Stress sorption

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

Stress Sorption. This mechanism supposes that the reaction between a species in the environment and the metal atoms at the crack tip can cause a redistribution of electrons in the orbits of the atoms so that the bond between them is weakened (19). It is not possible to cite experimental data at would support this concept for the fracture of metals although the absence may merely reflect the difficulties of obtaining such data. 

Stress-sorption cracking is the basic mechanism applying to stress cracking of plastics by specific organic solvents [41, 42] and to liquid-metal embrittlement—the cracking of solid metals by specific liquid metals. It is also the mechanism proposed earlier by Fetch and Stables [43] to account for stress cracking of steel induced by interstitial hydrogen (see Section 8.4). 

The anomalous increase of the water uptake observed in Fig. 10 when approaching equilibrium at 60 °C has been associated to the damage. The abrupt upturn of the sorption curve may be explained considering a possible crazing of the low crosslinked internodular matrix induced by the differential swelling stresses that can arise, at high water contents, between areas of different crosslinking density. 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

Hence, for a PAH with Kiov/ = 106 we would obtain a Knipoc of about 2 kg oc - kg 1 lip as compared to 24 kg oc-kg 1 lip for a PCB with the same Ki0Vf. This would explain the differences found in the field. We should, however, stress again that A, oc values as predicted from LFERs such as Eqs. 10-19 and 10-25 reflect sediment organic matter-water partitioning and not sorption to highly active sorbents (e.g., soot) that may be present in sediments. Thus, very low BSAFaipoc values found in the field not only may reflect disequilibrium but also may be due to the presence of such sorbents, which are particularly important for sorption of PAHs (see Illustrative Example 9.3). 

Humidity control. Moisture is an important factor in the stability of many pharmaceutical compounds and formulations (22,23). The decision to control moisture for comparative stress studies should depend on the characteristics of the compound and the information desired. The moisture content of typical samples, moisture sorption as a function of relative humidity, and the effect of moisture on degradation can help guide the decision to control humidity. At a minimum, samples being compared should be exposed to the same ambient humidity conditions in the storage chamber. 

Humidity control The question of humidity control was examined in several ways. The moisture sorption isotherm showed moisture levels from 3-6% over a relative humidity range of 20-95%. Also, samples held at 65°C for over 2 weeks remained within this range of water content, which was within pharmacopeial specification. Finally, samples which were heated did not show a change in crystal form by X-ray powder diffraction, such as partial conversion to an anhydrate. Based on these results, humidity control was not deemed necessary for comparative stress studies. Samples to be compared were all stored under identical conditions, i.e., in the same oven. Any local change in humidity around each sample was therefore a function of the sample itself. 

Surface sizing can induce structural changes in the paper sheet185 due to the interaction of water sorption (which causes a relaxation of internal stresses) and machine direction tension (which increases anisotropy and creates additional stresses). Anisotropy can be lowered by reducing tension on the web during sheet passage... 

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