# Intermolecular Interactions in Mixed Choline Acetate and Maleic Acid Systems
## 3 Results
### Infrared Spectroscopy Results
In particular, for the ChAc-MA 2-1 mixture, Figure 4b shows that all the three conformers (Figure 2a-c) present different peaks in correspondence to the broad band around 350 cm\({}^{-1}\). Conversely, the two configurations obtained for the ChAc-MA 1-1 mixture (Figure 2d,e) are characterized by a very similar spectrum (Figure 4c) and one cannot distinguish different contributions around 350 cm\({}^{-1}\). Actually, the shape of the peak around 350 cm\({}^{-1}\) in the spectrum of the ChAc-MA 1-1 mixture does not appreciably change with temperature and, indeed, this is highly expected since the two configurations are almost coincident and separated by an extremely low energy difference. To further investigate if the changes in temperature in the band intensity are given by the modification of the configurations distributions, the band profile was analyzed with the same model already used for other systems [53; 54]. Indeed, in a liquid, the ratio (\(r\)) between two specific configuration concentrations is proportional to the ratio of the intensities of their bands and can be defined as reported in Equation (3):  
\[r=\frac{I_{1}}{I_{2}} \tag{3}\]  
where \(I_{1}\) and \(I_{2}\) are the integrated IR intensity of the bands corresponding to the different configurations. The thermodynamic equilibrium constant K between configurations is proportional to \(r\) and depends on \(\Delta G^{0}\), as reported in Equation (4):  
\[-RTln(r)=\Delta G^{0}=\Delta H^{0}-T\Delta S^{0} \tag{4}\]  
where \(R\) is the gas constant, \(T\) is the temperature and DH\({}^{0}\) and DS\({}^{0}\) are the enthalpy and entropy difference in the configurations; it can be rearranged as  
\[ln(r)=-\frac{1}{T}\frac{\Delta H^{0}}{R}+\frac{\Delta S^{0}}{R}+c \tag{5}\]  
where \(c\) is a constant. Plotting \(ln(r)\) as a function of \(1/T\) allows to verify the van't Hoff dependence of the concentration from the temperature evolution. The slope of the linear regression of \(ln(r)\) provides the values of energy separation between the two configurations. In order to use this model to verify our hypothesis, the experimental band observed in the ChAc-MA 2-1 mixture was deconvoluted after a baseline correction, using three Gaussian functions centered at about 340 cm\({}^{-1}\), 360 cm\({}^{-1}\) and 380 cm\({}^{-1}\), respectively: indeed, the 2-1 composition presented three different possible geometric configurations within the energy range that was considered by us. Although the structures slightly differ from a geometric point, vibrational frequency calculations provided clues suggesting that each of them are capable of generating a different band in the spectral region between 300 and 400 cm\({}^{-1}\). Figure 6 reports the van't Hoff plot obtained from the calculated values of the \(ln(r)\) related to compositions 2-1. Two regression fits were performed using the ratio between the bands at 360 cm\({}^{-1}\) and 340 cm\({}^{-1}\) (black square, Figure 6) and 380 cm\({}^{-1}\) and 340 cm\({}^{-1}\) (red circle, Figure 6). In both cases, we observed a good linearity between \(ln(r)\) and \(1/T\), with the best fit obtained for 380/340 cm\({}^{-1}\). In particular, Figure 6 shows a linear relationship between \(ln(r)\) and \(1/T\) from room temperature to approximately 185K, while a notable deviation from linearity is observed below this value. This kind of dependence was typically observed in ionic liquids undergoing a glass transition: the liquid phase is characterized by a linear dependence of ln(r) while a substantial deviation from linearity is observed in the low temperature glassy phase. Indeed, the evolution of conformers according to a Boltzmann distribution is only applicable within the liquid phase: upon the formation of solid or vitreous phases, geometric conformations become fixed into a single geometry. It is worth noting that linear dependence is not observed below 185K, which is very close to the glass transition temperature reported by DSC (around 190K in Figure S1).