Spherical micelles concentration range

The geometric form of the micelles depends on the concentration of the surfactant and on additives. In the absence of additives the micelles are spherical for concentrations ranging from the critical micelle concentration (cmc) to at least ten times the cmc. - - At higher surfactant concentrations or with additives (added salt is the most efficient) rodUke or prolate micelles will form. At high surfactant concentration or with large amounts of additive, the micellar phase becomes unstable, and other phases will be present. In most cases a hexagonal phase or a lamellar phase will be the phases in equilibrium with the aqueous micellar phase. ... 

Micelles can be spherical or laminar or cylindrical. Micelles tend to be approximately spherical over a fairly wide range of concentrations above CMC (critical micelle concentration) but often they are marked transitions to larger, non spherical liquid crystal structures at high concentrations. For straight chain ionic surfactants, the number of monomer units per micelle ranges between 30 and 80. 

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

The exact shapes of the micelles are unknown, and this subject is open for discussion. Possible micellar structures could be spherical or nearly spherical over a wide range of concentrations not too far from the c.m.c. In a highly concentrated solution, the micellar shape is elongated and forms larger, nonspherical (i.e., cylindrical or lamellar) liquid structures, as illustrated in Figure 4.21. The size of a spherical micelle is determined by the length of the hydrocarbon chain in the... 

Surfactants are amphiphilic molecules which, when dispersed in a solvent, spontaneously self-assemble to form a wide variety of structures, including spherical and asymmetric micelles, hexagonal, lamellar, and a plethora of cubic phases. With the exception of the lamellar phase, each of these phase structures can exist in both normal and reverse orientations with the hydrophobic chains on the exterior of the aggregate, in contact with solvent or vice versa orientation. The range of structures a particular surfactant forms and the concentration range over which they form, depends upon the molecular architecture of the surfactant, its concentration, and the solvent in which it is dispersed. For example, some solvents such as ethanol do not support the formation of aggregates. As most pharmaceutical systems use water as their solvent, this entry will concentrate on aqueous-based systems, although other solvent systems, particularly other non-aqueous polar systems, will be mentioned where appropriate. 

One of the most widely used indexes for evaluating surfactant activity is the critical micelle concentration (CMC). The CMC is in effect the solubility of a surfactant within an aqueous phase, or the minimum surfactant concentration required for reaching the lowest interfacial or surface tension values y. At concentrations above the CMC, surfactants associate readily to form micelles that can be spherical, oblate, tablet shaped, or rod-like, with a hydrophilic surface and a hydrophobic interior. Such micellar structures usually have hydrodynamic radii, ranging from 200 to 400 A. The CMC value is estimated from the inflection point in the y vs log C curve. 

Within a broad range of concentrations above the CMC the surfactant molecules are associated into spherical micelles, the so-called Hartley -Rehbinder micelles. The hydrocarbon core of such micelles remains liquid, even though its state is different from that of the corresponding bulk hydrocarbon, such as in the emulsion droplets. The formation of mixed micelles containing different additives (even when the constituent molecules substantially differ in size), as well as the dissolution of liquid hydrocarbons (otherwise insoluble in water) in hydrophobic cores (solubilization, see Chapter VI, 4) points to a liquid-like state of micellar hydrocarbon cores. 

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