帶電Laponite懸浮液：凝膠與玻璃; Charged Laponite suspensions : gel and glass

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题名:	帶電Laponite懸浮液：凝膠與玻璃;Charged Laponite suspensions : gel and glass
作者:	蔡明勳;Ming-hsun Tsai
贡献者:	化學工程與材料工程研究所
关键词:	搖變性;流變學;玻璃;凝膠;帶電膠體懸浮液;thixotropic behavior;rheology;glass;gel;charged colloidal suspension
日期:	2011-06-23
上传时间:	2012-01-05 11:16:42 (UTC+8)
摘要:	膠體系統取決於膠體粒子不同的體積分率及作用力，可能會相轉變形成無序結構的兩種不同相態。這兩種相似而不易直接分辨的相態，分別是玻璃態跟凝膠態。以帶電的Laponite懸浮液，這個濃度超過3.5wt%就會相轉變成黏彈固體的系統為例，Laponite形成的固體態到底該歸類為玻璃或凝膠，是長久以來一直爭論不休的議題。玻璃態是指當粒子的體積分率增加超過某程度時，粒子間長距的靜電排斥力導致每個粒子被周圍粒子限制在一個假想牢籠中而形成。另一方面，凝膠態則是由粒子間互相吸引連結而形成的立體網狀結構。在本研究中，我們使用稀釋實驗來分辨系統內的作用力情形，以及搖變性測試來判定結構上的差異。最後，我們建構出Laponite懸浮液在不同Laponite濃度及不同離子強度下的相圖。在超過3.5 wt%之某固定的Lapontie濃度下，可以觀察到懸浮液先形成一個排斥性的玻璃態，接著當離子強度增加到某範圍內時，即轉變成凝膠態。系統從排斥力主導轉變成吸引力主導，導致玻璃到凝膠的相轉變發生。而除了改變離子強度外，混入低介電係數的溶劑到系統中，也可降低Laponite圓盤上的表面電荷，並使粒子間作用力改變，因此，同樣也能導致從玻璃態到凝膠態的相轉變發生。本研究對於Laponite膠體系統在相轉變前後的結構上及機制上，都有更深入的驗證及了解。 Colloidal dispersions may translate into two types of disordered solid states depending on the volume fraction of the particles and the interaction among them. These two types of states, which are very similar and hardly indentified, are glass and gel. Take the charged Laponite suspensions in water as an example, which will undergo a phase change and form a visco-elastic solid as the Laponite concentration above 3.5 wt%. It has caused a long-time controversy of which state, glass or gel, this solid should be classified as. Since repulsive glass is formed due to the long-range electrostatic repulsion between the particles, so each particle gets trapped by its nearest-neighbours and be arrested in a so-called cage as the volume fraction increased above certain concentration. On the other hand, gel stems from the network developed by the particles attracting and linking together. In this work, interactions among the systems were distinguished by the dilution experiments and the structure differences were defined by the thixotropic behavior test. As a result, a phase diagram of Laponite suspensions based on the Laponite concentration and ionic concentration was constructed. For a fixed Laponite concentration above 3.5 wt%, Laponite dispersions form the repulsive glass state first and change into the gel state for a finite range of ionic strength enhancement. This phase transition from glass state to gel state was induced by changing the inter-particle interactions from predominantly repulsive to attractive. Except by tuning the ionic concentration, mixing low dielectric constant solvent to the system can reduce the charges of the Laponite particle surfaces, therefore the inter-particle interaction can be modified and a phase change from glass to gel can be induced as well. In this study, more precise evidences and understandings in structural changes and mechanism are obtained for phase transforming in the charged Laponite colloidal system.
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