Construction of vibrational basis spectra of multicomponent systems 1. Aerosil

Aerosil (pyrogenous dispersed silica) is considered as a three-component system consisting of a silica core, hydroxyl shell, and adsorbed water. Factoring was carried out for its amplitude-weighted vibrational state density (AWSD) spectra obtained using inelastic neutron scattering at 10 K into the spectra of the components comprising this system. The factoring method used is based on the postulate of lack of correlation of the basis spectra. The factoring was carried out by the construction of physically significant basis spectra satisfying the zero linear correlation coefficient criterion for a set of AWSD spectra of the starting and modified samples of Aerosil A380.Russian International Friendship University and Joint Institute for Nuclear Research, Dubna. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 54–63, January–February, 1993.     

Adsorption mechanism of substituted pyridines on silica suspensions: an NMR study

Adsorption isotherms of 2,5-dimethylpyridine (2,5-DMP) on Aerosil 200 silica from water-2,5-DMP binary mixtures are known to exhibit special features indicative of surface phase transitions in the adsorbed layer. We have made similar observations on another substituted pyridine, 2,4,6-trimethylpyridine (2,4,6-TMP). By using 1D and 2D NMR spectroscopy, we investigated adsorbed layers on silica in suspension in water/substituted-pyridine mixtures and demonstrated the existence of adsorbed species specific signals. Comparison of signals with those displayed on NMR spectra of liquid binary mixtures under various pH conditions rules out adsorption via interaction of the surface silanol group and the lone electron pair of the nitrogen atom. A mechanism of adsorption through the interaction of surface siloxane oxygen and the aromatic pi-system is proposed; it is consistent with both thermodynamic measurements and stacking of substituted pyridines within the adsorbed layer.     

Effect of adsorbed water on the electrorheology of silica suspensions

Monodisperse silica particles were formed by hydrolyzing tetraethylorthosilicate in an ethanol solution and the silica suspensions dispersed in a silicone oil were prepared by a different procedure. The effects of adsorbed water on the electrorheological (ER) behavior were studied under oscillatory shear. The amounts of adsorbed water and surface silanol groups were determined by thermogravimetric analysis. The magnitude of the complex viscosity, |η^*|, increases with the application of electric fields. The physically adsorbed water is primarily responsible for the ER effect. However, the fluids containing large amounts of adsorbed water do not always show excellent ER performance. The surface silanol groups have an important role in promoting the ER effect. Not only the amount but also the situation of silanol groups determines the ER activity of adsorbed water.     

Study of radiation-induced polymerization of vinyl monomers adsorbed on inorganic substances. VIII. Polymerization of styrene and methyl methacrylate adsorbed on aerosil

Aerosil is silica having a purity which is very high compared with that of silica gel and having, unlike silica gel, no micropores. To investigate the effects of impurities and micropores on the radiation-induced polymerization of styrene and methyl methacrylate adsorbed on silica gel, the radiation-induced polymerization of styrene and methyl methacrylate adsorbed on Aerosil was carried out. The results of both the styrene–Aerosil 300 system and the methyl methacrylate–Aerosil 300 system were similar to those of the styrene–silica gel and methyl methacrylate–silica gel systems, respectively. This suggests that in the radiation-induced polymerization of both styrene–silica gel and methyl methacrylate–silica gel systems the impurity and the presence of micropores have almost no effects on the reaction mechanism. The effect of aluminum as an impurity was investigated on the styrene–Aerosil MOX 170 system. It was found that aluminum accelerated the cationic polymerization.     

Theoretical and experimental study of the adsorption of neutral glycine on silica from the gas phase.

