Pressure-Area Isotherms of Langmuirs Monolayers Two-Dimensional Phases

4 PRESSURE-AREA ISOTHERMS OF LANGMUIR S MONOLAYERS TWO-DIMENSIONAL PHASES 

FIGURE 7.3 Schematic representation of a Langmuir trough. The harrier BB is movable over the slidings R to vary the enclosed interfacial area. The corresponding force is measured by a torsion wire T that is connected to the barrier. As an alternative, the interfacial tensions at both sides of the barrier may be measured independently. 

FIGURE 7.4 Schematic example of a pressure-area isotherm of a spread monolayer showing different 2D phases gas (G), liquid expanded (LE), liquid condensed (LC4), and solid (S). 

At very low values of n, say 0.5 mN m , the monolayer exhibits gaseous (G) behavior. At these conditions, the molecules in the monolayer are so far apart from each other that they do not interact. Because of their amphiphilic character, the surfactant molecules have different interactions with the two phases at either side of the interface so that they adopt a preferential orientation. In the G state, the isotherm obeys the relation 

It requires only a small reduction of A to convert the monolayer from the LC into the solid (S) state. In the S state, the compressibility of the monolayer is essentially zero, which implies that the surfactant molecules are closely packed as in a 2D crystal. The cross-sectional area of the surfactant molecule may be estimated as the value of flni corresponding to the S state. 

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