【作者】 郑凤仙；

【导师】 汪文川； 张现仁；

【作者基本信息】 北京化工大学 ， 化学工程， 2009， 博士

【摘要】 表面活性剂是一类具有表面活性的化合物,溶于液体(特别是水)后,能显著降低溶液的表面张力或界面张力,并能改进溶液的增溶、乳化、分散、渗透、润湿、发泡和洗净等能力,因而广泛应用于纺织、食品、医药、农药、化妆品、建筑、采矿等工业领域。表面活性剂的独特性能与它的特殊结构是分不开的。一个表面活性剂分子通常包含性质不同的两部分,一部分亲水一部分亲油。因此在溶液中或界面上容易自组装成介观尺度下的有序结构(如层状、膜和液晶态等),从而体现出相应的独特宏观行为。正是这种介观尺度下的聚集态直接影响了表面活性剂的性能。表面活性剂的性质和功能都取决于自组装过程。目前,表面活性剂自组装是生物制品和各种功能材料制备的重要手段。因此,表面活性剂自组装特性和聚集体结构的研究已成为当前的研究热点。本论文采用晶格蒙特卡罗(Lattice Monte Carlo,LMC)模拟方法研究了表面活性剂在固液界面上及限制空间中的聚集性质,并对其聚集过程进行了探讨。从介观尺度上揭示表面活性剂在界面上及限制空间中的聚集行为,为理论研究和实际应用提供重要依据。论文的主要工作如下:1.采用LMC方法模拟研究了表面活性剂在固液界面(疏水表面)上的吸附形貌及形貌转变,重点考察吸附作用能、表面活性剂头基与水之间的相互作用(表面活性剂头的溶解度)、表面活性剂尾基与水及表面活性剂结构的影响,并给出了不同表面活性剂随吸附作用能和表面活性剂头基的溶解度变化的相图。模拟结果除了发现实验报道的半球状胶束和单层这两种聚集形貌外,还发现了另外四种聚集形貌,分别为非稳定胶束,半球一半柱形混合胶束,蠕虫状半柱形胶束和带孔单层。表面吸附作用能和表面活性剂头基与水之间的相互作用不仅影响表面活性剂在表面上的聚集形貌还影响表面吸附量的大小。表面吸附作用能越大,表面活性剂越倾向于形成曲率较低的表面聚集形貌,并且表面吸附量越大。与此相反,表面活性剂头基与水之间的吸引作用越强,表面活性剂越倾向于形成曲率较高的表面聚集形貌,并且表面吸附量越小。2.采用LMC方法研究表面活性剂溶液在限制空间内的吸附和聚集行为,包括由两个相同的平行疏水固体表面组成的狭缝孔和由固体粒子组成随机孔。首先考察了孔宽和表面活性剂浓度对狭缝孔中表面活性剂溶液的聚集形貌的影响,并给出不同表面活性剂聚集形貌随孔宽和浓度变化的相图。模拟结果表明,从表面单层结构到双层结构的转变过程中存在一中间态——桥形结构,并且它的存在依赖于转变路径及表面活性剂分子结构。该结构的存在可能是相距较远的两个表面间长程疏水吸引作用的一种来源。然后研究了随机孔中表面活性剂溶液的聚集性质,尤其是固体粒子(拥挤分子)对表面活性剂溶液的临界胶束浓度(CMC)的影响。主要考察固体粒子的排列方式,包括规则排列和随机排列两种方式以及固体粒子的体积分数对受限表面活性剂CMC的影响。模拟结果表明,固体粒子的存在使受限表面活性剂的CMC偏离其体相值。总体而言,因拥挤而产生的两种因素决定其偏离程度的大小,一种是由表面活性剂与固体粒子间的体积排斥效应引起的排空效应(depletion effect),另一种是胶束形成的旌可达体积(the available volume for micelle formation)。排空效应不可避免地导致表面活性剂在远离固体粒子的区域富集,从而导致CMC的减小。另一方面,固体粒子的存在会减小胶束形成的可达体积,这将减小体系的构象熵,阻止胶束的形成,从而导致CMC升高。正是这两种因素的竞争决定着受限表面活性剂的CMC的偏离程度。3.结合LMC和标准盒子(Gauge Cell)方法研究表面活性剂溶液在亲水表面上的吸附和相行为,考察了温度、吸附作用能以及表面活性剂分子结构的影响。模拟结果表明,不同体系存在两种不同的相分离方式,即宏观相分离和微观相分离。对于发生宏观相分离的体系,有两种不同的动力学机制,在相分离区域遵从成核生长机理。该体系存在临界温度和临界吸附作用能。在临界温度以下或临界吸附作用能以上,体系发生宏观相分离,微量吸附相和双层相两相共存,吸附等温线明显有一回滞环,这是一级相变的特征表现。对于发生微观相分离的体系,在对数坐标系里的吸附等温线分为四个区,分别为微量吸附区,半胶束区,结构转变区和平台区,这与实验结果一致。更多还原

