Temperature effects critical micelle concentration

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. 

Temperature has a large effect on the mass transfer between the micelle and the stationary phase and can therefore be used to improve the efficiency of the separation. Micellar chromatography should be carried out at elevated temperature, typically around 40°C. At elevated temperatures, the effects of flow rate and surfactant concentration on the efficiency of the separation are minimized. For optimum efficiency, however, the flow rate should be minimized while still maintaining a reasonable elution time.33 Likewise, a surfactant concentration close to but above the critical micelle concentration should be used.33... 

Another study on these variegated cells depicting an amphiphile revealed a temperature effect on the critical micelle concentration (cmc) that was minimal at about PB(W) = 0.25. Experimentally, the minimal cmc value occurs at about 25 °C.64 The onset of the cmc was also modeled and shown to be dependent on a modestly polar fragment of the amphiphile. 

Flochart, B.D. (1961) The effect of temperature on the critical micelle concentration of some paraffin-chain salts. /. Colloid Sci., 16, 484-92. 

The similarities between non-ionic micelles and globular proteins (Nemethy, 1967 Schott, 1968 Jencks, 1969) render micelles potentially useful as models for the investigation of hydrophobic interactions. Indeed, the stability of non-ionic micelles has been treated theoretically in terms of hydrophobic interactions (Poland and Scheraga, 1965). Since the critical micelle concentration is related to the degree and nature of the hydrophobic interactions of the amphiphile, its valne in the presence of additives and at different temperatures can be nsed as a quantitative measure of the effect of these variables on the hydrophobic interactions. In spite of the similarities between proteins and micelles, considerable caution is warranted in extrapolating the results obtained from micellar models to the more complex protein systems. 

However, surfactants incorporated into the electrolyte solution at concentrations below their critical micelle concentration (CMC) may act as hydrophobic selectors to modulate the electrophoretic selectivity of hydrophobic peptides and proteins. The binding of ionic or zwitterionic surfactant molecules to peptides and proteins alters both the hydrodynamic (Stokes) radius and the effective charges of these analytes. This causes a variation in the electrophoretic mobility, which is directly proportional to the effective charge and inversely proportional to the Stokes radius. Variations of the charge-to-hydrodynamic radius ratios are also induced by the binding of nonionic surfactants to peptide or protein molecules. The binding of the surfactant molecules to peptides and proteins may vary with the surfactant species and its concentration, and it is influenced by the experimental conditions such as pH, ionic strength, and temperature of the electrolyte solution. Surfactants may bind to samples, either to the... 

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. 

In sufficiently dilute aqueous solutions surfactants are present as monomeric particles or ions. Above critical micellization concentration CMC, monomers are in equilibrium with micelles. In this chapter the term micelle is used to denote spherical aggregates, each containing a few dozens of monomeric units, whose structure is illustrated in Fig. 4.64. The CMC of common surfactants are on the order of 10 " -10 mol dm . The CMC is not sharply defined and different methods (e.g. breakpoints in the curves expressing the conductivity, surface tension, viscosity and turbidity of surfactant solutions as the function of concentration) lead to somewhat different values. Moreover, CMC depends on the experimental conditions (temperature, presence of other solutes), thus the CMC relevant for the expierimental system of interest is not necessarily readily available from the literature. For example, the CMC is depressed in the presence of inert electrolytes and in the presence of apolar solutes, and it increases when the temperature increases. These shifts in the CMC reflect the effect of cosolutes on the activity of monomer species in surfactant solution, and consequently the factors affecting the CMC (e.g. salinity) affect also the surfactant adsorption. 

An examination of the composition and physical state of sebum suggests that several cleaning mechanisms can operate during its removal from hair. Since sebum is completely molten at body temperature [122], it can be effectively removed by the roll-back mechanism. Also, the presence of approximately 25% free fatty acids in sebum indicates, as discussed in Section IV.C.3, that it is subject to removal by emulsification and mesophase formation. Finally, because the concentration of detergents during shampooing is well above their critical micelle concentrations, sebum can also be cleaned from hair by solubilization. 

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

Retention in Porous Media. Anionic surfactants can be lost in porous media in a number of ways adsorption at the solid—liquid interface, adsorption at the gas—liquid interface, precipitation or phase-separation due to incompatibility of the surfactant and the reservoir brine (especially divalent ions), partitioning or solubilization of the surfactant into the oil phase, and emulsification of the aqueous phase (containing surfactant) into the oil. The adsorption of surfactant on reservoir rock has a major effect on foam propagation and is described in detail in Chapter 7 by Mannhardt and Novosad. Fortunately, adsorption in porous media tends to be, in general, less important at elevated temperatures 10, 11). The presence of ionic materials, however, lowers the solubility of the surfactant in the aqueous phase and tends to increase adsorption. The ability of cosurfactants to reduce the adsorption on reservoir materials by lowering the critical micelle concentration (CMC), and thus the monomer concentration, has been demonstrated (72,13). 

In aqueous solutions of surfactants at concentrations above the critical micelle concentration (CMC), the molecules self-assemble to form micelles, vesicles, or other colloidal aggregates. These may vary in size and shape depending on solution conditions. In addition to surfactant molecular structure, the effects of concentration, pH, other additives, cosolvents, temperature, and shear affect the nanostructure of the micelles. The presence of TLMs or cylindrical, rodlike, or wormlike micelles at concentrations > CMCii are generally believed to be necessary for surfactant solutions to be drag reducing [Zakin et al., 2007]. 

The choice of 20 mN m" as a standard value of surface tension lowering for the definition of adsorption efficiency is convenient, but, as mentioned, somewhat arbitrary. When one discusses the effectiveness of adsorption, as defined as the maximum lowering of surface tension regardless of surfactant concentration, the value of o-min is determined only by the system itself and represents a more firmly fixed point of reference. The value of oinm for a given surfactant will be determined by one of two factors (1) the solubility limit or Krafft temperature (Tk) of the compound, or (2) the critical micelle concentration (cmc). In either case, the maximum amount of surfactant adsorbed is reached, for all practical purposes, at the maximum bulk concentration of free surfactant. 

Shinoda, K., Kobayashi, M., and Yamaguchi, N., Effect of iceberg formation of water on the enthalpy and entropy of solution of paraffin chain compounds the effect of temperature on the critical micelle concentration of lithium perfluorooctane sulfonate, J. Phys. Chem., 91, 5292, 1987. 

The calorimeter has been utilized in emulsion polymerizations to obtain polymerization rates from temperature time curves, rather than conventional conversion-time curves. By this technique polymerization variables such as electrolyte level, its effects on the critical micelle concentration and kinetic features not otherwise detectable can quanti-... 

At low concentrations, the bile acids are freely soluble in water and yield molecular solutions. The concentration at which molecular solubility is reached is termed the critical micelle concentration. Above this concentration, aggregation of the molecules occurs and micelles are formed. Hofmann and Small (6) have summarized the effects of temperature, electrolyte concentration, impurities, and the pH of the solution upon the critical micelle... 

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