Solubility region

Conclusions from the PDB lYEW X-ray crystallographic study of pMMO are as follows (1) The pMMO structure reveals an unexpected trimeric arrangement (2) three metal ions, a mononuclear copper(II) center, and a dinuclear Cu-Cu site reside within the soluble regions of the pmoB subunit ... 

Flow rate I ml/min (note specifications mentioned for the chromatogram shown in Fig. 10). x Solvent composition at precipitation points from turbidimetric titration at 20 °C with n-hexane precipitant and THF solvent. These points determine the boundary of the solubility region... 

The solubility region can approximately be delimitated by a circle with a radius of about 5 5-units. The centre of this circle is indicated by the symbol it has the coordinate values dv = 18 5h = 5, both in (MJ/m3)1/2. It can be seen that the solubility increases approximately as the distance from the centre decreases. 

The phase regions for micellar solutions and lyotropic liquid crystals form a complicated pattern in water/amphlphile/hydrocarbon systems. The present treatment emphasizes the fact that they may be considered as parts of a continuous solubility region similar to the one for water/short chain amphiphilic systems such as water/ethanol/ethyl acetate. 

It is essential to realize that any thermodynamic evaluation of this solubility "maximum" with standard reference conditions in the form of the three pure components in liquid form is a futile exercise. The complete phase diagram. Fig. 2, shows the "maximum" of the solubility area to mark only a change in the structure of the phase in equilibrium with the solubility region. The maximum of the solubility is a reflection of the fact that the water as equilibrium body is replaced by a lamellar liquid crystalline phase. Since this phase.transition obviously is more. related to packing constraints — than enthalpy of formation — a view of the different phases as one continuous region such as in the short chain compounds water/ethanol/ethyl acetate. Fig. 3, is realistic. The three phases in the complete diagram. Fig. 2, may be perceived as a continuous solubility area with different packing conditions in different parts (Fig. 4). 

These inverse micelles will solubilize electrolytes in their aqueous core but the presence of the electrolytes also will influence the stability of the inverse micelle. A change in the stability of the inverse micelle will be reflected in modifications of the solubility region of the inverse micellar solution. This chapter will relate the changes in solubility areas from addition of electrolytes to the water to the structure of inverse micelles and other association complexes in the pentanol solution. 

Solubilization Limits. The solubility regions were determined by titration with the sodium chloride solution until turbidity and the results checked by long-time storage of suitable compositions. 

M NaCl) the solubility region of pure Hfi (- -) will change. 

The results showed distinct and regular changes for the aqueous solubility region in pentanol surfactant mixtures. With increased electrolyte content, the "minimum amount of water for solubility was enhanced, the solubility limit towards the pentanol water axis was shifted to higher soap concentrations, and the "maximum solubility of the aqueous sodium chloride solution was obtained for higher surfactant alcohol ratios (Figure 2). 

The result showing an increase of the minimum water concentration is directly understood from the association structures in the aqueous solution without electrolytes. The light-scattering determinations indicated that no surfactant association took place at low water concentrations for the system without electrolytes. An association structure of one surfactant molecule, a few associated water molecules, and one or two alcohol molecules is a reasonable conclusion. The experimental results showed no electrolyte solubility in the part of the solubility region where these non-associated structures were found for the electrolyte-free solution. A small structure containing only 10 water molecules cannot be expected to accommodate electrolytes and the structural analysis offers a satisfactory explanation of the results. 

The energy of the electric double layer is directly dependent on the square of the surface potential (Equation 4) and the observed increase of the potassium oleate alcohol ratio should enhance the stability of the inverse micelle. The stability of the inverse micelle is not the only determining factor. Its solution with a maximal amount of water is in equilibrium with a lamellar liquid crystalline phase (7) and the extent of the solubility region of the inverse micellar structure depends on the stability of the liquid crystalline phase. 

In Figures 5 and 6, one might expect to see two different solubility regions. At low fluid densities where intermolecular forces are reduced and the surfactant concentration is below the CMC, the solubility should increase gradually as the density increases. At higher densities, above the CMC, the solubility should increase rapidly because the total surfactant solubility is dominated by the saturation concentration of micelles in the fluid. This type of behavior is not apparent in Figures 5 and 6, perhaps because the CMC is below 10 M. 

