Fatty-acid monolayers and metal ions: First step towards a new look

We have studied the surface pressure versus specific molecular area (π–A) isotherms of heneicosanoic acid Langmuir monolayers, varying the pH and concentration of Cd-ions in the aqueous subphase. We have measured the discontinuities in A (ΔA) at the L 2 - L ′ 2 phase transition and in κ, the isothermal compressibility, i.e., Δκ at the L ′ 2 - S phase transition, and have shown that these have distinct zeroes at specific values of pH, which depend on the Cd-ion concentration. From these we have constructed the pCd–pH phase diagram, where pCd = −log10[Cd], [Cd] being the molar concentration of Cd-ions. This phase diagram clearly shows that this complex two-dimensional system has three distinct behavioral regions.