Studies On Supramolecular Self-Assembly Of Mesoporous Silica Via Pyrene Fluorescence Spectrum

Mesoporous materials are known for their unique textural properties and have found versatile applications in areas such as adsorption/separation, catalysis and assembly of host-guest materials. The mechanistic studies on the supramolecular assembly of mesoporous materials have been being a hotspot, while the synthesis methodology and the property characterization are well developed. Though several routes for the formation of mesoporous materials were suggested, no uniform cognition has been arrived up till now. It is well accepted that the identification of the critical micelle concentration (CMC) of surfactants plays a key role in deducing a mechanism of formation of mesoporous materials, and the fluorescence spectrum of pyrene probe molecule constitutes one of the most accurate methods to measure the CMC of surfactants. Thus, in this work, the process of synthesizing mesoporous silica, using tetraethyl orthosilicate (TEOS) as silica resource and cetyltrimethyl ammonium bromide (CTAB) as surfactant, was examinated systematically by means of fluorescence spectrum of pyrene probe molecule, X-ray diffraction (XRD) technology, and reaction kinetics analysis. The mechanism of formation of mesoporous silica was also discussed based on the experimental results in this work.A series of solutions of surfactant cetyltrimethylammonium bromide (CTAB) with different concentration were studied by pyrene fluorescence spectrum. It is found that the I1/I3 ratio of fluorescence spectrum was ca. 1.7～1.8 for pyrene in water and ca. 1.2～1.3 for that in CTAB micelles, and the curve of I1/I3 ratio vs. CTAB concentration exhibited a characteristic of Sigmiod function. The CMC of CTAB was then determined via fitting experimental data using Sigmiod function. Besides, the effects of additives, such as, hydrochloric acid, sodium hydroxide, sodium silicate, ethanol, isobutanol, hexanol, and decanol, on the CMC of CTAB were examined. It is found that the addition of inorganic additives, HCl, NaOH, or Na₂SiO₃, reduced the CMC of CTAB, and the more the amount of addition, the more the extent of reduction. The CMC of CTAB has an approximatively liner relationship with the logarithm of concentration of inorganic additives. The addition of alcohols did not affect the CMC of CTAB when the amount of addition was not too high. These results suggest that the influence of ethanol from the hydrolysis of TEOS to the CMC of CTAB could be ignored for an actual synthesis system of mesoporous silica.The process of supramolecular self-assembly of mesoporous silica was monitored by pyrene fluorescence probe method. It was investigated in detail that the band intensity at 374 nm in pyrene fluorescence spectrum changed differently with reaction time, dependent on different initial concentration of NaOH and synthesis temperature. Three characteristic stages for the surpramolecular assembly of mesoporous silica, as shown by the variation of pyrene fluorescence spectrum with the progress of synthesis reaction, were clearly identified, i.e., the band intensity at 374 nm increased slowly in stage I (< 2h), increases rapidly in stage II (2-7 h), and maintains almost constant in stage III (>7 h). It is also found, from the identification of concentrations of CTAB and Br^- and pH values for solution phase by pyrene fluorescence spectrum and chemical analysis, that the pH value decreased rapidly in the earliness of stage I and was almost unchanged thereafter; the concentration of Br^- kept a level of ca. 90 % of its initial value in batch, while the concentration of CTAB was lower than its CMC, all through the synthesis process. These results suggest that, in the stage I, there exists an exchange between Br^- ions from CTAB and negatively charged Si species from the hydrolysis of TEOS. As a result, Br^- ions are releases into solution and anionic-cationic pairs consisting of CTA⁺ and silicates are formed; in the stage II, the polymerization of silicates and accordingly the aggregation of CTA⁺ parts in the anionic-cationic pairs occur, resulting the formation of the surfactant micelles encapsulated by polymerized silicates; in the stage III, the silicates encapsulating the micelles increase in the degrees of polymerization and are deposited by free silicates from solution phase, and as a result, forming the organic-inorganic mesoporous composite.The reaction kinetics behaviors of supramolecular self-assembly process of mesoporous silica was studied. The monitoring to the changes in pH value of solution phase during the synthesis indicates that the first-order hydrolysis of TEOS is predominant in the stage I. As the pyrene fluorescence intensity of the synthesis system changed with synthesis time in a manner of a sigmoidal function, the finishing time of the stage II, i.e., the period of time consumed for the polymerization of silicates, is taken as indicative of the influence of synthesis parameters. It is found thatÏ„has an exponential relationship with NaOH concentration, ln(Ï„/h) = 2.29-0.48C/mmol.L^-1, and an approximatively linear relationship with temperature,Ï„/h = 56.3-0.163T/K.The reaction kinetics behaviors of Supramolecular Self-Assembly forming process of mesoporous silica have been studied. It found that first order hydrolyze of TEOS is the mainly process at the beginning by detecting pH change in solution phase via all the reaction process. According the curve character of pyrene fluorescence intensity vs. reaction time, the end time of polymerization (Ï„) can be used to character the effect of initial condition on the forming process of mesoporous silica. The result indicated thatÏ„has exponential relationship with NaOH concentration, ln(Ï„/h)=2.29-0.48C/mmol.L^-1, and has linear decreased relationship with temperature,Ï„/h=56.3-0.163T/K.In conclusion, it extrapolate that Supramolecular Self-Assembly forming process of mesoporous silica keep to Hybrid Polyelectrolyte Model that was noticed in literature.