Langmuir layers

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Tanford, C., Ben Franklin Stilled the Waves, Duke University Press, Durham, NC, 1989. (A nontechnical treatment. A historical perspective of Langmuir layers is intertwined nicely with a readable, popular introduction to surfactant films.)... 

Chapter 7 now includes an introduction to structural transitions in Langmuir layers and examples of applications of Langmuir and Langmuir-Blodgett films. 

Langmuir layer was repeatedly compressed and expanded until reproducible, equilibrium pressure-area (I1 A) curves were obtained. 

The dependence of the surface tension of a Langmuir layer containing a given amount of surfactant, as a function of its area, reveals many of the properties of the surfactant. (At constant temperature, such measurements are analogous to determining the P-V isotherm of a gas.) One device for making such measurements is the Langmuir balance, the essential principles of which are shown in Fig. 10. 

Consider first mixed Langmuir layers, built up from the two surfactants, si and s2. Both are insoluble, so for neither of them may be equated to /i (bulk). Such layers can only be made by spreading the mixture. Experimentally this offers some problems the right spreading fluid should be found, no pockets of one of the two should be formed (unless the system demixes spontaneously) and no non-equilibrium states should develop if one of the components spreads much faster than the other. Here, it will be assumed that the system obtained is fully relaxed, and that all other experimental problems are solved. 

Figure 2. SERS spectra of mitox deposited on TGF-340, covered by 1 (a), 3 (b) and 5 (c) Langmuir layers of cadmium behenate.

Heath and coworkers have recently reported the observation of a reversible, room-temperature metal-non-metal transition in organically-functionalized silver particle monolayers. A useful parameter for characterizing these monolayers is the quantity [d/D], where d is the interparticle separation, as measured between particle centers, and D is the particle diameter (c.f. Fig. 4). The ultraviolet-visible reflectance spectrum from a Langmuir layer of 40 A diameter silver particles, collected in-situ as the film is compressed, is shown in Fig. 13. Upon initial compression, the film becomes more reflective (see (1), (2), and (3) in Fig. 13). The final reflectance spectrum is similar to that reported for thin, metallic silver films ((4) and (5) in Fig. 13) and indicates conclusively that the Langmuir film has finally become metallic. 

Wang, J. Zou, L. Weissflog, W. Jakli, A. Mann, E. K. Anisotropy in Langmuir layers of a bent-core liquid crystal. Langmuir 2006, 22, 3198—3206. 

A currently very fashionable recipe for the preparation of well-controlled surface layers is the use of molecules with a reactive head-group amenable to undergo a covalent bond with a complementary chemical site on the surface of the substrate to be modified. This leads to the formation of so-called self-assembled monolayers SAM [1] with structural properties similar to Langmuir layers prepared from amphiphiles at the water-air interface [2]. Examples are silanes on oxide surfaces or thiols, and monosulfides or disulfides on noble metal surfaces (see also Chapter 7). 

---