This is reflected by the negative partial molar volume of a solute near the critical point of SC CO2. Also, it is reflected by endothermic adsorption of a solute near the solvent´s (CO2) critical point. Experimentally there are a few examples of this behaviour. This is known not to be caused by the solute, but to the solvent itself. One could conclude that the SCF has structure more compact (dense) than that of the gas.

Supercritical fluids are highly heterogeneous: they consist of large clusters in the gas-like zones and pores in the liquid-like zones. Their local density fluctuates from the density of the liquid phase to the density of the gaseous phase. The largest fluctuations take place at the critical density. The closer we come along the critical density isochor to the critical temperature, the larger are the density fluctuations. Here we see an analogy with the Curie-Weiss law: the maximal dimension of clusters or bubbles grows inversely proportional to the T - Tcr value.

Article Analytical Approach to Clusters in near Critical CO 2

Thanks Prof. Boris I. Sedunov for your comments. Nice article that you shared.

Dear!

I think that the best answer you already get from Prof. F. Recasens!

Best regards, Zeljko Knez

Dear Hari,

     If you are trying to understand the mechanism behind solubility enhancement/depletion (and related solvation behavior) at near-critical (highly compressible) solvent conditions from a microstructural viewpoint, you need to find the link between the driving force of the process (a free energy change during the solvation of the infinitely dilute solute) and its microstructures manifestation.  In other words, you need to investigate the ‘cause-effect connection’ based on sound stat. mech. considering the actual coexistence of phenomena involving two rather different length-scale processes, i.e., a short-range local density perturbation of the solvent around the infinitely dilute solute and the longer-range propagation of such a local density perturbation across a distance dictated by the solvent’s correlation length (~ proportional to the solvent’s compressibility, where the solvent compressibility can be interpreted in terms of the fluctuation of the number or particles in an open system).  

     The full development and theoretical justification for the alluded processes have been presented in details in a series of publications that you can download from our site in Researchgate, including the following extensive reviews:

Chapter: Solvation Phenomena in Dilute Solutions

Ariel Chialvo

Chapter: Molecular‐Based Modeling of Water and Aqueous Solutions at Supercritical Conditions

Ariel A. Chialvo · Peter T. Cummings

as well as the original manuscripts including,

Article: Solute-Induced Effects on the Structure and the Thermodynamics of Infinitely Dilute Mixtures

Ariel A. Chialvo · Peter T. Cummings

Article: Molecular approach to high-temperature solvation. Formal, integral equation and experimental results

AA Chialvo · P. G. Kusalik · P. T. Cummings · [...] · R. E. Mesmer

      The main idea underlying the rigorous developments in the above publications is that ‘solvent local density augmentation’ has nothing to do with the solvation of infinitely dilute solutes in near critical (or highly compressible) solvents; in other words, there is no such a cause-effect connection.  In fact, you even find solvent local density augmentation around any solvent molecule in a pure solvent environment, as a result of the non-zero intermolecular interactions among molecules (if no interactions, such as an ideal gas environment, then you find an uniform local density distribution that equals the bulk density value).  The solvation process of a highly dilute solute at any state condition (i.e., non-critical, near-critical, or supercritical) is governed by a difference of free energy that involves a finite local density perturbation, while the solvent compressibility plays no role beyond that of propagating the perturbation.  In fact, the free energy change (e.g., Figure 8.1 of Chapter: Solvation Phenomena in Dilute Solutions)  can be rigorously written in terms of finite direct correlation function integrals, and interpreted in terms of well-defined thermodynamic (measurable) properties, whose corresponding temperature- and pressure-derivative counterparts do not contain any compressibility driven contributions (as discussed in details in the above contributions).  

     The attempts to link the size of the first-peak of the solute-solvent radial distribution function, or assign a physically meaningful connection to solubility enhancement have been discussed extensively in the literature (see some of those references in the above publications), and while frequently used as a hand-waving argument to either “explain” some observations or correlate experimental measurements, they lack sound foundations needed in the accurate/reliable modeling of solvation of dilute solutes in highly compressible (near-critical) solvents.