The crevice model

Concerning the stabilization of bubbles by hydrophobic ions in water, Akulichev has hypothetized that ions such as Cl, F, etc. migrate to the bubble surface while others (like OH ) cannot.52 The basis of this model is that the repulsion between electrical charges of the same polarity on the bubble surface slows down or prevents its dissolution. Atchley repeated the experiments, extending the range of dissolved ions by 3 orders of magnitude (lO to 10 2 M), and provided the proof that the concentration of dissolved ions is important in the stabilization process. Until now all the experiments have been carried out with water, and the field remains open as far as organic solvents are concerned. 

Gernez was the first to suggest that crevices in solids kept in suspension by the Brownian motion may act as nucleation sites.54 jn 1944 Harvey et al suggested that small amounts of gas can be trapped in conical crevices in solid inhomogeneities present in liquids.55 In view of its general appearance, this model has received considerable attention. Solvents such as water contain numerous small 

57 Crum, L.A. in Cavitation and Inhomogeneities in Underwater Acoustics (Lauterbom, W. Ed.), Springer-Verlag, Berlin, 1980, pp. 84-89 id. Nature 1980,278, 148-149. 

59 Trevena, D.H. Cavitation and Tension in Liquids Hilger, Bristol, 1987. 

Atchley and Prosperetti noted that this description is incomplete. Indeed, as the meniscus recedes, not only does the Laplace pressure fall, but so does the partial pressure of the gas within the pocket. If the internal pressure decreases more rapidly than the Laplace pressure, the interface stops progressing to the crevice mouth. On the contrary, the growth will be unstable if the partial pressure in the gas pocket decreases less rapidly than the Laplace pressure, and the growth is explosive . 

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Problems with the crevice model for bubble nuclei... 

However, in addition to many problems in describing bubble nucleation in aqueous gels with the crevice model (see below, and Section 3.2), several fundamental observations suggest that this mechanism is not applicable to aqueous media in general (ref. 114). 

According to the lock-and-key model, an enzyme is pictured as a large, irregularly shaped molecule with a cleft, or crevice, in its middle. Inside the crevice is an active site, a small region with the shape and chemical composition necessary to bind the substrate and catalyze the appropriate reaction. In other words, the active site acts like a lock into which only a specific key can fit (Figure 24.10). An enzyme s active site is lined by various acidic, basic, and neutral amino acid side chains, all properly positioned for maximum interaction with the substrate. 

In summary, nuclear models of the crevice type consist essentially of gas phases stabilized in crevices in solid particles. While the crevice hypothesis represents a viable nuclear model, none of the existing mathematical treatments make predictions that are supported by the above-mentioned gelatin experiments (ref. 114). In addition to these problems with the mathematical devel-... 

For some materials (e.g., nickel alloys), the current is a direct measure of the rate of crevice propagation. For systems such as titanium alloys, however, internal cathodic reactions are also possible, as is illustrated in Fig. 29. This figure shows schematically the important electrochemical and chemical reactions occurring within the creviced area and on the coupled counterelectrode. This system will be used to illustrate the information that can be obtained from this galvanic coupling technique and how it can then be used directly in the development of models. 

Based on this information and these relationships a model framework can now be developed, Fig. 34. The four primary inputs to the model are the crevice propagation rate as a function of 02 concentration and temperature (A and B), a knowledge of the time-evolution of temperature and oxygen concentration within the waste vault (C), and a transport model for the flux of 02 to the container... 

The crystal structure of human albumin located Cysteine-34 at the turn between helices h2 and h3 with the side chain sulfhydryl group oriented toward the protein interior, consistent with EPR studies suggesting that it is 950 pm below the surface. Sadler has demonstrated by H NMR studies that the cys-34 residue must move outward from the crevice created by the helices before it can react to form disulfide bonds or bind to Et3PAu+ derived from auranofin. These structural observations are consistent with the kinetic mechanism for the reactions of albumin with auranofin and its triisopropylphosphine analogue, which revealed a slow crevice opening reaction in equilibrium between open and closed forms of albumin. The kinetic model accounts for a process that is first order in protein when the auranofin is present in excess. 

Stronger. Taking the rectangular crevice as a model, the fluid in the crevice essentially becomes quiescent at an aspect ratio of 5. 

The sigmoidal decomposition curves can be interpreted using the Prout and Tompkins model. This model assumes that the decomposition is governed by the formation and growth of active nuclei which occur on the surface as well as inside the crystals. The formation of product molecules sets up further strains in the crystal since the surface array of product molecules has a different unit cell from the original substance. The strains are relieved by the formation of cracks. Reaction takes place at the mouth of these cracks owing to lattice imperfections and spreads down into the crevices. Decomposition on these surfaces produces further cracking and so the chain reaction spreads. 

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