Micelles Krafft point

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). 

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],... 

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. 

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,... 

The temperature, abbreviated c.m.t., at which a deter-gent/solvent system or a lipid/solvent system passes from a hydrated crystalline state to an isotropic micellar solution. For a number of lipids, the c.m.t. is below the freezing point of the solvent. The Krafft point,, is the c.m.t. at the critical micelle concentration. 

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. 

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... 

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. 

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. 

Another important transition of surfactants involving micelles, the critical micellization temperature (CMT), has been found to be readily amenable to study by FT-IR, largely because of the relatively high surfactant concentrations involved (>0.1 M). The CMT is concentration dependent up to concentrations of about 0.1 to 0.3 M, above which the dependence decreases significantly. The Krafft point is thus found at lower temperatures than the CMT, and can be considered the CMT at the cmc (63-65). A thermostatted transmission cell for control of the temperature of the surfactant solutions, held between CaF2 or BaF2 windows, is necessary. Automation of the entire spectroscopic CMT experiment has been described (66). 

The solubilities of micelle-forming surfactants show a strong increase above a certain temperature, termed the Krafft point (Tk). This is explained by the fact that the single surfactant molecules have limited solubility whereas the micelles are very soluble. Referring to Figure 3.22, below the Krafft point the solubility of the surfactant... 

Non-ionic surfactants do not exhibit Krafft points. Rather the solubility of nonionic surfactants decreases with increasing temperature and the surfactants begin to lose their surface active properties above a transition temperature referred to as the cloud point. This occurs because above the cloud point a separate surfactant-rich phase of swollen micelles separates the transition is visible as a marked increase in dispersion turbidity. As a result, the foaming ability of, for example, polyoxyethyle-nated non-ionics, decreases sharply above their cloud points. The addition of electro-... 

Figure 3.22 Illustration ofthe solubility-micellization behaviour of surfactants in solution, showing the Krafft point.

The temperature (in practice a narrow range of temperatures) above which the solubility of a surfactant increases sharply (micelles begin to be formed). Below the Krafft point only single, unassociated surfactant molecules (monomers) or ions (ionomers) can be present, up to a given solubility limit. Above the Krafft point, a solution can contain micelles and thus allow much more surfactant to remain in solution in preference to precipitating. In the soap industry the Krafft point is sometimes defined as the temperature at which a transparent soap solution becomes cloudy upon cooling. 

Micelles are loose aggregates of amphiphiles in water or organic solvents which form above a certain temperature (Krafft point) and concentration (critical micellar concentration, cmc). Below the Krafft temperature, clear micellar solutions become turbid and the amphiphile forms three-dimensional hydrated crystals. Below the cmc, micelles dissociate into monomers and small aggregates. Above the cmc, the micelles of an aggregation number n are formed n then remains stable over a wide concentration range . Table 1 gives some typical cmcs and three Krafft point values. 

The first reason lies in the fact that the interaction between solvent molecules (usually water) is stronger than the interaction between the solvent and the solute. This effect alone would lead to a precipitation of the solute. In the case of amphiphiles which form micelles, however, the head groups are strongly hydrated and repulse each other. The hydration forces and steric forces which are made responsible for this repulsion effect prevent crystallization above the Krafft point and also above the cmc. Where the formation of 3D crystals is impeded, the smallest possible droplet is formed, removing the alkyl chains from the solvent. The interactions between solvent molecules are therefore disturbed to a minimal extent, allowing the head groups to be solvated with a minimal entropy loss. It is irrelevant whether the solvent contains clusters or not. Micelle formation only occurs as a result of a solvation of head groups and non-solvation of a solvophobic core. ... 

In MEKC, an ionic surfactant is used as a pseudo-stationary phase, and the Krafft point is also an important temperature. At a temperatures lower than the Krafft point, C f does not exceed the CMC, due to reduced solubility and, therefore, no micelle is formed. At the Krafft point, C f reaches the CMC and then the formation of the micelle is begun. The Krafft point of SDS is 16°C in a pure water, whereas it is 31°C for potassium dodecyl sulfate in pure water. Thus, a potassium salt is not an adequate buffer component for the SDS-MEKC system. 

Fig. 9 is a schematic phase diagram of a dilute aqueous cationic surfactant solution showing temperature and concentration effects on its microstructures. When the temperature is lower than the Krafft point [the temperature at which the solubility equals the critical micelle concentration (CMC)], the surfactant is partially in crystal or in gel form in the solution. At temperatures above the Krafft point and concentrations higher than the CMC, spherical micelles form in the surfactant solution. With further increase in concentration and/or on addition of counterions, the micelles form cylindrical rods or threads or worms with entangled thread-like and sometimes branched threadlike structures. 

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 Krafft point can be defined as the temperature T above which the amphiphile (surfactant) solubility in water greatly increases [3], The reason is that the water solubility of the amphiphile, which increases with temperature, reaches the amphiphile critical micelle concentration (Cm in Figure 3.6). When the solubility curve is above Cm the dissolved amphiphile forms micelles and the amphiphile... 

FIG. 3.6 The Krafft point is the temperature at which the solubility of the amphiphile becomes higher than its critical micelle concentration (Cm). 

The Krafft point is a triple point because at this temperature three phases coexist [4] hydrated solid amphiphile, individual amphiphile molecules in solution (unimers), and amphiphile molecules involved in micelles. 

Hydrotropy When there are strong chain-chain and head-head interactions between surfactant molecules (due to long, straight chains and close-packed heads), either insoluble crystal formation (low Krafft point, p. 214) or liquid-crystal formation (Chapter 3, Section IIC) may occur. Since there is much less space available for solubilization in rigid liquid-crystal structures than in the more flexible types of micelles, the onset of crystal formation usually limits the solubilization capacity of the solution. The tendency to form crystalline structures can be reduced by the addition of certain nonsurfactant organic additives called hydrotropes. 

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. 

Fig. 12. Partial phase diagrams for the dilute region of aqueous solutions of the disodium salts of sulfated monohydroxy bile salts glycolithocholate sulfate (GLCS at pH 10.0) and taurolithocholate sulfate (TLCS at pH 7.0, inset). The solid solubility curves and the interrupted CMC curves demarcate areas where crystals (and monomers), micelles (and monomers), and monomers alone are found. The critical micellar temperature (CMT) represents an equilibrium between micelles and hydrated crystals connected via the monomer concentration at the CMC. The Krafft point is a triple point and only represents the CMT at the CMC. (After ref. 6 with permission.)...

Due to micelle formation the total surfactant concentration undergoes an abrupt increase. Since true (molecular) solubility of surfactants, determined by the CMC, remains essentially constant, an increased surfactant concentration in solution is caused by an increase in a number of formed micelles. Micellar solubility increases with increase in temperature, and thus a continuous transition from pure solvent and true solution to micellar solution, and further to different liquid crystalline systems and swollen surfactant crystals (see below), may take place in the vicinity of the Krafft point. 

For many nonionic surfactants, that are liquid, the Krafft point does not exist, and one uses the cloud point as a thermal characteristic. An increased cloudiness above the cloud point is related to the increased size of micelles and the separation of system into two phases due to the dehydration of polar groups at elevated temperatures. 

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