## 4 Results and Discussion
### Raw and Heated at 100 \({}^{\circ}\)C Citric Acid Monohydrate Samples
#### 4.1.1 Thermal Behavior and IR Spectra
The peak at the higher wavenumber at 1753 cm\({}^{-1}\) cannot be assigned to acid groups since it is characteristic of aliphatic ester C=O or anhydride C=O, which typically appear at even higher wavenumbers, e.g., 1740-1800 cm\({}^{-1}\)[18]. It is known that the C=O stretching vibration of carboxylic acids appears at quite lower wavenumbers, e.g., 1700 cm\({}^{-1}\)[18]. Thus, the peak at 1753 cm\({}^{-1}\) cannot be assigned to acid C=O and points out the existence of ester C=O. In addition, the water bending vibration occurs at around 1650 cm\({}^{-1}\), but no such peak is present in the spectrum of citric acid monohydrate. In the case of gallic acid, shifting of the water bending peak was observed, which depended on if the water molecule interacted with the COOH or OH group [3]. Thus, it seems likely that the water bending band in the spectrum of the citric acid monohydrate is shifted at higher wavenumbers and overlaps with the C=O stretching of acid groups. Thus, the triple peak of the citric acid monohydrate in the C=O region contains contributions from acid C=O stretching, ester C=O stretching and water H-O-H bending. This means that the citric acid monohydrate does not exist in the solid state, it partially decomposes through esterification, and the substance we name citric acid monohydrate is actually a mixture of citric acid, water and citric acid ester, most likely in equilibrium. In the region of O-H stretching (3100-3700 cm\({}^{-1}\)), the characteristic bands of water O-H stretching at 3490  
Figure 3: (**a**) FTIR spectra of the raw and the heated at 100 \({}^{\circ}\)C samples and their subtracted spectra (heated-raw) in the 1500–4000 cm\({}^{-1}\) region, (**b**) Same spectra as in (**a**) in the 1500–2000 cm\({}^{-1}\) region and (**c**) FTIR spectra of gallic acid. and 3280 cm\({}^{-1}\)[18] cannot be distinguished in the IR spectrum of citric acid monohydrate since they overlap with the O-H stretching of the OH group at higher wavenumbers and the O-H stretching of COOH groups, which is known to occur at lower wavenumbers, e.g., in the region 2500-3300 cm\({}^{-1}\)[18]. In the heated sample, two sharp peaks at 3494 and 3290 cm\({}^{-1}\) are present. The two peaks are very close to the literature values of water at 3490 and 3280 cm\({}^{-1}\)[18]. These two peaks were also present in the IR spectrum of gallic acid and were found to be related to water O-H stretching [3]. The IR spectrum of gallic acid is presented in Figure 3c, in which the resemblance with the spectrum of the heated citric acid can be seen in the region 3000-3600 cm\({}^{-1}\). The above-mentioned peaks and especially the sharp peak at 3498 cm\({}^{-1}\) are clearly visible in both spectra. In the subtracted spectrum (Figure 3a,b), it is clear that these are two positive peaks inside the overall negative O-H stretching peak. This clearly points out that the overall OH groups have decreased, with a simultaneous increase in the water content. The esterification reaction could explain this alteration in the spectrum of the heated sample, since it should result in a decrease in O-H stretching of the COOH and OH groups and cause an increase in the water bands. Of course, in such a case, the water bending band should also be positive, and the ester C=O band should also increase. Indeed, in the subtracted spectrum, it can be seen that two positive peaks exist at 1745 cm\({}^{-1}\) (assigned to ester C=O) and at 1705 cm\({}^{-1}\) (assigned to water H-O-H bending). The negative peak at this region can be related to the reduction of the acid C=O groups due to the esterification reaction. Based on the above, we propose the band assignments that are presented in Tables 1 and 2 for citric acid monohydrate and citric acid anhydrous (heated at 100 \({}^{\circ}\)C), respectively. As a summary of this section, we can say that citric acid monohydrate undergoesesterification to some extent even at room temperature during its preparation/storage. Upon heating, it exhibits desolvation inability and does not dehydrate.