Deposited Monolayers and Multilayer Films

With modifications in the dipping technique, it is possible to produce multilayer films with each layer having the same orientation (head-to-tail or X films Fig. 8.21c). It is usually found that the Y films are more stable than 

FIGURE 8.21. Insoluble monolayer films can, with careful work, be deposited onto solid substrates from the liquid surface. The so-called Langmuir-Blodgett films will usually have alternating molecular orientations (a) first-layer deposition with the head (or tail) toward the solid substrate and (h) the second-layer head-to-head or tail-to-tail in a Y film orientation. Under some conditions it is possible to produce an X film with head-to-tail orientation (c). 

The adsorption of deposited monolayer films can be very tenacious, especially when the solid substrate can react with the deposited material. Carbox-yhc acid monolayers deposited on metal or metal oxide surfaces, for example, almost certainly form metal soaps that are extremely difficult to remove by any means other than direct chemical etching. For example, such strongly adsorbed films are very useful in friction and wear studies of boundary lubrication (see Chapter 18). 

Monolayer films on solid substrates need not necessarily be deposited from a monolayer system. They can also be deposited directly from solution or a melt of the material to be deposited, or by a vapor deposition process. Such films are generally referred to as self-assembled monolayers. There are several possible advantages to such monolayers, mostly associated with simplification of the process (no need to be concerned with depositing the monolayer on a liquid surface, maintaining a constant surface pressure, problems of surface contamination, etc.) 

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