Self-assembled structures having a regular hollow icosahedral form (such as those observed for proteins of virus capsids) can occur as a result of biomineralization processes, but are extremely rare in mineral crystallites. Compact icosahedra made from a boron oxide have been reported, but equivalent structures made of synthetic organic components such as surfactants have not hitherto been observed. It is, however, well known that lipids, as well as mixtures of anionic and cationic single chain surfactants, can readily form bilayers that can adopt a variety of distinct geometric forms: they can fold into soft vesicles or random bilayers (the so-called sponge phase) or form ordered stacks of flat or undulating membranes. Here we show that in salt-free mixtures of anionic and cationic surfactants, such bilayers can self-assemble into hollow aggregates with a regular icosahedral shape. These aggregates are stabilized by the presence of pores located at the vertices of the icosahedra. The resulting structures have a size of about one micrometre and mass of about 1010 daltons, making them larger than any known icosahedral protein assembly or virus capsid. We expect the combination of wall rigidity and holes at vertices of these icosahedral aggregates to be of practical value for controlled drug or DNA release.

Discs of finite size are a very rare form of stable surfactant self-assembly. It is shown that mixing of two oppositely charged single-chain surfactants can produce rigid nanodiscs as well as swollen lamellar liquid crystals with frozen bilayers. The crucial requirement for obtaining nanodisc self-assembly is the use of H+ and OH- as counterions. These counterions then form water and lower the conductivity to 10 microsiemens per centimeter. In the case of cationic component excess, a dilute solution of nanodiscs is in thermodynamic equilibrium with a lamellar phase. The diameter of the cationic nanodiscs is continuously adjustable from a few micrometers to 30 nanometers, with the positive charge located mainly around the edges.

Mixtures of cationic and anionic surfactants crystallized at various ratios in the absence of added salt form micrometer-sized colloids. Here, we propose and test a general mechanism explaining how this ratio controls the shape of the resulting colloidal structure, which can vary from nanodiscs to punctured planes; during cocrystallization, excess (nonstoichiometric) surfactant accumulates on edges or pores rather than being incorporated into crystalline bilayers. Molecular segregation then produces a sequence of shapes controlled by the initial mole ratio only. Using freezefracture electron microscopy, we identified three of these states and their corresponding coexistence regimes. Fluorescence confocal microscopy directly showed the segregation of anionic and cationic components within the aggregate. The observed shapes are consistently reproduced upon thermal cycling, demonstrating that the icosahedral shape corresponds to the existence of a local minimum of bending energy for facetted icosahedra when the optimal amount of excess segregated material is present.C ontrolling both size and shape of colloidal particles is a major challenge to the predictable formulation of mixed systems consisting of surfactants, polymers, and inorganic solids. Successful control of size and shape requires simultaneous knowledge of the mixture's equilibrium phase behavior and the mechanism of formation. The aim of this article is to describe and test hypotheses based on bending energy to control a general sequence of colloidal shapes, from large discs to punctured planes.If the elemental building blocks of a complex colloidal aggregate are amphiphilic molecules, the basic concept used to rationalize self-assembly is the concept of spontaneous curvature originating from the surface-to-volume ratio of the surfactant film (1). Common single-chain ionic amphiphiles have a spontaneous radius of curvature equivalent to one surfactant length (2) and therefore form globular micelles. Decreasing monolayer curvature obtained by mixing surfactants first produces giant cylindrical and finally locally flat bilayers.When the elementary building block is a fragment of a bilayer, line tension of pores, or rims of discs, and elastic energy associated to dihedral angles on the contact line between adjacent facets need to be considered (3). The crystallization͞ segregation should be consistent with the sequence of shapes observed with strongly interacting charged colloids in the absence of salt. Molecular segregation is demonstrated by specificity of labeling with a dye and direct observation, and we show finally that the resulting shapes correspond to local minima of energy of formation. Mechanism of Shape Control Through Molecular SegregationConsider an initial state of the dispersion with unilamellar vesicles in the fluid state. In the fluid state of mixed vesicles, the two components exhibit in-plane miscibility.Y Upon cooling, nucleation and growth of planar crystals occur in the form of polygonal frozen bilayers, which can only form at a fixe...