Nonionic Surfactant Systems

Many nonionic surfactants such as the alkylphenol-POE ethers form microemulsions. When compared to systems involving ionic surfactants, a number of important differences are evident  

In most case, no cosurfactant is needed, even for pure surfactant samples in which the POE chain has been purified, rather than the normal distribution of chain lengths found in commercial materials. 

Because of the temperature-solubility relationship for POE nonionic materials, the system temperature becomes an important variable in determining the character of the final microemulsion. 

Not surprisingly, nonionic microemulsion systems are much less sensitive to electrolytes than are ionic systems, although any effect will be in the same sense as that for ionic systems. 

For alkylphenol-POE nonionic surfactants of a constant HLB but different alkyl and POE substitutions, an increase in surfactant molecular weight (tad + head group) results in an increase in the amount of oil that can be included in the system before a second phase begins to form. Branching of the hydrocarbon tail, on the other hand, results in a decrease in oil incorporation capacity. 

Capek and Chudej [87] studied the emulsion polymerization of styrene stabilized by polyethylene oxide sorbitan monolaurate with an average of 20 monomeric units of ethylene oxide per molecule (Tween 20) and initiated by the redox system of ammonium persulfate and sodium thiosulfite. It is interesting to note that the constant reaction rate period is not present in this polymerization system. The maximal rate of polymerization is proportional to the initiator and surfactant concentrations to the -0.45 and 1.5 powers, respectively. The final number of latex particles per unit volume of water is proportional to the initiator and surfactant concentrations to the 0.32 and 1.3 powers, respectively. In addition, the resultant polymer molecular weight is proportional to the initiator and surfactant concentrations to the 0.62 and -0.97 powers, respectively. Some possible reaction mechanisms may explain the deviation of the polymerization system from the classical Smith-Ewart theory. Lin et al. [88] investigated the emulsion polymerization of styrene stabilized by nonylphenol polyethoxylate with an average of 40 monomeric units of ethylene oxide per molecule (NP-40) and initiated by sodium persulfate. The rate of polymerization versus monomer conversion curves exhibit two nonsta-tionary reaction rate intervals and a vague constant rate period in between. 

The rate of polymerization and the final number of latex particles per unit volume of water are proportional to the 1.4 and 2.4 powers, respectively, of the NP-40 concentration. The polymerization system does not follow the conventional micellar nucleation model, and some possible reaction mechanisms responsible for this deviation are discussed. 

Ouzineb et al. [89] carried out emulsion copolymerizations of n-butyl acrylate and methyl methacrylate with different types and concentrations of surfactants (Triton X-405 versus sodium dodecyl sulfate) to study particle nucleation and the resultant latex particle size and particle size distribution. The presence of relatively hydrophilic methyl methacrylate in the continuous aqueous phase has a significant influence on the CMC of Triton X-405. Furthermore, the relatively hydrophobic n-butyl acrylate predominates in the particle nucleation process involved in emulsion copolymerizations of n-butyl acrylate and methyl methacrylate, with the final number of latex particles per unit volume of water very similar to that of latex particles obtained from the homopolymerization of n-butyl acrylate. 

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The surfactants used in the emulsion polymerization of acryhc monomers are classified as anionic, cationic, or nonionic. Anionic surfactants, such as salts of alkyl sulfates and alkylarene sulfates and phosphates, or nonionic surfactants, such as alkyl or aryl polyoxyethylenes, are most common (87,98—101). Mixed anionic—nonionic surfactant systems are also widely utilized (102—105). 

P. Sakya, J. M. Seddon, R. H. Templer, R. J. Mirkin, G. J. T. Tiddy. Micellar cubic phases and their structural relationships the nonionic surfactant system Ci2EOi2/water. Langmuir 75 3706-3714, 1997. 

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

Benton, W.J. Raney, I.H, Miller, C.A. Enhanced Videomicroscopy of Phase Transitions and Diffusional Phenomena in Oil-Water-Nonionic Surfactant Systems, paper presented at the National AIChE Meeting, March 1985, Houston, Texas. 

Emulsion Polymerizations, eg. vinyl acetate [VAc]/ABDA, VAc/ethylene [VAE]/ABDA, butyl acrylate [BA]/ABDA, were done under nitrogen using mixed anionic/nonlonic or nonionic surfactant systems with a redox Initiator, eg. t-butyl hydroperoxide plus sodium formaldehyde sulfoxylate. Base monomer addition was batch or batch plus delay comonomer additions were delay. 

