Micelles and Vesicles

A surfactant (surface-active agent) is an amphiphilic chemical compound, so named because it contains a hydrophilic or water-seeking head group at one end, and a hydrophobic or water-avoiding (i.e., lipophilic or oil-seeking) tail group at die other end, as shown in Fig. 12.13. The hydrophilic part is polar, with a charge that renders it either anionic (—), cationic (+), zwitterionic (±), or nonionic in nature, and the lipophilic portion consists of one or perhaps two nonpolar hydrocarbon chains. Surfactants readily adsorb at an oil-water or air-water interface, and decrease the surface tension there. The hydrocarbon chain might consist of a monomer that can take part in a polymerization reaction.

A surfactant molecule is characterized by a dimensionless packing parameter p defined by where AH is the area of the polar head and Fx and Zq- are, respectively, the volume and length of the hydrocarbon tail (Nakache et al. 2000). If the average cross-sectional area of the tail (Vy/Lr) is appreciably less than that of the head (e.g., p < j), then the tails will pack conveniently inside the surface of a sphere enclosing oil with a radius r > Lr suspended in an aqueous medium, as indicated in Fig. 12.14a. Such a structure is called a micelle. The elongated or cylinder-shaped micelle of Fig. 12.14b appears over the range j < p < j. Larger fractional values of die packing parameter, | <p < 1, lead to the formation of vesicles, which have a double-layer surface structure, as shown in Fig. 12.14c. Far example, sodium di-2-ethylhexyl-phosphate can form nanosized vesicles with FT~0.5nm3, Lr~0.9nm, and Ah ~ 0.7 nm2, corresponding top ~ 0.8, which is in die vesicle range. If the packing

Hydrocarbon Tail

Polar Head

Hydrocarbon Tail

Polar Head

Figure 12.13. Amphiphilic surfactant molecule with a polar hydrophilic head and a nonpolar hydrophobic hydrocarbon tail.

water water

water

Figure 12.14. Sketch of structures formed by amphiphilic molecules at water-oil or water-air interfaces for various values of the packing parameter p of Eq. (12.6). [Adapted from E. Nakache et a)., in Nalwa (2000), Vol. 5, Chapter 11, p. 580.]

parameter is unity, p— 1, then the average transverse cross-sectional area of the tail VilLi will be the same as the area AH of the head, and the tails will pack easily at a planar interface, as shown in Fig. 12.14d, or they can form a bilayer. The inverse of a spherical micelle, called an inverse micelle, occurs with p > 1 for surfactants at the surface of a spherical drop of water in oil, as illustrated in Fig. 12.14e.

An emulsion is a cloudy colloidal system of micron size droplets of one immiscible (i.e., nonmixing) liquid dispersed in another, such as oil in water. It is formed by vigorous stirring, and is thermodynamically unstable because the sizes of die droplets tend to grow with time. If surfactants are present, then nanosized particles, ~100nm, can spontaneously form as a thermodynamically stable, transparent microemulsion that persists for a long time. Surfactant molecules in a solvent can organize in various ways, depending on their concentration. For low concentra-

'ions they can adsorb at an air-water interface. Above a certain surfactant concentration, called the critical micellar concentration, distributions of micelles in the size range from 2 to lOnm can form in equilibrium with free surfactant molecules, being continuously constituted and disassembled with lifetimes measured in microseconds or seconds. Synthetic surfactants with more bulky hydrophobic groups, meaning larger packing parameters, produce extended bilayers that can close in on themselves to form vesicles that are generally spherical. These structures form above a critical vesicular concentration. Vesicles typically have lifetimes measured in weeks or months, so they are much more stable than micelles.

