Structural Changes in Nonionic Surfactant Systems
Abstract: In this thesis we have studied structural changes of a nonionic surfactant self-assembly, as governed by changes in temperature and concentration. The techniques used in experimental investigations were dynamic and static light scattering, nuclear magnetic resonance and cryo-transmission electron microscopy (using a novel specimen preparation protocol). The model systems consist of nonionic surfactants (alkyl oligo poly(oxy)ethylene ethers) C12E4, C12E5, C16E6 and C10E3 in water. These surfactants form micelles in water at lower temperatures, and planar bilayers and inverted phases at elevated temperatures. In a dilute micelle phase, the linear increase in size of wormlike micelles as a function of concentration is investigated and quantified. At higher concentrations, intermicelle interactions start to be significant. Here, micelles start to branch and overlap. When a dilute solution of nonionic wormlike micelles is rapidly heated to the temperatures where a lamellar phase normally exists, vesicles are formed. These vesicles are large and polydisperse and the size distribution depends on the rate of heating. Below a certain temperature, vesicles appear to be stable. Upon rapid increase in temperature (T-jump) to temperatures in the vicinity and above a three-phase line, vesicles fuse. The rate of fusion is determined by the final temperature of the T-jump. At higher concentrations, the behavior of lamellar phase is investigated. An unexpected lamellar phase separation in a centrifugal field is observed and a method for measuring undulation forces is developed. Upon decrease in temperature, an intermediate phase between lamellar and cubic/hexagonal phase was imaged.
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