The FTIR spectra of all Sipernat 50s samples (Sip) (Fig. 1) showed absorption bands characteristic for stretching vibrations of O-H groups (blue bar; OH band) at WN 3500 to 3300 cm-1, and Si-O-Si groups (yellow bar; Si-O-Si band) at WN of 1000 to 1100 cm-1. Additionally, the spectra show a band at WN 1600 to 1650 cm-1 (blue arrow) which indicate the presence of molecular coordinated water within the SiO2-structure 30. The 1650er band is less intense for Sip50s as compared to the other samples. A broadening of this envelope, an increase in intensity and/or peak shifts to higher WN may be attributed to a more complex structure of the material under study, which might result in a higher binding affinity towards water 19. However, Sip50S showed a smaller particle size (18 µm) as compared to Sip 50 or Sip 320.
The FTIR spectra of Aerosil showed the lowest intensity of the O-H band, which is in accordance with the fact that Aerosil is a pyrogenic silica that may contain only small amount of water and OH-groups due to the conditions of synthesis. Pyrogenic or fumed silica is formed when silicon tetrachloride (SiCl4) reacts in a hydrogen flame (2100 K) to form amorphous silicon dioxide (SiO2) 31. With this single spherical droplets of silicon dioxide form and is followed by particle growth through collision and coalescence forming larger droplets whereas the aggregation is occurring via cooling 31.
As expected, the FTIR spectra of the Sipernat- and Aerosil-samples correspond mostly to that of silica gel (SG, Fig. 1: grey line). However, for the silica gel the Si-O-Si band is narrower as compared to that of Sipernat and Aerosil (Fig. 1, grey arrow). The spectra of Sipernat has of course more OH groups compared to the Aerosil due the formation process. The FTIR of quartz sand (QS, black line in Fig. 1) is also mostly comparable to that of the silica gel sample but shows a second maximum at the left-hand side of the Si-O-Si band (Fig. 1, grey arrow).
The FTIR spectra of ZEOfree (Fig. 2), a precipitate calcium-silicate, showed an additional band at WN of about 1500 cm-1 (Fig. 2; red solid arrow) compared to the ones of Sipernat 50 & 320 (Fig. 1) and that of Sipernat50s (green line in Fig. 2), a shift of the Si-O-Si band towards smaller WN, and a second maximum at the right hand side of the Si-O-Si band (Fig. 2; black solid arrow). Molecular IR vibrations are a function of binding strength and the mass of the atoms involved. A band shift towards lower WN occurs as the mass of the atoms (here m > 28 u) increases. Furthermore, peak broadening may indicate a lower internal order of the material under study1822. The additional band at about 1500 cm-1 in the FTIR of ZEOfree (Fig. 2; red solid arrow) is in a similar range of WN as the SiO44- band of sodium silicate (Fig. 2, grey line, Fig. 2: empty red arrow).
Compared to ZEOfree the FTIR spectra of the Andosol soils show no absorption band at about 1500 cm-1 (Fig. 3a, red arrow). The Si-O-Si bands of the Andosol soils look similar to those of the SROAS studied by Parfitt 28 (Fig. 3b). However, the FTIR spectra of SROAS show a the shift of the Si-O-Si band towards lower WN (Fig. 3b; Parfitt 32), which is found by Wada, et al. 33 to increase with Al content. However, there is currently some inconsistency in the definition of allophane. The definition of allophane commonly used in soils science from Parfitt 32 stating that: 'Allophane is the name of a group of clay-size minerals with short-range order which contain silica, alumina and water in chemical combination.' is misleading as allophanes are defined as particles consisting of spherical, hollow units 34,35. Short-range ordered silica, amorphous aluminosilicates and also amorphous silica are phases not belonging to the group of allophanes 4. Only Imogolith (Fig. 3b, line A) showed a second maximum of the Si-O-Si band like ZEOfree a. The FTIR spectra of SROAS studied by Wada, et al. 33 (designated therein as allophanes) showed also absorption bands at WN 550 to 720 cm− 1 (Fig. 3b) which are indicative for Si-O-Al bands. Most recently Lenhardt, et al. 20 reported on IR properties of SROAS with varying Al:Si ratios and identified SROAS specific absorption bands at 590–570 cm− 1 (Al-OH bending vibration) which decreased with increasing Si content and an absorption maximum at 690 cm− 1 for Si-rich SROAS. In contrast to the SROAS (Fig. 3b), the silica samples studied here (Fig. 1) did not show absorption bands at this WN.
The FTIR spectra of the water-soluble fraction of Sipernat 50s showed a smaller Si-O-Si band (ca. 1100 cm− 1) and an additional band (Fig. 4c, bluearrow) at about 1400 cm− 1 which is characteristic for Si-O− groups compared to the bulk Sipernat 50S sample (Fig. 4b). The additional band in the spectra of the water soluble Sipernat50s fraction is located at a lower WN (WN 1400 cm− 1) as compared to that of ZEOfree (1500 cm− 1; Fig. 4a) and Na4SiO4 (1475 cm− 1; Fig. 4d).
