Micelles, reverse, hydration

That crosslinking has indeed occurred is confirmed by the very existence of aggregates at 25 °C, as in its absence the diblock copolymers are completely soluble at this temperature. Tunability of the solubility of the PMEMA block in water arises from the fact that its lower critical solution temperature (LCST) lies between 25 and 60 °C. This reversible hydration of the core could be a very useful feature to trigger release of occluded guest molecules from the core interior. More recently, utilizing a similar methodology, zwitterionic shell-crosslinked systems have also been prepared wherein the core and shell domains contain amine and carboxylic acid groups, respectively, or vice versa. Such systems exhibit an isoelectric point, at a pH wherein the crosslinked micelles ( 40 nm) become electrically neutral and precipitate out in water addition of acid or base causes complete redissolution of these nanospheres [58]. 

The majority of practical micellar systems of Tionnal micelles use water as tire main solvent. Reverse micelles use water immiscible organic solvents, altlrough tire cores of reverse micelles are usually hydrated and may contain considerable quantities of water. Polar solvents such as glycerol, etlrylene glycol, fonnamide and hydrazine are now being used instead of water to support regular micelles [10]. Critical fluids such as critical carbon dioxide are... 

Sodium octanoate (NaO) forms reversed micelles not only in hydrocarbons but also in 1-hexanol/water. The hydration of the ionogenic NaO headgroups plays an important role in this case too. For this reason Fujii et al. 64) studied the dynamic behaviour of these headgroups and the influence of hydration-water with l3C and 23Na NMR measurements. Below w0 = [H20]/[NaO] 6 the 23Na line-width... 

As mentioned above, water structure in reversed micelles deviates considerably from the structure in the bulk-phase. Therefore, the hydration shell of macromolecules entrapped in reversed micellar systems should be changed and thus also their conformation. According to the results of several authors this is indeed the case. 

The effects of the intramicellar confinement of polar and amphiphilic species in nanoscopic domains dispersed in an apolar solvent on their physicochemical properties (electronic structure, density, dielectric constant, phase diagram, reactivity, etc.) have received considerable attention [51,52]. hi particular, the properties of water confined in reversed micelles have been widely investigated, since it simulates water hydrating enzymes or encapsulated in biological environments [13,23,53-59]. 

FIG. 4 Onion model of spherical water-containing reversed micelles. Solvent molecules are not represented. A, surfactant alkyl chain domain B, head group plus hydration water domain C, hulk water domain. (For water-containing AOT-reversed micelles, the approximate thickness of layer A is 1.5 nm, of layer B is 0.4 nm, whereas the radius of C is given hy the equation r = 0.17R nm.)... 

For many solubilized enzymes the greatest catalytic activity and/or changes in conformation are found at R < 12, namely, when the competition for the water in the system between surfactant head groups and biopolymers is strong. This emphasizes the importance of the hydration water surrounding the biopolymer on its reactivity and conformation [13], It has been reported that enzymes incorporated in the aqueous polar core of the reversed micelles are protected against denaturation and that the distribution of some proteins, such as chymotrypsine, ribonuclease, and cytochrome c, is well described by a Poisson distribution. The protein state and reactivity were found markedly different from those observed in bulk aqueous solution [178,179],... 

In reverse, the surfactant precipitates from solution as a hydrated crystal at temperatures below 7k, rather than forming micelles. For this reason, below about 20 °C, the micelles precipitate from solution and (being less dense than water) accumulate on the surface of the washing bowl. We say the water and micelle phases are immiscible. The oils re-enter solution when the water is re-heated above the Krafft point, causing the oily scum to peptize. The way the micelle s solubility depends on temperature is depicted in Figure 10.14, which shows a graph of [sodium decyl sulphate] in water (as y ) against temperature (as V). 

The amount of water solubilized in a reverse micelle solution is commonly referred to as W, the molar ratio of water to surfactant, and this is also a good qualitative indicator of micelle size. This is an extremely important parameter since it will determine the number of surfactant molecules per micelle and is the main factor affecting micelle size. For an (AOT)/iso-octane/H20 system, the maximum Wq is around 60 [16], and above this value the transparent reverse micelle solution becomes a turbid emulsion, and phase separation may occur. The effect of salt type and concentration on water solubilization is important. Cations with a smaller hydration size, but the same ionic charge, result in less solubilization than cations with a large hydration size [17,18]. Micelle size depends on the salt type and concentration, solvent, surfactant type and concentration, and also temperature. 

In a liquid/liquid biphasic system (Figure 9.1a), the enzyme is in the aqueous phase, whereas the hydrophobic compounds are in the organic phase. In pure organic solvent (Figure 9.1b) a solid enzyme preparation is suspended in the solvent, making it a liquid/solid biphasic system. In a micellar system, the enzyme is entrapped in a hydrated reverse micelle within a homogeneous organic solvent... 

Hydrates have further applications in bioengineering through the research of John and coworkers (Rao et al., 1990 Nguyen, 1991 Nguyen et al., 1991, 1993 Phillips et al., 1991). These workers have used hydrates in reversed micelles (water-in-oil emulsions) to dehydrate protein solutions for recovery and for optimization of enzyme activity, at nondestructive and low-energy conditions. 

Nguyen, H.T., Clathrate Hydrate Formation and Protein Solubilization in Reversed Micelles, Ph.D. Thesis, Tulane University, LA (1991). 

FIGURE 28.1 Chemical modification of the protein with a water-insoluble reagent in the reverse micelles of Aerosol OT in octane. The protein molecule is entrapped in the reverse micelle surrounded by a cover of hydrated surfactant molecules. The water-insoluble reagent is located in the bulk organic phase and can be incorporated into the micelle surface layer coming into contact with the reactive group in the protein. After completion of the reaction the reverse micelle system is disintegrated and the protein is precipitated by cold acetone. 

Three main effects are universal and do not depend on the system studied. The favorable effect of a cation on third-phase formation is measured by the slope of the energy of attraction between the reverse micelles plotted versus the cation concentration in the organic phase or the total nitrate concentration for different salt. Whatever the nature of the extracted cations, third-phase formation is observed when the energy of attraction is near 2kBT. Finally, the tendency toward phase splitting correlates well with the hydration enthalpy of the cations. 

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