Self-diffusion coefficient, surfactants

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]. 

Further information on the dependence of structure of microemulsions formed on the alcohol chain length was obtained from measurement of self diffusion coefficient of all the constitutents using NMR techniques (29-34). For microemulsions consisting of water, hydrocarbon, an anionic surfactant and a short chain alcohol and C ) the self diffusion... 

In that case the self diffusion coefficient - concentration curve shows a behaviour distinctly different from the cosurfactant microemulsions. has a quite low value throughout the extension of the isotropic solution phase up to the highest water content. This implies that a model with closed droplets surrounded by surfactant emions in a hydrocarbon medium gives an adequate description of these solutions, found to be significantly higher them D, the conclusion that a non-negligible eimount of water must exist between the emulsion droplets. 

Figure 7. Schematic diagram comparing the behavior of self-diffusion coefficients of oil (D0), water (Dw), and surfactant (Ds) expected for the droplet inversion transition and the bicontinuous transition of microemulsion depicted in Fig. 6.

Interactions between oppositely charged micelles in aqueous solutions spontaneously form vesicles. The self-diffusion coefficient of water and 2H relaxation of 2H-labeled dodecyl trimethyl ammonium chloride of the dodecyl trimethyl ammonium chloride-sodium dodecyl benzenesulfonate systems show that in these mixtures there is limited growth of the micelles with changes in composition. The vesicles abruptly begin to form at a characteristic mixing ratio of the two surfactants. The transition is continuous.205 Transformation from micelle to vesicle in dodecyl trimethyl ammonium chloride-sodium perfluoro-nonanoate aqueous solution has been studied by self-diffusion coefficient measurements, and it was found that at a concentration of 35 wt% with a molar ratio of 1 1, the self-diffusion coefficient of the mixed micelles is far smaller than that of the two individual micelles.206 The characteristics of mixed surfactant... 

We have studied a variety of transport properties of several series of 0/W microemulsions containing the nonionic surfactant Tween 60 (ATLAS tradename) and n-pentanol as cosurfactant. Measurements include dielectric relaxation (from 1 MHz to 15.4 GHz), electrical conductivity in the presence of added electrolyte, thermal conductivity, and water self-diffusion coefficient (using pulsed NMR techniques). In addition, similar transport measurements have been performed on concentrated aqueous solutions of poly(ethylene oxide)... 

NMR self-diffusion coefficients (Lindman, 1983), small-angle neutron scattering (SANS) (Cebula, 1982 Triolo, 1983 Corti, 1984), freezing point and vapor pressure methods (Herrington, 1986) and fluorescent probes (Atik, 1979) have been used to calculate aggregation numbers of several different types of surfactants (Zana, 1980 Lianos, 1980, 1981, 1982, 1983). Some aggregation numbers of surfactants are listed in Table 3-1. 

The self-diffusion approach relies on the fact that molecular displacements over macroscopic distances are very sensitive to confinement and thus to microstructure. For example, we found that at the same composition (water, oil, surfactant), the ratio between water and oil self-diffusion coefficients could differ by a factor of 100 000. This also illustrates that the microstructure is primarily determined by the spontaneous curvature of the surfactant film and not by the oil-to-water ratio. Contributions to a better understanding of microemulsion structures with FT spin-echo NMR self-diffusion starting with Stilbs, included also Nilsson, Olsson, Soderman, Khan, Guering, Monduzzi, Ceglie, Das and many others in Lund. In this work [49-63], the access to suitable systems was very important. Here, the contacts with Friberg, Shinoda, Strey and Langevin played a central role. 

Measurement of self-diffusion coefficients by means of PGSE techniques has evolved to become one of the most important tools in the characterization of surfactant systems. In particular, this is true of those surfactant systems that are isotropic liquid solutions such as micellar systems and microemulsions. The technique has been described in a number of review articles [9,11-13]. An account of the most recent developments of the method can be found in Ref 9. We do not dwell on the technical aspects here but merely note that the technique requires no isotopic labeling (avoiding possible disturbances due to addition of probes) furthermore, it gives component-resolved diffusion coefficients with great precision in a minimum of measuring time. 

Anderson and Wennerstrom [33] calculated the geometrical obstruction factors of the self-diffusion of surfactant and solvent molecules in ordered bicontinuous microstructures, which serve as good approximations also for the disordered bicontinuous microemulsions and L3 (sponge) phases. The geometrical obstruction factor is defined as the relative diffusion coefficient DIDq, where D is the diffusion coefficient in the structured surfactant system and Z)q is the diffusion coefficient in the pure solvent. In a bicontinuous microemulsion the geometrical obstruction factor depends on the water/oil ratio. An expansion around the balanced (equal volumes of water and oil) state gives, to leading order. 

Figure 7 Relative self-diffusion coefficients of water and oil plotted as a function of temperature in a three-component system, Ci2E5-water-tetradecane, at constant (16.6 wt%) surfactant concentration. Note the symmetry around the balanced temperature (here approximately 47°C) corresponding to zero spontaneous curvature. (Data taken from Ref 42.)...

Figure 10 Variations of self-diffusion coefficients of water, oil, surfactant, and cosurfactant in a salinity scan for a five-component microemulsion, SDS-butanol-water- NaCI-toluene. The experiments were performed with excess solvent(s) in the so-called Winsor sequence.The system...

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