Block effects

Masson F and Rabalais J W 1991 Surface periodicity exposed through shadowing and blocking effects Chem. Rhys. Lett. 179 63-7... 

In a pore system composed of isolated pores of ink-bottle shape, the intrusion curve leads to the size distribution of the necks and the extrusion curve to the size distribution of the bodies of the pores. In the majority of solids, however, the pores are present as a network, and the interpretation of the mercury porosimetry results is complicated by pore blocking effects. 

The molecular features of covalent hydration are also present in the dihydroxy series, i.e., in pteridine-2,6-dione (30) and in pteridine-4,6-dione. The latter compound is hydrated only at the C(7)—N(8) double bond, whereas (30) forms two hydrated species, 7-hydroxy-7,8-dihydro- (29) and 4-hydroxy-3,4-dihydro-pteridin-2,6-dione (31) (equation 8). Structure (29) is thermodynamically the more stable substance (31) is formed more rapidly in solution but disappears slowly with time (63JCS5151). Insertion of a 4-methyl group greatly reduces the extent of 3,4- in favour of 7,8-hydration by a blocking effect . 

The blocking effect does not apply to the Pitot-static tube alone. Any sensor/instrument immersed into a duct has a similar effect the larger the sensor is, the greater the problem. For other types of instruments an analysis must be made, so as not to block large proportion of the duct cross-section with the meter. A good rule of thumb to avoid corrections is to keep the cross-section of the meter less than 5% of the duct cross-section. 

To solve the replica OZ equations, they must be completed by closure relations. Several closures have been tested against computer simulations for various models of fluids adsorbed in disordered porous media. In particular, common Percus-Yevick (PY) and hypernetted chain approximations have been applied [20]. Eq. (21) for the matrix correlations can be solved using any approximation. However, it has been shown by Given and Stell [17-19] that the PY closure for the fluid-fluid correlations simplifies the ROZ equation, the blocking effects of the matrix structure are neglected in this... 

C. Blocking Effect of a Methyl Group One of the most valuable means for locating the site of hydration involves the blocking effect of a methyl group. 

The blocking effect of the methyl group in 4-methylpteridine has been found to decrease the ratio of the hydrated to the anhydrous species in the neutral molecule and in the cation,but the small proportion of hydrated cation rapidly undergoes the ring-opening reaction 19 20 and hence is steadily regenerated from its anhydrous form. 2-Methylpteridine behaves like pteridine, but in 7-methyl-pteridine the action at a distance (+M) effect of the methyl group (see Section III,C,2) produced a smaller ratio of the hydrated to anhydrous species. 

If a methyl group replaces a hydrogen atom on the carbon of the C==N bond across which addition of water occurs, a considerable reduction in the extent of water addition is observed. Conversely, the existence of such a blocking effect can be used as a provisional indication of the site at which addition of water occurs, while the spectrum and acid dissociation constant of the methyl derivative provide a useful indication of the corresponding properties of the anhydrous parent substance. Examples of the effect of such a methyl group on equilibria are given in Table IV. 

The guanidine function, when attached to an appropriate lipophilic function, often yields compounds that exhibit antihypertensive activity by means of their peripheral sympathetic blocking effects. Attachment of an aromatic ring via a phenolic ether seems to fulfill these structural requirements. Alkylation of 2,6-dichlorophenol with bromochloroethane leads to the intermediate, 58. Alkylation of hydrazine with that halide gives 59. Reaction of the hydrazine with S-methylthiourea affords the guanidine, guanoclor (60). ... 

Blocking of reaction sites The interaction of adsorbed inhibitors with surface metal atoms may prevent these metal atoms from participating in either the anodic or cathodic reactions of corrosion. This simple blocking effect decreases the number of surface metal atoms at which these reactions can occur, and hence the rates of these reactions, in proportion to the extent of adsorption. The mechanisms of the reactions are not affected and the Tafel slopes of the polarisation curves remain unchanged. Behaviour of this type has been observed for iron in sulphuric acid solutions containing 2,6-dimethyl quinoline, /3-naphthoquinoline , or aliphatic sulphides . 

Cu, Ni, Co, Cr, Fe, or Al, even in traces, must be absent when conducting a direct titration of the other metals listed above if the metal ion to be titrated does not react with the cyanide ion or with triethanolamine, these substances can be used as masking reagents. It has been stated that the addition of 0.5-1 mL of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with solochrome black and also with xylenol orange (see below). 

Drowsiness and sedation are common adverse reactions seen with the use of many of the antihistamines. Some antihistamines appear to cause more drowsiness and sedation than others. These dm may also have varying degrees of anticholinergic (cholinergic blocking) effects, which may result in dryness of the mouth, nose, and throat and a thickening of bronchial secretions. Several newer preparations (eg, loratadine) cause little, if any, drowsiness and fewer anticholinergic effects than some of the other antihistamines. Hiotosensitivity may occur with the use of the antihistamines. 

ADMINISTERING DISOPYRAMIDE. Because of the cholinergic blocking effects of disopyramide (see Chap. 25), urinary retention may occur. The nurse monitors the urinary output closely, especially during the initial period of therapy. If the patient s intake is sufficient but the output is low, the lower abdomen is... 

The rules are simple as long as we also keep site-blocking effects in mind and as long as we can identify the electron acceptor or electron donor character of the various adsorbates. 

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