I am studying the effect of different surfactants in the kinetic of formation of triiodide. We need to use 1-Pentanol as a co-surfactant . What is the role of 1-Pentanol in reverse micelles of CTAB or CTAC?
The formation and structure of microemulsions are considered as critical phenomena. Critical phenomena are observed near points of weak phase transitions of the first kind, where the entropy and density jumps are very small and the transition is close to a second-order phase transition. Fluctuations of the order parameter (concentration) near the critical point are large. The correlation radius (the size of the fluctuations) significantly exceeds the average distance between the particles. The role of 1-pentanol in increasing surface activity of surfactants. Surface tension can also play the role of an order parameter. Another important role of alcohol + surfactant in their co-dissolving property, which makes possible critical mixing of the two components at a relatively low concentration of surfactants.
Not many surfactants & especially ionic surfactants such as CTAB can readily form inverse micelles. They need cosurfactants such as short chain alcohols in order to form such aggregates and if oil is present in the system to form w/o miicroemulsions.
Prof. Mirgorod are right. It is necessary also to mention that the pentanol molecules, integrating radially on the microemulsion interface, reduce the repulsion between cations of surfactants and, in this connection, increase the number of surfactant molecules, as well as decrease the interfacial tension by thousand or more times. The microemulsions became thermodynamically stable and their solubilization of different substances increases thousand times too.
I agree with all the answers above. I think a more "molecular-packing" approach can also be very helpful.
CTAB has roughly the shape of a cone, with the base on the cationic headgroup and apex at the end of the tail. To form reverse phases, you need surfactants with an aproximate shape of a cone but with the base on the tail and apex in the headgroup - lets call it roughly a reverse cone. Because of the cationic charge this is very difficult.
Now what 1-Pentanol does is that since it is a co-surfactant, the alcohol headgroup has a tendency to go close to the interface, near the cationic headgroup of CTAB. This helps reducing the repulsion between the CTAB headgroups. At the same time, the oil molecules can penetrate more in the surfactant film, which leads to a reverse curvature, resembling more the shape of a reverse cone.
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