Unimolecular films

On the assumption of the uniinolecular character of the Gibbs film we can, as Langmuir has indicated, obtain some idea as to the size of the solvent molecules. If the thickness of the film be t and the bulk concentration G, the quantity of solute which has left the film, on the hypothesis of a unimolecular film of pure solvent existing at the surface, will be Gr this is equivalent to the negative surface excess or... 

Spreading may occur by a process of surface solution or by vaporisation from the lens and condensation on the water surface. This latter, indeed, is the only method of spreading on a solid. The adsorption of vapours from a liquid onto a second liquid surface to the point of equilibrium results in the formation of a primary (unimolecular) film and this is doubtless followed in many cases by secondary film formation or a banking up of the layers on the primary film to a thickness which may be several hundred molecules thick. The conditions which have to be fulfilled are two (1) the surface tension of the film whether primary or secondary o- must attain the value... 

Inflvsnce of temperature on time required to cover an area of 56 9 sq. cm. of water with a unimolecular film under zero compression of myristic acid. Length ofwaier acid interface 0 251 cm. 

It may be suggested that carbon monoxide, is adsorbed extremely tenaciously by platinum and that b represents a unimolecular film covering the platinum on the top of which a second film may be built up. 

We have so far discussed two possibilities alternative to the assumption of the existence of multimolecular films and enquiry is necessary to examine how far all existing data can be reconciled to the assumptions either of a capillary surface or a surface variable in accessibility. It must be admitted that these views do not seem adequate to explain all cases of adsorption. Thus in the data presented by Evans and George it is rather singular that the amount of nitrogen adsorbed on glass should be equal to the computed unimolecular film whilst the other easily liquefiable gases exceed this thickness. Langmuir s data on the adsorption of carbon... 

Schulman has shown that tannic acid in the solution below a protein film greatly alters the surface potential, probably through combination with the protein film. Here apparently we have the phenomenon of tanning a unimolecular film ... 

Addition of hydrogen to compounds with multiple bonds, such as the formation of ethane from ethylene or acetylene. Rideal1 found 137° C. to be the optimum temperature for the hydrogenation of ethylene, and considers the mechanism of the process to be explicable on the hypothesis of Langmuir 2 that the reaction proceeds in a unimolecular film on the surface of the nickel catalyst. 

Surface-Active Substances Surface Adsorption. Many organic solutes in aqueous solution, particularly polar molecules and molecules containing both polar and nonpolar groupings, considerably reduce the surface tension of water. Such solutes tend to accumulate strongly at the surface where, in many cases, they form a unimolecular film of adsorbed molecules. 

The phenomenon described with benzene on water occurs for most organic liquids of lower molecular weight. However, most experimental work with unimolecular films has involved molecules of higher molecular weight involving at least ten carbon atoms. These substances are often solids at room temperature and are introduced to the water surface as a solution in a volatile organic solvent. For example, a unimolecular film of octadecanoic acid can be spread on water by... 

Fig. 8.20 Schematic diagram showing the structural changes in a unimolecular film going from the liquid expanded state (Li) through the intermediate state (1) to the liquid condensed state (L2). The hydrophilic head group is designated by the circle at the end of a long hydrophobic hydrocarbon chain.

Unimolecular films provide a rich area for study in interfacial chemistry which has only been briefly introduced here. Other investigations include the kinetics of film formation, the viscosity of the monolayer, and measurements of surface potential changes [Gl]. Unimolecular films in condensed format may be removed from the surface of the liquid on which they are formed using a glass plate or other suitable solid substrate. The hydrophilic ends of the molecules in the film remain in contact with the solid surface, and the hydrophobic ends, with the air. This type of unimolecular film, which is called a Langmuir-Blodgett film, is also the subject of considerable research interest. 

The spreading pressure of many substances forming unimolecular films on water rises diarply when the film area is reduced to a certain value. For substances whose molecules consist of a paraffin chain tail with an active head such as —COgH, —CH2.NH2, —CO.NH2, —CO.CHg, — NH.CO.CHj, the sudden rise occurs at about 8 x 10 mole/cm regardless of the length of the tail or the nature of the head, (Adam, Physics and chemistry of surfaces", Oxford University Press, 1941, p. 50). 

From the behaviour of the unimolecular films we conclude that their cross-sectional area is determined by that of the hydrocarbon tails. By combining this information on cross-sectional area with the density of paraffins, we obtain a rough estimate of N. 

The interest of this example is that the experimental data used can be obtained by such simple experiments. The method of calculation is essentially that used by Rayleigh (Phil. Mag. 1899, 48, 337), who concluded from his experiments that the linear dimensions of an oil molecule are about 1 m that is to say about 10 A. Nowadays since the value of IV is known accurately, the calculation is performed in reverse to determine the cross-sections and heights of the molecules in the unimolecular film. 

The formation of monolayers of cerotic, be-henic, and stearic acids adsorbed from cis-decahydronaphthalene and w-hexadecane solutions onto glass was studied by electron microscopy. Platinum preshadowed carbon replicas of the monolayered glass surfaces were examined in a JEM 6-A electron microscope. The acids adsorb initially in discrete patches the average patch diameter depends on the length of the acid molecule, becoming smaller for shorter acids. The area covered by the unimolecular film increases as the adsorption time or concentration of adsorbate solution increases. Contact angle measurements on partially monolayered surfaces show the contact angle to be dependent upon the fraction of substrate covered. Direct evidence of metastable incorporation of hexadecane in acid mono-layers was obtained. 

By measuring the adsorption of gases on plane surfaces such as sheets of mica, Irving Langmuir found that it ceases when the surface is covered with a unimolecular film (one molecule thick), and if is the pressure, surface covered, and b and n> 1 are constants ... 

Hirai, Y. Haruta, M. Nishimura, Y Matsuda, H. Nakagiri, T. Electrooptical displays containing organic unimolecular films. Jpn. Kokai Tokkyo Kobo JP 60241273,1985. 

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