Multimolecular films

Natural macromolecular materials, which form multimolecular films around the disperse droplets of O/W emulsions. They are frequently called auxiliary emulsifying agents and have the desirable effect of increasing the viscosity of the dispersion medium. However, they often suffer from the disadvantages of being subject to hydrolysis and sensitive to variations in pH. 

The rate-controlling step in the growth of a multimolecular film is the transport of metal ions from the solution through the film. Diffusion and... 

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... 

Examination of the temperature gradients at the surfaces of heated metal in air at high pressures (ca. atmospheric) likewise reveals the existence of a thin biit multimolecular film in contact with the hot metal. 

In regions where each molecule covers an area approximately equal to its own area, tt is relatively constant. The size of these regions is highly dependent on the molecule making up the film. However, if the film is compressed beyond a certain point, the surface pressure increases dramatically, as shown on the left side of Figure 22.11. In these regions the surface film is forced into a multimolecular film, instead of a monomolecular film. The surface pressure thus represents the surface energy needed to force layers of molecules over each other. 

Figure 7.19 Sodium stearate molecules adsorbed on water-air Interface at high concentration (multimolecular film).

Bettelheim A, White BA, Murray RW (1987) Electrocatalysis of dioxygen reduction in aqueous acid and base by multimolecular films of electropolymerized cobalt tetra (orthoaminophenyl)porphyrin. J Electroanal Chem 217(271-286) 271-286... 

Reaction of adsorbed inhibitors In some cases, the adsorbed corrosion inhibitor may react, usually by electro-chemical reduction, to form a product which may also be inhibitive. Inhibition due to the added substance has been termed primary inhibition and that due to the reaction product secondary inhibition " . In such cases, the inhibitive efficiency may increase or decrease with time according to whether the secondary inhibition is more or less effective than the primary inhibition. Some examples of inhibitors which react to give secondary inhibition are the following. Sulphoxides can be reduced to sulphides, which are more efficient inhibitorsQuaternary phosphonium and arsonium compounds can be reduced to the corresponding phosphine or arsine compounds, with little change in inhibitive efficiency . Acetylene compounds can undergo reduction followed by polymerisation to form a multimolecular protective film . Thioureas can be reduced to produce HS ions, which may act as stimulators of... 

Modified electrodes. Where relevant, we have followed the recent lUPAC directive on the recommended list of terms for chemically modified electrodes (CMEs) [1]. A CME is thus an electrode made up of a conducting or semiconducting material that is coated with a selected monomolecular, multimolecular, ionic or polymeric film of a chemical modifier and that, by means of faradaic reactions or interfacial potential differences exhibits chemical, electrochemical and/or optical properties of the film . 

If the specific rate of evaporation v be very small and p large, evidently multimolecular layers or secondary film formation may readily be obtained. 

Similar conclusions may be drawn from the experiments of Hardy on lubrication. Hardy has obtained very convincing data in support of the hypothesis that on the adsorption of a vapour such as octyl alcohol by a metal surface, whilst the first layer is held very tenaciously the thickness of the film of vapour condensed on the metal surface which is in equiUbrium with the free surfeice of the liquid is certainly multimolecular in character and those layers forming the secondary film may be squeezed out by application of sufficient pressure. 

Figure 1 Electrocalalytic mechanism when a multimolecular layer film is present on the electrode...

Chemisorption requires direct contact between the chemisorbed molecule and the electrode surface as a result, the highest coverage achievable is usually a monomolecular layer. This may be contrasted with several of the methods to be discussed later that allow the electrode surface to be covered with thick films (i.e., multimolecular layers) of the desired molecule. In addition to this coverage limitation, chemisorption is rarely completely irreversible. In most cases, the chemisorbed molecules slowly leach into the contacting solution phase during electrochemical or other investigations of the chemisorbed layer. For these reasons, electrode modification via chemisorption was quickly supplanted by other methods, most notably polymer-coating methods. 

There are several reasons for the appeal of polymer modification immobilization is technically easier than working with monolayers the films are generally more stable and because of the multiple layers redox sites, the electrochemical responses are larger. Questions remain, however, as to how the electrochemical reaction of multimolecular layers of electroactive sites in a polymer matrix occur, e.g., mass transport and electron transfer processes by which the multilayers exchange electrons with the electrode and with reactive molecules in the contacting solution [9]. 

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