Single-chain amphiphiles

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

This inequality indicates the amphiphile adopts a shape essentially equivalent to that of a cone with basal area <3. Such cones self-assemble to fonn spheroidal micelles in solution or spheroidal hemimicelles on surfaces (see section C2.3.15). Single-chain surfactants with bulky headgroups, such as SDS, typify surfactants in this category. 

These results are quite compatible with the preferred conformations of the two-chain carbonyl diacids in aqueous media mentioned above (Porter et al, 1986b, 1988). The meso-compounds preferred collinear conformation, which places the hydrogens at the asymmetric carbons in a nearly eclipsed position relative to each other (Fig. 44), is more stable than that of the ( )-diastereomer by about 1.2kcalmol 1. In this conformation, the two carboxylic acid groups at the ends of the chain can be attached to the water surface side-by-side. The entire molecule can then behave as a good-amphiphile whose structure is similar to a pair of single-chain fatty acid molecules bound side-by-side, each chain mirroring the other about the molecules plane of symmetry. 

The W 2 and v values were obtained for various single-chain amphiphiles with the same chain length (Cig) and the different... 

We could also build up vesicles from a number of single chain amphiphiles (12,13,14) 9). 

More recently, Kunitake and Okahata (1978b) discovered that a stable bilayer structure could be formed from a series of single-chain amphiphiles which possess rigid segments and flexible hydrocarbon tails as in [6]. 

As discussed in the preceding sections, fluid, globular micelles are formed from monoalkyl surfactants, whereas the liquid-crystalline bilayer structure is formed from a variety of dialkyl amphiphiles and from single-chain amphiphiles with rigid hydrophobic segments. It may then be asked what structure is expected from amphiphiles with three alkyl chains. 

Figure 6 (Top) Chemical structure of dequalinium (bottom) possible conformations of single-chain bola amphiphiles. Amphiphiles in a stretched conformation (bola) would form monolayers, while amphiphiles in a bended conformation (horseshoe) would form bilayers.

The effect characteristic of a multi-chain hydrophobe, that is, increase in the cmc and simultaneous decrease in the cloud point, appears to be inconsistent with the well-known HLB concept in surfactants. Tanford has pointed out that based on geometric considerations of micellar shape and size, amphiphilic molecules having a double-chain hydrophobe tend to form a bilayer micelle more highly packed rather than those of single-chain types ( ). In fact, a higher homologue of a,a -dialkylglyceryl polyoxyethylene monoether has been found to form a stable vesicle or lamellar micelle (9 ). Probably, the multi-chain type nonionics listed in... 

Fig. 30 Left Side view of a single chain of amphiphilic polymer 52 at the air-water interface. Right End-on view of three adjacent n-stacking polymer chains...

Many single-chain amphiphiles form cubic phases when added to water in a given composition. Two of the most well known are didodecyl-phosphatidyl ethanolamine, and mono-olein. Figure 9.18 shows some idealized bicontinous cubic structures of the former, including typical inverse ones. This is also highly viscous and optically transparent as are most of the other cubic phases. 

Hargreaves, W. R and Deamer, D. W. (1978a). Liposomes from ionic, single-chain amphiphiles. Biochemistry, 17, 3759-68. 

It has been reported that a variety of single-chain amphiphiies spontaneously produce stable, membrane-forming aggregates when dispersed in water 258 260). Dialky 1-amphiphile l-III (l or d means l- or D-configuration of amino acid unit in compound III, respectively), which was prepared by condensation of didodecyl L-glutamate and p-(4-bromobutoxy)benzoic acid and the subsequent quarterization with tri-methylamine, produces bilayer vesicles in water as probed by electron microscopy 251 >. 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

Fig. 15 Binary phase diagrams showing the change of the mesophase structure depending on the ratio of single-chain to three-chain compounds (a) amphiphiles 41/43 [139] and (b) pentaery-thritol tertabenzoates 35/37 RP = (CH2)4C6F13 for all compounds (T/°C) [138], Reproduced with permission (a) [139] copyright 2002, American Chemical Society (ACS) (b) [138] copyright 2000, Wiley-VCH...

FIGURE 7.1. Aggregate morphologies of single-chain amphiphiles. 

As we will see in Sect. 3.4, such a relatively trivial modification of the standard HP model can lead to some nontrivial consequences when studying the collapse for the single-chain amphiphilic polymers and their aggregation in solution. 

In this review, hydrophilically and hydrophobically modified poly(N-iso-propylacrylamide) (PNIPAM) copolymers are mainly used to illustrate how amphiphilic copolymer chains can fold from an extended random coil to a collapsed globule in extremely dilute solutions and associate to form a stable mesoglobular phase which exists between single-chain globules and macroscopic precipitation. The copolymers used can be prepared by free-radical reaction. 

As the polymer concentration increases, interchain association inevitably occurs, but some amphiphilic chains can undergo a limited interchain association to form a stable mesoglobular phase that exists between microscopic single-chain globules and macroscopic precipitation. As expected, when the solvent quality changes from good to poor, intrachain contraction and interchain association occur simultaneously and there exists a competition between these two processes. Such a competition depends on the comonomer composition and distribution on the chain backbone and also depends on the rate of micro-phase separation. When intrachain contraction happens quickly and prior to interchain association, smaller mesoglobules are formed. A proper adjustment of the rates of intrachain contraction and interchain association can lead to polymeric colloidal particles with different sizes and structures. 

Fig. 9 Vesicles produced by single-chain amphiphiles such as fatty acids tend to be destabilized by certain environmental factors. If the fatty acid is protonated at low pH ranges, the membranes collapse into droplets. The vesicles also become increasingly unstable as temperature increases. In the presence of high salt concentrations, the vesicles undergo osmotic collapse and may also form nonmembranous aggregates if divalent cations react with the carboxylate head groups...

This review includes the structural information that has been carried out to date from films of single chain amphiphiles, enzymes, and proteins using both IRRAS and PM-IRRAS methods. 

Giulieri, F., and Krafft, M. R (2003), Tubular microstructures made from nonchiral single-chain fluorinated amphiphiles Impact of the structure of the hydrophobic chain on the rolling-up of bilayer membrane, J. Coll. Interf. Sci., 258, 335-344. 

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