Temperature Krafft

Soap Bars. In soap bars the primary surfactant is predominantly sodium salts of fatty acids. These products typically contain between 70 and 85% soap. Occasionally, potassium soap ( 5-30%) is included in the formulation to increase the solubiUty of the soap and, hence, the bar s lathering properties. The low Krafft temperatures for potassium soap are the basis for the lather enhancement, but also limits their content in bars. 

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 ... 

The curve shown in Fig. 6 for sodium dodecyl sulfate is characteristic of ionic surfactants, which present a discontinuous and sharp increase of solubility at a particular temperature [80]. This temperature is known as the Krafft temperature. The Krafft temperature is defined by ISO as the temperature [in practice, a narrow range of temperatures] at which the solubility of ionic surface active agents rises sharply. At this temperature the solubility becomes equal to the critical micelle concentration (cmc). The curve of solubility vs. temperature intersects with the curve of the CMC vs. temperature at the Krafft temperature. 

The Krafft temperature for a homologous series rises as the chain length increases but with independent and different curves for even or odd members [80]. Figure 7 shows the Krafft temperatures of sodium alcohol sulfates ranging from 11 to 18 carbon atoms where the two different curves for odd- and even-numbered series and the alternancy of values can be clearly observed. Lange and Schwuger explain this effect as caused by the different crystal structures of the even and odd numbers. 

Krafft temperatures depend not only on chain length but on the cation. Eth-oxylation of the base alcohol reduces the Krafft temperature due to the higher solubility of the sulfate. Calcium and other earth alkaline metals produce an increase of the Krafft temperature that is significantly reduced by ethoxylation of the alcohol. The decrease is more significant for alkaline earth metals than for alkaline cations as shown in Table 6 [81,82], although it should be noted that, according to other workers, sodium dodecyl sulfate has a Krafft temperature of 16°C. 

Different authors sometimes report different values, as commonly happens with many other physical properties, which may be explained by the purity of the product. Mixtures of isomeric surfactants generally have Krafft temperatures... 

FIG. 6 Solubility of sodium dodecyl sulfate vs. temperature. Krafft temperature [80]. 

TABLE 6 Krafft Temperatures (°C) of Dodecyl Ether Sulfates... 

TABLE 7 Krafft Temperatures of Sodium Alcohol and Alcohol Ether Sulfates... 

Table 6 shows the Krafft temperature of C18 AOS (about 70% alkenesulfonate and 30% hydroxyalkanesulfonate) and reference compounds in water. Although C18 AOS is a mixture, its Krafft temperature is clearly defined [41] and found to be 23-24°C. The Krafft temperatures of the pure main components, alkene-and hydroxyalkanesulfonates are significantly higher at 54 and 51 °C the value for the alkenesulfonate is in line with a Krafft temperature of 57 °C for octade-canesulfonate. 

The low Krafft temperature of C18 AOS may then be explained by the presence in the mixture of small amounts of highly water-soluble isomers and disulfonates [41]. 

AOS is a useful surfactant system for the formulation of soap bars. The effect of AOS on the Krafft temperature of soap is shown in Table 27 AOS lowers the Krafft temperature of soap. AOS can also be used to cosolubilize soap in water thereby reducing the waste of insoluble soap as shown in Table 28. 

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. 

Figure 10.14 Graph of [surfactant] (as y ) against T (as V) for sodium decyl sulphate in water. The Krafft temperature is determined as the intersection between the solubility and CMC curves, yielding a /K of about 22 °C. At lower temperatures, the micelles convert to form hydrated crystals, which we might call scum (Reproduced by permission of Wiley Interscience, from The Colloidal Domain by D. Fennell Evans and Hakan Wennerstrom)...

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. 

With short chain derivatives, the forces of repulsion are higher than the ones of attraction the curvature is high and spherical micelles are formed at a concentration called the critical micellar concentration (cmc). This concentration can be detected by a change in the physico-chemical properties of the solution (e.g. surface tension, Fig. 3 a). Above a characteristic temperature (referred as Krafft temperature), the tensio-active molecules are infinitely soluble in the form of micelles (Fig. 3 b). 

