Microemulsion interface, orientation

Recently an alternative approach for the description of the structure in systems with self-assembling molecules has been proposed in Ref. 68. In this approach no particular assumption about the nature of the internal interfaces or their bicontinuity is necessary. Therefore, within the same formahsm, localized, well-defined thin films and diffuse interfaces can be described both in the ordered phases and in the microemulsion. This method is based on the vector field describing the orientational ordering of surfactant, u, or rather on its curlless part s defined in Eq. (55). 

It is important to consider the different stages when producing microemulsions from macroemulsions. It was mentioned earlier that surfactant molecules orient with the hydrophobic group inside the oil phase, while the polar group orients toward the water phase. The orientation of surfactants at the interfaces cannot be measured by any direct method, although much useful information can be obtained from mono-layer studies of the air-water or oil-water interfaces. 

Much work has also gone into understanding the chemical composition requirements for microemulsification [229]. For example, surfactants having extended hydrocarbon tails (linkers) have been developed to induce additional orienting of oleic phase molecules at depth from the interfaces [230,231]. Table 11.3 gives some idea of the range of components that may be formulated into a microemulsion. 

Figure 2. Orientation of the RCOtCo(NH,)s complex at the interface of a microemulsion containing hexadecyltrimethylammonium bromide (CTAB).

The simplest representation of the structure of microemulsions is the droplet model in which microemulsion droplets are surrounded by an interfacial film consisting of both surfactant and cosurfactant molecules, as illustrated in Fig. 7.18. The orientation of the amphiphiles at the interface will, of course, differ in o/w and w/o microemulsions. As... 

With emulsions, nanoemulsions and microemulsions, the surfactant adsorbs at the oil/water (O/W) interface, with the hydrophilic head group immersed in the aqueous phase and leaving the hydrocarbon chain in the oil phase. Again, the mechanism of stabilisation of emulsions, nanoemulsions and microemulsions depends on the adsorption and orientation of the surfactant molecules at the Uquid/liquid (L/L) interface. Surfactants consist of a small number of units and are mostly reversibly adsorbed, which in turn allows some thermodynamic treatments to be applied. In this case, it is possible to describe adsorption in terms of various interaction parameters such as chain/surface, chain solvent and surface solvent. Moreover, the configuration of the surfactant molecule can be simply described in terms of these possible interactions. 

Figure 8 Schematic representation of the processes leading to birefringence (and turbidity) in a W/O microemulsion, in relation to an applied electric square pulse E. Below a (second) threshold value of the field strength and far from critical conditions, or under any conditions if the pulse is terminated at a time indicated by the dashed line, only birefringence is observed due to the formation of AJ, and Above the threshold of the field strength, close to critical conditions, and with a sufficiently long square pulse, turbidity contributes to the signal due to phase separation or/and percolation. The double wall of the particles symbolizes the water/oil interface. Symbols A, surfactant monomer An, microemulsion droplet (An), cluster LCmp, liquid-crystalline microphase or/and percolation structure. Primed symbols stand for polarized structures oriented parallel to E (- ) reversible step with respect to turning the field on or off (->) irreversible step. (Reprinted with permission from Refs. 6 and 41. Copyright 1989 and 1994 American Chemical Society.)...

In another model, de Gennes and Taupin [114] proposed the use of a simple cubic lattice the size of the cube was taken as the same as the droplet diameter or the persistence length The latter has been related to the interfaces in the following way the interface is flat at scales less than and (ii) interfaces with an area of ( k) have independent orientations. The cubes are, of course, composed of oil or water. When a series of adjacent cubes are made of either oil or water, there is no interface in between. Therefore, progressive increase in the number of adjacent cubes of the same type will lead to the conversion of one type of microemulsion to another via a bicontinuous structure. 

Comparisons of electrochemical values of surfactants 1 and 2 in microemulsions and micellar solutions helped establish qualitative pictures of dynamics at relevant electrode-fluid interfaces. Rate constants for oxidations of ferrocene (Fc) 2-Fc (1) and 5-Fc (2) were similar in homogeneous DMF and DM SO on Pt and glassy carbon electrodes [32, 33]. However, in aqueous CTAB micelles, electron transfer rates were in the order Fc > 2-Fc > 5-Fc, with tenfold differences in successive values. This was attributed to 2-Fc and 5-Fc achieving head down orientations on the electrode prior to electron transfer. Adsorbed CTA+ on the electrode seems to help order 1 and 2 on the electrode prior to electron transfer. 

In a bicontinuous CTAC microemulsion, ferrocene had a k° twice as large as 1 and 2 [33], but values for 2-Fc and 5-Fc were similar. These relatively small differences in electron transfer rates in the microemulsion cannot be explained by head down-tad up orientation of 1 and 2 at the time of electron transfer as proposed for micellar solutions. The results suggest an increased disorder and mobd-ity in the electrode-fluid interface in the CTAC microemulsion compared to micellar CTAB solutions. 

Fatty alcohols orient themselves at phase interfaces and can, therefore, be used in emulsions and microemulsions. They impart body to cosmetic creams and lotions and solvency to industrial emulsions. They also serve as lubricants in polymer processing. The products obtained from fatty alcohols, EO, and propylene oxide (PO) are weak foaming surfactants. 

The study of amphiphile ordering at interfaces is necessary to understand many phenomena, like microemulsions, foams or interfacial reactivity. It is expected that the preferential orientation taken by these compounds at interfaces is entirely determined by their interactions with the two solvants forming the interface and the intermolecular repulsion or attraction within the monolayer. As mentioned above, the SH response at liquid/liquid interfaces is dominated by electric dipole contributions and is therefore surface specific. Neglecting the contribution from the sol-vant molecules, which usually only have a weak nonlinear optical activity, the passage from the macroscopic susceptibility tensor xP to the microscopic molecular hyperpolarizability p of the adsorbate is obtained by merely taking the SHG response of the amphiphile monolayer as the superposition of the contribution from each single moiety. Hence, it yields... 

