EVALUATION OF MELTING AND CRYSTALLINE RELAXATION TEMPERATURES OF FATTY-ACID MONOLAYERS ON THE WATER-SURFACE AND THEIR IMPORTANCES FOR MOLECULAR AGGREGATION STATES IN MONOLAYERS

The melting temperature, T(m) and the crystalline relaxation temperature, T-alpha(c) of fatty acid monolayers on the water surface were evaluated by a combination of two kinds of measurements, the subphase temperature, T(sp) dependence of monolayer modulus based on a pi-A isotherm and also, the T(sp) dependence of the electron diffraction, ED patterns of monolayer. T(m)S of myristic, palmitic, and stearic acid monolayers were evaluated to be 278, 301, and 317 K, respectively, on the basis of both apparent decrease in the T(sp) vs. modulus curve and the change of the ED pattern from a crystalline Debye ring to an amorphous halo. The crystalline relaxation process corresponds to a change from elastic to viscoelastic characteristics in a crystalline phase due to a considerable contribution of anharmonic thermal molecular vibration. T-alpha(c)S of palmitic and stearic acid monolayers were evaluated to be 291 and 298 K, respectively, from both apparent decrease in the T(sp) dependence of modulus and appearance of the break in the thermal expansion coefficient of crystalline lattice constant. This break may be attributed to a remarkable increase in contribution of the anharmonic term with respect to the intermolecular potential energy. The magnitude of T(m) and T-alpha(c) of monolayers on the water surface is an important factor to decide the structure or aggregation state in the crystalline and amorphous monolayers and also, the formation process of a large-area monolayer, that is, the fusion or sintering behavior among crystalline monolayer domains.