We have already shown [ 36 ] that chemical interaction of dimethyl carbonate with sites of the dehydrated silica surface takes place at a temperature of 200 °C and higher. Chemisorption processes involve both structural silanol groups and siloxane bridges on the surface. In this paper, we investigated the peculiarities of diethyl carbonate chemisorption on the dehydrated silica surface. At the first stage, both features of the interaction of diethyl carbonate with structural silanol groups on the silica surface (see Scheme 2 I) and possibilities of the cleavage of siloxane bonds located directly on the dehydrated silica surface were investigated (see Scheme 2 II).
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5C2O)2CO) with the surface sites of dehydrated silica takes place when the temperature increases up to 200 °C (It was found that the chemical interaction of DEC ((HO)CO) with the surface sites of dehydrated silica takes place when the temperature increases up to 200 °C ( Figure 1 a).
−1 in grafted ethoxy groups are observed. It should be noted that the concentration of grafted ethoxy groups (–OC2H5) increases and full participation of silanol groups (O–H) in the chemical reaction with diethyl carbonate under these conditions is not observed. Therefore, it was logical to assume that the chemisorption of (H5C2O)2CO proceeds via the siloxane bond cleavage on the silica surface. To test this assumption and to better understand the processes of chemisorption of diethyl carbonate, the silanol groups were removed from the silica surface by being substituted with trimethylsilyl groups as a result of the reaction with hexamethyldisilazane (When the temperature rises further, the increase in the intensity of bands corresponding to the stretching vibrations C–H at 2987 and 2923 cmin grafted ethoxy groups are observed. It should be noted that the concentration of grafted ethoxy groups (–OC) increases and full participation of silanol groups (O–H) in the chemical reaction with diethyl carbonate under these conditions is not observed. Therefore, it was logical to assume that the chemisorption of (HO)CO proceeds via the siloxane bond cleavage on the silica surface. To test this assumption and to better understand the processes of chemisorption of diethyl carbonate, the silanol groups were removed from the silica surface by being substituted with trimethylsilyl groups as a result of the reaction with hexamethyldisilazane ( Figure 2 b).
−1). However, after the contact with vapors of (H5C2O)2CO at 300 °C and the subsequent vacuum treatment of the surface, the band with maximum at 2923 and 2987 cm−1 is very hard to distinguish from the valence vibrations of methyl groups as they absorb at the same frequency ranges (From Figure 2 b, we can see that silanol groups are not observed on the spectra with simultaneous appearance of the stretching vibrations of methyl groups (2908–2965 cm). However, after the contact with vapors of (HO)CO at 300 °C and the subsequent vacuum treatment of the surface, the band with maximum at 2923 and 2987 cmis very hard to distinguish from the valence vibrations of methyl groups as they absorb at the same frequency ranges ( Figure 2 c, see Scheme 3 ).
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In summary, we can say that DEC reacts with silanol groups on the silica surface forming the grafted ethoxy groups which can act as an additional reactive center for chemisorption of organosiloxanes. However, it is very hard to say that DEC reacts with the Si–O bond directly at the SiO2 surface, but we can assume that this reaction may occur due to the above-mentioned context.
2 nanoparticles (2 surface with neat poly(dimethylsiloxanes) (PDMS-20 or PDMS-50) and their mixture with DEC (−1, the peak attributed to absorbed water at 3700–3400 cm−1 and 1620 cm−1 and also vibration of silanol groups at 3750 cm−1. The spectrum of neat fumed silica (−1 and the valence and deformation vibration of absorbed water at 3700–3400 cm−1 and 1620 cm−1, respectively. Intensive band at 2965 cm−1 (asymmetric C–H vibrations in methyl group) and accompanying band at 2908 cm−1 (symmetric C–H vibrations) in IR spectra of modified silicas samples indicate high concentration of grafted methylsiloxane which is in accordance with the data on carbon content (Changes in surface structure of modified silicas are clearly visible in the IR spectra ( Figure 3 ). Figure 3 shows the IR spectra of neat SiOnanoparticles ( Figure 3 a), silica modified with neat DEC ( Figure 3 b) and composites prepared by modification of SiOsurface with neat poly(dimethylsiloxanes) (PDMS-20 or PDMS-50) and their mixture with DEC ( Figure 3 c–f). In the spectrum of the silica which was modified only with diethyl carbonate ( Figure 3 b), we can see the presence of valence vibrations of C–H bond in ethoxy groups at 2987–2908 cm, the peak attributed to absorbed water at 3700–3400 cmand 1620 cmand also vibration of silanol groups at 3750 cm. The spectrum of neat fumed silica ( Figure 3 a) is characterized by the presence of valence vibration of silanol groups at 3750 cmand the valence and deformation vibration of absorbed water at 3700–3400 cmand 1620 cm, respectively. Intensive band at 2965 cm(asymmetric C–H vibrations in methyl group) and accompanying band at 2908 cm(symmetric C–H vibrations) in IR spectra of modified silicas samples indicate high concentration of grafted methylsiloxane which is in accordance with the data on carbon content ( Figure 4 , discussed in Section 3.2 ) demonstrating high yield of methylsiloxane grafting.
−1 is no longer detected. The signal of the free silanol groups disappeared completely from the spectra of modified silicas confirms the passage of the reaction between the silica surface and modifier agents. Note that the intensity of a silanol band at 3660 cm−1 changes much less than the 3750 cm−1 free silanol band. The former is attributed to silanols that are less accessible [−1 corresponding to adsorbed water is detectable on the surface of all modified samples, but the intensity of this peak slightly decreases for silicas modified by mixtures of PDMS-x/DEC (However, the free silanol band at 3750 cmis no longer detected. The signal of the free silanol groups disappeared completely from the spectra of modified silicas confirms the passage of the reaction between the silica surface and modifier agents. Note that the intensity of a silanol band at 3660 cmchanges much less than the 3750 cmfree silanol band. The former is attributed to silanols that are less accessible [ 37 ] to siloxane molecules during the surface modification. In addition, we can see that the intensity of band at 3700–3400 and 1620 cmcorresponding to adsorbed water is detectable on the surface of all modified samples, but the intensity of this peak slightly decreases for silicas modified by mixtures of PDMS-x/DEC ( Figure 3 e,f). This could be explained by that fact that siloxane oligomers which were formed as the result of DEC and PDMS interaction have reacted with surface silanols more intensively than neat PDMS macromolecules.
At any rate, we still can see that individual molecules or clusters of water remained in the adsorption layer of oxide composites despite the presence of hydrophobic PDMS. The presence of adsorbed water clusters can be explained by the textural features of the nanosilica powders. Water molecules are much smaller than the cross-section of PDMS. Therefore, water can penetrate into narrow nanovoids in the contact zones between adjacent nanoparticles in aggregates but PDMS molecules cannot penetrate into these voids.
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