If the vesicles are formed from natural or synthetic phospholipids, they are called unilaminar or single-layer liposomes, that is, liposomes containing only one bilayer. A phospholipid is a lipid (fatty or fatlike) substance containing phosphorus in the form of phosphoric acid, which functions as a structural component of a membrane. The main lipid part is hydrophobic, and the phosphoryl or phosphate part is hydrophilic. The hydration or uptake of water by phospholipids causes them to spontaneously self-assemble into unilaminar liposomes. Mechanical agitation of these unilaminar liposomes can convert them to multilayer liposomes that consist of concentric bilayers. Unilaminar liposomes have diameters from the nanometer to the micrometer ranges, with bilayers that are 5-10 ran thick. Proteins can be incorporated into unilaminar liposomes to study their function in an environment resembling that of their state in phospholipid bilayers of a living cell.

If polymerizable surfactants are employed, such as those containing acrylate, acrylamido, allyl (CH2=CHCH2—), diallyl, methacrylate, or vinyl (CH2=?CH—) groups, then polymerization interactions can be carried out. When vesicles are involved, the characteristic time for the polymerization is generally shorter than the vesicle lifetime, so the final polymer is one that would be expected from the monomers or precursors associated with die surfactant. However, when micelles are involved, their lifetimes are generally short compared to the characteristic times of the polymerization processes, and as a result the final product may differ considerably from the starting materials. Surfactants with highly reactive polymerizable groups such as acrylamide or styryl have been found to produce polymers with molecular weights in excess of a million daltons. Those with polymerizable groups of low reactivity such as allyl produce much smaller products, namely, products with degrees of polymerization that can bring them close to the micelle size range prior to polymerization.

Micelles and bilayers or liposomes have a number of applications in chemistry and biology. Micelles can assist soap solutions to disperse insoluble organic compounds, and permit them to be cleaned from surfaces. Micelles play a similar role in digestion by permitting components of fet such as tatty acids, phospholipids, cholesterol, and several vitamins (A, D, E, and K) to become soluble in water, and thereby more easily processed by the digestive system. Liposomes can enclose enzymes, and at the appropriate time they can break open and release the enzyme so that it can perform its function, such as catalyzing digestive processes.

Many biological membranes such as the plasma membrane of a red blood cell or erythrocyte are composed of proteins and lipids, where a lipid is a fat or Mike

Figure 12.15. Sketch of the structure of a plasma membrane. The phospholipids arrange themselves into a lipid biiayer with their hydrophHic phosphoryl groups at the outside and their fatty acid lipophilic chains on the inside. There are also cholesterol chains in parallel with them. In addition, the membrane contains tightly bound intrinsic proteins that pass through the lipid biiayer, and loosely bound extrinsic proteins on the outside. Some intrinsic proteins have sugar side chains at their surface. (From C. de Duve, A Guided Tour of the Living Cell, Scientific American Books, 1984.)

Figure 12.15. Sketch of the structure of a plasma membrane. The phospholipids arrange themselves into a lipid biiayer with their hydrophHic phosphoryl groups at the outside and their fatty acid lipophilic chains on the inside. There are also cholesterol chains in parallel with them. In addition, the membrane contains tightly bound intrinsic proteins that pass through the lipid biiayer, and loosely bound extrinsic proteins on the outside. Some intrinsic proteins have sugar side chains at their surface. (From C. de Duve, A Guided Tour of the Living Cell, Scientific American Books, 1984.)

substance. Much of the lipid material is present as phospholipids and cholesterol that form bilayers, typically with the packing parameter p ~ 1. The overall structure of a plasma membrane is a lipid biiayer of the type illustrated in Fig. 12.15, which is formed by the self-assembly of lipid molecules. On die outside of the biiayer are found the hydrophiiic phosphoryl groups POj~ of the phospholipids, which are in contact with the blood plasma aqueous solution that surrounds the red blood cell, and in which it resides. The lipophilic groups are inside pointing toward the suspension of hemoglobin molecules that fills the interior of the red blood cell. We see from the figure that most of the membrane structure consists of fatty acid chains of the phospholipids, with the remainder of the biiayer formed from cholesterol chains. The figure also shows that the plasma membrane contains extrinsic proteins on the surface, intrinsic proteins that penetrate through the biiayer, and sugar side chains attached to the outside.

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