NANOSCALE CONTROL OF MOLECULAR DISPERSION-PHASE SEPARATION FOR TWO-DIMENSIONAL AGGREGATING SYSTEM

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Surface morphology of Langmuir mixed monolayer was investigated on the basis of a π-A isotherm measurement and a scanning probe microscopic observation. The molecular aggregating behavior of mixed monolayer markedly depended on difference in alkyl chain length, that is, cohesive energy between hydrophobic portions of fatty acids. The mixed monolayers for the cases of 4, 6 and 8 methylene units differences corresponding to 28, 42 and 56 kJ/mol for cohesive energy were in a molecular mixing state, phase separation in a miscibility gap and segregation in an immiscibility system, respectively. In particularly, the phase separated morphology was a co-continuous state, indicating the phase separation due to the spinodal decomposition. Moreover, the molecular aggregating state of the mixed monolayer changed from two-phase to one-phase with an increase in subphase temperature. This suggests that the mixed monolayer is assigned to the upper critical solution temperature type. The molecular aggregating behavior of the mixed monolayer was independent of the surface pressure for fatty acids in undissociated state, and also the monolayer phase of the single component system.