## 4 Discussion
#### Implications for Art Conservation
Secondly, the dynamics of solvent composition changes during evaporation are affected by the initial composition of the solvent mixture. Should a solvent mixture be used at ambient temperature with a concentration that is under the azeotropic composition, the ensuing evaporation will enrich the mixture in the more volatile solvent. Conversely, if a solvent mixture is prepared with a concentration that surpasses the azeotropic composition, the evaporation process will lead to an accumulation of the less volatile solvent. This phenomenon holds potential value for art conservators who may benefit from a solution that undergoes an alteration in potency over time, for instance becoming a \"weaker\" solvent as the solvent evaporates from the surface being cleaned. Finally, what is often overlooked in advocating for the use of azeotropes in the cleaning of painted surfaces is the preferential absorption of one component of the azeotrope into the coating material itself, thus throwing off the azeotropic composition of the evaporating liquid. Dissolution of organic material into the azeotrope also has the potential of altering the evaporation process, just as solvents with dissolved organic polymers have been shown in conservation research to alter the solvent's solubility characteristics [23]. Since the chemical composition of dirt, yellowed varnishes, and aged overpaint are difficult to know, it may be impossible to predict the evaporation behavior of even an appropriately formulated room temperature azeotropic cleaning solution. In an earlier cursory experiment performed by one of the authors, the room temperature evaporation of a 9.5% v/v isopropanol in n-hexane solution from the surface of a young cobalt blue hue linseed oil paint (Grumbacher) was compared to the evaporation of the same solution without an organic coating present [24]. The headspace over the evaporating solution was depleted of isopropanol in the sample with the oil paint, consistent with the expectation that isopropanol would be rapidly absorbed into the paint film, thus breaking the azeotrope. This complicating factor may prove the use of even room temperature azeotropes as insufficient for controlling solvent strength during evaporation from painted surfaces, although their use might ensure stock solutions do not change over the course of their use or storage. Other potential issues include evaporative cooling of the paint surface, the use of exhaust systems during artwork treatment, and the tendency of water to adsorb into polar organic solvents, all of which might change the evaporation behavior of the cleaning mixture to some extent. Future research will explore more closely how the evaporation dynamics of room temperature azeotropic solutions vary in the presence of organic coatings or solutions. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Azeotropic Mixtures \({}^{\ast}\)** & **T\({}_{\text{AZ}}\) (\({}^{\circ}\)C)** & **P\({}_{\text{AZ}}\) (mmHg)** & **Component A, x\({}_{\text{AZ}}\) (\% v/v)** \\ \hline (A) isopropanol & 62.7 & 760 & 20.05 \\ (B) n-hexane & 20.8 & 133 & 12.65 \\  & 14.8 & 100 & 11.69 \\ \hline (A) 2-butanone & 71.9 & 760 & 40.06 \\ (B) cyclohexane & 21.0 & 112 & 38.90 \\  & 18.5 & 100 & 38.85 \\ \hline (A) acetone & 55.6 & 760 & 10.48 \\ (B) n-heptane & 22.0 & 200 & 10.47 \\  & 7.1 & 100 & 10.46 \\ \hline (A) isopropanol & 65.3 & 760 & 32.06 \\ (B) cyclohexane & 21.4 & 120 & 24.40 \\  & 17.6 & 100 & 23.79 \\ \hline (A) 2-pentanone & 92.9 & 760 & 29.85 \\ (B) n-heptane & 39.0 & 100 & 28.88 \\  & 21.0 & 43 & 28.57 \\ \hline \hline \end{tabular} \({}^{\ast}\) Component A is the least volatile of the mixture.  
\end{table} 
Table 3: Calculated azeotropic data [22] for azeotropes of interest in art conservation [8].