Surface pressure-area isotherms surfactants

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... 

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. 7. Surface pressure/area isotherms of lysophospholipid analogous diacetylenic surfactants (45)-...

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. 

Monolayers of micro- and nanoparticles at fluid/liquid interfaces can be described in a similar way as surfactants or polymers, easily studied via surface pressure/area isotherms. Such studies provide information on the properties of particles (dimensions, interfacial contact angles), the structure of interfacial layers, interactions between the particles as well as about relaxation processes within the layers. Such type of information is important for understanding how the particles stabilize (or destabilize) emulsions and foams. The performed analysis shows that for an adequate description of II-A dependencies for nanoparticle monolayers the significant difference in size of particles and solvent molecules has be taken into account. The corresponding equations can be obtained by using a thermodynamic model developed for two-dimensional solutions. The obtained equations provide a satisfactory agreement with experimental data of surface pressure isotherms in a wide range of particle sizes between 75 pm and 7.5 nm. Moreover, the model can predict the area per particle and per solvent molecule close to real values. Similar equations were applied also to protein monolayers at liquid interfaces. 

The LB trough is commonly used to measure surface pressure/area isotherms for a particular surfactant film. To carry out this measurement, the thin surfactant film is compressed by moving a barrier across the fluid surface at a constant rate while monitoring the surface tension. At a constant temperature the surface pressure Tt is measured as a function of the interfacial area available to each molecule as... 

FIG. 22 Side view snapshots of a simulation of a 16-carbon hydrogenated surfactant chain with a carboxylate-like head group on a water surface at 300 K. The view iu (a) (top) is au area of 0.21 um molecule (b) (bottom) is at 0.21 um molecule . These two areas roughly bracket a first-order trausitiou with some features of the LE-LC transition. See also Figure 23 for the correspoudiug pressure-area isotherm. (Reproduced with permission from Ref. 364. Copyright 1992 American Chemical Society.)... 

The mass spectral data selected for multivariate analysis represented a suite of 30 microlayer and bulk surface seawater surfactant samples (fraction FI Frew et al. (2006)) including seven from waters off Monterey and Santa Barbara, California and 23 collected along a transect from Delaware Bay on the U. S. east coast to the Sargasso Sea. Sample extracts were analysed in triplicate by desorption-electron ionisation (DEI) mass spectrometry (Boon 1992 Frew et al. 2006). Individual DEI mass scans were summed over the full desorption/pyrolysis interval, reduced to integer masses, and averaged for processing by multivariate analysis. Elasticities were estimated quasi-statically from surface pressure-area (El A) isotherm measurements in a KSV 2200 Langmuir film balance (Frew et al. 2006). The elasticity data were subsampled at fixed surface pressure intervals (0.5 mN m"1) for comparison with the results of the multivariate analysis. 

Figure 17 shows the 11/A isotherms of racemic and enantiomeric films of the methyl esters of 7V-stearoyl-serine, -alanine, -tryptophan, and -tyrosine on clean water at 25°C. Although there appears to be little difference between the racemic and enantiomeric forms of the alanine surfactants, the N-stearoyl-tyrosine, -serine, and -tryptophan surfactants show clear enantiomeric discrimination in their WjA curves. This chiral molecular recognition is first evidenced in the lift-off areas of the curves for the racemic versus enantiomeric forms of the films (Table 2). As discussed previously, the lift-off area is the average molecular area at which a surface pressure above 0.1 dyn cm -1 is first registered. The packing order differences in these films, and hence their stereochemical differentiation, are apparently maintained throughout the compression/expansion cycles. 

Now we consider the relationship between the effective concentration(reff) and the surface pressure(tt) at the air/water interface. Ideally, the surface pressure is directly proportional to the concentration of surfactants. However, as the actual it-A isotherms show several specific effects, such as limiting area and points of inflexion, we shall assume the following relationships ... 

Surface-pressure/surface-area isotherms provide valuable insight into the molecular packing of surfactants in monolayers. A steep slope in the n-A curve... 

The subscript G specifies elasticity determined from isothermal equilibrium measurements, such as for the spreading pressure-area method, which is a thermodynamic property and is termed the Gibbs surface elasticity, EG. EG occurs in very thin films where the number of molecules is so low that the surfactant cannot restore the equilibrium surface concentration after deformation. 

A dynamic technique is described for obtaining surface elasticity (e0) vs. surface pressure (tt) curves which can be transformed into accurate tt—A curves for soluble monolayers. Small amplitude periodic area variations are used with a sufficiently high frequency to make monolayers effectively insoluble in the time of the experiment even though they behave as soluble in equilibrium measurements. plots are given for some nonionic surfactants. Straight line portions in these plots illustrate that surface interactions are too complex to be described by a Frumkin isotherm. In the limit of very low surface pressures there is no trace of an ideal gaseous region. Some examples show the implications of particular e0—rr curves for equilibrium and dynamic surface behavior. 

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