Counter-ions

Condensation and Phase Behaviour

A lot of conclusions from isotherm work in the past are compromised because they were based on the idea that there are only four monolayer phases. It is now known that there are many more[1]. The phase of stearic acid above 26 mN/m at room temperature on a pure water subphase was previously believed to be solid, in spite of the fact that the monolayer flows in response to small surface pressure gradients. It is not. There are monolayer phases which are true solids. The characteristic feature of a solid is that it shows a definite shear stress threshold for plastic flow. Solid monolayers can be recognized by the fact that they do not flow in response to small pressure gradients. When you get a part of an isotherm which is nearly vertical, i.e. incompressible, it does not mean that the monolayer is solid, but that the molecules are standing vertically and so cannot readily change their area by changing their tilt angle. The phenomenological aspects of the tilt transition are described quite well by a mean-field theory[2].

The true solid phases in fatty acid monolayers on pure water were first reported in 1945 [3]. In the CS phase the molecules stand upright while in the L₂" phase they are tilted. To get them at room temperature on a pure water surface you have to go to chain length 26 (hexacosanoic acid) and above. Lundquist also reported these phases in monolayers of various esters[4], and she commented that the phase transitions between them are only very poorly defined in the normal compression isotherms. In order to get more precise values for the transition pressures, take an expansion isotherm instead. On a Nima trough, you can do this by using the isocycle mode.

This transition has also been observed using BAM[5], and it was found that the change of morphology does not coincide exactly with the isotherm breakpoint. This is to be expected, because a solid phase can have 'locked-in' pressure gradients. If you arrive at these phases by compression, as is possible over a certain temperature range, then there will be regions at the interior of domains where the pressure is still equal to the freezing pressure, while at the boundaries it will be significantly above. For investigations of this sort the presence of a small amount of immiscible liquid impurity would help by distributing the pressure. Perfluorododecanoic acid is one possibility. It is available commercially, and is known to phase separate at a mesoscopic scale[6].

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