|M.Sc Student||Ido Ben Barak|
|Subject||Cryogenic-Temperature Electron Microscopy of Curvature-|
Induced Nanostructure in Ionic Surfactant
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Talmon Yeshayahu|
|Full Thesis text|
The nanostructure of microemulsions has been actively investigated for decades, using a variety of methods. While many microemulsion systems are known and are in commercial use today, some factors governing the nanostructure, and even types of microemulsions, are still not fully understood.
In previous work, we used cryogenic-temperature electron microscopy (cryo-EM), a technique suitable for direct imaging of liquid and other high-vapor-pressure samples, to investigate the effect of the surfactant layer curvature on the nanostructure. Curvature is known to be a key factor in ionic-surfactant microemulsions for many years. Using cryo-EM, we demonstrated this some time ago in a well investigated microemulsion system, of the surfactant didodecyldimethylammonium bromide (DDAB).
In recent years, a new type of microemulsion nanostructure was discovered - high internal phase microemulsion (HIPME). This is a thermodynamically stable microemulsion, in which a solvent of very high volume ratio is dispersed as droplets in the lower-volume solvent. In earlier work this nanostructure had been shown only for mixtures of nonionic and ionic surfactants. We hypothesized that the formation of HIPME nanostructure, which was known so far for only one type of surfactant system and for only two sets of solvents, is greatly affected by the curvature of the interfacial layer. We attempted to use this principle to produce HIPME nanostructure in two new systems.
In the first system we investigated, we replaced the original anionic surfactant, magnesium dodecylsulfate, with an analogous cationic surfactant, dodecyltrimethylammonium bromide (DTAB). By that, we were unable to produce a single phase microemulsion. The systems produced were of three phases, with the bicontinuous middle gel phase (a nanostructure in which both solvents are continuous, and not one being dispersed in another). We imaged this structure using cryo-SEM. The intermediate, non-thermodynamically stable structure of the system was indeed HIPME, but cannot be considered a microemulsion.
The second system we investigated was a purely ionic surfactant system, DDAB and DTAB. We showed that using the appropriate amounts of overall surfactant in the system and ratio between surfactants, it is possible to form a stable microemulsion for compositions in which only one of the surfactants does not form a single-phase system, and depending on the ratio between surfactants, the HIPME nanostructure forms in this system.
This leads to the following conclusions: curvature is indeed an important design tool for manipulating the structure of microemulsions; cryo-EM is an important technique for the investigation of nanostructure of microemulsions and complex liquids; and HIPME nanostructure is not specific to the previously known system, and can form in other surfactant systems, including purely ionic surfactant mixtures.