Nanopores as Switchable Gates for Nanoparticles: A Molecular Dynamics Study

Abstract

Using molecular dynamics simulations and scaling theory, we present a systematic study of the function of cylindrical nanopores which are decorated with polymer brushes. Our focus is on the regimes in which these systems are able to function as switchable gates for bulky nanoparticles. The process of switching is triggered through the addition of a smaller component to the solvent, which acts as a co-nonsolvent, causing a discontinuous collapse of the brush and opening a central passway through the nanopore. We quantify the conditions under which agents of different diameters are allowed to pass through the nanopore, through direct simulation of their fluxes as well as evaluations of the free energy profiles during a forceful inclusion of these particles into the pore. Our results quantitatively confirm how these nanopores serve as semipermeable gates which grant passage of small compounds, while blocking larger ones under certain conditions.

Polymer Brush Friction in Cylindrical Geometries

“…The reason for this is that opposing polymers on the interior of the cylinder start to interact with each other for these small radii. This also is consistent with earlier simulation results [40,41].…”

“…The wall-to-wall distance D (15.44 and 15.29 σ) and grafting densities α g (0.085 and 0.17 σ −2 ) have been chosen to be close to values employed in Ref [27], such that our results can be compared to their results for the parallel plate geometry. Our surface coverage is small enough to prevent crowding effects [41]. Yet, it is high enough to ensure we are in the brush regime (2.65x and 5.3x the critical grafting density for brush formation) [45].…”

“…This might seem counter-intuitive, considering that this relation for only either the inner or outer cylinder is non-linear. Yet, combined the average density is constant [41].…”

“…In contrast, brushes on the exterior of cylinders display a shallower decay in the density, which can become concave for small cylinder radii R [37,38]. Brushes grafted on the interior of cylinders can be stretched or compressed depending on the chain length, grafting density and R [39][40][41]. Due to these deviating density profiles, one can expect that the interaction of a brush on the interior of a cylinder that is in contact with a brush on the exterior of a cylinder (see Figure 1b) is different from that of a parallel plate geometry too.…”

Theoretical Modeling of Chemical Equilibrium in Weak Polyelectrolyte Layers on Curved Nanosystems

“…Scaling arguments predicted qualitative phase diagrams for neutral homopolymers grafted to the inner surface of nanochannel, as a function of the channel width and the grafting density of the polymers [ 169 ]. Such analyses have been applied to both the good and poor solvent conditions, which leads to the prediction that under certain coating conditions the nanopore can be switched between open and closed states by changing the solvent quality [ 170 ]. Guided by these predictions, MD simulations have been carried out to study the effect of co-nonsolvent on homopolymer-coated nanopores [ 170 ].…”