Inverse microemulsion approach

Over the past few years, a large number of experimental approaches have been successfully used as routes to synthesize nanorods or nanowires based on titania, such as combining sol-gel processing with electrophoretic deposition,152 spin-on process,153 sol-gel template method,154-157 metalorganic chemical vapor deposition,158-159 anodic oxidative hydrolysis,160 sonochemical synthesis,161 inverse microemulsion method,162 molten salt-assisted and pyrolysis routes163 and hydrothermal synthesis.163-171 We will discuss more in detail the latter preparation, because the advantage of this technique is that nanorods can be obtained in relatively large amounts. 

Neyret and Vincent [30] have developed such an approach for the formation of microgel particles, named inverse microemulsion polymerisation. The oil phase consisted of anionic 2-acrylamido-2-methylpropanesulfonate (AMPS) and cationic (2-(methacryloyloxy)ethyl) trimethylammonium (MADQUAT) monomers in addition to a BA cross-linker. The co-polymerisation was initiated using UV irradiation and the product isolated and re-dispersed in aqueous electrolyte solution to yield polyampholyte microgel particles. The particles became swollen in the presence of high electrolyte concentrations as a result of screening of the attractive electrostatic interactions between neighbouring chains. 

W/o microemulsion solutions are mostly transparent, isotropic liquid media with nanosized water droplets that are dispersed in the continuous oil phase and stabilized by surfactant molecules at the water/oil interface. These surfactant-covered water pools offer a unique microenvironment for the formation of nanoparticles. They not only act as microreactors for processing reactions but also exhibit the process aggregation of particles because the surfactants could adsorb on the particle surface when the particle size approaches to that of the water pool. As a result, the particles obtained in such a medium are generally very fine [76]. Inverse microemulsion droplets, however, are slightly polydisperse due to less strict transformation of... 

There are other variations of this approach that involve the phase inversion temperature (PIT) (see Section 3.6.1). In one method an emulsion is formed at a temperature a few degrees lower than the PIT, where the interfacial tension is quite low and small droplets can be formed. The emulsion can then be quickly cooled. Another method uses a controlled temperature change to cause an emulsion to suddenly change from a coarse oil-in-water (O/W) emulsion, through a microemulsion phase, and into a fine water-in-oil (W/O) emulsion [432]. 

At the oil-rich side, the phase behaviour is inverted temperature-wise as can be seen in the T( wA)-section provided in Fig. 1.7(c). Thus, the near-critical phase boundary 2 —1 starts at low temperatures from the lower n-octane-QoEs miscibility gap (below <0°C) and ascends steeply upon the addition of water. With increasing wA, this boundary runs through a maximum and then decreases down to the upper critical endpoint temperature Tu. The emulsification failure boundary 1 —r 2 starts at high temperatures and low values of wA, which means that only small amounts of water can be solubilised in a water-in-oil (w/o) microemulsion at temperatures far above the phase inversion. Increasing amounts of water can be solubilised by decreasing the temperature, i.e. by approaching the phase inversion. At Tu the efb intersects the near-critical phase boundary and the funnel-shaped one-phase region closes. 

New biocompatible oils from renewable resources have also been investigated. Acharya et al. investigated the impact of the addition of ricebran oil on the phase behaviour of microemulsions [28]. In combination with isopropylmyristate as second oil a large microemulsion domain is formed in the phase diagram, which makes ricebran oil a potential oil base for microemulsions. Another approach to improve skin friendliness is a reduction of the surfactant content of microemulsions. Diec et al. report on optimised surfactant-co-surfactant systems in combination with a phase inversion process to reach this goal [29]. The resulting formulations are clear, stable over the long term and contain less than 10% of surfactants. 

The formulation of food systems as microemulsions is not easy, since addition of triglycerides to inverse micellar systems results in a phase change to a lamellar liquid crystalline phase. The latter has to be destabilized by other means than adding co-surfactants, which are normally toxic. An alternative approach to destabilize the lamellar phase is to use a hydrotrope, a number of which are allowed in food products. 

Structure changes [6] and the W/0 tnicroemulsions were hence described as inverse micellar solutions. The approach was initially not received well by Schulman s successors [7], and it is remarkable that Schulman s initial publication on the concept described these miaoemulsions as colloid solutions. The term microemulsion was coined much later [8]. 

At the same time, significant changes in the state of a system can result from fairly moderate deviations in T, for example, the changes in the mutual solubility of both the disperse phase and the dispersion medium components, leading to a radical decrease in a. Typical examples include studies on systems approaching the critical point, (and yet still below the TJ, such as those carried out with binary mixtures of paraffins with moderately polar organic substances, such as oxyquinoline [26,67,68], In these works, the formation of direct, inverse, and bicontinuous microemulsions had been described and analyzed in comparison with the independently determined values of a down to 10" -10" mN/m,... 

Conversions in w/o emulsion or microemulsion asymptotically approach equilibrium values between 60 and 80% while those from s/i dispCTsions catalyzed by the cation exchange resin reach or exceed 95% despite the slower rates. Although not apparent in Figure 2, it will be shown later that equilibrium conversions show a weak inverse dependence on temperature for runs conducted in the NaDBS A/7.4N H2SO4 w/o emulsion. 

Microemulsion Techniqne Direct/inverse micelles or microemulsion represent an approach based on the formation of micro/nano-reaction vessels under a ternary mixture containing water, oil and a surfactant. Metal precursors on water will precede precipitation as oxo-hydroxides within the aqueous droplets, typically leading to monodispersed materials with size limited by the surfactant-hydroxide contact [50]. 

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