Bicontinuous structures form, amphiphile

In an intermediate concentration range, where the materials cannot decide if the water or the amphiphile aggregate structure should be the continuous matrix, often so-called bicontinuous structures form. In such bicontinuous cubic phases the interfaces have saddle-splay fype sfructures characterized by nonzero negative mean curvature and negative Gaussian curvature. The most common bicontinuous cubic phase is called gyroid... 

The kinds of structures adopted by these microorganisms as well as other mineral morphologies, are the subject of increasing study as chemists look at soft templated routes to nanoscale objects. Early work in the 1990s by a team at Mobile used supramolecular micelles, lamellae and bicontinuous phases4 formed by amphiphiles, to assemble inorganic materials, particularly silica and alumina. A range of new... 

Despite the reasonable tolerance of nonionic surfactants, particularly in topical applications, microemulsions prepared from (phospho)lipids seem to be preferred over those prepared by synthetic surfactants from a toxicity point of view. As discussed by Shinoda et al. [13], lecithin in water-oil systems does not spontaneously form the zero mean curvature amphiphile layers required for the formation of balanced microemulsions but rather forms reverse structures. On decreasing the polarity of the aqueous phase by addition of a short-chain alcohol, e.g., propanol, lecithin was found to form microemulsions at low amphiphile concentrations over wide ranges of solvent composition. The structure of the microemulsions formed was investigated by NMR self-diffusion measurements, and it was found that with a decreasing propanol concentration there was a gradual transition from oil droplets in water, over a bicontinuous structure, to water droplets in oil [13]. 

As we will see below, bicontinuous structures are very significant in many contexts of amphiphile self-assembly. Another type of bicontinuous structure in simple surfactant-water solutions is the sponge phase , formed also in quite dilute surfactant solutions (Figure 19.26). This structure forms for all classes of surfactants but in particular for nonionics. We will also mention that the structure of the sponge phase is related to that of many microemulsions. 

Although spherical and cylindrical micelles, as well as polymersomes, constimte the majority of structures formed through the self-assembly of amphiphilic copolymers, lately there has been a substantial effort to create new intriguing morphologies, including multicompartment, disc-like and bicontinuous micelles. These classes of novel micellar morphology have been recently reviewed by Holder and Sommerdijk [55]. 

Amphiphiles are intriguing materials that self-assemble in various forms when mixed with oil and water. In disordered phases they take a structure of spherical droplets of water-in-oil (or oil-in-water) or a bicontinuous structure, in which both water and oil are separated from each other by intertwined interfacial films. In ordered phases they take a rather regular structure of hexagonal arrays of cylindrical tubules made of oil (or water) in surrounding water (or oil) or parallel arrays of alternative oil and water sheets. In these mixtures amphiphile molecules are mostly located at the interface of oil and water with their polar heads directed into water media and with apolar acyl tails into oil media, thus forming interfacial monolayers. The phase diagram of these structures depends strongly on temperature, pressure, kinds and concentrations of the... 

A mixture of water, oil and amphiphile is known to form various structures under different external conditions, for example, oil-in-water or water-in-oil droplet structure, disordered bicontinuous structure, ordered lamellar structure and so on. Many researchers have directed their attentions to these structural formations and phase transitions among these structures [1, 2]. Their attentions have been focused on the formation of new structures of mesoscopic scale, the self-assembling mechanism of the systems, and interactions among different molecules. 

An amphiphilic film at the oil-water interface exists as a duplex film influenced by the properties of the oil and water phases (Schuhnan et al., 1959 Hanagan and Singh, 2(X)6). When oil molecules have the ability to associate with those within the interfacial film, a microemulsion readily forms. For instance, a mixed system consisting of cetyl alcohol, benzene and 2-amino-2-methyl-1-propanol (AMP) oleate does not form microemulsions due to the lack of complementarity between these compounds. However, if benzene is replaced by a straight-chained hydrocarbon chain (7-18 C), a microemulsion readily forms due to the ability of the oil molecules to associate with the alcohol and AMP oleate at the interface (Schulman et al., 1959). It is normally assumed that microemulsion formation requires the presence of a duplex film with differing tensions on either side of the interface, which impart curvature and thus the possibility of forming dispersed domains or bicontinuous structures (Prince, 1969). 

Amphiphilic molecules (surfactants) are composed of two different parts hydrophobic tail and hydrophilic head [1 ]. Due to their chemical structure they self-assemble into internal surfaces in water solutions or in mixtures of oil and water, where the tails are separated from the water solvent. These surfaces can form closed spherical or cylindrical micelles or bicontinuous phases [3,5]. In the latter case a single surface extends over the volume of the system and divides it into separated and mutually interwoven subvolumes. 

First, the lyotropic phase is used as a template for the preparation of a bicontinuous silica structure, from which the polymer is removed by calcination or extraction. In the second step the porous inorganic structure is filled with monomer and crosslinker which is polymerized to form a bicontinuous organic polymer network from which the silica template is removed by treatment with hydrofluoric acid. An example for the preparation of hierarchical structures is the synthesis of bicontinuous pore structures by using two templates simultaneously [115]. In this case a liquid crystalline lyotropic phase of an amphiphilic block copolymer is used as a template together with suspended latex particles. The sol-gel process with subsequent calcination leads to a bicontinuous open pore structure with pores of 300 nm and 3 nm. 

Liquid crystalline phases are also of interest from the point of view of controlled or sustained release, or even the absence (e.g. in the case of certain potent enzymes) of such release of bioactive molecules. For example, due to the presence of both water and oil channels in bicontinuous cubic structures, such systems are capable of solubilizing both hydrophilic, hydrophobic and amphiphilic drugs, the release of which can be sustained over extended periods of time. Particularly interesting in this respect is the incorporation of large oligopeptide or macromolecular drugs (e.g. enzymes). For example, Ericsson et al. investigated the incorporation of lysozyme in a cubic phase formed by monoolein and water, and found that a considerable amount could be solubilized in the liquid crystalline phase (182). Furthermore, the incorporation of c -lactalbumin, bovine serum albumin and pepsin was found to resemble that... 

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