SPECTROFLUOROMETRIC MEASUREMENTS OF LOCAL COMPOSITIONS AND COMPARISONTO THERMODYNAMIC MODELS

We present measurements of the local composition about the fluorescenc e probes pyrene and benzo[ghi]perylene in binary liquid solvents. The probes provide information about the solvation of the solute on a micr oscopic scale. The advantage of the fluorescence probes is that they a re relatively nonpolar and do not contain functional groups with which to form hydrogen bonds. Actual measurement of local compositions is i mportant as the concept is the basis of a number of useful thermodynam ic models. Thus, we compare the measured values with those predicted f rom both the nonrandom two-liquid (NRTL) equation and the Wilson equat ion. The standard recommended values of the nonrandomness factor in th e NRTL equation generally underestimate the magnitude of the local com positions. Larger nonrandomness factors provide better descriptions of the microscopic environment. On the other hand, the Wilson equation o verestimates the magnitude of the local compositions in all cases.