Cubosomes: Bicontinuous Liquid Crystalline Nanoparticles
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Surfactants, lipids, and polymer molecules that have both polar and nonpolar components are termed amphiphilic. The hydrophobic effect drives amphiphilic molecules in polar solvents to spontaneously self-assemble into a rich array of thermodynamically stable lyotropic liquid crystalline phases with characteristic lengths on the nanometer scale. Liquid crystalline phases possess a sufficient average degree of molecular orientational order to be characterized by their structural symmetry, despite their liquid state, and often form in aqueous surfactant systems at relatively high amphiphile concentrations. An interesting example of contemporary interest is the bicontinuous cubic liquid crystalline phase.
Bicontinuous cubic phases are optically isotropic, very viscous, solidlike liquid crystals with cubic crystallographic symmetry. Prior to their structural characterization, these phases were termed “viscous isotropic phases” and considered quite a nuisance in industrial processes. Bicontinuous cubic phases consist of two separate, continuous but nonintersecting hydrophilic regions divided by a lipid bilayer that is contorted into a periodic minimal surface with zero average curvature. The phase's regular structural connectivity results in a very high viscosity, whereas its tortuosity is useful for slowing diffusion in controlled transport applications. The bicontinuous nature of such cubic phases distinguishes them from the so-called micellar or discontinuous cubic phases containing micelles packed in cubic symmetry. In this work, all references to cubic phases refer to bicontinuous cubic phases, unless otherwise noted.
The last several decades have brought about a great understanding of the properties of cubic phases and a realization of their relevance in areas such as medicine, biology, and chemistry. An intriguing property of the cubic phases formed by certain classes of amphiphiles is their ability to be dispersed into particles, termed cubosomes. Cubosomes are liquid crystalline nanostructured particles with the same unique properties of the bulk cubic phase, although cubosome dispersions have much lower viscosity. Although fundamental research has been focused sharply on bulk cubic phases, it is commercial applications that drive much of the existing and still very active research into cubosomes. This chapter reviews historical research into cubic phases and cubosomes, describing the initial discovery and the realization that differential geometry allows the mathematical characterization of cubic phase structures. Cubosome properties are reviewed in the context of particle formation and applications. Where appropriate, interesting open problems are cited and discussed.