Overall permeability

Perhaps the most important feature of the plasma membrane is that it is selectively permeable. Overall permeability of the membrane to a chemical depends on the nature of the membrane, its surface charge and rigidity, and the chemical in question. Therefore, only certain substances are able to pass through the membrane, depending on particular physicochemical characteristics. It will be apparent throughout this book that the physicochemical characteristics of molecules are major determinants of their disposition and often of their toxicity. 

Membrane permeability overall Overall permeability coefficient <2r Volumetric flow rate r Internal radius of the hollow-fiber tube ri Hollow-fiber log-mean radius... 

In a second approach, the improvement of solubility and metabolic stability based on heterocyclic aglycones was studied. Thereby, triazole (25a-c, entries 10, 12 13), pyrazole (28a,b, entries 15 17), and six-membered heterocyclic moieties (34a-d, entries 19, 21, 23 25 and 39, entry 27) proved highly beneficial to the aqueous solubility but in turn reduced lipophilicity and membrane permeability. Overall, only poor oral absorption could be observed. By contrast, the pyrrolylphenyl mannosides - optimized by the introduction of a chloro or trifluoromethyl substituent on ring A and a methyl group on the heterocycle (42f and 50, entries 37 41) - exhibited sufficient permeability as well as aqueous solubility. Furthermore, incubations with rat liver microsomes, revealed low propensity to enzyme-mediated hydrolysis (ti/2 > 120 min), supporting slow elimination due to expected renal re-absorption of the metabolically stable esters. 

Overall board density will strongly affect core layer plasticization and density profile (Fig. 12), as at the highest overall board density a steep density gradient appears between the surface and core layers of the board. This is due to the greater difficulty encountered by the steam to penetrate and plasticize it. At lower density, the greater mat permeability enables a faster steam throughflow of the board, comparable to a steam injection process. The final result is similar as the overall board density is closer in value to both core and surface densities. 

A variety of molecules are released from cells and plasma proteins during acute inflammation whose net overall effect is to increase vascular permeability, resulting in tissue edema (Table 52—10). 

Overall, the mechanisms involved in the role of the mycobacterial cell wall as a permeability barrier are poorly understood and it is not known why MAI and M. chelonae, in particular, are more resistant than other species of mycobacteria. 

Since the membrane is permeable for cations but not for the anions A, it should intrinsically contain anions R . When these are fixed, their concentration, Cr, will remain the same everywhere. Hence in layers ( J,) and (ii) the overall cation concentration should also be the same, and the diffusion potential (which is caused by a possible difference in cation mobilities) is extremely small. In the left-hand part of the membrane system, the concentration of cations M + in each of the phases is equal to the given (invariant) concentration of anions A or, respectively the potential difference between the phases is determined, according to Eq. (5.10), by the cation concentration ratio. The right-hand part of the membrane system corresponds to the system (5.22), where phase (P) now takes the place of phase (a), and phase (rj) takes that of phase (y). As a result, we obtain for the membrane potential. 

Filter-cakes are hard to remove and thus can cause considerable formation damage. Cakes with very low permeability can be broken up by reverse flow. No high-pressure spike occurs during the removal of the filter-cake. Typically a high-pressure spike indicates damage to the formation and wellbore surface because damage typically reduces the overall permeability of the formation. Often formation damage results from the incomplete back-production of viscous, fluid loss control pills, but there may be other reasons. 

Asymmetric membranes have a tight, low-permeability, retentive zone that performs the desired separation and a more open, high-permeability zone that provides mechanical strength to the overall membrane. This structure is particularly critical to the economic viability of reverse-osmosis membranes. Asymmetric membranes operated in TFF mode must have the tight side facing the feed channel so that particles are retained on its surface and can be acted upon by the tangential flow. Asymmetric membranes operated in NFF mode can... 

