Solubilization by micelles

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

Caution should be exercised when considering temperature effects on solubilization by micelles, since the aqueous solubility of the solute and thus its micelle/water partition coefLcient can also change in response to temperature changes. For example, it has been reported that although tt solubility of benzoic acid in a series of polyoxyethylene nonionic surfactants increases with temperature, the micelle/water partition coefLci rt, shows a minimum at 2C, presumably due to the increase in the aqueous solubility of benzoic acid (Humphreys and Rhodes, 1968). The increasr in Km with increasing temperature was attributed to an increase in micellar size, as the cloud point temperature of the surfactant is approached (Humphreys and Rhodes, 1968). 

One of the primary uses of modified electrodes has been In the area of electrochemical synthesis. Again the Increased solubilization by micelles and emulsions has been the primary Interest In using cationic surfactants. However, micelle and phase transfer catalysis and the hydrophobic nature of the electrode film has contributed to Increased yields. Table II shows a comparison of yields obtained In the electrooxidation of benz-hydrol in the presence of different surfactants and a comparison of the yields obtained with several other compounds with and without Ify-amlne 2389 (50). It can be seen that without a surfactant there Is no yield In aqueous solutions. Anionic and neutral surfactants which solubilize the compound but do not film the electrode cause only small Increases In yield, but the cationic film forming surfactant causes a sharp Increase in 3rleld. 

The uniqueness of the ferritin structure arises from the metabolic requirement to organize and utilize dissolved iron at concentrations and pH levels that induce precipitation of potentially toxic solid phases. Not only is iron solubilized by micelle encapsulation, but homeostatic control is also maintained. The study of ferritin therefore provides an important example of biological control of solid state reactions that involve the formation and organization of nanometer-size inorganic solids in biological time and space. 

The totality of micelles represents a colloidal phase, into which a substance is dissolved in the aqueous phase partitions. The capacity of the micellar phase to solubilize a solute can therefore be expressed as a partition coefficient A n,. Hence, a linear relationship can be expected between the concentration of substance solubilized by micelles and the concentration of the surfactant Cs in the system. Because only micelles contribute to the solubilizing effect but not the monomeric surfactant molecules, the critical micelle concentration Ccmc must be subtracted from the total of the surfactant concentration. The resulting total concentration of solute in the micellar solution is then ... 

The ether group, secondary and tertiary amines, has group contributions that are negative i.e., they oppose solubilization by micelles. 

The micelle has too small an aggregation number to be considered as a phase in the usual sense, and yet normally contains too many surfactant molecules to be considered as a chemical species. It is this dichotomy that makes an exact theory of solubilization by micelles difficult. The primary theoretical approaches to the problem are based on either a pseudophase model, mass action model, multiple equilibrium model, or the thermodynamics of small systems [191-196]. Technically, bulk thermodynamics should not apply to solute partitioning into small aggregates, since these solvents are interfacial phases with large surface-to-volume ratios. In contrast to a bulk phase, whose properties are invariant with position, the properties of small aggregates are expected to vary with distance from the interface [195]. The lattice model of solute partitioning concludes that virtually all types of solutes should favor the interface over the interior of a spherical micelle. While for cylindrical micelles, the internal distribution of solutes... 

Compared to an aqueous solution, nearly all aromatic substances show small spectral changes of several nanometers in the absorption and emission spectra when they are solubilized by micelles. These shifts are mostly similar to shifts that occur when the solvent polarity is decreased. In principle all shifts can be used for the determination of critical micelle concentrations when spectral changes... 

Studies on micellar solutions began in the first half of the 20th century, and a summary of the results obtained was presented in Hartley s book [59] published in 1936, which describes many currently used terms. Further research focused primarily on micelle structures and properties, monomer-micelle equilibrium, and solubilization by micelles [60]. The research turned out to be extremely interesting, and the results were used in a number of applications. 

The effectiveness of a surfactant to stabilize a co-crystal or attain a CSC is determined by the relative magnitude of drug and co-former solubilization by micelles, and As illustrated in Figure 11.9, the greater the drug... 

The last term in this sum reflects the decrease of H concentration in the aqueous phase due to solubilization by micelles, and can often be neglected for usual experimental conditions. The second term is close to zero in cationic and neutral micelles, where A remains in the micellar phase and can be greater (but less than unity for investigated compounds) in anionic micelles, and where A" leaves the micelles of a rate constant of lOMO s ... 

Solubilization by micelles has been described in detail in Chapter 13, and only a summary is given here. As discussed in Chapter 13, solubilisation is the incorporation of an insoluble substance (usually referred to as the substrate) into surfactant micelles (the solubilizer). Solubilization may also be referred to as the formation of... 

