Collapse point

Figure 12. Pressure drop after regular intermittent compressions after film has been compressed beyond collapse point. From Thompson (101).

A racemic film was compressed nearly to its collapse point. It was then seeded by sprinkling crystals of pure enantiomeric amide on the surface. A rapid decrease in surface pressure was observed approaching the equilibrium spreading pressure of the enantiomer. A control experiment in which racemic crystals were sprinkled on the compressed racemic film produced a pressure drop that slowly approached, but did not reach, the ESP of the racemic film. The observed behavior was consistent with what would be expected if the enantiomer seed crystals had removed molecules of the same enantiomer from the racemic film, leaving a monolayer composed mainly of molecules of the opposite configuration. 

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. 

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,... 

In contrast to this, the system neutral lipid (2J)/DSPC shows considerably smaller deviations from the additivity rule and the surface pressure/area isotherms indicate two collapse points corresponding to those of the pure components62. Photopolymerization can be carried out down to low monomer concentrations and no rate change is observed. These facts prove that the system (23)/DSPC is immiscible to a great extent. The same holds true for mixed films of diacetylenic lecithin (18, n = 12) with DSPC, as well as for dioleoylphosphatidylcholine (DOPC) as natural component. 

Phase rule of Defay-Crisp describing the number of degrees of freedom for a system having one single plane surface (monolayer) Multi-component monolayers consisting of immiscible amphiphiles exhibit the same surface pressure for phase transitions and collapse points as the corresponding one-component monolayers, while these surface-pressures are different for mixtures of miscible amphiphiles. 

Just above the melting point the polymer is visually quite viscous and numerous observations have been made that the polymer exhibits a memory effect, that is to say, on recooling the melt crystallites will appear in the same sites where they had been before melting the polymer. Hartley, Lord and Morgan (1954) state It is reasonable to suppose that there will be a few localities in the crystalline polymer which have a very high degree of crystalline order, and therefore the melt can contain, even at considerable temperatures above the observed melting or collapse point, thermodynamically stable minute crystals of the polymer . Especially if the polymer has been irradiated so as to contain a few crosslinks as in irradiated polyethylene, then flow is inhibited and spherulites can be made to appear on recrystallization in the same sites that they had before the polymer was melted, Hammer, Brandt and Peticolas (1957). However, as mentioned above, the specific heat of irradiated polyethylene in the liquid state is identical with that of the unirradiated material, within the limits of experimental error. Dole and Howard (1957). 

Different molecular areas of Langmuir monolayers can be determined. They can be defined in three ways Ao is the area per molecule extrapolated to zero differential surface tension, Ac is the minimum area per molecule at the collapse point, at the point in the tt - A isotherms where the pressure is the maximum reversible pressure (or collapse pressure ttc) and Am is the area at the midpoint pressure rrm = 0.5 TTC. 

Further compression of the monolayer in the S-state leads to a sharp break in the t(A) isotherm, the so-called collapse point. This point Indicates the onset of molecules bulging out of the monolayer, which may lead to the formation of multilayers, i.e. of a new phase. This is schematically depicted in fig. 3.7e. The collapse point occurs around = 20-50 mN m". depending on the nature of the amphiphile and, particularly, on the interaction of the amphiphile with the subphase and/or super-phase. 

The occurrence of cis-double bonds hampers dense packing. Trans-double bonds do not have this effect elaldic acid (which has such a bond) packs like stearic acid. The effect of the cis-bond in the hydrocarbon chain is shown in fig. 3.1 lb, where it is observed that in the condensed phase the molecular area of Cj COOH increases from 0.28 nm for the fully saturated hydrocarbon chain (stearic acid), via 0.40 nm for the single unsaturated chain (oleic acid) to 0.49 nm for the doubly conjugated unsaturated chain (linoleic acid). In line with this, the collapse point, i.e. the value for where the monolayer breaks down to form a multilayer. Increases with decreasing degree of saturation. The pressure corresponding to the collapse point is lower when the fatty acid contains more double bonds (see the arrows in the figure). 

For insoluble monolayers of cholesterol and dipalmitoyl choline the relaxation at pressures below the collapse point were studied by Joos et al. ), using oscillatory and stress relaxation techniques. They found experimental evidence (and presented theory) for a double-exponential decay, representing two consecutive processes. The longer r s are 0(10 s) and 0(10 s) for cholesterol and the lipid, respectively, so these relaxations are relatively slow and may therefore be overlooked, especicJly in automated apparatus. No molecular mechanism was proposed the two r s did not exhibit a clear relationship with the surface pressure at which the experiments were carried out. 

Figure 3.79. Compression until beyond the collapse point a (3), followed by expansion and recompression. Tetracosanoic acid T = 25°, pH = 3. (Redrawn from McFate et al., loc. cit.)...

