Depletion of water-in-oil aggregates from poor solvents: Transition from weak aggregates towards reverse micelles

• W/o weak aggregation occur with extractants above a certain concentration
• Reverse micelles and weak aggregates are in dynamic equilibrium with monomers.
• A polar “solute” molecule is required to nucleate a reverse micelle.
• Determination of monomer concentration allows detection of both types of RM.
• Molecular dynamic simulations give new insights in weak aggregation process.

We assemble here all available descriptions of oil-soluble surfactant aggregates with or without solutes, assumed to be located in the polar cores of reverse micelles. The presence of solutes is crucial for the formation of a well-defined interface, thus inducing a transition from a loose reverse aggregate into a more structured micelle. This transition can be followed by the concomitant decrease of the “critical aggregation concentration” (c.a.c.). The less organized state as reverse aggregates is predominant when no “nucleating” species such as water, salts, or acids are present. One way to understand this weak aggregation is a depletion driving to aggregates as pseudo-phases introduced by Tanford. Analogues coexisting pseudo-phases seem to exist: weak oil-in-water (o/w) aggregation with the so-called surfactant-free microemulsions, containing loose aggregates, and re-entrant phase diagrams presenting a lowest aggregation concentration (l.a.c.), as described in the seventies.

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