Designing smart membrane nanopores with selective polymeric coatings

Polymer-functionalised nanopore membranes have emerged as "smart" materials in nanoscience due to their ability to respond to external stimuli, enabling precise control. A potential application of this technology is in wastewater treatment for the recovery of potable water. With water scarcity already affecting many regions globally—and projected to aggravate due to population growth, overuse, and climate change—developing new technologies for sustainable water use is essential. To this end, we investigate the behaviour of polymer-functionalised nanopores at the single-pore scale, where polymer brushes adsorb colloids from the fluid. This study combines theoretical analysis and computer simulations to explore their applicability in sustainable water treatment. First, we examine the conformation of polymer brushes grafted onto a flat plane under the influence of colloidal adsorption in equilibrium by performing Langevin Dynamics. As the brush increasingly adsorbs colloids it shrinks, that is, it decreases its height and then re-swells. We elucidate this behaviour by treating the adsorption of a colloid as a change in the effective local solvent quality, from a good solvent to a poor solvent, of the chains' monomers. Therefore, the more colloids adsorbed, the more compact the chains become, until a geometrical limit is reached, and the brush-colloid mixture needs to grow again. We capture this behaviour in a theoretical model which is based on the Alexander-de Gennes scaling theory and which is in good agreement with the simulation data. Next, we explore a new design perspective in which the monomer-colloid attraction becomes anisotropic through the introduction of attractive patches, offering greater tunability and where we can apply the same theoretical model as before. Matching anisotropic and isotropic potentials via the second virial coefficient, we simulate brushes using Langevin Dynamics. We observe a peculiar phenomenology which depends, in part, on the characteristics of the patches and on their ability to "share" colloids. In contrast to the isotropic case, there is no accumulation of colloids close to the wall inside the brush. In general, we find that this design provides a reduced adsorption capacity and leads to a milder collapse. In the last part, going towards practical applications, we simulate a polymer brush in a cylindrical channel utilising Dissipative Particle Dynamics, in order to consider the influence of an explicit solvent that mediates a flow, as well as the impact of the colloids that get adsorbed. As many different ingredients provide to this picture, we tested the model thoroughly: we reproduced the adsorption behaviour in equilibrium, as well as the typical flow profiles in the absence of colloids. Taking advantage of our preliminary studies, we then design a protocol to create a constant flux of colloids in the central part of the channel. With this in mind, we analyse the complex interplay between adsorption and flow by observing both an alignment of the chains in the flow direction and a collapse due to adsorption. Increasing colloid flux, either by an increase in applied pressure or colloid density, unexpectedly promotes adsorption. Rescaling our findings to practical nanopore dimensions provides insights for experimental validation and potential applications in wastewater treatment.