11.4.2 Amphiphilic Vesicles

Phospholipid molecules form bilayer films or membranes about 5 nm in thickness as illustrated in Fig. XV-10. Vesicles or liposomes are closed bilayer shells in the 100-1000-nm size range formed on sonication of bilayer forming amphiphiles. Vesicles find use as controlled release and delivery vehicles in cosmetic lotions, agrochemicals, and, potentially, drugs. The advances in cryoelec-tron microscopy (see Section VIII-2A) in recent years have aided their characterization [70-72]. Additional light and x-ray scattering measurements reveal bilayer thickness and phase transitions [70, 71]. Differential thermal analysis... 

At high MEGA-n concentration, the size is as small as a MEGA-n micelle itself suggesting that dialkyl amphiphile is solubilized in the nonionic micelle. At low concentration a dialkyl amphiphile vesicle keeps its size relatively constant and takes up MEGA-n molecules. 

The diffusion barrier. Much attention has been directed toward primitive amphiphile vesicles, inasmuch as they self-assemble from simple components and have an obvious ancestral connection with the more complex membranes that enclose modem cells. A review has been provided by Monnard and Deamer.55 The papers by Segre et al. and Hanczyc et al. contain additional discussion.56 57 Other prominent alternatives that would limit loss by diffusion have been electrostatic forces at mineral surfaces,58 iron sulfide membranes,59 and aerosols at the ocean-atmosphere interface.60 Section 2.7.1 discusses the function of compartmentalization in Earth life today. 

Liposomes—Spherical amphiphilic vesicles capable of sustained release of water-soluble substances. 

Besides issues related to the accuracy of force fields in spatially inhomogeneous systems comprising many chemically distinct components, the basic restriction related to the chemically detailed models is the rather small length and time scales that they can access. This limitation imposes severe restrictions for considering collective phenomena in amphiphilic vesicles, i.e., processes that involve large particle numbers. Typical examples include vesicle assembly, vesicle fusion, phase separation and shape transformations of multicomponent amphiphilic vesicles. For many of these processes, it is expected that the underlying atomistic details of the molecular constituents can be captured by a small number of relevant characteristics and universality classes, comprised of systems with a rather different atomistic structure, can be identified. These phenomena can be successfully investigated via minimal... 

It is of interest not only to perforate vesicle membranes but also to destroy them after they have served their purpose as transport vehicles, in particular for DNA. Natural vesicles, so-called endosomes, contain about 50% cholesterol. The disruption of such cholesterol-containing lipid bilayers by Triton XI00 or sodium deoxycholate, examples of artificial and natural detergents, results in a leaky membrane at low concentration and in a catastrophic rupture process above the cmc of the amphiphiles. Vesicles made of fluid phospholipid bilayers devoid of cholesterol showed only leakiness under the same conditions. Amphiphiles with a carboxylate end group and a very bulky hydrophobic end (e.g., with two tert. butyl groups) disrupt membranes at pH 5 and have no effect above pH 7 (harpoons). For an example, see Figure 6.5.3. 

M. K. Kawamuro, H. Chaimovich, E. B. Abuin, E. A. Lissi, 1. M. Cuccovia, Evidence that the effects of synthetic amphiphile vesicles on reaction-rates depend on vesicle size, J. Phys. Chem., 1991, 95, 1458-1463. 

The PBE calculations showing appreciable concentrations of counterions in the inner aqueous compartment of the vesicle is consistent with the results showing that the reactivity of OH in (DODA)C is comparable in the inner and outer compartments. Direct measurement of ion concentrations in the internal aqueous of synthetic amphiphile vesicles has not been reported. We have recently measured the concentration of counterions in the intermicellar aqueous phase using the dediazionation method first described by Romsted [50]. Basically, this method consists in determining the products from the fast reaction of a substituted... 

Cuccovia, I.M., Quina, E.H., Chaimovich, H. A remarkable enhancement of the rate of ester thiolysis by synthetic amphiphile vesicles. Tetrahedron 1982, 58(7),... 

J. C. Lang, Physics of Amphiphiles Micelles, Vesicles and Microemulsions, Soc. Italiana di Fisica, XC Corso, Bologna, 1985. 

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. 

These chain models are well suited to investigate the dependence of tire phase behaviour on the molecular architecture and to explore the local properties (e.g., enriclnnent of amphiphiles at interfaces, molecular confonnations at interfaces). In order to investigate the effect of fluctuations on large length scales or the shapes of vesicles, more coarse-grained descriptions have to be explored. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Mesoscopic structures and phases vesicles and vesicle shapes, structured phases and phase behavior of amphiphilic systems. 

The other class of phenomenological approaches subsumes the random surface theories (Sec. B). These reduce the system to a set of internal surfaces, supposedly filled with amphiphiles, which can be described by an effective interface Hamiltonian. The internal surfaces represent either bilayers or monolayers—bilayers in binary amphiphile—water mixtures, and monolayers in ternary mixtures, where the monolayers are assumed to separate oil domains from water domains. Random surface theories have been formulated on lattices and in the continuum. In the latter case, they are an interesting application of the membrane theories which are studied in many areas of physics, from general statistical field theory to elementary particle physics [26]. Random surface theories for amphiphilic systems have been used to calculate shapes and distributions of vesicles, and phase transitions [27-31]. 

FIG. 6 Configuration snapshot of a spontaneously formed vesicle from doubletailed amphiphiles in the Larson model (a) entire vesicle (b) vesicle cut in half in order to show its inner side. Black circles represent head particles (+1), gray circles tail particles (—1), white circles the neutral connecting particles (0). (From Bernardes [126].)... 

---