Spontaneous formation of vesicles

A typical kinetic profile for the reaction leading to spontaneous formation of vesicles is shown in Fig. 19.10, both for 6-SLABS and 7-SLABS. It can be seen that the kinetics are relatively simple in this configuration and the formation of vesicles occurs on the time scale of about a minute. 

Amphiphilic inorganic-organic block copolymers (PDMS-6-PEO) with variable block lengths were successfully S5rathesized. The obtained polymers revealed predominantly lamellar phases in their aqueous solutions. At low concentrations the spontaneous formation of vesicles was observed. The polymers can be applied as structure-directing agents in the synthesis of mesostructured silica-based thin films. 

S. C. Ji, J. D. Ding, Spontaneous formation of vesicles frommixed amphiphiles with dispersed molecular weight Monte Carlo simulation, Langmuir, 2006, 22, 553-559. 

Recently, Howse et al. demonstrated the spontaneous formation of vesicles via hydration and dewetting of a thin BCP film from a patterned surface. " The pattern determines the maximum vesicle size. Vesicles form via hydration of the BCP leading to microphase separation into lamellae, followed by budding from the surface (Figure 16). A low-molecular weight, relatively hydrophilic BCP was used (PE0i6-( -PB022, = 2300 Da). 

Polymer vesicles are considered to form in a two-step process. First, the polymer chains form a bilayer-type membrane, which then subsequently closes to form a hollow structure (Figure 9). This process involves an interfacial curvature change, which can correspond to a change in the packing parameter for the polymer and hence a change in the resultant morphology. However, theoretical calculations have revealed that some vesicle formation process may be more complicated than the above two-step procedure. These results can be summarized as two different proposed mechanisms for the spontaneous formation of vesicles from the homogeneous state. 

Jeon YJ, Bharadwaj PK, Choi S-W, Lee JW, Kim K. Supramolecular amphiphiles spontaneous formation of vesicles triggered by formation of a charge-transfer complex in a host. Angew Chem Int Ed 2002 41 4474-6. 

Banerjee, R., S. Dutta, S. Pal, and D. Dhara (2013). Spontaneous formation of vesicles by self-assembly of cationic block copolymer in the presence of anionic surfactants and their application in formation of polymer embedded gold nanoparticles. The Journal of Physical Chemistry B 117(13) 3624-3633. 

Lonchin et al. (27) have also studied the matrix effect of phospholipid vesicles. These authors investigated the formation of mixed phospholipid/fatty acid vesicles by using a combination of kinetic and structural methods to study the process of spontaneous formation of vesicles on addition of oleate micelles to preexisting vesicles of l-palmitoyl-2-oleoyl-.yyn-glycero-3-phosphocholine (POPC). 

Formation of Hposomal vesicles under controlled conditions of emulsification of Hpids with phosphoHpids has achieved prominence in the development of dmgs and cosmetics (42). Such vesicles are formed not only by phosphoHpids but also by certain nonionic emulsifying agents. Formation is further enhanced by use of specialized agitation equipment known as microfluidizers. The almost spontaneous formation of Hposomal vesicles arises from the self-assembly concepts of surfactant molecules (43). Vesicles of this type are unusual sustained-release disperse systems that have been widely promoted in the dmg and cosmetic industries. 

Faure, C., Derre, A. and Neri, W. (2003) Spontaneous formation of silver nanoparticles in multilamellar vesicles. Journal of Physical Chemistry B, 107, 4738-4746. 

Phospholipids are the most important of these liposomal constituents. Being the major component of cell membranes, phospholipids are composed of a hydrophobic, fatty acid tail, and a hydrophilic head group. The amphipathic nature of these molecules is the primary force that drives the spontaneous formation of bilayers in aqueous solution and holds the vesicles together. 

The time course of an actual experiment is shown in Figure 7.17, which shows the hydrolysis of oleic anhydride catalyzed by spontaneously formed oleate vesicles. Note the sigmoid behavior, typical of an autocatalytic process. The lag phase is due to the preliminary formation of vesicles, and in fact the length of the lag phase is shortened when already formed vesicles are pre-added, as shown in the hg-ure. Some mechanistic details of these processes will be discussed in Chapter 10. In this work, an analysis of the number and size distribution of vesicles at the beginning and the end of the reaction was also performed by electron microscopy. 

In our model study reported in this contribution, we have chosen two double-chained C-13 alkylbenzenesulphonate surfactants (SLABS) of closely-related structure, which form micelles in aqueous solution in the absence of salt. However, when small amounts of electrolyte are added (e.g., —20mM NaCl), vesicles are spontaneously formed over a time period of seconds/minutes. These vesicle structures are then reasonably stable over a period of hours/days. The onset of vesicle formation can be readily characterised by the determination of the critical salt concentration (esc), needed to induce the formation of vesicles, from smaller aggregates or monomers. This parameter is easily determined experimentally from the increase in light scattering associated with self-assembly. It has now been determined for a number of electrolyte systems. 

Cha JN et al (2003) Spontaneous formation of nanoparticle vesicles from homopolymer polyelectrolytes. J Am Chem Soc 125 8285... 

Reaction of dendrimers 38-[G2] with L-hexadecylamino-l-deoxylactitol led to the formation of the cationic dendrimers 39-[G2] (Scheme 27). A spontaneous supramolecular self-assembly leading to the formation of vesicles was observed by transmission electron microscopy (TEM). It was shown that the size of these vesicles was around 100 nm, and that their shell was constituted by a bilayer (thickness 10 nm) made of two lipophilic layers (thickness 3 nm) separated by a hydrophilic layer. Furthermore, preliminary experiments show that these cationic dendrimers possess an anti-HIV activity in vitro [41]. 

A. H. de Vries, A. E. Mark, S. J. Marrink, Molecular dynamics simulation of the spontaneous formation of a small DPPC vesicle in water in atomistic detail, J. Am. Chem. Soc, 2004,126, 4488-4489. 

Of course, the two layers of the bilayer do not have areas per molecule which are exactly equal, nor are their curvatures equal and opposite the areas are self-adjusting in equilibrium. This can result in a vesicle that is more stable than a flat, lamellar bilayer since it may be that the favorably curved layer e,g., the outer layer) has a smaller value of the area per molecule and hence more molecules in it than the inner layer. However, such corrections are only important for systems with large spontaneous curvatures and the resulting vesicles would have sizes comparable to molecular lengths. It therefore may turn out that micelles may be more stable than either vesicles or lamellae. In what follows, we consider the spontaneous formation of large vesicles whose sizes are much greater than a surfactant size and hence neglect these effects. 

Polymersomes, self-assembled polymer shells composed of block copolymer amphiphiles. These synthetic amphiphiles with amphiphilicity similar to lipids constitute a new class of drug carriers. They are spontaneously formed in aqueous media, as unilamellar vesicles up to tens of microns in diameter. Amphiphilic block copolymers form a range of self-assembled aggregates including spherical, rod-like, tubular micelles, lamellae, or vesicles, depending on polymer architectnre and preparation conditions. Polymers having low hydrophobicity (less than 50%) favor the formation of micelles, however, intermediate level of hydrophobicity (50%-80%) favors the formation of vesicles. Polymeric vesicles, which have a liposome-like structure with a hydrophobic polymer membrane and hydrophilic inner cavity, are called polymersomes. 

Anraku Y, Kishimura A, Oba M et al (2010) Spontaneous formation of nanosized unilamellar polyion complex vesicles with tunable size and properties. J Am Chem Soc 132 1631-1636... 

---