Thermodynamics of anisotropic surfactant micelles. II. A molecular interpretation of the micellar curvature free energy

• Explicit expressions have been derived for the two curvature constants k(1) and k(2) that mainly influence the size and shape, respectively, of generally shaped micelles (triaxial tablets with a thickness < width < length). It is found that geometrical packing constraints, together with an unfavorable hydrocarbon/water interfacial tension gamma(hc/w), give rise to a large and positive contribution to both k(1) and k(2) that increases with the micellar thickness (equivalent to surfactant tail length) as well as with gamma(hc/w). The constant k(1) that mainly influence the size of the micelles may be brought down by electrostatics to values where rather small micelles are able to form. However, electrostatics also have a tendency to increase the constant k(2) that mainly affects the length-to-width ratio of the micelles so that the smaller micelles are predicted to be rather disklike, i.e., with a low length-to-width ratio. In addition, residual head group effects as well as the free energy contribution due to the hydrocarbon chains may influence the micellar size and shape. In particular, it is demonstrated that mixing of two surfactants reduces k(2) and promotes the formation of elongated micelles with a large length-to-width ratio whereas k(1), and thus the overall size of the micelles, is only slightly influenced. The effect of mixing is predicted to increase with increasing asymmetry between the surfactants mixed in the micelles with respect to head group size and charge number as well as hydrophobic tail volume.

Keyword

angle neutron-scattering

ribbon-like micelles

cationic surfactants

bilayer vesicles

shaped micelles

phase-behavior

mixed micelles

mixtures

transitions

mechanics

Publication and Content Type

ref (subject category)

art (subject category)