Variation in gel composition

Notice that small compositional variations (preparations 1-3) are allowable. However, 25Z variations in gel composition do not crystallize pure DPA-VPI-5. It is interesting to note that the water content is important and that excess DPA (preparation 11) hinders the crystallization of VPI-5. 

With the recent commercial availability of ultrafiltration, it has become possible to increase the concentration of casein, thus levelling out seasonal variations in milk composition and consequently in gel characteristics and... 

In a later paper, De Haan et al. (1981b) report on the seasonal variation in the composition of fulvic acids determined by Curie point pyrolysis-mass spectrometry. Water samples were collected monthly between January and September, 1978, and fulvic acid fractions were isolated by gel permeation chromatography. Differences in the chemical composition (particularly striking in the pyrograms of fractions of high apparent molecular weight)... 

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... 

From this data it can be seen that the sol-gel technique allows for the rapid production of macroporous polyester xerogels produced via ambient drying in the medium chosen. In addition, these results demonstrate that the pore stmcture may be tuned by altering composition and / or concentration of the precursor solutions, allowing for the variation of composition with minimal structural variation or vice-versa. Additional variations in gel structure can also be produced by allowing gelation to occur in open versus closed containers, with the former accelerating the reaction due to more effective... 

Suzuki et al. (14) have shown that upon variation of the gel composition as many as five different zeolite structures (ZSM-5, ZSM-35, ZSM-39, ZSM-48 and KZ-1) can be synthesized in the presence of pyrrolidine (14). Hence, not only the nature of the organic template but also tFie gel chemistry and synthesis conditions have a marked influence on the structure of the crystalline product. In a recent systematic study it was pointed out that especially pH and Na+-content have to be carefully adjusted in order to avoid the formation of phases other than ZSM-23 (4). 

The form of the dissolved sulfur has not been characterized properly yet. While stable at ambient temperatures, a substantial amount can be converted to crystalline sulfur at elevated temperatures or by solvent separation. This observation led to the development of a rapid liquid chromatography method to determine elemental sulfur in SA binders. The procedure which has been described previously by Cassidy (17) is based on gel permeation principle and uses a Styragel column and a uv detector. Results showed that 2-14% of the elemental sulfur added reacted chemically with the asphalt. Petrossi (18) and Lee (19), who determined free sulfur by extraction with sodium sulfite followed by titration with iodine, calculated a higher percent of bonded sulfur in sulfur-asphalt compositions. The observed differences are most likely caused by variations in the asphalt composition with regard to polar aromatics and naphthene components as well as by reaction temperature and contact time. 

As it is well known [36,37], the natural lipid/protein mixtures (such as amniotic fluid) can undergo different phase transitions due to variation in temperature or composition. Of special importance for the natural bilayer lipid membranes is the so-called main phase transition between the lipid crystalline and gel states at which a melting of the hydrocarbon tails of the lipid molecules occurs. For example, it has been demonstrated [36] that there exists an upper limit of the gel phase content in membranes above which the membrane morphology and permeability change dramatically thus making the execution of the physiological functions of the membrane impossible. 

Other approaches to reducing the membrane pore size are being investigated. Many of them are based on the sol-gel process or chemical vapor deposition as discussed in Chapter 3. An example is the preparation of small-pore silica membranes. Amorphous silica membranes have been prepared from solutions of silicate-based polymers. More specifically, some strategies are employed aggregation of fractal polymeric clusters, variation of sol composition, the use of organic molecular templates and modification of pore surface chemistry [Wallace and Brinker, 1993]. 

The term amorphous silica encompasses an almost infinite variety of structural forms, from ordered opaline aggregates to extended gel-like materials. In all these structures, the mineral exists as a hydrated, covalent inorganic polymer of general formula [SiO /2 (OH)4- ]m 4). This formula, where n = 0 to 4 and m is a large number, indicates the variation in residual functional groups within the condensed structure. There is similar variation in the extent of hydration. This flexibility in composition and reactivity indicates that biogenic silica is not a stoichiometric mineral (in the way that CaCOs is, for example) and that the nature (density, hardness, solubility, viscosity) and composition of siliceous structures in biology may vary consid-... 

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