Cybotactic region

Probing the cybotactic region in gas-expanded liquids (GXLs). 

Kim and Johnston (27), and Yonker and Smith (22) have used solute solvatochroism to determine the composition of the local solvent environment in binary supercritical fluids. In our laboratory we investigate solute-cosolvent interactions by using a fluorescent solute molecule (a probe) whose emission characteristics are sensitive to its local solvent environment. In this way, it is possible to monitor changes in the local solvent composition using the probe fluorescence. Moreover, by using picosecond time-resolved techniques, one can determine the kinetics of fluid compositional fluctuation in the cybotactic region. 

Baker S N, Baker G A, Kane M A, et al. The cybotactic region surrounding fluorescent probes dissolved in l-butyl-3-methylimidazolium hexafluorophosphate Effects of temperature and added carbon dioxide. J. Phys. Chem. B. 2001. 105, 9663-9668. 

Narang, U. Jordan, J. D. Bright, F. V. Prasad, P. N., Probing the cybotactic region of PRODAN in tetramethylorthosilicate-derived sol-gels. Journal of Physical Chemistry 1994, 98(33), 8101-8107... 

Most of the solvent systems exhibit a non-ideal additive behaviour. This result can be ascribed to (a) the preferential solvation of the cybotactic region by some of the solvents present in the mixture, (b) the occurrence of strong solvent-solvent interactions that can generate complex intersolvent structures affecting solute-solvent interactions or (c) a combination of the two phenomena. 

Several studies of photochemical transformations of solutes have sought to exploit the unusually high viscosities, anisotropic cybotactic regions, and helical macrostructures that cholesteric phases provide. Only in selected cases have the... 

Knauer and Napier (34e) have suggested a division of current polarity parameters into two classes (I) those that involve no model reaction and which do not probe the solvent at the molecular level (in the cybotactic region) and (2) those that do involve a model and do probe the solvent at the molecular level. As examples of the former class they cite the dielectric constant dipole moment /a, and as examples of the latter class, Y,Z,EtOO) and ft. Nitrogen hyperfine splitting... 

The cybotactic region is the volume around a solute molecule in which the ordering of the soivent moiecuies has been influenced by the solute. 

A suitable probe for the general solvent effect must therefore be a polar compound. Around its dipole moment, the solvent molecules will arrange themselves as effectively as possible -in thermal equilibrium- and the interaction between the probe dipole and the solvent molecules that form the cybotactic region must be strongly altered by electronic excitation, which will result in an appropriate shift in charge the charge will then create a new dipole moment enclosed by the same cybotactic cavity as in the initial state of the transition. 

Analyzing the SPP, SB and SA scales in the light of spectroscopic data that are sensitive to the nature of the solvent poses no special problem thanks to the vertical nature of the transitions, where the cybotactic region surrounding the chromophore is hardly altered. The papers where the SPP, SA and SB scales were reported discuss large sets of spectroscopic data in terms of the nature of the solvent. Some additional comments are made below. 

For homogeneous environments only, averages out to zero. For liquids if a very structures solvation shell remains around the solute, it is convenient to introduce this region (cybotactic region) into the definition of the solute and treat the surroundings as homogeneous. 

FIGURE 2.5 (a) Solvation in flatland. A solvated anion (black circle) and its cybotactic region. 

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