Characterisation of monolayers recent techniques

The last decade has seen the introduction of several new characterisation techniques which have been of major assistance in understanding the structure of monolayers at a molecular level. The most important of these has been the use of synchrotron radiation to obtain diffraction patterns from films at the air/water surface. In principle it would always have been possible to use X-rays for this purpose but the high intensity and highly monochromatic nature of the radiation from a synchrotron source has made this technique far easier to use. A selection of recent papers based on this technique is given [79-88], not all of which refer to simple fatty acids. The information available from such experiments is of two distinct kinds, though, in several studies, both kinds of information have been obtained. 

Monochromatic radiation impinges on the water surface at an angle, a, to this surface and radiation which undergoes specular reflection leaves at an angle, a2, where a, and a2 are adjusted to be equal. The intensity of the reflected radiation as a function of a gives information about the variation of the electron density of the system in the direction normal to the surface. 

Monochromatic radiation is again directed at the water surface at a small angle, a, less than the critical angle for total external reflection. The intensity of the reflected radiation is explored as a function of the 

Bohanon et al. [86] studied heneicosanoic acid (which contains 21 carbon atoms) and Lin et al. [87] studied this material with particular reference to the effect of pH and the presence of divalent cations in the subphase. The former authors made use of in-plane diffraction (method 2 above) and obtained first order and second order diffraction peaks. They were able to show that, at high pressures ( r=35 mN m-1), at low pH (pH = 2) and at temperatures in the region of 0-5 °C, the material packs into a distorted hexagonal structure with the tilt towards the nearest neighbours. However, in the region 5-10°C the tilt is towards the next nearest neighbours. In the latter study [87] in-plane diffraction was studied as a function of pH and the presence of Ca2+ or Cu2+ in 

Recently Kenn et al. [89] have studied films of docosanoic acid and have used their results to construct a phase diagram for this material which they believe to be representative for most fatty acids. This work will be discussed in the next section. 

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