Equilibrium surface pressure

The dynamic surface tension of a monolayer may be defined as the response of a film in an initial state of static quasi-equilibrium to a sudden change in surface area. If the area of the film-covered interface is altered at a rapid rate, the monolayer may not readjust to its original conformation quickly enough to maintain the quasi-equilibrium surface pressure. It is for this reason that properly reported II/A isotherms for most monolayers are repeated at several compression/expansion rates. The reasons for this lag in equilibration time are complex combinations of shear and dilational viscosities, elasticity, and isothermal compressibility (Manheimer and Schechter, 1970 Margoni, 1871 Lucassen-Reynders et al., 1974). Furthermore, consideration of dynamic surface tension in insoluble monolayers assumes that the monolayer is indeed insoluble and stable throughout the perturbation if not, a myriad of contributions from monolayer collapse to monomer dissolution may complicate the situation further. Although theoretical models of dynamic surface tension effects have been presented, there have been very few attempts at experimental investigation of these time-dependent phenomena in spread monolayer films. 

Enantiomeric recognition was clearly displayed in films spread from solution and films in equilibrium with their crystals, and was sharply dependent on the acidity of the subphase. Protonation of the amide group appeared to be necessary for spreading to stable monolayers. For example, the crystals of the racemate deposited on a 10n H2S04 solution at 25°C spread quickly to yield a film with an ESP of 7.7 dyn cm"1, while the single enantiomers spread only to a surface pressure of 3.9 dyn cm-1 (Table 1). Similar effects are observed at 15 and 35°C. The effect of stereochemistry on equilibrium spreading is even more pronounced at lower subphase acidities. On 6n sulfuric acid, the racemate spread to an equilibrium surface pressure of 4.9 dyn cm-1, while the enantiomeric systems spread to less than 1 dyn cm-1. 

These results for spread film and equilibrium spreading suggest that films of racemic N-(a-methylbenzy 1) stearamide may be resolved by seeding the racemic film with crystals of either pure enantiomer. Indeed, when a monolayer of racemic jV- (a-methylbenzyl) stearamide is compressed to 45 A2/molecule (27 dyn cm-1), deposition of a crystal of either R( +)- or S( — )-enantiomer results in a decay of surface pressure from the initial 28 dyn cm-1 film pressure to 3.0 dyn cm-1, the ESP of the enantiomeric systems on a pure 10n sulfuric acid subphase (Table 1). When the experiment is repeated with racemic crystals, the system reaches an equilibrium surface pressure of 11 dyn cm-1, nearly the ESP of the racemic crystal on the clean acidic interface. In either case, equilibrium pressure is reached within a two hour time period. 

The differences in time-dependent adsorption behavior between 99% PVAC at 25° and 50°C demonstrate the influence of intra- and intermolecular hydrogen bonding in the adsorption process. The limiting surface pressure of the hydrophobic water-soluble polymer appears to be 33 mN/m, approximately 7 mN/m below that of commonly used surfactants. The rate of attainment of equilibrium surface pressure values is faster if there is uniformity of the hydrophobic segments among the repeating units of the macromolecule. 

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. 

Chiral discrimination effects within surface monolayers may also be employed for separating a racemic surface monolayer into domains of uniform chirality which occurs if the S S or R R interaction is more favourable than the S R interaction (homochiral discrimination). Such a two-dimensional resolution was triggered off by the sprinkling of pure enantiomer A -(a-/ -methylben-zyl)stearamide 5 crystals on to the corresponding racemic monolayer. A rapid decrease of surface pressure well below the equilibrium surface pressure of the racemate was observed . This result implies the deposition of / -configured molecules on the added crystals, leaving a partially resolved film which was composed predominantly of 5 -molecules. 

Relaxation phenomena of (—) LMWE, (-) proteins and ( ) protein-LMEW mixed films at (a) constant surface pressure (at tt < rre) and (b) at constant molecular area at the collapse point. The arrows indicate the equilibrium surface pressure for LMWE and proteins (ttK ). 

Equilibrium surface pressure (TTg) values were calculated as TTg = ct-q — (Te, where (a-g) is the equilibrium surface tension and oq is the solvent surface tension. They were measured by the Wilhelmy plate method, using a platinum plate attached to a Sigma digital tensiometer. The range of concentrations studied were 5 X 10 to 2% wt. 

Figure 29.1 shows the variation of equilibrium surface pressure of E4M, E50LV, and F4M with polymer concentration. The surface pressure increased with HPMC concentration tending to a pseudo-equilibrium in the case of E4M and F4M, whereas for E50LV the surface pressure continuously grew within the range of concentrations studied. 

The positive heats of mixing for lecithin-cholesterol mixtures indicate that interactions between unlike molecules are smaller than the interactions between like molecules, i.e., the hydrocarbon chain interactions with cholesterol are smaller than in each of the pure phases. If the excess heats of mixing become large enough, phase separation will occur. It may occur when the surface pressure is increased (i.e., as the films are compressed). The point at which phase separation occurs is difficult to predict, measure, or detect however, evidence of phase separation can be deduced from the following experiment. If excess amounts of two lipids are placed in water, the equilibrium surface pressure should reflect whether the surface film is a mixture. According to the phase rule (11,12, 13,14), if two bulk lipid phases are present, only one surface phase can be present at the air—water surface. Thus the composition of the equi-... 