The same effect is seen when a non—aromatic cationic surfactant/nonionic surfactant system is used. Since the nonideality of mixed micelle formation in this case is due almost entirely to the electrostatic effects and not to any specific interactions between the dissimilar hydrophilic groups, the geometrical effect just discussed will cause the EO groups to be less compactly structured... 

The equilibrium in these systems above the cloud point then involves monomer-micelle equilibrium in the dilute phase and monomer in the dilute phase in equilibrium with the coacervate phase. Prediction o-f the distribution of surfactant component between phases involves modeling of both of these equilibrium processes (98). It should be kept in mind that the region under discussion here involves only a small fraction of the total phase space in the nonionic surfactant—water system (105). Other compositions may involve more than two equilibrium phases, liquid crystals, or other structures. As the temperature or surfactant composition or concentration is varied, these regions may be encroached upon, something that the surfactant technologist must be wary of when working with nonionic surfactant systems. 

This equation is applied to the nonionic surfactant system as well. 

The cloud point phenomena as a lower consolute solution temperature is becoming better understood in terms of critical solution theory and the fundamental forces involved for pure nonionic surfactant systems. However, the phenomena may still occur if some ionic surfactant is added to the nonionic surfactant system. A challenge to theoreticians will be to model these mixed ionic/nonionic systems. This will require inclusion of electrostatic considerations in the modeling. 

J.M. Corkill and K.W. Herrmann, Solution structure in concentrated nonionic surfactant systems, J. Phys. Chem. 67 (1963) 934-937. 

Similar attempts were made by Likhtman et al. [13] and Reiss [14]. Reference 13 employed the ideal mixture expression for the entropy and Ref. 14 an expression derived previously by Reiss in his nucleation theory These authors added the interfacial free energy contribution to the entropic contribution. However, the free energy expressions of Refs. 13 and 14 do not provide a radius for which the free energy is minimum. An improved thermodynamic treatment was developed by Ruckenstein [15,16] and Overbeek [17] that included the chemical potentials in the expression of the free energy, since those potentials depend on the distribution of the surfactant and cosurfactant among the continuous, dispersed, and interfacial regions of the microemulsion. Ruckenstein and Krishnan [18] could explain, on the basis of the treatment in Refs. 15 and 16, the phase behavior of a three-component oil-water-nonionic surfactant system reported by Shinoda and Saito [19],... 

Based upon the use of nonionic surfactant systems and their cloud point phase separation behavior, several simple, practical, and efficient extraction methods have been proposed for the separation, concentration, and/or purification of a variety of substances including metal ions, proteins, and organic substances (429-441. 443.444). The use of nonionic micelles in this regard was first described and pioneered by Watanabe and co-workers who applied the approach to the separation and enrichment of metal ions (as metal chelates) (429-435). That is, metal ions in solution were converted to sparingly water soluble metal chelates which were then solubilized by addition of nonionic surfactant micelles subsequent to separation by the cloud point technique. Table XVII summarizes data available in the literature demonstrating the potential of the method for the separation of metal ions. As can be seen, factors of up to forty have been reported for the concentration effect of the separated metals. 

Future work in this area should focus on further development of novel extraction schemes that exploit one or more of the cited advantages of the nonionic cloud point method. It is worth noting that certain ionic, zwitterionic, microemulsion, and polymeric solutions also have critical consolution points (425,441). There appear to be no examples of the utilization of such media in extractions to date. Consequently, the use of some of these other systems could lead to additional useful concentration methods especially in view of the fact that electrostatic interactions with analyte molecules is possible in such media whereas they are not in the nonionic surfactant systems. The use of the cloud point event should also be useful in that it allows for enhanced thermal lensing methods of detection. 

In the nonionic system observed under EVM, the initial microemulsion showed no tendency of gelation until it reached 60 C. After reaching 60<>C, the system gels and starts to polymerize after 10-12 hours. As polymerization proceeds, the water separates out. After about 20-24 hours, the gel starts to become a solid with an excess emulsion phase formed at the bottom. The polymerization is essentially complete after 36 hours. Due to different modes of polymerization in the anionic and nonionic surfactant systems, the mechanical properties of the solid are different. The polymers obtsuned from anionic microemulsions are brittle, while those obtmned from nonionic microemulsions are ductile. 

