# Natural Clays as Potential Amino Acids Carriers for Animal Nutrition Application
## 3 Results
### Study of the Methionine-Carrier Interaction  
Organic molecule-mineral substrate interaction nature and strength can be deeper analyzed by combined TG-DTG and in situ FTIR analysis, as already reported in the literature for different organoclays [20,21]. Thermogravimetric analysis (Figure 7a-c) in air and in nitrogen (Supporting S6) were both performed on the solid phase of sample MET-ADS after three and six contacting steps (in the following, ADS 3C and ADS 6C). TG are reported in Figure 7a, while DTGs are reported in Figure 7b, where DTG of the pristine carrier and of a mechanical mixture of same composition of sample ADS 6C are plotted for comparison. DTG in air of pure methionine is plotted in Figure 7c. Figure 7: _Cont._In TG-DTG curves, different regions can be evidenced where different thermal phenomena occurred; analyzing it in detail it can be observed:  
1. A weight loss in the range 100-150 \({}^{\circ}\)C present in all the samples, but larger and more complex in pristine ADS, which is related to the release of water molecules allocated inside the clay interlayer and/or in the zeolite cages. Water evolution is lower in the case of the ADS 3C and 6C samples, as water molecules displacement occurred upon methionine intercalation [16]. Water evolution in the mechanical mixture is only barely evident due to the strong methionine decomposition. 2. A weight loss in the range 220-240 \({}^{\circ}\)C, always sharp and well defined, which is only present in the methionine-containing samples. This phenomenon is clearly associated with methionine decomposition, being centered at 250 \({}^{\circ}\)C, and corresponding to pure methionine degradation. The extent of the phenomenon increases on increasing methionine content (Figure 7b, compare ADS 3C and ADS 6C). Considering temperature and shape, this phenomenon corresponds to free and/or weakly bounded methionine. This attribution is also confirmed by DTG of the mechanical mixture (Figure 7b) (composition equal to ADS 6C), where no and/or very poor MET-ADS interaction closely resembles pure methionine decomposition peak (Figure 7c). 3. A weight loss in the range 500-550 \({}^{\circ}\)C, evident in ADS 3C and ADS 6C, which can be attributed to the decomposition of methionine strongly interacting with the carrier, e.g., intercalated in the bentonite and/or trapped in the zeolite cages. The stronger interaction may displace the decomposition at higher temperature. Alternatively, this thermal phenomenon could correspond to the decomposition of residues formed during methionine decomposition and re-adsorbed onto the clay. This last hypothesis is apparently also consistent with the small and barely evident peak, at the same temperature, in DTG of the mechanical mixture. 4. The broad peak centered at about 470 \({}^{\circ}\)C, and clearly evident in both ADS and MEC MIX, can be due to decomposition of the organic molecules already present in the pristine clay. The IR spectra of gas phase species at increasing temperatures for the same ADS 6C show the presence of bands in the 2400-2300 cm\({}^{-1}\) region and in the 2160-2130 cm\({}^{-1}\) one at the highest temperatures, consistent with the formation of COx from decomposition products from the organic molecules (both methionine and intercalated ammonium com  
Figure 7: Thermogravimetric analysis in air of MET–ADS samples at three and six contacting steps: (a) TG, (b) DTG, (c) DTG of pure methionine. ADS and mechanical mixture reported for comparison. pound). In particular, bands in the 2400-2300 cm\({}^{-1}\) region correspond to CO\({}_{2}\) production, while the ones in the 2160-2100 cm\({}^{-1}\) region are related to CO formation (Supporting S6).