Collapse desorption

There appear to be two stages in the collapse of emulsions flocculation, in which some clustering of emulsion droplets takes place, and coalescence, in which the number of distinct droplets decreases (see Refs. 31-33). Coalescence rates very likely depend primarily on the film-film surface chemical repulsion and on the degree of irreversibility of film desorption, as discussed. However, if emulsions are centrifuged, a compressed polyhedral structure similar to that of foams results [32-34]—see Section XIV-8—and coalescence may now take on mechanisms more related to those operative in the thinning of foams. 

With liquid and semi-solid dosage forms, the rate and the extent of desorption are influenced by the solvent system of the preparation, pH, and temperature conditions during processing and storage. If a severe problem exists, it will usually manifest itself via an outward sign, such as container collapse, product discoloration, or precipitation [14]. 

Pore size plays a key role in determining permeability and permselectivity (or retention property) of a membrane. The structural stability of porous inorganic membranes under high pressures makes them amenable to conventional pore size analysis such as mercury porosimetry and nitrogen adsor-ption/desorption. In contrast, organic polymeric membranes often suffer from high-pressure pore compaction or collapse of the porous support structure which is typically spongy . 

The two examples of adsorbed side chain substituted macromolecules, i.e., the poly(n-butyl acrylate) brush and the tris(p-undecyloxybenzyloxo) benzoate jacketed polystyrene, demonstrate two rather complementary aspects of the interaction of such molecules with a planar surface. In the first case the two-dimension to three-dimension transition results in a cooperative collapse of an extended coil conformation to a globule. The second case shows a rather high degree ordering with a distinct orientation of the backbone in the substrate plane. Combination of both effects and partial desorption can lead to a repta-tion-hke directed motion as depicted schematically in Fig. 36. 

The properties of both organic matter and clay minerals may affect the release of contaminants from adsorbed surfaces. Zhang et al. (1990) report that desorption (in aqueous solution) of acetonitrille solvent from homoionic montmorillonite clays is reversible, and hysteresis appears to exist except for K+-montmorillonite. This behavior suggests that desorption may be affected by the fundamental difference in the swelling of the various homoionic montmorillonites, when acetonitrile is present in the water solution. During adsorption, it was observed that the presence of acetonitrile affects the swelling of different homoionic clays. At a concentration of 0.5 M acetonitrile in solution, the layers of K+-montmorillonite do not expand as they would in pure water, while the layers of Ca +- and Mg +-montmorillonite expand beyond a partially collapsed state. The behaviors of K+-, Ca +-, and Mg +-montmorillonite are different from the behavior of the these clays in pure water. Na+-montmorillonite is not affected by acetonitrile presence in an aqueous solution. 

Fig. 45 When cylindrical brushes possessing a gradient grafting density along the backbone are adsorbed on a surface, one can observe a transition from a rod-like to a tadpole conformation upon partial desorption of side chains. The end with a higher grafting density, and thus with a greater extension of the side chains, is predicted to collapse more readily than the loose end (Reproduced with permission from [173])...

As indicated by XRD patterns, there exist just 2-3 broad peaks in the calcined acid-made materials (Fig. 3A). Moreover, the N2 adsorption/desorption isotherm shown in Fig. 3B, the calcined acid-made mesoporous silica indeed possesses a broad capillary condensation at the partial pressure p/p0 of ca. 0.2-0.4, indicating a broad pore size distribution with a FWHM ca. 1.0 nm calculated from the BJH method. This is attributed to the occurrence of partial collapse of the mesostructure during the high temperature calcination. The hexagonal structure completely collapsed when subjected to further hydrothermal treatment in water at 100 °C for 3 h. Mesoporous silica materials synthesized from the acid route are commonly believed to be less stable than those from the alkaline route [6,7]. 

Fig. 1 also shows the XRD patterns of the samples after hydrothermal treatment at 100°C for 4 days. It can be seen that samples prepared with additional TPA are quite stable to hydrothermal treatment. In contrast, the sample without TPA collapsed after such treatment. The nitrogen adsorption-desorption isotherms of calcined and hydrothermally treated MCM-... 

It should be pointed out at this juncture that strict thermodynamics treatment of the film-covered surfaces is not possible [18]. The reason is difficulty in delineation of the system. The interface, typically of the order of a 1 -2 nm thick monolayer, contains a certain amount of bound water, which is in dynamic equilibrium with the bulk water in the subphase. In a strict thermodynamic treatment, such an interface must be accounted as an open system in equilibrium with the subphase components, principally water. On the other hand, a useful conceptual framework is to regard the interface as a 2-dimensional (2D) object such as a 2D gas or 2D solution [ 19,20]. Thus, the surface pressure 77 is treated as either a 2D gas pressure or a 2D osmotic pressure. With such a perspective, an analog of either p- V isotherm of a gas or the osmotic pressure-concentration isotherm, 77-c, of a solution is adopted. It is commonly referred to as the surface pressure-area isotherm, 77-A, where A is defined as an average area per molecule on the interface, under the provision that all molecules reside in the interface without desorption into the subphase or vaporization into the air. A more direct analog of 77- c of a bulk solution is 77 - r where r is the mass per unit area, hence is the reciprocal of A, the area per unit mass. The nature of the collapsed state depends on the solubility of the surfactant. For truly insoluble films, the film collapses by forming multilayers in the upper phase. A broad illustrative sketch of a 77-r plot is given in Fig. 1. 

This simplified description of dealumination by steaming shows that the increase of the framework Si/Al ratio is accompanied by other changes in the zeolite characteristics. The partial collapse of the zeolite leads to the creation of secondary pores (supermicropores and mesopores) at the expanse of the initial micropores. The size and number of secondary pores increase with the degree of framework dealumination38. These secondary pores were shown to have a pronounced positive effect in liquid phase reactions which are often limited by product desorption. For these reactions (e.g. in alkylation of toluene with 1-heptene)32, the maximum in activity is significantly displaced to high values of the framework Si/Al ratio. These secondary pores were also... 

The daughter nuclide may be stable or unstable (radioactive), debris disk a circumstellar disk in which the majority of the dust is not derived from the collapsing molecular cloud, but from the collisions of minor bodies in the disk. The typical masses and optical depths of these disks are several orders of magnitudes lower than those typical to accretion disks, desorption changing from an adsorbed state on a surface to a gaseous or liquid state. 

An interesting approach was adopted by Annabi-Bergaya et al. (1979), who determined methanol and isopropanol desorption isotherms on a series of charge-deficient Ca-montmorillonites (prepared from Na- and Li-saturated montmorillonite). Each desorption isotherm was determined after a surface cleaning process with the particular alcohol in order to prevent the irreversible collapse of the interlamellar space. The adsorbent was exposed to the alcohol vapour atp/p° = 0.9 and the stepwise mass... 

A positive influence of the internal collapse (internal dephlegmation) on the accumulation ratio (i.e. its increase) was observed only in the cases where a very small cl.o value lies in the range cm < c 0 < cr (cr, corresponds to the saturated adsorption layer). The reason is that desorption from film surfaces during foam collapse increases surfactant concentration in the foam liquid phase, and, as a consequence, the degree of adsorption increases to values that ensure the formation of a small volume of a stable foam. Therefore, it becomes possible to separate the foam from the original solution with Rf>. ... 

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