Divalent Cation-Induced Phase Transition of Phosphatidylserine Monolayer at the Polarized Oil–Water Interface and Its Influence on the Ion-Transfer Processes

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Abstract
The phase properties of dipalmitoylphosphatidylserine(DPPS) monolayer at a polarized nitrobenzene–water interface have been studied by measuring the double-layer capacitance of the interface, Cdl, under precise control of the potential drop across the interface. When a saturated DPPS monolayer was in contact with an aqueous 0.1 mol dm−3 LiCl solution, the Cdl took a value of 9.5 μF cm−2. The presence of Ca2+ or Mg2+ at greater than 2 mmol dm−3 in the aqueous phase induced a decrease in Cdl down to 1.5 μF cm−2, corresponding to the phase transition of the monolayer from a liquid-expanded to a condensed phase. The condensed DPPS monolayer was stable against a change in the applied potential across the interface between ΔOWφ=−0.14 and 0.10 V, where ΔOWφ is the potential of the aqueous phase with respect to that in the nitrobenzene phase. Cyclic voltammetric measurements of ion transfer showed that the DPPS monolayer in the liquid-expanded state decreased the rate of ClO4− transfer from the aqueous to the nitrobenzene phase, but did not affect the rate of tetramethylammonium ion, TMA+, transfer. This indicates the importance of the electrostatic interaction between the negatively charged DPPS monolayer and the transferring ions in determining the rate of ion transfer across the monolayer. In contrast, the condensed monolayer significantly diminished the rates of ion transfer for both TMA+ and ClO4− ions, suggesting that the presence of the condensed monolayer altered the rate-determining step and that the condensed monolayer exerted an additional hydrodynamic friction on the ion-transfer processes.

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