Phase diagram water-sodium sulfate

The phase diagram of sodium dodecyl sulfate-water is representative of many ionic systems (Figure 3.7) [5], In Figure 3.7 Liquid is the aqueous micellar phase Ha is the hexagonal lyotropic liquid crystal, sometimes called the middle phase and La is the lamellar lyotropic liquid crystal, sometimes called the neat phase. On the surfactant-rich side, several hydrated solid phases are present. 

Figure 4 Binary phase diagram for sodium dodecyl sulfate as a function of temperature. Note that SDS has a substantial region with different hydrated solids in water and ice and that the Krafft temperature increases with temperature. At very high wt% SDS, solid hydrates are formed. (Redrawn from Ref. 17. John Wiley Sons, Ltd, 1998.)...

Figure 8.7 shows the ternary phase diagram for water, hexanoic acid, and sodium dodecyl sulfate at 25 °C. Seven different areas are shown in the figure, which has been used to describe the solubilization of polar dirt by surfactant solutions in detergency applications. The following comments refer to these seven different regions and explain the labeling used in Figure 8.7 ...

FIG. 8.7 Ternary phase diagram for water (W), hexanoic acid (A), sodium dodecyl sulfate (S) at 25°C. See text for a description of the various regions. (Redrawn, with permission, from A. S. C. Lawrence, Chem. Ind., 44, 1764 (1961).)... 

Figure 16.2. Some phase diagrams, (a) The water end of the system potassium chloride and water, (b) The water end of the system sodium chloride and water, (c) The water end of the system magnesium sulfate and water the heptahydrate goes to the mono at 150°C, and to anhydrous at 200°C. (d) /3-methylnaphthalene and /S-chloronaphthalene form solid solutions, (e) Mixtures of formamide and pyridine form a simple eutectic, (f) These mixtures form binary eutectics at the indicated temperatures and a ternary eutectic at mol fractions 0.392 dibenzyl, 0.338 diphenyl, and 0.27 naphthalene.

The calculated results in the absence of electrolyte will be now compared with the experimental results obtained regarding a lamellar lyotropic liquid crystal SDS (sodium dodecyl sulfate)/pentanol/water/dodecane swollen in a mixture of dodecane and pentanol.24 The weight fraction water/surfactant was 1.552 from the dilution line in the phase diagram, we calculated that the initial concentration of pentanol in the oil-free system was 29 wt % and the concentration of pentanol in the dodecane-based diluant was 8 wt %. The experimental values for the repeat distance were obtained from the X-ray diffraction spectrum (Figure 2 in ref 24) for various dodecane concentrations. 

Interfacial tension has been deduced from the spectrum of the light scattered by the interface. The results are relative to water-toluene-sodium dodecyl sulfate (SDS)-butanol mixtures either in the two phase, or in the three phase region of the phase diagram. Values down to 10 dynes/cm have been measured. Measurements down to 10 - 10 6 dynes/cm are expected to be achievable with this technique. 

The samples were made using toluene as oil and a mixture of sodium dodecyl sulfate (SDS) and butanol as emulsifier. The phase diagram of this system was extensively studied by Lalanne et al. (8). The composition of samples is indicated in Table 1. Their densities, viscosities and refractive indices are reported in Table 2. The three-phase samples were obtained by adding salt into water (the limits of the three-phase domain for the samples of Table 1 were 5.8% - 7.8% of salt in water). 

Real crystals are often not in the equilibrium state in aqueous surfactant mixtures. Sometimes, this is true simply because the equilibrium crystal is disordered. In other cases, a nonequilibrium situation exists because the crystal present is not the equilibrium coexisting phase [48]. If a crystal that is not the equilibrium phase is mixed with water, the solubility initially observed will typically be higher than the equilibrium solubility. An excellent illustration of this behavior is found in the sodium sulfate—water diagram (Fig. 5), where the equilibrium coexisting crystal below 32.4°C is the decahydrate (X Wio). A metastable heptahydrate crystal (X W,) also exists in this system, which can be seen from the diagram to be significantly more soluble and have a lower peritectic temperature (23.7 C) than the equilibrium decahydrate. 

Even if the micelle is regarded as a chemical species, the melting-point model is not correct. In this case, the system is monovariant (/= ), and the Krafft point is determined automatically at 1 atm pressure. That seems reasonable for a single surfactant solution, but when the model is applied to a mixed surfactant solution, it is found to be incorrect. Figure 6.2 shows the phase diagram of a water/sodium dodecyl sulfate (SDS)/manganese (II) dodecyl sulfate [Mn (08)2] system, with temperature as the ordinate. The CMC of the surfactant mixture gives a curved surface between the... 

Figure 6.2. Phase diagram of water/sodium dodecyl sulfate/manganese(II) dodecyl sulfate system. 5, mixed CMC surface. (Reproduced with permission of Academic Press.)...

The first system where the existence of the undulation forces was quantitatively established was in the lamellar phase of the quaternary mixture of sodium dodecyl sulfate (SDS, the surfactant), pentanol (cosurfactant), water, and dodecane as a function of dodecane dilution. Figure 6 shows a cut of the phase diagram discovered and mapped out by Roux and Bellocq S represented on a standard triangular phase diagram in the plane with a constant water/SDS weight ratio equal to 1.55. This... 

Fig. 6.6 Phase diagram of the sodium dodecyl sulfate-water system near the Krafft point Tk. (From Ref. 86. Reproduced by permission of Pergamon Press.)...

Phase diagrams of the sodium perfluorodecanoate-sodium decyl sulfate-water system have been constructed from the dependence of solubility and cmc on temperature [202]. Two kinds of micelles were found a fluorocarbon-rich mixed micelle and a hydrocarbon-rich mixed micelle. 

The effect of microemulsion structure on reaction rate has also been studied in relation to oxidation and reduction of cysteine residues in keratin [10]. The system sodium dodecyl sulfate (SDS)/n-pentanol/water/dodecane was chosen as a microemulsion because in this system the realm of the existence of the isotropic region in the pseudoternary phase diagram is a continuous domain, extending from the water apex to the close vicinity of the hydrocarbon-surfactant edge. It was shown experimentally that the microemulsion structure varied smoothly with composition within the isotropic region. 

In Figure 4.2, the concentration-temperature diagram is shown for sodium dodecyl sulfate (SDS) in water. The solid line denotes the solnbility limit the dashed line is the solubility that is expected based on extrapolation of the behavior at low temperatures. The rate of solubility increases with temperatnre changes abruptly at about 10°C, which is known as the Krafft point. Below this temperatnre, no micelles are present and the solid surfactant, in which the hydrocarbon chains are rather rigid, is formed at the solubility limit. However, above the Krafft point, micelles whose hydrocarbon chains are much more fiexible form at con-centrahons above the dashed line, which represents the CMC. At much higher concentrahons (not shown), the solubility limit is ultimately reached and a liquid crystalline phase also having fiexible chains separates. Snch phases are discussed in Sechon 4. 

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