Ionic-nonionic mixed micelles ideality

Mixed Micelles Showing Negative Deviation -from Ideality. In an aqueous solution containing a mixture o-f Cll an ionic sur-factant and a nonionic sur-factant, or C21 an anionic sur-factant and a cationic sur-factant, or C33 a zwitterionic sur-factant and an anionic sur-factant, the CMC o-f the mixed sur-factant system exhibits a CMC which is substantially less than that predicted by Equation 1 (9.12.18-37). This means that the mixed micelle -formation is enhanced and that the mixing process in the micelle shows negative deviation -from ideality. This is demonstrated -for a cationic/nonionic system in Figure 1. 

Below the CMC, the surfactant mixing in monolayers composed of similarly structured surfactants approximately obeys ideal solution theory. This means that the total surfactant concentration required to attain a specified surface tension for a mixture is intermediate between those concentrations for the pure surfactants involved. For mixtures of ionic/nonionic or anionic/cationic surfactants, below the CMC, the surfactant mixing in the monolayer exhibits negative deviation from ideality (i.e., the surfactant concentration required to attain a specified surface tension is less than that predicted from ideal solution theory). The same guidelines already discussed to select surfactant mixtures which have low monomer concentrations when micelles are present would also apply to the selection of surfactants which would reduce surface tension below the CMC. 

In order to define a ionic/nonionic surfactant solution with high salinity/hardness tolerance, the following criterion should be followed. The mixed micelle should have as large of a negative deviation from ideality as possible. Surfactant mixture characteristics which result in this have already been discussed. The nonionic surfactant should have a high cloud point. Otherwise the amount of nonionic surfactant which can be added to the system is limited to low levels before phase separation occurs. If possible, a mixed ionic surfactant should be used for reasons Just discussed. There is no such benefit to using mixed nonionic surfactants, although this is not necessarily detrimental either. 

Large negative deviations from ideality are well known when mixed micelles are formed between ionic and nonionic surfactants (11—15.21.24) Negative deviations from ideality have been reported for mixed ionic/nonionic admicelle formation (26), although the degree of nonideality was not quantified. Since this work has pointed out the similarities and differences between mixed micelles and admicelles, the study of these systems should elucidate this relationship even further and will be the subject of future publications. 

Others have studied the volumetric changes occurring in mixed micelles of anionic-anionic and nonionic-nonionic surfactants as a determinant of intermolecular interactions and a measure of the thermodynamic ideality of mixing. In particular, Funasaki et al. (1986) have studied the volumetric behavior of mixed micelles of ionic and nonionic surfactants and analyzed their results in terms of regular solution theory. They found that in water, anionic surfactants such as SDS bind to PEG,... 

With mixed surfactants, the CMC of the mixed micelle varies according to the CMCs of the individual surfactants, and their proportions. Clearly, micelle composition varies with concentration since the micelles that form at lowest concentration are rich in the lowest-CMC surfactant, while the higher-CMC materials become more abundant in micelles as the overall concentration is increased. The detailed dependence of CMC values on mixed surfactant composition varies according to whether there are specific interactions between head-groups which lead to non-ideal mixing in the micelle. This applies particularly with mixtures of nonionic and ionic surfactants, and ionic surfactants of opposite charge. Various treatments are available to describe the behaviour (see Chapter 19), for example, as outlined in the text by Clint (3) (Chapters 5 and 6). 

The ideal solution theory can predict the concentration of each surfactant in micelles or the monomeric state for mixed hydrocarbon surfactants of similar structure. However, hydrocarbon surfactant mixtures of dissimilar structures, such as ionic-nonionic or cationic-anionic surfactant mixmres, exhibit nonideal behavior. The deviation from the ideal relationship can be negative or positive (Fig. 7.4). 

Unlike mixed hydrocarbon-chain surfactants of similar molecular structure, mixtures of fluorinated surfactants and hydrocarbon-chain surfactants do not behave ideally, even when the surfactants have a similar hydrophilic group. Mixtures of anionic fluorinated surfactants with anionic hydrocarbon surfactants exhibit a positive deviation from the ideal relation (Fig. 7.4). In contrast, surfactant mixtures containing a nonionic surfactant or oppositely charged ionic surfactants exhibit a negative deviation from ideal predictions. The formation of mixed micelles is governed by hydrophobic interactions between hydrocarbon and fluorocarbon chains and electrostatic effects [66]. Introduction of nonionic surfactants into micelles of anionic fluorinated surfactants reduces electrostatic repulsion between the ionic head groups. Apparently, the resulting electrostatic effect overcomes the hydrophobic interaction between the fluorocarbon and hydrocarbon chains. 

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