ABSTRACT
This chapter and the next two chapters will focus on the general preparation methods for micelles, vesicles, liposomes, nanoparticles, nanocapsules, and nanobottles. In this chapter will cover the preparation of micelles, vesicles, and liposomes because these require only physical methods, compared to most of the preparation methods for nanoparticles and nanocapsules where polymerization reactions are required (except for the layer-by-layer approach, which is treated separately in Chapter 10). A relatively new method of preparing micelles and vesicles is the polymerization-induced self-assembly (PISA) method, which will be treated in Chapter 5.
4.1 Preparation of MicellesIn principle micelles are formed spontaneously on dissolution of the surfactant above the critical micelle concentration (CMC). For SDS at 60°C the CMC in pure water is around 10-2 mol L-1. The CMC is strongly influenced by the presence of salts but also by the presence of hydrophobic molecules that solubilize inside the hydrophobic domain of the micelles. For blockcopolymer micelles the CMC lies at much lower concentrations. Micelles can coexist with other surfactant aggregate morphologies like vesicles and worm-like structures. There are anionic, cationic, and nonionic micelles. In all case a hydrophobic and a hydrophilic part can be distinguished. Also mixed micelles are possible, for example, catanionic micelles. In general the model of hydrophilic lipophilic balance (HLB) is used to describe the properties of the surfactant (Mitchell, 1981). 4.2 Preparation of Vesicles and Liposomes Basically there is no difference between a vesicle and a liposome, other than that the liposome is based on lipids and a vesicle is based on any other (synthetic or natural) surfactant. The basis for the existence of vesicles and liposomes lies in the underlying phase diagram, which can be complex. To force these structures into a certain shape and size several methods are available. The formation of these structures needs some input of energy, and a number of different approaches have been used to assemble these lipids vesicles, including mechanical methods (thin-film hydration, ultrasonic irradiation, high-pressure homogenization, microemulsification, and extrusion) and methods based on the replacement of organic solvents by aqueous media (injection methods, reverse-phase evaporation, and proliposomes) (Wagner, 2011). Special methods are those based on the use of dense gas in the production of liposomes (supercritical fluid liposome methods). In the literature, one can find other approaches sometimes used to prepare liposomes; however, its use is limited at present. A description of such approaches can be found elsewhere
