Micellar solution diffusion

Micellization is a second-order or continuous type phase transition. Therefore, one observes continuous changes over the course of micelle fonnation. Many experimental teclmiques are particularly well suited for examining properties of micelles and micellar solutions. Important micellar properties include micelle size and aggregation number, self-diffusion coefficient, molecular packing of surfactant in the micelle, extent of surfactant ionization and counterion binding affinity, micelle collision rates, and many others. 

Mechanisms of micellar reactions have been studied by a kinetic study of the state of the proton at the surface of dodecyl sulfate micelles [191]. Surface diffusion constants of Ni(II) on a sodium dodecyl sulfate micelle were studied by electron spin resonance (ESR). The lateral diffusion constant of Ni(II) was found to be three orders of magnitude less than that in ordinary aqueous solutions [192]. Migration and self-diffusion coefficients of divalent counterions in micellar solutions containing monovalent counterions were studied for solutions of Be2+ in lithium dodecyl sulfate and for solutions of Ca2+ in sodium dodecyl sulfate [193]. The structural disposition of the porphyrin complex and the conformation of the surfactant molecules inside the micellar cavity was studied by NMR on aqueous sodium dodecyl sulfate micelles [194]. 

JR Crison, VP Shah, JP Skelly, GL Amidon. Drug dissolution into micellar solutions Development of a convective diffusion model and comparison to the film equilibrium model with application to surfactant-facilitated dissolution of carbama-zepine. J Pharm Sci 85 1005-1011, 1996. 

Fig. 19 Diffusion coefficients of the samples MACnE042-grafted microgels (1, 2), mixture of PVCL microgel El and the macromonomer (3), micellar solution of macromonomers (4). (Adapted from Refs. [177,181])...

Since their effective diffusivities are of the same magnitude as those of micellar solutions, the hquid crystalUne phases, though viscous, do not significantly hinder surfactant dissolution for these rather hydrophihc surfactants. Indeed, a drop of Ci2(EO)6 having Ro = 78 pm dissolved completely in only 16 s at 30 °C. Rapid dissolution is favored because free energy decreases as the surfactant is transferred from the Hquid surfactant phase L2 to liquid crystals) to aqueous micellar solution and the aggregate shape approaches that of a dilute Li phase, where its free energy is minimized at this temperature. 

Dungan et al. [186] have measured the interfacial mass transfer coefficients for the transfer of proteins (a-chymotrypsin and cytochrome C) between a bulk aqueous phase and a reverse micellar phase using a stirred diffusion cell and showed that charge interactions play a dominant role in the interfacial forward transport kinetics. The flux of protein across the bulk interface separating an aqueous buffered solution and a reverse micellar phase was measured for the purpose. Kinetic parameters for the transfer of proteins to or from a reverse micellar solution were determined at a given salt concentration, pH, and stirring... 

Pulsed gradient spin-echo (PGSE) NMR techniques have also been employed to study the structure of the oil phase [12]. This gives an idea of the mobility of each component in the HIPE, and showed that, for stable emulsions and HIPEs, the oil phase was indeed a reverse micellar solution which solubilises water. Further work using PGSE NMR has shown that water can diffuse between aqueous droplets in concentrated emulsions [101]. Presumably this involves solubilisation of the water molecules by the micellar oil phase. 

In dynamic light scattering (DLS), or photon correlation spectroscopy, temporal fluctuations of the intensity of scattered light are measured and this is related to the dynamics of the solution. In dilute micellar solutions, DLS provides the z-average of the translational diffusion coefficient. The hydrodynamic radius, Rh, of the scattering particles can then be obtained from the Stokes-Einstein equation (eqn 1.2).The intensity fraction as a function of apparent hydrodynamic radius is shown for a triblock solution in Fig. 3.4. The peak with the smaller value of apparent hydrodynamic radius, RH.aPP corresponds to molecules and that at large / Hs,Pp to micelles. 

The photolyses of 1,2-dipheny1-2-methyl-1-propa-none and its 2h and derivatives in micellar solution are now described and further demonstrate the enhanced cage and magnetic isotope effects of mlcelllzatlon. We report also the observation of CIDP during the photolyses of micellar solutions of several ketones, and demonstrate the validity of the radical pair model to these systems. Analyses of the CIDNP spectra in the presence and absence of aqueous free radical scavengers (e.g., Cu2+) allow us to differentiate between radical pairs which react exclusively within the micelle and those that are formed after diffusion into the bulk aqueous phase. In some cases this allows us to estimate a lifetime associated with the exit of free radicals from the micelles. 

Since the quantum yield for disappearance of ketone (triplet ketone nor the primary radical pair PhCH2CO CH2Ph is scavenged. As stated above, the proposed mechanism for the photolysis of DBK in micellar solution is illustrated in Fig. 5 13,21). The micellar environment inhibits the diffusion of radicals to the bulk aqueous phase the radical pair s distance maximum separation is maintained to a few tens of angstroms or less. The amount of escape being reduced, the radicals can then undergo more efficient intersystem crossing and recombination. 

Photolysis of 2,4-diphenylpentan-3-one (meso and d, 1, 6) produces different ratios of products26) in homogeneous and aqueous micellar solutions (Scheme VI). While DPE is the major product in both homogeneous and micellar solutions, only in micelles is the recovered starting material isomerized (meso - d,l d,l - meso). This indicates that while diffusion out of the micelle is hindered, rotation and diffusion inside the micelle is still facile. 

Pyrene can sensitize the photooxidation of 1,3-diphenylisobenzofuran (72) in DTAC micellar solutions 61>. The reaction involves sensitization of singlet oxygen by pyrene which diffuses into another micelle and reacts with (72). Indole and tryptophan, which also react with singlet oxygen, quench the above reaction in ethanol solutions. However, in micellar systems they enhance the rate of reaction. Because of the high local concentrations of the quencher, the pyrene excited state is quenched by indole and tryptophan which leads to the photooxidation of (72) by a Type I process. 

D. Fennell Evans is the director of the Center for Interfacial Engineering and professor of chemical engineering and materials science at the University of Miimesota. He is the author of more than 180 publications on self-assembly processes in water and nonaqueous solvents, microemulsions, diffusion in liquids and micellar solutions, and characterization of surfaces using scanning probe techniques. He has published two textbooks. The Colloidal Domain and The Fundamentals of Interfacial Engineering. 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

The formation of a dispersion front can be described in terms of the diffusion path. As oil diffuses into the liquid crystal dispersion, the continuous phase gradually increases in oil content from an initial composition which would ordinarily be called a micellar solution to compositions which would be called... 

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