Monolayers formed from carboxylic acids

In recent years study of the absorption of small molecules on well characterised single crystal surfaces has attracted many research workers. Here, however, we will only be concerned with relatively large molecules such as, for example, long chain fatty acids. In 1946 Bigelow et al. [16] showed that a carboxylic acid dissolved in a non-polar solvent will adsorb on to a hydrophilic surface immersed in this solvent and that, 

In 1985 Allara and Nuzzo [354, 355] published the results of an extensive investigation in which adsorption took place on to an aluminium oxide layer formed on a film of aluminium deposited in vacuo on to a silicon wafer. Various carboxylic acids were dissolved in high purity hexadecane and allowed to adsorb from this solution on to the prepared aluminium oxide surface. The monolayers so formed were examined by ellipsometry and infrared spectroscopy. Contact angle measurements were made on the monolayer surfaces and radioactive labelled (tritiated) compounds were employed to study the interchange of adsorbed molecules with those in solution. Various other techniques of less immediate relevance to our present interests were also employed and reference to these two papers should be made for further particulars. Aluminium 

It was found that, for carboxylic acids containing 12 or more carbon atoms, ellipsometry data indicated a him thickness which would be expected from nearly vertical orientation of the hydrocarbon chains and relatively tight packing. For shorter chain lengths it was not possible to form stable monolayers. It was shown that the kinetic processes involved in layer formation can take up to several days. Infrared studies lead to three important conclusions. 

The carboxylic groups were bonded to the aluminium oxide by the transfer of a proton to the oxide substrate. 

There was a randomness in the arrangement of these bonds which might indeed be expected from the amorphous nature of the substrate. 

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Monolayers are best formed from water-insoluble molecules. This is expressed well by the title of Gaines s classic book Insoluble Monolayers at Liquid-Gas Interfaces [104]. Carboxylic acids (7-13 in Table 1, for example), sulfates, quaternary ammonium salts, alcohols, amides, and nitriles with carbon chains of 12 or longer meet this requirement well. Similarly, well-behaved monolayers have been formed from naturally occurring phospholipids (14-17 in Table 1, for example), as well as from their synthetic analogs (18,19 in Table 1, for example). More recently, polymerizable surfactants (1-4, 20, 21 in Table 1, for example) [55, 68, 72, 121], preformed polymers [68, 70, 72,122-127], liquid crystalline polymers [128], buckyballs [129, 130], gramicidin [131], and even silica beads [132] have been demonstrated to undergo monolayer formation on aqueous solutions. 

Stearic acid, C17H35COOH, may be used to provide a monolayer to reduce the rate of water loss by evaporation from the surface of a reservoir. The molecule floats at the surface with the hydrophobic hydrocarbon chain upward and the hydrophilic carboxylic acid downward, and in so doing covers roughly a circular area of radius 2.82 A (0.282 nm) per molecule. How many kilograms of stearic acid would be required to form a solid monolayer over the surface of the Elk Lake reservoir, which covers an area of 289 acres (1 acre = 4,047m )... 

Polymeric association of different molecules through multiple hydrogen bonding has been used for the formation of non-liquid-crystalline bulk solids [12], fibrous solids for gelation in solvents [119], and as monolayers [120], Simpler H-bonding such as the interaction between carboxylic acid and pyridine has also been shown to be useful for one- or two-dimensional aggregates in solid states [121-124], For example, a one-dimensional polymeric complex from an A-B type monomer is formed in crystalline solids [121],... 

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