Chemical Engineering Seminar
Mixing aqueous solutions of polycations and polyanions under favorable salinity and pH conditions leads to the coexistence of a highly viscous liquid-like coacervate phase and a supernatant. The supernant is nearly depleted of polymers, whereas the coacervate usually contains twenty to thirty percent polymer. A major driving-force behind such transition is the electrostatic correlation between dissociated ionic groups, i.e. the neighborhood of one ion is more likely populated with ions of opposite charge. This effect has been conventionally treated using Debye-Hückel theory. We extend such treatments by incorporating three effects known to be important to coacervate formation. The first is the entropic effect of connecting charges to build a chain, the second is the reversible association and dissociation between ionic groups, and the third is the hydrophilicity of the backbone or solvent quality. These three components are combined into one explicit model of solution free energy, after their individual effects have been examined. The prediction of the combined theory is found to compare favorably with nine experimental phase diagrams obtained from solutions of polymers with systematically calibrated molecular weights and backbone hydrophilicity.