Atomistic Modeling of Silica Based Sol-Gel Processes

Density Functional Theory is used to study water, methanol, ethanol, TMOS, and TEOS molecules and the most important silica clusters participating in sol-gel processes. Calculated bond lengths, bond angles and electric dipole moments compare well with experimental data. The energy of these molecules is reported and used to discuss the energetics of the hydrolysis and condensation reactions. Molecular Dynamics is employed to simulate liquid water, methanol, ethanol, TMOS, TEOS and experimental sol-gel solutions. Calculated densities and enthalpies of vaporisation compare well with experimental data. Preliminary results are presented for MD simulations of sol-gel solutions.     

Antibody-Antigen Reactions in Porous Sol-Gel Matrices

Porous silica gels, synthesized via the acid hydrolysis and basic condensation of TMOS, have been used for the encapsulation of antigens. The pores of the matrix are large enough to allow the diffusion of antibodies through the gel. Antigen-antibody specific fixation occurs within the sol-gel matrix. It can be detected via the so-called enzyme linked immunosorbent assays (ELISA). Antigen-antibody associations occurring in the gel are optically detected via the reaction of a peroxidase conjugate with ortho-phenylenediamine leading to the formation of a yellow coloration. Immunoassays have been performed using the hydatid cyst fluid as the source of antigens and sera from human patients as the source of antibodies. Specific fixation appears to be as good in the sol-gel matrix as in antigen solutions.     

A Study of the Hydrolysis Pathway in Oxalic Acid Catalyzed Reacting TMOS-Water Mixtures Under Ultrasound Stimulation

The hydrolysis of TMOS in oxalic acid catalyzed reacting TMOS-water mixtures, under ultrasound stimulation, was studied by fitting a simplified dissolution and reaction modeling for samples, the hydrolysis rate of which had been measured in a previous work. The reaction pathway represented in a ternary diagram shows a heterogeneous step for the reaction which gradually progresses until complete homogenization of the system. Besides the water dissolved due to the homogenizing effect of the alcohol, ultrasound maintains a virtual and additional dissolution of water located at the interface between the TMOS and water during the heterogeneous step of the reaction. The mean radius of the heterogeneity represented by water dispersed in TMOS was evaluated as around 150 Å. The oxalic acid concentration accordingly increases the hydrolysis rate constant but its fundamental role on the solubility of water in TMOS could not unequivocally be established.     

Coupled mass transfer and reaction in water of an oil soluble chemical: A CT scan study

Chemical compounds of the family of alkoxysilanes, such as tetramethyl orthosilicate (TMOS), are soluble in oil but can be dissolved in water where they undergo a hydrolysis and a condensation reaction to form a stable silica gel. We have investigated in detail the coupled mass transfer–reaction process that takes place when oil with dissolved TMOS is brought in contact with water in a well-defined geometry. The main physical parameters (initial TMOS concentration in the oil phase, initial water to oil volume ratio, and water pH and salinity) were varied during the experiment. X-ray CT was used to visualize and quantify concentration changes in the oil and the water phases. We found that varying the initial concentration does not affect mass transfer rates out of the oil phase, but increasing initial concentrations do speed up gelation. The water to oil volume ratio has a larger impact on mass transfer, with a transfer rate that increases with increasing water to oil volume ratio. Low pH of the water phase promotes mass transfer, whereas high pH and addition of salt has no significant effect. Gelation is however more profoundly affected by both increasing pH and addition of salts.     

MORPHOLOGY OF INORGANIC-ORGANIC HYBRID MATERIALS DERIVED FROM TRIETHOXYSILYLATED DIETHYLENETRIAMINE AND TETRAMETHOXYSILANE

Triethoxysilylated diethylenetriamine (f-DETA) and its mixtures with tetramethoxysilane (TMOS) have been hydrolyzed and condensed to form abrasion resistant coatings for optical plastics such as bisphenol-A polycarbonate and CR39. In this paper, the influences of the pH and water concentration on the morphology of gels derived from f-DETA alone (f-DETA100) or equal weights of f-DETA and TMOS (f-DETA50TMOS50) were studied. For the f-DETA50TMOS50 system, with an equivalent ratio (r) of water to alkoxysilane (OH/SiOR) of 4.4, sol-gel reactions at pH 0–2 often led to a particulate gel structure. However, if the reaction was carried out pH 4–5, the resulting gel often had a uniform compact structure although some particulate features were found on the surface. For the f-DETA100 system at pH 0–2, as r increased from zero, the resulting f-DETA gels exhibited a morphological transition from a monolithic uniform structure, to a porous particulate structure, and then to a compact fine particulate structure. A similar transition also existed in the binary system f-DETA50TMOS50, but the particle size was much smaller and no obvious porous stage was observed. The aging time and drying rate were also important factors influencing the structure of the f-DETA/TMOS coatings.     

