低濃度電解質在奈米管內異常的擴散和導電性; Anomalous diffusivity and electric conductivity for low concentration electrolytes in nanopores

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题名:	低濃度電解質在奈米管內異常的擴散和導電性;Anomalous diffusivity and electric conductivity for low concentration electrolytes in nanopores
作者:	高政揚;Zhen-yang Kao
贡献者:	物理研究所
关键词:	傳輸性質;擴散;電導性;奈米管;diffusivity;conductivity;nanopores
日期:	2002-06-13
上传时间:	2009-09-22 10:53:05 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	摘要我們應用平衡與非平衡的分子動態(EMD 、NEMD)模擬研究電解質在奈米管內的傳輸性質。電解質由帶一價之正、負離子及中性水分子組成。在本論文中使用連續強制簡易模型 (Continuum restrictive primitivemodel(CRPM))、連續簡易模型 (Continuum primitive model(CPM))模擬電解質；將正、負離子浸入具極性的水分子環境，極性效應以巨觀介電常數(78.41)模擬成連續背景，並將整個系統放在電中性無限長的圓柱管。使用Gaussian isokinetic equation of motion 維持系統在室溫，同時也使用週期邊界條件 (periodic -boundary conditions)，固定離子的濃度。我們擬探討通道半徑大小、離子濃度對平衡擴散性質與非平衡導電性質的影響。CRPM 及CPM 的模擬結果顯示當離子濃度為0.1M ，擴散係數隨通道半徑縮小而減少；0.05M 時，擴散係數幾乎不變；但在低濃度0.025M ，隨著通道半徑縮小，擴散係數異常增加，此異常現象在加入電場的非平衡導電性模擬也有相同結果。上述現象在本論文中皆可藉由自由能概念定性解釋。論文最後模擬水分子為佔有體積的中性軟球，結果顯示離子之擴散係數約小2 個數量級。 Abstract We apply the equilibrium and the non-equilibrium molecular dynamics sim-ulationsto study the dynamic properties of electrolytes in nanopores. The primitive model and the restrictive primitive model widely used in the sta-tistical mechanics of liquid-state theory were used to model the electrolytes. The electrolytic ions were immersed in water, treated in this work as either a dielectric continuum ignoring the size of solvent molecules or a macroscopic dielectric continuum (polar property) plus neutral soft spheres, and the whole system is put in a con?ned space. To simulate a condition closer to processes of practical interest and yet maintain the imulation computationally manage-able, we consider an in?nitely long and uncharged cylindrical tube. The equi-librium property of self-di®usion coe± cent and the non-equilibrium property of electric conductivity are computed in terms of electrolytic concentration, particle size and cylindrical radius. Results of simulations for the continuum solvent restrictive primitive model and continuum solvent primitive modelshow normal behavior for the di®usion coeefcient D vs pore radius R, i.e., D decreases with decreasing R, at ionic concentration c¸ =0.1 M, display R-independence of D at certain threshold c¸ , and an anomalous increase in D with reducing R at a lower c¸ =0.025 M. The mechanism of the anomaly is interpreted to arise from the energetic and entropic factors. For the discrete solvent primitive model, the simulated D is about two order of magnitude less than the continuum solvent primitive model. This di®erence in D is attributed to the solvation e®ect. Similar disparities between these latter results were obtained by others for the discrete restrictive primitive model.
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