Micellar size measurements

In order to test further the applicability of 1-pyrene carboxaldehyde as a fluorescent probe, we applied Keh and Valeur s method (4) to determine average micellar sizes of sulfonate A and B micelles. This method is based on the assumption that the motion of a probe molecule is coupled to that of the micelle, and that the micellar hydrodynamic volumes are the same in two apolar solvents of different viscosities. For our purposes, time averaged anisotropies of these systems were measured in two n-alkanes hexane and nonane. The fluorescence lifetime of 1-pyrene carboxaldehyde with the two sulfonates in both these solvents was found to be approximately 5 ns. The micellar sizes (diameter) calculated for sulfonates A and B were 53 5A and 82 lOA, respectively. Since these micelles possesed solid polar cores, they were probably more tightly bound than typical inverted micelles such as those of aerosol OT. Hence, it was expected that the probe molecules would not perturb the micelles to an extent which would substantially affect the micellar sizes measured. 

Flocculation values achieved from turbidity measurements using the light scattering technique showed improvement with nonylphenol ether carboxylic acid (4 mol EO) in particular. The oil solubilization rate has been found to be proportional to the surfactant micellar size [190]. 

Pyrene carboxaldehyde has utility as a fluorescent probe in some Inverted micellar systems containing solubilized Inorganic species in the polar core. Its fluorescence lifetime is ca. 5 ns thus it is an appropriate probe for measuring micellar sizes which are approximately lOOA. 

We have no measurements of micellar size, since the translation of micelle size into the number of monomers in the micelle is not a simple task and requires assumptions not easily experimentally tested. We are hopeful of extending experimentation in this direction in future research. Table II lists dielectric constants, dipole moments and effective polarities for methanol, 1- and 2-octanol, and water at 25°C. 

The methods mentioned above do not measure the micellar aggregation number itself but rather some micellar size. A direct determination of aggregation numbers can be performed by analysing some physico-chemical parameter in terms of the equilibria involved equilibrium analyses on the basis of potentiometric data have been pioneered by Danielsson and co-workers who studied short-chain, not typically micelle-forming, amphiphiles in the presence of added electrolyte (see above). 

Fig. 16 displays the Rf, values derived by QLS measurements of two bile salt-lecithin (BS-L) systems at 10 g/dl as fucntions of the L to BS ratio and 3 temperatures. As L/BS ratio is increased, micellar sizes vary markedly. In the... 

To make the significance of the NMR technique as an experimental tool in surfactant science more apparent, it is important to compare the strengths and the weaknesses of the NMR relaxation technique in relation to other experimental techniques. In comparison with other experimental techniques to study, for example, microemulsion droplet size, the NMR relaxation technique has two major advantages, both of which are associated with the fact that it is reorientational motions that are measured. One is that the relaxation rate, i.e., R2, is sensitive to small variations in micellar size. For example, in the case of a sphere, the rotational correlation time is proportional to the cube of the radius. This can be compared with the translational self-diffusion coefficient, which varies linearly with the radius. The second, and perhaps the most important, advantage is the fact that the rotational diffusion of particles in solution is essentially independent of interparticle interactions (electrostatic and hydrodynamic). This is in contrast to most other techniques available to study surfactant systems or colloidal systems in general, such as viscosity, collective and self-diffusion, and scattered light intensity. A weakness of the NMR relaxation approach to aggregate size determinations, compared with form factor determinations, would be the difficulties in absolute calibration, since the transformation from information on dynamics to information on structure must be performed by means of a motional model. 

Initially, the decane and hexadecane microemulsion systems were studied at the emulsification boundary T m) to make an accurate comparison between the two systems. Deuterium-NMR relaxation using selectively deuterium-labeled surfactant is a useful method to detect variation of micelle size when changing composition and/or temperature. In a relaxation NMR experiment the combined motion of longitudinal (1/Ti) relaxation of the micelles and the transverse relaxation (I/T2) of the surfactants inside the film (internal diffusion) are measured. The measured parameters are therefore sensitive to the micellar size but essentially insensitive to intermicellar interaction. More information can be found in papers by Wennerstrom [133] and Halle [33]. At Tbpb the microemulsion droplets are generally spherical at infinite dilution [84]. The results from the relaxation measurements (expressed as l/r2-l/Ti) on the two systems are presented in Figure 3.3. 

Inspection of Figs 5 and 6 shows that at high probe concentration, limiting values of D tend toward Di. Thus, a reasonable estimate of D for monodisperse systems can be obtained by measuring D at relatively large probe concentrations, for example, 2 to 5 mM. For polydisperse systems, additional equilibria must be considered. A three-state model was used to develop a version of Eq. (12) for two micellar size distributions, and applied to systems with coexisting globular and rodshaped micelles [4]. 

Nambam and Philip (2012) studied the interaction of a nonionic surfactant (nonylphenolethoxyl-ate [NPg]) with a triblock copolymer of PEO-PPO-PEO in aqueous solution. The micellar size, zeta potential, and the rheological properties were measured. They also compared the results with the interactions of the same copolymer with an anionic surfactant (sodium dodecyl sulfate [SDS]) and a cationic surfactant (cetyltrimethylammonium bromide [CTAB]). They reported that the addition of NPg does not have any effect on the viscoelastic properties. The same result was observed in the case of CTAB whereas a drastic reduction was reported in the case of SDS. 

---