Phase collapse

The structure of M41S-type materials is built up of pores with amorphous walls that are formed around micelles of templating material (surfactants). One of the extreme structures of M41S-type materials (MCM-41) is a hexagonal ordering of the pores, an other extreme is a worm-hole disordered type of arrangement of the pores. A lamellar layered structure is another form in which these type of materials often (partially) appear, but this phase collapses to amorphous material upon removal of the surfactant (eg by calcination). A cubic ordering of the pores is also encountered. This form has been named MCM-48 and will not be discussed in the current paper. 

Determination of the phase purity of mesoporous molecular sieves (MMSs) [1,2] is important in synthesis, modification and application of these materials [3-7]. Many of the synthesis procedures reported so far involved various phase transformations [8-20] and thus the desired MMS product may be contaminated with some mesostructured impurities. One of the possible impurities is a lamellar phase, which readily forms under various synthesis conditions [1,8-25]. Because of its layered structure, the lamellar phase collapses upon calcination [1] and therefore constitutes a disordered impurity of calcined MMS samples. 

If the H and L phases present in HL mixtures have the same adsorption properties as those of the pure H and L phases, respectively, the fitting coefficients xc(H) and xc(L) provide the mass fraction of the H and L phases in the calcined samples. Otherwise, for instance when the pore volumes of the hexagonal phases differ, the fitting coefficients are proportional to the phase contents of H and L phases in the HL sample, as discussed in detail elsewhere [28]. It should be noted here that the lamellar phase collapses during calcination, thus losing its structural ordering [1]. So, the calcined lamellar phase is actually disordered [1,28]. 

Near a critical point, the parent p coexists with another phase that is only slightly different if, as we assume here, the free energy function is smooth, these two phases are separated—in p-space—by a hypothetical phase which has the same chemical potentials but is (locally) thermodynamically unstable. [This is geometrically obvious even in high dimensions between any two minima of f p)—p p, at given p, there must lie a maximum or a saddle point, which is the required unstable phase. ] Now imagine connecting these three phases by a smooth curve in density space p(e). At the critical point, all three phases collapse, and the variation of the chemical potential around p e = 0) = p must therefore obey... 

Przybyciel, M. Majors, R.E. Phase Collapse in Reversed-Phase Liquid Chromatography, LC-GC 20, 516-523 (2002). 

Figure 3.16. Chromatogram illustrating the phenomenon of phase collapse for convention C18 phases when used with highly aqueous mobile phases (chromatogram on the left-hand side). Polar-embedded phases are not prone to phase collapse and are better suited for the separation of very water-soluble analytes. Diagram courtesy of Dionex Corporation.

Foam is a gas/liquid dispersion system, with gas bubbles forming an inner non-continuous phase and liquid forming a continuous phase. As bubbles pass through a liquid solution, surface-active compounds preferentially adsorb onto a bubble surface. The surface-active compounds can be carried out from the liquid phase by these bubbles into a foam phase, which can be formed when these bubbles accumulate above the gas/liquid pool interface. The most strongly surface active component or component with the largest bubble net adsorption rate in the liquid solution will have the highest relative adsorption in the foam phase. When the foam phase collapses to form a new liquid phase, a liquid solution can be produced with a... 

W emulsions are formed at low electrolyte concentrations, < 0.3M NaCl, and W/0 emulsions are formed at high electrolyte concentrations, > 0.3M NaCl. The continuous phase at 0.3M NaCl was indeterminable, by the simple method previously discussed. This emulsion phase collapsed to a value of V /Vm of 0.1 between 10,000 minutes (limit of time scale of Figure 11) and 15,000 minutes (last observation point) all the other emulsion phases formed with univalent alkalis at high pH s flocculated to V /Vrj. = 0.5 over the observation period. The flocculation rate of the W/O emulsions is faster than the 0/V7 emulsions because of double layer effects (25). The W/O emulsions flocculated but did not coalesce because of the presence of interfacial films discussed by Wasan et al. (17). The relative rates of flocculation of either the O/W or the W/O emulsions appear to depend on the concentration of electrolyte however, the data are insufficient to make a more definitive statement at this time. 

At a normal chiral nematic to smectic A transition, the helical ordering of the chiral nematic phase collapses to give the layered structure of the smectic A phase. However, for a transition mediated by a TGB phase, there is a competition between the need for the molecules to form a helical structure due to their chiral packing requirements and the need for the phase to form a layered structure. Consequently, the molecules relieve this frustration by trying to form a helical structure, where the axis of the helix is perpendicular to the long axes of the molecules (as in the chiral nematic phase), yet at the same time they also try to form a lamellar structure, as shown in Fig. 21. These two... 

The initial sharp fall in bed height relates to the escape of bubbles immediately after the gas flux has been cut off Thereafter, the dense phase collapses linearly with time in the manner of a homogeneous bed as described in Chapter 5. Extrapolating this linear segment back to the H axis yields the height Hd of a notional homogeneous bed from which its void fraction, the required e, follows from a knowledge of the total volume Vp of particles per unit area of bed cross-section Vp = Hd ed). 

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