Micelles normal micelle

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. 

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). 

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. 

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... 

In recent years different types of surfactants have been used, in concentrations above their cmc, forming normal micelles, to improve different aspects of chemiluminescent reactions. Though the choice of the best surfactant depends on the characteristics of the chemiluminescent reaction, the surfactants most used have been the cationic (fundamentally quaternary ammonium salts) and to a lesser degree the anionic, the nonionic, and the zwiterionic compounds. 

As mentioned earlier, reversed micelles have different properties from normal micelles. These properties have the potential to favorably affect the sensitivity and other analytical aspects of CL reactions. Thus, reversed micelles have been used to prolong the duration of the observed CL of various oxalate ester (or acid)-hydrogen peroxide-sensitizer reaction systems for application as chemical light sources [62],... 

The original work was on ionic reactions in normal micelles in water, but subsequently there has been extensive work on reactions in reverse micelles (O Connor et al., 1982, 1984 Kitahara, 1980 O. A. El Seoud et al., 1977 Robinson, et al., 1979). There also has been a great deal of work on photochemical and radiation induced reactions in a variety of colloidal systems, and microemulsions have been used as media for a variety of thermal, electrochemical and photochemical reactions (Mackay, 1981 Fendler, 1982 Thomas, 1984). 

Reactions of 2,4-dinitrochloro-benzene and -naphthalene are speeded by DDDAOH and the corresponding chloride -I- NaOH (Cipiciani et at., 1984). The rate/surfactant concentration profiles and the rate constants are very similar to those for reactions in solutions of the corresponding C16 single chain surfactants which form normal micelles. The spontaneous hydrolysis of 2,4-dinitrophenyl phosphate dianion is also speeded by DDDAC1 and rates reach plateau values in very dilute surfactant (Savelli and Si, 1985). 

It is convenient to differentiate between oil-in-water (o/w) microemulsions and water-in-oil (w/o) microemulsions in which water and oil are the respective major components. It is reasonable to regard (o/w) microemulsions as akin to swollen normal micelles and w/o microemulsions as reverse micelles (Section 1). 

A microemulsion droplet is a multicomponent system containing oil, surfactant, cosurfactant, and probably water therefore there may be considerable variation in size and shape depending upon the overall composition. The packing constraints which dictate size and shape of normal micelles (Section 1) should be relaxed in microemulsions because of the presence of cosurfactant and oil. However, it is possible to draw analogies between the behavior of micelles and microemulsion droplets, at least in the more aqueous media. 

Fig. 6.10 Spherical cross-section of an idealized anionic (a) normal micelle and (b) reverse micelle ( ) polar head group ( ) counter ion ( vw) the hydrocarbon chain.

Generation 6 Internal poor solvent Terminal athermal solvent normal micelle ... 

The present study demonstrates the utility of the above probe in describing the constitution of aggregates of certain alkarylsulfonates in hydrocarbon media. It also demonstrates the use of the probe technique in measuring the micropolarity of these same aggregates as a function of distance from the polar core. The micro-viscosity of inverted or normal micelles in the past has been estimated only as an average value of either the polar or non-polar regions (6). 

Inverted Micelles. The study o-f inverted micelles has received little attention. The study o-f these aggregates can bene-fit by application o-f techniques used -for normal micelles such as -fluorescence probes as demonstrated by Jao and Kreuz in Chapter 7. With applications in oil recovery and dry cleaning, this topic will see increased research activity. 

Figure 5.2 Top-diagramatic representation of a detergent molecule, (a) Single tailed (b) double tailed (c) zwitterionic (d) bolamphiphilic. Bottom - different types of surfactant aggregates in solution (A) monolayer (B) bilayer (C) liquid-crystallin phase lamellar (D) normal micelles (E) cylindrical micelles (hexagonal) (F) vesicles (liposomes) (G) reversed micelles.

