Micellar theory has developed in a somewhat uncertain fashion and is still in many respects open to discussion. Possible micelle structures include the spherical, lamellar and cylindrical arrangements illustrated schematically in Figure 1.2 Living Cells can be considered as micellar-type arrangements with a vesicular structure.
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Figure 1.2: Micellar structures: (a) Spherical (anionic) micelle. This is the usual shape at surfactant concentrations below about 40 percent. (b) Spherical vesicle bilayer structures, which is representative of the living cell. (c) and (d) Hexagonal and lamellar phase formed cylindrical and lamellar micelles, respectively. These, and other structures, exist in highly concentrated surfactant solutions.
Typically, micelles tend to be approximately spherical over a fairly wide range of concentration above the CMC, but there are marked transitions to larger, non-spherical liquid-crystal structures at high concentrations. Systems containing spherical micelles tend to have low viscosities, whereas liquid-crystal phases tend to have high viscosities. The free energy of transition between miceller phases tend to be small and, consequently, the phase diagrams of these systems tend to be quite complicated and sensitive to additive.
Some of experimental evidence favoring the existence of spherical, liquid-like micelles is summarized, as follows:
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1. Critical micelle concentrations depend almost entirely on the nature of the lyophobic part of the surfactant. If micelle structure involved some kind of crystal lattice arrangement, the nature of the lyophilic head group would also be expected to be important.
2. The micelle of a given surfactant is of approximately mono-dispersed and their size depends predominantly on the nature of the lyophobic part of the surfactant molecules. One would expect the radius of spherical micelle to be slightly less than the length of the constituents units; otherwise the hydrocarbon chains would be considerably buckled or the micelle would have either a hole or ionic groups in the center. The radii of micelles calculated from diffusion and light scattering data support this expectation. For straight-chain ionic surfactants the number of monomer units per micelle, m, and the number of carbon atoms per hydrocarbon chain, n, are approximately related as follows:
N 12 14 16 18
m 33 46 60 78
Laminar and cylindrical models, in contrast, provide no satisfactory mechanism by which the size of the micelles might be limited.
Earlier Mc Bain assumed two kinds of micelles: one is a spherical ionic micelle of not more than ten monomers formed in dilute solution before the CMC and the second kind of micelle formed is lamellar in shape consisting of hydrocarbon chain arranged parallel to each other. The basic feature of roughly spherical micelles is worked out by G.S. Hartly (figure 1.3). He suggested that the surfactant molecules are completely dissociated and unaggregated below the CMC while at CMC the aggregation of the amphiphiles occurs to give, initially, relatively small micelles. These micelles then rapidly over a narrow concentration rang to a size which then becomes independent of concentration. Further addition of surfactant causes only an increase in the number of micelles. Thus Harley,s model predicts only one type of micelle, which above the CMC remain constant in aggregation number. The classical Hartley model assumes a liquid-like interior of the micelle approximating in structure to that of liquid paraffin surrounded by a polar layer of
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head groups, counter ions and water. The major features to the Hartley model have been verified through many experimental studies. However a number of workers [15-16] did not accord with the view of spherical structure of the aggregates the results of angular light scattering studies by debye and anqcker [16] indicated that neither a sphere nor a disc- shaped micelle is formed but rather a rod like micelle having the general shape of a stock of coins may be formed. This model also fill reasonably well into the kind of structure suggested by x ray measurement of more concentrated solutions surfactant having long alkyl chains may upon sonifications in aqueous media, form vesicles, this type of micelle formation depends upon the length of alkyl chains and the nature of the hydrophilic groups.
In 1955 Tartar [17] described an ellipsoidal model for aggregated surfactants, which due to constraint introduced by the length incapable of aggregating to form a sphere; this model was subsequently adopted and refined by others [18-19]. The smell angle x-ray scattering studied by Reiss Husson and Luzzlatil showed different surfactant system classified the ionic micelles as being either:
(a) Spherical at all concentration, (b) Rod shaped at all concentration,
(c) Spherical at low concentration and rod shaped at high concentration.
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Figure 1.3: A two-dimensional schematic representation of a model of an ionic micelle, x the counter ions. (Adapted from “Solution Chemistry” Ed.K I. Mittal, Planum press, New York 1979)
In an attempt to develop a consistent interpretation of the result of a variety of experimental studies, stigter [20] introduced a rather detailed model for an ionic micelle.
According to these model three regions, (Figure 1.3) can be identified; (a) a spherical micelle cores (b) an aqueous stern layer and (c) the Gouy Chapman diffuse double layer.
The spherical core of the micelle is formed from hydrocarbon part of the surfactants molecules with radius equal to the length of the hydrocarbon chain of the surfactants molecule. The ionic head of the micellized surfactants and a fraction of the counter ions form the stern layer and outside the sphere surface is the Gouy Chapman diffuse double layer.
14 1.8 Physical properties of water
Water has a very simple atomic structure. The nature of the atomic structure of water causes its molecules to have unique electrochemical properties. The hydrogen side of the water molecule has a slight positive charge. On the other side of the molecule a negative charge exists. This molecular polarity causes water to be a powerful solvent and is responsible for its strong surface tension.
When the water molecule makes a physical phase change its molecules arrange themselves in distinctly different patterns. The molecular arrangement taken by ice (the solid form of the water molecule) leads to an increase in volume and a decrease in density. Expansion of the water molecule at freezing allows ice to float on top of liquid water.