Chain orientation, monolayers

Salts of fatty acids are classic objects of LB technique. Being placed at the air/water interface, these molecules arrange themselves in such a way that its hydrophilic part (COOH) penetrates water due to its electrostatic interactions with water molecnles, which can be considered electric dipoles. The hydrophobic part (aliphatic chain) orients itself to air, because it cannot penetrate water for entropy reasons. Therefore, if a few molecnles of snch type were placed at the water surface, they would form a two-dimensional system at the air/water interface. A compression isotherm of the stearic acid monolayer is presented in Figure 1. This curve shows the dependence of surface pressure upon area per molecnle, obtained at constant temperature. Usually, this dependence is called a rr-A isotherm. 

As seen from Chapter 2, adsorbed molecules often form monolayers with chain orientational structures in which the chains with identically oriented molecules alternate (Fig. 2.4). Consider the Davydov splitting of vibrational spectral lines in such systems. Let molecular orientations be specified by the angles 6> and spherical coordinate system with the z-axis perpendicular to the lattice plane ... 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

Curve 3 in Figure 7.14 applies primarily to amphipathic species. Most long-chain amphipathic molecules are insoluble unless the hydrophobic alkyl part of the molecule is offset by an ionic head or some other suitably polar head such as a polyethylene oxide chain, — (CH2CH20)n—. Like their insoluble counterparts, these substances form an oriented monolayer even at low concentrations. Figure 7.15 shows some actual experimental plots of type 3 for the ether that consists of a dodecyl chain and a hexaethylene oxide chain (n — 6) in the general formula just given. Example 7.4 illustrates the application of the Gibbs equation to these data. 

Arden and Fromherz [51-53] have reported elegant work involving mono-layers of surfactants transferred to electrodes by the Langmuir-Blodgett method. Some results are shown in Figure 28.10. Two separate cationic dyes, thiacyanine (A) and oxacyanine (D), were synthesized with C18 alkyl chains, so that they would form oriented monolayers on water-air interfaces. By standard techniques, these were transferred alternatively or in sequence to n-ln203 electrodes, as... 

By a consecutive downstroke into the subphase through the floating monolayer a second layer can be transferred, with the alkyl chains oriented towards the solid substrate in a tail-to-tail configuration. 

Figure 4.9 Adsorption of surface-active molecules as an orientated monolayer at air-water and oil-water interfaces. The circular part of the molecules represents the hydrophilic polar head group and the rectangular part represents the non-polar hydrocarbon tail. At the air-water interface, the hydrocarbon chains will tend to lie horizontally on top of the water surface at low coverage, but will tend to assume more upright configurations at high coverage (see page 103)...

We suggested, as a first approximation to assume that the transition dipole moments of both the methyl and methylene vibrations are equal on a per hydrogen basis, and estimated the orientation of the chains in monolayers of 1, and also of OTS by consideration of the measured vs(CH2)A s(CH3) intensity ratio. 

The results for the calculation of the orientation distribution for the hydrocarbon chains in the transferred monolayer films are presented in Figure 8. As is the case with the orientation distribution of the in-situ monolayers, the transferred films have a similar tilt angle in the expanded and phase transition regions. For the transferred monolayer, however, the tilt angle is in the range 35-40° from the surface normal, a much more oriented monolayer than the calculations indicate for the in-situ film (Figure 5). Figure 8 also shows that the condensed phase transferred monolayers are more oriented than those films transferred in the LE and LE-LC... 

To take a closer look at the area per molecule in the condensed phase of the diblock monolayers, the x-axis of Fig. 4.14a is expanded and shown in Fig. 4.14c. The extrapolated value of the area per molecule and the theoretically expected value for the area are listed in Table 4.5. To calculate the theoretically expected area for the hybrid block copolymers in the condensed phase, it is assumed that the stearate end groups are extended into the air perpendicular to the air-water interface as shown schematically in Fig. 4.14d. This assumption is based on the behavior of pure stearic acid, which forms ordered monolayers with the alkyl chains oriented perpendicular to the air-water interface. The area per molecule for stearic acid with this orientation is 20 A2, [113] Here, the theoretically expected area was calculated by multiplying the area per stearate molecule (20 A2) with the number of stearate groups present at the ends of the dendrimer block. PEO(2k)-S having no dendrimer block but a single... 

They also studied the interaction of stearic acid (Cig) monolayers at the air-water interface with bivalent cations (Cd2+, Pb2 +, Ca2+, Ba2+, Cu2+, Ni2+, and Zn2+) in aqueous subphase using the IRRAS technique [45-47]. However, the information on molecular orientation was limited due to the use of unpolarized IR radiation. Recently, the headgroup interaction and chain orientation in the monolayers of stearic acid on pure water and ion (Ag+, Co +, Zn2+, and Pb2+)-containing subphases have been investigated using the IRRAS technique [48]. 

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). 

Serpinet, using the inverse gas chromatography method, demonstrated the existence of oriented monolayers of long-chain hydrocarbons on silica gel surface [13], on the other hand Untz [31] showed that hydrocarbons also form solid condensed and liquid expanded monolayers on glycerol but not on the water surface. However, the addition of some amount of amphiphilic molecules to the hydrocarbon provokes the mixed monolayer formation on the water surface. The phase transition in such a monolayer occurs at the temperature higher than the melting point of bulk hydrocarbon. It also appeared that the monolayers characterized by 1 1 ratio of hydrocarbon to alcohol molecules were particularly stable [41]. 

Lipkowski and coworkers [328] have employed STM to study the spreading of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcho-line vesicles into the film at the Au(lll) electrode surface. During the initial stage, phospholipid molecules were adsorbed flatly with the acyl chains oriented parallel to the surface and assembled into an ordered monolayer similar to that formed by alkanes. Later, the molecules reoriented and the monolayer was transformed into a hemimicellar film. 

C and n-octadecane-1,2-H at a concentration of 0.35 molar in stearic acid and studied the coadsorption of stearic acid and octadecane on polished surfaces of silver, platinum, copper and iron. The films were prepared by retraction from the melt at 40 C (at room temperature the mixture was solid). The proportion of n-octadecane in the film was assayed by differential extraction with cyclohexane. The results of the investigation adequately demonstrate that n-octadecane coadsorbs with stearic acid but not necessarily as a mixed oriented monolayer. Some of the data indicate that more than a single layer is present on the surface. Thus the structure of the long-chain material on the surface may be open to conjecture, but that each constituent adsorbs and in what relative amount is directly determined by radioactive assay. 

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

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