Solubilizates in Micelles

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. 

One particular property of micelles stands out above all others their ability to solubilize organic compounds in water. Benzene, for example, dissolves in SDS to the extent of 0.90 mol/mol surfactant, resulting in around 40 benzene molecules per micelle . NMR chemical shift data situate most of the benzene at the micelle-water interface, but the localization of small solubilizates in micelles is never uniform. 

The location of a solubilizate in micelles and its effect on micelle size depend much on the nature of the solubilizate. - Thus alkanes are solubilized in the micelle core. Aromatic molecules tend to be solubilized first at the micelle surface and then into the core once the surface is saturated. Cosurfactants (alcohols, amines, carboxylic acids, for instance) are solubilized with their head group at the micelle surface and their tail penetrating more or less deeply into the core, depending on its length. 

Table 3.4 shows that the association (entry) of aromatic solubilizates in micelles is veiy close to being diffusion-controlled, as indicated by the entries 1, 2, and 7-16 for which ks is in the 10 -10 range. The ks values are somewhat smaller for compormds having an alkyl chain as in entries 18-20 but remain close to those for a diffusion-controlled process. The difference may reflect the detail of the solubilization process aromatics are solubilized close or at the micelle surface, in the pahsade layer, whereas compormds with an alkyl chain must have this alkyl chain penetrating in the micelle core. 

The position of solubilizates in micelles, as well as in living membranes, provides very important information concerning the physicochemical properties and physiological functions of both solubilizate and micelle or membrane. This property can be investigated using probe molecules, the molecular spectrum of which indicates the surrounding conditions. ... 

Another method is based on the evaporation of a w/o microemulsion carrying a water-soluble solubilizate inside the micellar core [221,222], The contemporaneous evaporation of the volatile components (water and organic solvent) leads to an increase in the concentration of micelles and of the solubilizate in the micellar core. Above a threshold value of the solubilizate concentration, it starts to crystallize in confined space. Nanoparticle coalescence could be hindered by surfactant adsorption and nanoparticle dispersion within the surfactant matrix. 

Hydrophobic solubilizates such as styrene (S) decrease the saponification rate of the EUP. Accordingly, the EUP-molecules in micelles containing S are more resistant against hydrolytic degradation than molecularly dissolved EUP-mole-cules. Obviously, the access of the base to the hydrophobic interior of these micelles and microemulsion droplets is more difficult. 

In summary, our bioavailability study provided for fhe firsf time data for fhe shorf-ferm bioavailability of ot-tocopherol solubilizate in comparison to regular fat-soluble preparations. Our results pointed to a higher short-term bioavailability of vitamin E in micelles versus fat-soluble forms of fhis vifamin in healthy adult volunteers both with regard to AUCs and with regard to maximum increases in plasma vitamin concentrations. 

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. 

Xing and Mattice (1998) applied Monte Carlo simulation techniques to study the models of BAB triblock copolymeric micelles with solubilizates in a selected solvent. They focused on a microscopic picture regarding the locus of solubilizates in BAB triblock copolymer micelles when the... 

The very complex variation of the amount solubilized, as well as physico-chemical properties, with chemical structure of solubilizate and surfactant as well as with surfactant concentration cannot be adequately discussed solely in terms of the energetical conditions of the solubilizate in the micelles. Thus one should also consider the conditions in the phase which separates out at the solubilization limit this is in most cases a liquid crystalline phase. A fundamental basis for a proper understanding of solubilization in surfactant systems is, therefore, a detailed information on phase equilibria in three-component systems surfactant-solubilizate-water. Due in particular... 

The rate constant for the reentry is of the magnitude expected for a diffusion-controlled reaction as in Eq. (5.6). This means that the exit rate is determined by the partition coefficient of the solubilizate in its triplet state between the micelle and the aqueous solution. Table 5.2 shows the exit rate constants k for several systems. The water solubilities of the probes are also given to show the correlation between kt and the solubility in water. These studies give further support to the view that the micelle has a very dynamic structure, which makes it easy for the solubilizate to enter and leave the aggregate. 