Two main microemulsion microstructures have been identified droplet and biconti-nuous microemulsions (54-58). In the droplet type, the microemulsion phase consists of solubilized micelles reverse micelles for w/o systems and normal micelles for the o/w counterparts. In w/o microemulsions, spherical water drops are coated by a monomolecular film of surfactant, while in w/o microemulsions, the dispersed phase is oil. In contrast, bicontinuous microemulsions occur as a continuous network of aqueous domains enmeshed in a continuous network of oil, with the surfactant molecules occupying the oil/water boundaries. Microemulsion-based materials synthesis relies on the availability of surfactant/oil/aqueous phase formulations that give stable microemulsions (54-58). As can be seen from Table 2.2.1, a variety of surfactants have been used, as further detailed in Table 2.2.2 (16). Also, various oils have been utilized, including straight-chain alkanes (e.g., n-decane, /(-hexane),... 

Surfactants having a positive curvature, above a given concentration usually called the critical micellar concentration, cmc, self-assemble to form oil-in-water aggregates called normal micelles. The surfactant most often used is sodium dodecyl sulfate, Na(DS) or SDS. To make particles, the counterion of the surfactant is replaced by ions which participate in the chemical reaction. These are called functionalized surfactants. 

At all temperatures, asl-CN B and C are insoluble in calcium-containing solutions and form a coarse precipitate at Ca2+ concentrations greater than about 4 mM. asl-CN A, from which the very hydrophobic sequence, residues 13-26, is deleted, is soluble at [Ca2+] up to 0.4 M in the temperature range 1-33°C. Above 33°C, it precipitates but redissolves on cooling to 28°C. The presence of asl-CN A modifies the behaviour of asl-CN B so that an equimolar mixture of the two is soluble in 0.4 M Ca2+ at 1°C asl-CN B precipitates from the mixture at 18°C and both asl-CN A and B precipitate at 33°C. aBl-CN A does not form normal micelles with K-casein. Since asl-CN A occurs at very low frequency, these abnormalities are of little consequence in dairy processing but may become important if the frequency of asl-CN A increases as a result of breeding practices. 

Figure 4. Formation of normal micelles (left), the surfactant phase (middle), and inverse micelles (right) may be referred to the relative size of the interfacial tensions against the oil (yo/n) and the water (jw/p) a plane interface (32)...

One interfacial tension (upper left) is considered located between water and the polar parts (unfilled circles) of the surfactant (upper right) and one (middle left) between the nonpolar part (filled circles) of the surfactant and the hydrocarbon (middle right). The different convexities of the O/W interface giving normal micelles, a surfactant phase or an inverse micelle are formally referred to different ratios of these interfacial tensions (bottom of figure) at a plane interface. 

With nonionic PEO emulsifiers, intermolecular interactions vary with temperature and types of metal ions and solvents. At low temperatures, nonionic emulsifiers are hydrophilic and form normal micelles. At higher temperatures they are lipophilic and form reverse micelles. A weak interaction with metal ions favors the stability of associates against moisture. On the other hand, a strong interaction may lead to a completely amorphous system. Ethanol as a co-solvent is a moderate solvent for PEO at low temperatures, but its power improves as the temperature is raised [34]. This means that solutions of the PEO copolymers in water and ethanol have opposing temperature coefficients of solubility negative for water and positive for ethanol. 

The aggregation numbers Nagg is determined as 27 for C1-(EO)53-C4-VB and 38 for Cr(EO)53-C7-VB micelles by analysis of fluorescence curves. A micelle formation mechanism is proposed for nonionic polymeric surfactants with weakly hydrophobic groups. At low concentrations of PEO macromonomers, large loosely aggregated structures involving the PEO chains are formed. At higher concentrations normal micelles form. These are star-shaped, with a hydrophobic core surrounded by a corona of PEO chains. 

Figure 12.6 Liquid-crystalline phases and catalytic activity of peroxidase in ternary water/surfactant/organic solvent phases (Martinek, 1986). Phases a = normal micelles b = reversed micelles c = lamellar aggregates d = hexagonal aggregates.

Normal Micelles - Solubilizate Probes. The addition of a probe molecule, usually bearing a C=0 group, to a micelle has been used to asses die solubilization site of the probe (67) and to infer the extent of penetration of water into micelles (68,69). The basis of such studies is the well known decrease in the 0=0 band frequency upon hydrogen bond formation (70 -73). Two important concepts must be addressed, however, when using probes in studies of micelles the solubilization site of the probe (micelle core or palisade layer) and the possibility of probe-induced changes in the micelle. 

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