Esters, monolayers

The most common situation studied is that of a film reacting with some species in solution in the substrate, such as in the case of the hydrolysis of ester monolayers and of the oxidation of an unsaturated long-chain acid by aqueous permanganate. As a result of the reaction, the film species may be altered to the extent that its area per molecule is different or may be fragmented so that the products are soluble. One may thus follow the change in area at constant film pressure or the change in film pressure at constant area (much as with homogeneous gas reactions) in either case concomitant measurements may be made of the surface potential. 

Following the successful examination of stearoylserine methyl ester monolayers by a variety of techniques, a detailed study of the effects of several subtle changes in the headgroup has been performed by Heath (1991 Heath and Arnett, 1992). Figure 28 compares the structural variations as the hydroxyl group of SSME is replaced by a thiol in stearoylcysteine methyl ester (SCME) two SCME chains are linked through their sulfurs to produce... 

Cysteamine was used to couple redox-active carboxylalkyl-4,4 -bipyridinium salts to the gold surface . The nonordered monolayer assembly was then transformed into a densely packed monolayer with 1-hexadecanethiol and cyclic voltammetry of the surface bound viologen was performed. The electron transfer rate constants to the bipyridinium sites depended on the alkyl chain length Abridging the redox site to the electrode. Electron transfer rate constants followed the Marcus theory. Cysteic-acid-active ester monolayers chemisorbed on gold were used to electrode-immobilize the protein glutathione reductase, then a bipyridinium carboxylic acid was condensed onto the enzyme in the presence of urea to wire the protein towards electrochemical reduction (Figure 6.26). 

Figure 10 External reflection infrared spectra of behenic acid methyl ester monolayers on water, measured with s-polarized and p-polarized light at different incidence angles. (Reprinted with permission from Flach CR, Gericke A, and Mendelsohn R (1997) Quantitative determination of molecular chain tilt angles in mono-layer films at the air/water interface infrared reflection/absorption spectroscopy of behenic acid methyl ester. Journal of Physical Chemistry B101 58-65 American Chemical Society.)...

Angle-resolved XPS (AR-XPS) measurements were conducted at different takeoff angles (detection angle), namely 15,45, 75, and 90° with respect to the surface plane, to obtain depth-dependent information and to determine the o decylphosphoric acid ester monolayer thickness deposited on the tantalum ojdde substrate. 

Figure 10 Images of domains of monolayers of the o-enantiomer, L-enantiomo-, and racemic mixture of A-stearoylserine methyl ester clearing showing chiral discrimination effects. The domains of the enantiomers display unique curvature, and the domain of the racemic mixture shows featnres with both senses of curvature and hence evidence for chiral segregation. Reproduced from Ref. 62. American Chemical Society, 2003, and the figure caption reads as follows Chiral discrimination of the condensed-phase domains of N-stearoyl serine methyl ester monolayers spread on pH 3 water, (a) D-enantiomer (b) L-enantiomer (c) and (d) 1 1 dl racemate. Image size 80 x 80 pm.

Constmction of multilayers requires that the monolayer surface be modified to a hydroxylated one. Such surfaces can be prepared by a chemical reaction and the conversion of a nonpolar terminal group to a hydroxyl group. Examples of such reactions are the LiAlH reduction of a surface ester group (165), the hydroboration—oxidation of a terminal vinyl group (127,163), and the conversion of a surface bromide using silver chemistry (200). Once a subsequent monolayer is adsorbed on the "activated" monolayer, multilayer films may be built by repetition of this process (Fig. 8). 

Shinkai [15] concluded that p-zert-butyl calix[n]ar-ene tetra esters form stable monolayers at the air-water interface and the metal responds, therein, quite differently from that in solution. They reported that examination of the metal template effect on the conformer distribution established that when the metal cation present in the base used serves as a template, the cone conformer results are predominant [16]. Hence, Na in... 

The generation of semicondnctor nanoparticles below spread monolayers was first reported by Fendler [134]. Poly(styrenephosphonate diethyl ester) and its 1 1 copolymer with... 

The use of a polymer monolayer to form LB films to be gas treated to form CdS nanoparticles has been reported [174]. The polymer use was poly (maleic anhydride) ester-... 

Lipoproteins. A lipoprotein is an endogenous macromolecule consisting of an inner apolar core of cholesteryl esters and triglycerides surrounded by a monolayer of phospholipid embedded with cholesterol and apoproteins. The functions of lipoproteins are to transport lipids and to mediate lipid metabolism. There are four main types of lipoproteins (classified based on their flotation rates in salt solutions) chylomicrons, very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL). These differ in size, molecular weight, and density and have different lipid, protein, and apoprotein compositions (Table 11). The apoproteins are important determinants in the metabolism of lipoproteins—they serve as ligands for lipoprotein receptors and as mediators in lipoproteins interconversion by enzymes. 

The surface shear viscosity of a monolayer is a valuable tool in that it reflects the intermolecular associations within the film at a given thermodynamic state as defined by the surface pressure and average molecular area. These data may be Used in conjunction with II/A isotherms and thermodynamic analyses of equilibrium spreading to determine the phase of a monolayer at a given surface pressure. This has been demonstrated in the shear viscosities of long-chain fatty acids, esters, amides, and amines (Jarvis, 1965). In addition,... 

Ai-Stearoylamino acids and their methyl esters were synthesized from enantiomeric and racemic forms of tyrosine, serine, alanine, and tryptophan (Fig. 16). Analogs of these molecules were investigated initially over 30 years ago by Zeelen and Havinga, who found stereochemical differentiation in the monolayer HjA isotherms of these materials (Zeelen, 1956 Zeelen and Havinga, 1958). We have extended this study using more sensitive Langmuir balances, a wider array of dynamic and equilibrium techniques, and the A-stearoyl methyl esters of the amino acids (Harvey et al., 1989 Harvey and Arnett, 1989). 

In addition, it should be noted that none of the compression and expansion cycles for these films are coincident. The considerable hysteresis exhibited during the compression/expansion cycle is evidenced at every compression/expansion rate investigated, and is indicative of a stereoselective kinetic process that must occur upon film compression. Table 3 gives the monolayer stability limits of the amino acid methyl ester films as defined by... 

The instability of these chiral monolayers may be a reflection of the relative stabilities of their bulk crystalline forms. When deposited on a clean water surface at 25°C, neither the racemic nor enantiomeric crystals of the tryptophan, tyrosine, or alanine methyl ester surfactants generate a detectable surface pressure, indicating that the most energetically favorable situation for the interfacial/crystal system is one in which the internal energy of the bulk crystal is lower than that of the film at the air-water interface. Only the racemic form of JV-stearoylserine methyl ester has a detectable equilibrium spreading pressure (2.6 0.3dyncm 1). Conversely, neither of its enantiomeric forms will spread spontaneously from the crystal at this temperature. 

---