Structure Transition from Hexagonal Mesostructured Rodlike Silica to Multilamellar Vesicles

Abstract

We studied the synthesis of siliceous structures by using a nonionic block copolymer (Pluronic P123) and perfluorooctanoic acid (PFOA) as cotemplates in an acid-catalyzed sol-gel process. Different siliceous structures were obtained through systematically varying the molar ratio (R) of PFOA/P123, and the resultant materials were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen sorption analysis, and Fourier-transform infrared spectroscopy. The results are consistent and reveal a structure transition from a highly ordered 2D hexagonal (HEX) mesostructure with a rodlike morphology to multilamellar vesicles (MLVs) with sharp edges when R is increased. The fact that the MLVs are initiated from the end of hexagonally mesostructured rods provides key evidence in such a novel structure transition. Our finding indicates that, at least in our observations, the MLVs are developed gradually from HEX structures, rather than by a direct cooperative self-assembly mechanism. It is suggested that PFOA molecules with rigid fluorocarbon chains closely interact with PEO. This interaction model may well explain (1) the "wall-thicken" effect in HEX mesostructures by enlarging the hydrophilic PEO moiety (R = 0-1.4), (2) the subsequent HEX to multilamellar structure transition by modifying the hydrophilic/hydrophobic volume ratio (R = 1.4-2.8), and (3) the formation of MLVs with sharp edges by increasing the bending energy. This model provides insight into the fabrication of novel porous materials by the use of block copolymers and fluorinated surfactant mixed templates.

Periodic Mesoporous Organosilicas with Helical and Concentric Circular Pore Architectures

“…[41] They are more hydrophobic, more surface active compared to the hydrocarbon counterparts, and can reduce the surface tension of aqueous solutions to much lower values. [41] It has been reported that by the addition of fluorinated molecules as co-templates, the particle sizes of mesoporous materials can be greatly reduced.A C H T U N G T R E N N U N G [39,42] The size change observed in our study is also attributed to the property of PFOA molecules in reducing surface tension. On one hand, PFOA molecules interact with the surfactants and influence the cooperatively self-assembled mesostructure.…”

“…[39,40] When PFOA is mixed with block copolymer surfactants in strong acid condition, PFOA with stiff fluorocarbon chains is inclined to interact with poly(ethylene oxide) moiety of Pluronic nonionic block copolymer to adjust the hydrophilic/hydrophobic volume ratio (V H /V L ) and finally leads to the structure evolution from hexagonal to multilamellar vesicular phase. [39] It has also been reported that there is electrostatic interaction between a cationic surfactant (cetyltrimethylammonium bromide, C 16 TAB) and PFOA under basic conditions, [40] which is responsible for the observed structural transformation from the helical hexagonal mesostructure to the vesicular structures with increasing PFOA/C 18 TAB ratio. As can be seen from the FTIR spectrum (Figure 6), the interaction between PFO À and C 18 TA + is similar to that observed in the PFO À and C 16 TA + system.…”

Macro-pore Volume Mesoporous Alumina: A Promising Support for Mo-Ni-P Catalyst in Hydrodesulfurization

“…The catalyst only exhibits very broad XRD lines of γ -Al 2 O 3 and no additional lines belonging to crystalline Mo, Ni, and P compounds are observed. This means that supported species are highly dispersed on the alumina surface, neither completely amorphous nor composed of oxide crystallites (Rana et al, 2005;Yuan et al, 2008). Figure 3a is an SEM image and shows typical morphologies of MMA.…”

Controlled evolution from multilamellar vesicles to hexagonal mesostructures through the addition of 1,3,5-trimethylbenzene

“…However, the transition from uni-or multilamellar vesicles to highly ordered mesostructures is yet to be fully understood. Yuan et al [23] reported a structure evolution from a highly ordered 2-D hexagonal mesostructure to multilamellar vesicles with sharp edges by increasing the molar ratio of PFOA/P123 (where PFOA is perfluorooctanoic acid). Therein, different amounts of PFOA were introduced before the addition of tetraethyl orthosilicate (TEOS) as a silica source.…”

“…Generally, the inorganic/surfactant mesophase can be explained and predicted by the effective hydrophilic-hydrophobic volume balance in the context of the classical micellar packing parameter (g), g = V/a 0 l, where V is the total volume of the hydrophobic surfactant chain, a 0 is the effective hydrophilic headgroup area at the aqueous-micellar surface, and l is the kinetic surfactant tail length [14,15]. According to this model, various methods have been used to control the organization of mesostructures and achieve structural transitions, such as varying reaction temperatures [16,17], pH value [18][19][20][21], compositions of reaction mixtures [13], and use of different kinds of templates from ionic surfactants to block copolymers to mixtures of surfactants [10,22,23].…”