【Abstract】 Surfactants are a class of surface active compounds. Surfactants can substantially lower the surface tension of the liquid or interfacial tension when they are solved in liquids (especially water). As a result, they can improve the properties of solution, such as solubilization, emulsification, dispersion, penetration, wetting, foaming, cleaning, etc. Therefore, they are widely used in many industrial fields, such as textile, food, medicine, pesticides, cosmetics, building, mining, and so on. A surfactant contains both hydrophobic (the "tails") and hydrophilic groups (the "heads"). Due to their unique architetures, surfactants can easily self-assemble into the ordered structures at mesoscopic scale (such as layer, membrane, and liquid crystal state, etc.). It is the aggregates at mesoscopic scale that directly affect the properties of surfactants. Therefore, the properties and functions of surfactants depend on the self-assembly process. At present, the self-assembly of surfactants is important means to prepare the biological and functional materials. Therefore, the properties of self-assembly of surfactant and aggregate morphology have become a current research focus.In this thesis, Lattice Monte Carlo (LMC) simulation method was used to study the aggregation behaviors of surfactants on solid surfaces and in confined space, and to explore their aggregation process. We would like to reveal the mechanism of self-assembly of surfactants at mesoscopic scale, to offer useful information for theoretical research and practical applications. The main contents and findings are summarized as follows.1. The LMC method was used to study the adsorption morphologies and morphology transition of surfactants in a solid-liquid interface (hydrophobic surface). Several impact factors are considered, i. e. the adsorption energy, the interaction between head groups of surfactants and water (the solubility of head groups), the interaction between tail groups of surfactants and water, and the surfactant structure. The phase diagrams in adsorption energy-the solubility of head groups panel for the different surfactants are given. The simulation results show that there exist six adsorbed morphologies: (1) premature admicelle, (2) hemisphere, (3) hemisphere-hemicylinder mixture, (4) worm-like hemicylinder, (5) perforated monolayer, (6) monolayer, among which hemisphere and monolayer are observed by experimental works. The surface morphologies and the amount of adsorption on hydrophobic surfaces are found to be affected obviously by two interchange parameters. One is the attractive interaction between tail groups and surface (the adsorption energy), and the other is the solubility of head groups in bulk. When the adsorption energy of surface is stronger, the surfactants are inclined to form the surface aggregation morphology with smaller curvature, and the amount of surface adsorption is greater. On the contrary, when the attractive interaction between the head groups and water is stronger, the adsorbed surfactants are inclined to form the surface aggregation morphology with larger curvature, and the amount of surface adsorption is smaller.2. The LMC method was used to study the behaviors of adsorption and aggregation of surfactants in confined space, including narrow pores composed of two parallel hydrophobic surfaces and random pores composed of randomly arranged solid particles.Firstly, the effects of the size of narrow pores and surfactant concentration on the aggregation morphologies of surfactants are studied. The phase diagrams in the pore size-surfactant concentration panel for different surfactants are given. The simulation results show that an intermediate state, which is called the bridge structure, may exist during the phase transition from the monolayers on each solid surface to bilayer structures between the adsorbed monolayers. Moreover, the occurrence of the bridge phase during the monolayer-bilayer transition is found to be dependent on the transition path and the surfactant architecture. In addition, it is suggested that the bridge structure may be one of possible origin for the long-range hydrophobic force between two solid surfaces.Then, the self-assembly of surfactants confined in random pores which are composed of different arrangement of solid particles is studied. The effects of solid particles (crowding agents) on the critical micelle concentration (CMC) of surfactants are particularly investigated. Three different factors are considered, i. e., the size, arrangement, and volume fraction of solid particles. The simulation results show that the existence of solid particles strongly shifts the critical micelle concentration (CMC) of surfactants from the bulk value. Two effects originated from crowding are found to govern the CMC shift: one is the depletion effects by crowding agents and the other is the available volume for micelle formation. The depletion effects inevitably result in the enrichment of surfactants in crowding-free regions, and cause the decrease of CMC. On the other hand, the appearance of solid particles decreases the available volume for micelle formation, which reduces the conformational entropy, impedes the micelle formation, and causes the increase of CMC. The trends of CMC shifts are interpreted from the competition between the depletion effects and the available volume for micelle formation.3. The LMC combined with the gauge cell method was used to study the adsorption and phase behaviors of surfactants on a hydrophilic surface. The effects of temperature, adsorption energy, and surfactant structure are considered. The simulation results show that there exist two different phase separations for different systems, i. e., macrophase separation and microphase separation. For the case of macrophase separation, there exist two different physical mechanisms of phase separation. In the phase transition region, the layer growth proceeds through the nucleation mechanism, whereas above the limits this mechanism is not available. There exist a critical temperature and critical adsorption energy, below which macrophase separation occurs; the low-affinity adsorption and the bilayer phase coexist. Such a surface phase transition in adsorption isotherm is featured by a hysteresis loop, which is the characteristic of a typical first order phase transition. For the case of microphase separation, the adsorption isotherm in adsorption processes is divided into four regions in a log-log plot, being in agreement with experimental observations. They are the low-affinity adsorption region, the hemimicelle region, the morphological transition region, and a plateau region, respectively.更多还原