Effect of cations on the dissociation of arachidic acid monolayers on water studied by polarization-modulated infrared reflection-absorption spectroscopy

We report on the variation of the dissociation of a Langmuir monolayer of a rachidic acid at the air/water interface as a function of the subphase pH a nd for several cations (Cd2+, Ca2+, Mg2+, and Na+) with the help of the pol arization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) method. The infrared spectra give access to the relative concentration of a cid and salt molecules and allow us to determine the influence of the subph ase pH on the acid dissociation reaction for each cation. It is shown that Na+ obeys the purely electrostatic Gouy-Chapman theory quite well, whereas the behavior of Mg2+, and even more so that of Ca2+ and Cd2+, requires the introduction of some complexation constant to be understood.