Critical micelle concentration electrolyte effects

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

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... 

We have examined the stmcture of both ionic and nonionic micelles and some of the factors that affect their size and critical micelle concentration. An increase in hydrophobic chain length causes a decrease in the cmc and increase of size of ionic and nonionic micelles an increase of polyoxyethylene chain length has the opposite effect on these properties in nonionic micelles. About 70-80% of the counterions of an ionic surfactant are bound to the micelle and the nature of the counterion can influence the properties of these micelles. Electrolyte addition to micellar solutions of ionic surfactants reduces the cmc and increases the micellar size, sometimes causing a change of shape from spherical to ellipsoidal. Solutions of some nonionic surfactants become cloudy on heating and separate reversibly into two phases at the cloud point. 

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. 

Ionic surfactants are strong electrolytes in dilute aqueous solution, and non-ionic surfactants are monomers, but above the so-called critical micelle concentration (cmc) they spontaneously self-associate to form micelles [15]. Micellization in water is an example of the hydrophobic effect at work [18]. The phenomenon is more properly called the solvophobic effect, because it is important in associated solvents which have three-dimensional structure, and normal micelles form in 1,2-diols, or formamide [19] and micelles with a carbocationic head group form in 100% sulfuric acid [20], for example. However, we live in an aqueous world, and most normal micellar systems are studied in water, so we can reasonably retain the term hydrophobic with the hydrophobic bond dictated by water association. 

Polymeric micelles form stable pseudostationary phases with a critical micelle concentration of virtually zero (aggregation number of 1), and are tolerant of high organic solvent concentrations in the electrolyte solution. Mass transfer kinetics are slow compared with conventional surfactant micelles, and peak distortion from mass overloading is a problem for some polymer compositions. Preliminary studies indicate that polymeric surfactants are effective pseudostationary phases in micellar electrokinetic chromatography, but only a limited number of practical applications have been demonstrated, and uptake has been slow. 

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... 

Besides improving the absolute level of thickening efficiency, the interactions of the hydrophobes in aqueous solution were also seen to affect in a beneficial manner the other properties of formulations based on such polymeric thickeners. The micellar clusters formed by the assembly of hydrophobes in the aqueous phase aggregate even more strongly when simple salts are also present in solution. This arises due to the fact that the critical micelle concentration (CMC) of the surfactant forming the side chain is somewhat lower in electrolyte solutions than in water alone. This effect, which boosts viscosity, helps to offset the reduction in thickening efficiency that inorganic and other salts have on conventional polyelectrolytes. 

If the interfacial area is small, it can only accommodate a small number of molecules. When, as usual, many more surfactant molecules than this are present, the majority cannot escape from the bulk liquid to the interface and the affinities of the hydrophilic and lipophilic groups must be satisfied by other means if thermodynamic stability is to be achieved. This again occurs by a process of orientation. In an aqueous medium the hydrophobic groups turn towards and associate with one another, forming in effect their own oil phase, surrounded by the hydrophilic groups turned outwards and anchored in the water. This type of internal association and orientation has been termed micelle formation. Micelles are usually spherical in shape. The escape mechanism of micelle formation only becomes operative above a certain minimum surfactant concentration. This concentration has been termed the critical micelle concentration (CMC). CMCs vary from about 5 x 10 mol 1 for the most hydrophilic to about 5 x 10 mol 1 for the most hydrophobic types of surfactant. They are influenced by electrolytes, especially in the case of ionic surfactants, and also by other polar/non-polar chemical compounds such as alcohols, amides and, of course, other surfactants. 

In aqueous solution micelles are generally thought to be spherical as long as the surfactant concentration remains close to the critical micelle concentration. Rod-like micelles may form at higher surfactant concentrations [1, 2]. Addition of a third component such as neutral salt or non-electrolytes may favour longer micellar structures, for instance rod-like micelles [3-6]. An increase in temperature, on the other hand, seems to favour spherical micelles [7, 8]. The effect of pressure on the shape transition point is not known, though it appears that the aggregation number of micelles decrease with pressure at least up to about 160 MPa [9-12]. 

Because the effective HLB of a given surfactant will depend on the nature of the solvent, HLB numbers cannot be considered to be absolute, reaUstic measures of the emulsifying ability of a material under all conditions. The actual HLB of a surfactant in a system will depend on the nature of the solvent, the temperature, and the presence of additives such as cosolvents, electrolytes, and polymers. Although the relationship will not always be linear, the HLB may be expected to vary in a manner analogous to that found for the critical micelle concentration of the surfactant under the same conditions. 

Measuring the surface tension in order to determine the critical micelle concentration (cmc) is also straightforward. Figure 4 shows plots of the surface tension versus surfactant concentration for CTAB and sodium do-decyl sulfate (SDS) solutions. The cmc can be determined as the intersection of the straight lines through the two linear portions of the semilogarithmic plot the values determined are close to literature values. The familiar dip in surface tension at the cmc for SDS due to the presence of impurities is also clearly apparent. Although the data are not shown, this method can also be used effectively to measure the cmc in the presence of added electrolyte [13]. 

SDS/NaCI Mixtures. The effect of temperature on the micelles formed in 70 mM SDS + NaCl solutions is presented below. Mazer et al. (14) have found that the aggregation number, N, is at a maximum for supercooled solutions below the critical micellization temperature (cmt), and decreases towards the value expected for a spherical micelle as the temperature is increased. The variations in N with temperature are dependent on the concentration of added electrolyte, with the rodlike micelles formed in high salt (0.6 M) showing large variations, and the spherical micelles formed in little (0.3 M) or no salt showing only small variations. 

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