SPECTROSCOPIC DETERMINATION OF SOLUTE FLUID CLUSTER SIZE IN SUPERCRITICAL N2O

In supercritical fluids, near the critical point, there exists a region where the interactions between a solute and the surrounding solvent are such that the solvent molecules collapse about the solute. This enhancement of local solvent density, or solvent clustering, is an intriguing phenomena and may prove useful in separations, extractions, and reaction processes. We present here a means to experimentally determine the size of a solute-solvent cluster in the near-critical region using relatively straightforward spectroscopic techniques. In this work the solute, 6-propionyl-2-(dimethylamine)naphthalene (PRODAN), is fluorescent. Therefore, we can use fluorescence anisotropy and lifetime measurements to determine the rotational correlation time of PRODAN in supercritical N2O. The recovered rotational correlation time is in tum related to the volume of the rotating species if the viscosity of the medium is known. Two limiting cases describing the observed rotational diffusion are considered. In the first, we assume a situation with minimal interaction between the solute and solvent. Second, we investigate the case where the solute and solvent interact strongly. Analysis of the experimental data within this framework allows us to determine the lower and upper volume limits of a solvent cluster. By using the sensitivity of the PRODAN emission spectrum, to the local solvent environment, we determine the density directly surrounding the solute (i.e., the local density). In the region near the critical point, the local density of N2O about PRODAN is enhanced approximately 2.5-fold compared to the bulk. From the experimental local solvent density and cluster volumes, one has a means to calculate the average number of excess solvent molecules making up a cluster. Based on the two limiting cases, average cluster sizes between 25 and 103 N2O molecules occur near the critical pressure, and decrease with increasing pressure.