274755 Ionic Effects On the Stability and Morphology of AOT-WATER-Isooctane Reverse Micelles: Molecular Dynamics and Dynamic LIGHT Scattering Perspective

Tuesday, October 30, 2012: 4:45 PM
415 (Convention Center )
Victor R. Vasquez, Chemical and Materials Engineering, University of Nevada, Reno, Reno, NV

We use molecular dynamics (MD) and Dynamic Light Scattering measurements in the AOT-water-isooctane system to study the morphology and stability of the reverse micelle regime under the presence of various salts. These include zirconyl chloride, ammonium hydroxide, and aluminum nitrate. In this presentation, we discuss the results obtained with both methods. DLS measurements show that the average reverse micelle size decreases with the addition of the salts. Further increase in salt addition eventually destabilizes the reverse micelle structures, which is observed by the formation of cloudy solutions. An initial hypothesis suggested that the high valence free ions were causing the destabilization, but preliminary MD modeling results show that the presence of potential hydrolyzed structures might be playing a significant role on the stability and morphology of the system. MD simulations using only free ions in solution do not seem to destabilize the reverse micelles; however, the presence of structures such as zirconyl chloride hydrated dimers can cause instability and changes in the morphology. We discuss how we use MD combined with experimental data from DLS measurements to systematically analyze and gain physical insight into the behavior of reverse micelle systems for particle synthesis and other potential applications. In general, we find good qualitative agreement between MD and DLS in the size and morphology analysis of the system. The combination of MD with DLS allows a better interpretation of the experimental results, in particular for conditions where the structures are nonspherical, commonly observed at lower water-to-surfactant ratios or distorted by the presence of highly ionic solutions. Significant challenges still remain including the analysis of the effect of all possible hydrated states and the respective structures of the high valence ions in solution. We show preliminary progress with this, but better force fields are required to describe more accurately the complex chemistry and hydrated structures of these ions in solution. Other challenges include the typical modeling issues dealing with the long time and length scales characteristic of the dynamics in reverse micelle systems. 

Extended Abstract: File Not Uploaded