Molecular dynamics study of solvation phenomena to guide surfactant design
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Supercritical carbon-dioxide has long been considered an inexpensive, safe and environmentally benign alternative to organic solvents for use in industrial processing. However, at readily accessible conditions of temperature and pressure, it is by itself too poor a solvent for a large number of industrially important solutes and its use as solvent necessitates concomitant use of surfactants. Especially desirable are surfactants that stabilize dispersions of water droplets in carbon-dioxide. So far only molecules containing substantially fluorinated moieties e.g. fluoroalkanes and perfluorinated polyethers, as the CO₂-philes have proved effective in stabilizing dispersions in supercritical carbon-dioxide. These fluorocarbons are expensive, non-biodegradable and can degrade to form toxic and persistent environmental pollutants. Hence there is great interest in developing non-fluorous alternatives. Given the development of powerful computers, excellent molecular models and standardized molecular simulation packages we are in a position to augment the experiment-driven search for effective surfactants using the nanoscopic insights gleaned from analysis of the results of molecular simulations. We have developed protocols by which to use standard and freely available molecular simulation infrastructure to evaluate the effectiveness of surfactants that stabilize solid metal nanoparticles in supercritical fluids. From the results, which we validated against experimental observations, we were able to determine that the alkane-based surfactants, that are so effective in organic fluids, are ineffective or only partially effective in CO₂ because the weak C-H dipoles cannot make up for the energetic penalty incurred at the surfactant-fluid interface by CO₂ molecules due to loss of quadrupolar interactions with other CO₂ molecules. Though the effectiveness of purely alkane-based surfactants in carbon-dioxide can be improved by branching, they cannot approach the effectiveness of the fluoroalkanes. This is because the stronger C-F dipole can supply the required quadrupolar interactions and a unique geometry renders repulsive the fluorocarbons' electrostatic interactions with each other. We have also determined the source of the fluoroalkanes' hydrophobicity to be their size which offsets the effect of favourable electrostatic interactions with water. Hence we can provide guidelines for CO₂-philic yet hydrophobic surfactants.