ATOMISTIC EXPLORATION OF DENSELY-GRAFTED POLYELECTROLYTE BRUSHES: EFFECT OF APPLIED ELECTRIC FIELD AND MULTIVALENT SCREENING COUNTERIONS

Polyelectrolyte (PE) or charged polymers are ubiquitous under biological and synthetic conditions, ranging from DNA to advanced technologies. PE chains can be grafted on a surface and they extend into solution to form a "brush"-like configuration if the grafting density is high. PE brushes respond to external stimuli by changing their conformation and chemical details, which make them very attractive for numerous applications. Multivalent counterions (neutralizing PE charges) and external electric fields are known to significantly affect the brush behavior. Obtaining fundamental insights into PE brush’s response to ions and electric filed is of utmost importance for both industrial and academic research. In this dissertation, we use atomistic tools to improve our understanding of the PE brushes grafted on a single surface and two inner walls of a nanochannel under these two stimuli.We start by developing an all-atom molecular dynamics simulation framework to test the behavior of the PE brushes (grafted on a single surface) in the presence of externally applied electric fields. It is discovered that the charge density of PE monomers can have significant influence on their response; a smaller monomer charge density helps the brush to tilts along the electric field, while the PE brush with higher monomer charge density bends and shrinks. We found that counterion condensation to PE chains has a substantial impact in controlling these responses. In the subsequent study we discuss the effect of counterion size and valence in dictating counterion mediated bridging interaction of two or more negative monomers. By examining the solvation behavior, we identify that bridging interactions are not a sole function of the counterion valence. Rather, it depends on the counterion condensation on the PE chain, as well as the size of the counterion solvation shell. We also test the dynamic properties of the counterions and associated bridges. Later, we proceeded to simulate PE brush-grafted nanochannels to explore equilibrium and flow behavior in presence of nanoconfinement. We identify the onset of overscreening: there are a greater number of coions than counterions in the bulk liquid outside the brush layer. This specific ion distribution ensures that the overall electroosmotic flow is along the direction of the coions. Furthermore, for a large electric field, some of the counterions leave the PE brush layer into the bulk, resulting in disappearance of overscreening. If the number of counterions is greater than coions, electroosmotic flow reverses its direction and follows the motion of counterions. Finally, we discover that counterion-monomer interactions control the ion distribution. As a result, a diverse range of electroosmotic flow is found for counterions with different valence and size

Hydrogen Bonding Inside Anionic Polymeric Brush Layer: Machine Learning-Driven Exploration of the Relative Roles of the Polymer Steric Effect, Charging, and Type of Screening Counterions

This paper employs a combination of all-atom molecular dynamics (MD) simulations and unsupervised machine learning (ML) for studying the water–water hydrogen bonds (HBs) inside the anionic poly acrylic acid (PAA) brushes modeled using all-atom MD simulations. PAA brush layer with different charge fraction (f), namely, f = 0, 0.25, and 1, is considered. Water–water interactions, both inside and outside the brush layer, are represented through distinct clusters of tupules of variables representing distances associated with the interacting water molecules. While clusters representing the HBs are present for water inside and outside the brushes, several clusters representing the long-range water–water interactions are missing for the water molecules inside the highly charged (f = 1) PAA brushes. More importantly, inside highly charged brushes, the edge of the clusters representing the water–water HBs is progressively shortened as compared to that in the bulk. Both of these results stem from the presence of the PAA brushes imparting the steric effect and the charge effect, or the effect associated with enhanced interactions of water molecules with PE charges and counterions, thereby disrupting the water connectivity. This water-charged-species interaction also increases the water–water HB angle, i.e., makes the water–water HBs less stable inside the highly charged PAA brush layer. The narrowing of the clusters representing the HBs and the alteration of the angle characterizing the HBs confirm that the conditions defining the water–water HBs change inside the PAA brush layer as a function of the charges on the PAA brush layer. Furthermore, we show that the use of the generic definition of HBs, as compared to using our simulation-motivated modified definition of water–water HBs, overpredicts the number of water–water HBs inside the PAA brush layer. Finally, we employ this all-atom-MD-ML framework to quantify the effect of other types of screening counterions (Li+, Ca2+, and Y3+ ions) in determining the water–water interactions and water–water HB properties inside the PAA brush layer. The findings of the present study, confirming the weakening of water–water HBs inside the PAA brush layer, point to the possibility that the water molecules will be more available for hydrating the brush layer and counterions, thereby leading to a more pronounced wetting of the PAA brush layer