Krafft phenomenon

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... 

The Krafft phenomenon is best discussed from the interplay between the temperature-dependence of the surfactant unimer solubility and the temperature dependence of the CMC. As we have learnt above, the latter temperature dependence is very weak and we can consider here that the CMC is, to a good approximation, independent of temperature. On the other hand, we expect the dissolution of the surfactant into the constituent solvated ions to increase markedly with temperature as seen for simple salts. If this solubility is below the CMC, no micelles can form and the total solubility is limited by the (low) unimer solubility. If, on the other hand, the unimer solubility reaches the CMC, micelles may form. It is a characteristic feature of micellization, as we will see later, that as the micelle concentration increases there is virtually no change in the free unimer activity (or concentration). This, together with a very high micelle solubility, explains why a quite small increase in unimer solubility (resulting here from a temperature increase) leads to a dramatic increase in the overall surfactant solubility. 

The Kiafft point is determined by the energy relationships between the solid crystalline state and the micellar solutions. It appears that the micellar solutions vary only weakly between different cases, like different counterions, while due to packing effects the solid crystalline state may change dramatically. Looking for an understanding of the Krafft phenomenon we have therefore to examine the packing effects and ionic interactions in the solid state. 

Non-ionic Surfactants. These surfactants do not present the Krafft phenomenon. However, a nonionic micellar solution becomes turbid and separates in two phases when the temperature is raised. This is the clouding phenomenon. The polyoxyethylene chain, the polar part of most non-ionic surfactants, is progressively dehydrated as the temperature raises. Losing water molecules, the polyoxyethylene ehain becomes less polar and, at a particular temperature, a turbidity, the clouding, appears. This temperature is called the cloudpoint of the nonionic surfactant solution. Above the cloud point, the nonionic micellar solution separates in an aqueous phase saturated by the nonionic surfactant, and an organic phase saturated by water and containing the major part of the surfactant. 

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. 

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. 

Various approaches have been employed to tackle the problem of micelle formation. The most simple approach treats micelles as a single phase, and this is referred to as the phase-separation model. In this model, micelle formation is considered as a phase-separation phenomenon, and the cmc is then taken as the saturation concentration of the amphiphile in the monomeric state, whereas the micelles constitute the separated pseudophase. Above the cmc, a phase equilibrium exists with a constant activity of the surfactant in the micellar phase. The Krafft point is viewed as the temperature at which a solid-hydrated surfactant, the micelles, and a solution saturated with undissociated surfactant molecules are in equiUbrium at a given pressure. 

The solubility of the surfactant in decane is also quite small at 25°C, about 0.04 wt%, but over a narrow temperature range around 50°C it rises dramatically, as in the Krafft point range of a single-chain surfactant in water (11a). Such a phenomenon with a surfactant in a nonpolar solvent is not uncommon (35). Incidentally, the absence of a Krafft point range for the surfactant in water between 10 and 90°C argues for the absence of micelles in solution. Abrupt change in the slope of such a property as surface tension versus concentration (9) can be due to precipitation of a new phase as well as to onset of appreciable micelle formation, and so does not constitute conclusive evidence for the latter. 

In a subsequent series of experiments, Landes and Wei [2] demonstrated that the phenomenon is real, and modeled the crack growth response in terms of creep deformation rate within the crack-tip process zone. The effort has been further substantiated by the work of Yin et al. [3]. The results and model development from these studies are briefly summarized, and extension to probabihstic considerations is reviewed. It is hoped that this effort will be extended to understand the behavior of other systems, and affirm a mechanistic basis for understanding and design against creep-dominated failures. The author relies principally on the earher works of Li et aL [1], Landes and Wei [2], Yin et al. [3], Krafft [4] and Krafft and Mulherin [5]. The findings rely principally on the laborious experimental measurements by Landes and Wei [2], and the conceptual modeling framework by Kraftt... 

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