Adsorption and mixing behavior of ethers and alkanes at the solid/liquid interface

The behavior of binary mixtures of linear symmetrical ethers and alkanes adsorbed to a graphite surface from the bulk liquid mixtures is described on the basis of differential scanning calorimetry (DSC) data. Both the ethers and the alkanes are found to form solid monolayers when adsorbed from the liquid. In addition, the monolayer mixing behavior is addressed. The results indicate that there is good, essentially ideal, mixing in the monolayers for ethers and alkanes of the same overall chain length, where the chain length is equal to the total number of carbon and oxygen atoms in the molecule. However, a difference in chain length of more than one atom results in a variation of mixing behavior from nonideal mixing (for long pairs) to phase separation (for short pairs) on the graphite surface. Hence, we conclude that it is the relative chain lengths that control mixing behavior. The results are quantified using a regular solution model with a correction for preferential adsorption. The phase behavior of the mixed monolayers is also compared to the behavior of the bulk. Interestingly, we observe mixtures where the bulk and monolayer behavior are quite different, for example, phase separation in the bulk but essentially ideal mixing in the monolayer for mixtures of ethers and alkanes with the same chain lengths. At present, we attribute this mixing in the monolayer to dilution of the unfavorable ether oxygen-ether oxygen lone pair interactions by the coadsorbed alkanes. In addition, we find evidence for the preferential adsorption of the alkane over the ether. For example, heptane is preferentially adsorbed over dibutyl ether even though it contains two fewer atoms in the molecular chain. This contrasts with the preferential adsorption of alcohols over alkanes reported previously (Messe, L.; Perdigon, A.; Clarke, S. M.; Inaba, A.; Arnold, T. Langmuir 2005, 21, 5085-5093).