A-gel phase

Contrary to the phase separation curve, the sol/gel transition is very sensitive to the temperature more cations are required to get a gel phase when the temperature increases and thus the extension of the gel phase decreases [8]. The sol/gel transition as determined above is well reproducible but overestimates the real amount of cation at the transition. Gelation is a transition from liquid to solid during which the polymeric systems suffers dramatic modifications on their macroscopic viscoelastic behavior. The whole phenomenon can be thus followed by the evolution of the mechanical properties through dynamic experiments. The behaviour of the complex shear modulus G (o)) reflects the distribution of the relaxation time of the growing clusters. At the gel point the broad distribution of... 

Sixou et al. (101) showed the circular dichroism of cholesteric CTA solutions in TFA depends on the CTA molecular weight. The intensity of the circular dichroic peak increases with molecular weight. Meeten and Navard (97) studied gel formation and liquid crystallinity in TFA-H2O solutions of CTA. When water was added to a liquid crystalline solution of CTA in TFA a gel phase formed presumably by the formation of crosslinks due to hydrogen bonding. They interpreted their results that liquid crystalline ordering involves both inter- and intramolecular forces. 

The addition of water causes the formation of a coagulated phase of PBT solutions in any of the solvents named above (of course, the amount of water tolerated varies with the solvent, with the PPA solvent being the most tolerant toward water) (4). In very dilute solutions, the water causes enhanced depolarized scattering, interpreted to be the result of the formation of aggregates in which the rodlike chains are in parallel arrays (10). In more concentrated solutions, a gel phase is created (11,12). In either case, the electronic absorption spectra is altered from that characteristic of the protonated chain to that characteristic of the deprotonated, dry polymer (10). In the following we will report observations on this phase transition. 

The vast majority of biological membranes are in the liquid-crystalline phase. There are many experimental studies on model bilayer phase behavior [3]. Briefly, at low temperatures lipid bilayers form a gel phase, characterized by high order and rigidity and slow lateral diffusion. There is a main phase transition, as the temperature is increased, to the liquid-crystalline phase. The liquid-crystalline phase has more fluidity and fast lateral diffusion. 

The lamellar spacing of a monoglyceride gel phase as a function of water content is plotted in Figure 14. The gel phase of the neutral monoglyceride has a lipid bilayer thickness of 49.5 A, and it swells to a unit layer thickness of 64 A (20). If an ionic amphiphilic substance (e.g. a soap) is solubilized in the lipid bilayer, it is possible to obtain a gel phase with high water content. As with the gel phases with infinite swelling that were discussed above, there is, however, a minimum water layer thickness which in this monoglyceride gel is about 40 A. 

Free fatty acids, derived primarily from adipocyte triglycerides, are transported as a physical complex with plasma albumin. Triglycerides and cholesteryl esters are transported in the core of plasma lipoproteins [134], Deliconstantinos observed the physical state of the Na+/K+-ATPase lipid microenvironment as it changed from a liquid-crystalline form to a gel phase [135], The studies concerning the albumin-cholesterol complex, its behavior, and its role in the structure of biomembranes provided important new clues as to the role of this fascinating molecule in normal and pathological states [135]. 

Many other foods are mixed dispersions, like ice cream which is an emulsion, foam, and suspension. Others abound. Sausages and frankfurters may be considered to be solidified O/W emulsions in which the oil droplets are covered by a protein membrane and dispersed in a gel [293]. Similarly, cakes can be considered to be air bubbles dispersed in a gel phase. 

Food grade surfactants are, in general, not soluble in water, but they can form association structures in aqueous medium that are liquid crystalline in nature. These liquid crystalline structures are produced by heating the solid emulsifier (which is dispersed in water) to a temperature above its Krafft temperature. On cooling such a system, a gel phase is produced which becomes incorporated with the emulsion droplets. These gel phases produce the right consistency for many food emulsions. 

