Structure and stability of triglyceride monolayers on water and mica surfaces

The structure and the stability of tripalmitin (PPP), tristearin (SSS), and triarachidin (AAA) monolayers at the air-water interface are investigated with the Langmuir method. The Langmuir-Blodgett (LB) layers obtained by deposition on mica were investigated with atomic force microscopy (AFM). Our experiments show that the three triglycerides can form monolayers with molecules in trident conformation at the air-water interface. We determined the equilibrium spreading pressure πeq below which such monolayers are thermodynamically stable. Under isobaric conditions, a slow compression was sometimes observed for PPP and SSS, corresponding to crystal formation with molecules in tuning fork conformation on top of the monolayer. This isobaric compression takes place at pressures significantly larger than πeq but still smaller than the collapse pressure. The isobaric compression rate was highest for PPP and almost zero for AAA. Through the use of AFM, the thickness of the trident monolayers was measured. It is 1.49 for PPP, 1.75 for SSS, and 2.2 nm for AAA, corresponding to tilt angles of the molecules of about 46, 49, and 59°, respectively. The LB monolayers of PPP and SSS are thermodynamically unstable in air. Small crystals form on top of the monolayer, presumably in β-phase for SSS. Domains with R-like and β-like structure coexist in the LB film of PPP. The nucleation rate increases with increasing surface pressure π and with decreasing chain length of the triglyceride. For AAA, no well-defined crystals were found on top of the LB monolayer during the periods of days. The trident monolayer is the less mobile, and the crystal phase is the more stable the longer the alkyl chains are.