We study an unusual transition between two different types of self-assembled structures in aqueous solutions. Mixtures of a cationic surfactant, CTAB and the organic compound, 5-methyl salicylic acid (5mS) spontaneously self-assemble into unilamellar vesicles at room temperature. Upon heating, these vesicles undergo a thermoreversible transition to

“wormlike” micelles, i.e., long, flexible micellar chains. This phase transition results in a 1000-fold increase in the solution viscosity with increasing temperature. Small-angle neutron scattering (SANS) measurements show that the phase transition from vesicles to micelles is a continuous one, with the vesicles and micelles co-existing over a range of temperatures. The tunable vesicle-to-micelle transition and the concomitant viscosity increase upon heating may have utility in a range of areas including microfluidics, drug delivery, and enhanced oil recovery.         


Surfactant molecules spontaneously form various types of self-assembled structures in aqueous solution since self-assembly is controlled by thermodynamics, self-assembled structures will respond to changes in thermodynamic variables such as concentration and temperature. Other external variables such as light and electric fields may also influence the shape and size of the self-assembling molecules, and thereby control their assembly in solution. The goal of this thesis is to exploit the sensitivity of self-assembled structures in developing “smart” fluids or materials  that respond in an unusual or interesting way to an external input. Two desirable types of responses are targeted in particular.


The work described in this thesis deals with several unusual properties of a surfactant fluid. Firstly, the formation of vesicles in this type of a system is itself rare. As discussed in the next chapter, vesicles are usually formed from lipids or in mixtures of two surfactants. Secondly, reversible transitions from vesicles to wormlike micelles are

also very rare. We have only found a few papers in the literature which have described this behavior, and earlier systems were considerably more complicated than the present one. Thirdly, an increase in fluid viscosity with temperature is also quite unusual. As our intuition might suggest, most fluids tend to get less viscous upon heating. Here, we have an instance where the opposite is true – the viscosity of our fluids can increase more than

1000-fold with increasing temperature. 


The fluids described here may have some potential utility. Applications may either exploit the reversible disruption of containers (vesicles), or the viscosity increase exhibited by these fluids upon increasing temperature. An example of the first kind would be a biomedical application where vesicles would be used to encapsulate drugs in their aqueous interior. One could then trigger the disruption of these vesicles by increasing the temperature and the released drug would remain embedded in the viscous micellar fluid. Examples where switchable viscosities could find use are in the design of microfluidic valves, in capillary electrophoresis, or in enhanced oil recovery. For instance, hydraulic fracturing operations in oil recovery require the use of a fluid that becomes gel-like at the high temperatures experienced deep inside oil wells. At the same time, a low viscosity at ambient temperatures could enable the fluid to be pumped down easily.