The freezing step is conducted at atmospheric pressure. It is crucial to ensure that the product will be thoroughly frozen by the end. This entails maintaining the temperature below the eutectic or collapse point of the product. 

The polyelectrolyte brush shrinks strongly on addition of electrolytes. At low or moderately low salt concentrations (cs=0.01 mol L-1) the force profiles resemble those of a soft brush. At salt concentrations of cs= 0.03 mol L, however, the profile of the static force resembles more closely that of a hard surface. Interestingly, if the behavior of the PEL brush is studied close to the collapse point significantly increased compressibility can be observed. However, the compressibilty shows no bistability, which indicates that the transition between the brush and the collapsed state is not a true first-order transition, although this would be expected from mean-field theory. One possible explanation of this behavior would be that the polydisper-sity of the surface-attached chains smoothens the transition. 

In Figure 14.5b we show that a common trend of the tt dependence of for LMWE monolayers is that E increased with increasing tt up to the collapse point. This increase is a result of an increase in the interactions between the monolayer molecules, as deduced from monolayer reflectivity. However, for the more condensed monolayer (saturated-LMWE), this increase is higher than for the more expanded imsaturated-LMWE monolayer. In summary, I-TT and -tt curves (Figure 14.5) could reflect the surface equation of state... 

As for unsaturated-LMWE, the domains that residues of protein molecules adopt at the air-water interface appeared to be of uniform reflectivity, suggesting homogeneity in thickness and film isotropy. I increased with tt and reaches a maximum at the collapse point. At ir < TTg, I was independent of the protein but, at the collapse point, I was higher for (3-casein and caseinate than for WPI. That is, at higher tt and especially at the collapse point, the thickness of (3-casein or caseinate monolayer is higher than that for WPI. However, for globular protein monolayers, E increased with increasing tt up to the collapse point. However, for the more disordered proteins ((3-casein and caseinate) the E-tt dependence is more complex. E increases to a maximum with tt for Structure 1, but decreases with tt and... 

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 ). 

Surface dilatational rheology is a very sensitive technique to analyze the competitive adsorption/displacement of protein and LMWE emulsifier at the air-water interface (Patino et al., 2003). A common trend is that the surface dilatational modulus increases as the monolayer is compressed and is a maximum at the highest surface pressures, at the collapse point of the mixed film, and as the content of LMWE in the mixture increases. At higher TT, the collapsed protein residues displaced from the interface by LMWE molecules have important influence on the dilatational characteristics of the mixed films. The mechanical properties of the mixed films also demonstrate that, even at the highest tt, the LMWE is unable to displace completely protein molecules from the air-water interface. 

For systems whose particle-particle bonds are permanent (image (b) in Figure 23.1), the film tends to form a secondary layer of wrinkles. The displaced particles do not completely remove themselves from the interface, but remain coimected to the cross-linked network. Contrary to what happens with very breakable bonds, part of the interfacial stress is released — and the area fraction decreases significantly — once the collapse point is passed (see curve (b)). 

If the bonds are somewhat easier to break (curve (a)), the collapse point is reached sooner, and the stress at rupture is significantly lower. For comparison, we have included the response of a system with no bonds at all (thick curve). In this case, the stress relaxes continuously from a negative value (i.e., the film pushes the imaginary mobile barrier). 

A second way of looking at forced expiration is with a maximum expiratory flow-volume (MEFV) curve, which describes maximum flow as a function of lung volume during a forced expiration (Fig. 12). In healthy human subjects, flow rates or flow-volume curves reach a maximum and will not increase with additional effort after the lungs have emptied 20-30% of their volume (Fry and Hyatt, 1960). This phenomenon of flow limitation is due to airway compression over most of the lung volume. Thus, flow rate is independent of effort and is determined by the elastic recoil force of the lung and the resistance of the airways upstream of the collapse point. In obstructive diseases of the lung this curve is shifted to the left, whereas restrictive diseases shift the curve in the opposite direction (also shown in Fig. 12). 

The concluding remarks of this work are that the addition of salt to the solution will move the collapse point In the case of charged blockpolyampholytes the existence of a nonuniform diunbbell-like configuration distributed in space is possible. One half of this dumbbell configuration will have a surplus positive... 

Polymerization of these films was carried out with tricosanoyl-10 12-diynoic acid creating a red (less ordered) and blue (more ordered) form. Bulk polymerized films were observed at the air/water interface and on glass substrates though with slightly differing intensities in their visible absorption spectra. It was clear that when monolayer films were polymerized and then compressed beyond the collapse point, the resultant film was significantly less ordered, as indicated by the predominance of the red conformer. Thus the molecular order of a polymerized film can be adversely affected by subsequent mechanical manipulation. In contrast to monolayers, solution cast films showed no polymerization under comparable conditions, but did do so once the film had been annealed. It would appear that annealing allowed the formation of a better oriented film. 

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