Solutions of alkali soaps and water in fatty acids XIII. Circumstances that regulate the extension of theL 2-phase and the role of the acid-soaps rich in fatty acid for the occurrence of the phase

Abstract

A basic requirement for that type ofL 2-phase which exists in the system sodium octanoate-octanoic acid-water is the formation of acid-soaps. In order for the phase to be formed at all, the temperature must lie above the melting point of the fatty acid so that a reaction in non-aqueous milieu between neutral soap and fatty acid is possible. In order to obtain the characteristic shape and complete extension of the phase in direction of high water content the temperature must be so high that also the hydrated acid-soaps occur in fluid state. On the other hand the temperature cannot be so high that the acid-soaps become unstable. At temperatures at which the phase has obtained its full extension those circumstances differs which in different regions regulate the location of the phase borders; they depend on the composition of the acid soaps and on their amounts. In that part of the phase where the molar ratio between octanoic acid and sodium octanoate lies between 2 and 3 and where one has a continuous transition from “reversed” to “normal” structure only the two acid octanoates 1 NaC8 ∶ 2 HC8 ∶ x H2O and 1 NaC8 ∶ 3 HC8 ∶ x H2O occur and both are at 20 °C in fluid state. At water contents from about 22 % to 40 % the hydrate-water molecules belonging to the first mentioned soap are capable of contributing actively to the formation of large aggregates of acid-soap, a process which however is counteracted by the inmixing of the latter acid-soap. This mixture of the two acid-soaps decides in this region where the border of the phase will lie in direction towards an increased content of sodium octanoate; the result is that in spite of the fact that the hydration is increased, the border is only slowly displaced towards a higher content of fatty acid. As soon as the hydration of the acid octanoates has been completed and the additional water occurs as unbound bulkwater, the location of the phase boundary will no longer be influenced by the water content — now it will be the amphiphilic composition of the acid-soaps that determines the location of the border and it remains at the molar ratio 2.5 between octanoic acid and sodium octanoate at water contents from about 40% and up to 82%. In the direction of decreasing content of neutral sodium octanoate and increased content of water theL 2-phase both at the highest content of fatty acid and the highest contents of water will be in equilibrium with the water-richL 1-phase; in the first mentioned region with theL 1-phase below the lac where at the border it is saturated with octanoic acid and in the latter region with theL 1-phase just above the lac, where the dilute sodium octanoate solution contains dissolved 1 NaC8 ∶ 1HC8 ∶ x H2O. In the large central part of theL 2-phase, from about 20 % to about 86 % of water, the location of the border is dominated by the acid octanoate 1 NaC8 ∶ 3 HC8 ∶ x H2O and that makes an equilibrium with theL 1-phase impossible; instead one has an equilibrium via a two-phase zone between the amphiphile-rich region of theL 2-phase and its water-rich region. In the first region the location of the border is regulated by the decreasing capability of the hydrated acid octanoate 1 NaC8 ∶ 3 HC8 ∶ x H2O to dissolve octanoic acid; in the latter it is regulated by the fact that 1 NaC8 ∶ 3 HC8 ∶ x H2O is the most fatty acid-rich acid-soap that is formed and that the octanoic acid is very little soluble in water and in the aqueous solution of this acidsoap. The middle part of theL 2-phase, especially the region between about 55 % and 82 % of water, constitutes a direct continuation of the liquid crystalline lamellarD-phase. The liquid crystalline character of theD-phase is lost at the transition, but the lamellar organization is retained. That the molecules at least up to a water content of about 40 % are of the original “reversed” type and have an elongated shape with a central part of hydrated polar groups, from which core the hydrocarbon chains extend in two opposite directions, is the reason to that they, at crowding, form transient layer-like agglomerates of tightly packed more or less parallel molecules; this facilitates the transformation to coherent double amphiphilic layers, in which all molecules lie with the hydrated polar groups outwards toward coherent domains of bulk-water, without another liquid phase occurs.