Surfactant-water system monomeric

Fig. 5.14. Phase diagram of the monomeric surfactant-water system.

Fig. 45. Phase diagram of the system water-monomeric surfactant (see Table 11, No. 3)...

This very simplified model of micellization is illustrated in scheme 4 for a cationic surfactant. At concentrations below the cmc only monomeric surfactant is present, but at higher concentration the solution contains micelle, free surfactant and counterions which escape from the micelle. It is assumed that submicellar aggregates are relatively unimportant for normal micelles in water, although, as we shall see, this assumption fails in some systems. However it is probably reasonable for relatively dilute surfactant, although at high surfactant concentration, and especially in the presence of added salt, the micelle may grow, and eventually, new organized assemblies form, for example, liquid crystals are often detected in relatively concentrated surfactant [1]. However, this discussion will focus on the relatively dUute surfactant solutions in which normal micelles are present. 

In a generic W/O microemulsion (L2 phase), the monomeric surfactant and water in the continuous oil phase are in equilibrium with spherical droplets however, the monomeric concentrations are very low (often neglected in calculations). Depending on temperature and volume fraction, the droplets are in equilibrium with clusters of droplets, mainly because of sticlQ collisions. Such is the constitution of the original equilibrium system that is perturbed by the electric field. 

Kunieda intensified the studies on phase behavior and formation of microemulsions in mixed-surfactant systems [66-76], in order to understand the relationship between maximum solubilization of microemulsions and surfactant distribution of mixed surfactants at the water/oil interface in the microemulsion phase. He developed a method to calculate the net composition of each surfactant at the interface in the bicontinuous microemulsions assuming that the monomeric solubihty of each surfactant in oil is the same as in the oil microdomain of the microemulsions [69]. Using this approach, the distribution of surfactants in the different domains of bicontinuous microemulsions (Figure 9) could be quantified [70-75], even if the complete microstracture of these systems was not completely elucidated. 

The self-diffusion coefficient method was developed originally by Lindman and co-workers [42], This method is by far the most powerful and successful application of NMR in the study of surfactant systems. The self-diffusion method permits a full characterization of the system under investigation, since it can yield the values of the free monomeric surfactant concentration, the degree of counterion binding, and the amount of water bound (hydration). Moreover, these data can help one deduce the information on micelle size, shape, and composition from the monomer surfactant concentration in a surfactant mixture ... 

Initial efforts were focused on the optimization of the online sample conditioning and data interpretation. In order to make absolute measurements on a heterogeneous system containing oil, water, monomer, polymer, initiator, and surfactant, the monomeric and polymeric contents of the inverse emulsion or latex must be spilled out from the discrete phase droplet in a period of seconds and the resulting detectors signals must be interpreted to allow differentiating among the complex mixture of components. 

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. 

Whether an emulsion is 0/W or W/0 depends on a number of variables like oihwater ratio, electrolyte concentration, temperature, etc. For most of this centiuy, emulsion chemists have known that surfactants more soluble in water tend to make 0/W emulsions and surfactants more soluble in oil tend to make W/0 emulsions. This is the essence of Bancroft s rule, which states that the continuous phase of an emulsion tends to be the phase in which the emulsifier is preferentially soluble. The word soluble is misleading, however, for two reasons. Firstly, a surfactant may be more soluble in, say, oil than in water in a binary system, but in the ternary system of oil -I- water + surfactant it may partition more into water. A good example of this is with the anionic surfactant Aerosol OT (sodium bis-2-ethylhexylsulfosuccinate) which dissolves in heptane at 25 C up to at least 0.5 m but has a solubility limit in water of only 0.03 M. An emulsion made from equal volumes of water and heptane at 25 °C is 0/W, however. Secondly, no distinction is made between the solubility of monomeric or aggregated surfactant in oil or water. We will see that this is an important omission. 

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