## 3 Results  
The Raman and FTIR spectra of the neat chemicals and their binary mixtures are displayed in Figure 1. In the first step of the evaluation of the data, the individual signatures were assigned to their corresponding modes according to the previous work of our group [14; 18; 19; 20] and of others [21; 22; 23; 24; 25; 26]. In the second step, the data were screened in order to identify interesting compositional regimes, for instance where ion pair dissociation likely takes place. For this purpose, principal component analysis (PCA) was applied to the data sets as described in reference [27]. Figure 2a shows the cumulative variance of the first five principal components (PCs) of the FTIR data. PC1-3 account for almost the full variance of the data set, and therefore the further analysis is limited to those three. Their loadings spectra are plotted in Figure 2b, revealing that they have a reasonable signal-to-noise ratio, indicating that they are physically meaningful. The loadings of the individual PCs are then plotted as a function of the mixture composition: see Panel 2c. PC1 decreases monotonously across the mole fraction range, while PC2 shows the opposite behavior. This makes sense when we look at the loadings plot in Panel 2b. PC1 predominantly contains signatures of the IL, while PC2 appears more characteristic of the alcohol spectrum. Hence, the loading-vs.-composition profiles roughly resemble the amounts of the pure components. The curves are not linear though, because the PC spectra do not exclusively represent the pure component spectra. PC3, on the other hand, does not exhibit a monotonous behavior. It shows a clear minimum around an alcohol mole fraction of \(x\)(PrOH) = 0.66. In the previous study [27], it was shown that such extreme values are likely pointing towards stoichiometries, at which interesting phenomena can be expected. In the present case, the mole fraction of \(x\)(PrOH) = 0.66 translates to a situation in which the solution contains two alcohol molecules per ion pair. This suggests two possible, but very different, interaction scenarios: (1) The hydrogen-bonded ion pairs, which likely exist in the neat IL [19], dissociate, and the cation-anion interactions are replaced by cation-alcohol and anion-alcohol interactions. In scenario (2), the ion pair remains intact, and two alcohol molecules establish an interaction, e.g., at those sulfonate oxygen atoms that are not involved in the cation-anion hydrogen bonding. Further details cannot be deduced from this initial PCA analysis. In order to gain further insights, the excess spectra were computed. The concept of excess spectroscopy was introduced about a decade ago [28], and was found to be very useful in the analysis of solvent mixtures, including those comprising ILs [29; 30]. Figure 3a,b display the IR and Raman excess spectra, respectively. The excess spectra of the pure compounds are equal to zero by definition. In contrast, the mixture spectra show multiple signatures. Interestingly, the OH stretching region in both data sets reveals predominantly negative contributions. In other words, the dipole moment and  
Figure 1: Attenuated total reflection (ATR)-FTIR spectra (**a**) and Raman spectra (**b**) of the neat substances and the binary mixtures. Figure 2: Principal component analysis (PCA) screening results of the FTIR data set: (**a**) cumulative variance of the first five principal components (PCs), (**b**) loadings spectra, (**c**) loadings as a function of composition. the polarizability are reduced in the real mixture compared to the ideal counterpart. This is an indicator of changes in the hydrogen bonding interactions when the two fluids are mixed. For example, the strengthening of a hydrogen bond between two molecules usually leads to a weakening of the covalent bond. This weakening can be observed as a red-shift of the vibrational frequency of the corresponding stretching mode [31]. The excess spectra give additional information here, as they indicate how the charge distribution develops.