Study on the property of cylindrical micellar solutions with observation of the additional amphiphilic molecules effects

Abstract

Amphiphilic molecules spontaneously self-organize into various structures in the aqueous solution above a certain concentration with their hydrophobic ``tail`` parts pointed inward and their hydrophilic ``head`` parts pointed outward, by hydrophobic effect. While the hydrophobic interaction leads to the self-assembly of surfactants, the equilibrium geometry of an assembled structure is affected by the various factors such as a steric molecular shape, concentration and the electrostatic interaction. Among the self-assembled systems of surfactants, the cylindrical micellar system shows various interesting phenomena such as a micellar length growth, branch formation and the micelle-to-lamellar transition depending on the thermodynamic conditions. In this study, we observed the overall properties of the cylindrical micellar solutions and showed that these properties can be controlled by adding a small amount of other amphiphilic molecules. Also, we considered and discussed about a correlation between changes of these properties and the possibility for the curvature energy argument to describe the surfactant self assemble system with an aggregate level not a monomer molecular level. For these, we have measured the micellar size, the cloud temperature and the micelle-to-lamellar transition temperature when a small amount of other amphiphilic molecule was mixed to the neutral cylindrical micellar solution (penta-ethyleneglycol mono n-dodecyl ether; $C_{12}E_5$). The mixed surfactants were a cationic (Dodecyl trimethyl ammonium bromide; DTAB) and a neutral (1-Tridecanoyl-2-Hydroxy-sn-Glycero-3-Phosphocholine; LysoPC) spherical micelle-forming surfactant, and a bilayer-forming neutral lipid (DL- $\alpha$ -phosphatidylcholine dimyristol; DMPC). At first we observed an unusual length shortening of a nonionic/ionic mixed cylindrical micelle as the concentration($\sl{C}$) increases quantitatively by using SANS experiments. For this behavior occured in the nonionic surf...