The adsorption of neutral glycine onto amorphous silica was investigated both theoretically and experimentally. DFT calculations were performed at the BLYP-631++G** level using a cluster approach. Several possible configurations involving the formation of H bonds between glycine and one, two, or three silanol groups (SiOH) were considered. The most favorable bonding of glycine with one silanol group (45 kJ mol(-1)) occurs through the COOH moiety, thus forming a cycle in which the CO group is an H-bond acceptor whereas the acidic OH group is an H-bond donor. With two or three silanol groups, additional H bonds are formed between the amine moiety and the silanol groups, which leads to an increased adsorption energy (70 and 80 kJ mol(-1) for two and three silanol groups, respectively). Calculated nu(CO), delta(HNH), and delta(HCH) values are sensitive to the adsorption mode. A bathochromic shift of nu(CO) as compared to the nu(CO) of free glycine (calculated in the 1755-1790 cm(-1) range) is found for glycine in interaction with silanol(s). The more H bonds are formed between the COOH moiety and silanol groups, the higher the bathochromic shift. For delta(HNH), no shift is found for glycine adsorbed on one and two silanol groups (where the amine is either not bound or an H-bond donor), whereas a bathochromic shift is calculated with three silanols when the amine moiety is an H-bond acceptor. Experimental FTIR spectra performed at room temperature for glycine adsorbed at 160 degrees C on Aerosil amorphous silica exhibit bands at 1371, 1423, 1630, and 1699 cm(-1). The experimental/calculated frequencies have their best correspondence for glycine adsorbed on two silanol groups. It is important to note that the forms giving the best correspondence to experimental frequencies are the most stable ones.     

Deactivation of silica surfaces with a silanol-terminated polysiloxane; structural sharacterization by inverse gas chromatography and solid-state NMR

Retention gape deactivated with Silicone OV-1701-OH show good chromatographic performance and remarkable stability against water induced stationary phase degradrdation. In an attempt to better understand the findamentals off the deactivation process using silanol terminated polysiloxanes, a fumed silica was deactivated with Silicon OV-1701-OH. In contrast to fused silic capillaries, fumed silica (Aerosil A-200) can be studied by ²⁹Si cross-polarization magic-angle-spinning (CPMAS) NMR, thus serving as a model substrate for fused silica. Retention data from inverse gas chromatography at infinite dilurion and ²⁹Si CP MAS NMR data of five Aerosil phases, differing in residual silanol surface concentration, are correlated with the aim of validating this approach for stationary phase characterization. A comparatively detailed model of the deactivating polymer layer that explains the observed absorption activities is deduced. Surface silanols are shown to play a key role in the polymer layer, the structure of which is of primary importance for the absorption behavior after deactivation. Contrary to common belief, the absolute silanol surface concentration after deativation is only of secondary importance for the overall absorption activity. High silanol surface concentrations enhance degradation of the polysiloxane chains into small cyclic fragments as well as subsequent absorption and immobolization to the silica substrate surface. The mobility of linear polysiloxane chains in the kHz regime (as determined bby NMR cross-polarization dynamics) appears to determine the extent which the residual silanols are accessible for analytes. It is therefore anticipated that there is an optimum silanol surface concentration of fused silica surfaces to be deactivated with silanol terminated polysiloxanes; it should be lazrge enough to adsord polymer fragments, but not large to avoid excessive residual silanol activity.     

The structure of water and methanol adsorbed on silica gel by FT-NIR spectroscopy

The NIR adsorption spectra were analyzed quantitatively on the fundamental, combination and first overtone region of OH vibrations of silanol groups, water and methanol adsorbed on mesoporous silica gels. Adsorbed methanol constitutes first layer of about 3 molecules/um^–2 and second layer, the structure of which is similar to that of bulk methanol liquid. Adsorbed water consists of a first layer of about 3 molecules/nm^–2, the second layer of about 9 molecules/nm^–2 and the third layer has a structure similar to the that of bulk water. The molecular configuration at the interface is discussed.     

Role of aerosil surface in reaction of exciplex formation in the presence of the nonionic surfactant triton X-100

In the formation of a pyrene/N,N-dimethylaninline exciplex solubilized in micelles of the nonionic surfactant Triton X-100, under conditions of adsorption on the surface of Aerosil (pyrogenic silica), the features of the reaction are determined by the restructuring of the Aerosil surface in the aqueous micellar medium and by encapsulation of the fluorophore molecules.Institute of Surface Chemistry, National Academy of Sciences of the Ukraine, 31 Nauka Prospect, Kiev 252039, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 5, pp. 320–323, September–October, 1996. Original article submitted June 16, 1995.     

