Normal micelles

These are transparent or translucent systems covering the size range from 5 to 50nm. Unlike emulsions and nanoemulsions (which are only kinetically stable), microemulsions are thermodynamically stable as the free energy of their formation is either zero or negative. Microemulsions are better considered as swollen micelles normal micelles can be swollen by some oil in the core of the micelle to form O/W microemulsions. Reverse micelles can be swollen by water in the core to form W/O microemulsions. 

Waugh and Noble (181) have proposed a coat-core model whereby the core of the micelle is comprised of calcium ag-caseinate covered by a coat of calcium s- and x-caseinates. However, the x-casein in the coat is oriented toward the outer surface of the micelle so that the hydrophilic macropeptide portion of the kappa casein would interact with the aqueous environment of the milk serum to stabilize the integrity of the individual micelles so that the micelles normally do not interact. 

Micelles (normal and reverse) are aggregates of surfactants, which are formed spontaneously in a liquid phase when the surfactant concentration is increased than the critical micelle concentration (CMC). Micellar solutions are formed in aqueous continuous systems, whereas, the reverse micelles are formed in oily continuous systems. In micelles (oil-in-water micelle), the polar heads of the surfactant lie outside in the aqueous phase, whereas the lipophilic hydrocarbon chains lie inside (Figure 58.3). When the surfactant concentration is increased, the free energy of the system increases due to the inauspicious interactions between the water molecules and the lipophilic portions of the surfactant. The water molecules around the oil droplets structures themselves, thereby, resulting in the decrease in the entropy. The reverse micelles (water-in-oil micelle) have opposite structure, that is, the polar heads lie at the centre, while the lipophilic tails are present outside in the oil phase (Figure 58.3). The surfactant concentration need not necessarily be higher than the CMC for the formation of reverse micelle. Many scientists reported formation of reverse micelles by lecithin in different oil phases. " ... 

To tell the truth, you cannot usually get all five successive stages (Figure C5.2 a-e) with the same substance. Either the tails are too thick to form spherical or even cylindrical micelles, or, on the contrary, they may be too thin to construct inverted micelles. Normally one substance can exhibit only two or three of the structures in Figure C5.2. 

Aside from block copolymer composition and the quahty of the block-selective solvent, polymer concentration can also affect micellar morphology. For example, the length of cylindrical micelles normally increases with the concentration of diblock copolymers in a block-selective solvent [38-40]. If the aggregates are kinetic products, the morphologies will be affected by sample preparation conditions and history as well. 

The cubic phases are also known as viscous isotropic phases - because they are As the name implies, these phases have structures based around one of several possible cubic lattices, namely the primitive, face-centred and body-centred. There are two very distinct aggregate structures, i.e. one comprised of small micelles, normal or reversed, and one based on three-dimensional bicontinuous aggregates. The normal and reversed structures that occur for both make a total of four classes. It is still not certain exactly which structures can occur for the different classes, but the overall picture has become much clearer during the past few years (46, 49-57), with more and more structures being identified. The first set of structures comprised of small globular micelles is labelled I , while the second group, the bicontinuous three-dimensional (3-D) micellar network, is labelled V . 

Medium selection Organic co-solvent Organic solvent pH/acld-base stability Micelles (normal and reverse)... 

Neutron reflectivity experiments provide the most direct method for probing the organization of surfactant micelles normal to the solid-solution interface [28]. The experimental data are used to select the most consistent model of the surface. Unfortunately, neutron reflectivity data often do not allow selection among proposed lateral arrangements of surfactants. Therefore, the AFM and neutron reflectivity are complementary an AFM reveals the lateral structure and neutron reflectivity reveals the normal structure. The remainder of this chapter will focus on the use of the AFM. 

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

However, often the identities (aqueous, oleic, or microemulsion) of the layers can be deduced rehably by systematic changes of composition or temperature. Thus, without knowing the actual compositions for some amphiphile and oil of poiats T, Af, and B ia Figure 1, an experimentaUst might prepare a series of samples of constant amphiphile concentration and different oil—water ratios, then find that these samples formed the series (a) 1 phase, (b) 2 phases, (c) 3 phases, (d) 2 phases, (e) 1 phase as the oil—water ratio iacreased. As illustrated by Figure 1, it is likely that this sequence of samples constituted (a) a "water-continuous" microemulsion (of normal micelles with solubilized oil), (b) an upper-phase microemulsion ia equiUbrium with an excess aqueous phase, ( ) a middle-phase microemulsion with conjugate top and bottom phases, (d) a lower-phase microemulsion ia equiUbrium with excess oleic phase, and (e) an oA-continuous microemulsion (perhaps containing iaverted micelles with water cores). 

