Multi-phase gels

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

The stable multi-phases are found not only in polyampholyte gels but also in a homopolymer gel, e.g. poly(/V-acryloyl-4-aminosalicyclic acid) gel [130], and... 

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

Analyses of Pitch. Modern analytical facilities of high-pressure liquid chromatography, gel permeation chromatography, an(j 1 nuclear magnetic resonance and mass spectrometry, associated with 1R and UV spectroscopy enable a total molecular constituent analysis of pitch composition to be obtained. The use of such information could then possibly be the route to prediction of pitch quality on carbonization. It would appear that such an approach would not be successful (ignoring the cost factor for such detailed analysis). The pitch cannot be considered as an assembly of molecules which pyrolyse independently of each other. The pitch carbonizes as a multi-phase system and experience today would indicate the impossibility of predicting all interactions, physical and chemical. 

Therefore, alternative carrier substances have been investigated in recent years. Among them, lipidic materials have garnered growing attention. Successful peptide or protein incorporation and delivery has been reported for liposomes [7], multi-vesicular liposome preparations [8], cubic phase gels [9], hollow lipid microparticles [10], hollow lipid microcylinders [11], microparticles [12,13], and sohd lipid nanoparticles (SLN) for intravenous applications [14,15],... 

The model proposed by Tu and Ouano [43] for polymer dissolution assumes Fickian solvent penetration into the polymer. The polymer dissolution problem was modeled as a multi-phase Stefan problem [44], The key parameter in this model was the disassociation rate, R, which was defined as the rate at which the polymer transformed from a gel-like phase to a solution. It was proposed that the dissolution process was disassociation -controlled if the polymer diffusion rate in a liquid layer adjacent to the solvent-polymer interface was faster than the disassociation rate, or diffusion -controlled if the diffusion rate was slower than the disassociation rate. 

As y is further decreased the signals disappear almost completely (J4-J1 at 30 °C) due to chain freezing. The poor signal-to-noise ratio observed for the samples in the gel regime shows that only a very small fraction of the chains is not frozen. Furthermore, the shape of the CH2 peak of samples J8-J3 indicates the presence of several phases in which the components have slightly different chemical shifts. This proves the multi-phase structure also of mixtures which do not show a macroscopic phase separation. 

Fig. 10 Cryo-TEM images of sample J3 (gel) and J6 (multi-phase region) (top) and contours of the aggregates obtained by image tracing (bottom)...

Unfortunately, exclusion chromatography has some inherent disadvantages that make its selection as the separation method of choice a little difficult. Although the separation is based on molecular size, which might be considered an ideal rationale, the total separation must be contained in the pore volume of the stationary phase. That is to say all the solutes must be eluted between the excluded volume and the dead volume, which is approximately half the column dead volume. In a 25 cm long, 4.6 mm i.d. column packed with silica gel, this means that all the solutes must be eluted in about 2 ml of mobile phase. It follows, that to achieve a reasonable separation of a multi-component mixture, the peaks must be very narrow and each occupy only a few microliters of mobile phase. Scott and Kucera (9) constructed a column 14 meters long and 1 mm i.d. packed with 5ja... 

Typical characterization of the thermal conversion process for a given molecular precursor involves the use of thermogravimetric analysis (TGA) to obtain ceramic yields, and solution NMR spectroscopy to identify soluble decomposition products. Analyses of the volatile species given off during solid phase decompositions have also been employed. The thermal conversions of complexes containing M - 0Si(0 Bu)3 and M - 02P(0 Bu)2 moieties invariably proceed via ehmination of isobutylene and the formation of M - O - Si - OH and M - O - P - OH linkages that immediately imdergo condensation processes (via ehmination of H2O), with subsequent formation of insoluble multi-component oxide materials. For example, thermolysis of Zr[OSi(O Bu)3]4 in toluene at 413 K results in ehmination of 12 equiv of isobutylene and formation of a transparent gel [67,68]. 

Notwithstanding all its advantages, the principle of solid-phase synthesis cannot be applied to all kinds of chemical reactions. Although reactants are used in excess, reaction is not always quantitative. The resulting impurities cannot be separated on the solid phase, giving rise to separation problems particularly in multi-step systems. Moreover, only limited use can often be made of conventional analytical methods (NMR, MS). Recent methods of 13C-NMR spectroscopy on solid phases [21] or in gel phases [22] are ideally suited for solid-phase synthesis, but are not universally available owing to the expensive instrumentation. 

Natural abundance gel-phase 13C NMR spectroscopy is used for assessing intermediate structures in solid-phase synthesis [20], but requires long acquisition times and so is not useful for following reactions. Affymax scientists [21] have pioneered the use of specifically 13C-labeled starting materials in conjunction with fast 13C-gel phase NMR to follow multi-step reactions [17,22,23],... 

Thus, it was shown that gold on carbon was much better than the more complicated multi-metallic Pt/Pd/Bi catalysts evaluated previously [365,386-392] for the liquid-phase oxidation of D-glucose, giving 100% selectivity to gluconic acid [360,361], Claus and co-workers also studied a series of Au/C catalysts prepared by the sol-gel method and got conversions and selectivities... 

The DHA-containing phospholipids exhibit very low phase transition temperatures. For example, 18 0,22 6 PC has its at about-9°C (Stillwell etal., 2000), whereas 22 6,22 6 PC s transition is at -68.4°C (Kariel et al., 1991) and the transition enthalpy H for homo acid is extremely low ( H = 0.5 kcal/mol) (Kariel et al., 1991). The low T, s are the result of packing restrictions resulting from steric effects caused by DHA s multiple rigid double bonds. The restrictions result in reductions in intermolecular and intramolecular van der Waal s interactions. The broad, low H transitions are consistent with the notion that interaction between saturated sn-1 chains stabilize the gel state in hetero acids (Kariel et al., 1991). Furthermore, differential scanning calorimetry (DSC) isothenns are often multi-component, suggesting microclustering and domain formation (Niebylski Salem, 1994). 

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