【作者】 谭军军；

【导师】 孙德军；

【作者基本信息】 山东大学 ， 胶体与界面化学， 2011， 博士

【摘要】 乳液是一种液体以液滴形式分散于另一种与之不相混溶的液相中形成的胶体分散体系,表面活性剂稳定的乳液的研究已经有上千年的历史。自从Pickering和Ramsdon首次报道了胶体颗粒对稳定油水界面的作用后,人们开始慢慢了解到颗粒很有可能替代或者部分替代表面活性剂作为乳液稳定剂,这主要是因为颗粒作为乳化剂具有很多固有的优势,例如：(1)可以大大降低表面活性剂的用量,节约成本；(2)对生物体的毒性远小于表面活性剂；(3)环境友好；(4)乳液稳定性非常强,特别是抗聚结能力,一般受环境影响因素较少,所以被广泛应用于食品、医药、原油开采、农药、涂料等很多工业部门。同时随着纳米科技的兴起,颗粒稳定乳液还被应用到组装纳米颗粒、制备空壳结构、胶囊和其他具有特殊功能的结构和材料。因此,纳米颗粒在液液界面上的吸附特性及其关联的乳液稳定作用机理一直是国际学术界研究的热点之一。同时由于在很多工业生产环节中或者科学研究中,颗粒稳定的乳液只能暂时存在或者是需要消除的,比如粘稠原油的管道输运,高分子乳液聚合过程中的后期纯化,药物在生物体内的可控释放等方面都会涉及这个问题,如何设计出具有条件响应的颗粒乳化剂就成了解决这个问题的关键。近年来,人们对颗粒稳定的乳液进行了较为系统全面的研究,找到了乳液稳定性与助乳化剂、颗粒浓度、溶液pH值、盐浓度、颗粒自身基本性质等因素变化的规律,进而对这些因素对乳液稳定性的影响都有了较为深刻的认识。同时也开发出很多具有特定条件响应的乳液体系,比如磁场、温度、pH等。尽管如此,我们注意到仍有许多问题需要进一步深入探讨：(1)原位生成的无机纳米颗粒对乳液稳定性的研究；(2)不同结构和形貌的纳米颗粒对Pickering乳液性质的影响；(3)以往设计响应乳化剂的颗粒的思路主要是通过环境条件来改变颗粒表面润湿性,是否还有其它响应模式；(4)仍需大量新的表征方法来深入探讨Pickering乳液的稳定机理以及各种乳液现象(如Pickering乳液的非球形转变)的发生机理。基于乳液研究领域的上述背景,本文选择了两种金属氢氧化物亲水性颗粒：原位共沉淀法制备的氢氧化镁纳米颗粒(Magnesium hydroxide nanoparticles,简称MPs)和原位共沉淀法制备的氢氧化铝纳米颗粒(Aluminum hydroxidenanoparticles,简称APs)作为研究颗粒。首先研究了氢氧化镁纳米颗粒对石蜡油/水乳液体系的稳定性。在此基础上,探讨了氢氧化铝颗粒对石蜡油/水乳液体系的稳定性,最后研究了氢氧化镁颗粒稳定的乳液随老化温度变化发生的非球形转变。通过宏观观察、接触角、光学显微镜、扫描电镜(SEM)、透射电镜(TEM)和激光共聚焦显微镜(LFCM)等实验手段考察了颗粒在乳液滴表面(即弯曲的油/水界面)的吸附行为以及制备的乳液的性质,包括乳液类型、稳定性等。另外,通过总有机碳含量(TOC)测定、电位分析仪、界面张力等手段对上述表面活性剂与颗粒之间的相互作用进行了较为系统的研究。从而丰富了Pickering乳液的研究内容,提出了在所研究体系中乳液稳定性产生变化的机理。本文的主要内容包括：1.氢氧化镁颗粒对乳液稳定性及其pH响应性的影响以往,制备pH响应型颗粒乳化剂的主要方法是将具有pH敏感的有机分子吸附在无机颗粒表面或者是直接将一些pH敏感的有机分子或者聚合物直接交联成颗粒乳化剂,通过这些处理使颗粒乳化剂具有pH敏感性,其核心原理就在于利用水相中pH变化来调控这些有机分子的解离程度或者溶解性,从而来控制颗粒在水相中的润湿性,最终达到控制乳液稳定性和乳液类型。那么是否存在其它方式使得颗粒具有pH敏感性呢?鉴于此,我们考察了原位形成的氢氧化镁纳米颗粒对石蜡油/水体系乳液稳定性的影响,同时也评价了此体系的pH敏感性能。通过对不同颗粒浓度和不同pH值下乳液的分层稳定性和乳液滴尺寸的考察,证明了原位形成的氢氧化镁纳米颗粒具有非常优秀的乳化性能,而且所制备的乳液对体系pH值具有强烈的依赖性。通过激光共聚焦显微镜和乳液滴的透射电子显微镜观察确认了氢氧化镁纳米颗粒对乳液稳定性的作用及其颗粒层的存在。通过乳液的pH开关实验证明氢氧化镁纳米颗粒具有非常优良的开关性能,而且在多次开关循环中,乳液稳定性几乎没有受到影响。对氢氧化镁颗粒的基本性质进行了考察,包括pH对润湿性、界面张力、颗粒电位的变化等,证明了氢氧化镁纳米颗粒的pH敏感性主要是由于水相中pH值影响到水相中氢氧化镁纳米颗粒浓度,而其它因素对其敏感性的影响很小。基于得到的结果,可以推断乳液pH敏感性机理为：(1)在较低pH值下,由于镁元素以离子形态存在于水相中,此时对油水体系没有界面保护作用；(2)当pH值大于氢氧化镁的沉淀pH值,水相中存在氢氧化镁颗粒,此时可以得到稳定的水包油型乳液；(3)再一次将pH值调到小于氢氧化镁的沉淀pH值,乳液会快速破乳,完全分层,其原因是稳定乳液的氢氧化镁颗粒被完全溶解,恢复到离子形态。2.氢氧化铝颗粒对乳液稳定性及其pH响应性的影响前面研究了氢氧化镁纳米颗粒稳定乳液及其pH开关性的核心就在于通过调节水相pH来控制镁元素是处于离子形态还是固体形态；而氢氧化铝两性化合物不仅在偏酸的环境比较容易溶解,而且在偏碱的环境也容易溶解。如果将其应用到颗粒稳定乳液体系中必然会带来一些有趣的现象,于是我们研究了原位形成氢氧化铝颗粒对石蜡油/水体系稳定性的影响,结果发现无论调节颗粒浓度还是体系的pH,单纯使用氢氧化铝颗粒很难得到稳定的乳液。这可能和氢氧化铝颗粒很强的亲水性及其对油相亲和力很差有关。为此我们考察了在体系中加入少量的表面活性剂来帮助增强颗粒的亲油性。并对三种不同的表面活性剂进行了考察,分别是十二烷基硫酸钠、油酸钠、Brij 30。结果表明当加入的表面活性剂浓度为10-5M时,颗粒在界面上的吸附能力就能得到改善,乳液的宏观分层稳定性和聚结稳定性都有很大的提高；同时发现随pH变化,乳液体系表现出特定乳化pH区域。当表面活性剂浓度为10-4M时,可乳化的pH区域出现宽化现象。而当表面活性剂浓度为10-3M时,结果发现可乳化的区域已经覆盖了整个考察pH区域,乳液敏感性消失。通过测定界面张力、颗粒润湿性变化以及表面活性剂吸附变化等,我们认为氢氧化铝颗粒和表面活性剂复配体系表现出来的pH敏感性主要是与氢氧化铝颗粒的两性及表面活性剂的协同效应有关。当体系中加入过量的表面活性剂时,乳液的这种pH敏感性消失,主要是由于过剩的表面活性剂就可以在整个pH研究范围内稳定乳液,所以pH敏感性无法表现出来。3.温度诱导氢氧化镁稳定的乳液滴的非球形转变我们在对氢氧化镁颗粒稳定乳液的研究过程中,尝试考察温度对乳液会不会带来一些影响,结果很意外发现在氢氧化镁乳液中出现了球形乳液滴向非球形乳液滴转变的现象。在具体的实验考察中发现,常温下制备的氢氧化镁颗粒稳定乳液保持着完美的球形形状,即使放置几个月也不会发生形状改变；然而将刚制备的乳液放置在特定老化温度下一段时间后,原先球形液滴就会转变成非球形乳液滴。我们还考察了几个实验因素对其非球形化程度的影响,包括老化温度、颗粒浓度、油水比、老化时间。