Submonolayer Injections of Surfactants. Plots of surface pressure vs. time are shown in Figure 2. Time zero is the time at which the injection was completed. The surface pressure, initially 5 dynes/cm, rises sharply and subsequently decreases with time to an equilibrium value. The equilibrium surface pressure and potential change are summarized in Columns 2 and 3 of Table I. A nonlinear relationship exists between the log of the difference of the equilibrium pressure, 7re, the pressure at time t, 7Tt, and time [i.e., log (7re—tt ) vs. t the surface pressure change is therefore not a first-order phenomenon. 

Tnteractions at surfaces have long been at the center of interest in the study of surfactant monolayers and have been thought to influence both static and dynamic surface properties considerably (1,2). Although the theoretical interpretation and even the definition of surface interactions may be controversial, the experimental method has not been in doubt. Invariably, the equilibrium surface pressure vs. molar area relationship has been used as a criterion for assessing interactions in mono-layers since interactions, no matter what their precise definition, must appear in the measurable quantity of surface tension (y) or surface pressure (7r = y° — y) at a given surface concentration (r) or molar... 

Measurement of the high frequency modulus, c0, as a function of the equilibrium surface pressure, tt, should provide a sensitive criterion for interaction for monolayers that are quite soluble by normal standards, which involve much longer time spans than the inverse frequency of the compression/expansion experiment. A numerical example of the greater sensitivity of an e0 vs. tt plot, compared with that of the ir vs. log c relationship is shown in Figure 1 for a hypothetical case. The specific defini-nition of surface interactions used here to arrive at numerical values includes all mechanisms that produce deviations from Szyszkowski-Langmuir adsorption behavior. Ideal behavior, with zero surface interactions, then is represented by zero values of In fis in the equation of state ... 

We have also measured y(t) and e(t) during polymer adsorption for a given concentration. In Figure 6, the e-n curve, the equation state of the layer during the adsorption process, is presented. At low surface pressure, one observes a linear increase of the dilational elastic modulus with the surface pressure n. From the slope of the linear part of the e-n curve, a value of 0.66 was found for the excluded volume critical exponent. The same value has been measured elsewhere with another technique.12 This result indicates that, unlike the excluded volume chain behaviour in the bulk, the air water interface is not a good solvent for MeC. At intermediate surface pressures, the modulus levels off and then increases again until the equilibrium surface pressure is reached. 

Fig. 2.9 Equilibrium surface pressure for BHBCi solutions symbols - experimental data from [13,25], curves -theoretical calculations curve 1 - Langmuir-Szyszkowski equation curves 2 - 5 - reorientation model for 2, 3, 7 and over 50 adsorption states of the BHBCie molecule, respectively (n j =2.52 lO mVmol,...

Recently, Ferri and Stebe [62] proposed a scaling low in order to directly compare the adsorption dynamics of different surfactants. By plotting dynamic surface tensions in a dimensional format n( t/ToVrio, where no=y(t)-yo is the equilibrium surface pressure and the diffusion relaxation time tq is defined by the following relationship... 

A more advanced model was suggested very recently by [78] based on the adsorption isotherm for proteins given by Eq. (2.124). In addition to diffusion of the molecules in the bulk, a kinetic process was assumed equivalent to the mechanism used in the mixed kinetic model. The configuration changes, i.e. orientation of a globular protein molecule to the surface, were characterised by one rate constant k. The following Fig. 4.7 shows model calculations where the following parameters were used coi = 2.5-10 m /mol, W2 = 5.010 m /mol (i.e. coj/ ] = 2), a i = 200. These parameters correspond to those for HSA adsorbed at the water/air interface [79]. The diffusion coefficient was taken to be D = lO cmVs and the protein concentration as 10 mol/I. The equilibrium surface pressure of the protein solutions was taken to be 20 mN/m, typical for HSA at this concentration. It should be noted first that the time required for an experimentally observable decrease of the surface tension, say by 0.5 mN/m, is about 3100 s... 

Penetration into Monolayers at the Air-Water and Oil-Water Interface. Membrane extracts as described above penetrate monolayers at the air-water interface reaching equilibrium surface pressures in 10-30 minutes depending on the protein concentration in the substrate. Figure 8 shows the equilibrium spreading... 

Figure 8. The equilibrium surface pressure increases reached after interaction of protein extract with films of cholesterol ( ),...

Future studies in this area will require the radiolabelling of intrinsic membrane so that the amount of protein entering the monolayer can be accurately measured. More information should also be obtained on the dependence of the initial monolayer surface pressure on protein mediated fusion of vesicles with monolayers. The surface pressure at which the properties of a monolayer most closely mimic the properties of a bilayer is known to be relatively high. It is clear from our studies that protein extracts penetrate monolayers upto equilibrium surface pressures approaching the monolayer collapse pressure which suggests that data can be obtained from monolayer studies at surface pressures which are directly applicable to bilayers. 

Figure 7. Dependence of equilibrium surface pressure on the concentration of C EOs ( ) - data from Ueno et al. (1981) at 25°C. Theoretical curves were calculated in (Fainerman et al. 2003) from the Langmuir models (curve 1), and reorientation models (15)-(17) and (18)-(20), curves 2 and 3...

Ozone (0.8 ppm) affected the surface activity of surfactant in rats exposed for 2 or 12 h, whereas distinct morphological changes in bronchoalveolar lavage or in the surfactant subtypes were not observed (Putman et al. 1997). Adsorption experiments indicated that bronchoalveolar lavage from rats exposed for 12 h to O3 remained at lower equilibrium surface pressures than lavage from control rats. These observations suggest interference of in-... 

Table 5 Equilibrium surface pressure (ESP) and surface area for long chain fatty acids and alcohols...

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