Microemulsions model

Zackrisson, M., Anderson, R. and Bergenholtz, J. (2004) Depletion interactions in model microemulsions. Langmuir, 20, 3080-3089. 

Alander, J. and Warnheim, T. (1989) Model microemulsions containing vegetable oils. Part 1 Nonionic surfactant systems. /. Am. Oil Chem. Soc., 66( 11), 1656-1660. 

As an example the viscosity of microemulsions is an important factor in their ability to recover oil [1], which means that knowledge of it and, even more so, the capability to control it are important in the process of tertiary oil recovery. Therefore, it has been studied experimentally in some detail with respect to surfactant systems that are of interest for the oil recovery process [2-10]. Furthermore, there also exists theoretical work that models microemulsion viscosity as a function of the phase composition and phase type in order to predict properties of microemulsions under realistic conditions [11,12]. 

Sucrose esters have two attractive properties biocompatibility and temperature insensitivity [68-77]. We are investigating microemulsions based on sucrose esters in order to use them as microreactors for enzymatic and chemical reactions [42,78,79]. SZT-DSC has been applied to model microemulsion systems based on sucrose monostearate (HLB 15, also designated as S-1570). 

Soderman, O. and Nyden, M. 1999 NMR in microemulsions. NMR translational diffusion studies of a model microemulsion. Colloids Surf., A 158 273-280. 

First results show clearly that a microemulsion designed especially for the extraction of entrapped toxic compounds in surfaces can also be used as a nanoreactor for the decomposition of these chemicals by oxidizing agents and enzymes. Different degradation methods were already demonstrated to work in model microemulsion systems and can probably be applied also in practically important microemulsion systems. Here, technical grade components suitable for extraction and solubilization have to be used instead of purified surfactants and oils. 

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]. 

Lattice models have been studied in mean field approximation, by transfer matrix methods and Monte Carlo simulations. Much interest has focused on the occurrence of a microemulsion. Its location in the phase diagram between the oil-rich and the water-rich phases, its structure and its wetting properties have been explored [76]. Lattice models reproduce the reduction of the surface tension upon adsorption of the amphiphiles and the progression of phase equilibria upon increasmg the amphiphile concentration. Spatially periodic (lamellar) phases are also describable by lattice models. Flowever, the structure of the lattice can interfere with the properties of the periodic structures. 

It is of particular interest to be able to correlate solubility and partitioning with the molecular stmcture of the surfactant and solute. Likes dissolve like is a well-wom plirase that appears applicable, as we see in microemulsion fonnation where reverse micelles solubilize water and nonnal micelles solubilize hydrocarbons. Surfactant interactions, geometrical factors and solute loading produce limitations, however. There appear to be no universal models for solubilization that are readily available and that rest on molecular stmcture. Correlations of homologous solutes in various micellar solutions have been reviewed by Nagarajan [52]. Some examples of solubilization, such as for polycyclic aromatics in dodecyl sulphonate micelles, are driven by hydrophobic... 

Shioi A and Flarada M 1996 Model for the geometry of surfactant assemblies in the oil-rich phase of Winsor I microemulsions J. Chem. Eng. Japan 29 95... 

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. 

The period of the lamellar structures or the size of the cubic cell can be as large as 1000 A and much larger than the molecular size of the surfactant (25 A). Therefore mesoscopic models like a Landau-Ginzburg model are suitable for their study. In particular, one can address the question whether the bicontinuous microemulsion can undergo a transition to ordered bicontinuous phases. 

The model has been successfully used to describe wetting behavior of the microemulsion at the oil-water interface [12,18-20], to investigate a few ordered phases such as lamellar, double diamond, simple cubic, hexagonal, or crystals of spherical micelles [21,22], and to study the mixtures containing surfactant in confined geometry [23]. 

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]. 

FIG. 12 The behavior of the internal energy U (per site), heat capacity Cy (per site), the average Euler characteristic (x) and its variance (x") — (x) close to the transition line and at the transition to the lamellar phase for/o = 0. The changes are small at the transition and the transition is very weakly first-order. The weakness of the transition is related to the proliferation of the wormhole passages, which make the lamellar phase locally very similar to the microemulsion phase (Fig. 13). Note also that the values of the energy and heat capacity are not very much different from their values (i.e., 0.5 per site) in the Gaussian approximation of the model [47]. (After Ref. 49.)... 

Summarizing the detailed studies of the basic Landau-Ginzburg model presented in the preceding sections and in the present one suggest that this type of simplified model is not sufficient to describe all the effects related to the ordering in microemulsions. In particular, the only stable ordered phase in the model is the lamellar phase and all the cubic phases are only meta-... 

B. Widom. Lattice model of microemulsions. J Chem Phys 54 6943-6954, 1986. 

M. Schick, W. H. Shih. Simple microscopic model of a microemulsion. Phys Rev Lett 59 1205-1208, 1987. 

K. Chen, C. Ebner, C. Jayaprakash, R. Pandit. Microemulsions in oil-water-surfactant mixtures Systematics of a lattice-gas model. Phys Rev A 55 6240, 1988. 

A. Ciach. Bifurcation analysis and liquid-crystal phases in Landau-Ginzburg model of microemulsion. J Chem Phys 704 2376-2383, 1996. 

A. Ciach, J. S. Hoye, G. Stell. Microscopic model for microemulsion. I. Ground state properties. J Chem Phys 90 1214-1221, 1989. 

W. Gozdz, R. Hotyst. Triply periodic surfaces and multiply continuous structures from the Landau model of microemulsions. Phys Rev E 54 5012-5027, 1996. 

A. Ciach, J. S. Hoye, G. Stell. Microscopic model for microemulsion. II. Behavior at low temperatures and critical point. J Chem Phys 90 1222-1228, 1989. A. Ciach. Phase diagram and structure of the bicontinuous phase in a three dimensional lattice model for oil-water-surfactant mixtures. J Chem Phys 95 1399-1408, 1992. 

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