FIG. 12 Schematic representation of the location of solubUizates within water/isooctane microemulsions stabilized with AOT. (a) Without solubiUzate (b) ions and/or ionic clusters located within the water pools (c) aspartame molecules located at the water/isooctane interface, with the aspartyl end pointing toward the water pool and the phenylalanine end oriented toward the oil phase. 

The ability of aminated compounds to inhibit corrosion on metallic surfaces via adsorption phenomena has been already certified. Since operations taking place at interfaces are greatly affected by variations in surface tension, aminated surfactant molecules are expected to provide even better results. This has been the case, when self-assembled micellar or microemulsion systems are used as corrosion inhibitors. In that aspect, surfactants may be used as organic corrosion inhibitors, and act by forming a protective film onto surfaces which are exposed to corrosive media, like oxygen and saline or acidic solutions. When microemulsions are used, an oil film is also adsorbed onto the surface with the surfactants tails oriented towards it, in view of the usually positive character of the surface. In the petroleum industry, the oil itself may be the nonpolar component of such systems. Figure 15.10 is a schematic of these types of films. 

Arabi-Katbi O.I., Wegner K., Pratsinis S.E. Aerosol synthesis oftitania nanoparticles Effect of flame orientation and configuration. Ann. Chimie, Sci. Mater. 2002 27(6) 37-46 Barnickel P., Wokaun A., Sager W., Eicke H.-F. Size tailoring of silver colloids by reduction in W/0 microemulsions. J. Colloid Interface Sci. 1992 148(1) 80-90 Bender J., Wagner N.J. Reversible shear thickening in monodisperse and bidisperse colloidal dispersions. J. Rheol. 1996 40(5) 899-916... 

With emulsifiable concentrates, emulsions, and microemulsion, the surfactant adsorbs at the od/water interface, with the hydrophilic head group immersed in the aqueous phase, leaving the hydrocarbon chain in the oil phase. Again, the mechanism of stabilization of emulsions and microemulsions depends on the adsorption and orientation of the surfactant molecules at the liquid/liquid interface. 

The most convenient reactor for studying convective dynamics at the interface between two ME of different compositions consists of a vertically oriented Hele-Shaw cell, composed of two borosilicate glasses separated by a teflon spacer of 0.10 mm (see Fig. 5) [2,3,18]. Two different water-in-oil (W/0) reverse microemulsions were filled in the reactor through the inlet ports positioned at the top and the bottom of the cell. The excess of the solutions is pumped out through the cell s outlets until a flat interface between the two liquids is obtained. Each of the two microemulsions having different composition initially occupies half of the reactor height. The top and bottom solutions are prepared by using distilled... 

H-aggregation of methyl orange has been postulated to account for the hypsochromic shift of the dye in a water-in-oil microemulsion, with the dye molecules in a parallel orientation at the interface beween the water and the oil phase. The observation of salt-induced strongly red-shifted J-bands in aqueous solutions of Rose Bengal, an anionic xanthene dye, in the presence of zwitterionic surfactants was... 

Complexation in its various forms plays a key role in the homo- and copolymerization of 1-alky 1-4-vinylpyridinium ions. Intermonomer associations are believed responsible for the enhanced poly-merizability of monomers with long alkyl chains (C , n > 6) on nitrogen, the ability of the title monomers to copolymerize with anionic and Ti-rich monomers, and the strong dependence on concentration for homopolymerization of all these cationic monomers. Hydrophobic interactions between lipophilic monomers, electrostatic attraction between cationic and anionic monomers, and charge-transfer complexation between Ti-rich and Ti-deficient monomers have all been observed to control polymer formation. Monomer organization/orientation on polyanion templates, at organic solvent-water interfaces and in ordered multiple-phase systems such as micelles, membranes, vesicles, and microemulsions have been used with limited success in attempts to control the microstructure (e.g. tacticity, monomer sequence) in the related polymers. Interpolymer complexes of poly(l-alky 1-4-vinylpyridinium ions) with natural and synthetic poly anions represent a rich resource for the development of selective electroanalytical methods, for efficient new separation procedures, for manipulation of biomembranes in drug dehvery, and numerous other applications. 

Paleos [43] was the first investigator to attempt control of microstructure of 2 through orientation of 1 (R = CH3) at a water-toluene interface. The water-organic solvent interface provides a reaction site, that may orient amphipathic molecules and ions in a particular way. Paleos was interested in obtaining stereoregular 2, i.e. pure isotactic or syndiotactic forms of 2, in this fashion but the experimental results were inconclusive [43-45]. Fife and Hu [46] have reinvestigated the possibility for stereo-controlled polymerization of 1 (R = C12H25) as a component of aqueous suspensions of microemulsions. This work will be explored in detail in Sect. 6.2.4. 

There are three basic types of microemulsions, viz., direct (oil dispersed in water, denoted by o/w ), reversed or inverse (water dispersed in oil, denoted by w/o ), and bicontinuous (i.e., regions of water and oil). The domains of the dispersed phase are either globular or interconnected (giving a bicontinuous microemulsion), as shown schematically in Fig. 3.1. These are stabilized by an interfacial film of surfactant (usually in combination with a cosurfactant), its molecules being oriented at the interface such that the hydrophilic ends are in the aqueous phase and the hydrophobic ends are in the oil phase. It may be mentioned that the term microemulsion was first used by Hoar and Shuhnan [2], professors of chemistry at Cambridge University in 1943. However, other names are often used, such as transparent emulsion (implying optical clarity), micellar solution, and solubilized oil. 

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