Specific Counterion Effects on the Atomistic Structure and Capillary-Waves Fluctuation of the Water/Vapor Interface Covered by Sodium Dodecyl Sulfate

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• 作者：Meng Chen ; Xiancai Lu ; Xiandong Liu ; Qingfeng Hou ; Youyi Zhu ; Huiqun Zhou
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：August 21, 2014
• 年：2014
• 卷：118
• 期：33
• 页码：19205-19213
• 全文大小：550K
• ISSN：1932-7455

文摘

The structure of the water/vapor interface covered by a sodium dodecyl sulfate (SDS) monolayer can be remarkably affected by salts in aqueous phase. Molecular dynamics simulations have been performed to reveal the microstructure of the interface with different salts, including NaCl, CaCl₂, and MgCl₂. The bending modulus 魏 of the interface exhibits the order: with MgCl₂ < with CaCl₂ < with NaCl 鈮?without salt, while the surface tension 纬 almost remains unchanged. The smaller 魏 characterizes larger interfacial fluctuation. In the systems with CaCl₂ or MgCl₂, the intrinsic density of Na⁺ adsorbed beside the monolayer is much lower than that without salt or with NaCl due to the adsorption of Ca²⁺ or Mg²⁺. However, less Ca²⁺ or Mg²⁺ ions enter the hydration shells of sulfate groups while Na⁺ ions normally coordinate the sulfate groups together with water. So in the systems with CaCl₂ or MgCl₂, sulfate groups are less bridged by ions and more solvated by water. Thus, the alkyl tails become more disordered. The above microstructure change with the addition of divalent ions leads to more fluctuated interface. This finding helps in understanding the mechanism of the influence of salts on the stability of foam films.