INVESTIGATION OF FATTY ACID MONOLAYERS ON METALS BY CONTACT POTENTIAL MEASUREMENTS.
NAVAL RESEARCH LAB WASHINGTON D C
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Each of the homologous series of pure n-fatty acids from formic to hexacosanoic was studied as a condensed monomolecular layer adsorbed on Pt and NiO substrates. Both the stearic and nonadecanoic acids were also studied on various polished metal surfaces including Fe, Cr, Cu, Au, W, Mo, Ag, Nb, Al, Cd, Mg, Sn, Pb, Be, Zn, and Ni. The change in contact potential difference Delta V between the metal and an aged reference electrode was measured in air at 23C and 50 R.H. Surface packing in each monolayer was characterized by the methylene iodide contact angle theta. Systematic and reproducible values of Delta V and theta were found for each member of the homologous series. Values of theta for stearic acid varied with the nature of the metal substrate, and the results were in reasonable accord with the lattice spacing of the substrate atoms. Values of Delta V for stearic acid monolayers varied from a maximum of 0.365 volt on W to -0.095 volt on Pb, and it was concluded that the perpendicular component of the dipole moment of the adsorbed acid molecule was a function of the specific interaction of metal substrate and the acid carboxylic group. Stearic acid desorbed readily from Pt but not from NiO, and it was concluded that chemisorption had occurred on the NiO. Author