结果发现老化温度对非球形化具有很重要影响,是一个重要的驱动力；同时发现以下规律：颗粒浓度过低不利于整体的乳液稳定,过高不利与非球形化液滴形成；油水比也是出现了与颗粒浓度相似的结果,在中间某个小范围内容易得到较好的非球形液滴；延长老化时间有利于非球形液滴形成,但是老化温度必须大于80℃才有意义,低于此温度老化温度对乳液滴的非球形转变效果不明显,同时老化时间达到一定时间后对非球形化影响较小。利用透射电子显微镜和XRD等手段对老化前后的颗粒进行表征,发现颗粒老化前后发生明显的颗粒二次长大。同时利用显微镜和激光共聚焦显微镜对乳液在不同颗粒浓度下界面吸附形态的变化进行了表征,结果表明在低颗粒浓度下,乳液滴的吸附主要以单层或者少量多层颗粒吸附为主,在高颗粒浓度下,颗粒往往以小团聚体吸附在界面上居多。同时利用苯乙烯单体作为油相,原位本体聚合,利用扫描电子显微镜观察了非球形液滴表面颗粒吸附形态以及非球形液滴本身,发现颗粒形貌变化和液滴形态变化和其他表征方法得到的结果完全一致。对以上结果总结发现氢氧化镁颗粒稳定乳液发生非球形转变主要经历三个过程：(1)原位形成氢氧化镁颗粒吸附在液滴表面形成球形乳液滴。(2)在老化温度下,界面吸附氢氧化镁颗粒发生奥氏熟化,尺寸长大,总的颗粒覆盖面积降低,乳液滴表面出现颗粒空白区。(3)当液滴之间颗粒空白区碰撞在一起,乳液滴之间就会发生部分融合,即形成非球形乳液滴。(4)当空白区过大后,液滴内部的油相可能会逃逸出来,颗粒层完全破裂。更多还原

【Abstract】 An emulsion is a system of dispersed droplets of one immiscible liquid in another. Although it has been studied for over a thousand years, only when Pickering and Ramsdon reported the role of particles in the stabilization of oil/water interface one hundred year ago, did people start to realize that particles can totally or partially replace surfactant. As emulsifiers, particles have lots of natural advantages, for example,(1) cut down on the amount of surfactant and save cost;(2)lower toxicity to creature than surfactant; (3)friendly to environment;(4)strong emulsion stability, especially to coalescence and little effect by environment conditions, so they have been widely applied to industries like food, pharmaceutical, oil recovery, agrochemical industries and coatings etc.. what is more, with the rise of nanotechnology, particle stabilized emulsions have also been applied to the assembly of nanoparticles, the preparation of hollow structure, capsule and some other structures and materials with specific functions. Thus, the adsorption property of nanoparticles at the liquid-liquid interface and its related emulsion stabilization mechanism has always been a hot issue internationally. In industrial applications or scientific research, particle stabilized emulsions may be temporarily desirable or undesirable in cases like viscous oil transportation, emulsion and micro-suspension polymerization, drug controlling release etc.. Therefore, to design a stimulus-responsive particulate emulsifier is necessary as well as urgent.In recent years, systematical studies have been conducted on the particle stabilized emulsions. Some laws have been summarized about co-emulsifiers, particle concentration, pH, salinity and the natural features of the particles. And lots of comprehensive studies have been carried out on the role of these factors in stabilizing the emulsions. At the same time, a number of stimulus-responsive emulsion systems have been developed. However, there are still lots of problems which need further study:(1) the assessment of pure inorganic nanoparticles on emulsion stability; (2) the effect of nanoparticles with different structures and shapes on Pickering emulsion; (3) the key point for particles used as stimulus-responsive emulsifiers is the surface wettability, is there any other manners? (4) more new characterization methods should be employed to deepen the study on the mechanism of not only Pickering emulsion stability but also some other