Preparation of sol�Cgel hybrid materials from ��-methacryloxypropyltrimethoxysilane and tetramethyl orthosilicate: study of the hydrolysis and condensation reactions

Organic?Cinorganic hybrid materials suitable for the development of sol?Cgel coatings for metallic surfaces were prepared by hydrolysis and condensation of ??-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethyl orthosilicate (TMOS). The hydrolysis of MAPTMS/TMOS was carried out in an ethanol/water solution. The prehydrolysis stage of MAPTMS/TMOS system was monitored by Fourier transform infrared spectroscopy (FTIR) and liquid-state ²⁹Si and ¹³C nuclear magnetic resonance (²⁹Si and ¹³C NMR). FTIR analysis indicated that the hydrolysis of MAPTMS/TMOS was accomplished as far as the (SiOMe) band corresponding to unhydrolyzed silane disappeared. The concentration of the alkoxy groups and the extent of self-condensation of mono-, di-, and trisubstituted siloxanes (T species) in the sol were estimated by using liquid-state ²⁹Si NMR spectroscopy. The hydrolysis of the prepared sol was also evaluated by liquid-state ¹³C NMR spectroscopy. The results indicated that under the adopted synthesis strategy conditions, the hydrolysis process requires 4?h to be completed.     

Synthesis of inorganic gels in a lyotropic liquid crystal medium. I. Synthesis of silica gels in lamellar phases obtained from non-ionic surfactants

Some lamellar phases made with aqueous lyotropic liquid crystals were used as templates for the gelation of a silica inorganic network from tetramethylorthosilicate (TMOS). The aim was to synthesize materials with an anisotropic texture.Lamellar phases were obtained by using non-ionic surfacants. At first, structural, textural and rheological properties of the lamellar phases were studied. Then, the evolution of the system after introduction of the alkoxide, i.e., during the sol-gel transition, was followed by low angle X ray diffraction and rheological measurements. Finally, a textural study of dried gels was carried out.The formation of gels with a lamellar structure was analysed by compressing the total phase diagram (quaternary system) to a ternary system. The hydrolysis and condensation reactions of the TMOS are strongly influenced by the presence of the structured lamellar phase. Gelation seems to happen around the liquid crystal microdomains. A schematic model of gelation is proposed based on experimental observations.     

Mechanisms of amine-catalyzed organosilicate hydrolysis at circum-neutral pH

Mono- and polyamines can catalyze the hydrolysis and condensation of organosilicate starting materials in biomimetic silica synthesis pathways at circum-neutral pHs and room temperature. Our study is focused on understanding the mechanistic role of amines in catalyzing the hydrolysis process that precedes condensation. We have conducted (29)Si NMR experimental studies over a range of temperature and pHs for the hydrolysis rates of trimethylethoxysilane (TMES), a model compound with only one hydrolyzable bond, combined with quantum mechanical hybrid density functional theory calculations of putative intermediate and transition-state structures for TMES and tetramethyl orthosilicate (TMOS). Comparison of calculated energies with experimentally determined activation energies indicates that amine catalysis of TMES is primarily a consequence of the amine's acidity at neutral pH. The proton released by the amine is transferred to the organosilicate, producing a protonated ethoxy leaving group that can be displaced by water in an S(N)2 reaction. For TMOS, the activation energy of proton-transfer coupled with S(N)2 substitution is comparable to that for Corriu's nucleophile-activated nucleophilic displacement, such that the mechanism of amine-catalyzed hydrolysis is dependent mostly on the ambient pH conditions as well as the type of amine. The relevance of our results to biological silica precipitation is discussed.     

