Variation of Contact Angle and Structure with Molecular Length and Surface Density in Adsorbed Films of Fatty Acids

Films of f a t t y acids from 12 to 22 carbons were prepared by adsorp t ion on glass microscope slides from cetane solutions. Var ia t ions in dens i ty were achieved (1) by using a range of concent ra t ions wi th a fixed shor t t ime of immersion and (2) by subsequent immersion in cetane of the most dense films. Elec t ron diffraction pa t t e rns showed an increasing average t i l t of the molecules re la t ive to the surface normal wi th decreasing leng th of the molecules bu t no apparent change in t i l t wi th var ia t ions in surface densi ty . The contac t angles exhibi ted by drops of cetane on the films decreased marked ly from a max imum of 43-44 ° when the dens i ty of the adsorbed molecules on the surface was decreased, bu t were independen t of the var ia t ions in t i l t and length of the molecules.