Surfactants monodisperse

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

Sometimes it can be advantageous to use mixtures of alkanesulfonates and nonionic surfactants, such as Triton X-100, to prepare monodisperse polystyrene particles [95]. 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

Highly monodisperse reversed micelles are formed by sodium bis(2-ethylhexyl) sul-fosuccinate (AOT) dissolved in hydrocarbons that are in equilibrium with monomers whose concentration (cmc) is 4 X 10 M, have a mean aggregation number of about 23, a radius of 15 A, exchange monomers with the bulk in a time scale of 10 s, and dissolve completely in a time scale of 10 s [1,2,4,14], Other very interesting surfactants able to form reversed micelles in a variety of apolar solvents have been derived from this salt by simple replacing the sodium counterion with many other cations [15,16],... 

Independent of the nature of the apolar solvent, nearly spherical and monodisperse water-containing reversed micelles are formed by AOT, whose size is quite independent of the surfactant concentration and regulated mainly by the molar ratio R(R = [water]/[sur-factant]) [5,84,85]. 

Moreover, stable liquid systems made up of nanoparticles coated with a surfactant monolayer and dispersed in an apolar medium could be employed to catalyze reactions involving both apolar substrates (solubilized in the bulk solvent) and polar and amphiphilic substrates (preferentially encapsulated within the reversed micelles or located at the surfactant palisade layer) or could be used as antiwear additives for lubricants. For example, monodisperse nickel boride catalysts were prepared in water/CTAB/hexanol microemulsions and used directly as the catalysts of styrene hydrogenation [215]. 

Using a similar procedure, based on the thermal decomposition of a metal-surfactant complex followed by mild oxidation, we synthesized highly crystalline and monodisperse nanocrystals of cobalt ferrite (CoFc204), manganese ferrite (MnFe204) MnO, and Ni [5]. 

The efficient hydrogenation of various benzene compounds in biphasic systems has also been described by similar surfactant-protected irid-ium(O) nanoparticles [47]. The solubility of the nanoparticles was assured by 10 equivalents of water-soluble N,N-dimethyl-N-cetyl-Ar-(2-hydroxyethyl)-ammonium chloride salt. TEM observations show that the particles are monodispersed in size with an average diameter of 1.9 0.7 nm (Fig. 7). 

Particle separation can be characterized by the separation factor, Rp, which is the ratio of eluant to particle elution volumes, or, by the difference in elution voliame, AV, between particle and eluant marker turbidity peaks. For polystyrene monodisperse standards, a linear relationship occ irs between the log of the particle diameter and AV, with a series of parallel lines resulting for different concentration of either salt or surfactant below its critical micelle concentration (IT>18,19) The separation factor has also been shown to be independent of eluant... 

Calculations for Rp as a function of the relevant experimental parameters (eluant ionic species concentration-including surfactant, packing diameter, eluant flow rate) and particle physical and electrochemical properties (Hamaker constant and surface potential) show good agreement with published data (l8,19) Of particiilar interest is the calculation which shows that at very low ionic concentration the separation factor becomes independent of the particle Hamaker constant. This result indicates the feasibility of xmiversal calibration based on well characterized latices such as the monodisperse polystyrenes. In the following section we present some recent results obtained with our HDC system using several, monodisperse standards and various surfactant conditions. 

Electric-field-directed growth of gold nanorods in aqueous surfactant solutions. Advanced Functional Materials, 14, 571-579 (d) Jana, N.R. (2005) Gram-scale synthesis of soluble, near-monodisperse gold nanorods and other anisotropic nanoparticles. Small,... 

Figure 2 schematically presents a synthetic strategy for the preparation of the structured catalyst with ME-derived palladium nanoparticles. After the particles formation in a reverse ME [23], the hydrocarbon is evaporated and methanol is added to dissolve a surfactant and flocculate nanoparticles, which are subsequently isolated by centrifugation. Flocculated nanoparticles are redispersed in water by ultrasound giving macroscopically homogeneous solution. This can be used for the incipient wetness impregnation of the support. By varying a water-to-surfactant ratio in the initial ME, catalysts with size-controlled monodispersed nanoparticles may be obtained. 

The reverse ME technique provides an easy route to obtain monodispersed metal nanoparticles of the defined size. To prepare supported catalyst, metal nanoparticles are first purified from the ME components (liquid phase and excess of surfactant) while retaining their size and monodispersity and then deposited on a structured support. Due to the size control, the synthesized material exhibits high catalytic activity and selectivity in alkyne hydrogenation. Structured support allows suitable catalyst handling and reuse. The method of the catalyst preparation is not difficult and is recommended for the... 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

In the past few years, a range of solvation dynamics experiments have been demonstrated for reverse micellar systems. Reverse micelles form when a polar solvent is sequestered by surfactant molecules in a continuous nonpolar solvent. The interaction of the surfactant polar headgroups with the polar solvent can result in the formation of a well-defined solvent pool. Many different kinds of surfactants have been used to form reverse micelles. However, the structure and dynamics of reverse micelles created with Aerosol-OT (AOT) have been most frequently studied. AOT reverse micelles are monodisperse, spherical water droplets [32]. The micellar size is directly related to the water volume-to-surfactant surface area ratio defined as the molar ratio of water to AOT,... 

The reactive crystallization has some peculiar characteristics like insoluble product, initiation of reaction by change in pH and conductivity. In this case the solution becomes saturated and eventually supersaturated with respect to reactant nucleation [30], The ultrasound assisted decomposition precursors includes dissolving metal organic precursors in organic solvents/water with the assistance of surfactants leads to monodisperse and reduced metal/metal oxide nanoparticles. 

The classic studies of Saunders( 17) demonstrated that in the presence of excess surfactant methyl cellulose (MC) would desorb from monodispersed polystyrene latices. MC is one of the most surface active water-soluble polymers (W-SPs) and it will readily dominate the surface pressure 7T (7T = cre - cr t where cr is the surface tension of water and is the surface tension of the aqueous polymer solution) of the aqueous solution. For example, hydroxyethyl cellulose (HEC) lowers the surface tension of water much less than MC or HPMC, and when the combination of HEC and MC or HPMC in water is studied, there is no notable influence of HEC on the surface pressure (Figure 2). 

Ali, S. A. Sengupta, M. J., Preparation and characterization of monodisperse polystyrene latexes of varying particle sizes without the use of surfactants, Polym. Mater. Sci. Eng. 1991, 8, 243 250... 

Figure 2.18 The pair potential calculated for polystyrene particles of radius 500 nm, with a measured (.-potential of —12mV in 0.5moldm 3 electrolyte. A steric barrier of S = 3.5 nm was used as the particles had monolayer coverage of a monodisperse non-ionic surfactant. This was C 2E06 which represents a dodecyl hydrophobic moiety linked to hexaethylene glycol via an ether link...

Taniguchi T, Takeuchi N, Kobaru S, Nakahira T (2008) Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant. J Colloid Interface Sci 327 58-62... 

---