Micellization-dissolution process

A longer relaxation time Z2 (of the order of 10 -1 s), which is a measure of the micellization-dissolution process. Both ti and xi depend on the surfactant structure, its chain length, and these relaxation times determine some of the important factors in selecting adjuvants, such as the dynamic surface tension (discussed below). 

As with micelle-facilitated dissolution, emulsion-facilitated dissolution has gained renewed interest due to its application to water-insoluble drug delivery and enhanced absorption. Over the years, emulsion systems have been developed and used to either model the in vivo dissolution process or mimic the intestinal surfactant system to enhance drug delivery of poorly soluble compounds [54-66], Emulsions have also been used as vehicles for drug delivery, e.g., to protect... 

Fig. 26 A Model of the preferential dissolution process of PA at low SDS concentration. B Model for the existence of vesicle and mixed micelles at high SDS concentration...

Fig. 6, Variation of the reciprocal of the relaxation time T2 for the micelle formation-dissolution process upon addition of pentanol to O.IM TTAB (+) and O.lM tetradecylpyridinium bromide ( ) solutions in water at 25°. (The measurements were performed by means of T-jump Eosine was used as a probe for the relaxation process in TTAB solutions (see C. Tondre, J. Lang and R. Zana, J. Colloid Interface Sci., 52, 372 (1975).)...

Based on the above concepts, one would expect that the ratio of monomers Ci to micelles Cm, the aggregation number the rates of micelle formation (kf) and micelle dissolution (ka) will influence the rate of the adsorption process. Figure 11.20 gives a schematic picture of the kinetic process in the presence of micelles. 

In this case the dissolution process is only diffusion regulated. In the case of BS micelles> instead, the process of Ch dissolution is largely regulated by interfacial resistance the positive interception the vertical axis (D) of the correlation between k and o) indicates interfacial resistance. 

Solubility and dissolution are processes that take place in the gastric and the luminal fluids, not on the surface of epithelial cells. Measurement of solubility ideally needs to take place at pH 1.7 (stomach) and pH 5-8 (small intestinal tract). Ideally, the screen media should resemble intestinal fluids and contain bile acid-lecithin mixed micelles. Fast and reliable techniques for assessing solubility in... 

Surface-active agents used as adjuvants in pharmaceutical preparations to improve drug dissolution may affect the stability of /3-lactams. Thus, the presence of micelles of cetyl(trimethyl)ammonium bromide (CTAB) enhanced up to 50-fold the rate of alkaline hydrolysis of penicillins [140]. In the case of cephalosporins, micelle-promoted catalysis of the intramolecular degradation process (see Sect 5.2.2) was also observed [85][141], It has been proposed that the negatively charged penicillins and cephalosporins are attracted by the cationic micelles. This attraction increases substrate concentration in the micellar phase, in turn accelerating the rate of HO- ion attack. Ion exchange at the micellar surface and electrostatic stabilization of the transition state may also contribute to the increased rate [142][143],... 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

When the system has reached its quasi-equilibrium state a slower process, involving the relaxation to the true equilibrium, becomes measureable. This process involves a change in the number of micelles. The formation or dissolution of a micelle involves according to scheme (5.1) the appearence of aggregates of size at the minimum of the size distribution curve, and since these aggregates occur with low probability the process can be a very slow one. Aniansson and Wall showed that this process is also characterized by an exponential decay with a relaxation time r2,... 

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],... 

The basic processes of dissolution, acid-base interaction, micellization, solubilization, oxidation and reduction take place in oil formulation. During engine operation, additives of the lubricant interact continuously with engine surfaces and themselves. Thus, there is a progressive change in the surface due to the lubrication, friction, and wearing processes, tribofilm formation, and oxidation. All these processes are presented and discussed throughout this book. Surfactant additives are fundamental to reverse micelles (RMs) formation in oil... 

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)...

Solubilization of water. Detergency is defined as the ability of surfactant molecules to solubilize water molecules or polar substances in soft-core and hardcore RMs. Thus, micellization and solubilization are competitive processes. Any solubilized probe molecule causes a decrease in the CMC. Solubilization describes the dissolution of a solid, liquid or gas by an interaction with surfactant molecules. Addition of water has a dramatic effect on surfactant aggregation in hydrocarbons because hydrogen bonding has an appreciable stabilizing effect on reverse micelles. Solubilization for reverse micelles is phenomenologically similar to the adsorption processes (Eicke and Christen, 1978 Kitahara, 1980 Kitahara et al., 1976 Singleterry, 1955). 

Other processes responsible for formation of a fine copper tribofilm are mentioned in (Shpenkov, 1995a). In particular, copper ion deposition on steel is explained by contact substitution of iron for copper in solution. If sufficient metallic iron is in contact with the copper micelle, iron dissolution (friction will saturate the surface quicker) and copper deposition will continue until the activity ratio of their ions satisfies the equation ... 

Equilibria How do the following processes change in equilibria of surfactant molecules in mineral or synthetic oil (a) dissolution, (b) micellization, (c) solubilization, (d) interfacial process (see interpretation of Figure 3.3). 

Aqueous micelles are 40-80 A diameter spherical aggregates which are dynamically formed from surfactants in water above a characteristic concentration, the CMC (9). Depending on the chemical structure of their hydrophilic headgroups, surfactants can be neutral or charged (positively or negatively). The alkyl chain of the surfactants typically contains between 5-20 carbon atoms. Micelles rapidly break up and reform by two known processes. The first process occurs on the microsecond time scale and is due to the release and subsequent reincorporation of a single surfactant from and back to the micelle. The second process occurs on the millisecond time scale and is ascribed to the dissolution of the... 

Micelles are in dynamic equilibrium with their monomer surfactants. Two relaxation processes are related to this equilibrium, a fast one in the microsecond time domain associated with the exchange of individual monomers between the micelles and the bulk aqueous phase and a slower one on millisecond time-scale associated with the complete dissolution of the micelles into monomers [8], For example, the exit rate for the SDS anion from its micelle is about lO s, which is considered to be a diffusion-controlled process [8a]. Nonpolar molecules are usually attracted to the relatively hydrophobic inner core of micelles, whereas ionic reactants and products are either associated with the Stem and Gouy-Chapman layers or repelled from the micelles, depending on the sign of electrostatic interaction. For example, NMR studies show that nonpolar molecules such as benzene and naphthalene are... 

---