Details

Category: Selected Topics

Supercritical carbon dioxide (SC-CO2) is an useful and benign replacement for many organic solvents used in the traditional chemistry and oil industries. It is often considered as an ideal substitute to the standard organic solvents because it is non-flammable, essentially non-toxic, and the least expensive solvent after water. It has readily critical properties compared to those of water, being its critical density higher than that of most other supercritical solvents, and hence its solvent power can be enhanced several orders of magnitudes. In addition, it has a very low viscosity, low surface tension and low heat of vaporization, with highly tuneable properties by changes with pressure, and is readily recovered and recycled. It is, however, a poor solvent of many polar and organic compounds, including long-chain n-alkanes, water, polymers, and in general hydrophilic compounds, and this is reason becasue has not been used before.

This has led to an extensive search for effective surfactants that could be used to stabilize microdispersions of water or polymers in SC-CO2. In analogy to well-known surfactants which stabilize microdispersions in oil-water systems, surfactants for use in SC-CO2 would be amphiphiles with one part of the surfactant molecule being CO2-philic. Recent advances in surfactancy have shown that the most promising surfactants for use in SC-CO2 contain fluorinated chains as the CO2-philic part. Because surfactants in SC-CO2 form micelles in which the hydrophilic part of the surfactant is in the core, these micelles are generally referred to as reversed micelles by analogy to the nomenclature if convential oil-in-water systems.

Particularly interesting from industrial and practical point of view are surfactants that could be used to stabilize CO2 + water and CO2 + polymer binary mixtures. In the first case, fluoroalkane-polyoxyethylene diblock molecules constitute one the simplest non-ionic surfactants which stabilizes CO2 + water; in the second case, the ideal candidate could be fluoroalkane-alkane diblock non-ionic surfactants, which may stabilize CO2 + polymer mixtures. It has been demonstrated recently that these surfactants can emulsify insoluble solutes (water, polymers, etc.) into CO2, and hence, could be used as replacements for conventional solvent systems currently used in manufacturing and service industries, such as precision cleaning (metal finishing, microelectronics, optics or electroplanting), medical device fabrication and dry (garment) cleaning, as well as in the chemical manufacturing and coating industries. Unfortunately, these new surface active materials are relatively new, and hence, litle information on there properties is available. This is especially true for fluoroalkane-polyoxyethylene and fluoroalkane-alkane biblock surfactants, which are not commercially available and need to be synthesied specifically for there use and study.

Our group is involved in a preliminar study of this kind of systems using the SAFT-VR formalim which involves the understanding of the phase behavior of the corresponding binary and ternary mixtures. Different parts of the project are carried in collaboration with several groups: Multiphase Fluids System Group of Dr. Amparo Galindo at Imperial College London, UK; Dr. Clare McCabe at Vanderbilt University, US; and Dr. Eduardo J. M. Filipe at Instituto Superior Técnico, Lisbon.

• Critical behavior of CO2 + n-alkane binary mixtures.
• Determination of the vapor-liquid and liquid-liquid selected mixtures of CO2 + n-alkane and CO2 + perfluoroalkane binary mixtures.

• Extension of the SAFT-VR approach to deal with pure diblock surfactant molecules.