Compressed Ethane Research Articles

Aqueous heterogeneous radical polymerization of styrene under compressed ethane

Pressurization of emulsions with compressed gases potentially offers a low-energy route for preparation of miniemulsions that can be polymerized to produce polymeric nanoparticles. Conventional approaches typically require high-energy mixing to generate the miniemulsion and this impedes their industrial application. Compressed ethane is employed in this study to generate oil-in-water (mini)emulsions based on styrene/water and the anionic surfactant Dowfax 8390. The initial, turbid emulsion becomes transparent under compressed ethane at 60 °C and 6 MPa. Aqueous heterogeneous radical polymerization of styrene at these conditions yields nanoparticles with diameters of around 60 nm. However, mechanistic investigations reveal that particle formation proceeds via a classical emulsion polymerization mechanism (aqueous micellar nucleation) rather than direct polymerization of monomer droplets (droplet nucleation). The transparency phenomenon is attributed to matching refractive indices in the continuous and dispersed phases. The Lorentz-Lorenz mixing rule is used to estimate both the refractive index and ethane solubility in the dispersed phase at the transparent condition.

Advancements Toward the Greener Processing of Engineered Nanomaterials—Effect of Core Size on the Dispersibility and Transport of Gold Nanocrystals in Near‐Critical Solvents

The ability to process and purify engineered nanomaterials using near critical or supercritical fluids (NcFs or ScFs) has enormous potential for the application at various stages of the development of green nanomaterials. The dispersibility of octanethiol-stabilized gold nanocrystals of different core sizes is explored, which were chosen to serve as model nanomaterials of general interest in compressed ethane and propane over a wide range of fluid conditions. Both solvents have enormous potential for the environmentally benign processing and transport of engineered nanomaterials due to their nominal toxicity and high degree of tunability and processability that can essentially eliminate solvent waste. The dispersibility is determined by measuring the absorption spectra of dispersions of various sizes of nanocrystals in NcFs. To better understand the obtained results three models, the total interaction theory, the sedimentation coefficient equation, and the Chrastil method, are discussed. Nanoparticle dispersibility versus density plots are strongly dependent on nanoparticle size and solvent conditions, with the dispersion of larger nanocrystals more dependent on changes of pressure or density at a given temperature. For the range of nanoparticle sizes studied, compressed ethane at 25 degrees C leads to a greater tunability of nanoparticle dispersion when compared with compressed propane at 65 degrees C. For equivalent pressures, compressed propane is found to provide better solubility than ethane due to its higher density. The results quantitatively demonstrate that NcFs can offer pressure-tunable, size-selective control of nanoparticle solvation and transport at easily obtainable temperature and pressure conditions. These capabilities provide clear advantages over conventional solvents and direct application to various nanomaterials processes, such as synthesis, separation, transport, and purification of nanocrystals.

Tuning and Quantifying the Dispersibility of Gold Nanocrystals in Liquid and Supercritical Solvents

The application of nanomaterials relies on the ability to synthesize, purify, transport, and deposit them in a controllable fashion. The capacity to adjust the density, and thus the solvent strength, of a supercritical or near-critical fluid can be used to tune reaction and separation processes as well as to assemble nanomaterials in a controllable fashion. Herein we demonstrate and quantify density-tunable and reversible size-dependent dispersibility of octanethiol-stabilized gold nanocrystals with a size of 3.7 ± 2.2 nm in near-critical and supercritical solvents as a way to show the significant potential of these fluids for nanomaterials processing. This study introduced discrete variations on the pressure of nanocrystal dispersions in compressed ethane and propane at temperatures of 25, 45, and 65 °C until they reached a saturation region, at which point actual measurements of nanocrystal dispersibility were obtained using UV−vis absorption spectroscopy. Transmission electron microscopy (TEM) was employed to correlate the dispersibility results with the actual size of the nanoparticle fractions at different densities. The results showed that stable dispersions of nanocrystals could be obtained at pressures as low as 50 atm in both solvents. Compressed ethane in its liquid or supercritical state was found to provide better dynamic tunability, whereas propane provided higher dispersibility of these nanocrystals under the studied pressure−temperature conditions. Two theoretical models, the total interaction theory and Chrastil equation, are briefly presented as a means of interpreting the experimental observations. It was determined that dispersibility depends strongly on the nanocrystal size, solvent density, and carbon chain length of the solvent. These results clearly show that selected supercritical fluids can be remarkably effective for the manipulation of nanoparticles.

Solubility of Naphthalene in Compressed Ethane and Nitrous Oxide: The Correlation of Naphthalene–Gas Second Cross Virial Coefficients Using Pseudocritical Parameters

The solubility of naphthalene in compressed ethane and nitrous oxide has been measured over pressure ranges of 1–60 atm at temperatures ranging from 20°–68°C. Second cross virial coefficients representing naphthalene–gas pair interactions are evaluated from the data. It is shown that when allowance is made for the effect of quadrupole moment on the critical temperatures of quadrupolar gases, ordinary combining rules for pseudocritical parameters reduce naphthalene–gas cross virial coefficients involving five different gases to a single function of the reduced temperature. It is concluded from this that: (a) quadrupole–quadrupole interactions between naphthalene and quadrupolar gases are negligible in comparison to the dispersion forces involved; and (b) that equilibrium constants for formation of acid–base complexes of carbon dioxide and naphthalene are considerably smaller than thought previously.