Sol-Gel Route to Direct Formation of Silica Aerogel Microparticles Using Supercritical Solvents

A simple and versatile method to obtain silica aerogel particles based on the hydrolysis and subsequent condensation of silicon alkoxides in supercritical fluids is proposed. This microparticle production route reduces the number of steps of traditional microparticle sol-gel processing.A case example is explained in more detail. Spherical and fiber silica morphologies were obtained by a one step method using a sol-gel process with supercritical acetone as a solvent. Particle size was controlled by varying the relative amounts of alkoxysilane, water and acetone. The resulting materials are influenced by a large number of experimental parameters; it has been observed that a quite relevant one is the supercritical fluid venting rate. The morphology of the particles was characterized by electron microscopy (SEM and TEM) and Atomic Force Microscopy (AFM). Alternatively, a low temperature synthesis can be performed by using supercritical carbon dioxide as solvent and formic acid as condensation agent.     

Synthesis and spectroscopy of tpps-doped silica gels by the sol-gel process

Tetraphenylporphinetetrasulfonic acid (TPPS), which is well known as a photochemical hole-burning (PHB) dye, was incorporated in silica gels obtained by the sol-gel process from tetramethoxysilane (TMOS). The form of TPPS must be free base (H₂P), which exists in basic condition, to be active in PHB. To obtain transparent and higher density silica gels doped with free-base TPPS, two-step hydrolysis processes using solutions containing NaOH were developed, and the gels having bulk density of about 1.5 g/cm³ were synthesized. The form of TPPS in the gels was investigated by measuring the absorption and luminescence spectra, and it was found that in the silica gels almost all the TPPS retained free-base form at the molar ratio of NaOH/TMOS above 10^–3.     

Studies of alkoxysilane hydrolysis and condensation by fourier-transform infrared spectroscopy with a cylindrical internal-reflection cell

The relative concentrations of unhydrolyzed and hydrolyzed forms of (3-trimethoxysilyl) propyloctadecyldimethylammonium chloride (SiQAC) were determined in aqueous solutions as functions of pH and time. Subtraction of the concentrations of these two forms of SiQAC from the total concentration of SiQAC was assumed to give the concentration of the “condensed” form of SiQAC irrespective of its extent of oligomerization. The silane, SiQAC, is used to modify a variety of surfaces to impart antimicrobial properties. Knowledge of the speciation of SiQAC is required to understand its effect on the efficiency and durability of such chemically modified surfaces. The cylindrical internal- reflection/FTIR spectroscopy was also used to determine the rate constants for hydrolysis and condensation of monomethoxytrimethylsilane (MTMS) in 10% (v/v) water/acetone medium as a function of hydrogen-ion concentration. The hydrolysis of MTMS was found to be first order in both silane and water concentrations. the condensation of trimethylsilanol to form hexamthyldisiloxane was second order in silanol concentration.     

固定化β-葡萄糖醛酸酶反应器的制备及其在4-甲基亚硝胺基-1-(3-吡啶)-1-丁醇的O-糖苷化合物分析中的应用

采用溶胶-凝胶法,以丙烯酰胺作为单体,制备了基于有机-硅胶杂化整体柱的β-葡萄糖醛酸酶反应器。优化了硅酸甲酯和γ-(甲基丙烯酰氧)丙基三甲氧基硅的物质的量的比、丙烯酰胺和聚乙二醇的用量,以及水浴温度等制备条件,获得了孔隙均匀、通透性良好、机械强度高的有机-硅胶杂化整体柱。采用光学显微镜和扫描电镜表征杂化整体柱。进一步将β-葡萄糖醛酸酶共价键合在整体柱上,以4-甲基亚硝胺基-1-(3-吡啶)-1-丁醇(NNAL)的O-糖苷化合物(NNAL-O-Gluc)为底物研究酶反应器的水解效果,实验结果证明酶反应器在室温条件下的水解效率大大提高,实现了NNAL-O-Gluc高效水解与分析,解决了目前NNAL-O-Gluc分析中前处理水解效率低的问题。     

The influence of methacryloxypropyltrimethoxysilane on the sol-gel process of TEOS

The influence of MPS on the hydrolysis and condensation process of TEOS is studied by means of hydrolysis time (t _H) and gelation time (t _G) curves. The addition of MPS to a mixture of TEOS, ethanol and water results in a substantial increase in t _G. The increase is most pronounced when adding takes place in the acid step of the sol-gel process of TEOS.In acid environment hydrolysis of MPS will be dominant compared to hydrolysis of TEOS. This results in an effective decrease of the amount of water available for the hydrolysis of TEOS. However, this decrease in water concentration cannot explain the complete effect of the addition of MPS. The hydrolysed MPS will also be incorporated in the gel network and will strongly influence the cross-linking ability. The lesser functionality of MPS compared to TEOS and the steric hindrance of the acrylate group results in a large increase in t _G.     