phenomenon of emulsions, like Pickering non-spherical transformation.Based on the study on emulsion field above, this dissertation studied two types of hydrophilic metal hydroxide nanoparticles, magnesium hydroxide nanoparticles (MPs) and aluminum hydroxide nanoparticles (APs). Initially, we studied Mps and APs to serve as a particulate emulsifier on paraffin/water system. Then, we studied the non-spherical emulsion droplets transformation at different aging temperatures. The adsorption behaviors of particles at the Pickering emulsion surfaces, and the emulsion properties including emulsion stability and emulsion types are investigated systematically using visual observation, contact angle, optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM) and Laser-induced fluorescent confocal micrograph (LFCM) experiments. In addition, by total organic carbon (TOC) measurement, zeta potential analysis, interfacial intension measurement and Fourier transform infrared (FTIR) spectrometer, the interactions between particles and surfactants are further investigated in detail. The main contents of this dissertation are as follows:1. The assessment of Mps particles on the stability of emulsions and its pH sensitivityIn the past, to produce particulate emulsifiers were directly put the pH sensitive organic molecules or polymers with inorganic nanoparticles together. Through these processes, the particulate emulsifier would be pH sensitive; the principle behind it is that the emulsion stability as well as its type could be controlled by adjusting the particle wettability in the water phase. By adjusting the degree of dissociation or dissolubility of the organic molecules, particle wettability could be well controlled. However, is there any other approach to achieve such pH sensitivity? Guided by this issue, we studied in situ formed magnesium hydroxide nanoparticles as a stabilizer for paraffin/water system and evaluated its pH sensitivity. By studying the effects of particle concentration and pH on emulsion stability to resist creaming and the droplet size, it was demonstrated that in situ formed magnesium hydroxide nanoparticles are of superb emulsification ability and the emulsions produced strongly rely on the pH in the emulsion system. Through the observation of emulsion droplets with the laser scanning confocal microscope and transmission electron microscope, it was confirmed that Mps nanoparticles could affect the stability of emulsions and particle layers do exist in the system. Besides, it was also shown through a series of pH switch experiments that nanoparticles have excellent switchability, and the emulsion stability would hardly be affected by several switchable cycles. Studies have also been performed on the properties of Mps nanoparticles, including the effect of wettability on the interfacial tension, the zeta potential etc. The results proved that the pH sensibility of Mps nanoparticles was resulted from the pH of aqueous phase which affects the particle concentration of Mps nanoparticles. Other factors had minimum effect