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The enthalpy of solution (ΔsolvHm) and solubility of 1,4-naphthoquinone in CO2 + n-pentane were measured at 308.15 K in the critical region of the binary fluid. In order to study the effect of phase behavior of the mixed solvent on ΔsolvHm, the experiments were carried out in the supercritical (SC) and subcritical region of the binary solvent. The density of the mixed solvent in different conditions was determined. The isothermal compressibility (KT) of the mixed solvent, and the partial molar volume (Vn-pentane) of n-pentane in the solution were calculated. It was demonstrated that the ΔsolvHm was negative in all conditions. ΔsolvHm is nearly independent of pressure or density in all the solvents in a high-density region, in which compressibility of the solvent is very small; this indicates that the intermolecular interaction between the solvent and the solute is similar to that for liquid solutions. It is very interesting that ΔsolvHm in the mixed SC fluid differs from the ΔsolvHm in mixed subcritical fluids. The absolute value of ΔsolvHm in the mixed SC fluid is close to that in pure SC CO2 in the high-density region, and is much lower than that in pure SC CO2 in the low-density region. In the mixed subcritical fluids, the ΔsolvHm is also close to that in the pure CO2 in the high-density region. However, at the same density, the absolute value of ΔsolvHm in the binary subcritical fluid is larger than that in pure CO2 in the high-compressible region of the mixed solvent. The main reason for this is that the degree of clustering in the SC solutions is small at the density in which the degree of clustering is large in the subcritical solutions. It can be concluded that solubility enhancement by n-pentane in the mixed SC fluid is entropy driven. In contrast, the solubility enhancement by n-pentane in subcritical fluids is enthalpy driven. The intermolecular interaction in the SC solutions and subcritical solutions can be significantly different even if their densities are the same.

The enthalpy of solution (ΔsolvHm) and solubility of 1,4‐naphthoquinone in CO2+ n‐pentane were measured at 308.15 K in the critical region of the binary fluid. In order to study the effect of phase behavior of the mixed solvent on ΔsolvHm, the experiments were carried out in the supercritical (SC) and subcritical region of the binary solvent. The density of the mixed solvent in different conditions was determined. The isothermal compressibility (KT) of the mixed solvent, and the partial molar volume (Vn‐pentane) of n‐pentane in the solution were calculated. It was demonstrated that the ΔsolvHmwas negative in all conditions. ΔsolvHmis nearly independent of pressure or density in all the solvents in a high‐density region, in which compressibility of the solvent is very small; this indicates that the intermolecular interaction between the solvent and the solute is similar to that for liquid solutions. It is very interesting that ΔsolvHmin the mixed SC fluid differs from the ΔsolvHmin mixed subcritical fluids. The absolute value of ΔsolvHmin the mixed SC fluid is close to that in pure SC CO2in the high‐density region, and is much lower than that in pure SC CO2in the low‐density region. In the mixed subcritical fluids, the ΔsolvHmis also close to that in the pure CO2in the high‐density region. However, at the same density, the absolute value of ΔsolvHmin the binary subcritical fluid is larger than that in pure CO2in the high‐compressible region of the mixed solvent. The main reason for this is that the degree of clustering in the SC solutions is small at the density in which the degree of clustering is large in the subcritical solutions. It can be concluded that solubility enhancement by n‐pentane in the mixed SC fluid is entropy driven. In contrast, the solubility enhancement by n‐pentane in subcritical fluids is enthalpy driven. The intermolecular interaction in the SC solutions and subcritical solutions can be significantly different even if their densities are the same.

Silver nanoparticles were synthesized in sodium bis(2‐ethylhexyl)sulfosuccinate (AOT) reverse micelles in isooctane with tetraethylene glycol dodecyl ether (C12E4) as a cosurfactant. Recovery of the Ag particles from the reverse micelles by dissolving antisolvent CO2in the micellar solution was investigated. All the Ag particles in the reverse micelles could be precipitated by compressed CO2at suitable pressures, while the surfactants remained in the isooctane continuous phase, and well‐dispersed Ag nanoparticles were obtained. The effects of operating conditions on the size and size distribution of the Ag particles were investigated. The particle size decreased with decreasing molar ratio (w) of water to surfactant. A higher CO2pressure in the recovery process favored production of smaller particles. A decrease in the molar ratio of reductant KBH4to AgNO3resulted in larger Ag particles with higher polydispersity.

The solubility of Ls‐54 surfactant in supercritical CO2was determined. It was found that the surfactant was highly soluble in SC CO2and the water‐in‐CO2microemulsions could be formed, despite it being a non‐fluorous and non‐siloxane nonionic surfactant. The main reasons for the high solubility and formation of the microemulsions may be that the surfactant has four CO2‐philic groups (propylene oxide) and five hydrophilic groups (ethylene oxide) and its molecular weight are relatively low. The results of this work provide useful information for designing CO2‐soluble non‐fluorous and non‐siloxane surfactants. The phase behavior of the CO2/Ls‐54/H2O system, solvatochromic probe study, and the UV spectrum of lysozyme proved the existence of water domains in the SC CO2microemulsions. The method of synchrotron radiation small‐angle X‐ray scattering was used to obtain the structural information on the Ls‐54 based water‐in‐CO2reverse micelles. By using the Guinier plot (ln I(q) versus q 2) on the data sets in a defined small qrange (0.022–0.040 Å−1), the radii of the reverse micelles were obtained at different pressures and molar ratio of water to surfactant, W0, which were in the range of 20.4–25.2 Å.

Study on the properties of the fluids near the critical point of mixed systems is a key for the development of supercritical (SC) technology and for the further understanding of the features of supercritical fluids (SCFs). The constant-volume molar heat capacity (Cv) of a solution is directly related to the internal energy, intermolecular interaction, and the microstructure of the solution. In this work, the Cv of CO2–n-pentane and CO2–ethanol systems was determined at 308.15 K in different phase regions. This work focuses on how the properties of the mixtures change with pressure, composition, and the structure of the components near the critical point of the mixtures. It was found that at fixed composition, a maximum in Cv versus pressure curve exists (C$\rm{_{v}^{max}}$ that occurs at the pressure at which the isothermal compressibility (KT) is the largest. We deduced that breaking the “clusters” in the SC mixtures is an endothermic process. It is very interesting that Cv increases sharply as the pressure approaches the critical point (CP) or bubble point (BP), while Cv is nearly independent of pressure and composition at the pressures well above the CP or BP pressure, and that the Cv at CP or BP can be several times larger than that at the high pressures. It can be deduced that at fixed composition the degree of “clustering” changes significantly with pressure near the CP or BP, and is the largest at CP and BP.

A setup for a calorimeter for simultaneously measuring the solubility and the solution enthalpy of solid solutes in supercritical fluids (SCFs) has been established. The enthalpy of solution of naphthalene in supercritical CO2 was measured at 308.15 K in the pressure range from 8.0–11.0 MPa. It was found that the enthalpy of solution (ΔH) was negative in the pressure range from 8.0 to 9.5 MPa, and the absolute value decreased with increasing pressure. In this pressure range, the dissolution of the solute was enthalpy driven. However, the ΔH became positive at pressures higher than 9.5 MPa, and the dissolution was entropy driven. Monte Carlo simulation was performed to analyze the local structural environment of the solvated naphthalene molecules in supercritical CO2 under the experimental conditions for the calorimetric measurements. By combining the enthalpy data and the simulation results, it can be deduced that the energy level of CO2 in the high compressible region is higher than that at higher pressures, which results in the large negative enthalpy of solution and the larger degree of solvent–solute clustering in the high compressible region.