Micellar association

Disperse systems can also be classified on the basis of their aggregation behavior as molecular or micellar (association) systems. Molecular dispersions are composed of single macromolecules distributed uniformly within the medium, e.g., protein and polymer solutions. In micellar systems, the units of the dispersed phase consist of several molecules, which arrange themselves to form aggregates, such as surfactant micelles in aqueous solutions. 

An early review of micellization in block copolymers was presented by Tuzar and Kratochvfl (1976), and these authors have recently provided an excellent review of the literature up to 1992 (Tuzar and Kratochvfl 1993). Micellar properties of block copolymers were also reviewed by Price (1982). A discussion of micellization was included in the general reviews on block copolymers by Riess et al. (1985) and Brown et al. (1989). Excellent reviews focussed on the solution properties of a particular class of copolymer, i.e. copolymers of polyoxyethylene) with poly(oxypropylene) have been presented by Alexandridis and Hatton (1995) and by Chu (1995) and Chu and Zhou (1996). Micellization and micellar association in related poly(oxyethylene)/poly(oxybutylene) copolymers have been summarized by Booth et al (1997). 

PS-P(E/P) M = 48 and 80.38% Decane (selective for P(E/P)) irans-Decalin (neutral) and mixtures with decane Micellar D, RH and viscosity. Evidence for micellar association Viscometry, SLS, DLS Mandema ei at. (1979a, b)... 

Polycap rolactam-PS-poIycaprolac-tam = 31-53, 41-70% ToIuene/2,2,3.4- tetrafluoropropanol Determination of and M. Observation of large micellar associates SLS, DLS, DR T izar et al. (1988)... 

It may be noted that, denoting the fraction of A monomers in the corona belonging to the copolymer by t], and the micellar association number by p, incompressibility conditions give simple relations between R, RB, p and t) (de Gennes 1978 Leibler el al. 1983) ... 

These compounds differ from other surfactants in the pronounced sensitivity of their association structural organization to temperature. This characteristic feature was noted very early by Shinoda (3) with regard to their micellar association and solubilization. A corresponding sensitivity may also be observed in the strong dependence of the liquid crystalline regions in phase diagrams of the system water, surfactant, and hydrocarbon (4). 

One possible structure would consist of irregular forms among which spheres and cylinders with alternatively hydrophobic and hydrophilic outer surfaces are formed according to Figure 8. This suggestion has an immediate attraction its features are similar to critical phenomena [cf., two-dimensional Ising s model (40)]. The resemblance between micellar associations and the fast fluctuation aggregates before phase separations... 

According to the present results the marked changes of isotropic liquid regions at the PIT described by Shinoda and co-workers (29, 30, 31, 32, 33) appear limited to micellar associations and solubilization. The cosolvency of water and hydrocarbon in the emulsifier and the liquid crystalline phase region seem to undergo only small changes. 

Anderson, B. D., R. A. Conradi, and K. Johnson. 1983.ilBnce of premicellar and micellar association on the reactivity of methyl prednisolone 21-hemiesters in aqueous solutifRharm. Sci72 448-454. 

A very significant effect on the properties of proteins is exerted by their quaternary structures and micellar associations. 

The method is based on the ability of certain membranes to retain large molecules or aggregates, for example, micelles. A small part of the micellar solution is passed through the membrane, and the concentration of solute is determined in the filtrand and filtrate solutions. The partition coefficient of the solute can thus be calculated from the fraction of micellar-associated solute, which is given by the following relation ... 

FIG. 6.1 Structural information obtained from SANS for an 8 wt% EO37PO58EO37 solution at 60°C plotted as a function of the cosolvent (glycerol, formamide, or ethanol) content in the mixed solvent. First row micellar association number ( association) second row radii of core and core + corona (Wcore and / micelle) third row polymer volume fraction of core and corona (acore and acorona). (From Alexandridis, P. and Yang, L., Macromolecules, 33, 5574, 2000.)... 

In contrast to aqueous systems, micelle formation in non-polar media is driven by the benefit in energy rather than by an increase in entropy. The replacement of polar group - hydrocarbon interaction (as in the case of dissolution) with the interaction between polar groups upon their association into micellar core is thermodynamically beneficial. The benefit in energy upon association of polar groups is so large, that even at low concentrations true surfactant solutions contain small pre-micellar associates rather than individual surfactant molecules. 

The anionic AOT-heptane-water system is one of the best characterized microemulsions [40]. Up to 2 m AOT can be solubilized in heptane. The phase diagram was determined by Rouviere et al. [40]. It shows a particularly large L2 region (Fig. 1) in which water can be dispersed, even at very low surfactant concentrations. It is the wedge-shaped structure of the surfactant that favors the reverse micellar association. The reverse micelles can take up to 60 molecules of water per surfactant headgroup. 

A similar approach was taken by Aniansson et al. [81] who corroborated the above observations. Measurement of relaxation times (ps for T, and ms for X2) at different temperatures showed that the values decreased with increasing temperature and the total surfactant concentration. At 25" C, the value of T, was 320 ps, which decreased by a half to 155 ps at 35 C. The decrease in T2 through the same temperature range was more drastic, namely, from 41 ms to only 7 ms. Aniansson et al. [81 ] considered it likely that the process of micellar association/dissociation took place stepwise, i.e. it was marked by the entry and exit of one monomer at a time. Aggregated forms (dimers, trimers) with relatively low concentration could apparently contribute little to the process. 

The structure in the micellar region of the phase diagrams of potassium soaps has been analysed by Reiss-Husson and Luzzati (1969) using X-ray methods. A common feature in soaps of saturated fatty acids is that spherical micelles exist at low concentrations, and at increased concentrations a transition into rod micelles occurs. Sodium oleate, however, was found to give rod-shaped micelles at all concentrations. The micellar association and phase behaviour have been reviewed by Wenner-strom and Lindman (1979) and Lindman and Wennerstrom (1980). 

The possession of a cmc may be viewed as a criterion of micellar association. However, as discussed by Mukerjee (5), it is all too easy to imagine inflection points in experimental curves and it is preferable to examine the system using a variety of techniques and to compare cmc values. Table 1 shows such a comparison for antihistamine drugs containing a diphenylmethane hydrophobic group and shows reasonable agreement between values. 

Phenothiazine tranquillisers, of which chlorpromazine is an example, have been shown by a variety of techniques to undergo micellar association (8) (9). It is of interest however to note that nmr studies (10) (11) have suggested a stacked arrangement of molecules within the phenothiazine micelles. The association of the tricyclic antidepressants, for example amitriptyline, has similarly been shown to be micellar (12). 

Later studies on other drugs with non-micellar association patterns showed that the apparent c.m.c. detected by surface tension techniques arose because of the very limited change of monomer concentration with total solution concentration at high concentrations. 

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