Liquid-like state

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb s phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semicrystalline polymers are crystalline systems well removed from equilibrium. 

To obtain more detailed information on the ultrastructure of lipid dispersions and the morphology of the particles, electron microscopy is usually performed on replicas of freeze fractured or on frozen hydrated samples. These techniques aim to preserve the liquid-like state of the sample and the organization of the dispersed structures during preparation. By using special devices, the sample is frozen so quickly that all liquid structures, including the dispersion medium, solidify in an amorphous state. 

Condensed monolayer films of pure 6 polymerized rapidly, as did mixed 6/DSPE films of up to 75% DSPE, provided the monolayers were in the condensed state [33], In the liquid-expanded state, polymerization did not occur. In the condensed state, lateral diffusion of individual lipids within the monolayer is severely restricted compared to the liquid-like state. This precludes initiation of polymerization by diffusive encounter between excited-state and ground-state diacetylene lipids. In order for polymerization to occur in the condensed state, the film must be separated into domains consisting of either pure 6 or pure DSPE. A demonstration that the rates of photopolymerization for pure 6 and mixed 6/DSPE monolayers are equal would be a more stringent test for separate domains of the lipids, but no kinetic data have been reported for this system. 

Generally refers to the change from a crystalline to a liquid-like state in membranes and membrane-mimetic systems. The temperature (or the range of temperatures) at which the crystalline phase is converted to the liquid phase is referred to as the phase transition temperature. 

In his chapter, Mobility of Plasticizers in Polymers, R. Kosfeld describes his recent finding that a portion of a liquid plasticizer remains in the liquid-like state in the plasticized resin even below its glass transition temperature. 

Interpretation of the Calorimetric Results. There is little doubt that the transition observed in M. laidlawii membranes arises from the lipids since it occurs at the same temperature in both intact membranes and in water dispersions of membrane lipids. It is reasonable to conclude that in both membranes and membrane lipids the lipid hydrocarbon chains have the same conformation. The lamellar bilayer is well established for phospholipids in water (I, 20, 29) at the concentration of lipids used in these experiments. In the phase change the hydrocarbon core of the bilayer undergoes melting from a crystalline to a liquid-like state. Such a transition, like the melting of bulk paraffins, involves association between hydrocarbon chains and would vanish or be greatly perturbed if the lipids were apolarly bound to protein. We can reasonably conclude that most of the lipids in M. laidlawii membranes are not apolarly bound to protein. 

Interpretation of the NMR Spectra of Membranes. Let us first consider a model system of lysolecithin and serum albumin (74), shown in Figure 12. Lysolecithin in D20 is shown in Figure 12A, while the effect of increasing amounts of bovine serum albumin is shown in Figures 12B and 12C. When no protein is present, the lines for both the methylene protons on the lysolipid tails and the quaternary ammonium methyl protons on the choline moiety of the polar heads are narrow. Thus, the polar ends of the molecules are free to move, and the apolar hydrocarbons within the detergent micelles are in a liquid-like state (24). When protein is added, the hydrocarbon proton line broadens and shifts upfield slightly, but neither the width nor area of the quaternary ammonium line... 

A detailed study of the interaction of hydrocarbons with cetyl trimethyl ammonium bromide modified montmorillonite has been done using Raman spectroscopy [82]. The quaternary salt was observed to be in a liquid-like state and it was concluded that interaction of organic compounds in this system is best classified as absorption. 

The kink observed around 367 K corresponds to a change of the thermal expansion coefficient from a glassy to a liquid-like state and, by that, marks the position of the glass transition temperature. Usually, the 7g is calculated as a intersection point between two linear dependencies. Nevertheless, a more convenient method is the calculation of the first and second numerical derivatives of the experimental data (Fig. 15b,c). In this case, the Tg is defined as the minimum position in the second numerical derivative plot (Fig. 15c). Down to a thickness of 20 nm, no shifts of 7g as determined by capacitive scanning dilatometry were found (Fig. 16). 

For chemical applications, vibrational spectroscopy of high-pressure fluid phases, including liquids and compressed gases, is of special importance (Buback, 1991). The fluid, i.e., the non-solid region of a substance, is illustrated in Fig. 6.7-2. The packing density of the circles is approximately proportional to the density of a substance. The bottom left part of Fig. 6.7-2 shows the vapor pressure curve which, up to the critical point, separates the liquid phase from the gas phase. Above the critical temperature (7 ), the density of a substance may change continuously between gaseous and liquid like states vibrational spectroscopic methods make it possible to study the structure and dynamics... 

The above conclusions appear to account fairly well for the experimentally observed behaviour of egg PC vesicles. The qualitative features of the results, however, should be generally applicable to all one-component vesicles composed of amphiphiles whose hydrocarbon chains are in a liquid-like state. 

The energy that flows to the water or organic solvent interface is used in two ways. First, and most desirable, it is used to transform the water or organic solvent from a liquid or liquid-like state to a vapor state. The second use, often less desirable, is to raise the temperature of the interface. The distribution can be expressed in terms of an energy balance... 

More intuitively the elevating effect of the attractive pore potential can be understood as follows. In a pore with strongly attractive potential, a liquid-like state can hold even with lower vapor pressure than the saturated one. When this system is equilibrated with pure liquid or saturated vapor, the excess potential must be balanced with the increase in density... 

The results stated so far has been with saturated vapor or liquid as the equilibrium bulk phase. Liquid-like state in pore, however, can hold with reduced vapor pressure in bulk the well-known capillary condensed state. One of the most important feature of the capillary condensation is the liquid s pressure Young-Laplace effect of the curved surface of the capillary-condensed liquid will pull up the liquid and reduce its pressure, which can easily reach down to a negative value. In the section 2 we modeled the elevated freezing point as a result of increased pressure caused by the compression by the excess potential. An extension of this concept will lead to an expectation that the capillary-condensed liquid, or liquid under tensile condition, must be accompanied with depressed freezing temperature compared with that under saturated vapor. Then, even at a constant temperature, a reduction in equilibrium vapor pressure would cause phase transition. In the following another simulation study will show this behavior. 

Typical results are shown in Fig. 6 for U-methane in graphite pores of H =7.5 at T=114 K. At p/ps=l the system is solid-like at this temperature, but a discrete change in density occurs around p ps ca.0.5. The self diffiisivity along axial direction also shows drastic change at this point. Further examination of various characteristics of molecular state such as snapshots, in-plane pair correlations and static structure factors confirmed that this change in density is the result of a phase transition from solid-like state to liquid-like one, or melting. Since the critical condensation condition for this pore is far lower than this transition point to stay around p ps= ca.0.2, the liquid-like state is not on metastable branch but thermodynamically stable. Thus a solid-liquid coexistence point is found for this temperature. 

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