Bicontinuous properties

These fascinating bicontinuous or sponge phases have attracted considerable theoretical interest. Percolation theory [112] is an important component of such models as it can be used to describe conductivity and other physical properties of microemulsions. Topological analysis [113] and geometric models [114] are useful, as are thermodynamic analyses [115-118] balancing curvature elasticity and entropy. Similar elastic modulus considerations enter into models of the properties and stability of droplet phases [119-121] and phase behavior of microemulsions in general [97, 122]. 

Giannakas, AE Vaimakis, TC Ladavos, AK Trikalitis, PN Pomonis, PJ. Variation of surface properties and textural features of spinel ZnAl204 and perovskite LaMnOs nanoparticles prepared via CTAB-butanol-octane-nitrate salt microemulsions in the reverse and bicontinuous states. Journal of Colloid end Interface Science, 2003, Volume 259, Issue 2, 244-253. 

To make a solid foam we start from a liquid foam and induce solidification. This can be achieved by a chemical polymerization (Styrofoam), by lowering the temperature (pumice stone or a souffle), or by increasing the temperature to induce a structural transition (baking of bread). Porous solids can appear as solid foams because of their low density and their high content of gas. The difference between the two is that in a porous solid we have a bicontinuous structure while the individual cavities in a foam are closed. This is an important difference because porous solids tend to adsorb liquids due to capillary effects and then completely change their properties. 

TPs may provide some toughening for high-Tg networks without loss in thermomechanical properties the effect is modest below phase inversion and may increase significantly when bicontinu-ous morphologies are formed. 

The formation of bicontinuous microemulsions is conditioned by the nature of the monomer which is present in large amounts (up to by weight and which e.terts a great effect on the HLB and interfacial properties of the systems. Furthermore as the polynerirable microemulsicns contain a fairly large concentration of surfactant(s), interactions between surfactants and monomers cannot be neglected, especially when the latter are electrolytes. 

Addition of salting-out type electrolytes to oil-water-surfactant (s) systems has also a strong influence on their phase equilibria and interfacial properties. This addition produces a dehydration of the surfactant and its progressive transfer to the oil phase (2). At low salinity, a water-continuous microemulsion is observed in equilibrium with an organic phase. At high salinity an oil-continuous microemulsion is in equilibrium with an aqueous phase. At intermediate salinity, a middle phase microemulsion with a bicontinuous structure coexists with pure aqueous and organic phases. These equilibria were referred by Vinsor as Types I,II and III (33). 

This chapter focuses on the fixation of lyotropic liquid crystalline phases by the polymerization of one (or more) component(s) following equilibration of the phase. The primary emphasis will be on the polymerization of bicontinuous cubic phases, a particular class of liquid crystals which exhibit simultaneous continuity of hydrophilic — usually aqueous — and hydrophobic — typically hydrocarbon — components, a property known as bicontinuity (1), together with cubic crystallographic symmetry (2). The potential technological impact of such a process lies in the fact that after polymerization of one component to form a continuous polymeric matrix, removal of the other component creates a microporous material with a highly-branched, monodisperse, triply-periodic porespace (3). 

The polymerization of bicontinuous cubic phases provides a new class of micrpporous materials with properties that have never before been attainable in polymeric membranes. The most important of these properties are now discussed in turn, and for each an application is biiefly dicussed to illustrate the potential importance of the property in a technological, research, or clinical application. 

The molecules that formed the first most primitive form of life had to self-assemble in the "primeval soup". Besides the obvious requirement of selfreproduction, there must have been an encapsulating membrane, separating inside from outside. The cubosome (a dispersed bicontinuous cubic phase, cf. Chapter 5) provides a number of remarkable properties that make it a candidate as an organisational assembly for the earliest forms of life. 

Thus, self-diffusion - if studied over macroscopic distances - should reveal whether the process is rapid or slow, depending on the geometric properties of the inner structure. For example, a phase that is water-continuous and oil-discontinuous should exhibit a rapid diffusion of hydrophUic components, while the hydrophobic components should difiuse slowly. In contrast, an oil-continuous but water-discontinuous system should exhibit a rapid diffusion of the hydrophobic components. It would be expected that a bicontinuous structure should promote a rapid diffusion of all components. 

A general pattern of microemulsion phase behavior exists for systems containing comparable amounts of water and a pure hydrocarbon or hydrocarbon mixture together with a few percent surfactant. For somewhat hydrophilic conditions, the surfactant films tend to bend in such a way as to form a water-continuous phase, and an oil in water microemulsion coexists with excess oil. Drops in the microemulsion are spherical with diameters of order 10 nm. Both drop size and solubilization expressed as (VJVX the ratio of oil to surfactant volume in the microemulsion, increase as the system becomes less hydrophilic. At the same time interfacial tension between the microemulsion and oil phases decreases. Just the opposite occurs for somewhat lipophilic conditions. That is, a water in oil microemulsion coexists with excess water with drop size and solubilization of water (VJV,) increasing and interfacial tension decreasing as the system becomes less lipophilic. When the hydrophilic and lipophilic properties of the surfactant films are nearly balanced, a bicontinuous microemulsion phase coexists with both excess oil and excess water. For a balanced film (VJV,) and (VJV ) in the microemulsion are nearly equal, as are 7, 0 and... 

The research on microemulsions currently concentrates on even more complex mixtures. By adding amphiphilic macromolecules the properties of microemulsions can be influenced quite significantly (see Chapter 4). If only small amounts of amphiphilic block copolymers are added to a bicontinuous microemulsion a dramatic enhancement of the solubilisation efficiency is found [27,28]. On the other hand, the addition of hydrophobically modified (HM) polymers to droplet microemulsions leads to a bridging of swollen micelles and an increase of the low shear viscosity by several orders of magnitude [29]. 

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