Exploring the Dynamics and Structure of Soft Colloids at Oil-water Interfaces

• Other Authors: Kwok, Man Hin (author.)
• Format: Others
• Language: English; Chinese
• Published: 2016
• Online Access: http://repository.lib.cuhk.edu.hk/en/item/cuhk-1292144

自一百年前，膠體粒子於界面上的自發吸附被發現了後，它們穩定乳液的物理現象已經被研究了好一段長時間。這一類以固體微粒為穏定劑的乳液稱為皮克林乳液（Pickering Emulsion）。因為固體微粒在界面上的吸附能量非常高，使這一類乳液比起小分子乳化劑穏定的乳液有著一些有趣的差異。例如更高的穏定性和可以排除使用小分子乳化劑的一些不良效果。 === 在近幾年，科學家們的興趣都落在運用亞微米大小的聚N-異丙基丙烯酰胺（pNIPAM）水凝膠來穏定皮克林乳液。這些水凝膠可以來界面上變形，而且導致界面出現特別的特質和影響了粒子的排列結構。界面的張力和流變性質都會因而有所改變。而且，pNIPAM水凝膠的響應性使由它們所穏定的乳液可以被容易地破乳。雖然水凝膠穏定皮克林乳液有著不同的好處，但是乳液的穏定的機理還是未被完整地研究。然而有關的機理研究對於將來的應用發展卻是非常重要的。 === 第一章簡要地介紹了膠體粒子和pNIPAM的水凝膠微粒。第二章則詳細地研究了pNIPAM水凝膠微粒的製備。不同大小、交聯度、羧基含量和共聚物分佈的水凝膠微粒都被成功製備。第三章主要研究水凝膠微粒溫敏性和酸鹼響應性兩者相互之間的關係。 === 第四章概括地討論了乳液和皮克林乳液的一些基本知識。在第五章裡面，我們使用共聚焦雷射掃瞄顯微鏡拍下了水凝膠微粒在油水介面上的形態。以前被發表的相關研究主要都是以掃瞄電子顯微鏡去觀察粒子的形態。但是運用電子顯微鏡的時候樣品都需要在真空環境下準備，對於含水量高的水凝膠來說，這未必能夠充份表現水凝膠在水相中的原有形態。所以我們改進了水凝膠在共聚焦顯微鏡中的標定，而且發現在鹼性環境下，有酸鹼響應性水凝膠會在油水界面上出現形態改變。第六章研究了水凝膠在蘭米爾表面膜秤的一些現象。我們發現了水凝膠微粒是可以被槽柵擠壓而離開油水界面。最後，第七和第八章集中研究了水凝膠微粒在不同環境下的不同特性如何影響皮克林乳液的穏定性。而且，在當中了解到水凝膠微粒穏定皮克林乳液的一些相關機理。 === The ability of solid colloidal particles to physically stabilize emulsions, also referred to as Pickering emulsions, has been studied for a long time since their activity at the interface was discovered one hundred years ago. Pickering emulsions display various interesting phenomena because of it high desorption energy at the interface compared with conventional surfactant stabilizers. In addition, Pickering emulsions are considered to be ‘surfactant free’ emulsion and the adverse effects of using surfactants could be eliminated. === In the past few years, the use of submicrometer, poly(N-isopropylacrylamide) (PNIPAM)-based mcirogel particles for stabilizing emulsions has captured the interest of many scientists. Being soft, the microgels, which are spherical in solution, become deformed at the oil-water interface. This deformability leads to the special dynamic properties of interfacial layers and packing structures, which in turn alters the interfacial tension and the rheological properties of the interface. In addition, being responsiveness, PNIPAM microgels enable emulsions to be prepared and broken on demand. Despite all of the practical advantages and unique properties that have already been demonstrated, the mechanisms that govern emulsion stabilization and destabilization using microgels are far from completely understood. The study of soft colloids at the interface thus is of great academic interest and the fundamental understanding of them is the key to achieve the application potential of such novel materials. === This thesis focuses on the dynamics and structure of soft colloidal particles at the oil-water interfaces. First, in order to prepare tailored colloids for the study, the syntheses of multi-responsive PNIPAM microgels with different size, co-monomers, deformability and morphologies were thoroughly investigated. The combination of semi-batch synthesis and temperature-programmed technique resulted in a novel preparation of micron-sized PNIPAM microgels. Various experimental parameters were tested and modified in order to give microgels with optimized quality. The thermo- and pH- responsiveness of these microgels were characterized by laser diffraction and dynamic light scattering (DLS). === Next, a novel labeling technique of the soft PNIPAM microgel particles was developed. This technique was based on the physical adsorption of small fluorescent molecules. Instead of chemically bonded dye molecules, these adsorbed fluorescent dyes could move freely inside the polymer network of the microgel particle. It was also found that the fluorescent dye interacted with different parts of the microgel differently. Therefore, the internal structure and morphology of microgels could be directly visualized by confocal laser scanning microscopy (CLSM) in aqueous environment. The improvement of imaging techniques of microgel particles is essential for studying their behavior at the oil-water interface. It is because conventional scanning electron microscopy (SEM) requires dried sample, which might not reflects the actual states of microgels in aqueous environment. With the improved labeling method under CLSM mentioned above, the conformation of micron-sized PNIPAM microgel particles was captured at the oil-water interface. Particularly, anisotropic deformation of soft pH-responsive microgels was observed at the oil-water interface. Nevertheless, it was found that microgels were not likely to deform significantly unless they were extremely swollen. === We also use Langmuir trough to study the dynamics of microgel at an interface with changing area. Forced desorption-spontaneous adsorption cycles of microgel particles at the oil-water interface were successfully demonstrated. More interestingly, it was discovered that the microgel particles would be desorbed before having a significant deformation in Langmuir trough compression. Finally, the emulsion stability of the microgel stabilized Pickering emulsion was characterized by centrifugation. By comparing the stability of different microgels in different conditions, the correlation between the microgel properties and the corresponding Pickering emulsion stability was found. The emulsion stability and the interfacial behaviors of PNIPAM based microgels can now be better controlled and predicted, which gives great advantages for future applications using soft colloids as stabilizers. === Kwok, Man Hin. === Thesis Ph.D. Chinese University of Hong Kong 2016. === Includes bibliographical references (leaves ). === Abstracts also in Chinese. === Title from PDF title page (viewed on …). === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only.