Surface pressure-area isotherm

Surface pressure is the reduction of surface tension due to the presence of the monolayer  

A commonly used alternative to measure the surface pressure is the Wilhelmy plate. The force on this plate decreases as the surface tension decreases. 

The shape of the surface pressure isotherm depends on the lateral interactions between molecules. This in turn depends on molecular packing, which is influenced by factors such as the size of head group, the presence of polar group, the number of hydrocarbon chains and their conformation (straight or bent). 

Surface films also form on solutions of soluble surfactant. In that case, the surface tension isotherm is measured as a function of concentration of surfactant in the solution. The surface pressure isotherm can be basically understood by the Gibbs absorption isotherm, which in the particular case of air-water interface relates the change of the surface tension to the chemical potential of the surfactant as  

The chemical potential of the surfactant can be written as = RTdlna, where a is the activity of the surfactant in water, and R = 8.315 Joule/K = 1.986 cal/K is the gas constant. In the limit of an ideally dilute solution, a can be replaced by the concentration in the solution, c (in mol/dm ). Thus, from Eq. (2.6), the surfactant excess per imit area at the surface can be related to the surfactant concentration in the solution through the equation  

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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. 

FIG. 2 The surface pressure-area isotherms of PS II core complex with different concentrations of salt in the subphase. Subphase, lOmM tris-HCl, pH 8.0, 2mM sodium ascorbate and concentrations of 100, 200, and 500mM NaCl. Temperature, 23.0 0.5°C. 

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. 5 Surface pressure/area isotherm for the compression cycle of dipalmitoylphos-phatidyl choline (dashed line) and l-palmitoyl-2-(l2-hydroxystearoyl)phosphatidyl choline (solid line) on a pure water subphase at 25°C. Reprinted with permission from Arnett et al., 1989. Copyright 1989 American Chemical Society.

Fig. 17 Surface pressure/area isotherms for the compression and expansion cycles of racemic (dashed line) and enantiomeric (solid line) stearoylserine (A), stearoyl-alanine (B), stearoyltryptophan (C), and stearoyltyrosine methyl esters (D) on a pure water subphase at 25°C carried out at a compression rate of 7.1 A2/molecule per minute. Arrows indicate the direction of compression and expansion.

Fig. 22 Surface pressure/area isotherms for the compression cycles of stearoyltyrosine on a buffered pH 6.86 subphase carried out at a compression rate of 19.24 A2/molecule per minute at 16,19,22,25,28, 31, and 34°C. Reprinted with permission from Harvey et ah, 1990. Copyright 1990 American Chemical Society.

Fig. 24 Surface pressure/area isotherms for palmitic acid/stearoylserine methyl ester films at 25°C on a pure water subphase and compressed at 29.8 A2/molecules per minute. A, 16.7-33.3% B, 50% C, 66.6% D, 83.3% SSME.

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 ).

Fig. 38 Surface pressure/area isotherms for the compression/expansion cycles of meso- (dashed line) and ( )-(solid line) azobis-[6-(6-cyanododecanoic acid)] on a pH 3 subphase at 22°C. Compressed at a rate of 15.5 A2/molecule per minute. Reprinted with permission from Porter et al., 1986a. Copyright 1986 American Chemical Society.

Fig. 45 Surface pressure/area isotherms for the compression cycle of 12-ketooctadecanoic acid (A) and octadecanoic acid (B) on a buffered subphase (AR hydrochloric acid pH 4.0) at 30°C carried out at a compression rate of 2.0-3.0 A2/molecule per minute.

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.3. Surface pressure-area isotherms of azobenzene-linked amphiphiles 1 ...

Influence of subphase temperature, pH, and molecular structure of the lipids on their phase behavior can easily be studied by means of this method. The effect of chain length and structure of polymerizable and natural lecithins is illustrated in Figure 5. At 30°C distearoyllecithin is still fully in the condensed state (33), whereas butadiene lecithin (4), which carries the same numEer of C-atoms per alkyl chain, is already completely in the expanded state (34). Although diacetylene lecithin (6) bears 26 C-atoms per chain, it forms both an expanded and a condensed phase at 30°C. The reason for these marked differences is the disturbance of the packing of the hydrophobic side chains by the double and triple bonds of the polymerizable lipids. At 2°C, however, all three lecithins are in the condensed state. Chapman (27) reports about the surface pressure area isotherms of two homologs of (6) containing 23 and 25 C-atoms per chain. These compounds exhibit expanded phases even at subphase temperatures as low as 7°C. 

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