## 4 Results and Discussion  
### Raw and Heated at 100 \({}^{\circ}\)C Citric Acid Monohydrate Samples  
#### 4.1.1 Thermal Behavior and IR Spectra  
In Figure 1, the DSC and TGA curves of citric acid monohydrate are presented in the range of 40-165 \({}^{\circ}\)C. As can be seen, in this temperature range, citric acid monohydrate exhibits two endothermic effects (DSC curve), and both of them overlap with considerable mass loss (TGA curve). The first thermal effect is centered at 61 \({}^{\circ}\)C, while the second effect is located at 150 \({}^{\circ}\)C.  
According to the established interpretations, we could explain the above results as follows: citric acid monohydrate dehydrates, the mass loss up to 90 \({}^{\circ}\)C is water evaporation, and then the anhydrous citric acid melts at 150 \({}^{\circ}\)C [11; 12]. However, we dispute that such an interpretation is correct. Although the temperature of 150 \({}^{\circ}\)C is very close to the literature values [11; 12] of the melting point of citric acid, it is clear that this effect overlaps with mass loss. In Figure 2, photographs are presented that show inside the TGA pan of the raw citric acid monohydrate sample and the same sample after being heated up to 165 \({}^{\circ}\)C (the temperature of 165 \({}^{\circ}\)C was selected since at this temperature the thermal effect is completed, as can be seen in the DSC curve in Figure 1). It is clear from the photos of Figure 2 that the sample undergoes severe decomposition by heating up to 165 \({}^{\circ}\)C. As mentioned above, the center of the second peak in the DSC curve is located at 150 \({}^{\circ}\)C, and the end of this thermal effect is located in the range of 160-165 \({}^{\circ}\)C. If this thermal effect was melting, then no sign of decomposition should be observed after heating the sample at this temperature range. Thus, it is clear that the thermal effect that is centered at 150 \({}^{\circ}\)C and ends at 165 \({}^{\circ}\)C cannot be melting. Citric acid exhibits melting inability. Actually, it is not citric acid that exhibits melting inability but a mixture of citric acid, water and the ester of citric acid, as we will discuss next. More precisely, we will now argue that the broad endothermic peak at 61 \({}^{\circ}\)C does not arise solely from the heat, which is required for water evaporation, but there is a contribution from the heat, which is required for the esterification reaction that occurs upon the heating of the sample. In order to support this claim, we will focus on the thermal effect below 100 \({}^{\circ}\)C, which is centered at 61 \({}^{\circ}\)C. In order to study this effect, a citric acid monohydrate sample was heated up to 100 \({}^{\circ}\)C. The temperature of 100 \({}^{\circ}\)C was selected since at this temperature, the mass loss of the first stage has been completed (see TGA curve in Figure 1) and the thermal effect at 61 \({}^{\circ}\)C seems to be completed (see DSC curve in Figure 1). The FTIR spectra of the raw and the heated at 100 \({}^{\circ}\)C samples along with their subtraction spectrum are presented in Figure 3.  
Figure 1: DSC and TGA curves of raw citric acid monohydrate. Figure 2: Photographs of (**a**) raw citric acid sample and (**b**) the citric acid monohydrate sample heated at 165 \({}^{\circ}\)C.  
As can be seen in Figure 3a, the IR spectrum of the two samples is quite different. Obvious alterations can be seen in the region of O-H stretching (3100-3700 cm\({}^{-1}\)). Also, in Figure 3b, the same spectra are presented around the region of C=O stretching in order to show the double peak of the heated sample and the triple peak of the monohydrate sample, which were mentioned in the Introduction. The presence of multiple distinct C=O stretching vibrations indicates the existence of multiple types of C=O. The wavenumbers of the triple peak are 1690, 1727 and 1753 cm\({}^{-1}\).