My personal site
In my Master’s, I studied a smart wormlike micelle system composed of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) with cinnamate derivatives. These were chosen for their structural similarity to sodium salicylate (NaSal), which is the go-to cossolute to form wormlike micelles with cationic surfactants. The difference is a spacer group between the carboxylic acid and the aromatic ring, and the ortho group. This spacer group could be \(\ce{\phi-CH2 - CH2-COO-}\) or \(\phi-\ce{CH=CH-COO-}\) and the ortho group could be \(\ce{H}\), \(\ce{OH}\) or \(\ce{OCH3}\). These groups make the cossolutes sensitive to pH, with the most interesting one being ortho-hydroxy-cinnamate (OHCA). It was the strongest one, at around pH 9, but became totally liquid at higher pH. Other cossolutes showed little interest in pH variations (above pH 4), and the “most bare” cossolute, without the double bond or ortho group, barely even formed micelles. To study these systems, I employed a great deal of rheology, and also isothermal titration calorimetry.
A copy of my Master’s dissertation can be found here (in portuguese). One aspect of my work has been developed in an article, check it out here. We also described the calorimetric profile of wormlike micelles in another article, here.
In my PhD, I decided to tackle a bigger problem. Instead of looking directly at the components of the wormlike micelles, I looked at where they formed, i.e., the solvent. This was inspired greatly by the work of Prof. Heinz Hoffmann, who became a friend after Prof. Sabadini introduced us during ECIS Cyprus in 2014. Since then, we maintained contact. His work on adding glycerol to wormlike micelles and bilayers was fascinating. Even after his visit to our group, we spent quite some time debating and trying to understand exactly what was at stake. I admit I felt very incompetent during this part.
When deciding my topic for the PhD, I thought about continuing Hoffmann’s work, using other solvents. Laila Lorenzetti, another Master’s student, did some initial studies with sucrose, and I picked up where she left off, and studied, in total, glycerol, sucrose, dimethylsulfoxide, 1,3-butanediol and urea. These were chosen for their relatively high polarity. The concentration ranges were from 10% to 60% by mass. I decided to stick with the well known CTAB+NaSal system, and did NaSal and solvent concentration sweeps.
What we found was quite interesting. Sucrose had little effect on the micelles, even at concentrations as high as 50%. 1,3-butanediol was deleterious, and lead to very low viscosity fluids. Urea showed a similar behavior to glycerol, but the viscosity increased, instead of decreasing. In the end, we found that the Gordon parameter, related to solvent cohesivity (and surface tension) was the best descriptive parameter of the solvent to explain the changes observed, but the refractive index and the dielectric constant (which control the intermicellar attractions through the Hamaker constant) are also necessary to better explain the system.
During my PhD I fomented my interest in chemometric/data analysis methods, such as principal component analysis, clustering, etc. I also taught myself Python, and learned Matlab, in order to apply these techniques to my studies, which is something I was always interested in. At the moment, I do all my data analysis using these tools, and highly recommend it, for the breadth, speed and reproducibility you gain. My thesis reflects this, and I show many code snippets throughout it. I also gave a short class on using Python for data analysis, and the content can be found on Github
A copy of my thesis can be found here. I decided to write it in Portuguese, sacrificing international appeal, because I wanted my work to be more approachable to Brazilian students, who often struggle with texts in English. If you want a condensed version in English, check out my published article here. If you’re interested in the LaTeX source code, check out the repo. You need to request permission before using anything from it, be it text, figures or data!
During my PhD, my supervisor and I were invited to write a book chapter, which is partially available in Google Books.
I also spent quite some time trying to measure the kinetics of formation of my wormlike micelles, but didn’t have much luck, unfortunately. I used time-resolved fluorescence (long scales, in the order of milisseconds, not picoseconds), with the help from Prof. René Nome from Unicamp, and time-resolved small-angle X-ray scattering, with the help from Prof. Jan Skov Pedersen from Aarhus, Denmark. Since I didn’t get any positive result, no article was published. Nevertheless, this work spawned another one, which I collaborated some, and it can be found here. The model for wormlike micelles Prof. Pedersen developed in Fortran, and I ported to Python, can be found on Github.
In addition to all of this, I branched out to studying a biological system. Prof. Pessine introduced me to Dr. Carla Gomes, who was conducting a beautiful study on cystic fibrosis. We analysed hundreds of samples using the rheometer, and the combined results of those experiments with their data has been published here. My interest in automating analyses was very useful for this study, and I could run hundreds of fits in 1 minute, opposed to, at most, 5 or so, per minute, max. The code I used for this can be found here and here