Compressed monolayers

The second way of preparing L-B monolayer structures, the horizontal lifting method, was introduced by Langmuir and Schaefer. In this method, a compressed monolayer first is formed at the water-air interface, and a flat substrate is then placed horizontally on the monolayer film. When the substrate is lifted and separated from the water surface, the monolayer is transferred onto the substrate, as depicted in Fig. 15(d). 

FIG. 1 Schematic showing the preparation of Langmuir films of latex particles at the air-water interface (a) Spreading of the latex and formation of an expanded monolayer (b) formation of the compressed monolayer. 

Given such evidences of nonthermodynamic behavior of compressed monolayers, it was important to test film stability at various points along the ir-A isotherms for the normal rate of slow compression. The racemic film maintained a steady film pressure over at least 10 min after the barrier drive was stopped, showing little or no tendency to relax from the compressed state to one of lower energy. The enantiomer film in contrast showed a tendency to relax steadily from a compressed metastable state to a more stable and better packed condition approaching the equilibrium spreading pressure. 

In stationary phases commonly used in RPC the picket fence configuration is not likely because of the relatively low surface concentration of th ligates, oi. < S p.m6l/m. This value is much smaller tl n the surface concenlrutbn in compressed monolayers of compounds ccjfntaining alkyl chains with small polar terminal groups-(hydroxyl, carboxyl, amino groups) that was found to be 8 )ittiol/m at the Water sUrftice (///). 

Fig. 3.5.2 Schematic representation of the vertical dipping method of preparing LB films from compressed monolayers of fatty acids on aqueous subphases of divalent metal ions.

AFM images were obtained for films constructed, on freshly cleaved mica, from compressed monolayers of DDAB on a subphase of HMP-stabilized CdS (81). Particles, with dimensions of 8 3 nm, were seen to be evenly distributed. The determined area of 58 nm2/particle coincided well with the area per molecule determined for DDAB from its spreading isotherm, implying 1 1 particle/surfactant stoichiometry. This result is puzzling given that freshly cleaved mica is hydrophilic and therefore any particles would be buried under a layer of the hydrophobic tails of the DDAB and unaccessable to the AFM tip. 

Significantly, equilateral triangular PbS crystals have been grown under compressed monolayers in the same orientation (see Fig. 114). 

In 1925, E. Gorter and F. Grendel (J. Exp. Med. 41, 439) reported measurements in which they extracted lipid from red blood cell membranes with acetone, spread the lipids as a monolayer, and measured the area of the compressed monolayer. They then estimated the surface area of an erythrocyte and calculated that the ratio of the lipids (as a monolayer) to the surface area of the red blood cell was 1.9-2.0. More modern experiments gave the following each erythrocyte membrane contains 4.5 x 10 16 mol of phospholipid and 3.1 x 10-16 mol of cholesterol. 

Fig. 1 shows the surface pressure-area (n-A) isotherms of stearic acid monolayers on pure water and ion-containing subphases, respectively. The presence of bivalent cations in the subphase gives rise to condensation of the monolayers. On the Ag+-containing subphase, the isotherm shows extremely compressed characteristics with a limiting area of 0.12nm2/molecule, much smaller than the cross-sectional area of 0.20 nm2 of a saturated hydrocarbon chain, which suggests the formation of a three-dimensional structure of the compressed monolayer [48]. 

Monolayers and LB Films - Controllable Layered Assembly Some types of amphiphile form monolayer structures on the surface of water. These compressed monolayers can be transferred onto a sohd support in a layer-by-layer manner. This permits well-oriented multilayers to be created with defined numbers and sequences of layers. 

Figure 7. Adsorption of dissolved surface-active material onto the surface of an air bubble rising through sea water. (A) The upstream region (B) rough dividing line between compressed and non-compressed monolayer (C) compressed monolayer (D) bubble wake.

FK5.6.I9 Schematic representation of the structural formation and order-disorder transition for photoactive LBK showing (A) the compressed monolayer on the water surface with densely packed chromophore side chains oriented into the gas phase and the polymer backbone facing the water surface, and (B) LBK transfer from the water to a solid support, resulting in well-ordered smetic-tike (bilayered) multilayer assemblies. (C) After phcnoinduced trars to cts isomerization, a largely disordered struaure is obtained and the layered structure is completely lost (reproduced from reference 72 with permission from Wifey-VCH). 

Horizontal transfer (Schaefer s method). Another technique to prepare structures with LB (multi)layers is named after Schaefer [17]. This method is useful for depositing rigid films which can be described as two-dimensional solids. First, a compressed monolayer is established at the water-air interface. Subsequently a flat substrate is brought horizontally into contact with the film (figure C2.4.5). When the substrate is lifted and separated from the water surface a monolayer is transferred to the substrate while (theoretically) maintaining the molecular order. 

Figure 15. Out-of-plane tilt angle as a function of temperature for N2 on graphite. Circles nuclear resonance photon scattering of a compressed monolayer of coverage 1.05 0.02 [241]. Crosses molecular dynamics simulations of a complete monolayer [341]. (Adapted from Fig. 2 of Ref. 241.)...

Resonance photon scattering from N2 monolayers on Grafoil at 78 K has been used to determine the tilt of the molecular axes relative to the surface [240]. It was found that the molecules are preferentially aligned parallel to the graphite plane from near monolayer coverage down to about 0.8 monolayers, whereas no preferred orientation was obtained for compressed monolayers at a coverage of 1.5. 

Some of the results obtained for the commensurate herringbone structure are discussed in Section in.E because they served many authors as starting and test cases for calculations of compressed monolayers and bilayers. In particular, we refer to the study [19] of the lattice dynamics of the herringbone structure summarized in Table III. 

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