Middle phases

Fig. 1. Phase diagram of an amphiphile—oil—water system that forms a middle-phase microemulsion, definition of microemulsion, and illustration of the...

In Figure 1, the pairs (or triad) of phases that form ia the various multiphase regions of the diagram are illustrated by the corresponding test-tube samples. Except ia rare cases, the densities of oleic phases are less than the densities of conjugate microemulsions and the densities of microemulsions are less than the densities of conjugate aqueous phases. Thus, for samples whose compositions He within the oleic phase-microemulsion biaodal, the upper phase (ie, layer) is an oleic phase and the lower layer is a microemulsion. For compositions within the aqueous phase-microemulsion biaodal, the upper layer is a microemulsion and the lower layer is an aqueous phase. When a sample forms two layers, but the amphiphile concentration is too low for formation of a middle phase, neither layer is a microemulsion. Instead the upper layer is an oleic phase ("oil") and the lower layer is an aqueous phase ("water"). 

In three-phase systems the top phase, T, is an oleic phase, the middle phase, Af, is a microemulsion, and the bottom phase, B, is an aqueous phase. Microemulsions that occur ia equiUbrium with oae or two other phases are sometimes called "limiting microemulsions," because they occur at the limits of the siagle-phase regioa. 

However, often the identities (aqueous, oleic, or microemulsion) of the layers can be deduced rehably by systematic changes of composition or temperature. Thus, without knowing the actual compositions for some amphiphile and oil of poiats T, Af, and B ia Figure 1, an experimentaUst might prepare a series of samples of constant amphiphile concentration and different oil—water ratios, then find that these samples formed the series (a) 1 phase, (b) 2 phases, (c) 3 phases, (d) 2 phases, (e) 1 phase as the oil—water ratio iacreased. As illustrated by Figure 1, it is likely that this sequence of samples constituted (a) a "water-continuous" microemulsion (of normal micelles with solubilized oil), (b) an upper-phase microemulsion ia equiUbrium with an excess aqueous phase, ( ) a middle-phase microemulsion with conjugate top and bottom phases, (d) a lower-phase microemulsion ia equiUbrium with excess oleic phase, and (e) an oA-continuous microemulsion (perhaps containing iaverted micelles with water cores). 

Fig. 2. The phase diagrams and terminology of a microemulsion system close to its two critical end points, where the middle phase and one of the binodals...

Of all the characteristic points in the phase diagram, the composition of the middle phase is most sensitive to temperature. Point M moves in an arc between the composition of the bottom phase (point B) at and the composition of the top phase (point T) at reaching its maximum surfactant concentration near T = - -T )/2. (Points B and Tmove by much smaller amounts, also.) The complete nonionic-amphiphile—oh—water—temperature... 

Fig. 7. Cartoon illustration of the twelve theoretical morphologies of three-phase macroemulsions in which one the phases is a middle-phase...

Figure 28.23 Balancing of phase currents in a large three-phase system by introducing a reactor in the middle phase...

Consider a bus duct having a rated current of 4000 A and an unbalanced current in the middle phase of 4400 A. Determine the size of the reactor to achieve a balanced voltage system. 

Designing a reactor for the middle phase to balance a large unbalanced current-carrying system... 

The SCL2 structure is composed of three different embedded periodic surfaces. The middle surface is the Schwarz minimal surface P. Similarly, the middle phase surface in GLl (Fig. 8(c)) and GL2 (Fig. 8(d)) structures is the Schoen minimal surface G. 

An example for a partially known ternary phase diagram is the sodium octane 1 -sulfonate/ 1-decanol/water system [61]. Figure 34 shows the isotropic areas L, and L2 for the water-rich surfactant phase with solubilized alcohol and for the solvent-rich surfactant phase with solubilized water, respectively. Furthermore, the lamellar neat phase D and the anisotropic hexagonal middle phase E are indicated (for systematics, cf. Ref. 62). For the quaternary sodium octane 1-sulfonate (A)/l-butanol (B)/n-tetradecane (0)/water (W) system, the tricritical point which characterizes the transition of three coexisting phases into one liquid phase is at 40.1°C A, 0.042 (mass parts) B, 0.958 (A + B = 56 wt %) O, 0.54 W, 0.46 [63]. For both the binary phase equilibrium dodecane... 

Fatty alcohol- (or alkyl-)ethoxylates, CoE, are considered to be better candidates for LLE based on their ability to induce rapid phase separation for Winsor II and III systems. (Winsor III systems consist of excess aqueous and organic phases, and a middle phase containing bicontinuous microemulsions.) However, C,E,-type surfactants alone cannot extract biomolecules, presumably because they have no net negative charge, in contrast to sorbitan esters [24,26,30,31]. But, when combined with an additional anionic surfactant such as AOT or sodium benzene dodecyl sulfonate (SDBS), or affinity surfactant, extraction readily occurs [30,31]. The second surfactant must be present beyond a minimum threshold value so that its interfacial concentration is sufficiently large to be seen by... 

