Phase behaviour in binary mixed Langmuir–Blodgett monolayers of triglycerides

Binary mixed monolayers of the triglycerides (TAGs)-tripalmitin (PPP), tristearin (SSS) and triarachidin (AAA) at the air–water interface are investigated with the Langmuir method. Langmuir–Blodgett (LB) layers obtained by deposition on mica are investigated by Atomic Force Microscopy. Combining Langmuir and AFM results the relation between the phase behaviour of binary mixed TAGs and their chain length is established. TAG mixtures form monolayers with molecules in trident conformation at the air–water interface, like pure TAGs. The area Acond=63 Å2 and the pressure πcond=8–10 mN/m that separate “gas” and “condensed” film structures are the same for all mixtures and pure systems. In the π−A   isotherms the sharpness of the transition from “gas” to “condensed” phase decreases with the average chain length for all systems. Using AFM data the monolayer thicknesses for mixtures and pure systems is found to be linearly dependent on the average chain length of the TAG molecules. A linear relation between film thickness and applied AFM force is established. The corresponding coefficient K˜ is higher for mixed monolayers (K˜=0.08±0.01nN-1) than for pure systems (K˜=0.07±0.01nN-1). AFM images show phase separation in the systems PPP–SSS and PPP–AAA. The solubility of the shorter PPP molecules in the “long” (SSS- and AAA-rich) phase is significant. For the mixture SSS–AAA, phase separation is not observed. In that mixture the monolayer thickness varies linearly with composition, supporting the conclusion that SSS and AAA mix almost ideally. The main driving force for phase separation is the difference in the alkyl chain length. Indeed PPP–AAA (length difference 4 C atoms) shows the most clear phase separation. The relatively weak phase separation in PPP–SSS and the absence of phase separation in SSS–AAA show that the influence of chain length difference decreases with increasing average chain length. In air PPP–SSS and PPP–AAA mixed monolayers are unstable and crystals with α- and β-like structure are formed on top of the monolayer as in pure PPP and SSS systems.