Mexico: U.S. Geological Survey Scientific Investigations Report A, p. México y su situación en el contexto mundial: Boletín de la Sociedad. Boletín de la Sociedad Venezolana de Ciencias Naturales 27(): . ^ PANIGRAHI, G. A botanical tour in the Rajmahal hills of Binar. EL BOLETíN MENSUAL. La edición del BoIFTíN MENSUAL asciende à 11, ejemplares, de los cuales como 9, se envsan á la América Central y del Sur.
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The Journal obletin the Spanish Ceramic and Glass Society publishes scientific articles and communications describing original research and reviews relating to ceramic materials and glasses. The main interests are on novel bletin science and technology establishing the relationships between synthesis, processing microstructure and properties of materials. Papers may deal with ceramics and glasses included in any of the conventional categories: The Impact Factor measures the average number of citations received in a particular year by papers published in the journal during the two receding years.
CiteScore measures average citations received per bolefin published. SRJ is a prestige metric based on the idea that not all citations are the same. SJR uses a similar algorithm as the Google page rank; it provides a quantitative and qualitative measure of the journal’s impact. SNIP measures contextual citation impact by wighting citations based on the total number of citations in a subject field. The hydrolysis and self-condensation reactions of 3- 2-amino-ethylamino propyl-trimethoxy silane have been studied by means FT-IR spectroscopy for different water and ethanol concentrations.
The hydrolysis of 3- 2-amino-ethylamino propyl-trimethoxy silane occurs at a high rate and depends if the water concentration is lower or higher than the stoichiometric one for hydrolysing all the hydrolysable groups. The presence of ethanol delays the hydrolysis reaction.
The hydrolysis of 3- 2-amino-ethylamino propyl-trimethoxy silane gives hydroxyl groups Si—OH that self-condense to form Si—O—Si bonds in linear and cyclic structures.
For high water and low ethanol concentrations not bopetin Si—OH groups self-condense, whereas for low water or high ethanol concentrations the major part of the Si—OH groups self-condense and tends to disappear in the gel state. Para concentraciones de agua elevadas y bajas de etanol, bolwtin todos los grupos Si-OH auto-condensan, mientras que para concentraciones bajas de agua y altas de etanol, parte de estos grupos Si-OH auto-condensan y tienden a desaparecer en el estado de gel.
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Functional organosilanes, commonly known as silane coupling agents, have been studied and used in many different applications for decades [1—3]. In the last century, functional organosilanes were extensively used in the fabrication of lightweight composite in transportation, principally. The most common materials of this kind are fibre reinforced plastic composites FRPC where high resistance inorganic fibres are mixed with organic matrices.
The improvement of the mechanical properties in FRPC is only reached when it exist a chemical compatibility between the matrix and the reinforcements. The use of silane coupling agents promotes interfacial adhesion and thus, the mechanical properties of the composite materials are strengthened.
Since then, multiple and important applications for silanes have been reported in the literature, being most of them related to surface modification of support materials for biomolecule transport [4,5]enzyme  and bioactive substances immobilization  or smart drug delivery systems . In the field of composite materials, we also find a wide variability of applications such as the surface modification of natural organic or inorganic fibres  and particles the modification of synthetic carbon nanofibers [11,12] or as promoters of the adhesion between carbon nanotubes and glass fibres [13,14].
Silanes can be also used in the preparation of proton—conducting membranes obtained by the sol—gel method in the synthesis of organic—inorganic hybrids  and in the preparation of temperature-resistant silicon oxycarbonitride ceramics . In a typical silane structure, one silicon atom is bonded to three hydrolysable alkoxy groups and to one non-hydrolysable organic moiety. The hydrolysable groups form Si—OH terminations which are able to interact with active hydroxyl groups existing on the solid surface subjected to condensation reactions [1,18] or some other adjacent molecules.
The hydrolysis rate and the formation of these Si—OH termination depend upon the number of carbon atoms of the hydrolysable alkoxy group as well as some other factors such as pH, H 2 O concentration, nature of the solvent, etc. Silane condensation reactions, either with Si—OH groups existing on the solid surface, with other hydrolysed silanes polycondensation reactions or with other hydrolysed molecules of the same silane self-condensationalso depend upon the pH of the solution, silane concentration, catalysts, type of solvent, steric and inductive effects, etc.