Gelation of octadecyltrimethoxysilane at the air/water interface: effect of perfluorotetradecanoic acid and divalent cadmium ions

The effect of an acidic surfactant, perfluorotetradecanoic acid (PFTDA), on the gelation of octadecyltrimethoxysilane (ODTMS) monolayers at the air/water interface was investigated using a Langmuir film balance, attenuated total reflection (ATR) infrared spectroscopy, and atomic force microscopy (AFM). The gelation of ODTMS was greatly accelerated by adding only 2 mol % PFTDA into the monolayer; in this case, the hydrolysis of ODTMS was completed within a few minutes, whereas otherwise it took 18 h. ATR-IR spectroscopy clearly demonstrated that the gelation of a 49:1 ODTMS/PFTDA monolayer proceeded on a pure water subphase mainly via hydrolysis followed by condensation. In the presence of PFTDA, the local acidity at specific sites is presumed to be very high, catalyzing the hydrolysis of ODTMS. The extremely fast rate is attributed not only to the lateral diffusion of PFTDA in the mixed monolayer but also to proton propagation where the proton(s) involved in the initial hydrolysis are transposed rapidly to the nearby methoxy groups for subsequent hydrolysis. The catalytic activity of PFTDA can be neutralized, however, simply by the addition of multivalent metallic ions such as Cd2+ to form saltlike complexes with PFTDA; the rate of 2-D sol-gel processes can thus be easily regulated by a minute amount of PFTDA and/or Cd2+ added into the reaction system.     

The early stages of the sol-gel processing of TEOS

The reactivity of tetraethoxysilane (TEOS) used in many sol-gel syntheses can be increased by prehydrolysis. In contrast to one-step prehydrolysis acidic two-step prehydrolysis causes an increase of the reactivity of the low molecular species provided the total water/TEOS ratios are the same in both cases. By choice of the processing conditions of the hydrolysis—shown on variation of water content, hydrolysis steps and heat treatment—the degree of condensation, the ratio of hydrolyzable (-OC₂H₅) and hydrolyzed groups (-OH) and the structure of the low molecular species (short chains, 3-membered and 4-membered rings) can be influenced systematically. The development of the building units formed in the early reaction stages is ascertained by molybdate method and ²⁹Si NMR measurements.     

Characterization of sol-gel immobilized lipases

The immobilization of lipases within a chemically inert hydrophobic sol-gel support, which is prepared by polycondensation of hydrolyzed tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) or iso-butyltrimethoxysilane (iso-BTMS), results in heterocatalysts. The heterocatalysts so prepared showed a dramatically enhanced catalytic activity and stability as measured by the hydrolysis and transesterification of soybean oil. The lipase/sol-gel materials were characterized by nitrogen adsorption to determine their specific surface area. Solid state NMR was used to reveal the degree of cross-linking of the sol-gel materials. Scanning electron microscopy and atomic force microscopy were used to observe the morphology of the biocatalysts. Transmission electron microscopy and confocal microscopy were used to investigate the enzyme distribution within the sol-gel materials. The characterization studies showed that the most active lipase-containing sol-gel was a non-porous amorphous material with enzyme randomly distributed throughout the sol-gel material. The activity of the immobilized enzyme did not correlate to the degree of cross-linking or the specific surface area of the sol-gel materials. The highly retained activity of the immobilized enzyme was more likely attributed to the conformational changes of the enzyme during the immobilization, which result in enzyme's fixation in a more favorable conformation and to the lipophilic environment of the hybrid matrix structure which facilitates the transport of the hydrophobic substrate to the active sites.     

