The Effects of Divalent Ions on Langmuir Monolayer and Subphase Structure: A Grazing-Incidence Diffraction and Bragg Rod Study

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Abstract

Divalent ions dissolved in the aqueous subphase of fatty acid Langmuir monolayers have two types of effects on the structure of the organic film. The first and more familiar effect is to induce a structure similar to the high-pressure “S” phase on pure water, even at low pressures; ions of the first type include Ni2+(aq), Ba2+(aq), Co2+(aq), and Cu2+(aq). The presence of ions of the second type results in the appearance of superlattice structures: we see a 1 × 2 superlattice with Mn2+(aq) and a 2 × 2 superlattice with Mg2+(aq), and it is known that Cd2+(aq) and Pb2+(aq) also cause the formation of superlattices. Out-of-plane (Bragg rod) scans indicate that Mn2+(aq) and Mg2+(aq) interact with the headgroups so strongly that the organic film buckles, with a periodic out-of-plane density modulation (amplitude ∼2.5 Å). In addition, a thin (∼4 Å) ordered inorganic layer forms in the subphase under the Langmuir film.

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