(128c) Size-Independent Separation of Functionalized Nanoparticles on Porous Substrates

Authors: 
Neimark, A., Rutgers, The State University of New Jersey
Vishnyakov, A., Rutgers, The State University of New Jersey
Santo, K., Rutgers, The State University of New Jersey
Emerging technologies of production and processing of functionalized nanoparticles (NP) requite advanced methods of nanoparticle characterization and separation. While various methods are available for NP separation by size, there are no efficient methods for NP separation by surface chemistry. It is highly desirable to develop a technique for size-independent separation of functionalized NPs that would be similar to the method of liquid chromatography at critical conditions (LCCC) used for molecular weight independent separation of polymers by their composition and morphology.
In this work, we explore the conditions of chromatographic separation of NPs on porous substrates grafted with polymer brushes (PB) in a binary mixture of “good” and “poor” solvents. This process involves NP flow through pore channels with walls covered by PBs. NP separation is controlled by the competition between enthalpic attraction due to adsorption of polymer chains to NP surface and entropic repulsion caused by geometrical confinement. Using dissipative particle dynamics simulations in conjunction with the ghost tweezers free energy calculation technique, we calculate the free energy landscapes of NP-PB adsorption interaction and determine the Henry constant that characterizes adsorption equilibrium and NP partition between the stationary phase of PB and the mobile phase of flowing solvent. We find that with the increase of the poor solvent fraction and respective PB contraction, NP separation exhibits a transition from the hydrodynamic (size exclusion) to adsorption regimes of separation with the reversal of the sequence of elution. At good solvent conditions. the entropic repulsion dominates, causing the size exclusion separation with larger particles having shorter retention time. As the solvent quality worsens, the enthalpic attraction becomes dominant and the adsorption regime of separation is observed with the reverse order of elution with smaller particles moving faster along the column. This finding suggests a possibility of the existence of an analogous to LCCC regime in NP chromatography, at which NPs with like surface properties elute together disregarding of their size. The latter has important practical implications: NPs can be separated by surface chemistry rather than by their size employing the gradient mode of elution with controlled variation of the solvent composition.