Structure and dynamics of Langmuir-Blodgett tristearin films: Atomic force microscopy and theoretical analysis

The structure and temporal evolution of tristearin (SSS) monolayers at the air-water interface at 20±1 °C are investigated with the Langmuir method. The deposited Langmuir-Blodgett (LB) layers were investigated with atomic force microscopy (AFM). The LB experiments showed that adsorption isotherms obtained with commonly used compression rates do not correspond to thermodynamic equilibrium. Under isobaric conditions at π⩾10mN/m, a slow compression was found, corresponding to the formation of crystals on top of the monolayer. The AFM images reveal that SSS initially form trident monolayers at air-water interface. These layers are thermodynamically stable at surface pressure π⩽5mN/m. The thickness of the trident monolayer was found to be 1.6-1.8 nm, corresponding to tilt angles of the molecule chains varying from τ=43° at π=10mN/m to τ=53° at π=40mN/m. For π⩾10mN/m, growth of crystals takes place with a tuning fork conformation of the SSS molecules on top of the trident monolayer. The crystals grow with time, mainly in lateral directions. The growth rate increases with surface pressure. A new model is developed to quantitatively describe the crystal growth process. A lateral growth rate of 2.3 nm/min and a vertical growth rate of 0.005 nm/min were calculated for 1 individual crystal at π=10mN/m. The same growth process that was observed on the air-water interface was also observed in transferred monolayers at room temperature, though the growth was much slower.