## 3 Results
### Evaporation Studies of Solvent Mixtures  
In adherence to the methodologies delineated in industrial distillation literature [14], it was decided to assess the evaporation stability of solvent mixtures about the two azeotropes under scrutiny. An azeotrope maintains a consistent composition in both the liquid and gaseous states throughout evaporation [14; 15]. Consequently, the azeotropic composition can be determined by subjecting various compositional mixtures to evaporation at a constant temperature. The results presented in Table 1 are derived from this procedure and pertain to the two solvent systems under study, which exhibit the characteristics of homogeneous pressure-maximum azeotropes within a binary system characterized by complete miscibility. This implies that both systems possess an azeotrope maximum in vapor pressure and adhere to the behavior depicted in Figure 2. In contrast, when zeotropic mixtures evaporate, their liquid phase composition increasingly deviates from the azeotropic point (x\({}_{\text{AZ}}\)).  
Mixtures of known composition for the two solvent systems were prepared and allowed to evaporate at room temperature and pressure until they reached 50% or less of their original mass. Subsequently, the liquid phase's composition post-evaporation was ascertained using either gas chromatography (isopropanol-hexane and 2-butanone:cyclohexane) or refractive index measurements (2-butanone:cyclohexane). The results and their standard deviations are shown in Table 1.  
For the binary system comprising isopropanol and n-hexane, the empirical evaluation revealed that solutions with compositions below 9.5% v/v exhibited a reduction in isopropanol concentration post-evaporation as determined by GC analysis of the remaining liquid. In comparison, solutions with compositions exceeding 10.5% v/v showed an increase in isopropanol concentration. Furthermore, the deviation in the composition of the residual solution post-evaporation is more pronounced the further the initial composition deviates from the 9.5-10.5% v/v range. This observation suggests that the azeotropic composition at room temperature resides within this narrow range. For this study, pinpointing an exact composition is not as critical as furnishing empirical evidence that underscores the disparity between the azeotropic composition at room temperature and the literature-reported azeotropic composition of 19% v/v isopropanol at its boiling point of 62.7 \({}^{\circ}\)C, i.e., the composition used by conservators in previous cleaning treatments of artworks [8; 11; 12].  
Figure 3: Pressure–composition (P-x-y) equilibrium data plot for isopropanol and n-hexane. Error bars indicate the standard error from 3 replicates.  
In the binary system of 2-butanone and cyclohexane, similar observations were made. Solutions with compositions below 38.7% v/ v 2-butanone manifested a decrease in the percentage of that solvent post-evaporation, whereas those with compositions above 46.9% v/ v exhibited an increase in the percentage of 2-butanone post-evaporation. Notably, the solution with an initial composition of 38.7% v/ v demonstrated an insignificant alteration, implying that this composition is very close to the azeotropic composition at room temperature. It is pertinent to mention that in this example the magnitude of this 2-butanone:cyclohexane composition change is relatively small compared to the azeotropic composition of 40% v/ v reported at boiling that has been explored for applications in conservation [8; 10]. Of note, the assessment of the room temperature azeotrope was equally successful using the simple GC apparatus or the inexpensive RI meter.