Molecular Simulation Study On Adsorption And Aggregation Behaviors Of Surfactants In A Solid Liquid Interface And Confined Space

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.