Formation of Micelles in Aqueous Solutions

The basic experimental observation is the following. A surface-active molecule usually contains a polar head group and a nonpolar tail. Such molecules are known to reduce the surface tension of water when they are added to water—hence the term surface-active or surfactants. The main reason they do so is their tendency (more precisely, the tendency of their nonpolar part) to avoid contact with water and to seek a nonaqueous environment. These molecules are sometimes referred to as amphiphiles, but more recently the term amphipaths has been found more appropriate to describe the contrasting behavior of the two parts of the same molecule toward the solvent. 

When the concentration of the surfactant is gradually increased, one observes systematic deviations from the behavior of ideal dilute solution. This phenomenon may be ascribed to the formation of small aggregates, e.g., dimer, trimer, and so on, of solute molecules. This is a common phenomenon shared by many concentrated solutions. What makes aqueous surfactant solutions so remarkable is that over some small concentration range one finds an abrupt change in the properties of the solution. This phenomenon is ascribed to the formation of larger aggregates of solute molecules known as micelles. The 

Many physical properties may be followed in order to determine the CMC. The most common are surface tension and conductivity of the solution. Experimental evidence indicates that below the CMC micelles are not formed (or at least are undetectable by experimental means). Above the CMC it has been established that most of the added surfactant is used to build up more micelles. The concentration of the monomeric solute remains fairly constant. If it were exactly constant, then we should have a phenomenon similar to a phase transition at the CMC. 

We now turn to the thermodynamic description of surfactant solutions. First we note that if the micelles are viewed as a separate phase, then the chemical potential of the surfactant S in the two phases is (assuming dilute ideality of the aqueous solution) 

If 5(in micelle) is treated as the chemical potential of a pure phase, then one would have predicted that ps is constant and equal to the CMC. Hence, 

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Kim and coworkers observed the formation of micelles in aqueous solutions by synthesizing a amphiphilic linear-dendritic diblock copolymer based on poly(ethylene oxide) (PEO) (Figure 5)50,62. In this case, the PEO can be considered as the core upon which the dendrimer was synthesized via a divergent process. In an aqueous phase, the hydrophilic... 

The nonpolar portion of surfactant ions has an important role in promoting the adsorption process because it increases the affinity of these organic ions to the interfacial region. The effect derives from mutual attraction between the hydrophobic tails as well as their tendency to escape from an aqueous environment. That mechanism is precisely the same one which causes the spontaneous formation of micelles in aqueous solution and is known as the hydrophobic effect [78]. In the case of surfactant adsorption, it is responsible for the formation of surface aggregates. However, it is not easy to accurately predict the shape and the size of such molecular associations in the same way that the structure of bulk aggregates can be determined from the geometry of the molecule. This is because the surface imposes different restrictions on the organization of the adsorbed layer. 

Hydrogen-bonding-sensitive fluorescence of azaaromatics, such as 8b or 21, can also be utilized to probe the formation of micelles in aqueous solution. Because of their hydrophobic nature, these probes favor a distribution in the hydrophobic environment of a micelle core formed by nonionic surfactants such as Triton-X-100. Therefore, as shown in Figure 3.22a, the transfer of such a probe to the water-free region of the micelle results in a strong enhancement of its fluorescence. This feature makes these compounds promising as fluorescence probes for a critical micelle concentration (CMC) (Figure 3.22b). 

While the formation of micelles in the solution was expected, cryo-TEM studies proved that the short-chain PDMS-h-PEO diblock copolymers spontaneously form vesicles at low concentrations in aqueous solutions [4], Cryo-TEM images of the studied systems are shown in Fig. 1. 

Amphiphilic polysaccharide derivatives (APDs) consisting of hydrophilic polysaccharide chains and hydrophobic segments can form micelle structures with hydrophobic inner core and the hydrophilic outer shell in aqueous solutions [40-43]. The critical aggregation concentration (CAC) value, usually determined by pyrene probe fluorescence spectroscopy, is used to evaluate the thermodynamic stability of micelles in aqueous solutions, i.e. the small CAC is, the more stable micelles are [44,45]. Moreover, the formation and properties of the micelles may change with the structures of APDs [46,47]. 

Sha et al. applied the commercially available dual initiator ATRP-4 for the chemoenzymatic synthesis of block copolymers. In a first series of publications, the group reported the successful synthesis of a block copolymer comprising PCL and polystyrene (PS) blocks [31, 32]. This concept was then further applied for the chemoenzymatic synthesis of amphiphilic block copolymers by macroinitiation of glycidyl methacrylate (GMA) from the ATRP functional PCL [33]. This procedure yielded well-defined block copolymers, which formed micelles in aqueous solution. Sha et al. were also the first to apply the dual enzyme/ATRP initiator concept to an enzymatic polycondensation of 10-hydroxydecanoic acid [34]. This concept was then extended to the ATRP of GMA and the formation of vesicles from the corresponding block copolymer [35]. 