It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species. 

Benton, W.J., Raney, K.H., and Miller, C.A., Enhanced videomicroscopy of phase transitions and diffusional phenomena in oil-water-nonionic surfactant systems, J. Colloid Interface ScL, 110, 363, 1986. 

Alander, J. and Warnheim, T. (1989) Model microemulsions containing vegetable oils. Part 1 Nonionic surfactant systems. /. Am. Oil Chem. Soc., 66( 11), 1656-1660. 

Kunieda H, Yamagata M. Three phase behavior in a mixed nonionic surfactant system. Colloid Polym Sci 1993 271 997-1004. 

Fig. VI-18. Phase diagrams of water - hydrocarbon (oil) - nonionic surfactant system at three different temperatures. Winsor equilibria.

The behavior of protein-surfactant systems at the oil-water interface is similar to that at the air-water interface. For example, the interfacial tension-log[SDS] plot in the presence of sodium caseinate has very similar characteristics to that for gelatin plus SDS (Fig. 5) in showing an interfacial tension below that of SDS in the presence of caseinate at a low concentration followed by a plateau as the SDS concentration is raised [72]. As for the air-water interface, complexes are initially adsorbed and then displaced as the SDS-to-protein concentration ratio is increased. In the case of protein-nonionic surfactant systems at the oil-water interface, e.g., /3-casein plus C12E8, complexation is either very weak or nonexistent, and the protein and surfactant complete for adsorption sites at the interface [73], If the protein concentration is constant and the surfactant concentration is increased, the /3-casein, which in the absence of surfactant adsorbs at the interface, is progressively displaced until the interface contains... 

One limitation of the HLB concept is its failure to account for variations in system conditions from that at which the HLB is measured (e.g., temperature, electrolyte concentration). For example, increasing temperature decreases the water solubility of a nonionic surfactant, ultimately causing phase separation above the cloud point, an effect not captured in a temperature-independent HLB value. When both water and oil are present, the temperature at which a surfactant transitions from being water soluble to oil soluble is known as the phase inversion temperature (PIT). Below the PIT, nonionic surfactants are water soluble, while above the PIT. they are oil soluble. Thus, from Bancroft s rule, a nonionic surfactant will form an 0/W emulsion below its PIT and a W/0 emulsion above its PIT. Likewise, increasing salt concentrations reduces the water solubility of ionic surfactant systems. At elevated salt concentrations, ionic surfactants will eventually partition into the oil phase. This is illustrated in Fig. 13. which shows aqueous micelles at lower salt concentrations and oil-phase inverse micelles at higher salt concentrations. Increasing the system temperature will likewise cause this same transition for nonionic surfactant systems. 

Strey and coworkers [59] showed evidence that the structure of such a middle phase in a nonionic surfactant system resembles a molten cubic structure. This is a multiply connected structure, and this may indeed provide an answer to the question as to why a middle phase does not swell. It is not clear yet whether this structure is the same for all middle-phase microemulsions. 

From these phase diagrams, one important conclusion must be emphasized A comparison of the water-rich side and the oil-rich side of the phase diagram shows similar properties in their phase behavior and structures. In each side as the alcohol concentration is varied, the system exhibits the sequence micelle (or vesicle)-lamellar-sponge. This reveals that although the experimental situation seems opposite, the physics is the same and can be described with the flexible surface model [19]. Symmetry properties of phase behavior were found also with nonionic surfactants systems [104]. 

Figure 1 Phase changes of a system containing equal amounts of oil (O) and water (W) and a given (low) amount of surfactant (S). For a nonionic surfactant system, left-to-right transition may be induced by increasing temperature. (From Ref 33.)...

Several objectives motivated the extension of ACN studies to light compressible solvents [12]. Initial studies of AOT in such solvents had demonstrated the possibility of intriguing solvent effects [20,21,32], which could be clarified by additional experiments. A second objective was to test the concepts generated from the thermodynamic models that were developed for the AOT-brine-propane system [25,44]. A final objective was to study the behavior of nonionic surfactant systems as a complement to AOT systems. Nonionic systems provide an enhanced opportunity to study temperature effects on surfactant phase behavior, as nonionic surfactants are much more responsive to temperature than the anionic surfactant AOT. 

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