Phase behaviour surfactant concentrated solution

In a solvent, block copolymer phase behaviour is controlled by the interaction between the segments of the polymers and the solvent molecules as well as the interaction between the segments of the two blocks. If the solvent is unfavourable for one block this can lead to micelle formation in dilute solution. The phase behaviour of concentrated solutions can be mapped onto that of block copolymer melts [95]. Lamellar, hexagonal-packed cylinder, micellar cubic and bicontinuous cubic structures have all been observed (these are all lyotropic liquid-crystal phases, similar to those observed for nonionic surfactants). This is illustrated by representative phase diagrams for Pluronic triblocks in Figure 1.6. 

Experiment 5.2 Determination of the phase behaviour of concentrated surfactant solutions... 

The quantitative determination of surfactant concentration in solution is an essential part of any experimental work on surfactant adsorption or phase behaviour. In the field of experimental enhanced oil recovery the technique employed should be capable of determining surfactant concentrations in sea water, and in the presence of oil and alcohols, the latter being frequently added as a co-surfactant. 

In case of non-ionic surfactants in water, the behaviour of the water structure outlines three main concentration regions, which closely coincide with the three phases intersected by the experimental isotherms. In the micellar solution phase, no significant changes in the water structure are indicated, while, in the lamellar phase, rapid destruction of the tetrahedral hydrogen bond network occurs due to the confinement of the water between the hydrophilic surfaces of the lamellae. The dehydration of the surfactant head groups was found to start near the border between the lamellar and the reverse micellar solution phases. At higher concentrations, water demonstrates its trend to form clusters of tetrahedrally bonded molecules even at the very low content in the system. The results with surfactant solutions have been obtained by Raman spectroscopy (Marinov el al., 2001). 

At large surfactant concentrations emulsion films as well as foam films exhibit a layer-by-layer thinning (stratification) and metastable black films are formed [31,347,512], Such a behaviour has been reported for hydrocarbon films obtained from solutions of lecithin in either benzene or a mixture of chloroform and decane at concentration higher than 0.6-0.8% as well as in films from oxidised cholesterol in decane [31,512]. Manev et. al. [347] have reported stratification of O/W type emulsion films, toluene being added as a disperse phase, occurring within a surfactant (NaDoS) concentration range of 0.017-0.14 mol dm 3. The number of metastable states was 5-6. Compared to foam films of analogous composition, the respective emulsion films were thicker, due to the weaker intermolecular attraction and the stratification occurred at lower surfactant concentrations. 

A closer relationship between foam stability and HLB has been reported for two- or three-phase systems surfactant solution-oil or oil-surfactant phase-water [60,109-111]. The effect of various parameters changing HLB on the stability of foams and emulsions has been studied in [111]. These were the concentration of amyl alcohol and sodium chloride, the number of the ethylene oxide groups in the molecule of the oxyethylated octylphenol. As a general parameter of HLB the authors used the surfactant affinity difference concept (SAD) which is an empirical generalised formulation. It measures the deviation from the optimum formulation for three phase behaviour. For anionic surfactants... 

Crystallization-induced phase separation can occur for concentrated solutions (gels) of diblocks [58,59]. SAXS/WAXS experiments on short PM-PEO [PM=poly(methylene) i.e. alkyl chain] diblocks revealed that crystallization of PEO occurs at low temperature in sufficiently concentrated gels (>ca. 50% copolymer). This led to a semicrystalline lamellar structure coexisting with the cubic micellar phase which can be supercooled from high temperatures where PEO is molten. These experiments on oligomeric amphiphilic diblocks establish a connection to the crystallization behaviour of related nonionic surfactants. 

So far, all theoretical models are based on surfactant solutions with a distribution of surfactant molecules as monomers. One of the specific properties of surfactants is that they form aggregates once a certain concentration, the critical micelle concentration CMC, is reached. The shape and size of such aggregates are different and depend on the structure and chain length of the molecules. At higher concentrations, far beyond the CMC, the phase behaviour is often complex giving rise to novel physical properties (Hoffmann 1990). 

Consider a dilute solution of a mixture of an ionic surfactant and a cosurfactant in salt water. Figure 5.5 shows the typical phase behaviour for fixed temperature and salinity. The horizontal axis represents the total volume fraction (f) of amphiphile molecules and the vertical axis gives the ratio (pA/(f>s of cosurfactant concentration (f>A to surfactant concentration 0s- Almost all the amphiphile molecules aggregate together into micelles. 0 therefore represents the volume fraction of these objects, whilst 0a/0s represents their chemical composition with respect to the two chemical species which make them up. 

Figures 10.13-10.18 present the rheological behaviour of viscoelastic vesicle phases. Both the shear moduli Go and the yield stress values Gy increase with increasing total surfactant concentration that is seen in Figure 10.13. Below concentration of 1 wt%, both quantities drop to zero, what means that the vesicles are no longer densely packed under these conditions and thus are not restricted to a fixed position. Above 1 wt%, both quantities increase almost linearly with the concentration and Go is about one order of magnitude higher than Gy. This confirms that the vesicles must be deformed by about 10% until they can pass each other and the solutions start to flow like liquids.

Figure 5.9 General phase diagram of a surfactant solution, showing the CMC line, the Krafft point (temperature) and the lower consolute point (or lower critical temperature). As can be seen, the phase behaviour of aqueous surfactant solutions is rather complex and various phases are distinguished. At high concentrations, we can see various special surfactant phases (hexagonal, lamellar, cubic). These are called liquid crystalline phases and although there are 18 different types, the three mentioned are the most important. Many of these complex structures have found exciting applications (e.g. liquid crystal displays and study of biological membranes)...

The mechanism of 0/W gel emulsion formation was determined " by studying the phase behaviour of gel emulsion-forming systems as a function of temperature and following the emulsification process conductimetrically, as for W/0 gel emulsions. The phase diagram of 0.1 m NaCl aqueous solution/Ci2E06/monolaurin/ -decane as a function of temperature and brine concentration is shown in Figure 11.17. Monolaurin was added to the system to increase the lateral interactions of the surfactant layer and consequently to enhance the stability of the gel emulsions. ... 

In that case the self diffusion coefficient - concentration curve shows a behaviour distinctly different from the cosurfactant microemulsions. has a quite low value throughout the extension of the isotropic solution phase up to the highest water content. This implies that a model with closed droplets surrounded by surfactant emions in a hydrocarbon medium gives an adequate description of these solutions, found to be significantly higher them D, the conclusion that a non-negligible eimount of water must exist between the emulsion droplets. 

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