Globular microemulsion

S. A. Safran, Theory of Structure and Phase Transitions in Globular Microemulsions, in Micellar Solutions and Microemulsions, S. H. Chen and R. Rajagopalan, eds.. Springer-Verlag, New York, 1990, Chapter 9. 

As a summary, the change of the structure due to a small increase of temperature is the formation of a water core resulting from a partial dehydration of the oxyethylene sites. Coorelative-ly the addition of water promotes the transition between lamellar aggregate and globular microemulsion, till the area per polar head reaches a maximum value which is temperature dependent. 

The determination of the different domains in the phase diagram is a tedious and time-consuming task. In particular, bicontinuous and globular microemulsions are both transparent, isotropic, and of low viscosity and therefore are not distinguishable. Clear evidence of the bicontinuous structure can be provided by transmission electron microscopy (TEM) [11]. 

Another important aspect of the formulation is illustrated in the studies aimed at preparing porous materials. In this case, one has to formulate systems containing large amounts of hydrophobic monomers (up to 70 wt%) either in the continuous phase of globular microemulsions [32-42] or in the oil domains of bicontinuous microemulsions [43-55]. A typical example of a phase diagram is given in Fig. 4. It shows four detectable... 

The second difference lies in the structure of the initial systems. In an emulsion the monomer is located in large monomer droplets ( f l-10 / m) and in small micelles 5-10 nm) and is partially solubilized in the continuous phase. In a globular microemulsion, it is solubilized within swollen micelles of the same size = 5-10 nm). These features coupled with the dynamic character of microemulsions are the origin of the difference in the mechanisms observed in the two processes. 

Figure 5 Polymerization mechanism in AOT globular microemulsions. (I) Before polymerization AOT micelles (c/ 6 nm), (II) Polymer particle growth (a) by collisions between particles (b) by monomer diffusion through the toluene phase. (Ill) End of polymerization. Polymer particles ( / 40 nm) plus small micelles (r/ 3 nm). (From Ref 23.)...

The variety of structures encountered in microemulsions offers great versatility for choosing the locus of polymerization. Besides polymerization in globular microemulsions, several studies have dealt with polymerization of monomers in the other phases of microemulsions. One of the main goals underlying these studies was to use the microstructure of microemulsions as a template to produce solid polymers with similar characteristics. For example, incorporation of large amount of hydrophobic monomers in the continuous phase of W/O microemulsions should yield solid polymers with a Swiss cheese-like structure capable of encapsulating the disperse phase (water). This would allow the inclusion of materials (metallic colloidal particles as catalysts, photochromic compounds, etc.) in the disperse phase that would otherwise be insoluble in the polymer. 

For medical or pharmaceutical applications, attention must be paid to the problems that can be caused by the possible toxicity of the surfactant remaining in the final product. Antonietti et al. [89] proposed the use of natural, nontoxic, and nondenaturing surfactants based on mixtures of lecithin and sodium chlolate for the formation of globular microemulsions. Pure lecithin is known to form bilayers or liposomes. The role of sodium cholate is to increase the curvature and flexibility of the interfacial layer, allowing the formation of small droplets. The final microlatex particles have a size ranging from 22 to 40 nm, depending on surfactant composition and concentration. The ability to functionalize the surface of these particles was demonstrated by the incorporation of protein molecules. 

Since the PS reference sample is almost monodisperse, a cumulant analysis of that material would yield a very small Q, say Q < 0.03. That is, all the correction terms are negligible and Eqs. (17) collapse to Eqs. (12). But cumulant analysis is a useful way to handle practical samples such as pigments, inks, microemulsions, swollen micelles, globular proteins, and spherical virus particles, where there is a size distribution but one that is not very broad (say Q < 0.3). This analysis should be made for the milk data using a non-linem teast-squares fitting of Eq. (17a), neglecting /1.3 and all higher order terms. Report the F, D, and R values as well as the second cumulant /t2 aiid the polydispersity index Q. 