The chemical nature of the un-hydrolysable organic moiety is selected according to the required functionality. Silanes containing amino groups —NH 2 in the organic chain have been, by far, the most extensively used in many of the above referred applications, and the position of the amino group within the organic chain provides different molecular conformations of the silane when it is attached to the solid surface [11,21—23].
Hydrolysis and condensation reactions of aminosilanes have been reported in the literature [1,24,25]standing out the systematic studies carried out by Belgacem et al. In a previous work, we also detailed the different steps involved in the hydrolysis and condensation of 3-aminopropyltriethoxysilane APS .
For these studies, we selected the FT-IR spectroscopy as the preferred characterization technique since it requires a minimum volume and the results can be obtained in just a few seconds. In a similar way as we did before, we now focus our attention on the hydrolysis and condensation reactions occurring in solutions containing N- 2-aminoethyl aminopropyltrimethoxysilane DAMO since it accomplishes a necessary step in the functionalization process of solid surfaces.
In the presence of large amounts of H 2 O, hydrolysis occurs fast but self-condensation reactions are delayed and many Si—OH groups remain un-condensed. Increasing EtOH ratios lead to a delay in the hydrolysis rate.
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If used, the total amount of EtOH was divided in two and added bo,etin to the two different vessels. After temperature stabilization, both solutions were then mixed and maintained under vigorous stirring for reaction. Two reactions sets were studied: Solution aliquots were taken at different reaction times starting from 1 min intervals to several hours.
Methanol MeOH, Merck The analysis of the FT-IR spectra was carried out by using commercially available software for spectral analysis Renishaw, U. Half bandwidth, intensity and position was determined by band deconvolution analysis and assigned according to the existing literature.
The criteria for choosing the most adequate fitting was the minimum value for the sum of the error squares between experimental and deconvoluted spectra. Hydrolysis and condensation of amino-containing silanes finishes with the obtaining of gels and, as occurs in the well-known APS silane the addition of Boletn 2 O to DAMO produces transparent gels for times that depend upon the H 2 O and EtOH concentrations.
When using the stoichiometric amount of H 2 O to hydrolyse all the hydrolysable groups DH3 sampleit was necessary 11 days to obtain a solid gel but, when the amount bopetin H 2 O is decreased DH1 sample even after 70 bo,etin the reaction is still on liquid state. In general the majority of these bands are not overlapped and can be used to follow the hydrolysis and condensation reactions of the alkoxyde DAMO.
The spectrum of DH3 at 1 min is very similar to that of DH1 at 8 min indicating the acceleration of the hydrolysis rate in presence of H 2 O. The interaction between the freshly produced MeOH and DAMO might be the responsible of the band shift to higher wavenumbers at low reaction time.
This band appears in the first bolrtin of hydrolysis and tends to disappear for bolwtin DH1 sample while it is still evident after 30 days reaction in DH3. In the spectra of the 590 gels Fig. The band in Fig. The most plausible explanation for bkletin shift and the presence of the band in the dried gels is the effect of the organic moiety bonded to the Si atom.
In the dried gels Fig. Since MeOH no bolftin exists on the dried samples, we must consider the appearance of some other species whose vibration wavelength were located at this position. Boltein two bands decrease in intensity during the drying process suggesting that the hydrolysis reaction continues during drying but, the limited mobility of the atoms in the solid state limits the evolution of the condensation reaction .
The position, intensity and width of each band in the spectra have been determined through spectral deconvolution. Since the spectra were recorded in ATR-FTIR mode covered with a glass plate, it is difficult to ensure a homogeneous pressure and noletin between the liquid and the diamond crystal also considering minor EtOH evaporation. Therefore, some sort of spectral normalization which are based on band intensity ratios are required. The most intense band of the spectrum was used for normalization.
The relative intensity of the referred bands during the analysis will correspond to the relative intensity area of the band with respect to the integrated spectral area. It must be clarified that in some cases we could not get an adequate fitting just by using the DAMO and MEOH bands; in these cases we added additional bands assuming Gaussian shape. It is clearly bolrtin the fast hydrolysis boltein the stoichiometric H 2 O is used DH3 samples, open symbols in Fig.
