Rigid gels

The materials originally used as stationary phases for GPC were the xerogels of the polyacrylamide (Bio-Gel) and cross-linked dextran (Sephadex) type. However, these semi-rigid gels are unable to withstand the high pressures used in HPLC, and modern stationary phases consist of microparticles of styrene-divinylbenzene copolymers (Ultrastyragel, manufactured by Waters Associates), silica, or porous glass. 

On drying a rigid gel the aqueous phase enmeshed in the feltlike fibrils begins to evaporate, the gel is sufficiently rigid as not to collapse as the interfibrillar spaces become emptied of water and filled with air. Such a gel will, on immersion in other solvents, e.g. toluene, benzene or alcohols, imbibe the liquid. 

Acetylated wheat distarch phosphate pastes had improved clarity and better storage stability than the pastes of unmodified and crosslinked alone starches when 1% aqueous dispersions were heated in a boiling water bath for 30 minutes and stored at two different temperatures (4°C and 25°C) for 9 days.27 Storage at 4°C of 5% pastes of native and crosslinked wheat starch revealed that both were unstable after one day, the soft texture of the initial pastes being transformed into an opaque and rigid gel that exhibited syneresis. In contrast, the acetylated distarch phosphate was stable for at least... 

As water is withdrawn from a membrane during entry into anhydrobiosis, the strengths of interactions among acyl chains increase, and a shift from the fluid to the gel state is favored. This shift is noticeable as water content is reduced below about 20% (Crowe et al., 1997). Transition temperatures (Tm), the temperatures at which the fluid (liquid crystalline) to gel conversion occurs, increase significantly during dehydration. A membrane composed of palmitoyloleoylphosphatidyl choline has a Tm of —7°C when fully hydrated, but the Tm rises to approximately 60°C in the dry lipid (Crowe et al., 1997). Thus, a phospholipid membrane that would be in a fluid state at normal cell temperatures when hydrated acquires a rigid gel structure when dehydrated. 

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