Other possible experiments include compression-expansion hysteresis, isobaric, or

isochoric experiments. In the hysteresis experiment, the barriers compress the monolayer up to a

target surface pressure (or target MMA) and expand several times. In the isobaric experiment,

the monolayer is compressed up to a desired surface pressure which is then kept constant by

small barriers adjustments, and the evolution of the MMA versus time is recorded. In the

isochoric experiment, the barriers are stopped after reaching the desired surface pressure (or

MMA), and the surface pressure evolution is subsequently recorded versus time. These last 3

experiments are usually used to investigate monolayer stability versus time or its formation

reversibility as further exemplified in Chapters 3, 4, and 5.

 2.2 Langmuir-Blodgett Films and Atomic Force Microscopy

 One of the interesting properties of the Langmuir trough is that it is easy to transfer a

Langmuir monolayer prepared at the A/W interface onto a solid substrate for further analysis.

The most common technique for monolayer transfer is done by vertically lifting a substrate into a

Langmuir monolayer compressed at the desired surface pressure which is kept constant by small

barrier adjustments (Figure 2-4). The resulting transferred film is called a Langmuir-Blodgett

(LB) monolayer. The substrate used in this dissertation is freshly cleaved hydrophilic mica

because it mimics the water subphase during transfer. It is also possible to prepare LB films

consisting of multilayers by successively transferring several monolayers. This technique

provides thin films of supramolecular assemblies with well-defined molecular arrangement,s

chemical composition, and thickness, and with fewer defects than if prepared directly from the

bulk.

 The transfer ratio measures the degree of monolayer transfer. During LB monolayer

formation, part of the molecules initially adsorbed at the A/W interface are slowly transferred

onto the substrate, which results in a lower surface density in the Langmuir monolayer. To keep