Microemulsion structural features

The equations developed in previous sections can be used to calculate the structural features of microemulsions, provided explicit expressions for the standard free energies of transfer of surfactant and alcohol molecules from their infinitely dilute states in water and of oil molecules from the pure oil phase to the interfacial layer of the microemulsion droplets are available. Such expressions are given below for spherical layers of O/W droplets and W/O droplets and also for flat layers. The difference in the standard state free energy consists of a number of contributions ... 

These surfactants are found to exhibit a solubilization ability higher than those of their purely anionic and nonionic counterparts. It is found that for such surfactants the transition temperature from a liquid crystal mesophase to a (disordered) microemulsion structure is inversely related to the amount of alcohol cosurfactant, a quite useful feature in... 

W/o microemulsions of water, aniline, light petroleum, and nonionic surfactant Empilan NP-5 have been utilized for electrochemical polymerization of aniline to polyaniline [48]. Improved homogeneity and conductivity was achieved compared with polyaniline grovm in water. The heterogeneous nature of the microemulsion directed the mode of polymer growth and improved conductivity and structural features. 

An interesting application of ionic liquids (ILs) concerns their use in combination with classical surfactants [1,2]. Indeed, they can suitably replace each of the microemulsion components (aqueous phase, apolar phase, and surfactants) conferring peculiar features to self-assembled systems. Indeed, ILs are salts and as such have affinity for water, but they also typically possess a lipophilic moiety, and this means affinity for oils. Depending on their chemical structure, ILs can act as cosolvent either for water or for oil. In addition, when their hydrophilic and hydrophobic nature are both strong enough, a fraction of ILs will reside preferentially at the interface formed by the surfactant, and this can impact dramatically the interfacial physics, drastically changing the microemulsion structure and dynamics. 

The thermodynamics of microemulsion discussed in the beginning of the chapter has accounted for the basic conditions required for the formation and stability of reverse micellar systems. The energetics of formation in terms of Gibbs free energy, enthalpy, and entropy need to be quantified with reference to the system composition and the droplet structures. For the formation of w/o systan, a simple method called dilution method can exfiact energetic information for many combinations along with the understanding of their structural features. The method has been amply studied and presented in literature [4,27-32]. We, herein, introduce and present the method with basic theory and examples. 

Formariz, T.P., Chiavacci, L.A., Sarmento, V.H.V., SantiUi, C.V., Tabosa do Egito, E.S., Oliveira, A.G. 2007. Relationship between structural features and in vitro release of doxorubicin from biocompatible anionic microemulsion. Colloids Surf. B Biointerf. 60, 28-35. 

Lattice models seek to describe microemulsion structural domains down to the near-molecular level. These include models of amphiphilicity, imposed by mean field attractive and repulsive interactions applied to simplified diatomic or oligomeric amphiphiles, oil and water. Recent versions of these models include bending energy contributions for surfactants meeting at angles to approximate realistic molecular features and accurately capture microemulsion phase diagram. In all cases, microemulsion phase behavior is most accurately captured when the models consider key physical attributes (either exphc-itly or implicitly), including the balance between entropic (which tend to disperse oil-water into ever finer domains) and interfacial (which drive phase separation and place limits on domain curvature) contributions. 

SANS measurements. SANS measurements were employed to determine the structural features of microemulsions and the respective arrangement of IL and TX-100, by using three different contrasts. The first set of SANS data, named "full contrast", was obtained with microemulsions in deuterated toluene and allowed the determination of the global form of objects. Data obtained with deuterated ionic liquid in hydrogenated toluene and Triton XlOO show the objects formed by IL it is "core contrast" since IL is assumed to be in the core of the micelles. Third, data obtained in toluene mixture, which matches the deuterated ionic liquid, give the scattering from the TX-100 object it is named "shell contrast". 

Lattice models for bulk mixtures have mostly been designed to describe features which are characteristic of systems with low amphiphile content. In particular, models for ternary oil/water/amphiphile systems are challenged to reproduce the reduction of the interfacial tension between water and oil in the presence of amphiphiles, and the existence of a structured disordered phase (a microemulsion) which coexists with an oil-rich and a water-rich phase. We recall that a structured phase is one in which correlation functions show oscillating behavior. Ordered lamellar phases have also been studied, but they are much more influenced by lattice artefacts here than in the case of the chain models. 

