Monolayer pressure-area isotherm

Neumann has adapted the pendant drop experiment (see Section II-7) to measure the surface pressure of insoluble monolayers [70]. By varying the droplet volume with a motor-driven syringe, they measure the surface pressure as a function of area in both expansion and compression. In tests with octadecanol monolayers, they found excellent agreement between axisymmetric drop shape analysis and a conventional film balance. Unlike the Wilhelmy plate and film balance, the pendant drop experiment can be readily adapted to studies in a pressure cell [70]. In studies of the rate dependence of the molecular area at collapse, Neumann and co-workers found more consistent and reproducible results with the actual area at collapse rather than that determined by conventional extrapolation to zero surface pressure [71]. The collapse pressure and shape of the pressure-area isotherm change with the compression rate [72]. 

The three general states of monolayers are illustrated in the pressure-area isotherm in Fig. IV-16. A low-pressure gas phase, G, condenses to a liquid phase termed the /i uid-expanded (LE or L ) phase by Adam [183] and Harkins [9]. One or more of several more dense, liquid-condensed phase (LC) exist at higher pressures and lower temperatures. A solid phase (S) exists at high pressures and densities. We briefly describe these phases and their characteristic features and transitions several useful articles provide a more detailed description [184-187]. 

Fig. XV-3. Four types of pressure-area isotherms observed in end-functionalized PDMS monolayers. Up to six transitions (A-F) are observed and described in the text. (From Ref. 6.)...

Fig. XV-14. Surface pressure-area isotherms at 298 K for a DPPC monolayer on phos-photungstic acid (10 Af) at the pH values shown with 10 A/ NaCl added. (From Ref. 123.)...

Ruckenstein and Li proposed a relatively simple surface pressure-area equation of state for phospholipid monolayers at a water-oil interface [39]. The equation accounted for the clustering of the surfactant molecules, and led to second-order phase transitions. The monolayer was described as a 2D regular solution with three components singly dispersed phospholipid molecules, clusters of these molecules, and sites occupied by water and oil molecules. The effect of clusterng on the theoretical surface pressure-area isotherm was found to be crucial for the prediction of phase transitions. The model calculations fitted surprisingly well to the data of Taylor et al. [19] in the whole range of surface areas and the temperatures (Fig. 3). The number of molecules in a cluster was taken to be 150 due to an excellent agreement with an isotherm of DSPC when this... 

We studied the surface pressure area isotherms of PS II core complex at different concentrations of NaCl in the subphase (Fig. 2). Addition of NaCl solution greatly enhanced the stability of monolayer of PS II core complex particles at the air-water interface. The n-A curves at subphases of 100 mM and 200 mM NaCl clearly demonstrated that PS II core complexes can be compressed to a relatively high surface pressure (40mN/m), before the monolayer collapses under our experimental conditions. Moreover, the average particle size calculated from tt-A curves using the total amount of protein complex is about 320 nm. This observation agrees well with the particle size directly observed using atomic force microscopy [8], and indicates that nearly all the protein complexes stay at the water surface and form a well-structured monolayer. 

Our studies on the surface pressure-area isotherms of MGDG and the mixture of PS II core complex and MGDG indicate the presence of both PS II core complex and MGDG in the monolayer. MGDG molecules diluted the PS II core complex concentration in the monolayer. MGDG lipid functions as a support for the protein complex and the resulting mixture forms higher-quality films than PS II core complex alone. 

Fig. 19 Surface pressure/area isotherms for the compression/expansion cycle of enantiomeric (dashed line) and racemic (solid line) SSME monolayers on pure water subphase at (a) 20°C, (b) 25°C, (c) 30°C and (d) 40°C. The compression rate is 29.8 A2/ molecule/min. Reprinted with permission from Harvey et al., 1989. Copyright 1989 American Chemical Society.

Fig. 32 Surface pressure/area isotherms for the compression/expansion cycles of diastereomeric monolayers of (R or S)-iV-(a-methylbenzyl)stearamides mixed 1 1 with (R or S )-stearoylalanine methyl esters on a pure water subphase at 35°C. Dashed lines denote heterochiral pairs (R S or R S) and solid lines denote homochiral pairs (R R or S S ).

Figure 15. Pressure-area isotherms for spread diacetylene monolayer of cadmium...

Figure 4. Surface pressure - area isotherms of TFPP monolayers at 20 °C.

Figure 8. Surface pressure - area isotherms for stearylamine before (a) and after (b) adsorption of IgG at pH 8, 20 °C, and schematic representation for IgG molecule adsorbed to stearylamine monolayer.

Figure 12. Surface pressure - area isotherms (20 °C) of P-CDNHC12H25 monolayers included and/or adsorbed 1-NaphSC>3 at the air/aqueous solution interface under the different initial surface pressures ...

Figure 34. Surface pressure - area isotherms for monolayers of Ci 8TCNQ (a), the mixture of the dihydrothiophene and G 8TCNQ (b), and the complex (c), spread on distilled water, as compared with that on the aqueous subphase with 10 5M LiTCNQ (c ).

Fig. 30. Left Lateral compression of a monolayer of PBA brushes on water. Right Corresponding surface pressure-area Isotherm measured during compression...

Fig. 29 Pressure-area isotherms for 52 (solid line), 53 (dashed line), and 54 (dotted line) showing monolayer formation...

When a surfactant is injected into the liquid beneath an insoluble monolayer, surfactant molecules may adsorb at the surface, penetrating between the monolayer molecules. However it is difficult to determine the extent of this penetration. In principle, equilibrium penetration is described by the Gibbs equation, but the practical application of this equation is complicated by the need to evaluate the dependence of the activity of monolayer substance on surface pressure. There have been several approaches to this problem. In this paper, previously published surface pressure-area Isotherms for cholesterol monolayers on solutions of hexadecy1-trimethyl-ammonium bromide have been analysed by three different methods and the results compared. For this system there is no significant difference between the adsorption calculated by the equation of Pethica and that from the procedure of Alexander and Barnes, but analysis by the method of Motomura, et al. gives results which differ considerably. These differences indicate that an independent experimental measurement of the adsorption should be capable of discriminating between the Motomura method and the other two. 

In such an equilibrium study the surfactant is Injected beneath a monolayer, the surface is compressed in stages with equilibrium being established at each step, and the equilibrium surface pressure-area Isotherm is established. In this way, isotherms for a range of surfactant concentrations are produced. 

What is the meaning of surface pressure Describe one or two models of pressure-area isotherms for a monolayer. 

Fig. 6. Left Langmuir film balance, schematic right surface pressure/area isotherm of a monolayer at the gas/water interface (a-d, see text), n surface pressure A surface area33 ...

Fig. 29. Characterization of mixed monolayers left surface pressure/area isotherms of pure...

Differentiation between ideal miscibility and complete immiscibility is possible by evaluating surface pressure/area isotherms. According to the phase rule of Defay and Crisp 53-67) in a completely immiscible monolayer the surface pressures observed for phase transitions or collapse points are equal to those of the pure components. This case of a completely immiscible monolayer is schematically illustrated in Fig. 29 (left). In a completely miscible lipid monolayer these surface pressures vary with different molar ratios of the lipid components. 

Due to the topochemical restrictions of diacetylene polymerization, diacetylenic lipids are solely polymerizable in the solid—analogous phase. During the polyreaction an area contraction occurs leading to a denser packing of the alkyl chains. In addition to surface pressure/area isotherms the polymerization behavior of diacetylenic lipids containing mixed films give information about the miscibility of the components forming the monolayer ... 

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