Critical micelle concentration mixed surfactants

A spiropyran compound bearing a pyridinium group and a long alkyl chain behaves as a surfactant. The components shown in Scheme 1 exhibit reverse photochromism in polar solvents. The colored merocyanine form is more stable than the spiropyran form in the dark. Upon photoirradiation at A>510 nm, the polar merocyanine form is converted to the hydrophobic spiropyran form so that the CMC (critical micelle concentration) of the surfactant decreases. Consequently, when the initial concentration is set between the CMC of the two forms, photoirradiation induces a sudden formation of micelles at a certain conversion to the spiropyran form corresponding to the CMC of the mixed micelle of the two forms. 

The formation of mixed micelles in surfactant solutions which contain two or more surfactant components can be significantly affected by the structures of the surfactants involved. The observed critical micelle concentration (cmc) is often significantly lower than would be expected based on the erne s of the pure surfactants. This clearly demonstrates that interactions between different surfactant components in the mixed micelles are taking place. 

Ideal Mixed Micelles. The Critical Micelle Concentration (CMC) is the lowest surfactant concentration at which micelles form the lower the CMC, the greater the tendency of a system to form micelles. When the total surfactant concentration equals the CMC, an infintesimal fraction of surfactant is present as micelles therefore, the CMC is equal to the total monomer concentration in equilibrium with the micellar pseudo—phase. The CMC for monomer—micelle equilibrium is analogous to the dew point in vapor—liquid equilibrium. 

The variation of the mixture critical micelle concentration (CMCf ) with temperature and with overall surfactant composition has been studied using ultrafiltration for two binary mixed nonionic/anionic systems. 

In mixed surfactant systems, physical properties such as the critical micelle concentration (cmc) and interfacial tensions are often substantially lower than would be expected based on the properties of the pure components. Such nonideal behavior is of both theoretical interest and industrial importance. For example, mixtures of different classes of surfactants often exhibit synergism (1-3) and this behavior can be utilized in practical applications ( ).In addition, commercial surfactant preparations usually contain mixtures of various species (e.g. different isomers and chain lengths) and often include surface active impurities which affect the critical micelle concentration and other properties. 

Synergism in mixed micelle formation. Synergism in this respect is present when the critical micelle concentration of any mixture is lower than that of either pure surfactant. This is illustrated in Figure 2. 

The surface tension of the aqueous solution of dode-cylaitunonium chloride (DAC) — decylairanonium chloride (DeAC) mixture was measured as a function of the total molality m of surfactants and the mole fraction X of DeAC in the total surfactant in the neighborhood of the critical micelle concentration (CMC). By use of the thermodynamic equations derived previously, the mole fraction in the mixed adsorbed film was evaluated from the y vs. X and m vs. X curves. Further, the mole fraction in the mixed micelle was evaluated from the CMC vs. X curve. By comparing these values at the CMC, it was concluded that the behavior of DAC and DeAC molecules in the mixed micelle is fairly similar to that in the mixed adsorbed film. 

On the other hand, we showed that the coii5>osition of surfactant in a mixed adsorbed film can be estimated thermodynamically from experimental results without introducing such a supposition (9-11). Further, the composition of a mixed micelle was calculated assuming that the micelle behaves thermodynamically like a macroscopic bulk phase whose thermodynamic quantities are given by the excess thermodynamic quantities similar to those used for the adsorbed film (i8). Therefore, we can now compare the composition of surfactant in the mixed adsorbed film with that in the mixed micelle at the critical micelle concentration (CMC). 

The mixed admicelle is very analogous to mixed micelles, the thermodynamics of formation of which has been widely studied. If the surfactant mixing in the micelle can be described by ideal solution theory, the Critical Micelle Concentration (CMC) or minimum concentration at which micelles first form can be described by (21) ... 

The critical micelle concentrations (cmc) of the mixed surfactant systems were determined by measuring the surface tension as a function of total surfactant concentration on a du Noiiy ring balance at 25°C. 

Critical Micelle Concentration. In order to demonstrate the analogy between our treatment of mixed adsorption and earlier treatments of mixed micellization, we will briefly review the thermodynamics of mixed micelles. The thermodynamics of formation of ideal mixed micelles by two surfactants has been treated by Lange and Beck (9 ) and Cling (10). Rubingh ( ) extended the treatment to account for interactions between the surfactants, essentially by writing the cmc in the mixed surfactant solution as. 

An aggregate of surfactant molecules or ions in solution. Such aggregates form spontaneously at sufficiently high surfactant concentration, above the critical micelle concentration. The micelles typically contain tens to hundreds of molecules and are of colloidal dimensions. If more than one kind of surfactant forms the micelles, they are referred to as mixed micelles . If a micelle becomes larger than usual as a result of either the incorporation of solubilized molecules or the formation of a mixed micelle, then the term swollen micelle is applied. See also Inverse Micelle. 

Enhanced oil recovery always deals with two or more fluids. By implication these fluids are conjugate phases in equilibrium with each other, although Chapter 6 shows that nonequilibrium mixing can sometimes be important when surfactants are used. When one considers the role of the critical micelle concentration (CMC) in CO2 mobility control, it is the CMC of the aqueous phase saturated with CO2 that is important. As illustrated in Figure 10, this CMC may be much lower than the CMC of C02 free surfactant solutions (R. S. Schechter, University of Texas, personal communication, October 26, 1987). 

The adsorption of binary mixtures of anionic surfactants of a homologous series (sodium octyl sulfate and sodium dodecyl sulfate) on alpha aluminum oxide was measured. A thermodynamic model was developed to describe ideal mixed admicelle (adsorbed surfactant bilayer) formation, for concentrations between the critical admicelle concentration and the critical micelle concentration. Specific... 

This study is a continuation of our previous investigations, in which the aggregation phenomena of surfactant molecules (amphiphiles) in aqueous media to form micelles above the critical micelle concentration (c.m.c.) has been described based on different physical methods (11-15). In the current literature, the number of studies where mixed micelles have been investigated is scarcer than for pure micelles (i.e., mono-component). Further, in this study we report various themodynamlc data on the mixed micelle system, e.g., ci H25soi4Na (NaDDS) and sodium deoxycholate (NaDOC), enthalpy of micelle formation (by calorimetry), and aggregation number and second virial coefficient (by membrane osmometry) (1 6). 

Because the main driving force for surfactant self-association in polymer-surfactant mixed systems is the hydrophobic effect, the binding of surfactants to polyelectrolytes exhibits a similar dependence on the length of the alkyl chain as known for free micellization. Surfactants with longer hydrocarbon chains bind more strongly to polyions than those with shorter chains, and the binding starts a lower surfactant concentrations. In this context, a convenient parameter to characterize polyelectrolyte-surfactant systems is the critical aggregation concentration, cac, which is a counterpart of the well-known critical micellization concentration, cmc, but applies to solutions of surfactants in the presence of a polymer. It is defined as the... 

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