Rodlike reversed micelles

The cobalt, nickel, and copper bis(2-ethylhexyl) phosphate surfactants dissolved in n-heptane lead to quasi-one-dimensional association microstructures, i.e., rodlike reversed micelles that increase in size via water solubilization [111],... 

Z. J. Yu and R. D. Neuman. Giant rodlike reversed micelles formed by sodium bis(2-ethylhexyl) phosphate in n-heptane. Langmuir, 10(8) 2553-2558, 1994. 

FIG. 5 Representation of rodlike water-containing reversed micelles. 

The basic nanoreactor in particle formation is a reverse micelle in most cases, with a generally accepted spherical shape. The particles generated from these micelles transforming into W/O microemulsions are also often spherical in shape. However, other surfactant architectures may also yield particles. Thus, vesicles have been instrumental in the formation of particles in many cases (not discussed in this book) [98] similarly, cylindrical micelles could also generate elongated nanoparticles with the required manipulations in the system. Unfortunately clear-cut evidence on this offshoot procedure of synthesis, i.e. rodlike particle formation from rod-like micelles is apparently not so extensively available. 

Recently, Kaneko et aL [252] reported a method in which reverse micelles were first prepared from the system NP-6 (hexaoxyethylene nonylphenyl ether)/ cyclohexane, containing zinc di-n-butoxide. An aqueous ammonia solution was added to it, causing precipitation. When ammonia was not added to the system, the product was a mixture of ZnO and Zn(OH)2- In keeping with some earlier reports of synthesis by other techniques [33], the wurtzite-type zinc oxide nanoparticles were rodlike. The length and width of the particles varied as a function of the [H20]/[NP-6] and [NH3]/[NP-6] ratios, as also the concentration of the butoxide. For example, with an optimum water/surfactant concentration ratio of 4 and [NH3]/[NP-6] = 0.5, the average length and width of the particles showed values of about 21 and 13 nm respectively. 

The size and shape evolution of reversed micelles as a function of the water and surfactant concentrations are system-specific. The micellar size is mainly controlled by the strong tendency of the surfactant to be located at the interface between water and apolar solvent, which involves an enormous value of the interfacial surface and micelles of nanometric size. Spherical micelles result fl om a minimization of the micellar surface-to-volume ratio, i.e., a minimization of water-surfactant interactions less favorable than water-water and/or surfactant-surfactant interactions, while rodlike micelles, characterized by a greater surface-to-volume ratio, result from water-surfactant interactions more favorable than... 

Cooling a microemulsion may also cause a phase transformation. Thus, the reversed micelles in the AOT-isooctane-water system are spherical at 20°C but become rodlike at - 15°C and below [187]. Another instance is related to system A microemulsions, which, upon cooling, have been transformed to lamellar (L J and hexagonal (Hi) liquid crystalline phases as was confirmed by using small-angle X-ray scattering (SAXS) data (Figs. 26a, 26b). 

From the data presented in Chapter 10, it becomes evident that the extreme longevity of the artificial surfactant-stabilized microbubbles described therein is, in part, related to their continuous interaction with the simultaneously formed mixed micelle population in the saturated surfactant solution. More specifically, the surfactant-stabilized microbubbles produced by mechanical agitation of saturated solutions of either CAV-CON s Filmix 2 or Filmix 3 apparently undergo a cyclical (or reversible) process of microbubble formation/coalescence/fission/disappearance, where the end of each cycle is characterized by a collapse of the lipid-coated microbubbles into large micellar structures (i.e., rodlike multimolecular aggregates), only to re-emerge soon after as newly formed, lipid-coated microbubbles (see also below). 

Shrestha, LK., Sato, T., Acharya, D.P., Iwanaga, T., Aramaki, K., and Kunieda, H. (2006) Phase behavior of monoglyc-erol fatty add esters in nonpolar oils reverse rodlike micelles at elevated temperatures. /. Phys. Chem. B, 110, 12266-12273. 

Figure 2. Five proposed shapes of micelle (a) spherieal (b) inverted (or reversed) (e) lamellar (d) disk (e) cylindrical or rodlike.

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