Fases del Proceso Participativo

Vibrational spectroscopy (Infrared and Raman) are methods often used to obtain information from alkoxysilane hydrolysis and condensation reactions - as well as for evidence of bonding mechanisms to mineral substrates (Plueddemann 1991). Typically, infrared spectroscopy is used in conjunction with either Raman or NMR to identify

Specific oligomeric species in solution and to follow the evolution of the inorganic frameworks by comparison with control samples or model compounds of known structures (Brinker and Scherer 1990).

Matos et al. (1992) studied an acid catalyzed (with HCl, pH=3 .7) TEOS - ethanol - water system by Fourier Transform Infi’ared Spectroscopy (FT-IR). The hydrolysis and condensation reactions were monitored by comparing the changes in intensity and growth of some IR and Raman active bands of TEOS. The interesting aspect of this study is that Matos et al. (1992) tried to give a detailed account of the TEOS to silica gel to glass evolution. They were able to unambiguously assign all the main bands to characteristic modes of either TEOS or ethanol, except for IR peaks at 1086 cm’^ which correspond to the C - O stretching vibrations of ethanol and TEOS simultaneously. Due to the complexity of the reactions it is necessary to monitor the entire reaction sequence so that ambiguous regions may be determined. For instance, Matos et al (1992) found that the peak in the region between 790 cm'^ and 810 cm'^ was actually three peaks. The first peak, at 810 cm'^ (CHz rocking vibration) consistently decreases (after first reaching a maximum) while a new peak grows at 795 cm'^ (Si04 asymmetrical stretch of TEOS) as the local Tj symmetry of the Si04 groups changes to Csv and then to Ctv during hydrolysis. This new peak then gradually decreases as the condensation reaction progresses and a new peak, at 799 cm'^ forms due to the symmetrical stretch of the bridging oxygen atoms (Si - O - Si).

The study by Matos et al. (1992) essentially points out that there are at least three “different” spectra from the TEOS to silica gel evolution. The first may be considered as pre - hydrolysis and is dominated by the fingerprint regions of both the ethanol and TEOS in the solution. When the hydrolysis reaction begins, there is a disappearance of TEOS bands at 473 cm'^ (O - C - C deformation) 1102 cm'^ (C -O asymmetrical stretch), 1168 cm'^ (CH3 rocking), 1299 cm'^ (CH2 twisting) and 1400 cm'^ (CH2

wagging). Simultaneously, the intensity of the ethanol peaks at 881 cm'^ (CH3 or CH2

deformation) and 1048 cm*^ increase as ethanol is formed from the hydrolysis reaction and alcohol forming condensation reactions. These ethanol peaks begin to decrease in intensity as evaporation becomes the dominant process. The peak around 960 cm'^

(CH3 rocking) loses intensity and undergoes a frequency shift to 955 cm'^ (Si - OH stretch) due to the formation of Si - OH or Si - O species. Eventually, this peak shifts back to 960 cm*^ probably due to the SiO stretch in the wet gel (Matos et al. 1992). The condensation reaction begins before the hydrolysis reaction has gone to

completion. During the beginning of the condensation reaction the bands that

characterize the wet gel emerge. The Si - O - Si formation is marked by the occurrence of the 1078 cm*^ peak (transverse optic component of the Si - O - Si asymmetric stretch, with the oxygen atoms moving along a direction parallel to Si -O - Si plane) and a high frequency shoulder at 1163 cm'^ (related to the longitudinal optic component of the same vibrational mode). The peak at 799 cm'^ (symmetrical stretching of the bridging oxygens) slowly increases. The 795 cm'^ peak is now quite prominent due to, as discussed previously, the changing Si04 symmetry but will eventually disappear, and the 810 cm'^ component has decreased significantly.

Leyden et al. (1991) investigated the hydrolysis of a series of alkyl - substituted alkoxysilanes in order to qualitatively follow the sequential hydrolysis of the alkoxy groups and the subsequent condensation of the resulting silanol groups to form siloxane bonds. Trimethylmethoxysilane (TMMS) was chosen as the simplest, having only one alkoxy group to follow. The TMMS was diluted in water with acetone rather than an alcohol, so that the silanol and siloxane bands could be unambiguously assigned. After the initial mixing the spectrum showed two peaks due to the Si - O - C species, the asymmetrical stretch at 1083 cm'^ and the symmetrical stretch at 865 cm'\ After a delay, the length of which depends upon the pH (higher pH gave slower hydrolysis rates), the Si -O - C bands disappear. They are replaced by the C - O stretch of

methanol at 1031 cm'^ and the Si - OH stretch at 896 cm'^ of the silanol group thus indicating the hydrolysis of the methoxy group. After further standing, the Si - OH band is reduced and the Si - O - Si asymmetrical stretch at 1043 cm*^ appears.

Finally, Charola et al. (1984) used infrared spectroscopy to compare xerogels derived from neat MTMOS under conditions of differing relative humidity. Under conditions of low, ambient and high relative humidity they found that all the xerogels studied

contained un-reacted methoxy groups (850 cm'^) and un-condensed silanol groups (900 cm'^). Adsorptions in the Si - OH region were greater at higher relative humidities as one would expect from, essentially, adding water to the system to drive the hydrolysis reaction. The silanol region of the xerogels also decreases in all samples over time as condensation processes continue.