Stratum corneum lipids and lanolin share an important physical characteristic in that they can coexist as solids and liquids at physiological temperatures.33 A differential scanning calorimetry thermogram of lanolin is similar to that of stratum corneum lipids, showing two broad (heterogenous) phase transitions with midpoint melting temperatures at 21.9 and 38.3°C.16 The lower temperature peak may represent the transition from a liquid crystal to a gel phase, which has also been described for lanolin alcohols.34... 

In the 1970s, the fluid mosaic concept emerged as the most plausible model to account for the known structure and properties of biological membranes [41]. The fact that membranes exist as two-dimensional fluids (liquid disordered) rather than in a gel state (solid ordered) was clearly demonstrated by Frye and Ededin [42], who showed that the lipid and protein components of two separate membranes diffuse into each other when two different cells were fused. Since that time, numerous studies have measured the diffusion coefficient of lipids and proteins in membranes, and the diffusion rates were found to correspond to those expected of a fluid with the viscosity of olive oil rather than a gel phase resembling wax. 

The dimensions of the space available are in the nanometer range. At this length scale, water is supposed to behave as a constrained liquid, which follows rules of diffusion, flow and structuring more akin to those of gels than to those observed in free liquids [108]. Furthermore, if the silk is present as a gel phase, the structured nature of the water molecules is even more enhanced. This in turn affects the activities of the ions within this medium, especially where polyelectrolytes are also involved. This speculative scenario envisages that the chemical environment of nucleation is very different from a simple saturated solution, and that the thermodynamics and kinetics of nucleation are more akin to crystallization from hydrogels. The same situation exists also in collagen-mediated mineralization, where the tiny apatite crystals form inside the... 

It has been suggested that these ceramides form a gel-phase membrane domain within the skin. Straight fatty acid chains as well as the small polar head groups on the ceramides are thought to produce a tightly packed domain which is less fluid and thereby less permeable than other liquid crystalline domains which are also present. Recent evidence using differential scanning calorimetery (DSC) and infrared absorption spectroscopy analyses verifies the presence of gel phases within the stratum comeum. 

FIGURE 9.6 Low-angle neutron scattering intensity as a function of scattering vector Q (A-1) for propylammonium vermiculite immersed in a 0.25 M propylammonium chloride solution at four different temperatures, T = 309 K (continuous line), 311 K (dashed line), 313 K (crosses) and 315 K (solid circles). The low-angle peak at 0.1 A-1 is due to a gel phase with a clay layer spacing of 60 A. The data therefore show the temperature-induced gel-crystal phase transition occurring between 311 and 313 K. Note that body temperature is 311 K and that cell fluids are typically 0.2 M. 

As seen in Table 3.9, the difference between the experimental and calculated values is below 0.033 in the case of the Egyhazaskeszo, Istenmezeje (black and yellow), Mad Ujhegy, and Sajobabony samples. This value is in the range of the deviation of the determination of the mineral composition and the sorption experiments (5%-10%). So, the model, similar to cesium ion, fairly well describes the relation of the mineral composition and adsorbed quantity of strontium ion on different rocks. The deviation is higher in the case of the two samples (Kuzmice, Muiden), the main component of which is cristobalite, and it can be explained with the presence of a gel phase (Chapter 1, Section 1.3.6.1). 

Polacco, G., Semino D., Rizzo C., Feasibility of methyl methacrylate polymerization for bone cement by suspension polymerization in a gel phase, J. Mater. Sci. Mater. Med. 5 (1994) 587. 

Figure 2 Lipid bilayer composed of DPPC lipids in two different phases, (a) Gel phase below the main transition temperature (b) Fluid phase above the transition temperature. Water is not shown for clarity.

Macroporous Polymers Prepared by Bulk Polymerization. With toluene as diluent (DVB-50-B-T) the highly crosslinked networks closely parallel the pure solvent correlation line indicating the probe is highly solvated even in poor polymer solvents (Figure 3). This result indicates a gel phase with a lilgh degree of permanent micropore structure. There is essentially no difference between this material and that prepared by suspension polymerization (DVB-50-S-T). 

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