Molecular simulations to study thermodynamics of polyethylene oxide solutions

Polyethylene oxide oligomers are intrinsic to Corexit oil dispersants used in response to the Deepwater Horizon accident at the Macondo well in the Gulf of Mexico (2010). PEO chains are fragments of nonionic surfactants used. Versatile PEO oligomers also exhibit thermodynamically interesting and complicated closed loop phase diagrams.;Oil spill dispersants are produced in bulk and stockpiled near the potential oil spill locations for several years. Effective thermophysical modeling of these dispersants should assist the application of lab-scale results to ocean-scales. Construction of reliable thermophysical models demands molecular-scale structural and phase behavior data. Fully defensible molecular-scale theory of such complex materials will be challenging. This thesis is an initial step toward that challenge.;We study here telechelic, primarily methyl-capped, PEO oligomers but we also anticipated variations in capping groups and chain lengths. End-to-end distance distributions of aqueous single PEO chain solutions provide more information on conformations than conventional radius of gyration distributions. Closed loop phase diagram of these polymers is often attributed on hydrophobic attractions present in these polymers. End-to-end distance distributions, obtained here, confirm those interactions in an experimentally accessible context. Indeed those interactions can be targetted by neutron scattering experiments to gather experimental data on these polymer solutions. Simulation results below on these PEO solutions thus identify direct experimental observation of hydrophobic interactions.;The only available molecular scale theory of hydrophobic interactions---the so-called Pratt-Chandler theory---has never been tested with direct numerical simulation results for the hard-sphere solute cases on which the theory was founded. Second virial coefficient calculations of Ar sized hard spheres are obtained here for the first time, and directly compared with the results of the PC theory. The required calculations and extensive cavity search was never before attempted for this purpose, and this work provides world's best data on Ar sized cavity pair distribution functions in water at several temperatures.;The Flory Huggins model is a universally used molecular-model of the phase behavior of polymer solutions. It has been experimentally established that the Flory Huggins interaction parameter (chi) is operationally concentration dependent for PEO(aq) systems, in direct contradiction to the statistical thermodynamic justification of that theory. In the research below, we develop the more general theory for the concentration dependence of the Flory-Huggins chi parameter. Our theory also provides the osmotic pressure of these solutions, a point that has not been noted before, and this additional information should be valuable in a much broader context. We then carry-out molecular simulations of PEO(aq) solutions to evaluate the more general theory. Our analysis and results identify an unexpected phase separation for CH3(CH 2-O-CH2)mCH3 ( m=11) with coexisting oligomer volume fractions (0.21,0.91) at T = 300 K and p = 1 atm. Direct observation of the coexistence of these two phases then supports the correctness of the analysis for the systems studied.