Ishraaq Raashiq, Akash Tanmay Sarkar, Bera Arka, Das Siddhartha
Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States.
J Phys Chem B. 2024 Jan 11;128(1):381-392. doi: 10.1021/acs.jpcb.3c07520. Epub 2023 Dec 26.
We employ an all-atom molecular dynamics (MD) simulation framework to unravel water microstructure and ion properties for cationic [poly(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) brushes with chloride ions as counterions. First, we identify locally separate water domains (or first hydration shells) each around {N(CH)} and the C═O functional groups of the PMETAC chain and one around the Cl ion. These first hydration shells around the respective moieties overlap, and the extent of the overlap depends on the nature of the species triggering it. Second, despite the overlap, the water molecules in these domains demonstrate disparate properties dictated by the properties of the atoms and groups around which they are located. For example, the presence of the methyl groups makes the {N(CH)} group trigger apolar hydration as evidenced by the corresponding orientation of the dipole of the water molecules around the {N(CH)} moiety. These water molecules around the {N(CH)} group also have enhanced tetrahedrality compared to the water molecules constituting the hydration layer around the C═O group and the Cl counterion. Our simulations also identify that there is an intervening water layer between the Cl ion and {N(CH)} group: this layer prevents the Cl ion from coming very close to the {N(CH)} group. As a consequence, there is a significantly large mobility of the Cl ions inside the PMETAC brush layer. Furthermore, the C═O group of the polyelectrolyte (PE) chain, due to the partial negative charge on the oxygen atom and the specific structure of the PMETAC brush system, demonstrates strongly hydrophilic behavior and enforces a specific dipole response of water molecules analogous to that experienced by water around anionic species of high charge density. In summary, our findings confirm that PMETAC brushes undergo hydrophilic hydration at one site and apolar hydration at another site and ensure large mobility of the supported Cl counterions.
我们采用全原子分子动力学（MD）模拟框架，以氯离子作为抗衡离子，来揭示阳离子型[聚（2-（甲基丙烯酰氧基）乙基）三甲基氯化铵]（PMETAC）刷的水微观结构和离子性质。首先，我们确定了在PMETAC链的{N(CH)}和C═O官能团周围各自局部独立的水域（或第一水合层）以及在Cl离子周围的一个水域。这些围绕各自部分的第一水合层相互重叠，重叠程度取决于引发重叠的物种的性质。其次，尽管存在重叠，但这些区域中的水分子表现出由其所处原子和基团的性质所决定的不同性质。例如，甲基的存在使得{N(CH)}基团引发非极性水合，这通过围绕{N(CH)}部分的水分子偶极的相应取向得以证明。与构成C═O基团和Cl抗衡离子周围水合层的水分子相比，围绕{N(CH)}基团的这些水分子也具有增强的四面体结构。我们的模拟还确定，在Cl离子和{N(CH)}基团之间存在一个中间水层：该层可防止Cl离子非常靠近{N(CH)}基团。因此，Cl离子在PMETAC刷层内具有显著较大的迁移率。此外，由于氧原子上的部分负电荷和PMETAC刷系统的特定结构，聚电解质（PE）链的C═O基团表现出强烈的亲水性行为，并使水分子产生类似于高电荷密度阴离子物种周围水所经历的特定偶极响应。总之，我们的研究结果证实，PMETAC刷在一个位点经历亲水性水合，在另一个位点经历非极性水合，并确保所负载的Cl抗衡离子具有较大的迁移率。