In this section we characterize the minima of the functional (1) which are triply periodic structures. The essential features of these minima are described by the surface (r) = 0 and its properties. In 1976 Scriven [37] hypothesized that triply periodic minimal surfaces (Table 1) could be used for the description of physical interfaces appearing in ternary mixtures of water, oil, and surfactants. Twenty years later it has been discovered, on the basis of the simple model of microemulsion, that the interface formed by surfactants in the symmetric system (oil-water symmetry) is preferably the minimal surface [14,38,39]. 

Since some structural and dynamic features of w/o microemulsions are similar to those of cellular membranes, such as dominance of interfacial effects and coexistence of spatially separated hydrophilic and hydrophobic nanoscopic domains, the formation of nanoparticles of some inorganic salts in microemulsions could be a very simple and realistic way to model or to mimic some aspects of biomineralization processes [216,217]. 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

Our synthesis is based on the hydrolysis of a silicon alkoxide (TEOS Si(OCH2CH3)4) in a diluted solution of nonionic polyethylene oxide-based surfactants. The hydrolysis is then induced by the addition of a small amount of sodium fluoride [5], Depending on the initial mixing conditions, the size of the solubilized objects leads to either a colorless or milky emulsion. Small particles ( 300 nm) with a 3D worm-hole porous structure or small hollow spheres with mesoporous walls, are usually obtained [6]. The synthesis we report herein after exhibits an apparently slight but actually drastic change in the preparation conditions. The main feature of this approach is an intermediate step that utilizes a mild acidity (pH 2 - 4), in which, prior to the reaction, a stable colorless microemulsion containing all reactants is... 

In this paper, a molecular thermodynamic approach is developed to predict the structural and compositional characteristics of microemulsions. The theory can be applied not only to oil-in-water and water-in-cil droplet-type microemulsions but also to bicontinuous microemulsions. This treatment constitutes an extension of our earlier approaches to micelles, mixed micelles, and solubilization but also takes into account the self-association of alcohol in the oil phase and the excluded-volume interactions among the droplets. Illustrative results are presented for an anionic surfactant (SDS) pentanol cyclohexane water NaCl system. Microstructur al features including the droplet radius, the thickness of the surfactant layer at the interface, the number of molecules of various species in a droplet, the size and composition dispersions of the droplets, and the distribution of the surfactant, oil, alcohol, and water molecules in the various microdomains are calculated. Further, the model allows the identification of the transition from a two-phase droplet-type microemulsion system to a three-phase microemulsion system involving a bicontinuous microemulsion. The persistence length of the bicontinuous microemulsion is also predicted by the model. Finally, the model permits the calculation of the interfacial tension between a microemulsion and the coexisting phase. 

The general features of inverse microemulsion polymerization at the present state of knowledge are presented. The influence of various water-soluble monomers on the structural properties of the... 

A significant amount of work has demonstrated the feasibility and the interest of reversed micelles for the separation of proteins and for the enhancement or inhibition of specific reactions. The number of micellar systems presently available and studied in the presence of proteins is still limited. An effort should be made to increase the number of surfactants used as well as the set of proteins assayed and to characterize the molecular mechanism of solubilization and the microstructure of the laden organic phases in various systems, since they determine the efficiency and selectivity of the separation and are essential to understand the phenomena of bio-activity loss or preservation. As the features of extraction depend on many parameters, particular attention should be paid to controlling all of them in each phase. Simplified thermodynamic models begin to be developed for the representation of partition of simple ions and proteins between aqueous and micellar phases. Relevant experiments and more complete data sets on distribution of salts, cosurfactants, should promote further developments in modelling in relation with current investigations on electrolytes, polymers and proteins. This work could be connected with distribution studies achieved in related areas as microemulsions for oil recovery or supercritical extraction (74). In addition, the contribution of physico-chemical experiments should be taken into account to evaluate the size and structure of the micelles. 

To obtain a wide w/o microemulsion phase it is essential to adjust carefully the cosurfactant structure (usually its chain length) and its relative amount. Although trial and error is still the most commonly used method for obtaining microemulsions, a tentative rule is to combine a very hydrophobic cosurfactant (n-decanol) with a very hydrophilic ionic surfactant (alcohol sulfate) and a less hydrophobic cosurfactant (hexanol) with a less hydrophilic ionic surfactant (OTAB). For very hydrophobic ionic surfactants, such as dialkyl dimethylammonium chloride, a water-soluble cosurfactant, such as butanol or isopropanol, is adequate (this rule derives at least partially from the fact that an important feature of the cosurfactant consists of readjusting the surfactant packing at the solvent/oil interface). 

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