Preparation and evaluation of solid dispersions of nitrendipine prepared with fine silica particles using the melt-mixing method

Solid dispersions (SD) of nitrendipine (NTD), a poorly water-soluble drug, were prepared using the melt-mixing method with hydrophilic silica particles (Aerosil and Sylysia) with different particle size and specific surface areas as carriers. Powder X-ray diffraction and differential scanning calorimetry evaluation showed that NTD in the SDs treated with the melt-mixing method was dispersed in the amorphous state. FT-IR spectroscopy obtained with the SDs indicated the presence of hydrogen bonding between the secondary amine groups of NTD and silanol groups of silica particles. The dissolution property of NTD in the SDs was remarkably improved regardless of the grade of silica. At the end of the dissolution test (60 min) the concentrations of NTD for the SDs with Aerosil 200 and Sylysia 350 were 8.88 and 10.09 microg/ml, corresponding to 28 and 31 times that of the original NTD crystals, respectively. The specific surface area and the adsorbed water amount of the SDs were also significantly improved. The rapid dissolution rate from the SDs was attributed to the amorphization of drug, improved specific surface area and wettability than the original drug crystals. In the stability test, powder X-ray diffraction pattern indicated that amorphous NTD in the SD with Aerosil 200 was stable for at least 1 month under the humid conditions (40 degrees C/75% RH).     

Electron spin resonance studies of spin-labelled polymers—18: Adsorption of spin-labelled poly(vinyl acetate) on silica surfaces

The adsorption of spin-labelled poly(vinyl acetate) from dilute solutions in ethyl acetate, chloroform and toluene onto three silica adsorbents of different surface silanol contents was studied. The adsorption capacities of the three silica samples, which decreased with decreasing surface silanol content, were dependent on the nature of the solvent, being greatest in the poor solvent toluene and least in the good solvent ethyl acetate. The ESR spectra of the polymer adsorbed on the silica of highest silanol content suggested that the polymer had a relatively flat conformation when toluene or chloroform was solvent and a more looped conformation when ethyl acetate was solvent. With the silica of intermediate silanol content, the polymer adsorbed from chloroform solution also had a loopy conformation. The silica of lowest silanol content was prepared by treating the first silica absorbent with trimethylchlorosilane. The line-shapes in the ESR spectrum of the labelled polymer adsorbed on this modified silica indicated a change in mode of adsorption.     

Construction of vibrational basis spectra of multicomponent systems 2. Silica gel

Silica gel is considered a three-component system consisting of a silica core, hydroxyl shell, and pore-retained water. Inelastic neutron scattering was used to obtain a set of amplitude-weighted vibrational state density (AWSD) spectra of two silica gel families SG20 and SG100 with mean pore diameter of 20 and 100 Å, respectively. The AWSD spectra were factored into the components of this system. The factoring method used is based on the postulate of lack of correlation of the basis spectra. The factoring was carried out by the construction of physically significant basis spectra satisfying the zero linear correlation coefficient criterion.Russian International Friendship University and Joint Institute for Nuclear Research, Dubna. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 64–76, January–February, 1993.     

Use of kinetic method for determining the concentration of hydroxyl groups on a surface of silica

The reactions of surface hydroxyl groups of pyrogenic silica, Aerosil with chromium oxychloride and trichlorosilane are satisfactorily described at the initial sections by the first and second order equations. These reactions were used as an example and it was shown that the kinetic method is applicable for determining the concentration of OH groups on the SiO₂ surface.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 2, pp. 252–255, March–April, 1987.     

Thermostimulated diffusion-controlled reactions of formation of an exciplex between pyrene and N,N-dimethylaniline on the surface of silica

Pulsed-laser fluorometry has been used to study the features of formation of an excited-state donor-acceptor complex between pyrene and NN-dimethylaniline in the adsorbed state on an Aerosil surface, over a broad range of concentrations of the quenching agent.Institute of Surface Chemistry, National Academy of Sciences of the Ukraine, 31 Nauka Prospect, Kiev 252039, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 5, pp. 324–327, September–October, 1996. Original article submitted June 28, 1995.     

