Date of Award
Doctor of Philosophy
Peter T. Cummings
Hank Cochran, John Prados, Mark Dadmun
Carbon dioxide holds considerable potential as an environmentally and chemically benign alternative to the potentially hazardous conventional industrial solvents. In spite of its promise, one of the reasons that CO2 has so far failed to achieve its full potential is that few "CO2-philic" surfactant molecules are known at present that form stable reversed micelles in carbon dioxide. The aqueous or polymeric cores of these surfactant aggregates provide a medium for solubilizing substances (hydrophiles, polar molecule, proteins) that are otherwise insoluble in CO2. As a step in this direction, molecular dynamics simulation of a dichain (or hy-brid) surfactant + water + carbon dioxide (solvent) ternary system is presented in this work to gain valuable insight into the aggregation behavior of these surfactant molecules. Two different system sizes were investigated using detailed and quite real-istic molecular models for all the three chemical species involved. One of the system sizes investigated mimicked the overall composition studied in a recent experimental (SANS) work, while the other provided valuable insight into the effect of surfactant chemistry and architecture on surfactant aggregation. The simulations for the two system sizes used different solvent conditions (supercritical CO2 and high temperature liquid CO2,/sub>) to provide information into the effect of solvent condition on surfactant aggregation. The surfactant system showed a rapid and spontaneous propensity for aggregation of surfactant and water molecules into aggregates that resemble reversed micellar aggregates i.e. the aggregates consisted of an aqueous core surrounded by a layer of surfactant molecules with their head groups immersed in the core and the tails forming a corona. The aggregation mechanism observed in these simulations was a two-step mechanism involving rapid ion-hydration followed by gradual surfactant aggregation via hydrogen bond formation. The aggregation process was found to be diffusion-controlled i.e. dependent on the size of aggregates and the solvent density. Another factor influencing the aggregation process was the steric resistances offered by the surfactant tails. The structural properties of the aggregates were dependent on the water-to-surfactant molar ratio of the system. The size and shape of the aggregates predicted by these simulations were in reasonably good agreement with prior experimental results.
Salaniwal, Sumeet, "Self-assembly of di-chain surfactant in carbon dioxide. " PhD diss., University of Tennessee, 2000.