Solubility, micelle-forming surfactants

Micelle-forming surfactants exhibit another unusual phenomenon in that their solubilities show a rapid increase above a certain temperature, known as the Krafft point. The explanation of this behaviour arises from the fact that unassociated surfactant has a limited solubility, whereas the micelles are highly soluble. Below the Krafft temperature the solubility of the surfactant is insufficient for micellisation. As the temperature is raised, the solubility slowly increases until, at the Krafft temperature, the c.m.c. is reached. A relatively large amount of surfactant can now be dispersed in the form of micelles, so that a large increase in solubility is observed. 

The solubilities of micelle-forming surfactants show a strong increase above a certain temperature, termed the Krafft point (Tk). This is explained by the fact that the single surfactant molecules have limited solubility whereas the micelles are very soluble. Referring to Figure 3.22, below the Krafft point the solubility of the surfactant... 

It is also noteworthy that micelle-forming surfactants may solubilize organic compounds sometimes in a very low concentration of the surfactant (still above the CMC). This embedding depends on the charge of surfactant and the charge of reactant. Only hydrophobic reactants may permeate into the hydrophobic core. Important for good solubilization properties is the hydrophile-lipophile balance (HLB) of the surfactant because sufficient water-solubility is required [12] (cf. Sec-... 

The surface tension at the water-hydrocarbon interface may also drop to very low values when one introduces micelle-forming surfactants and surfactant mixtures, especially those that are oil- and water-soluble. The possibility to reach very low values of surface tension is a significant feature of liquid-liquid interfaces, which accounts for a substantial difference between such interfaces and gas-liquid and gas-solid ones, at which, even after reaching limiting values of adsorption, the surface tension remains high, i.e. the system remains lyophobic (See Chapter IV). 

The amount of substance present in the micellar state, cmjc = mnmic / NA may exceed the concentration of it in the molecular solution by several orders of magnitude. The micelles thus play a role of a reservoir (a depot) which allows one to keep the surfactant concentration (and chemical potential) in solution constant, in cases when surfactant is consumed, e.g. in the processes of sol, emulsion and suspension stabilization in detergent formulations, etc. (see Chapter VIII). A combination of high surface activity with the possibility for one to prepare micellar surfactant solutions with high substance content (despite the low true solubility of surfactants) allows for a the broad use of micelle-forming surfactants in various applications. 

The solubilities of micelle-forming surfactants show a strong increase above a certain temperature, termed the Krafft point (Tk). This increased solubility is explained by the fact that the single surfactant molecule has limited solubility, whereas the micelles are very soluble. As shown in Figure 6, at temperatures below the Krafft point, the solubility of the surfactant is too low for micellization, and solubility alone determines the surfactant—monomer concentration. As temperature increases, the solubility increases until at Tk the CMC is reached. At this temperature, a relatively large amount of surfactant can be dispersed in micelles, and solubility increases greatly. Above the Krafft point, maximum reduction in surface or interfacial tension occurs at the CMC, because now the CMC determines the surfactant—monomer concentration. Krafft points for a number of surfactants are listed in reference 11. 

The emulsion polymerization technique usually contains a micelle-forming surfactant and a water-soluble initiator in combination with a water-insoluble monomer. Polymerization takes place in the monomer-swollen micelles and latex particles. Therefore, the term emulsion polymerization is a misnomer the starting point is an emulsion of monomer droplets in water, and the product is a dispersion of latex particles. In the case of microemulsion polymerization, the monomer droplets are made very small (typical particle radius is 10-30 nm) and they become the locus of polymerization. In order to obtain such small droplets, a co-surfactant (e.g. hexanol) is usually applied. A microemulsion is thermodynamically stable... 

Figure 6 The temperature-dependent solubility of a micelle-forming surfactant (see text).

An emulsion polymerization system can comprise three phases (1) an aqueous phase, containing the water-soluble initiator, the micelle-forming surfactant, and a small amount of the sparingly soluble monomer (2) monomer droplets and (3) latex particles, consisting of the polymer and some monomer. The locus of polymerization is predominantly inside the latex particles. Usual free-radical water-soluble initiators are used, such as potassium persulfate for higher reaction... 

Both emulsion and suspension polymerization are heterogeneous polymerization techniques, in which the polymerization occurs in the monomer-swollen latexes and monomer droplets dispersed in a continuous liquid phase, usually water, respectively. The emulsion polymerization technique involves a water-soluble initiator, a water-insoluble monomer and a micelle-forming surfactant. The monomer is emulsified and stabilized in a continuous aqueous phase by the... 

Micelles only form above a crucial temperature known as the Krafft point temperature (also called the Krafft boundary or just Krafft temperature). Below the Krafft temperature, the solubility of the surfactant is too low to form micelles. As the temperature rises, the solubility increases slowly until, at the Krafft temperature 7k, the solubility of the surfactant is the same as the CMC. A relatively large amount of surfactant is then dispersed into solution in the form of micelles, causing a large increase in the solubility. For this reason, IUPAC defines the Krafft point as the temperature (or, more accurately, the narrow temperature range) above which the solubility of a surfactant rises sharply. 

Krafft point (for ionic surfactants) and cloud point (for nonionic surfactants) are both a limit to surfactant solubility. The solubility of ionic surfactants decreases significantly below the Krafft point, since its concentration falls below the CMC and individual surfactant molecules cannot form micelles. Therefore, the Krafft point of an ionic surfactant must be below the desired wash temperature for maximum soil removal. In contrast, the solubility of some nonionic surfactants decreases with increasing temperature. Above the cloud point, the surfactant becomes insoluble. Thus, the cloud point of a nonionic surfactant should be 15-30°C above the intended wash temperature [8],... 

In the one-phase reaction, complete conversion and ee values of about 72% were reached. In the biphasic system, the rhodium complex of the surfactant ligand 12 showed considerably higher activity than in the one-phase system, while retaining enantioselectivity (96). These results agree with results of earlier work that micelle-forming ligands enhance the solubility of lipophilic reactants in water. 

Micelles are colloidal dispersions that form spontaneously, under certain concentrations, from amphiphilic or surface-active agents (surfactants), molecules of which consist of two distinct regions with opposite afL nities toward a given solvent such as water (Torchilin, 2007). Micelles form when the concentration of these amphiphiles is above the critical micelle concentration (CMC). They consist of an inner core of assembled hydrophobic segments and an outer hydrophilic shell serving as a stabilizing interface between the hydrophobic core and the external aqueous environment. Micelles solubilize molecules of poorly soluble nonpolar pharmaceuticals within the micelle core, while polar molecules could be adsorbed on the micelle surface, and substances with intermediate polarity distributed along surfactant molecules in intermediate positions. 

The temperature (in practice a narrow range of temperatures) above which the solubility of a surfactant increases sharply (micelles begin to be formed). Below the Krafft point only single, unassociated surfactant molecules (monomers) or ions (ionomers) can be present, up to a given solubility limit. Above the Krafft point, a solution can contain micelles and thus allow much more surfactant to remain in solution in preference to precipitating. In the soap industry the Krafft point is sometimes defined as the temperature at which a transparent soap solution becomes cloudy upon cooling. 

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