Association-dissociation equilibrium micellization

Association-dissociation equilibrium, in micellization, 24 128, 129-131 Association for the Advancement of Medical Instrumentation (AAMI) Standards, 26 818-819 Association of American Feed Control Officials (AAFCO), 10 856 Nutrient Profiles, 10 857, 858-859t... 

Bile salts carry extensive hydrophobic (hydrocarbon) portions in each molecule that attempt to reduce their contact with water (4). This is reflected in rapid, dynamic association-dissociation equilibrium to form self-aggregates or micelles as the total concentration of bile salt solute is increased (the CMC) [2-6]. Experimentally, micelles are undetectable in dilute solutions of the monomers, and are detected in increasing numbers and often size above the CMC [98]. Because bile salt micelles are often small (i.e., dimers) [5], and since self-aggregation continues to proceed in many cases with increasing concentration above the CMC [17,18,20,52,98], the detection of the lowest concentration at which the first aggregates form depends particularly upon the sensitivity of the experimental probes employed [98] and the physical-chemical conditions [3-5]. 

In the treatment by the mass action model, micellization is considered as an association-dissociation equilibrium of individual molecules or ions with micelles in the concentration range above CMC. 

In the second approach, micelles and single surfactant molecules or ions are considered to be in association-dissociation equilibrium. In its simplest form, a single equilibrium constant is used to treat the process represented by Eq. (2.1). 

In the second approach, micelles and single surfactant molecules or ions are considered to be in association-dissociation equilibrium. In its simplest form, a single equilibrium constant is used to treat the process represented by Equation (20.2). The CMC is merely a concentration range above which any added surfactant appears in solution in a miceUar form. Since the solubility of the associated surfactant is much greater than that of the monomeric surfactant, the solubility of the surfactant as a whole will not increase markedly with temperamre until it reaches the CMC region. Thus, in the mass action approach, the Krafft point represents the temperature at which the surfactant solubility equals the CMC. 

Two main approaches to the thermodynamic analysis of the micellization process have gained wide acceptance. In the phase separation approach the micelles are considered to form a separate phase at the CMC, whilst in the mass-action approach micelles and unassociated monomers are considered to be in association-dissociation equilibrium. In both of these treatments the micellization phenomenon is described in terms o.f the classical system of thermodynamics. Theories of micelle formation based on statistical mechanics have also been proposed [16Q-162] but will not be considered further. The application of the mass-action and phase-separation models to both ionic and non-ionic micellar systems will be briefly outlined and their limitations discussed. More recent developments in this field will be presented. 

Micelles and unassociated surfactant ions are assumed to be in association-dissociation equilibrium and the law of mass action is applied. The mass-action approach was originally applied mainly to ionic surfactants [168-170]. Its application to non-ionic surfactants has been discussed by Corkill et al. [171]. 

Micellar colloids are in a dynamic association-dissociation equilibrium, and the kinetics of micelle formation have been investigated for a long time. " In 1974, a reasonable explanation of the experimental results was proposed by Aniansson and Wall, " and this conception has been accepted and used ever since. The rate of micelle dissociation can be studied by several techniques, such as stopped flow, pressure jump, temperature jump, ultrasonic absorption, NMR, and ESR. The first three methods depend on tracing the process from a nonequilibrium state brought about by a sudden perturbation to a new equilibrium state— the relaxation process. The last two methods, on the other hand, make use of the spectral change caused by changes in the exchange rate of surfactant molecules between micelle and intermicellar bulk phase. 

Micelles are not frozen objects. They are in equilibrium with free surfactants. In a micellar solution, surfactants are constantly exchanged between micelles and surrounding (intermicellar) solution. This implies processes of entry (or incorporation or association) of surfactants into micelles. Conversely, surfactants can exit (or dissociate) from micelles. The entry/exit processes are usually referred to as exchange processes. Owing to these processes a given surfactant resides in a micelle a finite time, which is the surfactant residence time. Likewise, one can define the residence time of a solubilizate in a micelle. 

Solubilization of lipid digestion products in intestinal mixed micelles enhances their dissolution and dramatically increases the GI lumen-enterocyte concentration gradient that drives absorption by means of passive diffusion. Micelles, however, are not absorbed intact [8, 9], and lipids are thought to be absorbed from a monomolecular intermicellar phase in equilibrium with the intestinal micellar phase [10], The dissociation of monomolecular lipid from the micellar phase appears to be stimulated by the presence of an acidic microclimate associated with the enterocyte surface [11,12], In addition to passive diffusion, growing evidence suggests that active uptake processes mediated by transport systems located in the enterocyte membrane are also involved in the absorption of (in particular) fatty acids into the enterocyte [4],... 

Two current approaches to explain the process of micellization are law of mass action and phase separation. In the law of mass action, micellization is treated as an equilibrium process between the progressive association and dissociation of the monomers (Figure 4.20). In phase separation, two phases (i.e., surfactant monomers in aqueous phase and micelles) are in equilibrium above the c.m.c. In this model, micellization takes place as a one-step process. 

A theory for the stepwise association and dissociation of surfactant micelles was developed a few years ago. Its application to a large quantity of experimental data has provided a consistent interpretation of these data and enabled the extraction of basic kinetic and equilibrium parameters for these systems In the following extension to mixed micelles the concepts and assumptions used are closely analogous to those of the previous treatment to which the reader is referred for more details. For simplicity the treatment is limited to two-component micelles but the extension to larger number of components is quite straightforward. 

Experimental investigations using fast kinetic methods such as stopped-flow, temperature and pressure jumps, and ultrasonic relaxation measurements have shown that there are two relaxation processes for micellar equilibrium [12-18], characterized by relaxation times ti and T2- The first, ri, is of the order of 10 s (10 to 10 s) and represents the life-time of a surface active molecule in a micelle, i.e. it represents the association and dissociation rate for a single molecule entering and leaving the micelle, which may be represented by Eq. (2.7). 

This model assumes a dissociation-association equilibrium between surfactant monomers and micelles - thus an equilibrium constant can be calculated. For a nonionic surfactant, where charge effects are absent, this equilibrium is simply represented by Eq. (2.1), which assumes a single equilibrium. In this case, the equilibrium constant Km is given by Eq. (2.15). 

The mass action model allows a simple extension to be made to the case of ionic surfactants, in which micelles attract a substantial proportion of counter ions, into an attached layer. For a micelle made of n-surfactant ions, (where n — p) charges are associated with counter ions, i.e. having a net charge of p units and degree of dissociation pjn, the following equilibrium may be established (for an anionic surfactant with Na+ counter ions),... 

Sams et al. [61] proposed a two-state kinetic model which assumed a monomeric state and an associated state consisting of aggregates in various sizes larger than the monomer. The model describes only the fast process and assumes that the rate constants for association and dissociation are independent of the micelle size. A revised version of the two-state model [62,63] assumed micelle formation to be an adsorption phenomenon, with micelles at equilibrium with monomers adsorbing and escaping from the surface of micelles. 

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