Krafft point/temperature

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. Temperature (°C) at which cogel or crystal formation takes place in a soap solution and produces opacity. 

Micelles can only form when the surfactant solubility is equal to or greater than the CMC. In general this occurs only above a particular temperature known as the Krafft point (temperature). Below this temperature surfactant solubility increases slowly with increasing temperature because the surfactant dissolves as monomers. The limit to monomer solubility occurs when the chemical potential of the monomers is equal to that of the pure (usually crystalline) surfactant. Above this temperature the solubility increases very rapidly because the surfactant dissolves as micelles the contribution of each micelle to the surfactant chemical potential being the same as that of a monomer. 

Figure 5.9 General phase diagram of a surfactant solution, showing the CMC line, the Krafft point (temperature) and the lower consolute point (or lower critical temperature). As can be seen, the phase behaviour of aqueous surfactant solutions is rather complex and various phases are distinguished. At high concentrations, we can see various special surfactant phases (hexagonal, lamellar, cubic). These are called liquid crystalline phases and although there are 18 different types, the three mentioned are the most important. Many of these complex structures have found exciting applications (e.g. liquid crystal displays and study of biological membranes)...

The Krafft point has practical implications for the solubility of surfactants. Only above the Krafft temperature can concentrated surfactant solutions be prepared. Otherwise, on cooling a hot surfactant solution a sudden precipitation may occur. A linear correlation between the Krafft temperature TK (°C) and the carbon number nc of sodium alkanesulfonates C10-C22 is given by the following equation ... 

For the disodium salts of the a-sulfo fatty acids it was found that the Krafft points are higher than those of the a-ester sulfonates. Because the CMC is also higher for the disalts they are more soluble than ester sulfonates at low temperatures but less soluble at higher temperatures [58]. 

In reverse, the surfactant precipitates from solution as a hydrated crystal at temperatures below 7k, rather than forming micelles. For this reason, below about 20 °C, the micelles precipitate from solution and (being less dense than water) accumulate on the surface of the washing bowl. We say the water and micelle phases are immiscible. The oils re-enter solution when the water is re-heated above the Krafft point, causing the oily scum to peptize. The way the micelle s solubility depends on temperature is depicted in Figure 10.14, which shows a graph of [sodium decyl sulphate] in water (as y ) against temperature (as V). 

The value of TK is best determined by warming a dilute solution of surfactant, and noting the temperature at which it becomes clear. Table 10.4 lists the Krafft points for a series of colloidal systems based on aqueous solutions of sodium alkyl sulphate (cf. structure III). 

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],... 

The Krafft Point may be defined as the temperature above which the solubility of a surfactant increases steeply. At this temperature, the solubility of the surfactant becomes equal to the critical micelle concentration (Cj ) of the surfactant. Therefore, surfactant micelles only exist at temperatures above the Krafft Point. This point is a triple point at which the surfactant coexists in the monomeric, the micellar, and the hydrated solid state (, ). 

An especially effective reduction of the Krafft Point results from the insertion of ether groups into the molecule of the anionic surfactant. In table I this is examplified with Na dodecyl sulfate and Na-tetra-decyl sulfate in comparison to various n-alkyl ether sulfates of the same chain length (10). As a measure of the Krafft Point, a temperature is deTined at which a 1 7o solution dissolves clearly. By the incorporation of oxyalkylene groups into the molecule, the Krafft -Point and the melting point are greatly depressed. This depression is especially effective if there is branching in the oxyalkylene groups. 

From these observations we will argue that the transition from the three-phase to the two-phase region with increasing NaCl takes place where the Krafft point is most likely higher than the experimental temperature (24). 

The temperature, abbreviated c.m.t., at which a deter-gent/solvent system or a lipid/solvent system passes from a hydrated crystalline state to an isotropic micellar solution. For a number of lipids, the c.m.t. is below the freezing point of the solvent. The Krafft point,, is the c.m.t. at the critical micelle concentration. 

FIGURE 3.6 Solubility (Krafft point KP) of ionic (anionic or cationic) surfactants in water (as a function of temperature). 

