Synthesis and Characterization of Nano-Aerogels

7.4. Conclusion

The in situ ATR-FTIR spectrometry was analyzed using the curve fitting and the self-modeling method, and the concentration profiles during the direct sol-gel process in CO₂ were obtained. The resulting relative concentration profiles of the precursors, complex structures and products are of importance for understanding the direct sol-gel process in CO₂. According to the calculated concentration profiles, the activities of TEOS, TIP and ZBO precursors with acetic acid were significantly different. For instance, the reactivity of TEOS was much lower than that of TIP and ZBO. Less than 100 % conversion from TEOS to SiO₂ was obtained in 360 minutes at 60 °C. On the other hand, the Ti and Zr complexes were formed in 10 minutes at 60 and 40 °C, respectively, indicating that these precursors were more reactive. Additionally, the condensation kinetics of the Ti and Zr complexes was different: the consumption of the former was abrupt after a critical point, and that of the latter was gradual through the sol-gel process. The different kinetics can be explained by the activity of the complexes and the competition of the two condensation reactions.

Chapter 8. Stabilization of the Colloidal Particles in CO₂

In this chapter, the interaction between CO₂ and the colloidal particles is studied using both the approaches of ATR-FTIR spectrometry and solubility parameter methods based on Fedors’ equations. The study results show that the acetate group is CO₂-philic and the SiO₂, TiO₂ and ZrO₂ colloidal particles containing an acetate group can be stabilized in CO₂.

8.1. Introduction

Due to the lack of polarity and dipole moment, scCO₂ or subcritical CO₂ is a poor solvent for most polar solutes and macromolecules. However, CO₂ has a large quadrupole moment and a polar C=O bond, making a variety of materials soluble in scCO₂.^294 The interaction between CO₂ and small molecules or polymers has been studied by ab initio calculations^{295, 296} and FTIR spectroscopy.^{289, 297, 298} CO₂ may behave as both an electron acceptor and donor.^{290, 296} For example, the interaction of CO₂ and the carbonyl functional group has been found to consist of a Lewis acid and Lewis base attraction (LA-LB) between the partial positive carbon of CO₂ and the lone pairs of the carbonyl oxygen.^289 As well, the C-HO hydrogen bond between CO₂ and the LB site has been attributed to the high solubility of polymers with a carbonyl moiety in CO₂.^290

Understanding the nature of the interactions between colloidal particles and CO₂ is of importance to design and synthesize well-defined oxide nanoarchitectures during the sol-gel process in scCO₂ and subcritical CO₂. During the synthesis of SiO₂, TiO₂ and ZrO₂ nanoarchitectures in CO₂ in the earlier part of this work, in situ FTIR spectra were used to study the mechanism and kinetics of the sol-gel processes. Figure 8.1 shows the in situ FTIR spectra during the alkoxides reacting with acetic acid, in which the absorbance peaks in the region of 1400-1600 cm^-1 indicate the formation of M-acetate bidentates (M= Si, Ti, and Zr).^229 Similar to the carbonyl group, the acetate bridging and chelating bidentate group may donate electrons to CO₂ and result in LA-LB interactions (Figure 8.2). Because of the high intensity of bulk CO₂ and interference of the organic species, it is not possible to directly observe LA-LB interactions between CO₂ and the M-acetate group through the in situ IR spectra. Our strategy was to use ATR-FTIR to study the CO₂-impregnated sol-gel product, i.e. TiO₂ and ZrO₂ aerogels, with the acetate bidentate groups.

Figure 8.73. In situ IR spectra: (a) 1.3 M TMOS reacting with 5.3 M HOAc in CO₂ at a reaction time of 360 minutes, at 50 °C and 4000 psig; (b) 1.10 M TIP reacting with 3.85 M HOAc in CO₂ at a reaction time 40 minutes, at 60 °C and 4500 psig; and (c) 0.547 ZBO reacting with 1.76 M HOAc in CO₂ at a reaction time of 40 minutes, at 40 °C and 4500 psig.


Figure 8.74. Schematic drawing of possible LA-LB interactions between CO₂ and (a) the bridging acetate bidentate, (b) the chelating acetate bidentate, and (c) the acetate monodentate. M: Si, Ti or Zr.

To study the interaction between CO₂ and the colloidal particles semi-quantitatively, the solubility parameter method was used. The solubility parameter () is defined as the square root of the cohesive energy density () by Hildebrand:^299

(8.42)

Solubility parameters provide a simple approach to predict the solubility behavior of the components, based on the knowledge of the individual components alone.^300 As a result, solubility parameters have been widely used for correlating the interactions of solutes and solvents. For instance, the solubilities of Cu(II) and Cr(III) complexes in scCO₂ have been studied using the Hildebrand equation and a group contribution method;^301 the low solubility parameters of siloxane and fluoroalkyl groups were partially attributed to their CO₂-philicity.^302 In this chapter, Allada’s method and mixing rules were used to estimate the solubility parameters of CO₂ and the solvent mixture, respectively.^{303, 304} The solubility parameters of macromolecules were estimated using Fedors’ group contribution method.^305 Using the solubility parameter approach, the previous experimental conditions for stabilizing the colloidal particles can be justified.