Packings, structured aqueous systems

The gel phase consists of crystalline lipid bilayers alternating with water layers. When the L -phase is cooled through the hydrocarbon chain crystallization temperature, a gel phase can be formed which is usually metastable. There are even amphiphile-water systems which exhibit thermodynamically stable gel phases as the only type of lipid-water phase. One example is the tetradecylamine-water system (Larsson and Al-Mamun, 1973) shown in Fig. 8.17. Other lipids which only give gel phases in their aqueous systems are cholesterol sulphate and cholesterol phosphate (Abrahamsson et al., 1977). The gel phase of tetradecylamine consists of bilayers with vertical chains in the orthorhombic chain packing. At low water content this structure swells to a water layer thickness of 14 A. At very high water concentrations, however, another gel phase with the same lipid bilayers but with a water layer several hundred A thick is formed. The reason for this seems... 

In essence, the packing parameter is a measure of the ratio between the effective areas occupied by the hydrophobic (5 ) and hydrophilic (a,) parts of the surfactant. This model is ideal when considering aqueous systems, but may also be applied to dispersions in oil. Depending on the surfactant structure, Pp assumes specific values and the packing constraints in the medium allow for the formation of a preferred aggregate shape configuration. This theoretical approach is also considered in microemulsion systems, as discussed below. 

The plastic gauze packing is i esented in Fig. 37A. It has been used indudrially with great success for maiQr years. The special gauze structure provides a very good wettability, even in aqueous systems. This pm king is used primarily for cohmms with low liquid lo. ... 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

It is well known that the aqueous phase behavior of surfactants is influenced by, for example, the presence of short-chain alcohols [66,78]. These co-surfactants increase the effective value of the packing parameter [67,79] due to a decrease in the area per head group and therefore favor the formation of structures with a lower curvature. It was found that organic dyes such as thymol blue, dimidiiunbromide and methyl orange that are not soluble in pure supercritical CO2, could be conveniently solubihzed in AOT water-in-C02 reverse microemulsions with 2,2,3,3,4,4,5,5-octafluoro-l-pentanol as a co-surfactant [80]. In a recent report [81] the solubilization capacity of water in a Tx-lOO/cyclohexane/water system was foimd to be influenced by the compressed gases, which worked as a co-surfactant. 

Monolithic silica can also be prepared by hydrolytic condensation of tetra-alk-oxysilanes, a process similar to the preparation of particles [13-19], The formation of the network structure utilizes spinodal decomposition of the polymerization system of tetramethoxysilane in aqueous acetic acid in the presence of a water-soluble polymer (e.g., polyethylene glycol) [20-22]. The preparation processes starting from monomers seem to be simpler than those starting from a particle-packed column, and the products can have higher permeability. 

These uncertainties question the validity of the above theory. Due to the poor understanding of this phenomenon, it is best to exercise caution with HETP predictions for all of the following types of systems on structured packings aqueous, high surface tension, high liquid viscosity, and high relative volatility. 

Underwetting (Sec. 8.2.16). With aqueous-organic systems, HETP tends to increase at the aquedus end of the column, both with random and structured packings. 

Summary. In the preloading regime, packing size, type, and distribution affect HETP. With aqueous-organic systems, HETP may be sensitive to underwetting and composition. HETP of structured packings may also be affected by pressure (at high pressure), and vapor and liquid loads. 

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