Critical micellar temperature

A temperature-composition phase diagram for a surfactant solution is a characteristic phase diagrarr that delineates the conditions under which crystalline surfactant, monomers, or micelles will exist. On the phase diagram shown in Figure 12.5 (Smirnova, 1995), L represents the liquid phase, S the solid phase, and )(the surfactant mole fraction. The critical micellar temperature, CMT, is deLned as the line between the crystalline and micellar phases. Micelle formation occurs at temperatures greater than the CMT. The critical micellar concentration, CMC, line separates the micellar and... 

A H NMR relaxation study of di-block and tri-block copolymers of ethylene oxide and 1,2-butylene oxide aqueous solutions has shown a phase transition from a micelle to a gel in the relaxation time of the ethylene oxide block, consistent with gel formation by close packing of micelles. NOE shows that the blocks of ethylene oxide and 1,2-butylene oxide interpenetrate at the core-fringe boundary.307 Similar phenomena have been observed for the tri-block copolymer of ethylene oxide and propylene oxide. At the critical micellar temperature, a marked transition in the relaxation times of the hydrophobic propylene oxide block occurs, which is attributed to a change from well-solvated mobile chains below the critical micellar temperature to a more restricted concentrated micelle-core environment above this temperature. However, no transition in the properties of the hydrophilic block of ethylene oxide has been observed. NOE data indicate that in the micelles there is considerable interpenetration of the... 

On the basis of surface and bulk interaction with water. Small [85] classified bile acids as insoluble amphiphiles and bile salts as soluble amphiphiles. On account of the undissociated carboxylic acid group, the aqueous solubility of bile acids is limited [35] in contrast, many bile salts have high aqueous solubilities as monomers [33] and, in addition, their aqueous solubilities are greatly enhanced by the formation of micelles [5,6]. Because many bile salts are weak electrolytes, their ionization and solubility properties are more complicated than those of simple inorganic or organic electrolytes [5,35]. For example, the p/Tj, values of bile acids in water vary markedly as functions of bile salt concentration and, because micelles formed by the A (anionic) species can solubilize the HA (acid) species [5,35], the equilibrium precipitation pH values of bile acids also vary as functions of bile salt concentration. Finally, certain bile salts are characterized by insolubility at ambient temperatures [2,5,6,86,87], only becoming soluble as micelles at elevated temperatures (the critical micellar temperature) [6]. 

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.)...

Fig. 1. The fatty acid soap-water phase diagram of McBain (58) modified (1) to show the molecular arrangement in relation to aqueous concentration (abscissa) and temperature (ordinate). Ideal solution, i.e., true molecular solution, is to the left of the vertical dashed line, indicating the critical micellar concentration (CMC), which varies little with temperature. At concentrations above the CMC, provided that the temperature is above the critical micellar temperature (CMT), a micellar phase is present. At high concentrations, the soap exists in a liquid crystalline arrangement, provided that the solution is above the transition temperature of the system, i.e., the temperature at which a crystalline phase becomes liquid crystalline. The Krafft point is best defined (D. M. Small, personal communication) as the triple point, i.e., the concentration and temperature at which the three phases (true solution, micelles, and solid crystals) coexist, but in the past the Krafft point has been equated with the CMT. The diagram emphasizes the requirement for micelle formation (a) a concentration above the CMC, (b) temperature above the CMT, and (c) a concentration below that at which the transition from micelles to liquid crystals occurs. Modified from Hofmann and Small (1).

Although soaps are called soluble amphiphiles, the formation of micelles is in fact a manifestation of the low molecular solubility of the soap molecule. Micelle formation can only occur above the critical micellar temperature of the system. Below this temperature, soaps form a crystalline phase (which may or may not contain water depending on the history of the system). This means that the classification of a given lipid as a soluble or insoluble amphiphile is conditional upon the experimental temperature. 

This equation indicates that the log CMC falls linearly with increasing chain length and electrolyte concentration. Thus addition of electrolyte lowers the CMC but lowers the concentration of soap anion even more greatly. The addition of electrolyte thus tends to salt out the soap solution rather than cause micelle formation. Micelle formation can be induced by raising the experimental temperature. Addition of electrolytes lowers the CMC but raises the critical micellar temperature, and the latter effect is greater (41). 

