Micellar processes

By 1980, research and development shifted from relatively inexpensive surfactants such as petroleum sulfonates to more cosdy but more effective surfactants tailored to reservoir and cmde oil properties. Critical surfactant issues are performance in saline injection waters, adsorption on reservoir rock, partitioning into reservoir cmde oil, chemical stabiUty in the reservoir, interactions with the mobiUty control polymer, and production problems caused by resultant emulsions. Reservoir heterogeneity can also greatly reduce process effectiveness. The decline in oil prices in the early 1980s halted much of the work because of the relatively high cost of micellar processes. 

Rate constant for exchange of solute through micellar processes... 

The copolymers of acrylamide and alkylacrylamide were prepared according to a micellar process in which sodium dodecyl sulfate was used to solubilize the water-insoluble monomer to enable copolymerization with the water-soluble monomer to occur (i). The mixture before the polymerization was clear and free of any emulsion particles or undispersed monomer. Potassium persulfate was used to initiate polymerization, which was taken to complete conversion at 50 °C for 24 h. The polymers were purified from the surfactant by acetone precipitation-redissolution and then dried in a vacuum oven at 25 °C. The amount of hydrophobe actually incorporated was taken to be equal to that used in the polymerization. 

Micellar processes (certain segments of some processes)... 

There is strong evidence from equilibrium studies, therefore, that association of the diphenylmethane derivatives occurs by a micellar process. Relaxation techniques are currently being used to further investigate the mode of association of these systems. 

The first theoretical model of surfactant adsorption from micellar solutions, proposed by Lucassen [142], uses the simplifying assumptions that the micelles are monodisperse and that the micellization happens as a single step, which is described as a reversible reaction of order n (the micelle aggregation number). Later, more realistic models, which account for the multi-step character of the micellar process, were developed [143-145]. The assumption for a complete local dynamic equilibrium between monomers and micelles makes possible to use the equilibrium mass action law for the micellization reaction [142,146,147]. In such a case, the surfactant transfer corresponds to a conventional diffusion-limited adsorption characterized by an effective diffusion coefficient, Deff, which depends on the micelle diffusivity, concentration, and aggregation number. Dgff is independent of the rate constants of the fast and slow demicellization processes and k. Joos et al. [146,147] confirmed experimentally that in some cases the adsorption from micellar solutions could be actually described as a diffusion-limited process characterized by an apparent diffusivity,... 

The computer modeling [150] shows that exhibits two exponential (kinetic) regimes, AB and CD, and two inverse-square-root (diffusion) regimes, BC and DE, see Figure 4.8. In particular, the point C corresponds to the moment Xq = (Dilhj)tc where is the characteristic time of the slow micellar process see Ref. [149]. x also serves as a characteristic relaxation... 

We recall that in the regime BC the rate constants of the fast and slow micellar processes, and ks, do not affect the surfactant adsorption kinetics, and cannot be determined from the fit of the data. In principle, it is possible to observe the kinetic regime AB (and to determine kj with faster methods or with slower surfactants. 

In summary, four distinct kinetic regimes of adsorption from micellar solutions exist, called AB, BC, CD, and DE see Figures 4.8 and 4.10. In regime AB, the fast micellar process governs the adsorption kinetics. In regime BC, the adsorption occurs under diffusion control because the... 

Amphiphilic diblock copolymers undergo a self-assembly micellar process in solvents that are selective for one of the blocks [100]. By choosing selective conditions for each block, conventional micelles and so-called inverse micelles can be formed. Examples of the so-called schizophrenic micelles were reported [101]. In this case hydrophilic AB diblock copolymers can form micelles in an aqueous solution, in which the A block forms the inner core and inverted micelles (with the B block forming the iimer core) [102]. A diblock copolymer with two weak polybases, (poly-[2-(N-morphoUno)ethyl methacrylate-i)Iock-2- and (diethyl amino)ethyl methacrylate) (PMEMA-block-DEAEMA), forms stable micelles with DEAEMA cores by adjusting the pH value of the solution. The formation of inverted micelles (MEMA core) was achieved by a salting out effect by adding electrolytes to the aqueous solution. 

Both the mass-action and phase-separation models, despite their limitations, are useful representations of the micellar process and may be used to derive equations relating the CMC to the various factors that determine it. Some insight into the role of the hydrocarbon chain in the micellization process may be gained from determinations of the free energy of micellization, AGS. A convenient method of determining AGS of ionic surfactants is from measurements of the effect of electrolyte on the CMC [172]. 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

In order to obtain more insight into the local environment for the catalysed reaction, we investigated the influence of substituents on the rate of this process in micellar solution and compared this influence to the correspondirg effect in different aqueous and organic solvents. Plots of the logarithms of the rate constants versus the Hammett -value show good linear dependences for all... 

An alternative to this process is low (<10 N/m (10 dynes /cm)) tension polymer flooding where lower concentrations of surfactant are used compared to micellar polymer flooding. Chemical adsorption is reduced compared to micellar polymer flooding. Increases in oil production compared to waterflooding have been observed in laboratory tests. The physical chemistry of this process has been reviewed (247). Among the surfactants used in this process are alcohol propoxyethoxy sulfonates, the stmcture of which can be adjusted to the salinity of the injection water (248). 

E. A. Knaggs and J. W. Hodge, Petroleum Sulfonates—Key Process Chemicals in Micellar Polymer Oil Recovery Systems, American Chemical Society,... 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

In the short term, we do not expect chiral membranes to find large-scale application. Therefore, membrane-assisted enantioselective processes are more likely to be applied. The two processes described in more detail (liquid-membrane fractionation and micellar-enhanced ultrafiltration) rely on established membrane processes and make use of chiral interactions outside the membrane. The major advantages of these... 

For example, for alkyl (8-16) glycoside (Plantacare 818 UP) non-ionic surfactant solution of molecular weight 390 g/mol, an increase in surfactant concentration up to 300 ppm (CMC concentration) leads to a significant decrease in surface tension. In the range 300 < C < 1,200 ppm the surface tension was almost independent of concentration. In all cases an increase in liquid temperature leads to a decrease in surface tension. This surface tension relaxation is a diffusion rate-dependent process, which typically depends on the type of surfactant, its diffusion/absorption kinetics, micellar dynamics, and bulk concentration levels. As the CMC is approached the absorption becomes independent of the bulk concentration, and the surfactant... 

Micellar medium has received great attention because it solubilizes, concentrates and orientates the reactants within the micelle core and in this way accelerates the reaction and favors the regio- and stereoselectivity of the process [68], In addition the micellar medium is cheap, can be reused, is more versatile than cyclodextrins and more robust than enzymes. With regard to Diels Alder reactions, we may distinguish between (i) those in which one or both reagents are surfactants which make up the micellar medium, and (ii) those that are carried out in a micellar medium prepared by a suitable surfactant. 

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