The importance of substrate chemistry and structure on supported phospholipid bilayer design and functionality is only recently being recognized. Our goal is to investigate systematically the substrate-dependence of phospholipid adsorption with an emphasis on oxide surface chemistry and to determine the dominant controlling forces. We obtained bulk adsorption isotherms at 55 degrees C for dipalmitoylphosphatidylcholine (DPPC) at pH values of 5.0, 7.2, and 9.0 and at two ionic strengths with and without Ca(2+), on quartz (alpha-SiO(2)), rutile (alpha-TiO(2)), and corundum (alpha-Al(2)O(3)), which represent a wide a range of points of zero charge (PZC). Adsorption was strongly oxide- and pH-dependent. At pH 5.0, adsorption increased as quartz < rutile approximately corundum, while at pH 7.2 and 9.0, the trend was quartz approximately rutile < corundum. Adsorption decreased with increasing pH (increasing negative surface charge), although adsorption occurred even at pH > or = PZC of the oxides. These trends indicate that adsorption is controlled by attractive van der Waals forces and further modified by electrostatic interactions of oxide surface sites with the negatively charged phosphate ester (-R(PO(4)-)R'-) portion of the DPPC headgroup. Also, the maximum observed adsorption on negatively charged oxide surfaces corresponded to roughly two bilayers, whereas significantly higher adsorption of up to four bilayers occurred on positively charged surfaces. Calcium ions promote adsorption beyond a second bilayer, regardless of the sign of oxide surface charge. We develop a conceptual model for the structure of the electric double layer to explain these observations.

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We have examined the effects of metal oxide surface chemistry and lipid phase on phospholipid adsorption affinity and self-assembly. Adsorption isotherms of ditridecanoylphosphocholine (DTPC) at 40 °C and pH 7.2 on quartz (α-SiO2), rutile (α-TiO2), and corundum (α-Al2O3) particle suspensions indicated oxide-dependent adsorption affinity that decreased as rutile > corundum ≅ quartz at low concentrations and corundum > rutile ≅ quartz at higher concentrations. Significantly, atomic force microscopy of DTPC and dipalmitoylphosphocholine (DPPC) at high concentration on planar oxide surfaces at 25 °C in liquid-crystal and gel phases, respectively, also showed oxide-specific adsorption. Multiple bilayers formed on corundum (100), indicating greater coverage compared to single bilayers, bilayer patches, or supported vesicle layers on the negatively charged surfaces of mica (001), rutile (100), and amorphous silica glass (fused quartz plate). Thus, the amount and self-assembly of adsorbed phospholipid was found to be oxide-dependent regardless of the lipid phase. Significantly, both experimental methods show multiple bilayer formation as a unique feature of the positively charged alumina surface. The observed oxide affinity sequences are interpreted as controlled by van der Waals and electrostatic forces between the oxide surface and the negatively charged (−R(PO4 −)R′−) portion of the phosphocholine headgroup. Our results have implications for the interactions of amphiphilic molecules with mineral surfaces in diverse biogeochemical, biomedical, and industrial processes, including membrane-bound biomineralization, cell membrane stability during early evolution of life, organic matter burial in ocean sediments, the design of supported lipid bilayers and biomimetic cell membranes for medical implant devices, enhanced oil recovery, and ore extraction by froth flotation.

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