Rodlike micelles solutions

In a very recent investigation, hydrophobic PFS (Sect. 7.1) was attached to a hydrophilic PEO block to form an amphiphilic PFSi2-[Ru]-PE07o block copolymer [331]. Rodlike micelles were observed in water for this copolymer (Fig. 24). These micelles have a constant diameter but are rather polydisperse in length, and DLS measurements indicate that they are flexible. Crystallization of the PFS in these micelles was observed and is thought to be the key behind the formation of rodlike structures. The cylindrical micelles can be cleaved into smaller rods whenever the temperature of the solution is increased or whenever they are exposed to ultrasound. 

In summary, while the nonionic surfactant dimethyldo-decylamine oxide forms only spherical micelles even in 0.20 M NaCl (see above and ref. 498), micelles of dimethyloleylamine oxide are subject to a sphere-rod equilibrium in aqueous solutions of NaCl as dilute as 10 4 M and even in water alone (ref. 500). Thus, Imae and Ikeda (ref. 479) conclude the rodlike micelles are stabilized more, as compared with the spherical micelles, when the hydrocarbon chain of the surfactant molecule is longer. This conclusion is, therefore, consistent with the earlier-mentioned belief of these authors that the rodlike micelles are more stable when the polar head group of the surfactant molecule is smaller and the chain length of its hydrocarbon part is longer (ref. 473). Since the surfactants referred to all behave as nonionics, these findings of rodlike micelle production have direct relevance to the formation of artificial gas microbubbles (see Section 10.3) with either the earlier-mentioned surfactant mixture Filmix 3 (see Chapter 9 and Section 10.4) or another, related surfactant preparation (see Section 10.4). 

S. Ikeda, S. Hayashi and T. Imae, Rodlike micelles of sodium dodecyl-sulfate in concentrated sodium-halide solutions, J. Phys. Chem. 85(1981) 106-112. 

T. Imae and S. Ikeda, Rodlike micelles of dimethyloleylamine oxide in aqueous NaCl solutions, and their flexibility and size distribution, Colloid Polym. Sci. 262 (1984) 497-506. 

SDS/NaCI Mixtures. The effect of temperature on the micelles formed in 70 mM SDS + NaCl solutions is presented below. Mazer et al. (14) have found that the aggregation number, N, is at a maximum for supercooled solutions below the critical micellization temperature (cmt), and decreases towards the value expected for a spherical micelle as the temperature is increased. The variations in N with temperature are dependent on the concentration of added electrolyte, with the rodlike micelles formed in high salt (0.6 M) showing large variations, and the spherical micelles formed in little (0.3 M) or no salt showing only small variations. 

Surfactant solutions with rod-like micelles can have notable viscosities up to six times higher than the water viscosity [37]. This can be explained by the presence of entangled rodlike micelles (often also called worm-like micelles or thread-like micelles) which arrange themselves in a supramolecular transient network [38-41 ]. Such solutions often have elastic properties but they do not show a yield stress. This means that even high viscous solutions flow under the influence of very small shear stress. In this situation they show a zero shear viscosity which is given by ... 

There are several ambiguities in the data analysis that can only be resolved with further experiments. Data at lower angles when added to those in Fig. 11 should provide a better fit to the hydrodynamic diameter of the rodlike micelles. In addition, DLS experiments on solutions of different concentrations are needed in order to be able to extrapolate the dependence of r/q2 as a function of qL to zero concentration. In this way one can avoid any influence of possible concentration dependence of micelle size on the DLS results. 

Viscoelastic Detergent Solutions from Rodlike Micelles... 

Many surfactant solutions are normal Newtonian liquids even up to rather high concentrations. Their viscosities are very small as compared with the viscosity of the solvent water. This is particulary the case for micellar solutions with concentrations up to 20% W/W in which spherical micelles are present. Even in the presence of rodlike micelles the viscosities can be rather low. Systems with rodlike micelles have recently been studied extensively. Missel et al. (1-2) studied alkylsulfate solutions and showed how the lengths of the rods can be varied by the addition of salt or by the detergent concentration. Under all these conditions the solutions are of rather low viscosity. On the other hand, we have studied a number of cationic detergent solutions in which rodlike micelles were formed and which all became quite viscous at rather low concentrations. In addition some of these solutions had elastic properties. The phenomenon of viscoelasticity in detergent solutions is not new. Exten-... 

Here T 0 means the viscosity of the solution at zero shear rate, r g the viscosity of the pure solvent, 6 the number of rods per unit volume and L the lengths of the rods. As long as L is still shorter than the mean spacing between the aggregates, the viscosity of the detergent solution is still very low and always small in comparison with the solvent viscosity. As a consequence, very accurate measurements are necessary to get some informations on the dimensions of the rodlike micelles. The Zimm-Crothers viscometer is a very sensitive instrument and it was possible to measure the viscosity of the dust-and airfree solutions with an accuracy of 0,2% at very low shear rates (15). 

Table I. The lengths of the Rodlike Micelles as a Function of the Concentration for Solutions of CPySal at Various Temperatures...

The concentration at which surfactants form rodlike micelles is called CMCII. Critical micelle concentration is almost independent of temperature while CMCII increases with temperature. Spherical micelles, rod-like or thread-like micelles, and vesicles are the three most common microstructures seen in dilute DR surfactant solutions. 

Prud homme, R.K. Warr, G.G. Elongational flow of solutions of rodlike micelles. Langmuir 1994, 10 (10), 3419-3426. 

At higher surfactant concentrations the ratio of alcohol to surfactant molecules does not exceed 2 at the phase boundary, and it appears that the alcohols promote structural changes of the micelles, for example from spherical to rodlike or ellipsoidal structures. This structural change is dependent both on the surfactant concentration and on the amount of solute, which is apparent from the study carried out by Backlund and co-workers who mapped the whole LI-phase of the SDS-1-hexanol and C gBr-l-hexanol systems demonstrating regions in the phase diagram relating to spherical micelles, spherical swollen micelles, and rodlike micelles. 

---