Phase rule micelle formation

The thermodynamic equilibria of amphiphilic molecules in solution involve four fundamental processes (1) dissolution of amphiphiles into solution (2) aggregation of dissolved amphiphiles (3) adsorption of dissolved amphiphiles at an interface and (4) spreading of amphiphiles from their bulk phase directly to the interface (Fig. 1.1). All but the last of these processes are presented and discussed throughout this book from the thermodynamic standpoint (especially from that of Gibbs s phase rule), and the type of thermodynamic treatment that should be adopted for each is clarified. These discussions are conducted from a theoretical point of view centered on dilute aqueous solutions the solutions dealt with are mostly those of the ionic surfactants with which the author s studies have been concerned. The theoretical treatment of ionic surfactants can easily be adapted to nonionic surfactants. The author has also concentrated on recent applications of micelles, such as solubilization into micelles, mixed micelle formation, micellar catalysis, the protochemical mechanisms of the micellar systems, and the interaction between amphiphiles and polymers. Fortunately, almost all of these subjects have been his primary research interests, and therefore this book covers, in many respects, the fundamental treatment of colloidal systems. 

Discussions of micelle formation and related phenomena based on the phase rule also lead to important conclusions, because this approach is thermodynamically correct. The concept of degrees of freedom, which is based on this approach, is employed frequently in this book. 

In the previous chapters, the dissolution and micellization of surfactants in aqueous solutions were discussed from the standpoint of the degrees of freedom as given by the phase rule. The mass-action model for micelle formation was found to be better for explaining the phenomena of surfactant solutions than the phase-separation model. Two models have similarly been used to explain the Krafft point, one postulating a phase transition at the Krafft point and the other a solubility increase up to the CMC at the Krafft point. The most recent version of the first approach is a melting-point model for a hydrated surfactant solid. The most direct approach to the second model of the Krafft point rests entirely on measurements of the solubility and CMC of surfactants with temperature. From these mesurements the concept of the Krafft point can be made clear. This chapter first reviews the concepts used to relate the dissolution of surfactants to their micellization, and then shows that the concept of a micelle temperature range (MTR) can be used to elucidate various phenomena concerning dissolution... 

The two-phase (phase separation) model [24,41-43] regards the micelle as a separate phase, albeit a small entity of microscopic dimensions. The cmc is considered to correspond to the maximum solubility of the monomeric surfactant. If the saturation concentration is exceeded, a new phase, the micelle, appears. The micelle is thermodynamically stable and reversible. The phase-separation model assumes that the activity of the surfactant molecule [44-51] and/or the surface tension [50,52] of the surfactant solution remains constant above its cmc. This assumption is not correct, however [29]. Furthermore, the phase-separation model is not consistent with the number of degrees of freedom given by the Gibbs phase rule [29]. In spite of these difficulties, the two-phase model has explained solubilization and mixed-micelle formation reasonably well and is therefore widely used. 

Catalysis of ethylbenzene oxidation initiated by Ni(ll)(acac)2 + CTAB system is not connected with formation of micro-phase by the type of inverse micelles since the micellar effect of CTAB revealing at f < 100° [77] is as a rule not important at f > 120°. Furthermore, as we saw the system Ni(ll)(acac)2 + CTAB was not active in decomposition of ROOH. 

Investigations of the effects of oil-soluble surfactants on the emulsification of paraffins in aqueous surfactant solutions led to the proposal that the formation of interfacial complexes at the oil-water interface could increase the ease with which emulsions could be formed and, possibly, explain the enhanced stability often found in such systems (Figure 9.9). By definition, an interfacial complex is an association of two or more amphiphilic molecules at an interface in a relationship that will not exist in either of the bulk phases. Each bulk phase must contain at least one component of the complex, although the presence of both in any one phase is not ruled out. The complex can be distinguished from such species as mixed micelles by the fact that micelles (and therefore mixed micelles) are not adsorbed at interfaces. According to the Le Chatelier principle, the formation of an interfacial complex will increase the Gibbs interfacial excess F/ [Eq. (9.2)] for each individual solute involved, and consequently, the interfacial tension of the system will decrease more rapidly with increasing concentration of either component. 

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