Location of solubilized molecules

Lin and Yang (1987) also calculated the thermodynamic parameters of diazepam for micellar solubilization in Pluronic surfactant solutions at different temperatures (Table 13.4). For all systems, AG was negative, indicating micellar solubilization was spontaneous. The sign of entropy has been associated with the location of solubilized molecules within the micelles. Positive values have been observed for molecules embedded in the micelle center and negative values for adsorption of the molecules on the micelle surface. The results in this paper indicate that in the F-108 and F-88 Pluronics, diazepam molecules can penetrate into the micelle interior, whereas for F-68 and lower concentrations of F-88, diazepam is adsorbed on the micelle surface without penetration into the micellar core. 

From the data presented here several conclusions may be reached regarding the effect of cholesterol on lipid bilayers. It is shown that, even if the presence of cholesterol in bilayers serves to moderate temperature-induced changes, its ability to affect the location of solubilized molecules is highly temperature dependent We have also shown, in accord with previous work (11), that the presence of cholesterol in the gel phase results in a larger separation between the lipid polar groups and this in turn allows water to penetrate into the lipid hydrophobic core. 

This effect of cholesterol on the location of guest molecules has been cited for chlorophyll a (36) and tetracaine (5) both molecules with specific biological function. In the present work we have been able to observe this effect over the physiological ranges of temperature and cholesterol concentration. Future experiments with other molecules would clarify if this property of cholesterol is generally applicable to other systems and, furthermore, if it extends to interactions between two molecules solubilized in a membrane. 

Even though chemical structures may dictate the preferred location for the additive, solubilized systems are dynamic, as are the parent micelles, and the location of specific molecules changes rapidly with time. It will always be important to remember that while a given region of the micelle may be preferred by an additive on chemical grounds, there is no guarantee that aU phenomena related to the system (catalysis, for example) will be associated with that region. 

Amphiphilic tertiary phosphines have their phosphorus donor atom located somewhere in the hydrophobic part of the molecule and should have at least one long alkyl or alkyl-aryl chain carrying a polar head group (Scheme 4. 10). Some of them, such as the sulfonated derivatives, are quite well soluble in water, others, such as Ph2P(CH) COOH (n = 3, 5, 7, 9, 11) are practically insoluble, however, can be easily solubilized with common surfactants (SDS, CTAB etc.). 

There is considerable interest in establishing the location within a micelle of the solubilized component. As we have seen, the environment changes from polar water to nonpolar hydrocarbon as we move radially toward the center of a micelle. While the detailed structure of the various zones is disputed, there is no doubt that this gradient of polarity exists. Accordingly, any experimental property that is sensitive to the molecular environment can be used to monitor the whereabouts of the solubilizate in the micelle. Spectroscopic measurements are ideally suited for determining the microenvironment of solubilizate molecules. This is the same principle used in Section 8.3, in which the ultraviolet spectrum of solubilized benzene was used to explore the solvation of micelles. Here we take the hydration for granted and use similar methods to locate the solubilizate. 

In addition to one part of a molecule influencing the NMR spectrum of another part, the medium in which the molecule is embedded also has an effect. Therefore the NMR spectrum of a solubilized molecule has the potential not only to reveal the location of a solubilized molecule in a micelle, but also to give information about its orientation. 

The reactivity of molecules bound to surfaces, located at various kinds of interfaces, solubilized in microheterogeneous media, or incorporated as "guests" in various "hosts" as inclusion complexes has been the subject of much recent study. Indeed the structure of the medium, the nature of "solubilization sites" and reactivity in these environments have all been the focus of independent or interrelated investigations (1-12). Photochemistry has played a major role in these studies both in terms of studies of the media and also in terms of modified or controlled reactivity (1,5,8,9). In the course of these investigations numerous questions have arisen many of these have developed from differing pictures of solute-environment interactions which are furnished by different studies using different molecules as "probes" (5,10-12). Controversies arising... 

A two-state model of solubilization may be used to describe the location of solutes in micellar systems. This model involves a distribution between a dissolved state, which is associated with the core, and an adsorbed state, associated with the micellar water interface. The molecules in thi dissolved state remain in the micelle because of the solvent properties of the core. Molecules in the adsorbed state are due to the surface activity of the dissolved species, similar to a surface exces (Mukerjee, 1979). 

