Molecular aggregation structure-intermolecular interaction relationships of fatty acid monolayer on the water surface

Abstract

The aggregation structure of arachidic acid monolayers on the water subphase of pH5.8(pure water) and pII12.6(adjusted with NaOH) was investigated on the basis of electron microscopic observations. The arachidic acid monolayer did not exhibited the phase transition in the case of a slightly dissociated state of hydrophilic groups. On the other hand, the monolayer showed the phase transition behavior from an amorphous state to a crystalline one by surface compression at subphase temperature, TSP below the melting temperature, Tm of the monolayer and the monolayer was still in an amorphous state without any phase transition at TSP above Tm in the case of a highly dissociated state. The aggregation structure of the monolayers on the water surface was systematically classified into “the crystalline monolayer”, “the amorphous monolayer” and “the compressing crystallized monolayer” with respect to thermal and chemical (intermolecular repulsive) factors.