Coating level mono-layer

In spite of the overall success of this particular scheme, there are some issues which need to be clarified especially when it comes to details of the structure of the electrode/monolayer/electrode crossings on an atomistic level [4]. In a recent review on the vapor deposition of metal atoms on organic monolayers [65], the complexity of the process and the subtle effects of the surface structure and composition on the outcome are illustrated. A reflection-absorption infrared spectroscopy study [66] of a system, which is similar to the actual cross-junction, suggests undamaged organic mono-layers with Ti coatings, but further research is needed for the complete characterization of these complex structures. 

There has been very little work on adsorption of stearic acid from aqueous solution, probably due to the low solubility of the acid. Suess has however made some measurements using carbon-14 labelled acid [16]. As with adsorption from organic solvent, a Langmuir type isotherm showing mono-layer adsorption was observed, however this was at a much lower level than for solvent adsorption. The reasons for this are not clear but were postulated to be due to adsorption as a hydrated complex salt. Presumably, this is converted to the stable coating layer during drying. 

The study of the adsorption of stearic acid onto calcium carbonate from alcohol solution by Ivanishchenko and Gladkikh [6] has already been mentioned. They also determined adsorption isotherms and the variation of ohmic resistance and water adsorption with coating level for shorter chain acids. Acids with ten and twelve carbon atoms appeared to form mono-layers on chalk with 1.5 x 10 moles g of filler while longer chain acids only required 0.9 x 10 moles g (unfortunately the surface area of the filler was not specified). With the shortest chain acid (Cjo), about three molecular layers were required to maximise hydrophobicity, but when more than 14 carbon atoms were present then only one mono-layer was needed. Their results also indicated that, for the shorter acids at least, the first molecular layer was packed differently from subsequent layers. 

As the concentration of the solute is increased, the specific amount adsorbed levels off to a constant value (Fig. 4-1). At the higher concentrations, the surface of the adsorbent is coated with a mono-molecular layer of the solute, and no further material may be adsorbed. One might say that the adsorbent was saturated. 

---