Molar volume has been studied widely [2, 128, 148, 149] but no simple relationship has been shown between molar volume and the amount of solubilizate dissolved. Stearns et al. [148], studying hexane, heptane, and octane, and benzene, toluene, ethylbenzene, propylbenzene, and butylbcnzene concluded that there was inverse proportionality between the volume of hydrocarbon solubilized and molar volume. The slope of the plots of ml hydrocarbon dissolved per 100 g solution against molar volume of hydrocarbon are different for the aliphatic and aromatic series. Klevens [16], with polycyclic compounds in sodium laurate, found linear relationships between the log volume solubilized and molar volume, the slope of plots for linear polycyclics varying from that for the nonlinear polycyclics. Schwuger [149] reported that the amount of naphthalene, anthracene, pyrene, perylene and dibenzanthracene solubilized by micelles of dodecylpentaglycol ether was inversely related to the molecular size of these solubilizates. 

Figure 10.12 The dependence of dye dispersed (absorbance units) on amounts of NaDS bound by 0.1% solutions of HSA and BSA. Arrows indicate the points at which F = CMC for the two proteins. Symbols HSA (O) BSA ( ). (b) The relation of total NaDS concentration (B -h F) of dye dispersed ( ) and of Na bound (B) by 0.1 % HSA. The solid diagonal represents the solubilization by micelles without protein. The broken line shows the free NaDS concentration (F). Symbols (O) dye solubilization ( ) binding. From Steinhardt et al [81] with permission.

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]. 

In some cases, dye-forming moieties attached to a polymeric backbone, called a polymeric coupler, can replace the monomeric coupler in coupler solvent (51). In other reports, very small particles of coupler solubilized by surfactant micelles can be formed through a catastrophic precipitation process (58). Both approaches can eliminate the need for mechanical manipulation of the coupler phase. 

A macroporous polystyrene-divinylbenzene copolymer is produced by a suspension polymerization of a mixture of monomers in the presence of water as a precipitant. This is substantially immiscible with the monomer mixture but is solubilized with a monomer mixture by micelle-forming mechanisms in the presence of the surfactant sodium bis(2-ethylhexylsulfosuccinate) (22). The porosity of percentage void volume of macroporous resin particles is related to percentage weight of the composite (50% precipitant, 50% solvent) in the monomer mixture. 

This approach was followed by Yushmanov for the localization of papaverine in ionic micelles.42 Another interesting application was reported by Chien43 who measured 19F NMR relaxation times of trifluor-omethyl labelled atrazine induced by paramagnetic probes gadolinium ethylenediamine tetraacetic acid and 2,2,6,6-tertramethyl-piperidine-N-oxyl. The results showed that atrazine solubilized by humic micelles occupied a hydrophobic domain accessible only to neutral hydrophobic molecules. 

Certain compounds are known to achieve higher absorption rates from the GI tract if they are taken with food, and this observation has been linked to their solubilization by bile salts [74], Bile salts, especially those of cholic and deoxycholic acids, have been used to solubilize steroid hormones [75], antibiotics [76], and nonsteroidal antiinflammatory drugs [77]. For example, amphotericin B (an antifungal agent) has been solubilized for parenteral use in micelles composed of sodium desoxycholate [78], As illustrated in Fig. 11, the degree of solubilization of carbamazepine by sodium desoxycholate is minimal below the critical micelle concentration but increases rapidly above this value [79]. At sufficiently high concentrations, when the micelles become saturated in carb-amezepine, the apparent solubility reaches a limiting value approximately seven times the true aqueous solubility in the absence of desoxycholate. 

One inherent property of peptides that interact with membranes is that self-association or even aggregation will interfere with solubilization by organic solvents or micelles. The preparation, purification and sample preparation of extremely hydrophobic (often transmembrane) peptides is nontrivial and has been addressed by only a few papers [74—79]. 

One of the most important characteristics of micelles is their ability to enclose all kinds of substances. Capture of these compounds in micelles is generally driven by hydrophobic, electrostatic and hydrogen-bonding interactions. The dynamics of solubilization into micelles are similar to those observed for entrance and exit of individual surfactant molecules, but the micelle-bound substrate will experience a reaction environment different from bulk water, leading to kinetic medium effects308. Hence, micelles are able to catalyse or inhibit reactions. The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentrations of the reacting species in or at the micelle. 

Organic molecules are solubilized by the organic constituents of a micelle. In small micelles the molecules lie close to the interface, while in large microemulsions considerable penetration into the core is observed (1.). Inorganic materials are quite polar and have little interaction with the organic components of the micelle. However, the micelle counterions,... 

---