Metastable gel

Fig. 11. Effects of pH in the colloidal siUca-water system (1), where A represents the point of zero charge regions B, C, and D correspond to metastable gels, rapid aggregation, and particle growth, respectively. Positive and negative correspond to the charges on the surface of the siUca particle.

The better understanding of the mechanisms of stability incomplex dermatological emulsions stabilized by surfactants and amphiphiles has enabled the development of a rapid microscopic method for evaluation of potential emulsifiers. The method is based on the observation that good emulsifier blends that stabilize emulsions by the formation of multilayers of stable gel phase also swell spontaneously in water at ambient temperature and this process can be observed microscopically. Mixtures that do not form gel phase or form metastable gels only after a heating and cooling cycle cannot be observed to swell spontaneously at ambient temperature. ... 

The slurries used to manufacture zeolite Y are extremely reactive and small variations in their compositions induce dramatic changes in either morphology, structure or Si/Al [41-44]. In a recent systematic study of the effect of organic and inorganic cations on such a metastable gel... 

Although we have proposed three working hypotheses (i)-(iii) for the development of gelators, the most difficult problem is how to stablize the formed gel, in other words, how to prevent crystallization from the metastable gel to the crystalline state. Actually, most of the compounds we synthesized were precipitated as crystals, but not gels. 

As an illustrative example the monolayer behaviour of glycerol 1-myristate (1-monomyristin) will be considered. With excess water monomyristin gives a lamellar liquid-crystalline phase at temperatures above about 42 °C and a metastable gel phase below this temperature. A Il-A isotherm of monomyristin is shown in Fig. 8.24. First a monolayer phase with liquid chains is obtained, referred to as form I, and at higher pressure a phase with crystalline chains, form II, is formed. At the first-order transition, represented by the plateau where the two forms co-exist, the molecular area of form I is 28.5 A, in good agree-... 

In surfactant systems the stable hydrated crystals have a structure different from that of the metastable gel phase. Thus, the melting characteristics of the two stiuctures will be different. Kaneshina [79] found two transitions in the dodecyl-ammonium bromide-water system. The first was at 32.3°C [AH = 49.7 kJ/mol, AS = 163 J/(mol K)], which is the Krafft temperature and corresponds to the transition from the stable coagel phase (hydrated crystals) to the micellar solution. The other transition was detected at 24.3 C [AH = 41.4 kJ/mol AS = 139 1/ (mol K)] in supercooled samples, which corresponds to the transition from the metastable gel phase to the micellar solution. 

Chemical stabilization involves removing the concentration of surface hydroxyls and surface defects, such as metastable three-membered rings, below a critical level so that the surface is not stressed by rehydroxylation in use. Thermal stabilization involves reducing the surface area sufficiently to enable the material to be used at a given temperature without reversible stmctural changes. The mechanisms of thermal and chemical stabilization are interrelated because of the extreme effects that surface hydroxyls and chemisorbed water have on stmctural changes. Full densification of gels, such as the... 

As-polymerized PVDC is not in its most stable state annealing and recrystaUization can raise the temperature at which it dissolves (78). Low crystallinity polymers dissolve at a lower temperature, forming metastable solutions. However, on standing at the dissolving temperature, they gel or become turbid, indicating recrystaUization into a more stable form. 

Finally, in instances in which a bulky solute molecule with several functional groups can be added to the system, a fragile sort of structure can be built up by simultaneous attachment of these molecules to create a network with the characteristics of a gel. This system is then permanently metastable toward settling and caking, but may not withstand the ravages of shear or high temperature. 

Solid polymer and gel polymer electrolytes could be viewed as the special variation of the solution-type electrolyte. In the former, the solvents are polar macromolecules that dissolve salts, while, in the latter, only a small portion of high polymer is employed as the mechanical matrix, which is either soaked with or swollen by essentially the same liquid electrolytes. One exception exists molten salt (ionic liquid) electrolytes where no solvent is present and the dissociation of opposite ions is solely achieved by the thermal disintegration of the salt lattice (melting). Polymer electrolyte will be reviewed in section 8 ( Novel Electrolyte Systems ), although lithium ion technology based on gel polymer electrolytes has in fact entered the market and accounted for 4% of lithium ion cells manufactured in 2000. On the other hand, ionic liquid electrolytes will be omitted, due to both the limited literature concerning this topic and the fact that the application of ionic liquid electrolytes in lithium ion devices remains dubious. Since most of the ionic liquid systems are still in a supercooled state at ambient temperature, it is unlikely that the metastable liquid state could be maintained in an actual electrochemical device, wherein electrode materials would serve as effective nucleation sites for crystallization. 

Zeolite formation depends on reaction conditions 2-4). It is generally believed that most zeolites are formed as metastable phases. According to Barrer (3), the course of the synthesis, beginning with the type of starting material, determines the structure of the zeolite formed. The studies of Zhdanov 2, 5) on the composition of liquid and solid phases of hydrogels indicate that the kind and composition of the zeolite formed depend on the hydrogel composition and that the results of crystallization of aluminosilicate gels obtained in the same way are reproducible. 

Fig. la, h. The elastic part (ne) and the negative of the mixing part (— Jtm) of the osmotic pressure as functions of polymer concentration < >. The intercepts of ae and — nm correspond to the equilibrium state of neutral gels. Numbers besides each curve of — represent Xi> which increases with temperature, (a) x2 = 0. Only one root at all temperatures, (b) Xi = 0.56. Three roots appear in the intermediate temperature range (around Xi = 0.465), which correspond to stable, unstable, and metastable states, respectively. (Reproduced with permission from Ref. 20)... 

The phase coexistence observed around the first-order transition in NIPA gels cannot be interpreted by the Flory-Rehner theory because this theory tacitly assumes that the equilibrium state of a gel is always a homogeneous one. Heterogeneous structures such as two-phase coexistence are ruled out from the outset in this theory. Of course, if the observed phase coexistence is a transient phenomenon, it is beyond the thermodynamical theory. However, as will be described below, the result of the detailed experiment strongly indicates that the coexistence of phases is not a transient but rather a stable or metastable equilibrium phenomenon. At any rate, we will focus our attention in this article only on static equilibrium phenomena. 

---