on its pH sensibility. The mechanism of emulsion pH sensibility could be concluded from the study as follows:(1) When the pH value was lower than the precipitation limit for Mps, the magnesium element existing in the water phase in the form of ion could not protect the interface of the oil-water system. (2) Over the precipitation condition for Mps, stable O/W emulsion could be produced due to the existence of Mps particles in the water phase. (3) Switching the pH value under the precipitation limit of Mps again, the emulsion would soon be demulsified and complete oil/water separation as the Mps particles in the stable emulsion would be completely dissolved to be ions again.2. APs particles as an emulsion stabilizer and its pH sensibilityPreviously we have studied the emulsion stabilized by MPs and its pH switchability, the key point is to control the form of the element of magnesium, being ion or solid, by controlling the pH in the water phase. However, the Mps are amphoteric, to be exact, it could be easily dissolved under both basic and acidic solution. Presumably it would be very interesting to apply Mps to the particle stabilized emulsion system, so we studied in situ formed APs used as an emulsion stabilizer for paraffin/water system and found that Mps particles alone can hardly produce stable emulsion either by controlling the particle concentration or the pH value in the system. The results may relate to the strong hydrophilic property of Mps particles and little interaction with oil phase. In order to find out the reason, we added a certain amount of surfactant to make the particles to be more hydrophobic and we observed three different types of surfactant, sodium dodecyl sulfate, sodium oleate and Brij30 separately. The results showed that the particle adsorption ability could be improved when the surfactant concentration was 10-5M, so does the stability of creaming and coalescence. With the change of pH, specific pH range would occur in the emulsion system. When surfactant concentration was 10-4M, the pH area, which could obtain stable emulsion would cover the whole pH range that were investigated, and the emulsion sensibility would disappear. By testing the interfacial tension, the change of particle wettability and the change of the adsorption of surfactant etc., we believe that the pH sensibility of mixture of APs and surfactant was mainly related to the synergy of the amphotericity of Mps particles and surfactant. The pH sensibility would disappear if excessive surfactant was added into the system. The primary reason for this phenomenon was that the emulsion could be stabilized by the extra surfactant in the whole pH range and therefore the pH sensibility could not be observed.3. The change of the stability of emulsion stabilized by Mps particles under different surrounding temperaturesIn our research on the emulsion stabilized by Mps particles, we considered the influence of temperature on the emulsion and surprisingly found that the Mps emulsion presented change from spherical droplets to non-spherical