Surfactant Flow and Foam Stability

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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.