Micelle core, liquid state

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. 

In this chapter, we shall be concerned with dilute solutions of surfactants in water, in the absence of oil. Above a certain concentration (the critical micellar concentration), which is generally very low, amphiphilic molecules spontaneously self-assemble to form small aggregates called micelles. As in the case of microemulsions, the spontaneous structuring of the mixture arranges all polar heads so that they are in contact with water, for which they have strong affinity. The hydrophobic tails, brought together into a condensed liquid state in the core of the object, thereby minimise contact with the aqueous solvent. 

Fig. 5.1. Schematic representation showing internal structure of a spherical micelle. Hydrophobic chains are in a condensed liquid state in the core of the micelle. Each one is linked at one end to a polar head on the other side of the interface. Typical diameter of a ball is between 30 and 50 A...

Two aspects of solubilization which remain to be investigated, are the variation of the apparent distribution coefficient with so-lubilizate concentration in the micellar ph.ase, and the mechanism of the incorporation of solubilizate into micelle. Since a micelle is assumed to consist of a hydrocarbon core (in liquid state), and surrounded by a palisade layer of hydrophilic group. Fig. 1, the following possible ways have been suggested for the incorporation of a solubilizate in a micelle [5,74] (a) adsorption on the surface of the micelle, (b) deep or short penetration into the palisade layer, and (c) dissolution into the (liquid like) hydrocarbon core. These mechanisms of incorporation are closely related to the structure of the micelle. However, the effect of solubilizate on the structure of the micelle has not been satisfactori 1y studied in the current literature [75,76,77]. 

In Eq. 4.2, r stands for the maximum possible radius for an anhydrous core of a spherical micelle (i.e. the extended chain length of the hydrophobic segment), p is the density of the hydrophobic segment, Na is Avogadro s number and m is the molecular weight of the hydrophobic part. Since no void can exist in the center of the micelles, and as the hydrophobic chains in the liquid state are not usually fully extended, one dimension... 

The micelle oily core is in a quasi-liquid state. This is demonstrated by several results ... 

The pseudophase separation model of micellar solutions considers a micelle to be a pseudophase in a liquid state. Because the micelles are assumed to have a liquidlike core, the mutual solubility of a fluorinated surfactant and a hydrocarbon surfactant in mixed micelles is, according to the pseudophase model, governed by the miscibility of the fluorocarbon and hydrocarbon chain. For example, heptane and perfluoroheptane are immiscible at 25°C, but above 50°C, these liquids are miscible in all proportions [75]. A terminal substitution of a hydrophilic group depresses the enthalpy of mixing and makes the components miscible at 25°C. 

Both ionic and non-ionic surfactants associate to form micelles. However, there are several property differences between the micelles formed. The CMC value depends on the length of the hydrophobic groups for both types, but it also depends on the state of charge for ionic surfactants and on the length of the hydrophilic part for non-ionic surfactants. The core of a micelle is assumed to be filled with liquid hydrocarbon, and the maximum size of the micelle radius is equal to the length of the fully extended hydrocarbon tail. In... 

Figure 11.12 Schematics of the proposed structural evolution during silica synthesis from reverse liquid crystals (a) Initial state. Micelles with a PDMS corona (thin lines) and solubilizing an aqueous phase (W) in their cores are arranged hexagonally, surrounded by TEOS molecules (thick lines). Arrows indicate the possible interfacial diffusion of W (b) After hydrolysis and partial...

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