Langmuir- Blodgett layers of amphiphilic molecules investigated by Atomic Force Microscopy

Abstract

Langmuir - Blodgett technique and Atomic Force Microscopy were used to study the phase behaviour of organic molecules (fatty alcohols and monoacid saturated triglycerides) at air-water and air-solid interfaces. The structure of binary mixed LB monolayers of fatty alcohols was reported. The dependence of phase separation phenomena on the difference between the chain lengths of the two components and the surface pressure was described. Phase separation occurs in compressed films, if the chain length of the two components differed at least with six carbon atoms. A strong dependence of the domain shape on the surface pressure was observed. At high surface pressure, the domains get tetragonal shapes. In line with thermodynamic expectation, the tendency of phase separation increased with increasing the excess Gibbs energy. Triglycerides are typical example for materials, which surface structure is different from the bulk structure, thus leading to different macroscopic properties as would be expected from the bulk structure. They adopt a chair conformation in crystals, but at air-water interface they rearrange in trident conformation (all hydrocarbon chains pointing toward the same direction). Because of the trident conformation they behave as amphiphilic molecules at the air-water interface, though in general they are lipophilic. By definition a Langmuir monolayer is thermodynamically stable if during long time presence at air-water interface it does not change its structure. Conditions for stability against collapse can in principle be established by measuring the equilibrium spreading pressure and corresponding surface area of the material. One of the surprising results, described in this work, is that the triglycerides showed thermodynamic instability at air-water interface at surface pressures far below the collapse pressure. Under isobaric conditions a molecular rearrangement process takes place which effectively thickens the film. Using Atomic Force Microscopy we have shown that this process involves the growth of 3D crystals of triglycerides on top of the monolayer for surface pressures above equilibrium spreading pressure. A new model to quantitatively describe this process was presented. The influence of the chain length of the triglyceride molecules on their stability on water and mica surfaces was described. The trident Langmuir - Blodgett monolayer is the less mobile and the crystal phase is more stable the longer the alkyl chains were. The nucleation rate increased with increasing the surface pressure at which the monolayer was compressed. The phase behaviour of binary mixed LB - monolayers of triglycerides was investigated. The relation between the phase separation and the chain length was shown. An incomplete phase separation was observed for the systems with two or more carbon atoms difference in the chain length of the two components. The solubility of the shorter PPP molecules in the "long" (SSS-rich and AAA-rich) phase was significant. In the mixture SSS-AAA, where the difference in the chain length is two carbon atoms as for PPP-SSS, phase separation was not observed. The conclusion was that, due to the stronger interaction between the longer alkyl chains, the sensibility for differences in the chain length decreases.