Stearic acid monolayers with various areas per molecule (σ) were deposited on SiO
2 (or Al
2O
3) surfaces by the Langmuir-Blodgett method under various surface pressures. The phase transitions from the two-dimensional gas state to liquid state (σ =0.24 nm
2) and the liquid state to the solid state (σ =0.20 nm
2) were clearly observed in the π-A isotherm of the monolayer spread on the subphase. From ellipsometric and reflectionabsorption FT-IR measurements, the thickness and the molecular tilt angle were determined to be 2. 28 nm and 27. 6° from the surface normal for the sample with a σ value of 0.20 nm
2, respectively. Wetting properties for water were evaluated as a function of molecular coverage. The static contact angle increased and the critical tilting angle (CTA) decreased as the coverage increased. Two discontinuous changes of the contact angle were found near the coverages where the phase transitions on the subphase occur. These findings were discussed in terms of the interaction between water and the surface of the sample quantitatively evaluated from the plots of tan CTA vs. {m
1/3 cos CTA}
-1 (m is the mass of water droplets).
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