ABSTRACT
This tutorial review deals with one of the most remarkable forms of surfactant aggregates, described as having a flexible, elongated cylindrical shape. Three structural scale lengths are pertinent to the flexibility and mobility of worm micelles: the cross-sectional radius, r(cs), the overall (contour) length, L, and the persistence length, l(p). The diversity of l(p) values in amphiphilic systems is demonstrated as well as the relation between L and l(p). The review also discusses the viscoelasticity of worm micelles and the relaxation mechanisms underlying this dominant property. Many aspects of viscoelasticity--such as non-linearity, shear banding, flow-induced phase transition, rheochaos--are only shortly described. The prevailing application of worm micelles, namely as fracture fluids and drag reducing agents are discussed in detail, stressing the effect of variations in the surfactant molecular structure on the efficacy of worm micelles. The vague possibility of using "smart" worm micelles in the foreseeable future is tersely outlined.
ABSTRACT
This article is the first part of a two-part study that exemplifies how to treat the solubilization of water in multicomponent surfactant-based systems. In particular, it aims at clarifying the role of cosurfactants in water solubilization in these systems. The judicious selection of the components in such systems to maximize water solubilization is occasionally thought to be dictated by the chain length compatibility principle, which may be expressed quantitatively by the BSO (Bansal, Shah, O'Connell) equation. Here we demonstrate some limitations of the equation. For example, in our best model system, C12(EO)8/dodecane+pentanol=1:1 (by weight)/water at 27+/-0.2 degrees C, the BSO equation predicts that no alcohol is needed for maximum water solubilization, contrary to our experimental findings. We discuss how to optimize the alcohol/oil weight ratio needed for stabilizing four-component microemulsions. In our model systems C12(EO)8 or C(18:1)(EO)10/pentanol/dodecane/water, this optimal weight ratio is 1:1. We also highlight the difference between the effect of normal alcohols on water solubilization-which passes via a maximum-and their effect on percolation processes and structured changes of proteins, which depends solely upon the alcohol hydrophobicity. For the investigation of the effect of branching on phase behavior the utilization of an extended form of the geometrical branching factor F(b) is suggested. The meaning of this factor is elucidated by comparing it with topological indices.
ABSTRACT
In this second part of a paper dealing with the effect of branched alcohols on solubilization, an attempt has been made to provide explanations of experimental data related mostly to the system Brij 97/branched alcohol + dodecane = 1:1 (by weight)/water at 27+/-0.2 degrees C. Applying the Hou-Shah mechanism it was shown that for many C4-C6 branched alcohol isomers having one methyl branch, solubilization behavior is readily interpreted by assuming control of the critical radius, R(c). Two parameters, both included in the definition of the branching factor, F(b) (which was treated in the first part of the paper), were also used to analyze solubilization data. The first, l(i), is defined as the distance from the free end of the alcohol molecule to the methyl branch. The second, d, is virtually N(A), the chain length of the alcohol. When l(i)>3, the solubilization becomes dominated by the natural radius of curvature, R0. Also, we have suggested that for R(c)-control, solubilization will be enhanced in direct proportion to the distance d-l(i), whereas for R0-control, solubilization will increase with decreasing d-l(i). The validity of our assumptions was demonstrated in many cases. Some examples of the more complicated case of double branching (two methyl groups along the alcohol chain) were also analyzed.