Polymerizations in Globular and Bicontinuous Microemulsions for Producing Microlatexes... 

The morphology of the microemulsion-polymerized solid after ethanol extraction, as revealed by SEM, is shown in Fig. 8. The extracted samples show globular microstructures and voids (pores). These pores might be derived from the interconnected water-filled voids generated from numerous coalescences of growing particles during polymerization. 

Polymerization in microemulsion systems has recently gained some attention as a consequence of the numerous studies on microemulsions developed after the 1974 energy crisis (1,2). This new type of polymerization can be considered an extension of the well-known emulsion polymerization process (3). Hicroemulsions are thermodynamically stable and transparent colloidal dispersions, which have the capacity to solubilize large amounts of oil and water. Depending on the different components concentration, microemulsions can adopt various labile structural organizations -globular (w/o or o/w tyne), bicontinuous or even lamellar -Polymerization of monomers has been achieved in these different media (4-18),... 

In the field of biology, the effects of hydration on equilibrium protein structure and dynamics are fundamental to the relationship between structure and biological function [21-27]. In particular, the assessment of perturbation of liquid water structure and dynamics by hydrophilic and hydrophobic molecular surfaces is fundamental to the quantitative understanding of the stability and enzymatic activity of globular proteins and functions of membranes. Examples of structures that impose spatial restriction on water molecules include polymer gels, micelles, vesicles, and microemulsions. In the last three cases since the hydrophobic effect is the primary cause for the self-organization of these structures, obviously the configuration of water molecules near the hydrophilic-hydrophobic interfaces is of considerable relevance. 

Battistel, E., Luisi, P.L. and Rialdi, G., Thermodynamic study of globular protein stability in microemulsions, Journal of Physical Chemistry, 1988, 92, 6680-6685. 

Bodet, J.F., Bellare, J.R., Davis, H.T., Scriven, L.E. and Miller, W.G. (1988) Fluid microstructure transition from globular to bicontinuous in midrange microemulsion. /. Phys. Chem., 92, 1898-1902. 

The formulation improves significantly vriien the globular water-in-oU microemulsions are replaced by microemulsions with a bicontinuous structure [46]. In the latter case, the amount of monomer(s) can reach 25% of the total mass and the amount of emulsifierfs) (nonionic) reduced to about 8% [58] neutral monomer (aciylamide) [59,60], anionic monomer (sodium aciylate (NaA)) [61,62] and sodium 2-aciylamido-2-methylpropanesulfonate (NaAMPS) [63] or cationic monomer (methaciyloyloxyethyltrimethylammonium chloride (MADQUAT)) [64-66] and copolymers of these monomers have been investigated. 

The initial structure of die globular or bicontinuous microemulsion is not preserved during polymerization the final system consists in both cases of a dispersion of spherical latex particles with a fairly low index of polydispersity 1.15), as seen from QELS and TEM experiments [48,53,61]. Several factors are responsible for this structural change ... 

Bicontinuous structures are also present in the well-investigated surfactant system didodecyldimethylammonium bromide (DDABr)-dodecane-water [95,103], where a percolation process occurs that is a function of the water content. Upon an increase in the water concentration, the interconnected water channels originally present (bicontinuous microemulsion) are transformed into globular water droplets (W/O microemulsion), and in this phase region the viscosity is somewhat lower than that of the bicontinuous microemulsion [104],... 

D. Fluid Microstructural Transition Studies from Globular to Bicontinuous Morphologies in Midrange Microemulsions... 

Figure 12 Micrographs from a series of control experiments to establish the globular-to-bicontinuous transition in microemulsion microstructures. Bar = 250 nm. (From Ref 9.)...

Microemulsions can also coexist in equilibrium with various phases, the most widely studied being the so-called Winsor phase equilibria Winsor I is a globular O/W microemulsion in equilibrium with excess oil, Winsor II a globular W/O microemulsion in equilibrium with excess water, and Winsor III a middle-phase bicontinuous microemulsion in equilibrium with both oil and water phases [10]. 

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