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- Evilevitch, Alex, et al.
(författare)
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Computer simulation of molecular exchange in colloidal systems
- 2002
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - AMER CHEMICAL SOC. - 1520-6106. ; 106:45, s. 11746-11757
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we introduce two computer simulation models to study molecular exchange between aggregates in a colloidal dispersion. The Brownian motion of the colloidal aggregates is simulated as a random walk with a Gaussian distributed step length. In model I, the exchanging molecules are simulated as discrete particles with the exchange process characterized by desorption, molecular diffusion, and adsorption. A molecule desorbed from an aggregate is registered in the simulation and allowed to undergo individual Brownian motion until it adsorbs onto another colloidal aggregate or returns to the same aggregate from which it originated. In this detailed simulation, random size fluctuations are obtained in addition to a net variation in a relaxing, nonequilibrium size distribution. For many processes, net variations are very slow compared to the random fluctuations, making this detailed method very time consuming for studying a relaxing size distribution. For this reason, we also consider, in model II, a more approximate method where only the net flow of molecules between aggregates is considered. Here, the flow of molecules is for each time step calculated assuming steady state conditions and pair wise additivity. The flow between an isolated pair of aggregates can be solved exactly. Although the pair flow is a good approximation at short separations, it becomes significantly reduced at larger separations because of the presence of other aggregates. This screening of the flow at larger separations is accounted for by introducing an exponential damping function. With these models, we have simulated the solubilization of larger oil drops by smaller micelles which has previously been experimentally studied in a nonionic surfactant-water-oil system. Besides comparing with experiments, the simulations provides a test of a previous mean-field cell model analysis of the solubilization process.
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| 2. |
- Evilevitch, Alex, et al.
(författare)
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Kinetics of oil solubilization in microemulsion droplets. Mechanism of oil transport.
- 2001
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Ingår i: Langmuir. - American Chemical Society. - 1040-0397. ; 16:23, s. 8755-8762
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the kinetics of the solubilization of oil through a temperature jump into a droplet microemulsion phase in the system water-pentaethylene oxide dodecyl ether-decane at 25 C. The initial state is formed by subjecting the equilibrium system at 25 C to a temperature quench to 22, 20, and 14 C, respectively. At this lower temperature, which at equilibrium corresponds to a two-phase system, oil droplets form and grow in size with increasing time. By varying the time between the quench and the T-jump, the size of the initial oil drops is varied in a systematic and known way in the relaxation study. The relaxation process is monitored by following the turbidity of the system. We find that for all the systems the relaxation back to equlibrium is much faster than the drop growth process observed after the temperature quench. This general observation is explained by realizing that the redissolution of the oil drops is analogous to the oil transfer phase, which in the quench experiment occurs prior to the Ostwald ripening phase. More significant is that we observed a qualitative transition in the relaxation behavior when the initial aggregate distribution is varied. In all cases we have the same initial temperature and overall composition and one population of many small droplets and fewer larger drops. The size of the larger drops only affects the relaxation in a quantitative way. If the small droplets are only slightly smaller than the equilibrium size, equilibration occurs through the diffusion of oil molecules in the bulk phase. When the initial droplets are sufficiently small, a new kinetic route is available where there is an efficient direct oil transfer between the small droplets and large drops. This allows for a fast relaxation of the oil distribution between the two populations of drops.
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| 3. |
- Evilevitch, Alex, et al.
(författare)
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Molecular transport in a nonequilibrium droplet microemulsion system.
- 2001
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Ingår i: Langmuir. - American Chemical Society. - 0743-7463. ; 17:22, s. 6893-6904
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we consider the problem of oil molecular transport between nonequilibrium microemulsion oil droplets in water. In particular, we have investigated the kinetics of solubilization of big oil drops by smaller microemulsion droplets transforming a bimodal size distribution into an equilibrium microemulsion with a unimodal size distribution. The process involves the diffusion of oil monomers across the aqueous solvent. Solubilization experiments are presented on a well-characterized nonionic microemulsion system, where interfacial area and dispersed volume are conserved and where the excess oil chemical potential is dominated by interfacial curvature energy. An analysis of the experiments shows that the solubilization kinetics depend significantly on the concentrations and sizes of big and small droplets. We formulate a quantitative model for the solubilization kinetics where the effects of size and concentrations are treated within the framework of a cell model. A quantitative agreement between model and experiment is obtained, and the analysis also shows that the majority of oil monomers are captured by small droplets already in the vicinity of the big drop surface when the concentration of small droplets is high.
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