The most widely employed techniques for the extraction of water samples for triazine compounds include liquid-liquid extraction (LLE), solid-phase extraction (SPE), and liquid-solid extraction (LSE). Although most reports involving SPE are off-line procedures, there is increasing interest and subsequently increasing numbers of reports regarding on-line SPE, the goal of which is to improve overall productivity and safety. To a lesser extent, solid-phase microextraction (SPME), supercritical fluid extraction (SEE), semi-permeable membrane device (SPMD), and molecularly imprinted polymer (MIP) techniques have been reported. 

The lack of understanding of the effect of the allelopathic agents on whole plant photosynthetic processes, namely, changes in stanatal opening, membrane permeability, water content and marry other processes that affect the overall photosynthetic processes. 

Gas-sensing electrodes. A gas-sensing electrode consists of a combination electrode that is normally used to detect a gas in its solution by immersion. The sensor contains the inner sensing element, usually a glass electrode or another ISE, and around this a layer of a 0.1 Af electrolyte, surrounded by a gas-permeable membrane. On immersion of the sensor this membrane contacts the solution of the gas which diffuses through it until an overall equilibrium is established, i.e., the partial pressure of the gas attains an equilibrium between sample solution and membrane and between membrane and sensor electrolyte. For a better understanding of the interaction between this electrolyte and the... 

The presence of -S02(OH) groups reduced the carbon dioxide permeability by a factor of three. This can be explained (15) by the decrease in local segmental mobility of the polymer chains due to the interactions arising from hydrogen bonding. However, the overall transport process for this polymer membrane is more complicated and involves a more pronounced discrimination against methane molecules due to the highly polar nature of the polymer. 

Similarly, the overall permeability coefficient is the reciprocal of the total resistance and thus Ptotai is ... 

Clearly the physicochemical properties of a drug are a decisive factor in its overall activity. Where possible, molecular structures should be optimized to obtain best clinical performance. Rarely does an oral drug have physicochemical features suitable for topical or transdermal therapy, and it can take a great deal of systematic research to identify where the best balance of activity and permeability lies. Experience with corticosteroids suggests that as much as a 100-fold improvement in clinical activity may be attainable through molecular design, for today s most potent topical corticosteroids are more active than hydrocortisone by a factor at least this large. 

For this reason much work has been done at the ALZA Corporation and elsewhere to increase the water permeation rates by various technologies. For example, ALZA scientists utilized a composite membrane in the development of their first commercial product with this technology [21,22], In this system they first applied a CA membrane containing a high concentration of porosigens. A second dense membrane containing only CA was added. In this way the overall fluid permeability was increased, since the thickness of the dense portion of the film could be proportionately reduced. 

The first set of data is for oil production from 22 wells. A quaternary ammonium salt polymer clay stabilizer was utilized in five of the well treatments. Otherwise the 22 well treatment designs were identical. Use of the clay stabilizer in 5 well treatments resulted in a 131% production increase compared to a 156% increase after stimulation of 17 wells without clay stabilizer. Although the initial overall production response of the five clay stabilizer treated wells was less, the overall production decline rate was 4% per year compared to 16%/yr for the treatments which did not include the clay stabilizing polymer. This decline rate was determined for the period 4 to 24 months after well treatment. It is tempting to speculate that the lower initial production response of the five polymer treated wells was due to the formation of an adsorbed polymer layer which reduced formation permeability (particularly of the Wilcox Formation) significantly. 

The determination of the evolution of the permeability of these rocks during acidizing is necessary when attempting to predict the evolution of the skin (Equation 2). Previous studies (6) have tried to model the shift of the pore size distribution due to acid attack. Then, permeability profiles were computed by integrating the contributions to the overall flow of each of the rock pores, all over the considered volume of rock. The main limitation of this method lies in the disregarding of the spatial correlation between rock pores. 

Another approach of the problem has been proposed recently (9). Based on a comprehensive experimental study, it considers that wormholes have an almost infinite conductivity in comparison with the original pores of the rock. Consequently, a fair approximation of the permeability profile of an acidized piece of rock of length L is a step function. From its inlet up to almost the tip of the wormholes, the permeability is infinite. In the rest of the rock, it is equal to the original one, i.e. ko, and the overall permeability of this core becomes ... 

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