Bicontinuous phase reactions

PHEMA solubility decreases with increasing ion concentration. As a result, Mikos et al. used salt solutions of varying ionic strength to dilute the reaction mixtures (Liu et al., 2000). It was noted that increasing the ion content of the aqueous solution to 0.7M, interconnected macropores were obtained at 60 vol% water. Surfactants may also be used to control the network pore structure. However, not much work has been done in this area, since surfactants typically work to reduce the surface repulsions between the two phases and form a uniform emulsion. These smaller emulsion droplets when gelled will create a network with an even smaller porous structure. Yet, this is still a promising area of exploration, since it may be possible to form alternate phase structures such as bicontinuous phases, which would be ideal for cellular invasion. 

The initial mixture is homogeneous, and phase separation takes place during the cure of the thermoset. This second technique is called reaction-induced phase separation (Williams et al., 1997) and may lead to several types of morphologies a dispersion of modifier-rich particles in a thermoset matrix a dispersion of thermoset-rich particles in a modifier matrix (phase-inverted morphology), or two bicontinuous phases. 

In this section, we summarize our results and discuss their physical implications on the reaction kinetics in the bicontinuous phases. The RG analysis confirmed our mean-field arguments and thereby predicts that the dynamic fluctuations ofthe microemulsion will be irrelevant in impacting the hydrodynamic behavior (long length and time scale) ofthe system. Thus, the kinetics ofthe reaction A B Q is expected to follow the anomalous mean-field regime (AMF), one wherein concentrations decay as ca,cb (nQyi (Dt) l at long times. This constitutes the central result of this part of the article. 

Gutfelt et al. (1997) have evaluated various ME formulations as reaction media for synthesis of decyl sulphonate from decylbromide and sodium sulphite. The reaction rate was fast both in water-in-oil and in bicontinuous ME based on non-ionic surfactants. A comparison was made with this reaction being conducted in a two-phase. system with quats as phase-transfer catalyst but was found to be much less efficient. However, when two other nucleophiles, NaCN and NaNOj, were used the PTC method was almost as efficient as the ME media. It seems that in the case of decyl sulphonate there is a strong ion pair formation between the product and the PTC. The rate in the ME media could be further increased by addition of a small amount of a cationic surfactant. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

There are multiple possible current pathways through a DSSC, as shown in Fig. 1, because the nanoporous cell consists of two interpenetrating, bicontinuous chemical phases. The relative conductivity of these two phases and of the connection between them, Rct, depends on the illumination intensity, applied potential, kinetics of the redox couple, and so forth. Therefore, the distribution of current pathways depends also on these variables. In the DSSC, the dark current will take the distributed path of least overall resistance (Sections III. A-III.C), meaning it will flow primarily through solution [50] under the expected conditions of Rct < / 2- The dark current is thus mainly a measure of reaction (5) in this potential range, even though reaction (4) is expected to be the dominant recombination... 

A variety of catalysts work well in the typical hydrolyses reactions used, including acids, bases, and salts.29 Basic catalysts give precipitated phases that are generally well-defined particles, whereas the acidic catalysts give more poorly defined, diffuse particles.4,30 In some cases, particles are not formed at all, and bicontinuous (interpenetrating) phases result.17,31... 

Oh et al. [16] have demonstrated that a microemulsion based on a nonionic surfactant is an efficient reaction system for the synthesis of decyl sulfonate from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2). Whereas at room temperature almost no reaction occurred in a two-phase system without surfactant added, the reaction proceeded smoothly in a micro emulsion. A range of microemulsions was tested with the oil-to-water ratio varying between 9 1 and 1 1 and with approximately constant surfactant concentration. NMR self-diffusion measurements showed that the 9 1 ratio gave a water-in-oil microemulsion and the 1 1 ratio a bicontinuous structure. No substantial difference in reaction rate could be seen between the different types of micro emulsions, indicating that the curvature of the oil-water interface was not decisive for the reaction kinetics. More recent studies on the kinetics of hydrolysis reactions in different types of microemulsions showed a considerable dependence of the reaction rate on the oil-water curvature of the micro emulsion, however [17]. This was interpreted as being due to differences in hydrolysis mechanisms for different types of microemulsions. 

The polymerization of one or more components of a lyotropic liquid crys in such a way as to preserve and fixate the microstructure has recently been successfully performed. This opens up new avenues for the study and technological application of these periodic microstructures. Of particular importance are the so-called bicontinuous cubic phases, having triply-periodic microstructures in which aqueous and hydrocarbon components are simultaneously continuous. It is shown that the polymerization of one of these components, followed by removal of the liquid components, leads to the first microporous polymeric material exhibiting a continuous, triply-periodic porespace with monodisperse, nanometer-sized pores. It is also shown that proteins can be immobilized inside of polymmzed cubic phases to create a reaction medium allowing continuous flow of reactants and products, and providing a natural lipid environment for the proteins. 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

III microemulsion system in the presence of water and an appropriate oil, without requiring cosurfactant nor electrolyte. They play a dual role as [1] Oj generating catalyst and as surfactant stabilizing the bicontinuous microemulsion. From a physicochemical point of view, they must have an effective packing parameter close to one [73]. Bis(dimethyldialkylammonium)molybdates are typical examples of such balanced catalytic surfactants, which lead to a three-phase system as shown in Figure 22.6. In such reaction media, 02 is exclusively generated in the aqueous nanodomains of the middle-phase microemulsion where the reaction takes place. Under stirring, the excess oil phase transfers the substrate S to the... 

The contribution of faradaic and double-layer capacitances in the response of DG-structured V2O5 can be estimated by comparing the CV curves of devices that use RTIL electrolyte with and without lithium salt, shown in Fig. 5.14b. Since V2O5 does not react with either of the ions of pure RTIL, the lithium-free experiment tests the EDLC response. With lithium salt added, the significant increase in capacitive current and the appearance of peak pairs indicates that redox reactions are taking place. These faradaic processes are kinetically facile and thus considered pseudocapacitive, but phase transitions may occur. Although it is difficult to distinguish between redox and intercalation pseudocapacitance, the latter is likely to be present in DG bicontinuous materials. 

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