Carrying out an emulsion polymerization requires an awareness of the krafft point of an ionic surfactant and the cloud point of a nonionic surfactant. Micelles are formed only at temperatures above the Krafft point of an ionic surfactant. For a nonionic surfactant, micelles are formed only at temperatures below the cloud point. Emulsion polymerization is carried out below the cloud temperature of a nonionic surfactant and above the Krafft temperature of an ionic surfactant. 

The state of the hydrocarbon chains in mesophases and micelles is reflected in the Krafft phenomena. In aqueous solutions of surfactants the Krafft point is defined as the temperature at which the solubility reaches the critical micelle concentration when the temperature is increased further, the solubility rises rapidly since the monomers form micelles (Figure 5) (10). Lipids that do not form micelles frequently start to swell by the uptake of water at a well-defined temperature they are transformed into a mesomorphous state (Figure 6) (11) The relation between these two Krafft phenomena is explained to some extent by the... 

For ionic surfactants micellization is surprisingly little affected by temperature considering that it is an aggregation process later we see that salt has a much stronger influence. Only if the solution is cooled below a certain temperature does the surfactant precipitate as hydrated crystals or a liquid crystalline phase (Fig. 12.4). This leads us to the Krafft temperature1 also called Krafft point [526]. The Krafft temperature is the point at which surfactant solubility equals the critical micelle concentration. Below the Krafft temperature the solubility is quite low and the solution appears to contain no micelles. Surfactants are usually significantly less effective in most applications below the Krafft temperature. Above the Krafft temperature, micelle formation becomes possible and the solubility increases rapidly. 

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. 

Fig. 2.8. Temperature dependence of surfactant solubility in the region of the Krafft point. (From Ref.2 )...

There exists, actually, another aspect regarding a temperature variation of surfactant solutions the well-known Krafft-point determination128). Since, however, not a micellar property is concerned but the temperature dependence of the monomer activity of the soap molecules, this section is considered more as an appendix to the foregoing discussion. 

Another important transition of surfactants involving micelles, the critical micellization temperature (CMT), has been found to be readily amenable to study by FT-IR, largely because of the relatively high surfactant concentrations involved (>0.1 M). The CMT is concentration dependent up to concentrations of about 0.1 to 0.3 M, above which the dependence decreases significantly. The Krafft point is thus found at lower temperatures than the CMT, and can be considered the CMT at the cmc (63-65). A thermostatted transmission cell for control of the temperature of the surfactant solutions, held between CaF2 or BaF2 windows, is necessary. Automation of the entire spectroscopic CMT experiment has been described (66). 

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... 

Non-ionic surfactants do not exhibit Krafft points. Rather the solubility of nonionic surfactants decreases with increasing temperature and the surfactants begin to lose their surface active properties above a transition temperature referred to as the cloud point. This occurs because above the cloud point a separate surfactant-rich phase of swollen micelles separates the transition is visible as a marked increase in dispersion turbidity. As a result, the foaming ability of, for example, polyoxyethyle-nated non-ionics, decreases sharply above their cloud points. The addition of electro-... 

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. 

In MEKC, an ionic surfactant is used as a pseudo-stationary phase, and the Krafft point is also an important temperature. At a temperatures lower than the Krafft point, C f does not exceed the CMC, due to reduced solubility and, therefore, no micelle is formed. At the Krafft point, C f reaches the CMC and then the formation of the micelle is begun. The Krafft point of SDS is 16°C in a pure water, whereas it is 31°C for potassium dodecyl sulfate in pure water. Thus, a potassium salt is not an adequate buffer component for the SDS-MEKC system. 

Fig. 9 is a schematic phase diagram of a dilute aqueous cationic surfactant solution showing temperature and concentration effects on its microstructures. When the temperature is lower than the Krafft point [the temperature at which the solubility equals the critical micelle concentration (CMC)], the surfactant is partially in crystal or in gel form in the solution. At temperatures above the Krafft point and concentrations higher than the CMC, spherical micelles form in the surfactant solution. With further increase in concentration and/or on addition of counterions, the micelles form cylindrical rods or threads or worms with entangled thread-like and sometimes branched threadlike structures. 

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