Surfactant monomer concentration

The differences between the two curves can be explained by the sulfonate (the most adsorbed surfactant) monomer concentrations at equilibrium, which were reached in both cases, considering the amounts of surfactants, liquid and solid present. Figure 4 shows a distinct evolution of monomer concentrations for the two solid/liquid ratios considered. 

In order to illustrate the eFFect oF micellar nonidealities oF mixing on total surFactant monomer concentrations and micelle compositions in a system at the CHC, consider a hypothetical binary surFactant pair, A and B. Assume CMCa = 1 mli and CMCb = 2 mil. For a equimolar mixture oF A and B as monomer, the values oF Cn and micelle compositions are tabulated in Table I at various values oF W/RT. 

In the case of non—eutectic systems, the solid phase shows nearly ideal mixing, so that the surfactant components distribute themselves between the micelle and the solid in about the same relative proportions (i.e., both the mixed micelle and mixed solid are approximately ideal). However, in the case of the eutectic type system, the crystal is extremely non-ideal (almost a single component), while the micelle has nearly ideal mixing. As seen in earlier calculations for ideal systems, even though the total surfactant monomer concentration is intermediate between that of the pure components, the monomer concentration of an individual component decreases as its total proportion in solution decreases. As the proportion of surfactant A decreases in solution (proportion of surfactant B increases) from pure A, there is a lower monomer concentration of A. Therefore, it requires a lower temperature or a higher added electrolyte level to precipitate it. At some... 

If the mixed micelle model already presented is used to predict the ionic surfactant monomer concentration, and a simple concentration—based solubility product is assumed to hold between the unbound counterion and monomer, the salinity tolerance of an anionic/nonionic surfactant mixture can be accurately predicted (91). supporting this view of the mechanism of tolerance enhancement by nonionic surfactant. 

As already discussed in Chapter 1, the relative tendency of a surfactant component to adsorb on a given surface or to form micelles can vary greatly with surfactant structure. The adsorption of each component could be measured below the CMC at various concentrations of each surfactant in a mixture. A matrix could be constructed to tabulate the (hopefully unique) monomer concentration of each component in the mixture corresponding to any combination of adsorption levels for the various components present. For example, for a binary system of surfactants A and B, when adsorption of A is 0.5 mmole/g and that of B is 0.3 mmole/g, there should be only one unique combination of monomer concentrations of surfactant A and of surfactant B which would result in this adsorption (e.g., 1 mM of A and 1.5 mM of B). Uell above the CMC, where most of the surfactant in solution is present as micelles, micellar composition is approximately equal to solution composition and is, therefore, known. If individual surfactant component adsorption is also measured here, it would allow computation of each surfactant monomer concentration (from the aforementioned matrix) in equilibrium with the mixed micelles. Other processes dependent on monomer concentration or surfactant component activities only could also be used in a similar fashion to determine monomer—micelle equilibrium. 

Synthetic surfactants are commonly used in shampoos, sometimes for reasons of cost and sometimes for performance. Non-ideal mixing in micelles can result when the repulsions between different surfactant head-groups are not uniform, such as when an anionic sulfonate is mixed with a non-ionic ethoxylate or when an anionic is mixed with a betaine. This causes the cmc of the mixture to be smaller than would be the case for ideal mixing, or for either surfactant alone. Such a reduction in cmc can be used to reduce the surfactant monomer concentration in a shampoo. This is an advantage since reducing the monomer concentration reduces the amount of eye and skin irritation experienced when the shampoo is used [904], Other synthetics offer other benefits. For example, some silicone surfactants can not only function as emulsifiers in hair and skin care products, but also act to improve feel, gloss, sheen, emolliency, conditioning and foam stabilization [905]. 

