An Investigation of Critical Micelle Concentration and Aggregative Behavior of Amphiphilic Molecules Using Dissipative Particle Dynamics

雙親分子形成微胞(micelle)的現象一直以來都是膠體科學的重要研究領域。一般而言當溶液濃度高於定義上的臨界微胞濃度(CMC)時，具有規整性結構的微胞才會存在於雙親分子溶液中。微胞在醫學上與工業上皆有重要的應用，但是它的性質深受許多因素的影響，例如:雙親分子的濃度、雙親分子的結構，以及溶液中添加劑，因此全面性地研究雙親分子在溶液中的行為是非常重要的。 本研究利用耗散粒子動力學法(Dissipative Particle Dynamics)研究雙親分子的臨界微胞濃度與聚集行為。我們在研究中有系統地探討了雙親分子之尾基與頭基長度，分子結構(直線型、星狀、樹枝狀、環狀以及不同鏈的硬度)，非極性添加劑，以及在非極性溶劑下對其臨界微胞濃度與聚集方式所造成的影響。在尾基與頭基長度的研究部分，我們發現臨界微胞濃度隨著尾基長度的增加而下降，但隨著頭基長度的增加而上升，兩者相較之下顯示尾基對於臨界微胞濃度的影響是勝過於頭基的。而根據微胞的分析結果得知，具有較大的尾基長度/頭基長度比例與較長的雙親分子會形成具有較大聚集數的微胞。除了尾基長度與頭基長度外，雙親分子結構對其溶液性質也造成重大的影響。我們發現雙親分子不論其疏水端或親水端的分枝或環狀結構皆會造成臨界微胞濃度的上升，然而當其疏水端或是親水端成為具有硬度的鏈段時，卻會造成臨界微胞濃度的下降。由微胞的型態與分析結果發現，雙親分子的尾基結構改變與頭基結構改變會造成不同的聚集結果。另外針對非極性添加劑的部分，我們改變非極性添加劑的濃度、長度以及結構(直線型與環狀)來探討這些變化對於雙親分子水溶液的影響，在此之中我們發現了許多有趣的聚集型態。除了以水作為溶劑之外，我們也研究在非極性溶劑下所形成的反微胞(inverted micelle)之性質。我們發現隨著非極性溶劑長度的增加，臨界微胞濃度會先急遽的下降再轉為平緩。 此研究成果針對雙親分子溶液在各項內部變因與外部變因下所產生的變化進行了全面性的探討，其中大部分的臨界微胞濃度與各項變因之間的關係和諸多實驗結果十分吻合，對於某些具有特殊結構與在特別條件下的雙親分子，雖然在實驗上尚未被深入探討，但是我們的研究結果將可成為未來實驗研究上的參考。The micellization phenomena of the amphiphiles have been an important area of the research in colloid science for a long time. The ordered structure, micelle, which is formed in the amphiphile solution, exists only above a fairly well-defined concentration, CMC. Micelles could offer a powerful multifunctional platform for medical and industrial applications. Various factors, such as amphiphile concentration, amphiphile architecture, and additives, all significantly affect the properties of the micelles. Thus, to thoroughly investigate the solution behavior of the amphiphiles is of great importance. The Dissipative Particle Dynamics was employed to study the CMC and aggregative behavior of the amphiphilic molecules. The effects of the tail/head length, amphiphile architecture (linear type, miktoarm star type, dendritic type, cyclic type and chain rigidity), nonpolar additives, and nonpolar solvent on the CMC and the aggregative patterns of the amphiphiles are systematically investigated. For the tail/head length, it is found that the CMC declines with increasing tail length but rises with increasing head length. Compared to head group, tail group has comparatively significant influence on the CMC. According to the analyses of the micelles, we found that amphiphiles with higher tail/head ratio or longer total length form the micelle in larger quantities. In addition to the tail and head length, amphiphile structures also have a considerable impact on the amphiphile solution properties. The branching and cyclic structures of either the hydrophobe or hydrophile lead to a striking increase in the CMC; nevertheless, as to the cases with rigid chain, amphiphiles with the greater degree of the rigidity on the tail group or head group have lower CMC. Form the analyses and morphologies of the micelles, we observed that the aggregative patterns of the amphiphiles with the changes in the tail structure and head structure are quite different. On the other hand, the changes in the concentration, length, and structure (cyclic and linear type) of the nonpolar additives were performed to study their effects on the behaviors of the amphiphiles in aqueous media, where some interesting supramolecular structures were observed. Besides the aqueous solvent, we investigated the properties of the inverted micelles formed in the nonpolar solvent. As the nonpolar solvent length increases, the CMC decreases sharply and then gradually. This work could provide a thorough investigation on various factors which influence the amphiphile solution. The relationship between the CMC and these factors from our investigation is consistent with most experimental findings. For the amphiphiles with unique architectures or in special condition which have not been fully investigated, what we study could be a valuable reference for the future experimental works