Other Surfactant Phases

The self-assembly of surfactants is not restricted to spherical micelles, and a variety of different phases may form depending on the molecular 

FIGURE 3.7 Bilayer tubules prepared in solution from the lipid DOPC (l,2-dioleoyl-sn-glycero-3-phosphocholine, a biological surfactant) and imaged using fluorescence microscopy. 

The lamellar phase consists of bilayer sheets and becomes particularly important in biological systems where the surfactants present are lipids. Either side of the lamellar phase in concentration, we often observe the complicated bicontinuous phases. These phases have cubic symmetry and consist of periodically curved lamellar sheets. The lamellar phase occurs at roughly the center of the phase diagram in concentration. At even higher 

---

Figure 8.13b shows the same data as Figure 8.13a, replotted with the variables interchanged. Note how the homogeneous area expands in size with increasing potassium oleate concentration up to 0.44 mole kg. At still higher concentrations of potassium oleate the homogeneous area shrinks as other surfactant phases compete for the components. 

The solution to the problem came in the late 1970s with the pioneering work of Scriven [30], introducing the bicontinuous structures based on minimal surfaces. Scriven s work, which included considerations of other surfactant phases (e.g. bicontinuous cubic phases), considerably stimulated the field and his ideas, based on theoretical arguments, were soon confirmed by experimental work, using mainly self-diffusion, electron microscopy and neutron scattering measurements. 

I consider my most important contribution to the field of microemulsion as being the first, together with coworkers, to demonstrate microemulsion bicontinuity. However, this woric also nicely demonstrates how important it is in microemulsion research to have a broader perspective, in particular considering other surfactant phases. 

In contrast to SDS, CTAB and C12E7, CufDSjz micelles catalyse the Diels-Alder reaction between 1 and 2 with enzyme-like efficiency, leading to rate enhancements up to 1.8-10 compared to the reaction in acetonitrile. This results primarily from the essentially complete complexation off to the copper ions at the micellar surface. Comparison of the partition coefficients of 2 over the water phase and the micellar pseudophase, as derived from kinetic analysis using the pseudophase model, reveals a higher affinity of 2 for Cu(DS)2 than for SDS and CTAB. The inhibitory effect resulting from spatial separation of la-g and 2 is likely to be at least less pronoimced for Cu(DS)2 than for the other surfactants. 

Like other surfactants, alkanesulfonates generate lyotropic liquid-crystalline phases. But the phase equilibria can only be inadequately described because of the enormous experimental difficulties in, for instance, establishing an appropriate equilibrium. Nevertheless, for simple ternary systems the modeling of surfactant-containing liquid-liquid equilibria has been successfully demonstrated [60],... 

Phosphorus-containing surfactants are amphiphilic molecules, exhibiting the same surface-active properties as other surfactants. That means that they reduce the surface tension of water and aqueous solutions, are adsorbed at interfaces, form foam, and are able to build micelles in the bulk phase. On account of the many possibilities for alteration of molecular structure, the surface-active properties of phosphorus-containing surfactants cover a wide field of effects. Of main interest are those properties which can only be realized with difficulty or in some cases not at all by other surfactants. Often even quantitative differences are highly useful. 

The interfacial tension behavior between a crude oil (as opposed to pure hydrocarbon) and an aqueous surfactant phase as a function of temperature has not been extensively studied. Burkowsky and Marx T181 observed interfacial tension minima at temperatures between 50 and 80°C for crude oils with some surfactant formulations, whereas interfacial tensions for other formulations were not affected by temperature changes. Handy et al. [191 observed little or no temperature dependence (25-180°C) for interfacial tensions between California crude and aqueous petroleum sulfonate surfactants at various NaCI concentrations. In contrast, for a pure hydrocarbon or mineral oil and the same surfactant systems, an abrupt decrease in interfacial tension was observed at temperatures in excess of 120°C 1 20]. Non ionic surfactants showed sharp minima of interfacial tension for crude... 

Regarding other pseudostationary phases for measurement of lipophilicity or lipophilicity-related properties (e.g., intestinal absorption, brain penetration), there are several reports on the use of vesicles such as phospholipid bilayer liposome (56-58), lysophospholipid micelle (59), DTAB/SDS vesicle (60), and double-chain synthetic surfactant vesicle (61), which are described in other chapters. 

