Solutions of alkali soaps and water in fatty acids XI. Correlation between the acid sodium octanoate in the most water-rich part of the L2-phase and the acid soaps in two adjacent phases

Abstract

The special nature of the outer-most water-rich region of theL2-phase in the ternary system sodium octanoate-octanoic acid-water is evidenced by its somewhat turbid appearance and by the character of its equilibria with adjacent phases. The phase contains aggregated acid sodium octanoate which is dispersed in a very dilute aqueous solution of sodium octanoate. The acid octanoate has the composition 1 NaC8∶2 HC8∶x H2O and is composed of closely packed amphiphilic units, all with the polar groups in the same direction. This acid soap obviously forms double-layered aggregates with the lipophilic hydrocarbon chains pointing inwards and the polar groups pointing outwards towards the surrounding bulk-water. The phase is formed when octanoic acid is added to theL1-phase of the system just above the l.a.c.; in this aqueous solution, the acid reacts with dissolved acid octanoate 1 NaC8∶1 HC8∶x H2O and that results in the formation of the slightly soluble acid soap 1 NaC8 ∶2 HC8∶x H2O that separates as a new phase, the turbidL2 phase. On further addition of octanoic acid, the content of the mentioned acid soap increases until the solution phase is transformed into a liquid crystalline lamellarD-phase with the same acid soap composition. This formation of acid soap 1 NaA∶2 HA on addition of fatty acid to the dilute soap solution just above the l.a.c., has been known for a long time to occur in various systems containing a long-chain sodium soap. However, at suitably low temperatures, the reaction in these systems does not result in separation of the acid soap in the liquid crystalline, but in the solid crystalline state.