Cholesterol can modify both the hydrophobic attraction between lipid hydrocarbon chains and electrostatic interactions between lipid polar groups. The influence it has on the location of 9HP reflects this dual effect At low temperature, the "spacer" effect of cholesterol allows the ketone to gain access directly to the lipid-water interface. At high temperatures, a more disordered hydrocarbon core favors the solubilization of the guest molecule. 

The kinetics and thermodynamics of the solubilization and localization of guest molecules into reverse micelles in lubricants are little known. Questions such as What are the driving forces responsible for the uptake of molecules into reverse micelles What are the kinetic steps in solubilization Where is the location of the guest molecules in a water pool or interface - all still await a definitive answer (Luisi et al., 1988 Pawlak, 2001 Willermet, 1998). 

The solubilization phenomenon in hydrocarbon surfactant solutions is defined as a lowering of the activity of any solubilizates. The location of solubilizates in micelles can be investigated using probe molecules which indicate the surrounding conditions. 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

A micelle is a colloidal aggregate of amphiphilic molecules (50-100 molecules per micelle) which forms at a specific concentration termed the critical micelle concentration. As illustrated in Fig. 1, in polar media such as water, the hydrophobic part of the amphiphilic molecule tends to locate away from the polar phase while the polar groups of the molecule tend to locate in the water phase, forming the micelle aggregate. Micellar systems are able to solubilize both hydrophobic and hydrophilic compounds. 

The location of a solubilized molecule in a micelle is determined primarily by the chemical structure of the solubilizate. Solubilization can occur at a number of different sites in a micelle ... 

A preferred location of the solubilizate molecule within the micelle is largely dictated by chemical structure. However, solubilized systems are dynamic and the location of molecules within the micelle changes rapidly with time. Solubilization in surfactant aqueous systems above the critical micelle concentration offers one pathway for the formulation of poorly soluble drugs. From a quantitative point of view, the solubilization process above the CMC may be considered to involve a simple partition phenomenon between an aqueous and a micellar phase. Thus the relationship between surfactant concentration Cm and drug solubility Ctot is given by Eq. (3). 

The stability of a lamellar liquid crystal with such a large amount of oil is an intriguing problem. The main structural entity to be clarified before a serious attempt at an examination of the problem may be made is the degree of order of the hydrocarbon chains. This factor in turn depends on the location of the solubilized hydrocarbon chains are they penetrating the amphiphilic layer or are they forming a liquid layer between the layers of amphiphilic molecules. 

A neutral molecule solubilized in the micelle can be located in several positions or microenvironments. As early as the 1930s it was suggested by Lawrence that the site of a solubilized molecule would be dependent on the hydrophobic/hydrophilic composition of the solubilizate. Two extremes are easily identified the core of the micelle providing a hydrocarbon-like microenvironment, and the palisade layer providing an aqueous or water-rich interfacial environment. It seems logical to assume, then, that nonpolar solutes like alkanes would prefer the micellar core and that polar molecules would be anchored at the surface. However, this is an oversimplification available data tend to contradict it. First, the solubility of alkanes in micelles is significantly lower than expected if compared to solubility in hydrocarbon solvents. Second, the size of a micelle is normally such that part of the solute would be close to the surface at any time. Sepulveda et al. state that for SDS micelles at least half of the solute will be within 4 to 5 A of the surface. We should also consider the timescale of the experiments, as the timescale for intramicellar migration is short. The rate constants of entry and exit of molecules to and from micelles is of the order 1(F and... 

Further work is clearly needed to unravel the detailed mechanisms of the hydrolysis and polymerization reactions in these micellar systems. Of particular interest is a better description of the initial stages of growth, which are believed to be responsible for the narrow monodispersity of the particles obtained within the entire R range. Other issues of interest are the location of hydrolysis products and other intermediates, changes in the aggregation number of surfactant molecules due to changes in the nature of the solubilized aqueous phase, and a quantitative description of the particle-filled and empty reverse micelle populations. 

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