Lateral interactions with carboxylic acids

Finally, even if these criteria are satisfied, there remains the question of whether the product will adhere to form a film or just precipitate homogeneously in the solution. This is the most difficult criterion to answer a priori. The hydroxide and/or oxy groups present on many substrates in aqueous solutions are likely to be quite different in a nonaqueous solvent (depending on whether hydroxide groups are present or not). Another factor that could conceivably explain the general lack of film formation in many organic solvents is the lower Hamaker constant of water compared with many other liquids this means that the interaction between a particle in the solvent and a solid surface will be somewhat more in water than in most other liquids (see Chapter 1, van der Waals forces). From the author s own experience, although slow precipitation can be readily accomplished from nonaqueous solutions, film formation appears to be the exception rather than the rule. The few examples described in the literature are confined to carboxylic acid solvents (see later). 

This is mainly on account of the electron-withdrawing ruthenium(in) centre destabilising the protonated nitrogen centre. This is exactly parallel to the effect of placing more conventional electron-withdrawing substituents on a carboxylic acid (c.f the pKa of acetic versus that of trifluoroacetic acid). We will observe in a later section that this effect may be modified by 7i-interactions with the ligand. 

The data points to the importance of two parameters, that of adhesive interactions between adsorbate and surface, and lateral interactions between adsorbate molecules on the surface. In general, the free energy of adsorption (at zero coverage) is of the order of 10-30 kJ mol for alcohols, carboxylic acids and esters on ferrous surfaces, consistent with hydrogen bonding between surface and adsorbate. 

In the case of boundary layer lubrication, in which the adsorption of mono-molecular films is required, the best protection is provided by materials such as fatty acids and soaps that can adsorb strongly at the surface to form a solid condensed film. Less durable but effective protection can be obtained with polar groups such as alcohols, thiols, or amines. The least effective protection is obtained with simple hydrocarbons that adsorb more or less randomly and through dispersion forces alone. For adsorbed monomolecular films, best results are obtained when the hydrocarbon tail has at least 14 carbons. In some cases fluorinated carboxylic acids and silicones may provide a lower initial coefficient of friction, but their weaker lateral interaction sometimes results in a less durable surface film that melts at a lower temperature, ultimately resulting in less overall protection. If a polar lubricant can form a direct chemical bond to the surface, as in the formation of metal soaps, even better results can be expected. 

One of the first applications of flow adsorption microcalorimetry has been the study of interactions of n-heptane solutions of long chain alcohols, carboxylic acids, and amines with metal surfaces and metal oxides. Strong correlation was found between the heats of adsorption of their long chain compounds and anti-wear action. Later work was extended to the study of adsorption on freshly formed metal surfaces, which behaved quite differently from the metal oxides [46]. Freshly formed... 

---