Colloidal State. The principal outcome of many of the composition studies has been the delineation of the asphalt system as a colloidal system at ambient or normal service conditions. This particular concept was proposed in 1924 and described the system as an oil medium in which the asphaltene fraction was dispersed. The transition from a coUoid to a Newtonian Hquid is dependent on temperature, hardness, shear rate, chemical nature, etc. At normal service temperatures asphalt is viscoelastic, and viscous at higher temperatures. The disperse phase is a micelle composed of the molecular species that make up the asphaltenes and the higher molecular weight aromatic components of the petrolenes or the maltenes (ie, the nonasphaltene components). Complete peptization of the micelle seems probable if the system contains sufficient aromatic constituents, in relation to the concentration of asphaltenes, to allow the asphaltenes to remain in the dispersed phase. 

A large variety of drug delivery systems are described in the literature, such as liposomes (Torchilin, 2006), micro and nanoparticles (Kumar, 2000), polymeric micelles (Torchilin, 2006), nanocrystals (Muller et al., 2011), among others. Microparticles are usually classified as microcapsules or microspheres (Figure 8). Microspheres are matrix spherical microparticles where the drug may be located on the surface or dissolved into the matrix. Microcapsules are characterized as spherical particles more than Ipm containing a core substance (aqueous or lipid), normally lipid, and are used to deliver poor soluble molecules... 

Even entrapment of entire cells within reversed micelles without loss of their functionality has been achieved. For example, mitochondria and bacteria (Actinobacter cal-coaceticus, Escherichia coli, Corynebacterium equi) have been successfully solubilized in a microemulsion consisting of isopropyl pahnitate, polyoxyethylene sorbitan trioleate [162], Enhanced hydrogen photoproduction by the bacterium Rhodopseudomonas sphaeroides or by the coupled system Halobacterium halobium and chloroplasts organelles entrapped inside the aqueous core of reversed micelles with respect to the same cells suspended in normal aqueous medium has been reported [183,184],... 

Below some critical surfactant concentration, the system is two-phase with excess oil or water depending on the oil/water concentration. On adding more surfactant, the system moves into a one-phase region with normal micelles forming in water-rich systems. The water constitutes the continuous phase, solvating the headgroups of the surfactant whose hydro-phobic tails solubilise oil in the core of the micelle. In oil rich systems, reverse-micelles form. With further increases in surfactant composition. 

FIG. 4 Time-resolved fluorescence Stokes shift of coumarin 343 in Aerosol OT reverse micelles, (a) normalized time-correlation functions, C i) = v(t) — v(oo)/v(0) — v(oo), and (b) unnormalized time-correlation functions, S i) = v i) — v(oo), showing the magnitude of the overall Stokes shift in addition to the dynamic response, wq = 1.1 ( ), 5 ( ), 7.5 ( ), 15 ( ), and 40 (O) and for bulk aqueous Na solution (A)- Points are data and lines that are multiexponential fits to the data. (Reprinted from Ref 38 with permission from the American Chemical Society.)... 

Because of the spherical shape of the PS-PI micelles, the equation of motion for the local displacement u(r, t) of the normal or breathing modes, where all the radical shells or layers move in phase, is given by the radical part of... 

Fig. 51 Phase diagram for PS-PI diblock copolymer (Mn = 33 kg/mol, 31vol% PS) as function of temperature, T, and polymer volume fraction, cp, for solutions in dioctyl ph-thalate (DOP), di-n-butyl phthalate (DBP), diethyl phthalate (DEP) and M-tetradecane (C14). ( ) ODT (o) OOT ( ) dilute solution critical micelle temperature, cmt. Subscript 1 identifies phase as normal (PS chains reside in minor domains) subscript 2 indicates inverted phases (PS chains located in major domains). Phase boundaries are drawn as guide to eye, except for DOP in which OOT and ODT phase boundaries (solid lines) show previously determined scaling of PS-PI interaction parameter (xodt

It was mentioned previously that the narrow range of concentrations in which sudden changes are produced in the physicochemical properties in solutions of surfactants is known as critical micelle concentration. To determine the value of this parameter the change in one of these properties can be used so normally electrical conductivity, surface tension, or refraction index can be measured. Numerous cmc values have been published, most of them for surfactants that contain hydrocarbon chains of between 10 and 16 carbon atoms [1, 3, 7], The value of the cmc depends on several factors such as the length of the surfactant chain, the presence of electrolytes, temperature, and pressure [7, 14], Some of these values of cmc are shown in Table 2. 

Most of the applications of the micelles in aqueous medium are based on the association or solubilization of solutes. The interactions between both can be electrostatic, hydrophobic, or, more normally, a combination of both effects [23, 24], It was thought initially that hydrophobic solutes dissolve in the core of the micelle in the same way in which they would do so in an organic solvent, but... 

Thus far reference has only been made to the so-called normal micelles that are formed in polar solvents such as water. However, when the surfactants are dissolved in organic nonpolar solvents, the hydrophilic groups are found in the interior of the aggregate, while the hydrophobic chains extend toward the... 

---