Abstract
Adsorption behavior of counterions under a Langmuir monolayer was investigated by
sum-frequency generation (SFG) spectroscopy. By comparing SFG spectra of arachidic acid (AA)
Langmuir monolayer/water interface with and without added salt, it was found that the simple
trivalent cation La<sup>3+</sup> adsorbed on AA monolayer only when the carboxylic
headgroups are charged (deprotonated), implying that counterion adsorption is induced by Coulomb
interaction. On the other hand, metal hydroxide complex Fe(OH)<sub>3</sub> adsorbed
even on a charge-neutral AA monolayer, indicating that the adsorption of iron hydroxide is due to
chemical interaction such as covalent or hydrogen bonding to the headgroup of the molecules at the
monolayer.
© 2013 Optical Society of Korea
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