Neutron and X-ray reflectivity study of Ba salts of alternating bilayers of deuterated and hydrogenated stearic acid

Neutron and X-ray reflectivity measurements are used to characterize Langmuir-Blodgett films composed of twenty bilayers of barium stearate. To facilitate contrast for neutron measurements, molecules with deuterated aliphatic tails were used for the deposition of every odd-numbered bilayer. Model fitting of both data sets shows the following: (1) the bilayer thickness is 50.4 ± 0.1 A ̊, consistent with an untilted aliphatic tail; (2) the area per molecule is 20.2 ± 0.1 A ̊², consistent with the area derived from pressure-area isotherms for the Langmuir-layer; (3) the hydrophilic layer consists of a mixture of Ba salts and undissociated carboxylic acid groups. Its stoichiometry is given as Ba_1-yH_2y(COO)₂, where y = 0.45 ± 0.04; (4) neighboring bilayers are 30% intermixed, an amount too large to be explained solely by incomplete transfer coefficients during growth. Models are presented to explain the high degree of interdiffusion.