Formation of silica nanoparticles in microemulsions

Silica nanoparticles for controlled release applications have been produced by the reaction of tetramethylorthosilicate (TMOS) inside the water droplets of a water-in-oil microemulsion, under both acidic (pH 1.05) and basic (pH 10.85) conditions. In-situ FTIR measurements show that the addition of TMOS to the microemulsion results in the formation of silica as TMOS, preferentially located in the oil phase, diffuses into the water droplets. Once in the hydrophilic domain, hydrolysis occurs rapidly as a result of the high local concentration of water. Varying the pH of the water droplets from 1.05 to 10.85, however, considerably slows the hydrolysis reaction of TMOS. The formation of a dense silica network occurs rapidly under basic conditions, with IR indicating the slower formation of more disordered silica in acid. SAXS analysis of the evolving particles shows that approximately 11 nm spheres are formed under basic conditions; these are stabilized by a water/surfactant layer on the particle surface during formation. Under acidic conditions, highly uniform approximately 5 nm spheres are formed, which appear to be retained within the water droplets (approximately 6 nm diameter) and form an ordered micelle nanoparticle structure that exhibits sufficient longer-range order to generate a peak in the scattering at q approximately equal to 0.05 A-1. Nitrogen adsorption analysis reveals that high surface area (510 m2/g) particles with an average pore size of 1 nm are formed at pH 1.05. In contrast, base synthesis results in low surface area particles with negligible internal porosity.     

Hierarchically structured meso-macroporous aluminosilicates with high tetrahedral aluminium content in acid catalysed esterification of fatty acids

A simple synthesis pathway has been developed for the design of hierarchically structured spongy or spherical voids assembled meso-macroporous aluminosilicates with high tetrahedral aluminium content on the basis of the aqueous polymerisation of new stabilized alkoxy-bridged single molecular precursors. The intimate mixing of an aluminosilicate ester (sec-BuO)(2)-Al-O-Si(OEt)(3) and a silica co-reactant (tetramethoxysilane, TMOS) with variable ratios and the use of alkaline solutions (pH 13.0 and 13.5) improve significantly the heterocondensation rates between the highly reactive aluminium alkoxide part of the single precursor and added silica co-reactant, leading to aluminosilicate materials with high intra-framework aluminium content and low Si/Al ratios. The spherically-shaped meso-macroporosity was spontaneously generated by the release of high amount of liquid by-products (water/alcohol molecules) produced during the rapid hydrolysis and condensation processes of this double alkoxide and the TMOS co-reactant. It has been observed that both pH value and Al-Si/TMOS molar ratio can strongly affect the macroporous structure formation. Increasing pH value, even slightly from 13 to 13.5, can significantly favour the incorporation of Al atoms in tetrahedral position of the framework. After the total ionic exchange of Na(+) compensating cations, catalytic tests of obtained materials were realised in the esterification reaction of high free fatty acid (FFA) oils, showing their higher catalytic activity compared to commercial Bentonite clay, and their potential applications as catalyst supports in acid catalysed reactions.     

Silicon tetraacetate as a sol-gel precursor

Variations in the starting materials, solvent, catalyst, and water to alkoxide ratio affect gel times and gel properties in sol-gel reactions. Silicon tetraacetate-derived sols gel surprisingly quickly. This paper discusses the acid catalyzed hydrolysis and condensation of silicon tetraacetate in an attempt to understand this rapid gelation. ²⁹Si and ¹H NMR spectra show that tetraethoxysilane is an intermediate, and ethyl acetate is produced via a secondary reaction. A correlation between gel times and H⁺ concentration is demonstrated.     

Spectroscopic studies of triethoxysilane sol-gel and coating process

Silica sol-gels have been prepared under different conditions using triethoxysilane (TES) as precursor. The prepared sol-gels have been used to coat aluminum for corrosion protection. Vibrational assignments have been made for most vibration bands of TES, TES sol-gel, TES sol-gel-coated aluminum and xerogel. It has been noticed that air moisture may have helped the hydrolysis of the thin coating films. Xerogels have been obtained from the sol-gel under different temperature conditions and the resulting samples have been characterized by using infrared and Raman spectroscopic methods. IR data indicate that the sol-gel process is incomplete under the ambient conditions although an aqueous condition can have slightly improved the process. Two nonequivalent silicon atoms have been identified from the collected (29)Si NMR spectra for the sol-gel, supporting the result derived from the IR data. The frequency of SiH bending vibration has been found to be more sensitive to the skeletal structure than that of the SiH stretching vibration. A higher temperature condition could favor the progression of hydrolysis and condensation. A temperature higher than 300 degrees C would cause sample decomposition without seriously damaging the silica network. From infrared intensity measurements and thermo-gravimetric analyses, the fractions of incomplete hydrolysis and condensation species have been estimated to be 4% and 3%, respectively. Electrochemical data have shown that the sol-gel coating significantly improves the corrosion protection properties of aluminum.