Monolayers surface pressure—area isotherm

A second example of the influence of the casting solvent on the morphology of EC film is given by the work of Rosilio et al. [153]. Monolayer surface pressure-area isotherms, wettability and permeability data clearly indicated that the EC films cast from various solvent systems differ not only in terms of their overall properties but also from one side to the other. 

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 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...

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. 

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 ... 

Surface pressure/area isotherms of mixtures of the cationic lipid (20, n = 12) with distearoylphosphatidylcholine (DSPC) are shown in Fig. 30. For all mixtures only one collapse point is observed. The collapse pressure increases continuously with increasing amount of DSPC, indicating miscibility of the two components. Plotting A versus molar ratio (Fie. 3D results in considerable deviation from linearity, which also suggests miscibility of the two compounds in monolayers. This is also confirmed by the fact that the polymerization rate, as measured by the increase of optical density at 540 nm, is reduced by a factor of 100 when the DSPC molar ratio is increased from 0 to 0.52,... 

Another surface parameter of interest is the hysteresis area (AG), which is indicative of energy trapped in a monolayer. The hysteresis area is the difference between the free energy of compression and free energy of expansion which is calculated from the area under corresponding surface pressure - area isotherms. 

The surface behavior of poly(4-vinylpyridine) quaternized with tetradecyl bromide (P4VPCi4) as function of the quaternization degree has been reported [81], The percentage of vinylpyridine moieties quaternized was found to be 35-75%. Surface pressure-area isotherms (tt - A) at the air-water interface were determined. The polymer monolayer have shown particular shapes at different quaternization degrees. Figure 3.14 shows the (tt - A) isotherms of P4VPCi4. 

An amphiphilic diblock copolymer spread from a solution of organic solvent onto the water surface, normally were found to form a stable monolayer [129], The surface monolayer has been successfully transferred onto a substrate by the Langmuir - Blodgett technique. Some times the surface pressure - area isotherms exhibited a plateau region, suggesting a structural change taking place on the water surface at specific pressures. 

The surface pressure - area isotherms were collected at different temperatures and the energy relationship between rod and coil as a function of rod length was analized [134], The microstructures of these monolayers based on the energy relationship were also investigated using Atomic Force Microscopy (AFM). 

It should be pointed out at this juncture that strict thermodynamics treatment of the film-covered surfaces is not possible [18]. The reason is difficulty in delineation of the system. The interface, typically of the order of a 1 -2 nm thick monolayer, contains a certain amount of bound water, which is in dynamic equilibrium with the bulk water in the subphase. In a strict thermodynamic treatment, such an interface must be accounted as an open system in equilibrium with the subphase components, principally water. On the other hand, a useful conceptual framework is to regard the interface as a 2-dimensional (2D) object such as a 2D gas or 2D solution [ 19,20]. Thus, the surface pressure 77 is treated as either a 2D gas pressure or a 2D osmotic pressure. With such a perspective, an analog of either p- V isotherm of a gas or the osmotic pressure-concentration isotherm, 77-c, of a solution is adopted. It is commonly referred to as the surface pressure-area isotherm, 77-A, where A is defined as an average area per molecule on the interface, under the provision that all molecules reside in the interface without desorption into the subphase or vaporization into the air. A more direct analog of 77- c of a bulk solution is 77 - r where r is the mass per unit area, hence is the reciprocal of A, the area per unit mass. The nature of the collapsed state depends on the solubility of the surfactant. For truly insoluble films, the film collapses by forming multilayers in the upper phase. A broad illustrative sketch of a 77-r plot is given in Fig. 1. 

Abstract. Surface pressure/area isotherms of monolayers of micro- and nanoparticles at fluid/liquid interfaces can be used to obtain information about particle properties (dimensions, interfacial contact angles), the structure of interfacial particle layers, interparticle interactions as well as relaxation processes within layers. Such information is important for understanding the stabilisation/destabilisation effects of particles for emulsions and foams. For a correct description of II-A isotherms of nanoparticle monolayers, the significant differences in particle size and solvent molecule size should be taken into account. The corresponding equations are derived by using the thermodynamic model of a two-dimensional solution. The equations not only provide satisfactory agreement with experimental data for the surface pressure of monolayers in a wide range of particle sizes from 75 pm to 7.5 nm, but also predict the areas per particle and per solvent molecule close to the experimental values. Similar equations can also be applied to protein molecule monolayers at liquid interfaces. 

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