Krafft point range

The solubility of the surfactant in decane is also quite small at 25°C, about 0.04 wt%, but over a narrow temperature range around 50°C it rises dramatically, as in the Krafft point range of a single-chain surfactant in water (11a). Such a phenomenon with a surfactant in a nonpolar solvent is not uncommon (35). Incidentally, the absence of a Krafft point range for the surfactant in water between 10 and 90°C argues for the absence of micelles in solution. Abrupt change in the slope of such a property as surface tension versus concentration (9) can be due to precipitation of a new phase as well as to onset of appreciable micelle formation, and so does not constitute conclusive evidence for the latter. 

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. 

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. 

Micelles are loose aggregates of amphiphiles in water or organic solvents which form above a certain temperature (Krafft point) and concentration (critical micellar concentration, cmc). Below the Krafft temperature, clear micellar solutions become turbid and the amphiphile forms three-dimensional hydrated crystals. Below the cmc, micelles dissociate into monomers and small aggregates. Above the cmc, the micelles of an aggregation number n are formed n then remains stable over a wide concentration range . Table 1 gives some typical cmcs and three Krafft point values. 

They are very stable in a large temperature range, usually from the Krafft point up to the boiling point. Moreover, the phase boundaries are almost insensitive to temperature. 

These are of two kinds related to each other by the difference in association structure as illustrated by the temperature variation of surfactant solubility and association. Figure 6 provides a schematic description of the interdependence. At low temperatures the solubility limit of the xmimers (s, solid line. Fig. 6) is lower than the limit for amphiphilic association (cmc, dashed line. Fig. 6), and, hence, the latter is not reached and a two-phase equilibrium, aqueous solution of monomers—hydrated surfactant, is established. At temperatures in excess of the Krafft point, Tj (Fig. 6), the association concentration (cmc, solid line, Fig. 6), is now beneath the solubility limit (s, dashed line. Fig. 6). Association takes place and the total solubility (ts. Fig. 6) is drastically increased. Hence, the water—siufactant phase diagram shows a large solubility range for the isotropic liquid solution (unimers plus micelles. Fig. 6) because the association structure, the micelle, is soluble in water. This behavior is characteristic of smfactants with Ninham R values less than 0.5. 

The other group, those with R values in the range 0.5— 1.0, also associate at temperatures in excess of the Krafft point, but the molecules are now not spherically packed but rather close to parallel. As a consequence, there is no limit to the size of the association structure, as in the spherical micelles, and a phase separation occurs to form a lamellar liquid crystal. The principle features of the phase diagram in Fig. 6 remain flie Krafft point marks the intersection of... 

A number of studies have been reported concerning the solution behavior of some divalent earboxylate complexes ((91) M = Zn, Mn, Pb, Hg, Cd). Krafft points (the Krafft point is the temperature at which micelles become soluble) and critical micelle concentration (cmc) were determined in a variety of long-chain alcohols, and for different earboxylate chain lengths. However, the demonstration of their lyotropic mesomorphism was not clear and it appeared that lamellar phases were induced, although over small ranges of temperature and concentration. 

Because of their proclivity to associate in solution, it is to be expected that surfactants and polymers can influence each other s solubility. There is abundant evidence of the ability of a surfactant to increase the range of solubility of polymers in water (see the next section). Less well known is the ability of a water-soluble polymer to increase the solubility range of a surfactant Schwuger and Lange (103), for example, reported that polyvinylpyrrolidone can reduce the Krafft point of sodium hexadecyl sulfate by close to 10°C—an effect evidently linked to a lowering of the monomer concentration required for aggregation in the presence of this polymer. 

In distilled water at pH in the range 2-9, all three types of lipopeptides displayed good solubility at room temperature (0.5-25%, w/v) except for Ci2ArgGlu (fully soluble only at pH 2.0) and CiaArg aa, which, because of its very hydrophobic character, was soluble only at pH 9.0. Ci2ArgGly and Ci2ArgPhe showed Krafft points around 0°C. All the synthetic lipopeptides were readily soluble in methanol and ethanol at room temperature. 

The c.m.c. is merely a concentration range above which any added surfactant appears in solution in a micellar form. Since the solubility of the associated surfactant is much greater than that of the monomeric surfactant, the solubility of the surfactant as a whole will not increase markedly with temperature until it reaches the c.m.c. region. Thus, in the mass action approach, the Krafft point represents the temperature at which the surfactant solubility equals the c.m.c. 

The solubility of ionic surfactants is strongly dependent on temperature. The solubility is often very low at low temperatures but increases rapidly in a narrow range as the temperature increases. The point at which the solubility curve meets the critical micelle concentration curve is termed the Krafft point, which defines the Krafft temperature (Fig. 4.17). The dramatic increase in solubility on increasing temperature above the Krafft temperature is due to an interplay between the temperature-dependent solubility of amphiphilic molecules and the temperature dependence of the CMC. The latter is generally very weak. If the amphiphile solution is below the CMC then the... 

In the previous chapters, the dissolution and micellization of surfactants in aqueous solutions were discussed from the standpoint of the degrees of freedom as given by the phase rule. The mass-action model for micelle formation was found to be better for explaining the phenomena of surfactant solutions than the phase-separation model. Two models have similarly been used to explain the Krafft point, one postulating a phase transition at the Krafft point and the other a solubility increase up to the CMC at the Krafft point. The most recent version of the first approach is a melting-point model for a hydrated surfactant solid. The most direct approach to the second model of the Krafft point rests entirely on measurements of the solubility and CMC of surfactants with temperature. From these mesurements the concept of the Krafft point can be made clear. This chapter first reviews the concepts used to relate the dissolution of surfactants to their micellization, and then shows that the concept of a micelle temperature range (MTR) can be used to elucidate various phenomena concerning dissolution... 

As can be seen, concepts of the Krafft point fall ito two different categories those involving a phase transition of solid surfactant and those involving a solubility increase up to the CMC. The two views are incompatible according to the former, the Krafft point is a definite temperature (a point), whereas according to the latter, it is a narrow temperature range. This conflict is addressed in the following sections. 

It can be concluded that the Krafft point is the temperature at which the solubility of surfactants as monomers becomes high enough for the monomers to commence aggregation or micellization. Recall from Chapter 4 that the CMC depends on the method used for its determination, and that the CMC value should therefore be defined as a narrow temperature range even though the solubility is definitely determined by temperature... 

The term micelle temperature range expresses the relation between solubility and micelle formation better than Krafft point... 

The Krafft point increases with increasing Tanaka pressure (Fig. 6.30). Sodium perfluorodecanoate (SPFDe) has the highest Krafft point of the surfactants included in Fig. 6.29 at any pressure applied at a constant temperature, such as 50°C. Hence, the range where micelles can exist is narrower for sodium perfluorodecanoate than for the other surfactants shown. 

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