In an earlier study calorimetry achieved this objective for the compositional boundaries between two and three phases (2). Such boundaries are encountered both in "middle-phase microemulsion systems" of low tension flooding, and as the "gas, oil, and water" of multi-contact miscible EOR systems (LZ). The three-phase problem presents by far the most severe experimental and interpretational difficulties. Hence, the earlier results have encouraged us to continue the development of calorimetry for the measurement of phase compositions and excess enthalpies of conjugate phases in amphiphilic EOR systems. 

The middle phase of this process may be turbulent as a new equilibrium is being established, and this period typically coincides with aftercare for the client. Many of the issues discussed previously become a central focus for the family during the aftercare period. As the issues become resolved and family members become more familiar and comfortable with the new, drug-free loved one, then family equilibrium may be restored at a new level. Some family systems do not survive these changes, meaning that a partnership may dissolve. However, many families and partnerships do weather the storms and grow much closer because of this process of separate and collective recovery that takes place after a drug problem has been overcome. The final phase for a family that weathers the storm... 

Bulk Properties. Because of viscosity and stability requirements for product manufacture and processing, the appearance of mesomorphous phases in mixtures with water is very important. In the case of dodecyl monoglycol ether sulfate and dodecyl sulfate, a highly viscous middle phase is observed up to a concentration of 80 %. For the corresponding diglycol ether compound, however, the middle phase is present only in a concentration range up to about 65 % ( ). Above this concentration, a lamellar neat phase exists. 

The middle phase is much more viscous than the neat phase, though the concentration of the latter is higher. If a concentrated system of an alkyl ether sulfate and water existing initially as a neat phase is stepwise diluted, the range of the middle phase will be reached. This is accompanied by a steep increase in viscosity. The formation of the neat phase allows the manufacture and handling of highly concentrated fluid preparations. This property, however, can lead to processing problems upon dilution. 

Table VI shows the results of polarized light microscopic observations. Sometimes isotropic regions and the middle phase exist simultaneously. The region of the middle phase is marked by heavy lines. The range of the especially viscous middle phase narrows with transition from two to three oxyethylene groups in the surfactant molecule. Up to 27 %, the system appears optically isotropic. In this concentration range the viscosity can be increased strongly by addition of NaCl, as shown in table VII.

In the case of sodium dodecyl sulfate, there is no corresponding effect. This thickening is at any rate not associated with the formation of the middle phase since the products remain isotropic. However, x-ray diffraction measurements indicate the presence of a crystalline, randomly oriented phase (39). It is still... 

The formulation of the three phases must be such that the liquid membrane extracts the solute from one of the phases and the third phase strips it from the membrane. Thus extraction and stripping take place in the same contactor, and the stripping phase is where the solute is accumulated, instead of the organic phase as in the case of conventional solvent extraction. This allows for a middle phase of small volume that, being thin, behaves like a membrane. 

The partitioning can be revealed by different patterns that are found in experimental data. The first one is a reduction in solubilization because a part of the surfactant mixture is no longer at interface. For the same amount of surfactant in the system at optimum formulation, the volume of microemul-son middle phase is lower, just because a certain proportion of the surfactant is no longer in the microemulsion, but has partitioned into one of the excess phases. However, it is worth noting that there are other reasons for the solubilization to decrease, hence this is only a hint. 

At relatively low concentrations of surfactant, the micelles are essentially the spherical structures we discussed above in this chapter. As the amount of surfactant and the extent of solubilization increase, these spheres become distorted into prolate or oblate ellipsoids and, eventually, into cylindrical rods or lamellar disks. Figure 8.8 schematically shows (a) spherical, (b) cylindrical, and (c) lamellar micelle structures. The structures shown in the three parts of the figure are called (a) the viscous isotropic phase, (b) the middle phase, and (c) the neat phase. Again, we emphasize that the orientation of the amphipathic molecules in these structures depends on the nature of the continuous and the solubilized components. 

The middle phase, which is characterized by a hyperchromic shift and the formation of acid-soluble oligonucleotides monoesterified phosphate can be detected. In this phase, the increase of ultraviolet absorption and of acid-soluble oligonucleotides is linear with the reciprocal... 

These have generally been investigated by hyperchromic shift or acid solubility assays and therefore bear on the middle phase of the DNA degradation. 

During the middle phase the composition of the 3 -P terminal and of the 5 -OH terminal and penultimate nucleotides of oligonucleotides obtained from calf thymus DNA by acid DNase digestion, in the incubation conditions of Fig. 1, is practically constant. Some results obtained in this phase are given in Table III. Purine nucleotides form about 75% of the 3 -terminals and of the 5 -OH penultimates with a predominance of G in... 

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