MODEL-CALCULATIONS ON THE TRANSITIONS BETWEEN SURFACTANT AGGREGATES OF DIFFERENT SHAPES

In a series of papers published earlier, we presented theories about the formation of surfactant micelles which include detailed treatments of the mechanics, surface thermodynamics, and small system thermodynamics of spherical, rod-shaped, and disk-shaped aggregates. In the present paper, we develop a calculation scheme which yields the volume fractions of the various kinds of sodium dodecyl sulfate (SDS) micelles and also of SDS vesicles, at different solution states. The familiar hierarchy of surfactant aggregates of different shapes is generated automatically upon raising the salt concentration, i.e., without introducing any extraneous packing or geometrical constraints, and is due to the more rapid decrease of the electrostatic free energy for the less curved aggregate surfaces. Moreover, we find that when spherical and rod-shaped micelles coexist, the weight-average aggregation number as a function of the total surfactant concentration shows positive deviations from a linear relationship at high salt concentrations and negative deviations at low salt concentrations, in broad agreement with the available experimental evidence on the sphere-to-rod transition. © 1990, American Chemical Society. All rights reserved.