Alkaloids ofVeratrum,Petilium, andKorolkowia

The review is devoted to a study of the steroid alkaloids of plants of the generaVeratrum, Petilium, andKorolkowia. Characteristic reactions, spectra, and some chemical transformations of the alkaloids subdivided into groups according to structure are discussed. Some methods for determining their structures are considered.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 3–22, January–February, 1980.     

DFT and direct ab-initio MD study on hyperfine coupling constants of methyl radicals adsorbed on model surface of silica gel

Methyl radicals interacting with silica gel surfaces have been investigated by means of DFT and direct ab-initio molecular dynamics (MD) methods using cluster models. Four typical binding sites of CH₃ on the cluster models were found in the geometry optimization from several initial geometries of CH₃ around the silica gel clusters. These were two silanol Si–OH sites and two siloxane Si–O–Si sites. In both sites, magnitude of hyperfine coupling constants of the methyl radical (a_H) was smaller than that of free CH₃ (a_H = 23.04 G). Temperature effects on a_H of the methyl radical were investigated by means of the direct ab-initio MD method. The hyperfine coupling constant of CH₃ interacting with the SiOH group decreased with increasing temperature. The methyl radical interacting with alkali metal supported silica gel was also investigated for comparison. The electronic states of methyl radicals on silica gel were discussed on the basis of theoretical results.     

An infrared study of the interaction of γ-aminopropyl and gb-cyanoethyl derivatives of triethoxysilane with aerosil

Aerosil has been altered in surface properties by treatment with these compounds as vapors and in solution in benzene. The surface OH groups give rise to an organosiiyl layer that is stable up to high temperatures under vacuum. The condensation occurs mainly on surfaces not blocked by hydrogen bonds. Cyano groups linked to these layers provide a new type of adsorption center. The CN band at 2249 cm^–1 does not shift when various compounds are adsorbed.     

Calculation of the vibrational spectrum of crystalline zinc oxide and modelling interstitial zinc impurities

The vibrational spectrum of zinc oxide has been calculated, taking into account the periodicity of the structure, and an analysis has been carried out using different models to investigate the nature of the ZnO lattice vibrations. It is demonstrated that it is possible in principle to model the vibrational diagram for a crystal containing defects. The vibrational spectrum of zinc oxide containing interstitial zinc has been calculated. The results obtained are in excellent agreement with experimental data.Saratov State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 133–140, January–February, 1993.     

Cyclic evolution of hydrogen during low-temperature thermolysis of water by adsorbed Re3Cl9 complex

The ability was studied of an Re₃Cl₉ cluster complex adsorbed on a silica surface to cause the thermochemical decomposition of water at a temperature of 670–770°K. In the multiply repeated cycles: water sorption, heating, water sorption, etc., the complex retains its ability to evolve hydrogen on heating to the same level. It was found that at room temperature, during the sorption of water, which occurred after heating, evolution of oxygen takes place. A reversible character of the interaction of the adsorbed rhenium complex with water in a two-stage thermochemical cycle has been postulated.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 365–367, May–June, 1990.     

Changes in Porosity and Amounts of Adsorbed Water in Sol-Gel Derived Porous Silica Films with Heat Treatment

Porous silica films were obtained by a heat treatment of poly(ethyleneglycol)-containing gels for micro-patterning. Changes in porosity and the amount of adsorbed water with the heat treatment temperature in the silica films were evaluated from the refractive index of the films. The apparent refractive index of the silica films in an ambient atmosphere was much higher than the intrinsic refractive index of the film due to the large amount of adsorbed water in the films. The amount of adsorbed water in the films decreased with an increase in the heat treatment temperature and became almost zero after a heat treatment at 850°C. The decrease in the amount of adsorbed water in the films was caused by the decrease in silanol groups, which acted as the sites for water adsorption in the films. The drastic decreases in thickness and in porosity of the films at temperatures from 850 to 950°C occurred after the disappearance of silanol groups in the films. The densification of the films due to collapse of the pores at high temperatures should have resulted from the viscous flow of silica network.