Krafft boundary

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. 

C). Similarly, above 72% surfactant the Vi phase melts to an La lamellar phase. At 79%, surfactant the La phase forms at 90°C. Note that the Krafft boundary has a rather large slope. Usually for monovalent surfactants, the Krafft boundary is flat or has only a small slope (50). 

A specific example, using model surfactant structures, was studied in the P G laboratories. Snoody found that the Krafft boundary could be altered dramatically by addition of a small methyl substituent on the chain of a methyl-substituted octadecylsulfate. P G has published some work on mixtures of midchain methyl-substituted alcohol sulfate surfactants. The resulting increased solubility of these alcohol sulfates called highly soluble alcohol sulfates (HSAS) allows use of long-chain alcohols, such as in laundry detergents under today s cold and hard water environments. The solubility effect is maximized when the methyl substitution is away from the end of the surfactant chain as shown in Figure 6.13. 

At a particular temperature, the solubility becomes equal to the c.m.c., i.e. the solubility curve intersects the c.m.c. and this temperature is referred to as the KrafFt temperature of the surfactant, which for sodium decyl sulphate is 22 °C. At the KrafFt temperature an equilibrium exists between solid hydrated surfactant, micelles and monomers (i.e. the KrafFt point is a triple-point ). Since the KrafFt boundary represents the region below which crystals separate, the energy of the... 

FIG. 8 The C MG-water phase diagram obtained via DIT-NDX studies. Calibration was carried out with a 49.5% liquid phase using data obtained at temperatures above the Krafft boundary, and adjusted on the basis of calorimetric data. Data on the temperatures of isothermal discontinuities obtained by calorimetric studies are indicated by solid squares. 

Five phases were found to exist between 25 °C and 130°C in the equilibrium Ci2MG-water system a crystal phase (XI), a liquid phase (to the dilute side of the diagram), and three liquid crystal phases (lamellar, cubic, and hexagonal, in order of decreasing surfactant composition). The liquid phase is contiguous with the water border, and the liquidus boundary has a classical form. The plateau region of the Krafft boundary also has a familiar form, except that it displays an unusually shallow slope [33]. This feature is important with respect to the interpretation of DSC data. 

The liquid region boundary displays (as expected) a sharp kink at the intersection of the Krafft boundary with the hexagonal liquid crystal solubility boundary. The hexagonal phase disproportionates at its upper... 

The first onset temperature does not correspond to a thermodynamic discontinuity, as one usually expects. Instead, its existence is a consequence of the shape of the Krafft boundary. Below the knee of this phase boundary, it is nearly vertical and the rate of dissolution of crystals with increasing temperature is extremely small. Above the knee, the slope is extremely small, and the rate of dissolution is very fast. Dissolution requires the absorption of heat—and a heat effect is observed. Calorimetry measures heat effects, not phase behavior. 

In most surfactants, crystals in equilibrium with the liquid below the Krafft boundary temperature in dilute mixtures disappear rapidly as temperature is... 

V. THE CRYSTAL-LIQUID (KRAFFT) BOUNDARY INFLUENCE OF THIRD COMPONENTS... 

An important result of this analysis was the concept that those interactions that influence the Krafft boundary temperature occur strictly within the solution phase. It is unnecessary to invoke changes in the crystal phase to understand the results. This inference is supported by data which show that a plot of A/fi (determined using DSC) vs. composition is linear, and the line passes (almost) through the origin [15] (Fig. 10). While unsurprising, this result had not previously been established experimentally. 

Binary Soap-Water System Mixtures of soap in water exhibit a rich variety of phase structures (4, 5). Phase diagrams chart the phase structures, or simply phases, as a function of temperature (on the y-axis) and concentration (on the x-axis). Figure 2.1 shows a typical soap-water binary phase diagram, in this case for sodium pahnitate-water. Sodium palmitate is a fully saturated, 16-carbon chain-length soap. At lower temperatures, soap crystals coexist with a dilute isotropic soap solution. Upon heating, the solubility of soap increases in water. As the temperature is increased the soap becomes soluble enough to form micelles this point is named the Krafft point. The temperature boundary at different soap concentrations above which micelles or hquid crystalline phases form is named the Krafft boundary (5). 

With the decreasing of the hydrocarbon chain length the Krafft point and Krafft boundary move toward lower temperatures since the solubility of the soap increases with decreasing the hydrocarbon chain length while the basic features of the phase behavior remains more or less the same. The unsaturation of the hydrocarbon chain also impacts in a similar way as caused by the decreasing of hydrocarbon chain length (4, 5). 

