Influence of hydrophobic cosolutes on the associative/segregative phase separation of aqueous cationic surfactant-polymer systems

The effect of hydrophobic cosolutes on the phase behavior for a cationic surfactant and charged polymer in water has been studied. With a similarly charged polymer (cationic), the phase separation is of the segregative type with one surfactant-rich phase in equilibrium with a polymer-rich phase. Addition of small amounts of octane to such a system significantly increases the compatibility; that is, the two-phase region is reduced. If a long-chain alcohol like octanol is added, the incompatibility is enhanced. The effect is attributed to changes in the micellar aggregation number and is essentially a molecular weight effect. Octane reduces the micellar molecular weight, while octanol causes an increase. With an oppositely charged polymer (anionic), the phase separation is associative with a concentrated polymer/surfactant phase in equilibrium with a water phase. For a system that has phase-separated associatively, the addition of hydrophobic cosolutes produces little effect.