One of the possible alternative to micelles are spherical dendrimers of diameter generally ranging between 5 and 10 nm. These are highly structured three-dimensional globular macromolecules composed of branched polymers covalently bonded to a central core [214]. Therefore, dendrimers are topologically similar to micelles, with the difference that the strnctnre of micelles is dynamic whereas that of dendrimers is static. Thus, unlike micelles, dendrimers are stable nnder a variety of experimental conditions. In addition, dendrimers have a defined nnmber of fnnctional end gronps that can be functionalized to prodnce psendostationary phases with different properties. Other psendostationary phases employed to address the limitations associated with the micellar phases mentioned above and to modnlate selectivity include water-soluble linear polymers, polymeric surfactants, and gemini snrfactant polymers. 

The fact that these phospholipids-stabilized emulsions are sterilized by heat may be surprising since most emulsions stabilized by almost any other surfactant is readily destabilized by heat. Indeed, the fact that the droplet size of phospholipids-stabilized emulsions actually decreases on the application of thermal stress is probably due to the behavior of the phospholipids which move from the aqueous phase to the oil phase, especially to the interfacial mesophase, during the heating process. 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

These compounds differ from other surfactants in the pronounced sensitivity of their association structural organization to temperature. This characteristic feature was noted very early by Shinoda (3) with regard to their micellar association and solubilization. A corresponding sensitivity may also be observed in the strong dependence of the liquid crystalline regions in phase diagrams of the system water, surfactant, and hydrocarbon (4). 

Figure 7. Close to the PIT value two phases with a lamellar structure exist. One of these, the surfactant phase (S), is an isotropic liquid, the other one, (N), is an optically anisotropic liquid crystal with a lamellar structure.

Values of the solubility of surfactants in equilibrium with crystal phase rarely have been measured. In detergent mixtures, other surfactants present in the mixture solubilize the 2-n-(p-sulfophenyl)-alkanes and the fabric softener cationics. A mixing effect which reduces the CMC can explain this. 

The removal of liquid oily soils from surfaces is generally understood in terms of three basic mechanisms the roll - back of droplets of oily soil, the surfaces of which are modified by the presence of an adsorbed layer of surfactant direct emulsification of macroscopic droplets of soil and the direct solubilization of the oily soil into surfactant micelles or other interfacial phases formed (1-3). 

Understanding surfactant phase behavior is important because it controls physical properties such as rheology and freeze-thaw stability of formulations. It is also closely related to the ability to form and stabilize emulsions and microemulsions. Micelles, vesicles, mi-croemulsions and liquid crystal phases have all been used as delivery vehicles for perfumes or other active ingredients. 

Under some conditions, mlcroemulslons form (284.285). In addition to these surface active extractants, many extraction schemes also have some other surfactants present (such as those given in Table V) that can form reversed micelles in the organic phase (1.4.5.283.330). The dialkylnaphthalene sulfonates (see Table V, anionic surfactant section) have been especially useful in this regard (263). 

As far as I am aware, independent experimental evidence for the values of the surface potential and salt fractionation factor have not been obtained for any system other than the n-butylammonium vermiculite gels. For this isolated system, the predicted values of 5 from the Donnan equilibrium and the new equilibrium based on the coulombic attraction theory, namely 4.0 and 2.8, respectively, are definitely distinguished by the experimental results. It would be highly desirable to obtain further tests of our prediction for 5 in systems of interacting plate macroions, both in clay science and lamellar surfactant phases. 

These show that alkyl chains play an important role in the protein extraction ability. Bulky two-tailed surfactants are favourable in the protein transfer from an aqueous phase into an organic phase, while the other surfactants, having saturated-straight alkyl chains, dioctyl phosphoric acid (DOPA) and dioctyl sulfosuccinate (AOC), or a single alkyl chain, monooleyl phosphoric acid (MOLPA), are limited by their solubility into an organic solvent or their ability to form reverse micelles. A tridecyl group in di-tridecyl phosphoric acid (DTDPA) and di-tridecyl sulfosuccinate (ATR) comes from the tridecyl alcohol which includes several isomers, and therefore these surfactants also have bulky tails. 

---