Polymers at interfaces and colloid stability in compressible fluids: Theory, simulation, and spectroscopy

Many emerging reaction and separation technologies in environmentally benign supercritical carbon dioxide involve polymers at interfaces and stabilization of emulsions and latexes. The structure of polymers grafted or adsorbed at a colloidal surface in a highly compressible solvent is poorly understood. Theory and simulation are used to gain a fundamental understanding of polymer conformation at a surface and the interaction of colloidal particles in a compressible solvent. Adsorption and layer structure are characterized versus solvent density for various polymer chain lengths, solubilities, adsorption energies, and concentrations with lattice-fluid self-consistent field theory (LFSCF). The free energy of interaction of two polymer coated surfaces is calculated and used to identify the critical flocculation density (CFD). The LFSCF adsorption and CFD calculations are used to identify design criteria for effective polymeric stabilizers. Lennard-Jones chains grafted to a solid surface in a supercritical solvent are simulated with a continuum Monte Carlo method. A novel expanded grand canonical ensemble Monte Carlo method is adapted to simulate adsorption of homopolymers from dilute supercritical solution. Chain conformational properties and the force of interaction between two surfaces coated with grafted chains are calculated as a function of solvent density and temperature. Both the LFSCF theory and simulation show that CFD coincides with the critical solution density of the stabilizer in bulk solution, which in turn corresponds to the coil-to-globule transition of a single stabilizer chain in bulk solution. This correspondence between surface and bulk properties is in agreement with emulsion stability experiments in supercritical CO{dollar}sb2.{dollar} Near the solvent critical point, local solvent density enhancement increases solvent quality between the surfaces adding resistance to flocculation. In good and poor solvents the colloidal forces are long ranged compared to grafted chain dimensions. The long range is caused by solvent expulsion from between the approaching surfaces. A novel FTIR spectroscopic technique is developed to measure equilibrium constants for weak Lewis base-CO{dollar}sb2{dollar} electron donor-acceptor interactions. The weak Lewis acidity of CO{dollar}sb2{dollar} may enhance CO{dollar}sb2{dollar} sorption in basic polymers, but does not contribute significantly to the solubility of basic polymers in CO{dollar}sb2.{dollar}...