The solubilities of micelle-forming surfactants show a strong increase above a certain temperature, termed the Krafft point (T ). This increase in solubility is explained by the fact that the single surfactant molecules have limited solubility, whereas the micelles are very soluble. Referring to Figure 10, below the Krafft point the solubility of the surfactant is too low for micellization, and solubility alone determines the surfactant monomer concentration. As temperature increases, the solubility increases until at the CMC is reached. At this temperature a relatively large amount of surfactant can be dispersed in micelles, and solubility increases greatly. Above the... 

Krafft point, maximum reduction in surface or interfacial tension occurs at the CMC because now the CMC determines the surfactant monomer concentration. 

Water-miscible surfactant molecules contain both a hydrophobic and hydrophilic portion, and can solubilize many poorly water-soluble drugs. Surfactants can also self-assemble to form micelles once the surfactant monomer concentration reaches the critical micelle concentration. Thus surfactants can solubilize drug molecules by either a direct cosolvent elfect or by uptake into micelles. The non-ionic surfactants in commercially available solubilized oral formulations include polyoxyl 35 castor oil (cremophor EL), polyoxyl 40 hydrogenated... 

The sharp reduction in the adsorption saturation level when one wt.% NaCl is added to the solution seems to be associated principally with the effect of electrolyte on the structure of the electrical double layer and on the influence of NaCl on the CMC of the surfactants. The CMC decreases as the ionic strength of the solution increases and this has the effect of reducing the maximum surfactant monomer concentration. 

Figure 20.4. Association between a homopolymer and a surfactant in different concentration domains (I) at low surfactant concentrations, there is no significant association at any polymer concentration (II) above the CAC, association increases up to a particular surfactant concentration, which increases linearly with the polymer concentration (III) association is saturated and the surfactant monomer concentration increases (IV) There is a coexistence of surfactant aggregates at the polymer chains and free micelles. This picture is schematic but gives a good description for aqueous mixtures of an ionic surfactant and a nonionic homopolymer. (Redrawn from B. Cabane and R. Duplessix, J. Phys. II (Paris), 43 (1982) 1529)...

Surface Tension Measurement. The most commonly used cmc determination method is the surface tension measurement [22]. Figure 2.2 shows that the surfactant molecules orientate at the solution air-water interface. This surfactant adsorption decreases the surface tension. The magnitude of the tension decrease depends on the free monomer surfactant concentration. As shown by Figure 2.5, the free surfactant monomer concentration reaches a plateau for surfactant concentrations above the cmc. As the surfactant concentration is increased in the aqueous solution, the surface tension... 

In a LTPWF, the concentration of surfactant and polymer is much lower than in a traditional micellar slug flood (surfactant 0.1-0.5 wt% and polymer <500 ppm) and incompatibilities between the chemicals resulting in associative or segregative phase separation are normally not observed even at high salinities [14], It is, however, very important that no association between surfactant and polymer takes place in solution. In the presence of excess polymer, the surfactant monomer concentration will then become lower than the CMC. The monomolecular packing of surfactants at the interface decreases, and the IFT will increase drastically. 

DSC has also been used to evaluate the surfactant monomer concentration in equilibrium with micelles and its temperature dependence for triblock copolymers [160]. 

The surfactant monomer concentration is almost equal to the CMC, and n is much larger than 1 for the usual process of micellization. The, (4.48) reduces to the approximate equation... 

In Fig. 7.14, the surface tension is plotted against the mole fractions of STrS in coexisting monomers (curve e) and micelles (curve f). The micellar composition was calculated assuming that (1) the surface tension of the pure surfactant remains constant above cmc and (2) the surface tension is a function of only the surfactant monomer concentrations regardless of coexisting micelles. In Fig. 7.14, the surface tension is constant at 18.73 mN/m between the mole fractions and Xs- The compositions at both ends of this region, Xa and Xb, represent the mutual solubility in the mixed NF-STrS micelles. When the total concentration of the mixed surfactants of a fixed composition between Xa and Xb is increased, the number of mixed-micelle types increases from zero to one, then to two, and finally, decreases to one. The concentration at which the number of mixed-micelle types begins to decrease is the cdc. 

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