Once a Langmuir monolayer is formed at the A/W interface, a variety of surface pressure

related experiments are possible. The simplest experiment is the isotherm, where the monolayer

is compressed toward the center of the trough and the surface pressure recorded versus surface

density or mean molecular area (MMA, average area occupied by one molecule at the interface,

calculated from Equation 2-5).


 M VA = "(2 -5 )


A is the area between the barriers, Mn is the molecular weight of the sample, NA is the

Avogadro' s number, and C and V are the concentration and the volume of the spreading solution,

respectively. Upon compression, the adsorbed molecules move closer to one another, the surface

pressure increases, and the phase transitions taking place in the monolayer are characterized by

pseudoplateaus or inflection points in the surface pressure versus MMA (or surface density)

isothermal plot as exemplified in Figure 2-3 for small amphiphilic molecules. The monolayer

undergoes phase transitions from the gaseous state (G) to the liquid state (L) and eventually to

the solid state (S) upon high compression.








 Liqu~id








 0.20 0 23 Oy 50 10 0
 a (nmw molcule*)

Figure 2-3. Schematic isotherm for small amphiphilic molecules.14