Lamellar phases of surfactants

Another force [57, 58] occurs in a multilayered system, like a swollen lamellar phase of surfactant bilayers or phospholipid vesicles. Shape fluctuations in the bilayers can give rise to steric effects that are supposed to stabilise such systems where the van der Waals and double-layer forces are very weak, as they often are. The magnitude of such fluctuations depends on the "stiffness" of die bilayer. The status of these forces is the subject of an active debate and imclear. 

Vesicles are closed bilayers that can be observed in two forms. At low surfactant concentration, the vesicles are unilamellar and behave like a colloidal suspension of polydisperse particles. At more concentrated surfactant solutions, small multilayered vesicles are formed [134], Multilamellar vesicles (known also as spherulites) have also been observed in the lamellar phases of surfactant-brine (or even pure water-alcohol) systems [218]. The surfactant may be SDS [218,223] or DDAB (didodecyldimethylammonium bromide) [224]. In alcohol-containing systems the bilayer structural transformations are controlled by the alcohol/surfactant ratio [134].Thus, in many SDS-brine (or water)-alcohol systems, a vesicle (L4) phase is located between the micellar phase and the lamellar (L ) phase. At fixed surfactant concentration, the sequence of phases L4 -La-L3 (in water) is obtained by increasing the alcohol content, and the sequence L2 -La-L3 (in oil) is obtained by decreasing the alcohol content [ 134]. 

Lamellar phases of phosphohpids and other HpophiHc surfactants can be dissolved, i.e., converted into micellar solutions, by contacting them with aqueous phases of hydrophihc surfactants, a phenomenon that is sometimes desirable and sometimes undesirable in studies of biological membranes. Often myehnic figures form during the dissolution process. Simoes et al. [33] reported rates of dissolution of various mixtures of phosphatidylchohne and the nonionic surfactant Tween 80. 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

Figure 3.17 Patterns of liquid crystalline phases of surfactants under the polarisation microscope (a) hexagonal phase the typical fan-like structure can be seen, (b) lamellar droplets with typical Maltese crosses and (c) lamellar phase.

Zirconia. Mesoporous zirconia was first reported by Knowles and Hudson with cationic aUcyltrimethylam-monium siufactants. Starting from zirconium sulfate or propoxide, Schiith and Ciesla et al. synthesized hexagonal mesoporous zirconia using aUcyltrimethylammonium surfactants as templates. Reddy and Sayari obtained hexagonal or lamellar phases of mesostructured zirconia using quaternary ammonium surfactants and acidified primary aUcylamine,... 

The same progression can be observed if one adds progressively a single-chained surfactant (S) (v/al = 1/3) to a lamellar phase of a double-chained surfactant D v/al=l) of the same head-group area and chain length. Here it is the effective volume which changes according to. 

Large quantities of hydrocarbon oils may be solubilized by aqueous lamellar dispersion of nonionic surfactants of the type n-dodecyl polyethylene glycol ether (1) in the case of the lamellar phase of n-dodecyl tetraethylene glycol ether UP to 55% (W/W) of n-... 

An interpretation of NMR data from solubilized alkanes in aqueous lamellar phases of a nonionic surfactant is found consistent with the presence of a layer of oil located at the center of the bilayer. This supports a similar novel conclusion derived from small angle X-ray data (1). The presence of such an oil layer will have consequences, as yet not understood, on the osmotic pressures within the bilayer and the interlayer interactions responsible for the stability of the lamellar phase. 

Bemi M, Lawrence C, Machin D. A review of the rheology of the lamellar phase in surfactant systems. Advances in Colloid and Interface Science. 2002 98(2) 217-243. 

Note added in proof, recently, two review papers covering intermediate phases of surfactant-water mixture [406] and fluctuating Euler characteristics in lamellar and microemulsion phases [407] were published. 

Surfactant micelles and bilayers are the building blocks of most self-assembly structures. One can divide the phase structures into two main groups [1) (1) those that are built of limited or discrete self-assemblies, which may be characterized roughly as spherical, prolate or cylindrical. (2) Infinite or unlimited self-assemblies whereby the aggregates are connected over macroscopic distances in one, two or three dimensions. The hexagonal phase (see below) is an example of onedimensional continuity, the lamellar phase of two-dimensional continuity, whereas the bicontinuous cubic phase and the sponge phase (see later) are examples of three-dimensional continuity. Figure 3.8 illustrates these two types schematically. 

Figure 6 Schematic diagram of (a) hexagonal phase of aligned, ordered rod-like micelles, and (b) lamellar phase of aligned, ordered surfactant bilayers.

Only a fraction of the C j -APC/SLES mixture needs to be replaced by the hydrophobic cosurfactant GMO to obtain a lamellar phase of medium viscosity (L, point (a) with a value of 23,000 mPa s atls ) hi terms of practical application, this means that the high-viscosity surfactant paste changes into a pumpable surfactant concentrate. Despite the inaeased GMO content, the lamellar phase remains intact. 

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... 

The structure of the lamellar phase of aqueous pentaethylene glycol mono- -dodecyl ether (C12E5) surfactant at various temperatures and... 

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