The narrower Si-O-Si band in the spectrum of silica gel compared to Sipernat or Aerosil samples (Fig. 1, grey arrow) can be explained by differences in the specific surface area (SSA) of Sipernat and Aerosol samples (150 to 500m2 g-1) compared to the silica gel and quartz sand samples. With increasing specific surface area, the number of Si-OH increases and the number of Si-O-Si groups located near to the surface is increasing. Such the ratio between the number of Si-O-Si group located near to the surface (Fig. 5b) and that of the ones located in the particles center (Fig. 5a) will increase. This may be important since the presence of Si-OH groups is affecting the binding strength of neighbored Si-O-Si groups. This effect may decrease with increasing distance such that a closely neighbored Si-O-Si group will be affected more strongly (green colored bonds, Fig. 5c) than Si-O-Si groups that are located at larger distances (brownish to black colored bonds, Fig. 5c from the particles surface (i.e., within the center of a non-porous silica particle like silica gel). Since the number of Si-O-Si bond located near to the particles surface relative to the ones located within the particles center is increased with the specific surface area, the number of “affected” Si-O-Si groups increase with SSA. Such number of Si-O-Si groups different in binding strength increases with SSA that it may become detectable in the FTIR spectra of samples with higher SSA resulting in broader Si-O-Si bands for the Sipernat and Aerosil samples. Bands around 955 cm-1 may also act as an indicator for specific surface area (SSA) as they are attributed to the asymmetric vibration of Si-OH. An increase in intensity of this band indicated an increase in SSA as hydroxyl groups are mainly situated at the surfaces of the respective Si species 18.
Sand grains (QS) with a low SSA also have Si-OH groups at the particles surface, which of course affect the binding strength of the neighbored Si-O-Si bond. But the low porosity and the relatively large particle size of such grains result a low SSA such that the number of Si-O-Si groups located near to the surface relative to the ones located within the particles center (Fig. 2) is much smaller for quartz sand as compared to amorphous silica like Sipernat. Consequently, mostly the Si-O-Si groups located within the particles center will show up in the FTIR spectra resulting in a small Si-O-Si band. Additionally, quartz has the highest internal order compared to the other materials analyzed which results in a narrow Si-O-Si band.
In the FTIR of quartz sand (black line in Fig. 1) the second maximum at the left-hand side of the Si-O-Si band (grey arrow, Fig. 1) can be explained by the fact that the quartz sand studied here is a commercially available material that is obtained by ashing and HCl treatment of sea sand. However, this procedure will not remove cations that are fixed within the Si-O-Si structure. Thus, the quartz sand will contain small amount of cations like iron within its Si-O-Si structure in contrast to silica (Fig. 5d): this cation substitutes a Si within the Si-O-Si structure. The presence of such cations within the Si-O-Si structure (Fig. 5d) will also affect the binding strength of the neighbored Si-O-Si groups and may explain the observed second maximum of the Si-O-Si band in FTIR (Fig. 1; black solid arrow) which may be interpreted instead as a Si-O-Fe band.
The band at about 1500 cm-1 (Fig. 2; red solid arrow) in the FTIR of ZEOfree may results from its high calcium content and its high pH value: both, the Ca content and the high pH suggest the presence of deprotonated -negatively charged- silanol (Si-O-) groups suggesting the formation of Si-O—Ca2+ groups at the surface of the ZEOfree particles. Those Si-O—Ca2+ groups will show Si-O- bands in FTIR at a comparable but not identical WN range as the Si-O- band of the sodium silicate (Na4SiO4). However, the Si-O- band of ZEOfree showed up at WN 1500 cm-1 while that of the Na4SiO4 showed up at 1440 cm-1. The difference between the Si-O- bands from sodium silicate and that of the ZEOfree samples can be explained by different chemical environment of the Si-O- groups within the Na4SiO4 and the ZEOfree: in contrast to Na4SiO4 ZEOfree is not a monomer. Such Si-O- group in ZEOfree is neighbored by Si-O-Si groups that will affect the binding strength of the Si-O- band thereby causes a shift of the Si-O- band in FTIR compared to that of Na4SiO4.
Like ZEOfree the Imogolith (Fig. 3b, line A) showed a Si-O-Si band with a second maximum. This may result from Al inserted into the Si-O-Si structure of SROAS which affects -like the iron traces in quartz sand (black line; Fig. 1; see above)- the binding strength of the surrounding Si-O-Si groups. Such Si-O-Al groups may explain the second maximum of the Si-O-Si bands. The SROAS showed also absorption bands at WN 550 and 720 cm-1 (Fig. 3b) indicative for Si-O-Al bands 32,Wada, et al. 33 but, these bands did not did not show a second maximum for the Si-O-Si band (Figs. 3b and 3c). The same study found that increasing Al content increases a shift in the Si-O-Si band by comparing the FTIR spectra of SROAS with different Si:Al ratios (Fig. 6a). A similar shift was observed by Lenhardt, et al. 20 when analyzing short-ranged ordered aluminosilicates with varying Al:Si ratios (Fig. 6b). Stein, et al. 19observed a peak shift towards lower WN and a broadening of the entire envelope (1000–1300 cm-1) when analyzing silica metal compounds which were synthesized at undersaturation of silicates (Fig. 6). As the definition of allophane by Parfitt 32 is wrong, we suggest that those compounds may be simple SROAS, amorphous aluminosilicas or ASi.
The FTIR spectra of the water soluble Sipernat 50s fraction (Fig. 4d) showed an additional band that is in the WN range of the Si-O− band found for ZEOfree (1500cm− 1; Fig. 4a) and Na4SiO4 (1475 cm− 1; Fig. 4d) compared to that of the bulk Sipernat (Fig. 4b). This band indicated that the FTIR spectral signature of water soluble Sipernat 50s-fractions can be distinguished from that of the bulk Sipernat 50s, and suggests the water soluble fraction of Sipernat 50s consist mostly of small sized amorphous, anionic silica particles, The pH values is about 6, such deprotonation of silanol groups may be neglectable small. The bands at 1400 & 1100 cm− 1 in the FTIR of the water soluble SIP50s fraction possibly indicate the presence of dimeric silica, since they correspond to the respective ones in the FTIR of a dimeric organo-silica (adopted from Igarashi, et al. 36; Fig. 4e).