Surface potential-area isotherms

FIG. 3 The surface pressure-area (jt A) and surface potential-area (AV-A) isotherms of PSII membranes. 

Tphe force-area (tt-A), surface potential-area (AV-A), and surface vis-cosity-area (-q-A) characteristics of long-chain fatty acid monolayers at the air-water interface have been extensively investigated over acid and neutral aqueous solutions (17). Studies of fatty acid monolayer isotherms at high pH and the specific cation effect on the isotherms are less numerous. The difficulties inherent in working at high pH are ... 

To calculate surface pressure-area isotherms, following the Gibbs convention, we used the adsorption equation relating surface tension to surface excess, T, and chemical potential, /a, in the form... 

Monolayers of adsorbed molecules at the air-water interface display phase transitions. Monolayers of octadecylamine salts of perfluoroalkanoates on 4.4M NaCl solution exhibit two or three types of phase transitions in their tt-A (surface pressure-area) isotherms [43]. The phases were classified and phase diagrams drawn from the phase-transition pressures and surface potentials. 

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... 

An unusually extensive battery of experimental techniques was brought to bear on these comparisons of enantiomers with their racemic mixtures and of diastereomers with each other. A very sensitive Langmuir trough was constructed for the project, with temperature control from 15 to 40°C. In addition to the familiar force/area isotherms, which were used to compare all systems, measurements of surface potentials, surface shear viscosities, and dynamic suface tensions (for hysteresis only) were made on several systems with specially designed apparatus. Several microscopic techniques, epi-fluorescence optical microscopy, scanning tunneling microscopy, and electron microscopy, were applied to films of stearoylserine methyl ester, the most extensively investigated surfactant. 

The interpretation of surface-pressure and surface-potential measurements.8 Surface pressure F is best plotted against area A, as this shows up the resemblance between the films and three-dimensional PV isothermals. For gaseous films, however, curves with FA as ordinates and F as abscissae are very useful. 

The water was distilled twice from glass (initially from alkaline permanganate). It was subsequently twice distilled from quartz. Surface areas at most pressures were reproducible within 0.5 A2/molecule however, at low pressures, for gaseous isotherms, the error was somewhat greater. Surface potentials were also obtained but are not reported here. Surface potential values, however, confirm observations made with surface pressure measurements (8). 

Films of 5-NS and 5-NS(Me) have a remarkable coincidence of isotherms over a 30 °C range a negative expansion temperature dependence only appears in the inflection as the films approach collapse. The inflection seems to be real and can be differentiated clearly from the collapse behavior for 8-NP(Me). Thus the plots of surface pressure vs. area/mole-cule and surface potential vs. area/molecule (not shown) for 5-NS, 5-NS (Me), and 2-NP(Me) differ from each other. Moreover, the inflections for 5-NS and 5-NS (Me) remain invariant with a varying rate of compression whereas the collapse pressure of 8-NP(Me) is slightly pressure sensitive. It appears, therefore, that films of 5-NS and 5-NS (Me) exhibit a zero temperature dependence over most of the pressure range but a negative temperature dependence as they approach collapse. 

Surface energy is defined as the increase in energy per unit area of the surface in an isothermal change. According to the classical theory, the mean value of the kinetic energy of a molecule is the same whether it is in the interior or on the surface of the liquid. Therefore we have only to calculate the increase in the total potential energy of the system which results from an increase of. the area of the surface. 

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