Saturated-LMWE monolayers

For saturated-LMWE monolayers the liquid expanded (LE), liquid-condensed (LC), solid (S) stmctures and, finally, the collapse at the highest surface pressure take place as a function of surface pressure and... 

At TT > TTg the relaxation phenomena for insoluble monolayers are caused by the transformation of a homogeneous monolayer phase into a heterogeneous monolayer-collapse phase system. However, some differences exist between saturated-LMWE and unsaturated-LMWE monolayers (Eigure 14.6b). Relaxation phenomena in saturated-LMWE monolayer are controlled predominantly by the collapse mechanism because the surface pressure relaxes to TTg. Eor these systems the monolayer collapses by nucleation and growth of critical nuclei. Unsaturated-LMWE monolayers behave differently to saturated-LMWE monolayers. As the surface pressure relaxes from the collapse value, which is close to TTg, towards values lower than TTg at longer times, the collapse competes with a desorption mechanism (Patino and Nino, 1999). 

In contrast with saturated-LMWE, imsaturated-LMWE monolayers present only the liquid expanded structure and the collapse. BAM images corroborate that only the homogeneous LE phase is present during the compression of unsaturated-LMWE monolayers. From the observation with BAM along the film balance no fractures were visualized after the monolayer collapse. These monolayers collapse with the formation of lenses. 

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... 

The existence of protein-LMWE interactions depends on the interfacial composition and on the protein/LMWE ratio. In general, the surface activity of the mixed films is determined by the LMWE as the surface pressure of the mixed film is the same as the LMWE equilibrium spreading pressure, and the monolayer is not saturated by the protein. However, the protein determines the surface activity of mixed films as the protein saturates the monolayer. In the intermediate region there exists coexistence of protein and LMWE at the interface. 

From a systematic study focused on fhe tt-A isofherm of protein-LMWE mixed monolayers (including fhe application of fhe additivity rule on miscibility and the quantification of inferacfions between monolayer components by excess free energy ( if has been concluded that, at a macroscopic level, these compounds form a pracfically immiscible monolayer at the air-water interface, af tt < Tlf At higher tt the collapsed protein is displaced from the interface by LMWE (monoglycerides, phospholipids, etc.). The existence of low profein interactions in disordered proteins ((3-casein and caseinate) facilitates the protein displacement by LMWE from fhe air-water interface. However, the lower surface acfivify of unsafurafed-LMWE explains the fact that this lipid has a lower capacity than saturated-LMWE for protein displacement. 

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