Molecular orientation at the solid-liquid interface

The idea of molecular orientation at an interface was conceived by Benjamin Franklin who, in 1765. spread olive oil on a water surface and estimated the thickness of the resulting film as one ten-millionth of an inch. Lord Rayleigh [8] in England and Miss Pockels [9] in Germany established that the film was only one molecule thick. Langmuir introduced novel experimental methods which resulted in new conceptions regarding these films. 

Some investigators suggest that the effective area occupied by fatty acid molecules at solid-liquid interfaces is the same as that occupied by these molecules in Alms on water. The quoted areas for stearic acid, for example, ranges from 0.205 nm to 0.251 nm, the former being the area of closest packing for ellipses and the latter the area for free rotation [11,12]. 

However, a greater variation than this is expected for an immobile interface since the adsorbate is not constrained to take up any deAnite orientation. In adsorption on caibon blacks Kipling and Wright [13] suggest that stearic acid is adsorbed with the hydrocarbon chain parallel 

In early work on oxides [22,23], it was suggested that oleic acid and butyric acid adopted a perpendicular orientation on titania, as did stearic acid on aluminum hydroxide [24]. In these experiments it was not clear whether adsorption was physical or chemical in nature. This now seems an important distinction to draw, especially with basic solids. In chemisorption the orientation of the solute molecules generally presents no problem, as the functional group determines the point of attachment. Thus the long chain fatty acids are attached to the surface by the carboxyl group, - OOH, with the hydrocarbon chain perpendicular to the surface. 

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