Fig. 26. Increased fluorescent yield (arbitrary scale) per Tb3+ ion at 545 nm as produced by decreasing the volume of solubilizate in each micelle. V = volume of mixture (1 1 by volume) of 0.5M TbCl3 and 0.5M HPA (= hydroxy-phenylacetic acid) solutions which has been solubilized in 1 ml of 2 10 2mol dm-3 AOT in isooctane. [J. Colloid Interface Sci. 56, 168 (1976))...

Although no quantitative description of the mechanism of solubilization is available, a knowledge of the location of the solubilizate in the micelle is necessary for an understanding of micellar catalysis. In the ensuing discussion, therefore, the attention will be focused on the possible locations of the different solubilizates in the molecular aggregates and on the experimental techniques used for determining this location. 

N.m.r. and e.s.r. techniques similar to those used for the determination of the location and environment of solubilizates in micellar systems have also been employed in investigations of solubilization by lipid micelles and of protein-substrate interactions (McDonald and Phillips, 1967 Meadows et al., 1967 Spotswood et al., 1967 Chapman, 1968 Penkett et al., 1968 Mildvan and Weiner, 1969 Raftery et al., 1969 Roberts et al., 1969a, b Rosenberg et al., 1969 Small et al., 1969). 

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... 

Predict the locations of the following solubilizates in a micelle of Ci2H25S04Na in aqueous medium ... 

From surface tension studies of bola-surfactants, it is concluded that they exhibit wicket-like conformations at the gas-water interface [239,428,430,431], In micelles and liquid crystals however, a stretched conformation is preferred [436,437] this implies that surface tension data and interfacial tension data do no more describe the micellar interface with all the implications for solubilization (compare Sect. 3.4). In fact, some reports stress the extremely low solubilization capacity of bola-surfactants [431,432, 437,438], although others obtain capacities comparable to the ones of the monomers [430]. Also noteworthy, solubilized fluorescence probes indicate a more polar environment for the solubilizates than in micelles of the monomers [430-432], but micellar aggregation numbers of the bola-surfactants are comparable or only slightly lower [429,432, 438, 439]. In exceptional cases, very high aggregation numbers and the existence of an additional pre-CMC are observed [440]. 

Solubilization Media. Many investigations have been reported on the interaction between a solubilizate and a micellar system. Several solubilization media can constitute interesting drug delivery systems. In other cases, solubilization in micelles represents a simplified model for mimicking interactions with biological membranes as an alternative to bicelles, liposomes and PL bilayers. 

Two aspects of solubilization which remain to be investigated, are the variation of the apparent distribution coefficient with so-lubilizate concentration in the micellar ph.ase, and the mechanism of the incorporation of solubilizate into micelle. Since a micelle is assumed to consist of a hydrocarbon core (in liquid state), and surrounded by a palisade layer of hydrophilic group. Fig. 1, the following possible ways have been suggested for the incorporation of a solubilizate in a micelle [5,74] (a) adsorption on the surface of the micelle, (b) deep or short penetration into the palisade layer, and (c) dissolution into the (liquid like) hydrocarbon core. These mechanisms of incorporation are closely related to the structure of the micelle. However, the effect of solubilizate on the structure of the micelle has not been satisfactori 1y studied in the current literature [75,76,77]. 

Water, aqueous solutions and many other strongly hydrophilic substances can be solubilized within the micellar core [19,20], Water solubilization involves hydration of the surfactant headgroup accompanied by an increase in the head-group area, a micellar swelling, a marked increase in the surfactant aggregation number, and, at constant surfactant concentration, a decrease in the number density of reversed micelles [21], A representation of a spherical reverse micelle entrapping a polar solubilizate in the core is shown in Fig. 3. 

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