| Phospholipid (PL) adsorption and bilayer formation on the oxides quartz (alpha-SiO2), rutile (alpha-TiO2), and corundum (alpha-Al2O3) was investigated over a range of pH, ionic strength, and Ca2+-bearing conditions using experimental and theoretical methods. The primary aim was to identify effects related to oxide chemistry and surface charge on total PL adsorption and bilayer stacking, and to determine the forces responsible.Bulk adsorption isotherms revealed that adsorption of dipalmitoylphosphatidylcholine (DPPC) on oxide particles decreased with increasing negative oxide surface charge, implying that DPPC bilayers behaved as though they were slightly anionic despite possessing net neutral charge. At least two bilayers formed on all particles regardless of the solution conditions, which suggests the role of van der Waals attraction. Electrostatic interaction is proposed to act over long distances due to solution displacement by the impermeable bilayers, thickening the electric double-layer near the oxide and accounting for additional adsorption, or lack thereof, beyond the second bilayer. The number of adsorbed stacked bilayers on quartz and corundum particles was predictably altered by changing ionic strength and by adding Ca2+. Isotherms of anionic dipalmitoylphosphatidylserine and cationic dipalmitoylethylphosphatidylcholine show that adsorption of highly charged bilayers is decreased, or prevented altogether, likely due to strong bilayer-oxide and/or bilayer-bilayer repulsion.Rupture of DPPC vesicles on the oxides was found to occur, under conditions identical to those of the bulk adsorption experiments, using fluorescent dye-loaded vesicles, confirming interpretation of the isotherms as representing adsorption of planar bilayers rather than intact vesicles. Furthermore, interaction energies calculated using extended DLVO-theory predict the same trends in bilayer stacking seen from the bulk adsorption results, and indicate the important role of electrostatic interaction over large separation distances.Neutron reflectivity data show that fractional surface coverage of DPPC bilayers decreases with increasing negative surface charge on the (112¯0) face of sapphire (alpha-Al2O3), in agreement with the bulk adsorption results on oxide particles. Multiple bilayer stacking was not observed on planar sapphire, however, indicating an effect of substrate morphology.Results of this work are potentially relevant to biomedical applications involving supported phospholipid bilayers, and to origin of life hypotheses regarding evolution and stability of pre-biotic membranes. |
