Pore expansion

Figure 4 The free energy of pore formation in a DPPC bilayer. The dashed line is a quadratic function, while the dotted line is a fit to a model of pore expansion with a line tension of 40 pN, and is close to linear (Adapted from ref. 78 courtesy of O. Edholm).

The process utilizing supramolecular organization involves pore expansion in silicas. A schematic view of such micelles built from the pure surfactant and those involving in addition n-alkane is shown in Figure 4.9. Another example of pore creation provides a cross-linking polymerization of monomers within the surfactant bilayer [30]. As a result vesicle-templated hollow spheres are created. Dendrimers like that shown in Figure 4.10 exhibit some similarity to micellar structures and can host smaller molecules inside themselves [2c]. Divers functionalized dendrimers that are thought to present numerous prospective applications will be presented in Section 7.6. 

In addition to functioning as Ca2+-sensors for vesicle exocytosis, synaptotag-mins may be involved in vesicle endocytosis, particularly the decision between kiss-and-run versus full exocytosis. Such a role would be economical in linking fusion-pore opening (which is triggered by Ca2+-binding to synaptotagmin) to fusion-pore expansion or contraction, but the precise mechanisms involved have not yet been explored. 

Figure 8.43 Schematic representation of MCM-41 pore-expansion, selective extraction, and calcination. Reproduced with permission from [201]. Copyright (2005) American Chemical Society...

Post-synthesis hydrothermal treatment in salt solution[202] could be a convenient method for pore expansion and silica-wall thickening for improvement of its stability. The pore size and wall thickness vary with the type of anion in the salt and its concentration. The salt effect follows the well known binding strength of the Hofmeister series of anions for the cationic surfactant, N03 > Br > Cl- > SO 2 FT The anion binds with cationic surfactant molecules in solution to shift the equilibrium of surfactant/silicate binding, leading to less surfactant and water in the pore, and hence less pore expansion. 

The unchanging amount of A1 in the zeolite MOR samples indicates that the isomorphous substitution of the Fe cations takes place at the expenses of the framework Si. Most likely, the observed pore expansion of the Al-Fe-MOR zeolite, demonstrated from the data of Fig. 1, is due by the substitution of the longer Fe-0 bond (1.84 A) in the place of the Si-O bond (1.61 A). 

A similar behaviour was also shown by the BEA type zeolite. Fig. 2. The results show that the iron-poor sample (B3) present an adsorption capacity equal to the iron-free form (sample B2). Only at high pressure the adsorption capacity of the sample B3 is greater than that of the sample B2. On the contrary the iron-rich form (sample B4) presented, in the entire pressure range, a higher adsorption capacity. As in the case of the MOR type zeohte, we can assume that also in the BEA zeolite iron is incorporated in the framework. As a support to this hypothesis there are the differences in the morphology and crystal size and the the increased adsorption capacities interpreted in terms of pore expansion due to the formation of Fe-0 bond in the BEA structure. 

Behnabkhout Y, Sayari A (2009) Effect of pore expansion and amine functionalization of mesoporous sdica on CO2 adsorption over a wide range of conditions. Adsorption 15 318-328... 

Motkuii, R. K., Xhallapally, R. K., Nrme, S. K., Fernandez, C. A., McGrail, B. R, and Atwood, J. L. Role of hydrocarbons in pore expansion and contraction of a flexible metal-oiganic framework. Chem Commun 47, lQni-l(fl9 (2011). 

The application of pore-expended sulfonic acid SBA-15 has been demonstrated for the first time by Dacquin et al. (2012). In their smdy, the impact of pore-expended sulfonic functionalized SBA-15 toward palmitic acid in esterification and transesterification of tricaprylin and triolein has been explored. Large-pore SBA-15 was obtained by incorporation of trimethylbenzene (TMB) into Pluronic P123/tetraethyl orthosilicate (TEOS) and been aged for 1—3 days. Results showed pore diameters up to 14 nm were achieved through this method, with pore-expansion conferring >3-fold activity toward Cie FFAs esterification and Cg/Cig transesterification reaction (Dacquin et al., 2012). 

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