Surface Potential of DPPC Monolayers on Aqueous Solutions at Atmospherically-Relevant Concentrations
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Date
2015-05
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The Ohio State University
Abstract
Here the surface potential of DPPC monolayers, a phospholipid found in the sea surface microlayer (SSML), is measured on various salt subphases in an attempt to elucidate the organization of DPPC on air/aqueous interfaces found on marine aerosols. The surface potential results suggest that metal ion binding to the phosphate located in the DPPC head group play an important role in DPPC monolayer organization. For monolayers on subphases containing monovalent cations Na+/K+, a small increase (~20−40 mV) in the surface potential with respect to DPPC on neat water is observed. For monolayers on subphases containing divalent cations Mg2+/Ca2+ a larger increase in the surface potential (~70−170 mV) is observed. These rises in surface potential are discussed in terms of changing DPPC dipole moments Δµhead and Δµtail with respect to the surface normal, the packing ability of the monolayer, saturation effects, and the surface potential contribution (Ψ0) of the electrical double layer (EDL). A 1:1 binding of monovalent cations to phosphate is thought to expand the monolayer by changing the tilt angle of the head group, whereas a 2:1 bridging complex is proposed to explain the increase in surface potential on divalent subphases. Mg2+ results agree well with this model. DPPC monolayer results on CaCl2 subphases agree only partially with the bridging model and need to be further explored.
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Keywords
surface potential, DPPC, monolayers, ions, aerosols