Monolayer surface potential

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. 

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]. 

The primary classical method of study of Langmuir monolayers is clearly that of recording H-A isotherms. Another classical method applied to the study of Langmuir monolayers is the measurement of surface potential [8,9], which is sensitive to changes in the orientation and density of the molecular dipoles of the monolayer. In addition, surface potential fluctuations were clearly observed in the coexistence region of palmitic acid [35]. 

The popular applications of the adsorption potential measurements are those dealing with the surface potential changes at the water/air and water/hydrocarbon interface when a monolayer film is formed by an adsorbed substance. " " " Phospholipid monolayers, for instance, formed at such interfaces have been extensively used to study the surface properties of the monolayers. These are expected to represent, to some extent, the surface properties of bilayers and biological as well as various artificial membranes. An interest in a number of applications of ordered thin organic films (e.g., Langmuir and Blodgett layers) dominated research on the insoluble monolayer during the past decade. 

The measurement of change in the surface potentials of aqueous solutions of electrolytes caused hy adsorption of ionophore (e.g., crown ether) monolayers seems to he a convenient and promising method to ascertain selectivity and the effective dipole moments of the ionophore-ion complexes created at the water surface. 

The presence of adsorbed Langmuir monolayers may induce very high changes in the surface potential of water. For example, A% shifts attaining ca. -0.9 (hexadecylamine hydrochloride) and ca. + 1.0 V (per-fluorodecanoic acid) have been observed. " ... 

The physical meaning of the g" (ion) potential depends on the accepted model of ionic double layer. The proposed models correspond to the Gouy Chapman diffuse layer, with or without allowance for the Stern modification and/or the penetration of small counterions above the plane of the ionic heads of the adsorbed large ions [17,18]. The presence of adsorbed Langmuir monolayers may induce very high changes of the surface potential of water. For example. A/" shifts attaining ca. —0.9 (hexadecylamine hydrochloride), and ca. -bl.OV (perfluorodecanoic acid) have been observed [68]. 

The oscillation at a liquid liquid interface or a liquid membrane is the most popular oscillation system. Nakache and Dupeyrat [12 15] found the spontaneous oscillation of the potential difference between an aqueous solution, W, containing cetyltrimethylammo-nium chloride, CTA+CK, and nitrobenzene, NB, containing picric acid, H" Pic . They explained that the oscillation was caused by the difference between the rate of transfer of CTA controlled by the interfacial adsorption and that of Pic controlled by the diffusion, taking into consideration the dissociation of H Pic in NB. Yoshikawa and Matsubara [16] realized sustained oscillation of the potential difference and pH in a system similar to that of Nakache and Dupeyrat. They emphasized the change of the surface potential due to the formation and destruction of the monolayer of CTA" Pic at the interface. It is... 

The surface potential as function of particle area (AV A isotherm) is another indicator of the quality of the monolayer structure. The surface potential at the air-water interface changes as the film-forming molecules reorient themselves during the compression process. For a closely packed monolayer, the surface potential is directly proportional to the surface dipole moment (/r ) by [13] ... 

The A F-A isotherm of PS II core complex is rather different from that of PS II membrane (Fig. 4). The surface potential of a monolayer of PS II core complex increases slightly as the molecular area is compressed from 600 to about 150nm, while surface pressure changes from 5 to 35mN/m. Further compression results in a sharper increase in surface potential. The surface potential starts to decrease only after the surface area is compressed to about 80 nm or surface pressure becomes larger than 40mN/m. This is consistent with the previous discussion that PS II core complexes form a more ordered monolayer structure at relatively high surface potential and will not form multilayered... 

Fig. 37 Surface potential versus molecular area plot for monolayers of meso- and (+ )-azobis[6-(6-cyanododecanoic acid)] spread statically on a pH 3 subphase.

Figure 17. The change in the surface potential by step illumination for ca. 2-3 s with a He-Cd laser of 441.6 nm in a total reflection mode on (a) a type a alternate and (b) a type b alternate LB film containing 15 unidirectionally oriented A-S-D triad monolayers, and (c) a black LB film.

Tb confirm the long-lived charge separation by the lateral diffusion mechanism, we also measured the surface potential change on alternate LB films containing A-S dyads similar to types a and b. Tb increase singnal intensities, both types of LB films contained 30 bilayers of the alternate monolayers. As shown in Figs. 19a and... 

Another property of monolayer films that can be continuously recorded as function of surface area is the surface potential (38,64). Although the exact interpretation of this property is not entirely clear, it is related to the orientation of the surfactant dipoles with respect to the subphase surface and is often followed simultaneously... 

The surface potential arises because the lipid molecule orients with polar part toward the aqueous phase. This effects a change in dipole at the surface. There would thus be a change in surface potential when a monolayer is present, as compared to a clean surface. The surface potential, AV, is... 

The transport of K+ ions through cell membranes by antibiotics (valinomycin) has been a very important example. Addition of K+ ions to the subphase of a valinomycin monolayer showed that the surface potential became positive. This clearly indicates the ion-specific binding of K+ to valinomycin (Birdi, 1989). 

Although the surface potential, /, the electrical potential due to the charge on the monolayers, will clearly affect the actual pressure required to thin the lamella to any given dimension, we shall assume, for the purpose of a simple illustration, that 1 Ik, the mean Debye-Huckel thickness of the ionic double layer, will influence the ultimate thickness when the liquid film is under relatively low pressure. Let us also assume that each ionic atmosphere extends only to a distance of 3/k into the liquid when the film is under a relatively low excess pressure from the gas in the bubbles this value corresponds to a repulsion potential of only a few millivolts. Thus, at about 1 atm pressure ... 

When the electrostatic surface potential becomes high or when the monolayer capacity of the surface is approached. So far the models agree with experiments up to 60-70 mV but the upper limit is probably below 100 mV. 

Monolayer properties of octadecyldimethylamine oxide alone and in combination wth sodium alkyl sulfate on aqueous substrate have been investigated. Nonionised amine oxide produces more expanded film than the ionised species minimum film expansion and highest surface potential are obtained at half ionisation. 

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