NOVEL CONCEPT FOR THE AGGREGATION STRUCTURE OF FATTY-ACID MONOLAYERS ON THE WATER-SURFACE AND DIRECT OBSERVATION OF MOLECULAR ARRANGEMENTS IN THEIR MONOLAYERS

The aggregation structure of fatty acid monolayers on water subphases of different pH's was investigated by means of transmission electron m icroscopy. Fatty acid monolayers exhibited the phase transition from a n amorphous state to a crystalline one by surface compression in the c ase of a highly dissociated state of hydrophilic groups, whereas they did not show the phase transition in the case of a slightly dissociate d state. The aggregation structure of monolayers on the water surface was systematically classified into ''the crystalline monolayer'', ''th e amorphous monolayer'' and ''the compressing crystallized monolayer'' with respect to thermal and chemical (intermolecular repulsive) facto rs. Molecular-resolution images of fatty acid molecules in the monolay ers on mica substrate were successfully observed with an atomic force microscope (AFM) for the first time. The AFM image of a lignoceric aci d monolayer prepared at a surface pressure of 5 mN m(-1) showed a two- dimensional periodic structure with locally disordered molecular arran gements. Also, the nondestructive AFM image observation was successful for a stearic acid monolayer which was prepared by a multistep creep method, indicating that a high mechanical stability of the monolayer i s inevitably required for the nondestructive AFM observation.