Monolayer polar

The preceding evidence for orientation at the interface plus the considerations given in Section III-3 make it clear that the polar end is directed toward the water and the hydrocarbon tails toward the air. On the other hand, the evidence from the study of the Gibbs monolayers (Section III-7) was that the smaller molecules tended to lie flat on the surface. It will be seen that the orientation... 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

Resonance Raman reflection spectroscopy of monolayers is possible, as illustrated in Fig. IV-14 for cetyl orange [157]. The polarized spectra obtained with an Ar ion laser allowed estimates of orientational changes in the cetyl orange molecules with a. 

Fig. IV-13. Example of a p-polarized reflection spectrum from Ref. [154] for a stearyl alcohol monolayer on water. The dashed line is the baseline to be subtracted from the spectra. [Reprinted with permission from Joseph T. Buontempo and Stuart A. Rice, J. Chem. Phys. 98(7), 5835-5846 (April 1, 1993). Copyright 1993, American Institute of Physics.]...

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

If an ionic surfactant is present, the potentials should vary as shown in Fig. XIV-5c, or similarly to the case with nonsurfactant electrolytes. In addition, however, surfactant adsorption decreases the interfacial tension and thus contributes to the stability of the emulsion. As discussed in connection with charged monolayers (see Section XV-6), the mutual repulsion of the charged polar groups tends to make such films expanded and hence of relatively low rr value. Added electrolyte reduces such repulsion by increasing the counterion concentration the film becomes more condensed and its film pressure increases. It thus is possible to explain qualitatively the role of added electrolyte in reducing the interfacial tension and thereby stabilizing emulsions. 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Figure Bl.5.8 Random distribution of (a) non-chiral adsorbates that gives rise to a surfaee having effeetive oo m-synnnetry (b) ehiral moleeules that gives rise to effeetive oo-synnnetry. (e) SH intensity versus the angle of an analyser for a raeemie (squares) and a non-raeemie (open eireles) monolayer of ehiral moleeules. The pump beam was p-polarized the SH polarization angles of 0° and 90° eorrespond to s- and p-polarization, respeetively. (From [70].)...

Zhao X L, Ong S W and Eisenthal K B 1993 Polarization of water-molecules at a charged interface. Second harmonic studies of charged monolayers at the air/water interface Chem. Phys. Lett. 202 513-20... 

The so-called self-assembly technique has its origin in 1946, when a paper was published by Bigelow et a] [116] and tluis is slightly younger tlian tlie LB teclmique. The autliors noted tliat a hydrophilic surface exposed to an amphiphilic compound dissolved in a non-polar solvent induces tlie amphiphilic material to fonn a monolayer on it. 

Bigelow WC, Piokett D L and Zisman W A 1946 Oleophobio monolayers. 1. Films adsorbed from solution in non-polar liquids J. Colloid Interface Scl. 1 513-38... 

It was estabhshed ia 1945 that monolayers of saturated fatty acids have quite compHcated phase diagrams (13). However, the observation of the different phases has become possible only much more recendy owiag to improvements ia experimental optical techniques such as duorescence, polarized duorescence, and Brewster angle microscopies, and x-ray methods usiag synchrotron radiation, etc. Thus, it has become well accepted that Hpid monolayer stmctures are not merely soHd, Hquid expanded, Hquid condensed, etc, but that a faidy large number of phases and mesophases exist, as a variety of phase transitions between them (14,15). 

The stmcture of SAMs is affected by the si2e and chemical properties of surface functionahties. Indeed, the introduction of any surface functionaUty reduces monolayer order. The impetus toward disorder may result from stericaHy demanding terminal groups, eg, —O—Si(CH2)2(C(CH2)3) (245) and —C H N Ru(NH2)5 (345,346), or from very polar surface groups, eg, OH, COOH, etc. In both cases, the disorder introduced may be significant and not confined only to the surface. 

Fig.4.53. Experimental and simulated PM (polarization modulated) IRRAS spectra of single monolayers of (A) PEG and (B) K(LK)7 at the air-water interface. The surface pressure was 20 mN m [4.281],...

Fig. 4.54. IR reflection spectra from an ODS monolayer on silicon for s- and p-polarized radiation at different incident angles ft Symmetric (s), asym-...

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

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