Sun Wenyuan, Zeng Hongbo, Tang Tian
Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada.
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada.
J Phys Chem A. 2021 Feb 4;125(4):1025-1035. doi: 10.1021/acs.jpca.0c08797. Epub 2021 Jan 25.
Adsorption of polymers from an aqueous solution onto clay minerals is of great interest to many applications such as water purification and soil conditioning. Molecular dynamics simulations were performed to study the adsorption of anionic polyacrylamide (APAM) on anionic montmorillonite, in an aqueous solution containing monovalent or divalent salts. Compared with monovalent salts (NaCl), the enhancement of APAM adsorption brought by divalent salts (CaCl) was significant, which could not be explained by the Poisson-Boltzmann theory alone. Each solvated Ca was coordinated by 4-6 water oxygens in its first coordination shell. One to two of these water molecules were displaced when APAM formed a complex with Ca. Ca ions in the adsorbed Ca-APAM complexes did not serve as bridges sandwiched between APAM and Mt; instead, the complexes carried a residual positive charge and were subsequently attracted to montmorillonite. The number of adsorbed Ca-APAM complexes changed with salinity in a nonmonotonic manner, due to the modulation of apparent charges of montmorillonite and APAM by Ca. Increasing adsorption of Ca-APAM complexes also promoted APAM adsorption through direct hydrogen bonding with montmorillonite. The findings provided new molecular insights into the long-standing debates on the role of divalent ions in promoting polymer adsorption on like-charged solid surfaces.
从水溶液中吸附聚合物到粘土矿物上，对许多应用（如水净化和土壤改良）具有重要意义。进行了分子动力学模拟，以研究在含有单价或二价盐的水溶液中，阴离子聚丙烯酰胺（APAM）在阴离子蒙脱石上的吸附。与单价盐（NaCl）相比，二价盐（CaCl₂）对APAM吸附的增强作用显著，这仅靠泊松 - 玻尔兹曼理论无法解释。每个溶剂化的Ca在其第一配位层中与4 - 6个水分子的氧配位。当APAM与Ca形成络合物时，其中1 - 2个水分子被取代。吸附的Ca - APAM络合物中的Ca离子并非夹在APAM和蒙脱石之间起桥梁作用；相反，这些络合物带有残余正电荷，随后被蒙脱石吸引。由于Ca对蒙脱石和APAM表观电荷的调节作用，吸附的Ca - APAM络合物数量随盐度呈非单调变化。Ca - APAM络合物吸附量的增加还通过与蒙脱石直接形成氢键促进了APAM的吸附。这些发现为关于二价离子在促进聚合物在带相同电荷固体表面吸附中作用的长期争论提供了新的分子见解。
