Lamellar micelle

The effect characteristic of a multi-chain hydrophobe, that is, increase in the cmc and simultaneous decrease in the cloud point, appears to be inconsistent with the well-known HLB concept in surfactants. Tanford has pointed out that based on geometric considerations of micellar shape and size, amphiphilic molecules having a double-chain hydrophobe tend to form a bilayer micelle more highly packed rather than those of single-chain types ( ). In fact, a higher homologue of a,a -dialkylglyceryl polyoxyethylene monoether has been found to form a stable vesicle or lamellar micelle (9 ). Probably, the multi-chain type nonionics listed in... 

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 phenomena of association colloids in which the limiting structure of a lamellar micelle may be pictured as composed of a bimolecular leaflet are well known. The isolated existence of such a limiting structure as black lipid membranes (BLM) of about two molecules in thickness has been established. The bifacial tension (yh) on several BLM has been measured. Typical values lie slightly above zero to about 6 dynes per cm. The growth of the concept of the bimolecular leaflet membrane model with adsorbed protein monolayers is traceable to the initial experiments at the cell-solution interface. The results of interfacial tension measurements which were essential to the development of the paucimolecular membrane model are discussed in the light of the present bifacial tension data on BLM. 

It is believed that polymerization of hydrophobic monomers is initiated by free radicals in the aqueous phase and that the surface-active oligomers produced migrate to the interior of the emulsifier micelles where propagation continues. Monomer molecules dispersed in the water phase also solubilize by diffusing —to the expanding lamellar micelles. These micelles disappear as the polymerization continues and the rate may be measured by noting the increase in surface tension of llie system. 

Two other shapes of micelles may be considered, namely, the rod-shaped micelle suggested by Debye and Anacker and the lamellar micelle suggested by McBain. The rod-shaped micelle was suggested to account for the light-scattering results of cetyl trimethyl ammonium bromide in KBr solutions, whereas the lamellar micelle was considered to account for the X-ray results in soap solutions. A schematic picture of the three type of micelles is shown in Fig. 2. 

Large, flat lamellar micelles (disklike extended oblate spheroids)... 

Vesicles, more or less spherical structures, consisting of lamellar micelles arranged in one or more concentric spheres. 

In many of these studies the structure of the middle phase is not established, but it is clearly immiscible in water or oil and its electrical conductivity is closer to water than oil. Phase diagram studies of oil-water-emulsifier systems Ekwall, (5), indicate that surfactant-rich phases immiscible in oil or wa"ter have rodshaped or lamellar micelles with some degree of optical anisotropy or flow birefringence, and these phases have much greater elec-rical conductivity than oil. Figure 1 illustrates that the middle phase composition varies smoothly from a water-rich composition to an oil-rich composition as the emulsifier partition changes from mostly water-soluble to mostly oil-soluble. If lamellar structures are present the relative thickness of oleophilic and hydrophilic layers must vary smoothly from the water-rich compositions to the oil-rich compositions. 

Redispersion of the flocculate and other evidences for the hydrophilic character of the support coated with the adsorbed surfactant in the neighbourhood of the cmc indicate that bilayer coverage represents complete saturation of the surface. Commonly, the term bilayer is applied to an adsorbed structure in which the surfactant molecules are oriented perpendicular to the surface and fully extended [5,9,20,37,81,89]. The hydrocarbon tails of both layers form a hydrophobic core between the heads. At both sides counterions accumulate between the ionic head-groups. The result looks like a lamellar micelle. For certain physical regimes, the adsorbed amount is only a fraction of what is expected for a tightly packed bilayer [37,48] the structure which best fits the experimental data can... 

Although, the spherical micelle model accounts for many of the physical properties of solutions of surfactants, a number of phenomena remain unexplained, without considering other shapes. For example, McBain [9] suggested the presence of two types of micelle - spherical and lamellar - in order to account for the drop in molar conductance of surfactant solutions. The lamellar micelles are neutral and hence they account for the reduction in conductance. Later, Harkins et al. 

At low V /V, a clear W/O microemulsion is produced with a high resistance (oil continuous), but as V /V increases the resistance decreases and, in the turbid region, hexanol and lamellar micelles are produced. Above a critical ratio, inversion occurs and the resistance decreases, producing an O/W microemulsion. 

The experimental results obtained called for some theoretical interpretation. Some authors - suggested that a possible explanation of the phenomenon can be the formation of lamella liquid-crystal stracture inside the film. Such lamellar micelles are observed to form in surfactant solutions, but at concentrations much higher than those used in the experiments with stratifying films. The latter fact makes the lamella-liquid-crystal explanation problematic. Nikolov et observed... 

When the value of the parameter Vh /lcao reaches a value of approximately 1, the surfactant can form either normal lamellar micelles in aqueous media or reversed micelles in nonpolar media. As the value of the parameter gets larger and larger than 1, the reverse micelles in nonpolar media tend to become less asymmetrical and more spherical in shape. 

The addition of medium-chain alcohols that are solubilized close to the surface of the micelle in the palisade layer increases the value of aQ, resulting in a greater tendency to form spherical micelles. Increase in the ionic strength of the aqueous solution or increase in the concentration of an ionic surfactant in the aqueous phase, on the other hand, decreases the value of aQ and promotes the tendency to form cylindrical or lamellar micelles. 

FIGURE 4-4 Effect of solubilization content and other molecular environmental factors on micellar structure. Note that interconversion of normal and reverse lamellar micelles involves only small changes in distances between hydrophilic and hydrophobic groups. 

In concentrated solutions, at ten times the CMC or more, micelles are generally non-spherical. At least in some cases, the surfactant molecules are believed to form extended parallel sheets of two molecules thick (lamellar micelles) with the individual molecules oriented perpendicular to the plane of the sheet. In aqueous solutions, the hydrophilic heads of the surfactant molecules form the two parallel surfaces of the sheets and the hydrophobic tails comprise the interior region. 

As surfactant concentration in solutions with c0 > CMC increases, not only the concentration of spherical micelles becomes higher, but also their shape changes. Spherical micelles turn into anisometric ellipsoidal and cylindrical micelles and further to rod-like, thread-like and lamellar micelles... 

Lamellar gels are also formed by soaps under appropriate conditions of concentration and temperature. These may be regarded as essentially micellar systems [Figure 11.2(d)] in which at high concentrations the lamellar micelles extend over considerable distances and interlock to form a continuous gel network. 

The predictions of ( )CMC were extended to cylindrical and lamellar micelle morphologies [Shull et al., 1991]. In all cases, geometric parameters, e.g., core radius and corona thickness, can be computed, assuming that the interfacial thickness between the core and the corona is equal to Al, . ... 

McBain and Harkins, who studied concentrated soap solutions (20-30 per cent) with the aid of x-ray diffraction, suggest that lamellar micelles are formed by soap molecules in such a way that the hydrocarbon chains of these molecules line up parallel to each other and form double layers, as shown in Fig. 15-5a. The hydrophilic ends of the soap molecules face outward, and each double layer is separated from the next one by a zone of water. If a hydrocarbon is now added to a strong soap solution, it is solubilized by locating in the hydrocarbonic part of the micelles. Fig. 15-5b. It is assumed that a similar solubilization occurs when a hydrocarbon monomer is added to a dilute soap solution (0.5-2 per cent), as used in emulsion polymerizations. In this case though, the aqueous Zone between the layers is assumed to be much wider than in concentrated soap solutions. 

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