When the solute becomes the solvent: Orientation, ordering, and structure of binary mixtures of 1-hexanol and cyclohexane over the (0001) α-Al ₂O₃ surface

Mixtures of 1-hexanol in cyclohexane over the (0001) α-Al ₂O₃ surface were probed in the CH stretching region using vibrational broadband sum frequency generation (SFG). Below 10 mol % 1-hexanol, the alcohol adsorbs to the surface through interactions that result in observed free adsorption energies of around 14-15 kJ/mol obtained from the Langmuir adsorption model. Polarization-resolved SFG spectra indicate ordering of the alkyl tails with increasing surface coverage. Highly correlated with the ordering of 1-hexanol is an orientational change of the cyclohexane solvent from flat to most likely tilted, suggesting that cyclohexane mediates the adsorption of 1-hexanol via intercalation. Above 10 mol %, the SFG signals for 1-hexanol and cyclohexane decrease with increasing concentration of 1-hexanol, consistent with the notion that cyclohexane is excluded from the interfacial region while 1-hexanol becomes increasingly disordered. At the interface, the alcohol solute becomes the solvent at a mole fraction of only 10%, i.e., five times below what is considered the solute-to-solvent mole fraction transition in the bulk. These results provide an important benchmark for theory, inform reaction design, and demonstrate that bulk thermodynamic properties of binary mixtures are not directly transferable to interfacial environments.