Monolayers of triple long-chain salts on the concentrated sodium chloride solution.

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The surface pressure–area (π–<I>A</I>) curves for monolayers of four kinds of cation-anion triple long-chain salts [dialkyldimethylammonium alkanesulfonates] on the aqueous solution of 4.4 M NaCl were measured at various temperatures by the Langmuir method. Some π–<I>A</I> curves of these salts showed two transition points. Judging from the apparent molar entropy change on the transition, the first was assigned to the transition from the expanded state (E phase) to the condensed state (C-I phase) and the second to the transition from the C-I phase to another condensed state (C-II phase). In this paper, another condensed phase (C-I′) was newly observed over a certain temperature range on the phase diagram, which is the intermediate phase between the C-I phase and the C-II phase. There exist two triple points on the phase diagram at which three phases (i.e., E, C-I, and C-I′, and C-I, C-I′, and C-II) coexist. From the apparent molar entropy change, limiting area, compressibility, and phase diagram, the orientation of (2–1) long-chain salts in the monolayer state was assumed.
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