Determination of the local environment surrounding pyrene in supercritical alkanes: A first step toward solvation in supercritical aviation fuels

We report quantitative data on the local microenvironment surrounding a model organic solute (pyrene) dissolved in several supercritical components of aviation fuels (n-pentane, n-hexane, n-heptane, n-octane). Toward this end, we use static fluorescence spectroscopy to determine whether these supercritical alkanes influence the local microenvironment surrounding pyrene in a way similar to other supercritical fluids (e.g., CO₂, CF₃H, C₂He, H₂O). Steady-state fluorescence measurements indicate that, in all alkanes studied, there is an increase in local fluid density surrounding the pyrene molecule relative to the fluid bulk density (solute-fluid density augmentation). The maximum in this local density augmentation occurs at approximately one-half the fluid critical density and subsequently decreases at higher fluid densities. These results are fully consistent with observations for pyrene in other supercritical fluid systems (i.e., CO₂ H₂O). The maximum relative local density augmentation increases with increasing alkane chain length. These results correlate with changes in alkane polarizability. Another, somewhat more tenuous, correlation suggests that the maximum degree of local density augmentation may decrease as chain length increases in supercritical linear alkanes above n-octane.