INTERACTION OF WATER WITH A FATTY-ACID LANGMUIR FILM

Using sum-frequency vibrational spectroscopy we have studied the struc ture of the fatty acid monolayer-water interface as a function of pH o f water. At low pH values, the monolayer is neutral and the surface wa ter structure is disordered by hydrogen bonding of water molecules wit h the acid head-groups. At high pH values, acid head-groups are ionize d and the resulting surface field helps the surface water molecules fo rm a more ordered hydrogen-bonding network. At pH similar to 3, a smal l change in pH leads to a reorientation of the acid OH groups while th e alkyl chains remain conformationally ordered at all pH values. Publi shed by Elsevier Science B.V.