Plate-shaped micelle

Figure 8.4 (a) Atypical molecule that behaves as lyotropic liquid crystal (b) schematic representation of a plate-shaped micelle (c) a spherical micelle (d) a cylindrical micelle. 

One would not expect this a priori. Indeed the dissociation of the polar groups decreases, thus the supposed decrease of the association of the micelles cannot be attributed to this. A consideration of a very schematic picture of the course of the events furnishes the solution of the poblem. We know (Fig. 39a) that an association of the soap micelles occurs in a soap solution of a fairly high concentration. On the other hand it is probable that large plate-shaped micelles are produced (Fig. 39c)... 

The size of micelles is determined by energetic factors and concentration factors. The decrease of CCMF in a viscous system must result in decrease of the size of the micelles. For increase of the surfactant concentration, the micelle size increases, and spherical micelles transform into elliptic ones and then into plate-shaped ones as increase in the surfactant concentration in common solvents is accompanied by a free energy advantage. It can be assumed that such a process occurs with the increase of the degree of conversion up to 30%. In this case CCMF shifts into the area of lower surfactant concentrations without any change in the surface tension (see Fig. 2.7). 

When the dissociation decreases considerably this has still another effect as a consequence. The force A (Fig. 36) continues to decrease further, the tendency of the micelles to round themselves off becomes steadily smaller. Eventually — with the inclusion of more soap ions — plate shaped structures are produced, which exhibit the well known... 

When we start from the Hartley micelles there are thus two possibilities of association a loose association of the micelles and an increase in the si2 e of the micelles, whereby plate-shaped aggregates are formed. It must be possible to understand all the phenomena of soap solutions with the aid of these two types of association. 

The major drawback of practical separations applying MLC is still the low chromatographic efficiency, caused by the resistance to mass transfer in the processes involving micelles and a surfactant-modified stationaiy phase as exposed in Chapter 6. This is especially important since the increase in micelle concentration causes a decrease in plate count, resulting in a varying efficiency over the variable space. It is thus worthwhile to examine the inclusion of the expected peak shape in the expression of the chromatographic quality. 

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