On the mechanism of surfactant adsorption on solid surfaces: free-energy investigations.
On the mechanism of surfactant adsorption on solid surfaces: free-energy investigations.
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DOI：
10.1021/jp8055009
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发表时间：
2008-10
期刊：
影响因子：
--
通讯作者：
Zhijun Xu;Xiaoning Yang;Zhen Yang
中科院分区：
文献类型：
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作者：
Zhijun Xu;Xiaoning Yang;Zhen Yang
以表面活性剂/溶剂体系为模型，用分子动力学(MD)方法计算了表面活性剂在固体表面的吸附自由能。采用伞形抽样的加权直方图分析方法(WHAM)。为了评价表面活性剂在固/液界面的吸附热力学，得到了表面活性剂在固/液界面上的全平均作用力(PMF)的熵贡献和热焓贡献。虽然我们观察到表面活性剂的吸附过程主要是由有利的热变驱动的，但仍然存在非常不利的熵贡献。通过将自由能分解为溶剂诱导贡献项和表面活性剂壁项，区分了表面和溶剂对吸附自由能的影响。表面效应对PMF的贡献在热力学上是有利的，而溶剂项则表现出明显的不利成分，并随着表面活性剂接近表面而单调增加。比较和讨论了表面(亲溶剂和憎溶剂)和溶剂之间的各种相互作用对表面活性剂吸附PMF的影响。与亲溶剂表面相比，疏溶剂表面为表面活性物质的吸附产生了更稳定的位置。然而，溶剂-疏水体系的PMF分布在体界面处有一个明显的正极大值，这导致了相当大的长程自由能势垒对表面活性物质的吸附。这些结果从局域界面结构的角度进行了分析。综上所述，这一综合研究有望揭示决定表面活性剂在固体表面吸附的微观相互作用机制。
The adsorption free-energy of surfactant on solid surfaces has been calculated by molecular dynamics (MD) simulation for a model surfactant/solvent system. The umbrella-sampling with the weight histogram analysis method (WHAM) was applied. The entropic and enthalpic contributions to the full potential of mean force (PMF) were obtained to evaluate the detailed thermodynamics of surfactant adsorption in solid/liquid interfaces. Although we observed that this surfactant adsorption process is driven mainly by a favorable enthalpy change, a highly unfavorable entropic contribution still existed. By decomposing the free energy (including its entropic and enthalpic components) into the solvent-induced contribution and the surfactant-wall term, the effect of surface and solvent on the adsorption free-energy has been distinguished. The contribution to the PMF from the surface effect is thermodynamically favorable, whereas the solvent term displays an obviously unfavorable component with a monotonic increase as the surfactant approaches to the surface. The impact of various interactions from the surfaces (both solvent-philic and solvent-phobic) and the solvent on the adsorption PMF of surfactant has been compared and discussed. Compared to the solvent-philic surface, the solvent-phobic surface generates more stable site for the surfactant adsorption. However, the full PMF profile for the solvent-phobic system shows a clear positive maximum value at the bulk-interface transition region, which leads to a considerable long-range free-energy barrier to the surfactant adsorption. These results have been analyzed in terms of the local interfacial structures. In summary, this comprehensive study is expected to reveal the microscopic interaction mechanisms determining the surfactant adsorption on solid surfaces.