Rotational Relaxation in the Compressible Region of CO2: Evidence for Solute-Induced Clustering in Supercritical Fluid Solutions

Abstract

Rotational correlation times of two solutes, trans, trans-1 ,Cdiphenylbutadiene (DPB) and rrans-4-(hydroxymethy1)stilbene (HMS), have been measured in supercritical C02 as a function of density at a reduced temperature of 1.01 (35 "C) across the compressible region. The DPB rotational correlation time increases by 50% upon increasing density from 0.3 to 0.8 g/cm3, whereas the HMS rotational correlation time increases by more than 300% over the same density regime. The results are compared with a model that is developed to describe the influence of local solvent density on the rotational Correlation time of the solute. The model predicts a strong dependence of the rotational correlation time on the local solvent density as the liquid density of C02 is approached. On the basis of a comparison of the data with the model, the experimental results are interpreted as evidence for solute-induced clustering of C02 around HMS as a result of attractive solute-solvent interaction. The relative magnitude of the observed effect in HMS compared with DPB is taken as evidence that the operative interaction occurs between C02 and the hydroxyl group of HMS.Clustering of supercritical fluid solvents around solute molecules is thought to be responsible for enhanced solubility of solutes, particularly in the compressible region of the solvent's phase Considerable the~retical~-~ and experimental8-I3 effort has been aimed at developing a detailed understanding of this phenomenon. Spectroscopic studies have been widely applied to these systems because they are inherently sensitive to local solvent effects around chromophoric solute molecules. Such studies have the potential for distinguishing between theoretical models describing fluid clustering. Solvatochromic studies are most commonly employed, resulting in empirical measures of the local solvent reaction field around the solute.8-11~13-14 However, solvatochromic studies suffer from the fact that compounds which exhibit strong solvatochromic behavior contain highly polar functional groups that often experience charge transfer isomerization, either in the ground or excited electronic states. Thus it is difficult to develop relationships between solute functionality and the strength of solvent-solute interactions through these measurements because only specific solutes exhibit the correct properties to be useful candidates for such examinations.Studies of molecular rotational relaxation are not restricted by this shortcoming. A wide variety of solutes are amenable to this method, offering the possibility of examining the influence of solute functionality on solvent clustering in supercritical fluid solutions. This approach has been applied to the study of solvent-solute interactions in liquid solution^,'^^'^ and a few studies of solute rotational relaxation have been carried out in supercritical fluids solutions to For example Bright and co-workers have observed remarkable variation in the rotational correlation time of PRODAN in supercritical N20 as a function of density.I2 However th...

Near-Critical Behavior of Mutual Diffusion Coefficients for Five Solutes in Supercritical Carbon Dioxide

“…The local density enhancement in supercritical fluid has been thoroughly studied by theoretical and experimental means. 19,20,[41][42][43][44][45] In particular, Bai et al 43 determined the self-diffusion coefficients of methanol and 1-decanol in supercritical CO 2 by pulsed field-gradient NMR and concluded that a large local solvent density around the alcohol molecules under supercritical conditions will make the solute diffusion coefficient decrease. Similarly, the local density enhancement may be in part responsible for the observed decrease in mutual diffusion coefficients.…”

CO2 Clustering of 1-Decanol and Methanol in Supercritical Fluids by 13C Nuclear Spin−Lattice Relaxation

“…8 Such solvent-solute interactions may initiate CO 2 solvent clustering. 34 The spin-lattice relaxation time measurements were performed for this mixture in our laboratory, and the results are reported elsewhere. 22 Calculations of f stick and f slip for methanol are found to be 0.69 and 0.10, respectively, and the van der Waals volume is 41.8 Å 3 .…”

Rotational Diffusion and Trans-cis Isomerization of Substituted Diphenyl Polyenes in the Compressible Region of Supercritical Fluids.

“…However, only a few such studies have been published with the goal of examining density augmentation in the compressible region of supercritical fluids. [3][4][5] In this paper we examine the rotational diffusion of two The lifetime spectrometer has been described in detail previously, [6] and the instrument response function is 50 psec. Rotational diffusion times are determined from the fluorescence lifetimes and fluorescence anisotropies using the Perrin equation.…”