https://scholars.lib.ntu.edu.tw/handle/123456789/62485
標題: | 膠體粒子在油/水界面二維系統中相變化之研究 A study of phase transitions in two-dimensional colloidal particle systems at an oil/water interface |
作者: | 林伯俊 Lin, Bo-Jiun |
關鍵字: | 膠體粒子;二維相變;界面;位能模型;colloidal particles;two-dimensional melting;interface;potential function | 公開日期: | 2007 | 摘要: | 自然界中所常見的熔化現象(melting phenomenon),如冰熔化成水,在二維系統中有相當大的不同,在KTHNY 理論的預測中,二維系統的熔化過程中會經過稱之為六角液相(hexatic phase)的中間相,使得熔化過程會先由固相轉變為六角液相,再由六角液相轉變為液相。當膠體粒子處於油/水界面時,粒子與粒子之間具有長距離的排斥力,由於此種長距離的作用力的特性使得膠體粒子在特定的條件下可形成晶格般結構,使其成為觀察二維熔化現象的一個合適的系統,在本研究中我們利用此系統,觀察溫度、濃度與在系統中加入小粒子所引起的相變化現象,並對粒子之間的位能函數進行量測。 利用數位顯微技術,可直接觀察到粒子在油/水界面的排列狀況,並以影像分析求得粒子的在影像上的位置後,計算系統的繞射圖樣(diffraction pattern) 與第一波峰的切面形狀,來決定系統的相行為,同時計算在KTHNY理論中所用來判定系統相行為的兩個有序量,即系統的徑向分佈函數 與鍵方位相關函數 ,由這兩函數強度的衰減型式,來決定系統的相行為。並以Voronoi圖進行缺陷分析來檢視系統中不同配位數的缺陷隨著相變化的發生的分佈狀況。利用Boltzmann關係式,可藉由徑向分佈函數 的量測反推其位能函數,求得膠體粒子在界面之間的作用位能。 由觀察結果硫酸基聚苯乙烯膠體粒子在油/水界面間,我們可以發現膠體粒子在油/水界面所形成的二維系統的相變化次序隨著密度的增加為: 液相 液相-六角液相共存相 六角液相 固相。在所實驗的三個溫度(20℃、30℃與40℃)下具有相同的相變化次序,隨著溫度的上升可以發現各相的邊界線有往更高密度偏移的趨勢。 帶電的膠體粒子在界面之間的靜電作用位能,對距離的衰減型式皆可用三次方衰減的形式來描述,且20℃、30℃與40℃三個不同溫度下所求得的作用位能型式皆隨著距離呈三次方衰減,而不同溫度之間的量測結果也有相當高的一致性,帶電密度越小的膠體粒子,其位能函數越偏離三次方衰減的型式,隨著帶電密度的減少,越接近自然指數型式的衰減。 在雙成份系統中,加入小粒子所造成的相變與原來主要系統的相變次序相同。液相的邊界線隨著系統中小粒子數目數量的增加而往高密度的方向偏移,同時造成其位能函數會向前偏移,而系統中的引力也會逐漸的消失。且系統中的空乏位能開始出現排斥力,與A-O模型中所預測的趨勢有一致的結果。 本研究利用吸附在界面的膠體粒子,藉由其二維系統的特性,觀察熔化現象在二維系統中的行為,並對粒子之間的作用位能進行量測,期望對膠體粒子在油/水界面的相行為有進一步認識。 Melting phenomenon in two dimensions exhibits a different scenario from that in three dimensions. According to KTHNY theory there exists an intermediate phase called “hexatic phase” between solid phase and liquid phase and that makes the melting process into two stages. First stage is from solid phase melting into hexatic phase and followed by hexatic phase melting into liquid phase. Due to the long range repulsive force, the colloidal particles adsorbed at an oil/water interface could form crystal structure that makes it suitable for observation of melting phenomena in two dimensions. We observed the melting phenomena in two dimensions induced by concentration change, temperature and addition of small particles by trapping colloidal particles at an oil/water interface. The potential function between the colloidal particles was also experimentally determined. Digital video microscopy method was used to calculate two dimensional structure factor and transverse line shape of first peak. In addition we also evaluated the radial distribution function and bond orientation correlation function of the system which are used in KTHNY theory to distinguish the phase behavior. Further defect analysis was accomplished by the Voronoi diagram to analyze the percentage of different coordination number of particles. The sequence of phase transitions of charged sulfate polystyrene particle at oil/water interface, a pure solid phase pure hexatic phase liquid-hexatic coexisting pure liquid phase, was observed at three different temperatures. In addition, the boundary line of each phase shifts to a higher density with raising temperature. The pair interaction potential of the two-dimensional colloidal particles was found to be a very long-ranged repulsion. The potential function decays with distance to the power of –3 and exhibits the same feature at three different temperatures (20℃、30℃ and 40℃). When the surface charge of particles is decreased, the potential function is no longer decay with distance to the power of -3. In contrast, the potential function decays exponentially. Sequence of phase transition induced by addition of small particles in binary mixture was found to be exactly the same as the sequence of phase transition observed in the pure single component system. The boundary line of liquid phase was found to shift to a higher density with increasing the number of small particles. The depletion potential of the colloidal particles at oil/water interfaces exhibits a repulsive force when the number of small particles gets concentrated, consistent with A-O model. |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/52255 | 其他識別: | zh-TW |
顯示於: | 化學工程學系 |
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ntu-96-D90524009-1.pdf | 23.53 kB | Adobe PDF | 檢視/開啟 |
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