  Aqueous two-phase systems (ATPSs), that is, phase-separating solutions of
water soluble but mutually immiscible molecular species, offer fascinating
prospects for selective partitioning, purification, and extraction. Here, we
formulate a general Brownian dynamics based coarse-grained simulation model for
a polymeric ATPS comprising two water soluble but mutually immiscible polymer
species. A third solute species, representing, e.g., nanoparticles (NPs),
additional macromolecular species, or impurities can readily be incorporated
into the model. We demonstrate that the model captures satisfactorily the phase
separation, partitioning, and interfacial properties of a model ATPS composed
of a polymer mixture of dextran and polyethylene glycol (PEG) in which magnetic
NPs selectively partition into one of the two polymeric phases. The NP
partitioning is characterized both via the computational model and
experimentally under different conditions. The simulation model captures the
trends observed in the experiments and quantitatively links the partitioning
behavior to the component species interactions. Finally, the response of the
simulation model to external magnetic field, with the magnetic NPs as the
additional partitioned component, shows that the ATPS interface fluctuations
can be controlled by the magnetic field at length scales much smaller than
those probed experimentally to date.
