Self-assembled aggregates of amphiphilic molecules in aqueous media can vary their size and shape in response to thermodynamic variables, such as concentration and temperature. Often, the energy scale of the interactions between self-assembled aggregates in solution is comparable to the energy scales that determine the structure of a single aggregate. This leads to a rich variety of structures and phases that allow these materials to use in a variety of applications.
In this study, we controlled the self-assembled micelle structures using additives and have studied their properties.
At first, we studied the mixed micellar solution of $C_{12}E_5$(nonionic surfactant, penta-ethyleneglycol mono n-dodecyl ether) and DMPC (nonionic lipid, DL - α- phosphatidylcholine Dimyristol). We observed the increase in size of micelles by adding DMPC to $C_{12}E_5$ micellar solutions. A one dimensional growth model had been applied for mixed micelles of $C_{12}E_5$ and DMPC to explain the growth of the micelles. Based on the CMC (critical micellization concentration) and thermodynamic - molecular theory, the micellization free energies and end-cap energies of mixed micelles were estimated. The end-cap energy of mixed micelles increased as the amount of DMPC increase. The end-cap energy increase made the average contour length of mixed micelles grow. The estimated diffusion coefficients based on the one dimensional growth model from SLS (static light scattering) measurements, decreased with increasing the amounts of DMPC. These findings from SLS measurements were in an excellent agreement with diffusion coefficients obtained from DLS (dynamic light scattering) measurements. Furthermore, nonionic lipids (DOPC and DMPE) gave enhanced effects on the micellar growth in size and in the temperature dependence.
On the other hand, the charged molecules (DOTAP, DTAB) decreased the size of micelles. The presence of salt also could control the micellar size by screening charge interaction. The flexibility and the size of the cylindrical micelles made of $C_{12}E_5$ could be controlled by varying the doping level of charged molecules (DTAB) without additional salt. We observed exactly the same behavior as reported for classical polyelectrolytes and salt-free ionic wormlike micelles that $q_{max}$ follows a power law of the form $q_{max} ~ C^{1/2}$ with 3 and 6mol% doping levels. However, we found a power law of $q_{max} ~ C^{1/3}$ for the micellar solution with 10mol% doping level, resulting from the shortening of the micelle. Here $q_{max}$ and C is the correlation peak position in the scattering curve and the concentration, respectively. And we found that it was in particular possible to easily tune the micellar charge density and the length of the cylindrical body without altering the local micellar structure.
Using the $C_{12}E_5$ / DMPC micellar solutions of which the end-cap energies had been tuned finely, we have shown for the first time that the topological transition (from disconnected cylindrical micelle to branched micelle) of the nonionic micellar solutions was strongly affected by defect energies and spontaneous curvature. This finding was in good agreement with theoretical calculations. And the junction energies of the branched micelles were determined experimentally for the first time.
Finally, we would show a good example in new material science using the self-assembled structure (micelle mesophase). Using the charge controlled micelles ($C_{14}E_{8}$ (nonionic surfactant) / TTAB (cationic surfactant)), a nanoporous silica thin film was made. The degree of orderliness of nanoporous silica thin film had been enhanced by introducing the charged molecules. Surface charge of the micelles was beneficial to the formation of a well ordered micelle pore array.
