Simple Langmuir-Blodgett films formed from carboxylic acids

2 Simple Langmuir-Blodgett films formed from carboxylic acids 

Blodgett s original paper [13] on LB films was devoted to systems formed from fatty acids and there has been a strong tendency to return to the study of these materials, particularly by those workers who are interested in the basic physics of the subject. It is natural therefore that a discussion of LB films should start with these materials. The literature of this topic is very extensive and the reader is referred to the recent book edited by Roberts [119] for a discussion of earlier papers in this field. In Section 2.4 the study of these materials by X-ray diffraction was discussed. Such studies establish the fact that the order of LB films in the direction 

Recent relevant work has made use of transmission electron diffraction, the theory of which we discussed in Section 2.5. This technique has a very straightforward interpretation which, combined with improved experimental methods, has provided very precise results. Two papers are of particular importance in this context. Garoff et al. [137] studied monolayers of cadmium stearate. Their substrates were 2 nm layers of SiO coated on 10 nm layers of amorphous carbon on 200 mesh Ni elec- 

Whereas the disclination picture provides a convincing explanation of the properties of multilayers of fatty acids, being consistent with both the electron diffraction and optical evidence, it is not yet proven that, at room temperature, such systems are really in the hexatic state. It is equally probable that the existence of an initial hexatic monolayer on which subsequent layers are grown by epitaxy produces a material which is far from thermal equilibrium and has more of the nature of a glass state rather than of a mesophase. Indeed, the relative hardness of multilayers and their resemblance to true three-dimensional crystals of fatty acids tends to support this view. 

The behaviour reported above has to be reconciled with the other well known properties of multilayers of cadmium stearate and cadmium arachidate. One possible picture is that these materials deposit in a hex-atic structure which, for a multilayer, represents a metastable state. The upper part of the multilayer, where the molecules are free to move, tends 

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