ELLIPSOMETRIC STUDIES OF BEHENIC ACID AT THE AIR-WATER-INTERFACE

The rich phase behaviour of monolayers of behenic acid (= docosanoic acid) has been analyzed by null ellipsometry in isothermic and isobaric measurements. By using structural data known from x-ray measurements the anisotropy and refractive indices in the vicinity of the S-CS phase transition have been calculated with a minimum of model assumptions. The calculated values are: n(x)s = 1.47 . n(x)cs = 1.48 . n(z)s = 1.54 . n(z)cs = 1.55 . and anisotropy gamma = n(x)/n(z) almost-equal-to 0.95. It is also shown that ellipsometry is a very sensitive and convenient method of detecting phase transitions. Its value for quantifying the roughness of the monolayer in the close vicinity of phase transitions due to thickness and density fluctuations is shown by the measurement of the parabolic intensity variations of the ellipsometric angle degrees-DELTA. The intensity minimum at the phase transition is much wider than in the adjacent homogeneous phases.