Besides influencing the critical micellar temperature, the number and position of hydroxy groups also influence the critical micellar concentration. The critical micellar concentration of dihydroxy bile acids is significantly below that of trihydroxy acids (1,63), but no values have been reported for monohydroxy acids. Conjugation with glycine or taurine does not appear to have any particular effect on those properties of bile acids which are related to the steroid nucleus, provided that experiments are carried out at pH sufficiently alkaline that all bile acid molecules present are ionized (64). 

Saturated fatty acid soaps dissolve in water at their critical micellar temperature to form micellar solutions. The addition of sodium ion raises the critical micellar temperature of this system, as discussed. One can measure the critical micellar temperature in systems composed of varying molar ratios of two ionic detergents, and the phase equilibria present in such... 

We have carried out similar experiments with saturated fatty acid soaps as the first soap and bile acids (as the sodium salt) as the second soap (66). The bile acids behave as typical ionic detergents with critical micellar temperatures well below 0°C (Fig. 10). For any saturated fatty acid soap, its critical micellar temperature decreases as the chain length shortens. Thus, at 37°C, more of a short-chain soap will be solubilized for a given amount of bile acid present, or, to paraphrase, the molar ratio of soap/bile acid will be much higher for short-chain soaps at 37°C. One carries out these experiments by incubating a series of different molar ratios of soap/bile acid. The bile acid is micellar, and the soap is crystalline. Over a few degrees of temperature range, the crystalline soap completely dissolves. No satisfactory quantitative description of these experiments has, as yet, been proposed. 

Fig. 10. Phase equilibria of the bile acid (as sodium salt)-fatty acid soap-water phase diagram at constant water concentration in relation to temperature. Mixtures with varying molar ratios of bile acid/sodium soap (total concentration 40 mM) were incubated, and the temperature at which the system became clear was plotted solutions were buffered top i 12. The curves indicate the critical micellar temperature of the system and have also been termed mixed Krafft points (46). The CMT of the bile acids is extremely low.

Since saturated fatty acids are insoluble in bile acid solutions, and since saturated fatty acid soaps are only soluble in terms of the mole fraction of a soap-bile acid mixture having a critical micellar temperature of 37° C, one would anticipate saturated fatty acid-soap mixtures to have negligible solubility in bile acid solutions. Some years ago, we compared the behavior of sodium, palmitate, and stearate at pH 5.8, 6.2, 6.6, and 7.0 in buffer or buffer containing bile acid. In the absence of bile acid, the saturated fatty acids remained as unwetted crystals. When bile acid was added, the solubility increased measurably but only very slightly. 

The trans fusion of rings A and B in the allo-acids produces a more planar molecule than the 5 3 acid and contributes to the poorer detergency of glyco allodeoxycholate and consequent poorer solubility of the calcium salt (36). The Krafft point (critical micellar temperature) of several allo-acids has been determined and discussed (64). In contrast to the notorious character of deoxycholic acid to complex with a large variety of other substances, no evidence has been reported for the formation of choleic acids by allodeoxycholic acid. 

Fig. 36. Critical micellar temperature (CMT) of alkaline metal salts of lithocholic acid as a function of the atomic volume of the alkaline metal. CMT vertical axis, atomic volume horizontal axis. Percent solids refers to the total amount of bile salt in g/100 ml water. Note that as the atomic volume of the alkaline metal decreases, the Krafft point for any given concentration of bile salt increases. There is a striking rise with lithium, which has the smallest atomic volume.

Critical Micellar Temperature (CMT) The temperature to which a given mixture of detergent and water must be raised to transform the detergent from a suspension of crystals in water or gel to a clear micellar phase. Since the critical micellar temperature may vary over large concentrations of detergent, the detergent concentration should be specified. 

Krafft Point The critical micellar temperature at the critical micellar concentration of the detergent (see Section VII. D). 

Another term, the critical solution temperature (CST), was introduced to designate the temperature beyond which the solubility of nonionic surfactants in organic solvents increases markedly, as marked by an inflection in the solubility curve. Mazer and Benedek used the critical micellar temperature (CMT) to refer to the phase boundary between a hydrated solid phase and a micellar phase." The CMT value was taken as the midpoint of the temperature range over which the hydrated solid phase clarified on slow warming with vigorous shaking. 

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