Micelle formation

Micelles form only above the critical micelle concentration (CMC) and above the Krafft temperature. The CMC is detected by noting a pronounced change in physical properties of the solution, particularly the molar conductivity (Fig. 19.40). There is no abrupt change in properties at the CMC; rather, there is a transition region corresponding to a range of concentrations around the CMC where physical properties vary smoothly but nonlinearly with the concentration. The hydrocarbon interior of a micelle is like a droplet of oil. Nuclear magnetic resonance shows that the hydro carbon tails are mobile, but slightly more restricted than in the bulk. Micelles are important in industry and biology on account of their solubilizing function: matter can be transported by water after it has been dissolved in their hydrocarbon interiors. For this reason, micellar systems are used as detergents, for organic synthesis, froth flotation, and petroleum recovery. Non-ionic surfactant molecules may cluster together in clumps of 1000 or more, but ionic species tend to be disrupted by the electrostatic repulsions between head groups and are normally limited to groups of less than about 100. The micelle population is often polydisperse, and the shapes of the individual micelles vary with con centration. Spherical micelles do occur, but micelles are more commonly flattened spheres close to the CMC. Under certain experimental conditions, a liposome may form, with an inward pointing inner surface of molecules surrounded by an outward pointing outer layer (Fig. 19.41). Liposomes may be used to carry nonpolar drug molecules in blood. In concentrated solutions micelles formed from surfactant molecules may take the form of long cylinders and stack together in reasonably close-packed (hexagonal) arrays. These orderly arrangements of micelles are called lyotropic mesomorphs and, more colloquially, ‘liquid crystalline phases’. The enthalpy of micelle formation reflects the contributions of interactions between micelle chains within the micelles and between the polar head groups and the sur rounding medium. Consequently, enthalpies of micelle formation display no readily discernible pattern and may be positive (endothermic) or negative (exothermic). Many non-ionic micelles form endothermically, with ∆H of the order of 10 kJ per mole of surfactant. That such micelles do form above the CMC indicates that the entropy change accompanying their formation must then be positive, and measurements suggest a value of about +140 J K⁻¹ mol⁻¹ at room temperature. The fact that the entropy change is positive even though the molecules are clustering together shows that hydrophobic interactions (Section 18.*) are important in the formation of micelles.

Fig. 19.40 The typical variation of some physical properties of an aqueous solution of sodium dodecyl sulfate close to the critical micelle concentration (CMC).

Fig. 19.41 The cross-sectional structure of a spherical liposome.

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مواضيع ذات صلة

Lattices and unit cells

Surface pressure

Membrane formation

The electrical double layer

Classification and preparation

The stability of proteins and nucleic acids

The structure of nucleic acids

Helices and sheets

Conformational energy

The Corey–Pauling rules

The different levels of structure

Viscosity