Metal ions binding to tri-n-octylphosphine oxide (TOPO) monolayer spread at the air/water interface

Our recent investigations proved excellent film-forming properties of tri-n-octylphosphine oxide (TOPO) spread at the free surface of water (K. Hąc-Wydro, P. Wydro, P. Dynarowicz-Łątka, Pol. J. Chem., 79 (2005) 773). Since TOPO is known for its affinity to metal ions, it is of interest to study the behavior of TOPO monolayers spread on aqueous subphases containing different ions. For our investigation we have selected mono- and divalent alkali metal ions, differing in nuclear charge and size. Additionally, the influence of La3+ and VO2+ on TOPO monolayer has been studied. In order to characterize the monolayers we have applied the Langmuir monolayer technique and recorded surface pressure–area (π–A) isotherms and analyzed the stability by monitoring the decrease of surface pressure (at constant area) vs time. Moreover, the stability of monolayers was examined by comparing the collapse pressure values. Quantitatively, the changes in the Gibbs free energy in passing from water to metal ion-containing subphase were determined from the isotherms datapoints. Our results indicate that TOPO shows the highest affinity towards La3+ and VO2+ and among alkali metals — towards magnesium and lithium ions, which posses the smallest ionic radius among investigated cations. Our results, based on the Langmuir monolayer technique, are in good agreement with the extraction efficiency results.