The melting temperature (T
m) and the crystalline relaxation temperature (Tα
c) of 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 the π-A isotherm and also, the T
sp dependence of the electron diffraction (ED) patterns of monolayer. T
m, and T
α c of monolayers on the water surface are important to decide the phase structure or molecular aggregation state in the monolayers and also, formation of a large-area defect-diminished monolayer. Monolayer prepared from fatty acid molecules with nonionic hydrophilic groups is classified into a crystalline monolayer and an amorphous one in the case of T
sp below and above T
m of the monolayer on the water surface, respectively. Formation of the crystalline or amorphous monolayers is independent of the magnitude of surface pressure. The π-A isotherm for fatty acid monolayer represents the aggregating process of isolated domains grownright after spreading a solution on the water surface. The crystalline monolayer is further classified into the two types; crystalline domains are assembled, resulting in aligning along their crystallographic axes due to an induced sintering at interfacial region among monolayer domains by surface compression at T
sp below T
α c and also, domains are gathered without any special orientation among domains above T
α c. Monolayer prepared from amphiphilic molecules with ionic hydrophilic groups is classified into a compressing-crystallized monolayer (pressure-induced crystallization) and an amorphous one at T
sp below and above T
m, respectively. In a region of very low surface pressure, even though T
sp is below T
m, spontaneous crystallization is suppressed from strong repulsion among hydrophilic groups. This indicates that a pressure-induced crystallization is induced by compressing the amphiphilic molecules on the water surface.
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