Krafft points

The solubility of surfactants in water increases gradually with increasing temperature until, at the ICrafft point, the solubility increases abruptly [83,84]. The Krafft point is the temperature at which the solubility of monomeric surfactant molecules is equal to the cmc at that temperature. The Krafft point can also be defined as the temperature at which the solubility versus temperature curve intersects the cmc 

Shinoda and Hutchinson treated the micelle as a separate phase [41,45.87] and proposed that the Krafft point is the temperature above which the solid hydrated surfactant melts and dissolves as micelles in water [88,89]. In a phase diagram of an aqueous surfactant, the Krafft point is the triple point at which monomolecular surfactant coexists with micelles and the solid surfactant [42]. At the Krafft point, micelles are in equilibrium with monomeric surfactant molecules 

The mass action model [Eqs. (1) and (2)] gives two degrees of freedom at the Ki afft point, because a solution phase is in equilibrium with a solid [93]. The number of components is three water, molecularly dissolved surfactant, and micelles  

In spite of some complications, the mass action model serves as a useful approximation for describing micellization processes. 

Branching of the carbon chain lowers the Krafft point [59]. A moderately branched surfactant has a lower Krafft point and melting point than a straight-chain surfactant (Table 6.4). 

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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]. 

FIG. 2 Krafft points 7" of hqucous surfactant solutions vs. mixing ratio of c16/c, a-ester sulfonates. (From Ref. 58.)... 

Sodium a-sulfonated fatty acid esters of long-chain alcohols have a structural effect on the Krafft point different from that of amphiphiles with short alkyl chains [60]. In a series of homologs with the same total carbon number the Krafft points are highest when the hydrophilic alkyl chain lengths in the a-sulfonated fatty acid and the alcohol are fairly long and equal. In this case the packing of the molecules becomes close and tight. 

The Krafft point, i.e., the aqueous solubility, also depends greatly on the type of counterion [30,60,61]. For example, potassium salts have higher Krafft points than sodium or ammonium salts. Bivalent cations, like calcium and magnesium, raise the Krafft point. The rise is smaller for ester sulfonates than for alkyl sulfates [61]. 

Weil and coworkers [34] studied the Krafft point behavior of soap-LSDA mixtures. They were able to show that soap and LSDA solubilize each other. 

Addition of as little as 5% soap to an amphoteric LSDA of limited water solubility (high Krafft point) brought about a substantial lowering of the Krafft point and thus markedly improved water solubility. On the other hand, addition of 10% of amphoteric LSDA to sodium palmitate lowered the Krafft point of the soap by 10-14°C. Addition of anionic LSDA to sodium palmitate showed smaller Krafft point depression. Addition of a builder-type salt, such as sodium metasilicate, had essentially no effect on the Krafft points of soap-LSDA mixtures. 

In another study of the physical behavior of soap-LSDA blends, Weil and Linfield [35] showed that the mechanism of action of such mixtures is based on a close association between the two components. In deionized water this association is mixed micellar. Surface tension curves confirm the presence of mixed micelles in deionized water and show a combination of optimum surface active properties, such as low CMC, high surface concentration, and low surface concentration above the CMC. Solubilization of high Krafft point soap by an LSDA and of a difficulty soluble LSDA by soap are related results of this association. Analysis of dispersions of soap-LSDA mixtures in hard water shows that the dispersed particles are mixtures of soap and LSDA in the same proportion as they were originally added. These findings are inconsistent with the view that soap reacts separately with hard water ions and that the resulting lime soap is suspended by surface adsorption of LSDA. The suspended particles are responsible for surface-active properties and detergency and do not permit deposits on washed fabric unlike those found after washing with soap alone. 

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. 

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). 

Table 10.4 Krafft point temperatures8 for sodium alkyl sulphates in water...

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). 

Domain Where Physics, Chemistry, Biology, and Technology Meet (see above) p. 11. The paper, Use of quantitative structure-property relationships in predicting the Krafft point of anionic surfactants by M. Jalali-Heravi and E. Konouz, Internet Electronic Journal of Molecular Design, 2002, 1, 410, has a nice introduction and useful references. It can be downloaded at http //www.biochempress.com/av01 0410.html. 

In the process of realizing product quality factors by changing product formulation, the relevant performance indices have to be determined. The determination process in turn requires experience and technical expertise. For detergent products the performance indices need to be considered include (1) optimum hydrophilic-lipophilic balance, HLB0p (2) critical micelle concentration, CMC (3) soil solubilization capacity, S (4) Krafft point,... 

Performance Indices Quality Factors Optimum E1LB Critical micelle concentration (CMC) Soil solubilization capacity Krafft point (ionic surfactants only) Cloud point (nonionic surfactants only) Viscosity Calcium binding capacity Surface tension reduction at CMC Dissolution time Material and/or structural attributes... 

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],... 

Krafft point, Tyaff, Surfactant chain length, l Increases with increasing l... 

Alkyl ether sulfates are/after alkyl benzene sulfonates(LAS),the group of technically important anionic surfactants with the largest production voluJne and product value. They have in comparison with other anionic surfactants special properties which are based on the particular structure of the molecule. These are expressed,for example,in the general adsorption properties at different interfaces, and in the Krafft-Point. Alkyl ether sulfates may be used under conditions, at which the utilization of other surfactant classes is very limited. They possess particularly favorable interfacial and application properties in mixtures with other surfactants. The paper gives a review of all important mechanisms of action and properties of interest for application. 

The great technical and economic Tmportance of this product group was reached despite its higher price only because of its special properties. Due to the ionic sulfate group and the adjacent ether groups, ether sulfates combine the classical elements of ionic and nonionic surfactants in one molecule. This provides a number of properties, one of which, the Krafft-Point, is of special importance for the technical application of these compounds. 

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 (, ). 

Below the Krafft Point, the surfactant dissolved in a molecularly dispersed manner until the saturation concentration is reached. At higher concentrations, a hydrated solid is in equilibrium with individual molecules. Above the Krafft Pointy the hydrated solid is in equilibrium with micelles and individual molecules. 

Therefore, the physical meaning of the solubility curve of a surfactant is different from that of ordinary substances. Above the critical micelle concentration the thermodynamic functions, for example, the partial molar free energy, the activity, the enthalpy, remain more or less constant. For that reason, micelle formation can be considered as the formation of a new phase. Therefore, the Krafft Point depends on a complicated three phase equilibrium. 

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. 

The lower Krafft Points resulting from the incorporation of oxyethylene groups into the surfactant molecule is an essential, but not sufficient, property for the utilizationof alkyl ether sulfates. 

Several variations in chemical constitution, which lead to a depression of the Krafft-Point (for example, branching of the hydrophobic part of the molecule), frequently result in diminished hydrophobicity of the molecule. At constant molecular weight, the critical micelle concentration (Cj.) is shifted with increased branching to higher concentrations, the surface activity diminishes, the tendency to adsorb at hydrophobic interfaces decreases, etc. (j, 14, 15). Therefore, the nature of the oxyethylene groups in aTkyl ether sulfates is of major importance. 

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

In this system, in the aqueous phase, the micellar system, NaDDS, on addition of butanol would change in free energy due to mixed micelle formation (i. je. NaDDS+n-Butanol), as we showed in an earlier study (23). The cahnge in free energy is also observed from the fact that both the critical micelle concentration (c.m.c.) and the Krafft point of NaDDS solution change on addition of n-Butanol (23,... 

Further, it is well known that the addition of electrolytes to ionic surfactant aqueous solutions increase the Krafft point (24,... 

This indicates that as one increases the NaCl concentration, the Krafft point is most likely higher than the experimental tem-... 

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