Solution equilibria of surfactants

Reagents used in flotation, collectors, frothers, depressants, flocculants and inorganic modifiers can interact with each other in the flotation pulp and at the mineral-solution interface. The chemical equilibria involved in these interactions and the nature of the products will have a significant effect on their adsorption and the resultant flotation processes. 

Most of the theories on interactions of surfactants with minerals are closely related to their solution chemistry. For example, the ion-exchange adsorption theory proposed by Gaudin (1932, 1934) and Wark (1938) and the molecular adsorption theory proposed by Cook and Nixon (1950) are based on the dissociation equilibria and states of the collectors in water. More recently, Somasundaran (1976) observed that ion-molecule complexes of long-chain surfactants in flotation systems can have high surface activity depending upon the association equilibria of the surfactants in solutions (Ananthapadmanabhan et al., 1979 Kulkarni and Somasundaran, 1980). Also the cationic flotation behavior of salt type minerals is closely related to the formation of alkyl amine salt (Hu and Wang, 1990). In this chapter, solution equilibria of reagents relevant to selected flotation systems are examined. 

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The thermodynamic equilibria of surfactant molecules in hydrocarbon solutions involve four fundamental processes dissolution, micellization, solubilization and interfacial processes, see Fig. 3.3 (Kertes and Gutman, 1976 Kon-no, 1993 Moroi, 1992). 

Fig. 3.3. Four fundamental processes for thermodynamic equilibria of surfactant molecules in hydrocarbons (1) dissolution of molecules into solution, (2) micellization (or aggregation) of dissolved molecules, (3) adsorption (solubilization) of molecules at an interface, and (4) interfacial processes of surfactant molecules (oVW = surfactant molecule), (Moroi, 1992)...

Solution equilibria of flotation reagents. Solution equilibria of various surfactants such as collectors, frothers, modifiers, and flocculants, encompass acid-base equilibria, dissociation equilibria, association equilibria and polymer equilibria. Using these equilibria and also various parameters such as the pH of the solutions and the critical pH of the dissolution of insoluble flotagents, pA(a can be calculated and the state of active flotation reagent species in solution predicted. 

Adsorption equilibria of surfactants on activated carbon from aqueous solutions... 

Taking Simultaneous Micellizadon and Adsorption Phenomena into Consideration In the presence of an adsorbent in contact with the surfactant solution, monomers of each species will be adsorbed at the solid/ liquid interface until the dual monomer/micelle, monomer/adsorbed-phase equilibrium is reached. A simplified model for calculating these equilibria has been built for the pseudo-binary systems investigated, based on the RST theory and the following assumptions ... 

Monomer—Micelle Equilibria. The distribution of surfactant components between micelles and monomeric state in aqueous solutions depends on surfactant structures as well as on overall solution composition. For example, for a binary system of surfactants A and B in solution, the micelle may contain SO mole % A/SO X B while the monomer may be 90 /. A/10 X B. Since either the monomer or the micelle composition may be crucial to behavior of the system, the ability to predict the relative distribution of surfactant components between monomer and micelle, given the overall solution composition, is an important one. 

Considering the importance of micellar aggregates in separations, it is unfortunate that our knowledge of solute-micelle equilibria is quite limited, both as regards the dependence of solute activities on the intramicellar mole fractions of surfactant and organic compound, and in relation to the influence of total... 

Inside the column, solutes are affected by the presence of micelles in the mobile phase and by the nature of the alkyl-bonded stationary phase, which is coated with monomers of surfactant (Fig. 1). As a consequence, at least two partition equilibria can affect the retention behavior. In the mobile phase, solutes can remain in the bulk water, be associated to the free surfactant monomers or micelle surface, be inserted into the micelle palisade layer, or penetrate into the micelle core. The surface of the surfactant-modified stationary phase is micelle-like and can give rise to similar interactions with the solutes, which are mainly hydrophobic in nature. With ionic surfactants, the charged heads of the surfactant in micelles and monomers adsorbed on the stationary phase are in contact with the polar solution, producing additional electrostatic interactions with charged solutes. Finally, the association of solutes with the nonmodified bonded stationary phase and free silanol groups still exists. 

The decrease of surfactant solubility in solution may also modify the phase equilibria in the system [26],... 

At lower surfactant concentrations (line b) regions of phase separation appear. In such a phase-separated state there is a sequence of equilibria between phases, commonly referred to as Winsor equilibria [13,23]. In point A two macroscopic phases are formed. These are a microemulsion of composition B and an aqueous solution containing dissolved surfactant and micelles with solubilized hydrocarbon. The volumes of these macroscopic phases can be estimated in the usual way by applying the lever rule with a correction for the densities of these phases. In such a state of separation into two macroscopic phases, the equilibrium between the microemulsion and the aqueous solution is referred to as the Winsor II (WII) equilibrium. 

Currently, we are studying in our group the adsorption of polyelectrolytes from aqueous solutions on charged and uncharged surfaces. Investigations of thiol adsorption on surfaces (e.g., Ref 257, 258) conformational equilibria of polymers near surfaces (e.g.. Ref 259), of surfactants with surfaces and micelles [260-263] and others were mostly done without explicit modeling of the solvent. 

Aggregation of surfactants in apolar solvents, e.g., aliphatic or aromatic hydrocarbons, occurs provided that small amounts of water are present [1,126,127], These aggregates are often called reverse micelles, although the solutions do not always appear to have a critical micelle concentration, and surfactant association is often governed by a multiple equilibrium, mass action, model vith a large spread of aggregate sizes [130,131], It has recently been suggested that the existence of a monomer f -mer equilibrium should be used as a criterion of micellization, and that this term should not be applied to self-associated systems which involve multiple equilibria [132],... 

The complexity of the equilibrium phases and nonequilibrium phenomena exhibited by multicomponent oil-water-surfactant systems is amply demonstrated in numerous contributions in this volume. Therefore, the need for theoretical (and computational) methods that make the interpretation of experimental observations easier and serve as predictive tools is readily apparent. Excellent treatments of the current status of theoretical advances in dealing with microemulsions are available in recent monographs and compendia (see, e.g., Refs. 1-3 and references therein). These references deal with systems consisting of significant fractions of oil and water and focus on the different phases and intricate microstructures that develop in such systems as the surfactant and salt concentrations are varied. In contrast, the present chapter focuses exclusively on simulations, particularly on a first level introduction to the use of lattice Monte Carlo methods for modeling self-association and phase equilibria in surfactant solutions with and without an oil phase. Although results on phase equilibria are presented, we spend a substantial portion of the review on micellization in surfactant-water mixtures, as this forms the necessary first step in the eventual identification of the most essential parameters needed in computer models of surfactant-water-oil systems. 

This chapter is intended to deal with the elucidation and characterization of structures occurring in monophasic Winsor IV equilibria. However, in many cases we use examples of binary amphiphilic systems whose isotropic solutions cannot, obviously, be considered microemulsions. This is because many of the microstructures found in three-component systems are simple extensions of their binary counterparts [16,27]. Furthermore, we included important results concerning liquid crystalline phases, as this information provides, we believe, the fundamental basis for more profound understanding of surfactant organization in multicomponent systems. 

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