Structure of soap micelles

Fia. 15-5. Structure of soap micelles (a) without and (6) with solubilized monomer. (From Burnett, Mechanism of Polymer Reactions," p. 350, Interscience Publishers, Inc., New York, 1954.)... 

Fig. 10. Structure of soap micelles according to Thiessen and Spychalski, 1931.

Although McBain suggested over 80 years ago that soap molecules form micellar structures of lamellar and spherical shape (McBain 1913), most of the subsequent work focused on spherical micelles. The earliest concrete model for spherical micelles is attributed to Hartley (1936), whose picture of a liquidlike hydrocarbon core surrounded by a hydrophilic surface layer formed by the head groups, has been essentially verified by modern techniques, and the Hartley model still dominates our thinking. We present an overview of the structure of the micelle first and then go on to examine the details a little bit more closely. 

The kinetic studies indicated that the rate of equilibrium between large monoolein-bile salt aggregates and small monolein-bile salt aggregates was very fast. Information is needed on the rate of exchange of polar lipids between micelles since these aggregates have different structures than soap micelles, where the exchange rates are believed to be rapid (15, 21). 

A mesomorphic (liquid-crystal) phase of soap micelles, oriented in a hexagonal array of cylinders. Middle soap contains a similar or lower proportion of soap (e.g., 50%) as opposed to water. Middle soap is in contrast to neat soap, which contains more soap than water and is also a mesomorphic phase, but has a lamellar structure rather than a hexagonal array of cylinders. Also termed clotted soap . See Neat Soap. 

The chemical structure of soap explains its cleaning ability. There are two main parts of a soap s structure. Soap molecules contain a nonpolar alkyl tail and a polar head that can interact with the polar water molecules. A soap solution is not a true solution, it doesn t have individual fatty acid anions in the water, but rather groups of these ions called micelles. 

Greases, like emulsions, contain stable colloidal dispersions, but when thickner (gelling agent) u.scd is a soap (soap-ba.sed greases), the particles are not spherical but crystallites of fibrous, sha )c which result from polymerisation or linear aggregation of. soap micelles (colloidal soap molecules). The fibres get tangled and a three-dimensional interconnected structure results. The oil particles gel physically entrapped in the interstices of the tangled structure and/or adsorbed on the fibres. 

In liquid crystals or LC-glasses one looks for orientational order and an absence of three-dimensional, long-range, positional order. In liquid crystals, large scale molecular motion is possible. In LC-glasses the molecules are fixed in position. The orientational order can be molecular or supermolecular. If the order rests with a supermolecular structure, as in soap micelles and certain microphase separated block copolymers, the molecular motion and geometry have only an indirect influence on the overall structure of the material. 

Mesophases of supermolecular structure do not need a rigid mesogen in the constituent molecules. For many of these materials the cause of the liquid crystalline structure is an amphiphilic structure of the molecules. Different parts of the molecules are incompatible relative to each other and are kept in proximity only because of being linked by covalent chemical bonds. Some typical examples are certain block copolymers50 , soap micelles 51 and lipids52. The overall morphology of these substances is distinctly that of a mesophase, the constituent molecules may have, however, only little or no orientational order. The mesophase order is that of a molecular superstructure. 

The greasy tails can dissolve in each other, forming a spherical structure called a micelle. This is how soaps work in washing your hands or doing dishes. Figure 7.31 shows a two-dimensional slice out of a micelle. Formation of a micelle creates a nonpolar microenvironment in the water. So, when you are scrubbing... 

The favored structure for most phospholipids and glycolipids in aqueous media is a bimolecular sheet rather than a micelle. The reason is that the two fatty acyl chains of a phospholipid or a glycolipid are too bulky to fit into the interior of a micelle. In contrast, salts of fatty acids (such as sodium palmitate, a constituent of soap), which contain only one chain, readily form micelles. The formation of bilayers instead of micelles by phospholipids is of critical biological importance. A micelle is a limited structure, usually less than 20 nm (200 A) in diameter. In contrast, a bimolecular sheet can have macroscopic dimensions, such as a millimeter (10 nm, or 10 A). Phospholipids and related molecules are important membrane constituents because they readily form extensive bimolecular sheets (Figure 1211). 

Micelle formation is briefly discussed in Section 2.2.5, item 4 see especially Figure 2.8. Soap micelles typically contain 50 to 100 molecules, and the radius is roughly 2nm (about the length of a surfactant molecule). The core of a micelle contains a little water, at most one molecule per surfactant molecule. The size and shape of the micelles closely depend on the molecular configuration of the surfactant. Micelles are dynamic structures. They are not precisely spherical, and surfactant molecules move in and out. Characteristic times for these processes are a matter of debate, but they seem to be of the order of 10 ps. Presumably, a micelle can disappear in 10-100 ms upon dilution. 

In an earlier review [3], mixed micelles formed by bile salts were classified into those with (i) non-polar lipids (e.g., linear or cyclic hydrocarbons) (ii) insoluble amphiphiles (e.g., cholesterol, protonated fatty acids, etc.) (iii) insoluble swelling amphiphiles (e.g., phospholipids, monoglycerides, acid soaps ) and (iv) soluble amphiphiles (e.g., mixtures of bile salts with themselves, with soaps and with detergents) and the literature up to that date (1970) was critically summarized. Much recent work has appeared in all of these areas, but the most significant is the dramatic advances that have taken place in our understanding of the structure, size, shape, equilibria, and thermodynamics of bile salt-lecithin [16,18,28,29,99-102,127, 144,218,223,231-238] and bile salt-lecithin-cholesterol [238,239] micelles which are of crucial importance to the solubihty of cholesterol in bile [1]. This section briefly surveys recent results on the above subclasses. Information on solubilization, solubilization capacities or phase equilibria of binary, ternary or quaternary systems or structures of liquid crystalline phases can be found in several excellent reviews [5,85,207,208,210,211,213,216,217] and, where relevant, have been referred to earlier. 

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