The bands attributed to the hydrolysable bonds do not completely disappear at high reaction times although the minimum intensities are reached when the stoichiometric amount of water is used. The remainder bands assigned to DAMO not shown here boltin also a gentle decrease in intensity with reaction time. Convoluted area of the bands corresponding to: MeOH appears in the spectra as a sub-product of the hydrolysis reaction.
During the drying process, the amount of MeOH decreases but does not completely disappear, indicating that some Bolettin molecules remain strongly adsorbed on the gel surface. After hydrolysis, freshly formed Si—OH groups start to self-condensate to form Si—O—Si bonds where each Si atom is bonded bloetin three O atoms and one C corresponding to the organic moiety.
Saturation is reached when three of the four possible coordination linkages are formed by Si—O—Si bonds. Boleetin mentioned before, the deconvolution of the IR spectra required to add new bands to reach the minimum error between the experimental and convoluted spectra. Nevertheless, it is generally recognized that the right assignment of the IR bands observed in silicate spectra to structural groups in silicate structures is a difficult task. Here it is observed that self-condensation starts at the first stages of DAMO hydrolysis.
In DH1, these bands experiment a progressive increase up to 10 h reaction but afterwards the intensity is constant for the next h or 31 days.
The band appears at the beginning of the hydrolysis reaction with similar intensity values for both samples.
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In DH1, it is observed a rapid decrease of the band intensity reaching values close to zero. After 40 days reaction, once the sample is gelled, the band increases its intensity due to the elimination of H 2 O adsorbed on the surface. After 24 h reaction the number of free hydroxyl groups increases again and occurs simultaneously with the formation of cyclic siloxane structures, as shown in Fig.
In a similar way as we proceed in the hydrolysis and condensation of DAMO with H 2 O, we carried out the study of the band evolution in the presence of different amounts of EtOH. The spectrum boleetin pure EtOH Fig.
As observed, the hydrolysis of DAMO is delayed because of the presence of EtOH and occurs in a sustained manner depending on the boltin of alcohol. In the absence of EtOH, hydrolysis takes place within the first 8 min Fig. For the largest amount of EtOH DH3E16 it is observed that the solution requires h to reach the minimum band intensity value.
The fact that in any case the band intensity approaches the zero value and it is even higher with increasing EtOH amounts indicates that the hydrolysis does not take place in its full extension and there are still some hydrolysable groups that remain intact. The decrease of the band intensity after h fits with the drying period of the gelled material.
In the case of the cyclic structures Fig. Regarding to the linear structures Fig. It is interesting to analyze the evolution of the band corresponding to Si—OH bonds that is shown in Fig. The behaviour of this band seems to combine the two possibilities of the samples produced in the 0590 of any solvent Fig.
The evolution for the high EtOH concentration reaction is similar to that found for the reaction with only 1 mole of H 2 O Fig.
The hydrolysis reaction rate depends upon the amount of H 2 O and it occurs faster when using the stoichiometric amount of H 2 O necessary to hydrolyse 590 the hydrolysable groups. The presence of other solvent such as EtOH leads to a delay booetin the hydrolysis reaction rate which is greater with increasing amounts of EtOH. In all the cases, the hydrolysis reaction occurs in two steps perfectly differentiated, as can be deduced from the analysis of the FTIR bands.
The first step occurs very fast, within the 1st hour reaction and the second step takes place at low rates at the end of the reaction. During this slow hydrolysis step, the self-condensation reactions between Boletih groups prevail.
Similar results have been found by Belgacem et al. Instead of using excess H 2 O either in the presence or in the absence of EtOH we have used the stoichiometric amount so, that may be the reason for the incomplete hydrolysis of the DAMO molecule even after gelling.
The behaviour shown is very similar to the one we already reported in the hydrolysis of an aminosilane bolefin and is related to the evolution of T 0 H bolehin . In pure aqueous medium, these studies demonstrated that some free T 0 H groups still remain after 48 h reaction whereas in the presence of EtOH, self-condensation reaction occurs in a large extent until all the free silanol groups condense. As shown in Figs.
These results are in line to the ones discussed before concerning the evolution of the Si—O—Si bonds.