Fig. 3. Schematic diagrams a surface tension versus concentration of a surfactant, b phase diagram of a surfactant near the Krafft temperature, c phase diagram of 3-0-dodecyl-D-glucitol [11]...

The liquids to be studied in this experiment are water, hexane, n-octanol and aqueous solutions of CTAB. It is recommended that they be measured in the order written, where the most critical with respect to contamination is first. The water used should be the best available, such as double distilled, and should be stored in a sealed flask before use. Pure samples of the other liquids should also be used as well as top-quality water to make up the CTAB solutions. The CTAB solutions should be measured at concentrations of 0.01, 0.1, 0.3, 0.6, 1 and 10 mM at a temperature above 21°C. CTAB has a Krafft temperature around 20°C - below this temperature the surfactant will precipitate from aqueous solution at the higher concentrations (see later). 

Ionic surfactants actually only form micelles when their hydrocarbon chains are sufficiently fluid, that is at temperatures above their chain melting temperature. Below a specific temperature for a given surfactant, the Krafft temperature, the surfactant becomes insoluble rather than self-assembles. For CTAB this temperature is around 20 °C and only above this temperature are micelles formed. In general, the longer the hydrocarbon chain length, the higher the Krafft temperature. For this reason, shorter-chain-length surfactants or branched chain soaps... 

Precipitation. When two surfactants of like charge are mixed together, and the structures are very similar (e.g., close members of an homologous series) and containing the same counterion, the Krafft temperature of the mixture is intermediate between the pure components or shows only a very shallow minimum (86.87) The crystals are mixed (contain both surfactants) in this case (86.87). 

If the surfactants have a more dissimilar structure or if the counterion is different with the same surfactant ion (e.g., sodium dodecyl sulfate and calcium dodecyl sulfate), the Krafft temperature of the mixture can be much less than either pure component (87—89) These systems show the classical eutectic type behavior and the crystals contain only one kind of surfactant or counterion in substantial amounts (87-89). 

We may consider precipitation in these systems in the context of competitive aggregate formation between micelles and precipitate. Even systems forming ideal mixed micelles can exhibit synergisms in salinity/hardness tolerance in such systems, the more components present, the higher the tolerance. This is the reason that mixtures of isomeric surfactants generally have Krafft temperatures considerably lower than those of the individual compounds (90). 

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. 

Nonionic surfactants tend to show the opposite temperature effect As the temperature is raised, a point may be reached at which large aggregates precipitate out into a distinct phase. The temperature at which this happens is referred to as the cloud point. It is usually less sharp than the Krafft temperature.2 The phenomenon that nonionic surfactants become less soluble at elevated temperature will be important when we discuss the phase behavior of emulsions. 

Generally, the phase diagram of surfactants has a triple point at which the solid, the dissolved, and the micellar phase coexist. The temperature at this triple point is the Krafft temperature (Tk). For most surfactants, the TK is below ambient temperature and, therefore, their CMC can be regarded as their molecular solubility. Everything said above about the CMC therefore holds true for aqueous solubility. However, TK of the cationic surfactants used in fabric softeners is above ambient temperature. The same is true for the individual 2-n-(p-sulfophenyl)-alkanes which are constituents of LAS. 

The hydrophilic part of the most effective soluble surfactants (e.g. soaps, synthetic detergents and dyestuffs) is often an ionic group. Ions have a strong affinity for water owing to their electrostatic attraction to the water dipoles and are capable of pulling fairly long hydrocarbon chains into solution with them for example, palmitic acid, which is virtually un-ionised, is insoluble in water, whereas sodium palmitate, which is almost completely ionised, is soluble (especially above its Krafft temperature - see page 93). 

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. 

Table 4.5 Krafft temperatures for sodium alkyl sulphates in water...

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