Figure lA. The solubility region for the pentanol solution in the system Water, sodium dodecyl sulfate (SDS) and pentanol. 

Figure IB. Replacing water by a 0 gave a smaller solubility region.

For the 1 M NaCl system the solubility region was further reduced. Fig. 13, and the water solubilization maximum found at even higher surfactant/cosurfactant ratio. The series with the lower ratios of surfactant to cosurfactant showed an uptake of the aqueous solution somewhat similar to the series in the system with 0.5 M NaCl. The series with the surfactant/(cosurfactant + surfactant) ratio equal to 0.4 gave an initial liquid crystal formation lasting for 2-3 days folllowed by a middle phase lasting a longer time. The liquid crystalline and the middle phase layer were both more pronounced for the sample with initial salt concentration equal in the water and in the microemulsion. Fig. 14A, than for the sample with all the salt in the water. Fig. 14B. 

Fig. 20A illustrates this behavior. In the beginning of the experiment the upper four layers and part of the fifth layer consisted of the W/0 microemulsion while the layers 6 and 7, as well as 1/3 of the layer 5 were water. Figs. 20A,3. After 71 days the composition in layers 6 and 7 was moved from the original pure water to positions to the right of microemulsion solubility region. Fig. 20B. 

Figure 20A. The compositions at the internal interface (A,B,C, Fig. 3) were located outside the W/0 microemulsion solubility region (A, 9 days B, 27 days and C, 31 Days). The composition of the birefringent layer (D,E,F, Fig. 3) was also outside the solubility region but towards high surfactant concentration (D, 31 days and E, 49 days).

Figure 20B. At 71 days the internal interface in Fig. 3 had disappeared and all the W/0 microemulsion compositions were now within the solubility region (1-5, 71 days), but the birefringent layer still persisted (Fig. 3) and layers 6 and 7 were still outside the solubility limit.

However, this Is not the case (21). The presence of minor amounts of polymer leads to a pronounced reduction In the solubility region. Figure 4 Illustrates this fact for pentanol as a cosurfactant. 10% of the styrene replaced by polystyrene (M = 1,000) reduced the solubility region to less than one third of the one for the styrene mlcroemulslon. 

Figure 3. (a) The pentanol (C5OH) solubility region of water... 

A comparison between the solubility region of water In W/0 mlcroemulslons of styrene and of Its dimer. Figure 6 (22), reveals a significant difference. The maximum solubility of water was reduced from 29 to 11% by weight and the maximum surfactant concentration declined by 30%. In fact, the solubility region of the dimer was only 20% that of the monomer. 

Figure 4. Replacing part of the styrene in Figure 3C with polystyrene gave further a reduction in the solubility region ...

Figure 6. The solubility region for conditions in Figure 3B (- -) was considerably reduced when the styrene was replaced by its dimer (---).

The data can now be represented more conveniently in a triangular diagram, as in Fig. 12-2. This plot shows the approximate limiting solubility boundaries for polyfmethyl methacrylate). The boundary region separates efficient from poor solvents. The probable solubility parameters of the solute polymer will be at the heart of the solubility region. The boundaries are often of greater interest than the central region of such loops because considerations of evaporation rates, costs, and other properties may also influence the choice of solvents. 

Identical conditions exist if the corresponding solubility region is determined at constant hydrocarbon content in a microemulsion. Such compositions mean that a fourth component is introduced, and a tetrahedral representation is necessary such as in Figures 2a, b, and c. From this and other diagrams (8,9,10) an important conclusion concerning microemulsion conditions may be drawn— the alcohol/soap ratio necessary to obtain maximum water solubihzation remains identical at different hydrocarbon contents. 

These systems have not been investigated as thoroughly as the W/O microemulsions have been. One determination (II) has been reported, and Prince has suggested (6) that the O/W microemulsions exist within a limited oil/emulsifier ratio in a sectorial solubility region emanating from the aqueous comer. This is tme only for nonionic systems (12) the combination ionic substance and alcohol gives a more complicated pattern. 

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