Combined Machine Learning and Molecular Dynamics Reveal Two States of Hydration of a Single Functional Group of Cationic Polymeric Brushes

Journal Article · Wed May 22 00:00:00 EDT 2024 · Macromolecules

DOI:https://doi.org/10.1021/acs.macromol.3c02539· OSTI ID:2568490

Ishraaq, Raashiq ^[1]; Akash, Tanmay Sarkar ^[1]; Das, Siddhartha ^[1]

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1. University of Maryland, College Park, MD (United States)

The state of hydration of a macromolecular system regulates a plethora of different properties of such a system. In this article, we develop a novel machine learning (ML) approach, based on the unsupervised clustering algorithm, for probing the hydration behavior of the {N(CH₃)₃}⁺ functional group of the PMETAC [Poly(2-(methacryloyloxy)ethyl trimethylammonium chloride] polyelectrolyte (PE) brush system. The PE brushes and the brush-supported water molecules and counterions (chloride ions) are first described using all-atom molecular dynamics (MD) simulations. The simulation data is subsequently used in our ML framework to identify that (1) the {N(CH₃)₃}⁺ functional groups of the PMETAC brushes have two distinct hydration states with one state (state 1) being characterized by less structured water molecules and the other state (state 2) being characterized by more structured water molecules and (2) an enhancement in the brush grafting density leads to the progressive dissapparenace of state 2. An increase in the grafting density increases the number of chloride counterions in a given volume around the {N(CH₃)₃}⁺ functional group and increases the number of shared water molecules between the {N(CH₃)₃}⁺ and Cl^-. The chloride counterions are associated with a hydration layer with much less structured water molecules. Therefore, with an increase in the grafting density, an increase in the percentage of shared water molecules leads to the prevalence of the hydration state [of the {N(CH₃)₃}⁺ moiety] with less structured water molecules. Finally, we explain how the present findings are commensurate with two key previous related results, namely a significantly large chloride ion mobility inside the PMETAC brush layer and the {N(CH₃)₃}⁺-Cl^- average distance remaining independent of the PMETAC brush grafting density. Furthermore, we anticipate that the combined ML-MD-simulation approach proposed in this study can be adapted to probe other soft matter systems to reveal new insights of the underlying mechanisms of emergent phenomenon.

• Research Organization: University of Maryland, College Park, MD (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: SC0017741
• OSTI ID: 2568490
• Journal Information: Macromolecules, Journal Name: Macromolecules Journal Issue: 11 Vol. 57; ISSN 1520-5835; ISSN 0024-9297
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Algorithms
Cluster chemistry
Counterions
Hydration
Molecules