Phase collapse phenomenon

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.

Taking into account the usual conditions of the hydrogel application, we shall limit our discussion to that part of the phase diagram which depicts the swollen gel and touch upon the collapse only in passing since a detailed analysis of this phenomenon is given by Tanaka [96, 97], Ilavsky [19, 98], and Khokhlov [99],... 

The list of the new gels for which phase transitions are possible is supplemented in the paper by Amiya and Tanaka, who discovered discrete collapse for the most important representatives of biopolymers - chemically crosslinked networks formed by proteins, DNA and polysaccharides [45]. Thus, it was demonstrated that discrete collapse is a general property of weakly charged gels and that the most important factor, which is responsible for the occurrence of this phenomenon, is the osmotic pressure of the system of counter ions. 

Figures 4-b and 4-d depict the pore size distribution curves of the SBA samples after these different treatments. For the sample SBA-A treated in acidic medium, the BET surface area (869 m2g" ), the mean pore diameter (6.4 nm) and the pore size distribution curve are similar to those from the pure parent silica SBA. For neutral treatment, the surface area (667 m2 g 1) decreases slightly. This can be related to the reduction of the microporous phase of the sample as shown in the pore size distribution curve. However, the mean pore diameter remains unchanged. Conversely, the structural properties of SBA-B are modified after treatment in basic solution. In this case, we observe a strong decreasing of the specific surface (454 m2 g 1) accompanied by a total loss of the microporous phase and an increasing of the mean mesoporous diameter (7.2 nm). It seems that in basic medium, a leaching phenomenon inside the mesoporous channels does occur, leading to a partial dissolution of the wall and resulting in smaller wall thickness (4.3 nm). Compared with the results on MCM-41, which show that the mesoporous structure collapses in basic solution [9,10], we can say that the stability of SBA materials in this medium is much higher.

Periodic reactions of this kind have been mentioned before, for example, the Liese-gang type phenomena during internal oxidation. They take place in a solvent crystal by the interplay between transport in combination with supersaturation and nuclea-tion. The transport of two components, A and B, from different surfaces into the crystal eventually leads to the nucleation of a stable compound in the bulk after sufficient supersaturation. The collapse of this supersaturation subsequent to nucleation and the repeated build-up of a new supersaturation at the advancing reaction front is the characteristic feature of the Liesegang phenomenon. Its formal treatment is quite complicated, even under rather simplifying assumptions [C. Wagner (1950)]. Other non-monotonous reactions occur in driven systems, and some were mentioned in Section 10.4.2, where we discussed interface motion during phase transformations. 

The volume change in these gels is not due to ionic effects, but rather to a thermodynamic phenomenon a lower critical solution temperature (LCST). The uncrosslinked polymer which makes up the gel is completely miscible with water below the LCST above the LCST, water-rich and polymer-rich phases are formed. Similarly, the gel swells to the limit of its crosslinks below the LCST, and collapses above the LCST to form a dense polymer-rich phase. Hence, the kinetics of swelling and collapse are determined mostly by the rate of water diffusion in the gel, but also by the heat transfer rate to the gel. 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the collapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of applications of ultrasound to enhancing chemical reactivity has been explored, with important applications in mixed-phase synthesis, materials chemistry, and biomedical uses. 

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