I am getting negative partial molar volume (PMV) of a solute solvated in a supercritical fluid at infinite dilution. The nature of interaction of entire system is purely van der Walls interaction. On increasing the density, PMV gradually increases and later, it becomes negative to positive in higher density regime. In literature, it has been explained on the basis of density augmentation (as much I understood). But I want to understand this variation at molecular level. For example, we can think this variation as an interplay of solute-solvent and solvent-solvent interaction. If density augmentation is cause of negative PMV in lower density regime, we can think of more favorable solute-solvent interaction in this regime. Then what happens further? Still I am not clear how this variation can be explained on the basis of molecular interaction. Please comment on this and give an interpretation at molecular level.
In my opinion your observation is not unusual for supercritical fluids. At low densities of SC solvent the solute molecules can be introduced into the cavities between SC fluid clusters. Due to solute-solvent interactions density augmentation and negative PMV occur. Under the high density conditions the distances between solvent molecules or clusters are not enough to incorporate solute molecules, so their penetration into fluid leads to increasing of solution volume (positive PMV) independently of nature of solvent-solute interactions.
In my opinion your observation is not unusual for supercritical fluids. At low densities of SC solvent the solute molecules can be introduced into the cavities between SC fluid clusters. Due to solute-solvent interactions density augmentation and negative PMV occur. Under the high density conditions the distances between solvent molecules or clusters are not enough to incorporate solute molecules, so their penetration into fluid leads to increasing of solution volume (positive PMV) independently of nature of solvent-solute interactions.
We looked at this some years ago for naphthalene in sc-CO2
http://pubs.acs.org/doi/abs/10.1021/jp0502656
Negative PMV is not unusual and relatively simple systems as magnesium sulphate in water show negative PMV at room temperature. I think that Dr. Kosyakov provide a good explanation in thermodynamic terms for this behavior. A very recent and more molecular example appears in P.VenkateswaraRao et al. (J.Mol.Liq. 219,289-296,(2016)) where some references may be tracked.
How can we explain the negative partial molar volume? - ResearchGate. Available from: https://www.researchgate.net/post/How_can_we_explain_the_negative_partial_molar_volume#view=57c97d6f48954cf273510d94 [accessed Sep 3, 2016].
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