Krafft eutectic temperature

The Krafft eutectic temperature was also estimated from the return-to-baseline temperature of DSC scans at compositions below the dilute limit of the Krafft eutectic. The temperature of the Krafft eutectic from DIT studies was 48°C. The first DSC onset temperatures observed across the entire composition range averaged 45.2°C, while the return-to-base-line temperatures averaged 48.2°C. The 3°C difference between these two temperatures is significant. 

To produce equilibrium phase data, several cells were loaded, sealed only at the filled end, and stored at room temperature within a 90% RH chamber (prepared from a saturated zinc sulfate solution in equihbrium with crystals and water vapor). After 2 weeks, initiation and DIT studies using these cells displayed none of the above-described behavior. Instead, swelling to form liquid crystal phases occurred at and above the Krafft eutectic temperature. Similar procedures were followed and similar observations made during the dioctadecyldimethylammonium chloride (DOD-MAC)-water DIT study [39]. The difference, in the case of DODMAC, was that much longer equilibration times were required to achieve equilibration (4 months). 

The nature of the separating phase above the Krafft eutectic temperature can be anticipated if the overall phase behavior of the system is known (or can be predicted). The most dilute liquid crystal is always the first separating phase which is encountered, but it may be replaced as the temperature is increased by other more concentrated liquid-crystal phases. 

A few comparisons of molecules which differ stereochemically are available. Aqueous-phase data on P-hydroxysulfoxides have been quoted which show that whereas the two racemic diastereomeric compounds display virtually identical liquid- and liquid-crystal-phase boundaries, they differ substantially in the temperature range of their crystal solubility boundaries [39]. In particular, the higher melting isomer displays the higher Krafft eutectic temperature. The situation is one in which the l drophilicity of the two isomers appears to be virtually identical—but their ability to pack into a crystal differs substantially as a result of their differences in shape. Molecular shape is very important to crystal thermodynamics. 

In compounds that have relatively small and weakly hydrophilic functional groups, the lamellar phase is very often the solubility-limiting phase. The same is true of many surfactants containing two or more lipophilic groups—irrespective of the hydrophilicity of the fiindlional groups present. Catanionic surfactants (ionic surfactants in vduch-both ions are amphiphilic [87]) are also reported to display the lamellar liquid-crystal solubility boundary. Dioctadecylammonium cumenesulfonate (mentioned earlier) may be regarded as a cationic surfactant in which the anion has a hydrotropic molecular structure [78]. It displays a very unusual lamellar liquid-crystal solubility boundary above the Krafft eutectic temperature. 

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. 

This analysis would not likely apply at the temperatures of the knee or plateau regions, as the dependence of activities on concentration is greatly complicated by the existence of micellar phenomena. Because no ternary aqueous surfactant phase study has been performed to date at a temperature below the Krafft eutectic of the binary aqueous surfactant system, little information exists regarding the influence of a third component on the crystal solubility of surfactants (except for the influence of added salts, as described above). It has been shown that plane sections through ternary diagrams of two surfactants and water have an appearance similar to the familiar eutectic diagram found in numerous nonsurfactant binary systems [45]. 

As noted earlier, the factors that affect crystal solubility differ substantially from those that affect other solubility boundaries. Foremost among these are the shape of molecules (symmetry and stereochemical structure) and conformational freedom. The thermodjmamic stability of the crystal phase is strongly influenced by the ability of molecules to pack densely together in this phase. Structural features which inhibit this packing may dramatically alter the temperature range of the crystal solubility. An historic example is the effect of the cis double bond in oleates, which lowers the Krafft eutectic significantly relative to that of stearates. The trans double bond has a smaller effect, which parallels the influence... 

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]. 

At a temperature just above the Krafft eutectic (Fig. 3), the solubility boundary that is encountered is almost invariably a liquid-crystal solubility boundary. (The only circumstance when this is not true is when a liquid-liquid miscibility gap intrudes, in which case a liquid solubility boundary is found [72].) In highly soluble surfactants, the span of the miscibility gap beyond a liquid-crystal solubility boundary is typically very small ( a few percent). The miscibility gap at the Krafft eutectic may span three-quarters of the diagram, and in the vertic part of this boundary the miscibility gap is nearly 100% (unless crystal hydrates intervene). In a typical anionic surfactant such as SDS, for example, the tie-line lengths in the low-temperature region are about 88.9%. Just below the ICrafft eutectic they are 50%, ut just above this eutectic they are about 2.5% [42]. 

Cubic phases are often the solubility-limiting phase above the Krafft eutectic of monoglycerides [84] and possibly of other polyol surfactants. It is claimed that these cubic phases have an inverted (head group in) structure [85]. Often, they coexist with a more concentrated bicontinuous cubic phase [86] which lies next to the lamellar phase. Cubic phases are the solubility-limiting phase in the iV-acyl-JV-methylglucamines, but only at temperatures above the Ejafft eutectic (see later). Just above the Krafft eutectic, the solubility boundary of this surfactant is defined by the familiar hexagonal phase. 

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.)... 

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). 

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