Bending free energy and spontaneous curvature for micelles and microemulsions with weak and strong surfactants

Abstract

For mixtures of water, oil and non-ionic surfactant the temperature-induced phase separation of a droplet-phase microemulsion into a phase of smaller sized droplets coexisting with a water-or oilrich phase is studied by differential-scanning microcalorimetry. A comparison of theoretical estimates and experimental findings for specific-heat measurements on mixtures containing different types of surfactants (weak and strong) support a broad range of applicability of an interfacial modeling ("Helfrich free energy"), provided a more general expression for the spontaneous curvature of the interface is considered than suggested so far. The model provides then a quantitative interfacial description even for the strongly curved diffuse interface of micelles of weak surfactants.

Investigating Internal Structural Differences between Micelles and Unilamellar Vesicles of Decanoic Acid/Sodium Decanoate

“…Relating our experimental data to differences in the curvature of the micelle and vesicle structures [37][38][39][40] is not a straightforward task. Several theoretical treatments that consider the Helfrich free energy are based on the minimization of the interaction energy between the solution and the amphiphile interface, 40 and there is little detailed experimental information on the issue of packing in the nonpolar region and curvature of the resulting structure. Clearly, the organization and interactions of the nonpolar regions of the micelle and vesicle structures will determine what we observe in our perylene reorientation.…”

“…Micelles are expected to have a diameter on the order of 5 nm, depending on the amphiphile used, while the unilamellar vesicles we use have diameters on the order of 100 nm. It is important to consider the different radii of curvature for the two types of structures − and to understand from an experimental standpoint the implications of the curvature of these systems on the internal organization of the aliphatic regions. Indeed, the issue of curvature has been discussed extensively in the context of phospholipid bilayer structures, but we are not aware of any direct comparisons of internal organization and dynamics between micelles and unilamellar vesicles comprised of the same amphiphiles.…”

Structural Evolution of Complexes of Poly(styrenesulfonate) and Cetyltrimethylammonium Chloride

“…Surfactants with high spontaneous curvature tend to form spherical micelles, while those with lower spontaneous curvatures tend to form cylindrical and laminar micelles. Because spontaneous curvature is governed by intrinsic properties of the surfactant molecule, micelle shape at and near the cmc is difficult to alter, , but constraining the micelles to small volume offers one possible means. A volume effect is observed when surfactant micelles are packed at high concentrations or placed inside a confinement geometry. – For the former, a series of morphological transitions occur as concentration increases .…”

Mesoscale structures in microemulsions

“…Here, the horizontal axis is surfactant concentration φs , and the vertical axis is dimensionless spontaneous curvature (the dimensionless temperature is t = 0.151) [50]. This phase diagram is analogous to the 'fish' diagram in figure 5 if the spontaneous curvature is a smooth function of temperature [51]. One can confirm that the spontaneous curvature vanishes when the threephase body meets the one-phase region.…”