Bilayer aggregates

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

Alternative and more complex surfactants continue to be explored as a resourceful option for MEKC separations. In situ generated micelles, which are anionic complexes formed by alkyl-or steroidal-glycoside surfactants and borate ions, the use of sodium 10-undecylenate (SUA) and sodium 10-undecylsulfate (SUS) oligomers as well as surfactants composed of two ionic groups and two liphophilic chains, such as sodium 5,12-h (dodecyloxymethyl)-4,7,10,13-(tetraoxa)-1,16-hexadecanedisulfonate (DBTD), bilayered aggregates such as vesicles and liposomes, and bilayer micelles are a few examples. 

Surfactants tend to accumulate at the solid/liquid interface just as they accumulate at the vapor/liquid and liquid/ liquid interfaces. Surfactants also tend to form aggregates at the solid/solution interface, just as they tend to form micelles in solution. Bilayered aggregates of adsorbed surfactants are called admicelles. 

The hydrophobic and hydrophilic portions of the surfactant should be of equal size (P = 1) enabling the formation of bilayer aggregates. This should lead to a lower interfacial tension substrate/water and a faster spreading kinetics. 

FIGURE 15.13. Surfactants that form closed bilayer aggregate structures such as vesicles usually produce multilayered systems such as (a). Smaller, single bilayer vesicles such as b) can be formed by disruption of the multilayer systems. 

Both the amount of CieTAB and the thickness of the bilayer aggregates are sensitive to the condition of the underlying silica surface, which gives rise to a certain variability of the amount adsorbed on polished silica sam-... 

However, photoreactivity of butadienes is not restricted to the bulk crystalline state, but also occurs in less ordered systems, as for example in the liquid-expanded state of monolayers in bilayer aggregates above the phase transition point and in host guest complex systems, in which the guest monomers frequently are not located at fixed lattice sites... 

The stracture of the cell plasma membrane is illustrated in Figure 7.7. The phosphohpid molecules aggregate into a bilayer which serves to remove the hydrophobic chains from the aqueous environment and place a polar, hydrophilic head group at eaeh side of the bilayer which is exposed to water. The bilayer aggregate is liquid crystalline in nature, in that the head groups do not have any periodic ordering and the hydrocarbon chains are not rigid. The liquidity of the stracture aUows the movement of phospholipid molecules about the cell membrane. Of course, the associated proteins can also move within the cell membrane but they do so more slowly. This liquid crystalline stractrrre of the cell membrane provides form and allows the selective movement of materials in and out of the cells. 

Surface force and ellipsometric experiments indicate that the surfactants self-assemble in the form of bilayer aggregates whose thickness was found to be consistent with the molecular structure, and, furthermore, ellipsometric measurements indicated that no complete bilayers were formed but rather bilayer aggregates were present on the surface even at concentrations well above the cmc. 

Surface force data for low fluorosurfactant concentrations demonstrated that upon compression the bilayer aggregates assembled on the isolated surfaces are transformed and as a result monolayer structures build up between the surfaces in contact (Figure 16). 

These compounds give clear to slightly turbid solution when dispersed in water by sonication. The flexible alkyl tail must be Cy or longer for the bilayer formation. The molecular weight of the bilayer aggregate(vesicles and lamellar) is 10 - 10. Similar results were obtained from amphiphiles (12) which contained the biphenyl unit as the rigid segment(lO). 

CTAB micelles + SOS monomers Mixed micelles (in less than 1 ms) Mixed micelles Floppy bilayer aggregates (in s to minutes)... 

Floppy bilayer aggregates Small closed vesicles (in minutes to hours) Small closed vesicles Equilibrium vesicles (in weeks)... 

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