ones. In our experiment, the Mps particles prepared under room temperature could perfectly keep being spherical, even being placed for several months. However, when the newly prepared emulsion was placed under certain aging temperature for a period, the droplet form would change from being originally spherical to non-spherical. In typical experiments, we studied several factors that may affect the non-spherical degree, including aging temperature, particle concentration, oil/water ratio and aging time. The results showed that aging temperature working as an important driving force, could significantly affect the non-spherical degree. Too low particle concentration is adverse to the stabilization of the whole emulsion system, while too high particle concentration is adverse to the formation of non-spherical droplets. The effect of oil/water ratio on the formation of non-spherical droplets is similar to that of particle concentration; droplets with high non-spherical degree could be produced when the oil/water ratio was controlled in a certain range. Extending the aging time would have a positive effect on the formation of non-spherical droplets only on condition that the aging temperature would be higher than 80℃. Below this aging temperature, the effect would be significantly decreased. Besides, when aging time reached a certain point, the influence of it on the formation of non-spherical droplets would become less significant. Through the observation of MHps with the XRD and transmission electron microscope, it was found that the particles experienced obvious growth after the aging process. Moreover, we used the laser scanning confocal microscope and microscope to observe of the particles on the surface emulsion droplets, the results showed that at low particle concentration, the particles adopted on the surface of emulsion droplets are mainly in the form of monolayer or multiple layer, while at high particle concentration, most of the particles on the interface are in small aggregates. Same results could be got when oil phase was replaced by styrene. The oil phase is polymerized through bulk polymerization at aging temperature, and then the appearance of particle layer and non-spherical droplets could be clearly observed with scanning electron microscopy (SEM). Based on the results above, it could be concluded that three key processes are necessary for Mps particle stabilized emulsion droplets to undergo non-spherical transformation. (1) Spherical emulsion droplets would be formed when in situ formed particles are adsorbed on the surface of the emulsion droplets. (2) The MHps on the surface droplets would experience Ostwald ripening at the aging temperature which results for particle size increase and the total surface area decrease, so does the emulsion coverage. In this case,’vacant area’takes on. (3) Partial coalescence occurs when droplets collide with each other and form non-spherical droplets. (4) If the vacant area is too large, the oil phase would release from the particle shell and particle layer would break up.更多还原