【作者】 杨涛；

【导师】 晁自胜；

【作者基本信息】 湖南大学 ， 分析化学， 2007， 硕士

【摘要】 介孔材料因其独特的织构特性,在吸附/分离、催化以及主-客体材料组装等领域中具有重要而广泛的应用。有关介孔材料的形成机理,一直是介孔材料领域内的研究热点和难点。虽然研究者们已经提出了许多可能的机理,但至今学术界尚未有统一的定论。在超分子自组装合成介孔材料过程中,合成体系中表面活性剂的临界胶束浓度参数对于确定介孔材料形成机理具有重要意义;另一方面,芘荧光探针法是测定表面活性剂临界胶束浓度最为准确的方法之一。因此,本论文主要基于芘荧光探针法,并结合XRD和反应动力学分析,对采用正硅酸乙酯（TEOS）为硅源和十六烷基三甲基溴化铵（CTAB）为表面活性剂合成介孔二氧化硅的过程进行了较为系统的跟踪检测,并探讨了介孔二氧化硅可能的形成机理。采用芘荧光探针光谱法对不同浓度的十六烷基三甲基溴化铵（CTAB）表面活性剂水溶液进行了研究,发现芘在水溶液中和在CTAB胶束中的荧光光谱的I1/I3值分别介于1.7～1.8和1.2～1.3之间,而I1/I3～CTAB浓度曲线表现出了Sigmiod曲线特征。通过对实验数据进行Sigmiod曲线拟合,准确测定了表面活性剂的临界胶束浓度。此外,还详细考察了CTAB溶液体系中添加HCl、NaOH、硅酸钠、乙醇、异丁醇、正己醇或正癸醇后对CTAB的临界胶束浓度的影响。结果表明, HCl、NaOH、Na₂SiO₃降低了CTAB的CMC,无机离子浓度越大,影响越大。CTAB的CMC与浓度的对数近似呈线性关系。醇类在浓度不是很大时,对影响CTAB的CMC影响较小,因此,对于实际的介孔二氧化硅体系而言,由TEOS水解所产生的乙醇对于CTAB的影响可以忽略不计。采用芘荧光探针法跟踪检测了介孔二氧化硅的超分子自组装过程。详细考察了在不同的NaOH初始浓度和合成温度条件下,芘荧光强度随合成时间的变化规律。发现介孔二氧化硅的超分子自组装过程反映在芘荧光光谱中,可分为三个阶段:I)芘荧光强度基本不变阶段（<2h）;II)芘荧光强度迅速增加阶段（2-7 h）;III)芘荧光强度稳定不变阶段（>7）。进一步地对于整个合成过程不同阶段液相中的CTAB和Br^-浓度以及pH值进行了芘荧光探针法光谱和化学分析研究,发现在反应的初始阶段,合成体系的pH值迅速下降,随后则几乎不再改变;在整个合成过程中,溶液相中的Br^-浓度约等于与其初始加入浓度90%,并保持不变,而CTAB的浓度则始终小于其CMC。这些研究结果暗示着,在由超分子组装合成介孔二氧化硅的过程中涉及了以下一些主要的历程:在合成过程的第一阶段,反应开始后CTAB中的Br^-迅速与TEOS的水解产物硅氧负离子发生离子交换,Br^-进入溶液相,CTA⁺离子与硅氧负离子缔合,形成阴-阳离子电对;在合成过程的第二阶段,随着反应的进行,硅氧负离子之间发生缩聚,诱导了CTA⁺随之发生聚集,导致了被硅物种所包裹的表面活性剂胶束的形成;在合成过程的第三阶段,包裹在表面活性剂外部的硅物种进一步缩聚,同时溶液相中的硅氧负离子在上述缩聚物表面沉积,最终形成了有机-无机介孔复合物。考察了介孔二氧化硅超分子自组装过程的反应动力学行为。通过对反应过程中溶液相的pH变化的检测,发现在合成反应的初始阶段,TEOS主要发生一级水解。根据合成体系的芘荧光强度随时间变化曲线的特征,采用第二阶段（缩聚）的结束时间τ来表征体系初始条件对介孔二氧化硅形成的影响。研究结果表明,缩聚时间结束时间τ与NaOH浓度存在着指数关系ln（τ/h）=2.29-0.48C/mmol.L^-1,与温度存在着近似线性关系τ/h=56.3-0.163T/K。综上所述,有理由推测二氧化硅超分子自组装过程可能遵循了近似于文献中所述的混合聚电解质模型机理。更多还原

【Abstract】 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 thatτhas an exponential relationship with NaOH concentration, ln（τ/h） = 2.29-0.48C/mmol.L^-1, and an approximatively linear relationship with temperature,τ/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 （τ） can be used to character the effect of initial condition on the forming process of mesoporous silica. The result indicated thatτhas exponential relationship with NaOH concentration, ln（τ/h）=2.29-0.48C/mmol.L^-1, and has linear decreased relationship with temperature,τ/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.更多还原