Diacetylenes pressure area isotherm

Figure 15. Pressure-area isotherms for spread diacetylene monolayer of cadmium...

Figure 3. Pressure area isotherm at room temperature for the diacetylene under...

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. 

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

Due to the topochemical restrictions of diacetylene polymerization, diacetylenic lipids are solely polymerizable in the solid—analogous phase. During the polyreaction an area contraction occurs leading to a denser packing of the alkyl chains. In addition to surface pressure/area isotherms the polymerization behavior of diacetylenic lipids containing mixed films give information about the miscibility of the components forming the monolayer ... 

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. 

Pressure-area isotherms indicate the an hiphiles of I and II on pure water both had identical take-off areas of 25 AVmolecule, corresponding to the molecular cross-section of the hydrocarbon-diacetylene stmcture. The film of I collapses at low pressure ( 12 ihN/m), but upon over-conpression reaches a stable solid phase with a limiting molecular area of 8 AVmolecide. This overcompressed state corresponds to a stable trilayer structure. The film of II was stable as a monolayer with a collapse pressure of ca. 35 mN/m and an extrapolated molecular area at zero pressure of 25 A /molecule. After equilibration, films were polymerized to the blue-phase by e qrosure to incidence powers of 40 pW/cm for I and 23 pW/cm for II over a period of 30 sec. Red-phase films were produced by exposing the trilayer of I to 500 pW/cm and the monolayer of II to 40 pW/cm for 5 min. 

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