Surfactant Flow and Foam Stability

ABSTRACTCTS_0086751M。这项研究是利用光学和流变学技术的结合来研究朗缪尔单层膜的流动行为。它由两组实验组成：研究水亚相中多价离子对Langmuir单层膜流动行为的影响；并对混合表面活性剂体系的流动特性进行了研究。重点是系统及其流动特性，直接影响泡沫和乳液的稳定性。拉格缪尔单层膜本质上是二维的，由两亲分子组成，这些分子被限制在空气-水界面上。这项工作中使用的朗缪尔槽由两个垂直定向的同心圆柱体组成。内筒固定，外筒自由转动。水的上表面是自由的，单层放在这个表面上。因此，单层模拟了标准含水泡沫中的气液界面。内筒由两部分组成。一个静止的圆柱体在水的亚相中，一个扭摆只与水面接触。直流粘度是通过转动外缸，用扭摆测量内缸上的应力来测量的。通过固定外筒并摆动扭摆来测量ac粘度。通过使用浏览器角度显微镜直接观察速度分布和区域动力学，可以获得有关单层流动特性的附加信息。在某种程度上，由于对其液体凝聚（LC）相的阐明，人们对朗格尔布雷层的神学重新产生了兴趣。LC相是三维近晶液晶的二维类似物。它们具有六向秩序，并且在分子相对于表面倾斜的相中，倾斜方位角表现出取向秩序。由于LC相在Langmuir单层中普遍存在，因此了解它们的流变学与涉及表面活性剂单层在界面处流动的一系列过程有关。近年来的工作重点是脂肪酸单层，因为它们的相行为已经被深入研究，并且已经观察到广泛的有趣的流动现象。尽管如此，即使是纯脂肪酸单层的流动行为也不完全清楚。这里的工作建议将这些测量扩展到脂肪酸盐（在水亚相中存在离子的脂肪酸单层）和混合脂肪酸/酯体系。将讨论一些基本问题。平衡相的结构和单层的粘弹性之间有什么联系？带电荷的单层膜和多价离子之间的相互作用如何影响单层膜的流动特性？亚相中的流动如何影响单层的结构，这对整体流动有什么影响？选择这两种体系的动机是它们在泡沫稳定性中的作用。溶液中离子的存在常被认为是稳定泡沫。另一方面，脂肪酸盐也可以用来减少泡沫。这里提出的脂肪酸盐的研究将有助于阐明电解质作为泡沫稳定剂和不稳定剂的双重作用。它甚至可能产生一种控制泡沫稳定性作为时间函数的方法。脂肪酸/酯混合物常被用作泡沫稳定剂。此外，表面粘度与泡沫稳定性之间存在相关性。然而，对稳定机制还没有基本的认识。它可能是粘度和稳定性之间的简单关系，也可能涉及LC相的非牛顿性质。在固定的温度和压力下，使用混合物可以改变系统的相和相应的流动行为。对亚相流与单分子膜的相互作用进行了初步研究。

ABSTRACTCTS_0086751M. DenninUniversity California @ IrvineThis research is a study of the flow behavior of Langmuir monolayers using a combination of optical and rheological techniques. It consists of two sets of experiments: a study of the effects of the multivalent ions in the water subphase on the flow behavior of Langmuir monolayers; and a study of the flow behavior of mixed surfactant systems. The focus is on systems and their flow properties that directly impact foam and emulsion stability.Lagmuir monolayers are intrinsically two-dimensional and consist of amphiphilic molecules that are confined to the air-water interface. The Langmuir trough used in this work consists of two concentric cylinders that are oriented vertically. The inner cylinder is fixed and the outer cylinder is free to rotate. The top surface of the water is free, and the monolayer is placed on this surface. As such, the monolayer models the gas-liquid interfaces in a standard aqueous foam. The inner cylinder consists of two parts. A stationary cylinder in the water subphase, and a torsion pendulum that just makes contact with the water surface. The dc viscosity is measured by rotating the outer cylinder and measuring the stress on the inner cylinder with the torsion pendulum. The ac viscosity is measured by holding the outer cylinder fixed and oscillating the torsion pendulum. Additional information about the flow properties of the monolayer is obtained by direct observation of velocity profiles and domain dynamics with a Browser angle microscope.There has been a renewed interest in the theology of Languor minelayers, in part, due to the elucidation of their liquid condensed (LC) phases. The LC phases are two-dimensional analogs of three-dimensional smectic liquid crystals. They posses hexatic order, and in phases where the molecules are tilted with respect to the surface, the tilt azimuth exhibits orientational order. Because LC phases are ubiquitous in Langmuir monolayers, understanding their reheology has relevance to a range of processes that involve surfactant monolayer flow at interfaces. The focus of recent work has been on fatty acid monolayers, as their phase behavior has been thoroughly studied, and a wide range of interesting flow phenomena has been observed. Despite this, even the flow behavior of the pure fatty acid monolayers is not completely understood. The work here proposes to extend these measurements to fatty acid salts (fatty acid monolayers in the presence of ions in the water subphase) and mixed fatty acid/ester system. A number of fundamental question will be addressed. What is the connection between the structure of the equilibrium phases and the viscoelastic properties of the monolayer? How do interactions between charged monolayers and multivalent ions affect the flow properties of the monolayer? How do flows in the subphase impact the structure of the monolayer, and what effect does this have on the bulk flow?The motivation for choosing these two systems is their role in foam stability. The presence of ions in solution is often found to stabilized foams. On the other hand, fatty acid salts can also be used to decrease foaming. The study of fatty-acid salts proposed here will help elucidate this dual role of electrolytes as foam stabilizers and destabilizes. It may even result in a method for controlling foam stability as a function of time. Fatty acid/ester mixtures are often used as foam stabilizers. Also, there is a correlation between surface viscosity and foam stability. However, there is no fundamental understanding of the stabilizing mechanism. It may be a simple relationship between viscosity and stability, or it may involve non-Newtonian properties of the LC phases. The use of mixtures allows one to alter the phase of the system and the corresponding flow behavior, for a fixed temperature and pressure. Preliminary studies the interaction between subphase flows and the monolayer will be made.