Formed from micellar surfactant solutions

The depletion interaction is present always when a film is formed from micellar surfactant solution the micelles play the role of the smaller particles. At higher micellar concentrations, the volume exclusion interaction becomes more complicated it follows the oscillatory curve depicted in Figure 5.26. In this case only, the first minimum (that at ft —> 0) corresponds to the conventional depletion force. 

At high surfactant concentrations (i.e., much above the CMC), thin foam films were observed (48—53) to become thinner in a stepwise fashion that is, thinning foam films formed from micellar surfactant solutions exhibit a number of metastable states before attaining an equilibrium film thickness. This process can be followed in Figure 13, which shows a photocurrent (film thickness)—time interferogram of a horizontal flat film... 

Differential interferometry in reflected light allows for the measurement of the shape of the upper reflecting snrface. This method was nsed by Nikolov et al. [253,273-275] to determine the contact angle, film, and line tension of foam films formed at the top of small bubbles floating at the surface of ionic and nonionic surfactant solntions. An alternative method is the holographic interferometry applied by Picard et al. [276,277] to study the properties of bilayer lipid membranes in solution. Film contact angles can also be determined from the Newton rings of liquid lenses, which spontaneously form in films from micellar surfactant solutions [217],... 

Microstructure of Film Formed from Micellar Solution. The dependence of foam stability upon surfactant concentration is well known. Specifically, above a certain surfactant concentration (after cmc), the stability of a foam increases sharply with surfactant concentration. This fact is used in industry, where stable foams are created with surfactant concentrations far above the cmc. 

There are different types of fluorescence experiments that can be performed in micellar systems in order to obtain both static and dynamic properties. Which method one chooses depends on the type of information being searched for (and of course on the availability of a particular instrument). For example, the fluorescence technique is very useful for the study of the CMC of surfactants in solution. The experimentalist then uses a fluorescence probe and simply measures the emitted light from a continuously illuminated sample containing very small amounts of the probe. Since the emission spectrum has the feature of being sensitive to the polarity of its surroundings, one can follow a situation when hydrophobic aggregates are formed in a surfactant solution. 

In this study we examined the influence of concentration conditions, acidity of solutions, and electrolytes inclusions on the liophilic properties of the surfactant-rich phases of polyethoxylated alkylphenols OP-7 and OP-10 at the cloud point temperature. The liophilic properties of micellar phases formed under different conditions were determined by the estimation of effective hydration values and solvatation free energy of methylene and carboxyl groups at cloud-point extraction of aliphatic acids. It was demonstrated that micellar phases formed from the low concentrated aqueous solutions of the surfactant have more hydrophobic properties than the phases resulting from highly concentrated solutions. The influence of media acidity on the liophilic properties of the surfactant phases was also exposed. 

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). 

Figure 4.14 Micellar structures, (a) Spherical (anionic) micelle. This is the usual shape at surfactant concentrations below about 40 per cent, (b) Spherical vesicle bilayer structure (see also Figure 4.28), which is representative of the living cell, (c) and (d) Hexagonal and lamellar phases formed from cylindrical and laminar micelles, respectively. These, and other structures, exist in highly concentrated surfactant solutions...

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

Brinker et al. developed a simple evaporation-induced self-assembly (EISA) process that allows the rapid production of patterned porous or nanocomposite materials in the form of films, fibers, or spheres. Starting from a homogeneous solution of soluble silica and surfactant below the critical micellar concentration, evaporation of ethanol increased surfactant concentration, driving self-assembly of silica-surfactant micelles and their further organization. By... 

H -tetramethylbenzidine in anionic-cationic mixed micelles has been studied in detail by ESR . The photochemistry of the semi-oxidised forms of eosin Y and rose bengal have been investigated in colloidal solutions. Relevant to the fluorescence of proteins is a study of fluorescence quenching of indolic compounds by amino-acids in SOS, CTAB, and CTAC micelles O Rate constants for proton transfer of several hydroxyaromatic compounds have been measured in a variety of surfactant solutions. Photoprotolytic dissociation does not require exit of the reactant molecules from the micelles. Micellar solutions can be used to improve the fluorescence determination of 2-naphthol by inhibiting proton transfer or proton inducing reactions z2. jpe decay of the radical pair composed of diphenylphosphonyl and 2,4,6-trimethyl benzoyl radicals in SDS is affected by magnetic... 

Micellar-polymer flooding relies on the injection of a surfactant solution to lower interfacial tension to ultralow levels, on the order of 10 mN/m. The resulting increase in capillary number allows the recovery of residual oil from porous media. The term micellar is used because the concentrations of injected surfactant solutions are always above their critical micelle concentration. That is, they are always above the concentration at which micelles form. 

Surfactant molecules are in dynamic equilibrium among three possible states (monomers adsorbed at the interface of the aqueous solution with a non-polar phase, monomers molecularly dispersed in the solution, and micellar aggregates formed when the CMC is reached). From various theoretical considerations, as well as experimental results, it can be said that micelles are dynamic structures whose stability is in the range of milliseconds to seconds.2223 Thus, in an aqueous surfactant solution, micelles break and reform at a fairly rapid rate, in the range of milliseconds.24 26... 

In the case of liquid detergents, surfactants are almost always present. At low to intermediate concentration, most neat surfactant solutions have low viscosity and are close to Newtonian in flow. Only at higher surfactant concentrations, when structured micellar bilayers and other complex phases are formed, do systems tend to differ greatly from Newtonian. This behavior also helps drive the viscosity of finished formulations. In the great majority of liquid detergent formulations, concentrations of surfactant are such that little structure is developed by the surfactants themselves, resulting in formulations of low viscosity. As such, thickeners and/or rheology modifiers are often required to obtain the desired viscosity and flow characteristics. 

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