Consider the formation of a mixed micelle in aqueous solution from a binary surfactant solution consisting of a nonionic and an anionic surfactant. The process is depicted as the aggregation of ng molecules of nonionic surfactant B, of n molecules of anionic surfactant A", and in addition there will be counterions, C" ", of the anionic surfactant in the amount of an where a is the fraction of the counterions associated or bound with the surfactant anions in the micelle. The process as depicted is... 

Figure9.8 The absorbance of 1.05 x 10 M pinacyanol chloride at 610.0 min pH 9.59 sodium borate buffer (I = 0.1) at 50 °C vs. dodecanoate concentration. The absorption spectrum of pinacyanol chloride in aqueous solution of anionic soaps changes sharply to one characteristic of its solutions in organic solvents within a small range of soap concentration (X ax 610 nm). This effect is attributed to the formation of micelles, in whose hydrocarbon-like layers or cores the dye is solubilized. The concentration of soap at which this spectral change occurs is taken as the cmc. The use of dyes for the determination of cmc values may lead to micelle formation at a concentration below the true cmc. In practice, the method gives only a rough approximation of the cmc. (Adapted, with some modifications, from Corrin et al., 1946.)...

Thus far the solvent systems we have discussed are typical protic organic media, such as, for example, water-ethanol mixtures containing an added supporting electrolyte. These solvents are presumably quite homogeneous on a microscopic level. However, a number of solvents have been developed in recent years which are heterogeneous on a microscopic scale. Micellar media are one example of such solvents. The electrochemical reduction of nitrobenzene in aqueous solutions containing polyoxyethylene lauryl ether, a substance known to produce neutral micelles, produces azobenzene (4) even at pH somewhat less than 723. This is apparently the first case of formation of a dimeric product from electrolysis of nitrobenzene (1) in acidic media. Another striking example of this phenomenon... 

Fig. 2.2. The solubility of naphthalene in aqueous solutions of sodium cholate (c) and of Orange OT in solution of sodium decane-sulfonate ( ). The ratio between the increment in solubility (AS) and in concentration (AC) is plotted. The sodium decanesul-fonate solution displays the behavior typical of micelle formation with a well-defined CMC. (From Ref.24))...

In our model study reported in this contribution, we have chosen two double-chained C-13 alkylbenzenesulphonate surfactants (SLABS) of closely-related structure, which form micelles in aqueous solution in the absence of salt. However, when small amounts of electrolyte are added (e.g., —20mM NaCl), vesicles are spontaneously formed over a time period of seconds/minutes. These vesicle structures are then reasonably stable over a period of hours/days. The onset of vesicle formation can be readily characterised by the determination of the critical salt concentration (esc), needed to induce the formation of vesicles, from smaller aggregates or monomers. This parameter is easily determined experimentally from the increase in light scattering associated with self-assembly. It has now been determined for a number of electrolyte systems. 

The most important property of micelles in aqueous or nonaqueous solvents is their ability to dissolve substances that are insoluble in the pure solvent. In aqueous systems, nonpolar substances are solubilized in the interior of the micelles, whereas polar substances are solubilized in the micellar core in nonaqueous systems. This process is called solubilization. It can be defined as the formation of a thermodynamically stable isotropic solution with reduced activity of the solubilized material (8). It is useful to further differentiate between primary and secondary solubilization. The solubilization of water in tetrachloroethylene containing a surfactant is an example of primary solubilization. Secondary solubilization can be considered as an extension of primary solubilization because it refers to the solution of a substance in the primary solubilizate. 

Several papers compare the properties of sulfobetaine (meth)acrylic polymers. NMR spectra and solution properties of 23a and 23b [59,60] are correlated with data from the corresponding polycarbobetaines [26]. The photophysical and solution properties of pyrene-labeled 23c were studied in terms of fluorescence emission. Addition of surfactants induces the formation of mixed micelles in aqueous solution [61]. Excluded volume effects of the unlabeled polymer were measured by light scattering [62], its adsorption on silica was studied by adsorbance measurement and ellipsometry [62,63], and the electrostimulated shift of the precipitation temperature was followed at various electric held intensities [64]. Polysulfobetaines may accelerate interionic reactions, e.g., oxidation of ferrocyanide by persulfate [65]. The thermal and dielectric properties of polysulfobetaines 23d were investigated. The flexible lateral chain of the polymers decreased Tg, for which a linear relationship with the number of C atoms was shown [66,67]. 

Fig. 6. Schematic view of the formation process of metallic nanoshells around organic micelles in aqueous solution. °> ...

In this chapter we will see how the surface activity of a molecule is related to its molecular structure and look at the properties of some surfactants which are commonly used in pharmacy. We will examine the nature and properties of films formed when water-soluble surfactants accumulate spontaneously at liquid/air interfaces and when insoluble surfactants are spread over the surface of a liquid to form a monolayer. We will look at some of the factors that influence adsorption onto solid surfaces and how experimental data from adsorption experiments may be analysed to gain information on the process of adsorption. An interesting and useful property of surfactants is that they may form aggregates or micelles in aqueous solutions when their concentration exceeds a critical concentration. We will examine why this should be so and some of the factors that influence micelle formation. The ability of micelles to solubilise water-insoluble drugs has obvious pharmaceutical importance and the process of solubilisation and its applications will be examined in some detail. 

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