Critical surface pressure

For both high and low molecular weights, there is a temperature dependence. It is also clear from these Figures that the critical surface pressure values (ttc) depend markedly on temperature. The collapse state is characterized by the collapse pressure or critical surface pressure (ire)- This surface pressure can be defined as the maximum pressure value that the monolayer can reach without expulsion or rejection of the material in order to form a new tridimensional phase. 

Ns and No the number of molecules in the regular and aggregated state, respectively. f n) is a discontinuous function of surface pressure (n). In a previous paper, we have introduced f n) as a discontinuous S -shape function which includes critical surface pressure (tc ) and it is a simple form to represent the cooperative aggregation between lipid molecules at an air/water interface. 

In the right-hand side of Fig. 3 is shown the simulation results obtained by using the above kinetic equations. Figure 3B shows the oscillation of surface pressure around the critical surface pressure. If we include the damping factors such as viscous properties of DNP-DPPE mono-layer in the simulation, the oscillation would be represented by the dotted Hne. 

Figure 3B is a simulation result corresponding to the actual n-A curve in Fig. 2b. We have considered that the increase of the subphase temperature induces an increase in the critical surface pressure. In other words, the increase changes the shape of the nonlinear S -shape function. If we simulate the experimental results (Fig. 2c), it should be used as another appropriate function. In addition to this, it is clear that the rate coefficients (fcj and k-i) depend on the subphase temperature. In the present work, for convenience of calculation, we used the parameters, ki and k-i, as a constant, and estimated the value of k to be much larger than k-i which was arbitrarily determined as 1.0 X 10 . ...

The overshoot-hump in the n-A curve of DNP DPPE monolayer was foimd with constant-speed compression. From the absorption spectra of the lipid thin film, the hump reflected the formation of condensed layers at the critical surface pressure. A computer simulation was carried out on the basis of a cooperative aggregation process between the DNP-DPPE molecules at the air/water interface, the characteristic feature in the n-A curve was able to be reproduced. 

Physical characteristics Molecular weight Vapour density Specific gravity Melting point Boiling point Solubility/miscibility with water Viscosity Particle size size distribution Eoaming/emulsification characteristics Critical temperature/pressure Expansion coefficient Surface tension Joule-Thompson effect Caking properties... 

Figure 8. The effect of surface viscosity on the critical capillary pressure.

Fig. 32 a-d. Phase transition a from the extended coil b to a globule state as found by scaling analysis [83] c the transition is caused by lowering the surface pressure below a certain critical value TTc at which the fraction of adsorbed monomers 3=N2d/N undergo discrete changes d hereby, the depends critically on the side chain length... 

In biological systems, the lung fluid exhibits surface pressure characteristics that are related to its lipid composition. The ratio between two different lipids has been shown to be critical for lung function in newborn babies. Recent studies of mixed monolayers have been made using AFM (Birdi, 2002a). 

Accounting for the influence of surface-active contaminants is complicated by the fact that both the amount and the nature of the impurity are important in determining its effect (G7, L5, Rl). Contaminants with the greatest retarding effect are those which are insoluble in either phase (L5) and those with high surface pressures (G7). A further complication is that bubbles and drops may be relatively free of surface-active contaminants when they are first injected into a system, but internal circulation and the velocity of rise or fall decrease with time as contaminant molecules accumulate at the interface (G3, L5, R3). Further effects of surface impurities are discussed in Chapters 7 and 10. For a useful synopsis of theoretical work on the effect of contaminants on bubbles and drops, see the critical review by Harper (H3). Attention here is confined to the practically important case of a surface-active material which is insoluble in the dispersed phase. The effects of ions in solution or in double layers adjacent to the interface are not considered. 

Most of the above membrane-oriented studies were carried out for peptides in multilayer systems that were collapsed or transferred onto a sample cell surface. An alternative and very interesting way to study membrane systems is by IRRAS (infrared reflection absorption spectroscopy) at the air-water interface. In this way, unilamellar systems can be studied as a function of surface pressure and under the influence of various membrane proteins and peptides added. Mendelsohn et al.[136] have studied a model series of peptides, [K2(LA) ] (n = 6, 8, 10, 12), in nonaqueous (solution), multilamellar (lipid), and unilamellar (peptide-IRRAS) conditions. In the multilamellar vesicles these peptides are predominantly helical in conformation, but as peptide only monolayers on a D20 subphase the conformation is (1-sheet like, at least initially. For different lengths, the peptides show variable surface pressure sensitivity to development of some helical component. These authors further use their IR data to hypothesize the existence of the less-usual parallel (i-sheet conformation in these peptides. A critical comparison is available for different secondary structures as detected using the IRRAS data for peptides on H20 and D20 subphasesJ137 ... 

For solutions of AEg with different distributions of hydrocarbon chain lengths, the Y log C curves appear to be different than mono-component system. The surface pressure at critical micelle concentration (iTcjic) AEg with a long hydrocarbon chain (C gEg) is Increased by adding the short AEg, but the effect is not significant if the hydrocarbon chain is in a wide distribution (i.g. coconut fatty radical) (Figure 2,3,4). As for the efficiency of surface tension reduction there is a synergestic effect for the mixed... 

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