In soap bar processing free fatty acid is usually added in formulations to create so-called super-fatted soap. An acid-soap complex with a fixed stoichiometric ratio between alkaline soap and the fatty acid is formed. For example, the ratio of potassium acid soap is 1 1 while sodium soap forms acid soaps with various ratios. The fixed ratio complex exits not only in anhydrous crystalline phase but also in a hydrous liquid crystalline phase (11, 12). Oleic acid and its potassium soap form a 1 1 complex acid soap when equal molar acid and soap are mixed. Above the Krafft boundary, the acid soap in water forms a lamellar liquid crystal phase at low surfactant concentration, from a few percent, and the lamellar liquid crystal phase extends to ca 60% surfactant concentration. A hexagonal liquid crystal phase is formed after the lamellar liquid crystal phase with further increasing the surfactant concentration. This phase behavior is different from the soap and water phase behavior, in which the hexagonal liquid crystalline phase is formed first followed by the lamellar liquid crystalline phase. Below the Krafft boundary the acid soap complex forms a solid crystal and separates from water (4). 

In Fig. 3, a prototypical phase diagram for a soluble surfactant whose Krafft eutectic lies above 0 C is shown. The Krafft boundary is the whole of the crystal solubility boundary, which lies below the Kraffi eutectic. This boundary may be arbitrarily divided into three regions. At low temperatures, a near-vertical region exists, where solubilities are low and do not vary rapidly with temperature. As the temperature is raised, a knee is encountered within the knee, the dependence of solubility on temperature (the slope) changes rapidly with temperature. Just above the knee, a plateau region exists within which the rate of change of solubility with temperature is very high but Nearly constant. The low-... 

Figure 3 A prototypical phase diagram of a surfactant whose Krafft boundary lies above 0 C and which displays typical hexagonal and lamellar liquid-crystal phases. The temperature regions within which the crystal, the hexagonal liquid-crystal, and the lamellar liquid-crystal solubility boundaries exist are shown. The crystal solubility boundary (below the temperature of the Krafft eutectic) is the Krafft boundary. The magnitude of the solubility below the knee is greatly exaggerated in this figure for the sake of clarity.

It was shown some time ago that in ionic surfactants (such as SDS), the trajectory of CMCs versus temperature intersects the Krafft phase boundary at the CMC Krafft point [7,44,45]. Just as the CMC itself is not a thermodynamic discontinuity [46], there is no kink or cusp in the Krafft boundary at this intersection. Nevertheless, this behavior is important because below the temperature of the CMC Krafft point micellar structure does not exist in equilibrium surfactant solutions. Metastable micellar solutions may, however, easily be formed below the Krafft boundary by cooling concentrated liquid phases [47]. Cooling liquid-crystal phases below the Krafft eutectic typically yields metastable liquid-crystal (not liquid) phases. 

The solubility of i nic surfactants in the Ipw-temperature region of the Krafft boundary and their response to added salts is, as a result, susceptible to analysis via classical... 

When the Krafft boundary is metastable (lies below the temperature where ice crystals separate when liquid solutions are cooled), the surfactant crystal solubility boundary does not exist [51]. Then, the solubility boimdary at temperatures down to 0°C and below is another kind of boundary (usually liquid or liquid crystal). 

It is also well known that increasing lipophilic chain lengths increases the temperature of the Krafft eutectic [55]. It is worth noting that increasing the lipophilic group volume actually increases the miscibility of water with surfactants within liquid-crystal phases, whereas, at the same time, this decreases the miscibility of the surfactant with liquid water. Among water-soluble surfactants, this has the effect of reducing the solubility at the liquid-crystal boundary just above the Krafft boundary [56]. 

In summary, the hexagonal phase is often the separating phase at the liquid-crystal solubility boundary just above the Krafft boundary in many commercially important surfactants- but it is not the only one. 

Solubility boundaries respond to variation in temperature in a similar maimer. Where the Krafft boundary exists as an equilibrium boundary it is always found at low temperatures, whereas a solubility boundary governed by a disordered fluid state is found at higher temperatures. The Krafft eutectic is extremely important in surfactant phase science, for this discontinuity divides surfactant diagrams into a high-temperature region within which equilibrium liquid crystals may exist, and a low-temperature region within which they are not found as equilibrium states [89]. (They are commonly encountered within the low-temperature region as metastable phases.)... 

From 0 C to 42°C, one encounters in this diagram the low-temperature region of the Krafft boundary. The knee in this diagram is quite sharp and falls between 42 C and 45°C. (The knee is much more gradual and spans a wider range of compositions and temperatures in many surfactants—especially ionic ones.) The plateau of the Krafft boundary is also readily visible in this diagram it extends from 45°C to 48°C. 

A fundamental requirement of a satisfactory recrystailization solvent is that the solute must have a high solubility at high temperatures and a very low solubility at somewhat lower temperatures. Phase studies of many solute-solvent recrystailization systems [23] have revealed that the desired change in solubility typically occurs within a narrow temperature range. This may sound familiar, as this behavior is precisely that which is found at the Krafft boundary in aqueous surfactant systems. It has been noted [40] that the form of the Krafft boundary is, in fact, not restricted to aqueous surfactant systems. 

The form of the generic recrystallization-phase diagram is illustrated by the diagrams in Fig. 10. It differs from the form of the Krafft boundary (Fig. 3) in only one major respect the upper limit of the crystal solubility boundary in nonaqueous systems is the... 

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