable to uniformly grind the samples to particle sizes of <50 fim. Sample homogeneity is also important as is the need for dilute concentrations ia the aoaabsorbiag matrix. 

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

Various factors affecting the nonpremixed edge speed, such as flame stretch, preferential diffusion, and heat loss, have also been investigated, including cellular and oscillatory instabilities of edge flames [1,39 3]. 

The following factors affect net diffusion of a substance (1) Its concentration gradient across the membrane. Solutes move from high to low concentration. (2) The electrical potential across the membrane. Solutes move toward the solution that has the opposite charge. The inside of the cell usually has a negative charge. (3) The permeability coefficient of the substance for the membrane. (4) The hydrostatic pressure gradient across the membrane. Increased pressure will increase the rate and force of the collision between the molecules and the membrane. (5) Temperature. Increased temperature will increase particle motion and thus increase the frequency of collisions between external particles and the membrane. In addition, a multitude of channels exist in membranes that route the entry of ions into cells. 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

Intramuscularly administered products typically form a depot in the muscle mass from which the drug is slowly absorbed. The peak drug concentration is usually seen within 1-2 hours. Factors affecting the drug-release rate from an IM depot include the compactness of the depot (the less compact and more diffuse, the faster the release), the rheology of the product, concentration and particle size of drug in the vehicle, nature of the solvent or vehicle, volume of the injection, tonicity of the product, and physical form of the product. 

A biologically important factor affecting drug absorption is drug metabolism or reaction coincident with diffusion across a membrane. The reaction often produces inactive or less potent products than the parent drug. It is conceivable that the reaction will also reduce the drug flux into the systemic circulation. We are interested in the effect of reaction on membrane diffusion. 

The quantitative role of electrical factors affecting the transport of charged molecules is obtained by comparing permeability coefficients with the permeability coefficient P(Lcdi for molecular size-restricted diffusion independent of the charge on the molecule (i.e., the neutral image). With Eqs. (45) and (46) one obtains... 

Diffusion of small molecular penetrants in polymers often assumes Fickian characteristics at temperatures above Tg of the system. As such, classical diffusion theory is sufficient for describing the mass transport, and a mutual diffusion coefficient can be determined unambiguously by sorption and permeation methods. For a penetrant molecule of a size comparable to that of the monomeric unit of a polymer, diffusion requires cooperative movement of several monomeric units. The mobility of the polymer chains thus controls the rate of diffusion, and factors affecting the chain mobility will also influence the diffusion coefficient. The key factors here are temperature and concentration. Increasing temperature enhances the Brownian motion of the polymer segments the effect is to weaken the interaction between chains and thus increase the interchain distance. A similar effect can be expected upon the addition of a small molecular penetrant. 

Westwater (W4, W5) has written a detailed review of boiling in liquids with emphasis on nucleation at surfaces. Although written in 1956, this is still very useful and it provides a detailed description of the factors affecting nucleation. In a more recent review, Leppert and Pitts (L2) have described the important factors in nucleate boiling and bubble growth, and Bankoff (B2) has reviewed the field of diffusion-controlled bubble growth in nonflowing batch systems. 

The effect of temperature on distribution ratios has already been mentioned on page 91. Although the separation proceeds more quickly at elevated temperatures, resolution suffers because of increased rates of diffusion. However, in adsorption TLC only small increases in Rt values are observed even with a 20°C rise. Strict temperature control is not necessary if samples and standards are run at the same time, although large fluctuations should be avoided. The quality of the thin-layer materials, and in particular the presence of impurities in them, determine the extent to which partition, adsorption, ion-exchange and exclusion participate in the sorption process. These factors affect Rr values in an unpredictable manner. Thin layers should be of uniform thickness, between 0.2 and 0.3 mm with thinner layers, local variations in thickness can result in appreciable variations in Rf values. 

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