Cavity formation

Postma J P M, H J C Berendsen and J R Haak 1982. Thermodynamics of Cavity Formation ui Wat - Faraday Symposium of the Chemical Society 1755-67. 

Fig. 20. Cavity formations behind impeller blades where (a) illustrates clinging cavities, (b) a large cavity, (c) 3—3 cavities, (d) alternating large and larger...

Fig. 20.4. Investment casting of turbine blades. This produces a fine-grained material which may undergo a fair amount of diffusion creep, and which may fail rather soon by cavity formation.

However, in LC solutes are partitioned according to a more complicated balance among various attractive forces solutes interact with both mobile-phase molecules and stationary-phase molecules (or stationary-phase pendant groups), the stationary-phase interacts with mobile-phase molecules, parts of the stationary phase may interact with each other, and mobile-phase molecules interact with each other. Cavity formation in the mobile phase, overcoming the attractive forces of the mobile-phase molecules for each other, is an important consideration in LC but not in GC. Therefore, even though LC and GC share a considerable amount of basic theory, the mechanisms are very different on a molecular level. This translates into conditions that are very different on a practical level so different, in fact, that separate instruments are required in modern practice. 

Where FCl is the solute gas-liquid partition coefficient, r is the tendency of the solvent to interact through k- and n-electron pairs (Lewis basicity), s the contribution from dipole-dipole and dipole-induced dipole interactions (in molecular solvents), a is the hydrogen bond basicity of the solvent, b is its hydrogen bond acidity and I is how well the solvent will separate members of a homologous series, with contributions from solvent cavity formation and dispersion interactions. 

Examine pressure heads and maximise whenever reasonably practicable to minimise vapour cavity formation. 

The effect of carbon on the corrosion of stainless steels in liquid sodium depends upon the test conditions and the composition of the steels . Stabilised stainless steels tend to pick up carbon from sodium, leading to a degree of carburisation which corresponds to the carbon activity in the liquid metal. Conversely, unstabilised stainless steels suffer slight decarburisation when exposed to very pure sodium. The decarburisation may promote corrosion in the surface region of the material and, under creep rupture conditions, can lead to cavity formation at the grain boundaries and decreased strength. 

Where AG is the free energy term associated with cavity formation, N is Avogadro s number, AA is the change in surface area due to... 

The nonelectrostatic components of the free energy such as the energy of cavity formation AGcav or components that take into account atomistic details of the medium (interactions between atoms inside the cavity and those in the medium) are calculated using empirical approximations (see Reference 164 for review or 165 for recent developments). These terms are do not affect the SCF procedure since their dependence on electron density p is usually neglected. 

Postma, J. Berendsen, H. Haak, J., Thermodynamics of cavity formation in water. A molecular dynamics study, Far. Symp. Chem. Soc. 1982,17, 55-67... 

The preceding calculations can also be performed for finite cavity sizes. For this case, there are some additional sources of small amounts of energy associated with cavity formation arising from surface tension, pressure-volume work, and electrostriction. Because of the Franck-Condon principle these do not affect the transition energy, but they have some influence on the heat of solvation. Jortner s (1964) results are summarized as follows ... 

Our extension of the LIE approach to calculate free energies of hydration (AGhyd) incorporated a third term proportional to the solute s solvent-accessible surface area (SASA), as an index for cavity formation within the solvent.19,27 The latter term is needed for cases with positive AGhyd such as alkanes and additional improvement occurred when both a and P were allowed to vary. Equation 8 gives the corresponding LIE/SA equation for... 

Hydrophobic interactions appear when a non-polar compound is transported into aqueous media. They include the following steps separating the non-polar molecule from its non-polar surrounding, filling up this empty space in the non-polar medium with water, cavity formation accounting for the interactions between water and the non-polar molecules, and reorganizing the water molecules around the non-polar solute. 

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