Surfactants positive deviation from

Having shown that ionic/nonionic surfactant mixtures show negative deviations from ideality (when both components are hydrocarbon—based) and fluorocarbon/hydrocarbon—based surfactant mixtures show positive deviations from ideality, what would a ionic fluorocarbon/nonionic hydrocarbon surfactant pair be expected to do In one example of this case (57). the electrostatic stabi1ization forces overcome the hydrophobic group phobicity effects and negative deviation from ideality is observed. 

Nishikido (21) has done a systematic study o-f mixed sur-factant solubilization. In that study, solubilization in mixed systems was compared to that predicted by application o-f a linear mixing rule to the solubilizations in the pure surfactant component micelles. For example, in this "ideal case, a micelle composed of a 50/50 molar mixture of two surfactants would have a solubilization capacity which is an average of that of the two pure surfactants involved. A system showing negative deviation from ideality would have less solubilization than this ideal system a system having positive deviation from ideality would have more. 

From Figures 3 and 6—9, the predicted total adsorptions For surFactant mixtures are higher than observed values. ThereFore, the mixed admicelles showed positive deviation From ideality at all compositions. This remarkable behavior has not been observed beFore because data oF the accuracy and range reported here has not (to our knowledge) previously been reported. Observation oF the expected ideal behavior For the CMC data indicate that this is probably not due to a peculiarity oF the surFactants used. 

It would be of scientific interest to study the adsorption of mixed surfactant systems showing positive deviations from ideality, as has been discussed for mixed micelles and monolayers. 

The adsorption of binary mixtures of anionic surfactants in the bilayer region has also been modeled by using just the pure component adsorption isotherms and ideal solution theory to describe the formation of mixed admicelles (3 ). Positive deviation from ideality in the mixed admicelle phase was reported, and the non-ideality was attributed to the planar shape of the admicelle. However, a computational error was made in comparison of the ideal solution theory equations to the experimental data, even though the theoretical equations presented were correct. Thus, the positive deviation from ideal mixed admicelle formation was in error. 

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. 

A mixed system of two fiuorinated surfactants was studied by Yoda et al. [123]. Mixtures of LiPFO and LiFOS behaved almost like ideal systems, with an interaction parameter [72] j8 = —0.48 (Fig. 7.22). In contrast, the LiFOS and LiDS mixtures exhibited a positive deviation from ideal predictions. The j8 value of 1.36 was attributed to repulsive interactions between the hydrocarbon and fluorocarbon groups (Fig. 7.23). 

With 1200 ppm polymer present in the brine, the position of the invariant point M does not shift much, but we observe further important deviations from phase behavior in the absence of polymer the microemulsion/brine interfacial tensions are about 3-10 times higher than in the absence of polymer (Figure 3). This may lead to incomplete displacement of the microemulsion by the polymer drive, i.e., a certain amount of microemulsion will remain trapped by capillary forces. This contributes to the high surfactant retention observed in experiments 1-3. 

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). 

The mixed cmc values for mixtures of DEFUMAC with a cationic hydrocarbon surfactant DTAC were determined by plotting equivalent conductivity versus the square root of concentration. The cmc values agreed reasonably well with cmc values calculated from the regular solution theory assuming an interaction parameter value of j8 = 1. The positive (3 value indicates a repulsive interaction between the two surfactants. Hence, the cationic-cationic surfactant mixture deviates more from the ideal regular solution theory than the cationic-nonionic system. 

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