PubMed · 12989159
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M LACHAUX. [Non-aqueous injectable solvents].. https://pubmed.ncbi.nlm.nih.gov/12989159/
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By comparison of neopentane pair potentials of mean force (PMFs) in room temperature water and 6.9 molar aqueous urea, it was recently shown that urea molecules affect the PMF minima in an unexpected way (Lee, M.-E.; van der Vegt, N. F. A. J. Am. Chem. Soc. 2006, 128, 4948). While the first PMF minimum in urea solution has an identical shape and depth to those of the corresponding minimum in water, the second minimum in urea solution is broader, deeper, and shifted out to a slightly larger distance. Here, we present a study of the enthalpic and entropic contributions to these PMFs. Its significance for understanding the driving forces responsible for thermodynamically favorable neopentane contact and solvent-separated distances in urea solution is discussed. We propose that the solute-solvent entropy and solute-solvent enthalpy changes should be analyzed for obtaining an unambiguous molecular-scale picture. In urea solution, enthalpy-entropy compensation effects associated with structural solvent reorganization processes are large, causing changes of the system's enthalpy and entropy with hydrophobic pair separation to be very different from the solute-solvent enthalpy and entropy changes. The entropies are discussed in terms of the molecular-scale solvent reorganization processes.
It has been noted that the work of cavity creation in water exhibits a crossover behavior, in that its cavity size dependence changes from volume dependence for small cavities to area dependence for larger cavities [Lum, K.; Chandler, D.; Weeks, J. D. J. Phys. Chem. B 1999, 103, 4570]. It is shown here that this behavior can be reproduced using the scaled particle theory in a straightforward manner for six different liquids (water, methanol, ethanol, benzene, cyclohexane, and carbon tetrachloride). It has also been suggested that the crossover is due to a change in the physical mechanism of the process, from one entropy-dominated to another enthalpy-dominated. However, the crossover behavior can be produced using the scaled particle theory without invoking any change in any physical mechanism. Also, the crossover occurs at a length scale of the size of the liquid molecules, as has been pointed out by others. This is the length regime where the work of cavity creation bears little relation to the bulk liquid surface tension. In addition, it is pointed out that cavity creation can always be considered as a purely entropy-driven process, which is usually accompanied by another process with compensating enthalpy and entropy changes.
The ultrafast internal conversion (IC) dynamics of the carbonyl carotenoid 12'-apo-beta-caroten-12'-al has been investigated in solvents of varying polarity using time-resolved femtosecond transient absorption spectroscopy. The molecules were excited to the S(2) state by a pump beam of either 390 or 470 nm. The subsequent intramolecular dynamics were detected at several probe wavelengths covering the S(0)--> S(2) and S(1)--> S(n) bands. For the S(1)--> S(0) internal conversion process, a remarkably strong acceleration with increasing polarity was found, e.g., lifetimes of tau(1) = 220 ps (n-hexane), 91 ps (tetrahydrofuran) and 8.0 ps (methanol) after excitation at 390 nm. The observation can be rationalized by the formation of a combined S(1)/ICT (intramolecular charge transfer) state in the more polar solvents. The effect is even stronger than the strongest one reported so far in the literature for peridinin. Addition of lithium salts to a solution of 12'-apo-beta-caroten-12'-al in ethanol leads only to small changes of the IC time constant tau(1). In addition, we estimate an upper limit for the time constant tau(2) of the S(2)--> S(1) internal conversion process of 300 fs in all solvents.