Structure and Dynamics of Water Surrounding the Poly(methacrylic acid) (PMAA) in the Presence of Multivalent Salts: A Molecular Dynamics Study
Date Issued
2007
Date
2007
Author(s)
Chung, Yong-Ting
DOI
zh-TW
Abstract
By doing computer simulations and theoretical calculations on specific polyelectrolyte chain systems, we obtain reference values for structure related properties and interactions of the system. By this, we can further understand the structure of polyelectrolyte chain systems and the relationship between different interactions in the system.
In a dilute aqueous solution, we add a salt (CaCl2) to the PMAA (poly (methacrylic acid)) polyelectrolyte chain system. When metal ions are added to the system, the conformation of the polymer will tend to collapse with the increase of the ion valence. This is due to the increase in ratio of ions condensed on the polymer chain as ion valence increases, and therefore the ion-bridging effect ratio increases as well. However, when the valence of added salt ions is larger than the charge of the polyelectrolyte chain, the polymer chain will gradually stretch out again, as there will be excess salt ions on the polyelectrolyte chain, which creates electronic repulsion.
Before now, there have been few studies on real water molecule systems. Although the screening effect of added salt ions is a known phenomenon, it has seldom been simulated with MD. We investigate by using all-atom simulations. Real water molecules are added to the polyelectrolyte chain solution system and treated explicitly. Although this greatly increases the time needed for computer calculations, it can give more precise understanding of the position of water molecules and its effect on the system.
By radial distribution function analysis and schematic observation, we find that the equilibrium distance between water molecules and COO- is smaller than the usual hydrogen bonds, proving that there exists a strong interaction between the two. By this interactive force, water molecules exhibit special adsorption arrangements, thus having strong stabilization on the polyelectrolyte chain conformation. Analysis of dynamics of water molecules under different adsorption conditions indicates that when water molecules are adsorbed on COO- or ions, the overall diffusion of the water molecules becomes slow, and changes occur in the vibrational spectroscopy of oxygen and hydrogen atoms.
Subjects
分子動力學
聚甲基丙烯酸
聚電解質
架橋效應
屏蔽效應
徑向分佈函數
振動頻譜
MD
PMAA
polyelectrolyte
bridging effect
screening effect
radial distribution function
vibrational spectrum
Type
thesis
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